Mars: Algae, Lichens, Fossils, Minerals, Microbial Mats, and Stromatolites in Gale Crater
Mars: Algae, Lichens, Fossils, Minerals, Microbial Mats, and Stromatolites in Gale Crater
Journal of Astrobiology and Space Science Reviews, 3, 40-111, 2020; ISSN 2642-228X, DOI: 10.37720/jassr.03082020

Mars: Algae, Lichens, Fossils, Minerals, Microbial Mats, and Stromatolites in Gale Crater

R. G. Joseph1, L. Graham2, Burkhard Büdel3, Patrick Jung4,
G. J. Kidron5, K. Latif6, R. A. Armstrong7,
H. A. Mansour8, J.G. Ray9, G. J. P. Ramos10, L. Consorti11, V. Rizzo12,
R. Schild13

1. Astrobiology Associates of California,
2. Dept. of Botany, University of Wisconsin-Madison, USA,
3. Plant Ecology and Systematics, Biology Institute, University of Kaiserslautern, Germany,
4. University of Applied Sciences Kaiserslautern, Applied Logistics and Polymer Sciences, Germany,
5. Institute of Earth Sciences, The Hebrew University of Jerusalem, Israel,
6. National Centre of Excellence in Geology, University of Peshawar, Khyber Pakhtunkhwa, Pakistan
7. Aston University, Birmingham, UK.
8. Dept. of Botany, Ain Shams University, Cairo, Egypt
9. School of Biosciences, Mahatma Gandhi University, Kerala, India
10. Phycology Laboratory, State University of Feira de Santana, Brazil
11. Department of Mathematics and Geosciences, University of Trieste, Trieste, Italy.
12. National Research Council (Emeritus), I.S.A.FO.M. U.O.S., Cosenza, Italy
13. Dept. Astrophysics, Harvard-Smithsonian (Emeritus), Cambridge


Gale Crater was an ancient Martian lake that has periodically filled with water and which may still provide a watery environment conducive to the proliferation and fossilization of a wide range of organisms, especially algae. To test this hypothesis and to survey the Martian landscape, over 3,000 photographs from NASA's rover Curiosity Gale Crater image depository were examined by a team of established experts in astrobiology, astrophysics, biophysics, geobiology, microbiology, lichenology, phycology, botany, and mycology. As presented in this report, specimens resembling terrestrial algae, lichens, microbial mats, stromatolites, ooids, tubular-shaped formations, and mineralized fossils of metazoans and calcium-carbonate encrusted cyanobacteria were observed and tentatively identified. Forty-five photos of putative biological specimens are presented. The authors were unable to precisely determine if these specimens are biological or consist of Martian minerals and salt formations that mimic biology. Therefore, a review of Martian minerals and mineralization was conducted and the possibility these formations may be abiogenic is discussed. It is concluded that the overall pattern of evidence is mutually related and that specimens resembling algae-like and other organisms may have colonized the Gale Crater, beginning billions of years ago. That some or most of these specimens may be abiotic, cannot be ruled out. Additional investigation targeting features similar to these should be a priority of future studies devoted to the search for current and past life on Mars.

Key Words: Mars, Algae, Cyanobacteria, Lichens, Fungi, Extremophiles, Minerals, Calcium Carbonate, Trace Fossils, Microfossils, Mineralized Fossils, Hydrated Minerals, Gale Crater, Water, Ooids, Nostoc Balls, Astrobiology, Geobiology, Lake Thetis


1. Life on Mars and in the Gale Crater
The authors of this report provide visual evidence of specimens, photographed within the Gale Crater, which resemble terrestrial algae, lichens, microbial mats, stromatolites, ooids, tubular-shaped and calcium carbonate encrusted organisms, and fossil-like formations reminiscent of metazoans. The authors' interpretations are speculative and Martian minerals and salts may have contributed to these observations as detailed in this report. Our purpose in presenting these observations is to identify and target specimens for future research and additional examination and investigation, and for possible extraction and analyses by robotic missions to Mars.

There is substantial evidence for life on Mars (Dass, 2017; Joseph 2014; Joseph et al. 2019, 2020a; Krupa 2017; Levin and Straat, 1976, 2016; Rabb, 2018; Small, 2015; Thomas-Keprta et al. 2002, 2009). For example, vast colonies of mushroom-shaped, lichen-like organisms, possibly engaged in photosynthesis, have been observed, attached by thin stems, to hundreds of rocks photographed in Eagle Crater by the rover Opportunity; whereas over a three-day period, 23 puffball-shaped organisms grew up out of the ground and expanded in size in surrounding areas of Meridiani Planum in the absence of wind or other abiotic contributing factors (Joseph et al. 2020a). No other specimens resembling living organisms have yet been reported for Meridiani Planum, which includes Eagle Crater. By contrast, green specimens have been observed in Utopia Planitia and Chryse Planitia (Levin et al. 1978), and those with features similar to algae and stromatolites have been photographed in Gusav Crater and Gale Crater in particular (Joseph, 2014, 2016; Krupa 2017; Noffke 2015; Rabb, 2018, Rizzo and Cantasano 2016; Rizzo 2020; Ruffi and Farmer 2016; Small, 2015).

2. Water and Gale Crater

Gale Crater may have and may still provide a moist environment conducive to the proliferation of life with temperatures as high as 6°C (43°F), as measured by the rover Curiosity's onboard Remote Environment Monitoring Station. Gale Crater has all the characteristics of a dried lake that is periodically replenished with water (Williams et al. 2013; Grotzinger et al. 2014; Vaniman et al. 2014; Bridges et al. 2015; Steele et al. 2017) as indicated by its numerous fluvial valleys and water pathways (Grotzinger et al. 2015). An analysis by the rover Curiosity's suite of sampling instruments also indicates that minerals, clays and mudstones have been repeatedly hydrated (Williams et al. 2013; Grotzinger et al. 2014; Vaniman et al. 2014); and there is also evidence that moisture may be available, on a daily basis, at ground level, during the summer months (Castro et al. 2015; Martin-Torres et al. 2015; Steele et al. 2017) and that pure water ice has been trapped inside the Curiosity rover wheels (Joseph et al. 2020b).

Two layers of clouds have also been observed above Gale Crater (Moores et al. 2015). On Earth, clouds consist of water with saturation ranging from 81% to 100% (Pruppacher and Klett 2010; Hu et al. 2010). Above Earth, water content levels of at least 95% have also been measured in ice-clouds and supercooled clouds with temperatures between −40°C and 0°C (Hu et al. 2010). Therefore, the clouds above Gale Crater also likely have a significant water content, despite even subzero temperatures, thereby inducing precipitation and providing moisture for Martian organisms especially when temperatures rise above freezing.

Columns of water vapors also form every spring and summer as based on data collected from orbital observations (Smith, 2004; Read and Lewis, 2004), and these vapors are transported from the north toward the equator by southerly winds, and thus, toward Gale Crater (Harri et al. 2014). Moreover, these vapors have a precipitable water content of at least 10–15 pr um (Smith, 2004). The vapors also appear to reach saturation, depending on humidity (Harri et al. 2014). Hence, given these vapors and layers of clouds, precipitation may moisten the surface on a daily basis, especially during the summer (Martin-Torres et al. 2015; Steele et al. 2017).

Martin-Torres and colleagues (2015), based on data from the Rover Curiosity's suite of sampling instruments, including measurements of ground temperature, air pressure and humidity, have reported the possible "formation of night-time transient liquid brines in the uppermost 5 cm of the subsurface that then evaporate after sunrise." Steele et al (2017) have come to similar conclusions and have argued that during "the evening and night, local downslope flows transport water vapour down the walls of Gale crater. Upslope winds during the day transport vapour desorbing and mixing out of the regolith up crater walls, where it can then be transported a few hundred metres into the atmosphere" (Steele et al. 2017).

Therefore, as reviewed here, there is evidence that the Martian water cycle, within the Gale Crater, follows the standard hydrologic cycle which characterizes water recycling on Earth (Fedorova et al. 2020), i.e. evaporation, vaporization, precipitation, condensation, freezing, melting, evaporation, thereby periodically forming frost, ice, and surface moisture, depending on temperature and relative humidity.

It is also believed that Martian water may be stored in underground aquifers (Malin and Edgett 2000), and is sequestered in rocks and hydrated minerals, or is locked within frozen ground (Biemann et al. 1977; Plaut et al. 2007; Mustard et al. 2012; Kieffer et al. 1976; Farmer et al. 1977), only to melt and pool upon the surface when temperatures rise above freezing. Geological features also indicate that flowing water, streams and run-off have carved fluvial pathways down the hillsides and across the surface of Gale Crater, only to eventually seep beneath the surface, evaporate, or freeze (Grotzinger et al. 2015; Steele et al. 2017).

Gale Crater may have first filled with water and became "Gale Lake" nearly 4 billion years ago, soon after the crater was formed. Based on morphological observations, Fairen et al. (2014) described what they believed to be "evidence for ancient glacial, periglacial and fluvial (including glacio-fluvial) activity within Gale crater, and the former presence of ground ice and lakes." Likewise, based on morphology, Oehler (2013) argued that landforms indicate a "major history of water and ice in Gale crater, involving permafrost, freeze-thaw cycles, and perhaps ponded surface water." Masson et al. (2001) have come to similar conclusions.

The substantial evidence of hydrated minerals also indicates that Gale Crater has been repeatedly inundated with large bodies of water which could have sustained a variety of species, some of which may have become fossilized or mineralized (Grotzinger et al. 2012, 2015). These possibilities are supported by the findings of Eigenbrode and colleagues (2018) who reported organic matter preserved in three billion year old mudstone at Gale crater--and which may represent the residue of Martian organisms.

3. Water, Minerals, the Search for Life in Gale Crater

It appears that there is sufficient water available in the Gale Crater to support the growth and proliferation of a variety of species, beginning billions of years ago when the crater was first formed and filled with water. Moreover, early in its history, when Gale Crater was "Gale Lake," this watery environment may have facilitated the construction of stromatolites (Noffke, 2015; Joseph et al. 2019; Rizzo 2020), and could have induced the fossilization of various species (Grotzinger et al. 2012, 2015).

To determine if Gale Crater may be host to living organisms and to search for evidence of fossils, over three thousand images photographed by the rover Curiosity were examined. The authors of this report include experts in algae, cyanobacteria, lichens, microbial mats, stromatolites, microfossils, mineralogy and geology.

As detailed here, the authors have observed Martian specimens which resemble fossilized or trace fossils of various organisms including metazoans, and those with features similar to or which closely resemble green algae, cyanobacteria, lichens, fungi, and mat-forming organisms. The authors also identified three domical-shaped and mat-like structures which are morphologically similar to microbial mats and the living stromatolites of Lake Thetis, Australia, a possible analogue environment for Mars and the Gale Crater when (and if) flush with water (Baldridge et al. 2009; Bridges et al. 2015; Graham et al. 2016; Nguyen et al. 2014).

Various terrestrial minerals may have a green color similar to algae, and minerals and clumps of sand and salt may come to have unusual shapes secondary to wind, moisture, and weathering. Therefore, to assist in determining possible abiogenic contributions to these life-like observations, all reports related to the discovery of salts and relevant minerals in the Gale Crater are reviewed in Section IV of this article.


Image Search: The rover Curiosity landed on Aeolis Palus inside Gale Crater, Mars, on August 6, 2012 (Sol 1) and has since transmitted over 20,000 images to Earth. Over 3000 images photographed between Sol 1 and Sol 1000, by the rover Curiosity's suite of cameras, and over 200 images photographed between Sol 1800 to 1900 were examined for features suggestive of living and fossilized organisms. Two hundred and fifty-three photographs, selected from these 3000+ images based on the target features were subject to additional visual analyses which included enlarging these photos by 300%. Of these 253 NASA-Curiosity photographs, 95 were judged to clearly depict the targeted features, 87 of which were in color and subsequently subject to color extraction protocols and additional visual inspection. It should be noted, that often, when obvious biological features were photographed, dozens to several hundred photos photographed immediately before or after these specimens were imaged were blanked out and could not be viewed on the NASA website.

Color Extraction: According to NASA, the rover Curiosity cameras have been calibrated to take “approximate true color” photos of the Martian landscape which are "imaged in 3 science color filters." They are "integrated" via an "RGB Bayer pattern filter" and the "RGB filter array" which makes possible "RGB imaging" (see However, NASA has admitted the emphasis given to the colors red, green and blue, are not equal, and certain (unidentified by NASA) color "pixels are poorer than in other pixels," and there is color leakage such that certain colors NASA has not identified were given less prominence in the resulting RGB images. Moreover, colors are adjusted based on "color calibration targets," such that the assumed colors of certain targeted abiotic features became the standard color which were given prominence over all other colors. As detailed by R. L. Levin of Lockheed Martin, NASA's Mars cameras have been calibrated such that there is an "excess of red" in the color photos transmitted back to Earth (Levin 2016) and that blues and greens become less evident or obvious. Hence, there is good reason to suspect that reds are emphasized in all NASA Curiosity color photos, and greens and blues which are more likely to depict living organisms or water on the surface, are deemphasized by the RGB pattern filter array.

For the purposes of this study, and to emphasize contrasting biogenic vs abiogenic surface features, the 87 NASA-Curiosity color photos judged to depict biological features, were subject to Red-Blue-Green color extraction, employing Fotor High Dynamic Range (HDR) image enhancement software which restored color balance and emphasized the natural colors within the original NASA images. No false colorization or tinting was applied to these NASA photos by the authors or their associates. Rather, greens and blues which were present in the original NASA photos were extracted and given near-equal balance compared to reds.

To ensure the accuracy of the HDR extraction process, six color photos of the Negev Desert, photographed by one of the authors (GJK) were subject to color extraction protocols. In five of the Negev Desert photographs, where no greens or blues were evident in the original photo or observed by the photographer when viewing the landscape, no greens or blues emerged following HDR extraction. In the sixth photo the HDR process was found to emphasize in equal measure, the green color of a Negev Desert bush and the red-brown color of the reddish-brown Negev surface.

Biological Features Analyses. These 87 HDR color photos and the eight black and white photos were subject to additional visual inspection by the authors. Based on the presumed biological features depicted (e.g. algae, lichen, fossil), relevant photos were distributed to outside experts with an expertise in those specific areas of biology and who were asked to provide feedback.

Of the 87 HDR photos and eight black and white photos, 45 were determined to depict the most obvious biological features. These 45 photos were then subject to additional visual analyses by the authors of this study, each of whom wrote summaries of their observations and which included identification of the specimens depicted.

Results: The authors of this study, each of whom has an expertise in the relevant subject matter, identified specimens that clearly resemble terrestrial algae, lichens, microbial mats, stromatolites, ooids, and tubular-shaped and calcium-carbonate encrusted fossil-like organisms. In addition, specimens resembling calcium-carbonate biosignatures and fossilized open-cone-like gas-bubble vents which are associated with photosynthesis-oxygen respiration (Bengtson et al. 2009; Sallstedt et al. 2018) were identified, as well as masses of tubular-shaped formations which resemble colonies of mat-forming filamentous organisms (Figures 1, 12, 16, 24-28). In total, these putative biological specimens were photographed by the rover Curiosity, in various locations of Gale Crater, on Sols 122, 173, 271, 298, 304, 305, 309, 319, 322, 528, 529, 809, 840, 853, 871, 880, 890, 1905.

The putative microbial mats presented in this report were also compared with terrestrial mats. All were found to have a macro-morphology nearly identical to terrestrial thrombolites and nodular and microbial mats photographed in Lake Thetis (Earth) in particular.

Domical Stromatolite Image Search: The ancient lakes of Gale Crater and Mars have been likened to the lakes of Western Australia, including Lake Thetis (Baldridge et al. 2009; Bridges et al. 2015; Graham et al. 2016; Nguyen et al. 2014) which host a variety of algae and living domical stromatolites. Therefore, a search was conducted for terrestrial domical stromatolites in Lake Thetis, and for specimens in Gale Crater which resemble these Australian stromatolites. Six concentric Martian formations were identified and their micro- and macro- structure, including laminae, fenestrae (gas bubbles) and concentric organizational geometry were visually examined, traced, and measured.

Results: Six concentric formations photo- graphed by the rover Curiosity's Mast Camera on Sol 122, 173, 309, 528, 529, were identified as resembling concentric and domical stromatolites (Figures 36, 38, 38-45). Three were found to be comparable, morphologically, to concentric stromatolites growing in Lake Thetis (Figures 34, 36, 38, 39, 42). Attached and/or adjacent to these specimens, photographed on Mars and in Lake Thetis, are numerous nodular microbial mat-like formations and thrombolites with the characteristic "peanut brittle" appearance of fossilized terrestrial mats (Figures 1, 8, 9, 16, 36, 37, 44). These putative Martian microbial mats were compared to their terrestrial counterparts and those in Lake Thetis and similarities noted.

Three of the domical specimens, photographed on Sol 122, 308, 528, 529, were found to meet most of the criteria for a biological formation (see Buick et al. 1981, 1995; Lowe, 1994), whereas a fourth (Sol 173) met all the criteria (Figures 40-43) which includes five layers of laminae, a lack of continuity of the stromatolite laminae, the presence of fine-scale crinkly and wavy laminae with several orders of curvature, an abundance of detrital material, the presence of a central (albeit collapsed) axial zone, the presence of what appears to be numerous fenestrae/gas bubbles, and numerous nodular mats attached to the walls and the inner collapsed surface and as part of adjacent structures.


4. Water, Algae, Lichens, Fossils, Microbial Mats, Stromatolites in Gale Crater

The Gale Crater (Aeolis Palus), located near the equator, was formed around 3.7 bya and presumably filled with water (Le Deit et al. 2013; Thomson et al. 2011). With a diameter of 154 km (96 mi), Gale Crater has the appearance of a dry lake, at the center of which rises a 5.5 km (18,000 ft) high mountain. There are foothills, a vast plain, and numerous gullies and channels that flow down from the hills and which appear to have been carved by flowing water. There are fluvial valleys rich in potassium (Grotzinger et al. 2015), mounds and peaks (Le Deit et al. 2013; Palucis et al. 2014; Schwenzer et al. 2012; Thomson et al. 2011) sedimentary rocks dated to 4.2 bya (Farley et al. 2014), a wide range and assortment of cobbles, small stones, and alkaline igneous rocks (Sautter et al. 2014; Schmidt et al., 2014; Stolper et al. 2013) including basalt tholeitic and alkaline basalts (Sautter et al. 2014), and rocks comparable to Earth rocks which formed around 2.7 bya or later (see Blichert‐Toft et al. 1996; Taylor and McLennan, 2009), i.e. during the Archean-Proterozoic transition.

Throughout this time period leading into the present, the Gale Crater appears to have repeatedly filled with water (Bibring et al. 2006; Cabrol et al. 1999; Fairen et al. 2014; Murchie et al. 2009; Siebach and Grotzinger 2014; Buz et al. 2017). With daytime air temperatures as high as 6°C (43°F) and the likelihood of daily exposure to moisture (Martin-Torres et al. 2015; Steele et al. 2017) the Gale Crater may have provided a habitable environment for the last 3.7 billion years.

As detailed in this report, the authors have identified specimens with features similar or identical to green algae, cyanobacteria, lichens, fungi, stromatolites and mat-forming organisms, and those which resemble fossilized or trace fossils of various organisms including metazoans. Many of the algae-like specimens appear to be moist or covered by a thin layer of ice.

Masses of filamentous vegetative-like material (Figure 10) and tubular shaped formations and those which resemble colonies of mat-forming filamentous organisms (Figures 1, 12, 15, 16, 24-28) were also observed in Gale Crater. In addition, formations resembling calcium-carbonate biosignatures (Figure 19), nostoc balls (Figures 2-23) and open-cone-like gas-bubble-vents (Figures 3, 5, 7, 13, 17, 18) which are associated with photosynthesis-oxygen respiration (Sallstedt et al. 2018) were identified. In support of this interpretation: it's been reported that colonies of hundreds of skyward oriented mushroom-shaped specimens, observed in Eagle Crater, and attached by thin stems and jutting upward from rocks, may also be engaged in photosynthesis (Joseph et al. 2020a).

We are unable to provide scale-bars of the exact size of these specimens as that information was not provided by NASA or made available following inquiries. Therefore, only size-estimates are provided based on generalized camera pixel and focal length specifications and other parameters made available by NASA.

5. Algae

The authors of this study, ten of whom are established experts in algae or algae-fungi symbiotes and related organisms, identified Martian specimens resembling algae, photographed at ground level, atop and alongside rocks, mudstone and sand (Figures 1-9, 12-16). The algae-like formations were often associated with or adjacent to specimens similar to ooids, lichens, fungi, microbial mats, and stromatolites.

These algae-like specimens, depending on substrate, appear as green clumps, spherules, cake-like layers, thin sheet-like layers and thick layered leafy vegetative masses of material which partially cover Martian rocks, sand, and fungi-like surface features. Many specimens appeared to be moist or covered by a thin layer of ice (Figures 2-7, 40-43). Although their exact identity is unknown, if biotic, they may include blue-green algae (cyanobacteria), green algae, diatoms, and mosses.

Figure 1. Sol 890: Greenish and yellow substances covering Martian sand and rock and specimens resembling microbial mats and a carpet of yellow-green substances within which are embedded patterns similar to mat-making filamentous cyanobacteria and which have also been found in association with stromatolites within Cambrian strata in Northern China (Latif, 2010). Although some features resemble abiotic exfoliation processes, it's been determined that exfoliation is not entirely abiotic, but due to the endolithic growth of unicellular cyanobacterium--thereby giving these surfaces a green color--as determined in Antarctic Beacon Sandstone (Friedmann 1980, Friedman and Weed 1987) and by Büdel and colleagues in South Africa and Antarctica (Weber et al. 1996, Büdel et al. 2004, 2008). Chrooccocidiopsis and other species of algae including Saphophyte cyanobacteria also display changing color patterns (Ferrari et al. 2002), particularly in response to insufficient water (Pattanaiki et al. 2007). Cyanobacteria or blue-green algae contain chlorophyll and phycobiliproteins, the latter comprising pigments ranging from the blue phycocyanin to the red phycoerythrin. The early-diverging, rock-dwelling cyanobacterium genus Gloeobacter is often colored purple (Graham et al. 2016). In high irradiance environments, cyanobacteria are often colored brown or black due to their protective scytonemin sheaths. They also produce carotenoids (yellow to orange pigments) that provide protection against irradiance (Pattanaiki et al. 2007). Therefore, if these specimens are in fact algae/cyanobacteria, they would have adapted to this high radiation environment. It should be stressed that the chemical composition of these mosaics is unknown. However, mineral and salt concentrations so far detected in the Gale Crater are insufficient to account for these patterns, colors and morphologies.

Figure 2. Sol 871. Green sphericals upon Martian sand, soil, rocks and pinnicle-columnar structures resembling terrestrial stromatolites and thrombolites (see Graham et al. 2014) and algae growing in shallow water (see Ray and Thomas 2012). On Earth, the greenish-coloration of sand and rock is due to green cryptoendolithic cyanobacteria (Büdel et al. 2004, 2008; Jung et al. 2019; The darkening in soil coloration may indicate moisture. Thin sheets of sphericals covering Martian sand, rock and stromatolite-like structures/protrusions, similar to the layered structures of terrestrial stromatolites and thrombolites resemble fossilized as well as living stromatolites and thrombolites on Earth (see Graham et al. 2014). These sheets are also similar to algae growing in shallow water, but may be frozen – the arrangement of sandy substance in between rocks is the typical nature of shallow rocky pools (see Ray and Thomas 2012).

Figure 3. Sol 840: Algae-like spherules and open cone-like protrusions similar to fossilized gas bubbles (upper left) on broken rocks photographed in the Gale Crater. These specimens are similar to those subjected to stressed conditions (Gaysina et al. 2019) and appear to have been recently moistened. Terrestrial algae similar to these specimens may assume these features when subjected to highly stressful conditions (Gaysina et al. 2019)

Figure 4. Sol 812 (top and bottom): The greenish-coloration may be due to green cryptoendolithic cyanobacteria (see (Büdel et al. 2004, 2008; Jung et al. 2019). The darkening in soil coloration may indicate moisture. Thin layer of green-colored substances on layered formations photographed in the Gale Crater. The overall appearance is similar to sand strata layers with fluvial water pathways. The contrasts between upper vs lower soils indicates the presence of moisture. Speculation: these strata may be harboring epipelic cyanobacteria (Hasler and Poulickova, 2010; Luesova, 2001) and algae recently exposed to water (see Ray et al. 2009). The possibility these are hydrated minerals is not likely due to the locations and insufficient quantities of minerals so far detected vs the widespread and somewhat homogenous distribution of the specimens depicted here. Surface weathering of sandstone, such as occurs in some arid and desert environments, also support a biological interpretation, as colonies of green cryptoendolithic cyanobacteria are typically found in association (Büdel et al. 2004; Wierzchos et al. 2011). Enhanced with HDR processing.

Figure 5. Sol 305: Epilithic algae-like crust-like layers covering fungi-like specimens and microbial mat-like substance and mudstone marked by numerous apertures resembling oxygen-vents secondary to photosynthetic activity. The surface appears to be moist or covered by a thin layer of ice.

Figure 6. Sol 305: Epilithic algae-like material covering Martian sand, rock and fungi-like specimen (lower center).

Figure 7. Sol 305 (Top): Epilithic algae-like substances and fungi-like specimens. (Bottom) Sol 298: Specimens resembling green algae, lichens, and ooids.

Figure 8. Sols 840, 319, 271, 853. (Clockwise from the top): Thick-layered clumps of algae-like substance on the top of an overturned, multi-layered Martian rock. Bacterial mats with fluvial water pathways between them. Mushroom-shaped Algae-Symbiotes. Stromatolite-like protrusions. On Earth, cyanobacteria (microalgae) are the most abundant of all aerial and sub-aerial epilithic and hypo-lithic organisms that grow on rocks and which produce multiple mat formations (Eldrin, 2016), as well as stromatolites, and which form symbiotic relationships with fungi.

Figure 9. (Top): Algae-Microbial Mat. Image Credit, University of Waterloo. (Bottom) Sol 309: Formations resembling layers of elevated and possibly fossilized microbial mats dusted with algae.

Figure 10. Sol 322: Possible flowing tangles of filamentous cyanobacteria/algae or Pseudoparenchymatous (plant roots pressed tightly to rock surfaces marked by porosity. The filamentous algae Klebsormidium and Oedogonium are comparable to the specimens seen here.

Within scientific literature, the term "algae" may be used broadly to include cyanobacteria (aka blue-green algae) plus oxygenic-photosynthetic protists such as green algae and non-photosynthetic relatives with a wide range of metabolic and reproductive processes, or more narrowly to exclude cyanobacteria (Graham et al. 2016). For our purposes, and as the exact identity of the specimens presented here cannot be determined with precision, these specimens (depicted or included in Figures 1-9, 12-16) will be referred to as "algae."

As will be explained (Section IV. The Geology of Martian Minerals...), it is not likely that these algae-like specimens are exfoliated weathered rocks containing masses of green minerals or salts, as no gross surface features resembling these abiotic substances have been previously identified by other scientists. The concentrations of the minerals and salts detected are insufficient, lack crystallization and are not visible in Gale Crater photos. By contrast, these algae-like and related specimens have been photographed in numerous locations and resemble their presumed terrestrial counterparts.

Nor can weathering or other geological processes account for these algae-like surface features. In fact, the greenish-coloration associated with surface weathering of terrestrial sandstone or other rocks, as occurs in some arid and desert environments on Earth, is evidence of biology, as colonies of green cryptoendolithic cyanobacteria are typically found in association (Büdel et al. 2004, 2008; Jung et al. 2019; Wierzchos et al. 2011). On Earth, minerals and desert salts and rocks are infested with cyanobacteria (Stivaletta, Barbieri and Billi, 2012; Finstad et al. 2017; Wierzchos, Ascaso and McKay, 2011), which help bind these substances together (reviewed by Joseph et al. 2019), and provide greenish coloration (Büdel et al. 2004, 2008; Jung et al. 2019). Therefore, although salt formations have not been visually observed on the surface of Gale Crater and salts detected are of insufficient quantity to account for these observations, even if some of these algae-like formations were masses of greenish-salts, the green coloration would be an indication of biology.

Other investigators have also observed what may be green algae growing on Mars. In 1978, and based on specimens photographed during the 1976 Mars Viking Missions, Levin, Straat and Benton published evidence of changing patterns on "greenish rock patches" which were "green relative to the surrounding area." Levin et al (1978) speculated that these greenish areas may represent "algae" growing on Mars. Thirty-five years later Joseph (2014), Rabb (2015), and Small (2015) published photos taken by the Mars rovers Spirit and Curiosity, depicting what they believed to be green algae. Soon thereafter, at the Lunar and Planetary Society, Krupa (2017) presented evidence of what appear to be green photosynthetic organisms growing in the Columbia Hills area of Gusev Crater, adjacent to what may be water pathways which intermittently fill with water. He noted that "the covered by a very thin layer of green material" and "green spherules" which resembles algae in the soil. Thus, seven different investigators have observed and provided evidence that algae may be flourishing on the Red Planet.

6. The Lichens of Gale Crater vs Eagle Crater

Lichens are a symbiotic organism of at least one alga that can be green algae or cyanobacterium (photobiont) and at least one fungus (mycobiont), the latter of which is largely responsible for the lichens' mushroom shape, bulbous cap, thallus, and fruiting bodies (Armstrong 2017, 2019; Brodo et al. 2001; Tehler & Wedin, 2008). Molecular analyses, however, indicate that lichen consortia include a wide range of bacterial communities on the lichen-surface and within the photobiont zone, such as "Sphingomonas, Methylobacterium, and Nostoc," as well as "eukaryotic representatives of Rhizaria, Amoebozoa, Alveolata, Metazoa, and Viridiplantae" and the rhizarian protist Protaspa (Graham et al. 2018).

Eagle Crater Mushrooms: Colonies of thousands of skyward-oriented mushroom-shaped, lichen-like organisms, up to 8 mm in length, topped by bulbous mushroom caps, with 1 mm in diameter stems attached to rocks and have been identified in Eagle Crater (Joseph et al. 2020a). The upward orientation of these formations was interpreted as possible evidence that these organisms are engaged in photosynthesis.

Joseph and colleagues (2020a) did not find any evidence of algae in Eagle Crater. However, 23 specimens identical to "puffball" fungi--photographed by the rover Opportunity--were observed to increase in size with 12 emerging from beneath the soil during a three-day period in the absence of any contributing wind or weathering process. Other scientists have also identified spherical "puffballs" on the surface of Meridiani Planum (Dass, 2017; Joseph 2016; Rabb, 2018) whereas evidence that fungi may have contaminated the rover Opportunity has been reported (Joseph et al. 2019). Nevertheless, if the formations observed in Eagle Crater are fungi, lichens (algae-fungal symbiotes) or non-lichenized fungi, is unknown.

It's been falsely claimed that these Eagle Crater mushroom-shaped specimens consist of hematite. However, Burt, Knauth and Woletz (2005) in a presentation at the Lunar and Planetary Society and paper published in the journal Nature, dismissed the spherical hematite claims as "inappropriate." In addition, Joseph et al (2020a) have shown that the hematite hypothesis was based on inference and speculation, that no spheres and not a single mushroom-shaped specimen were examined for hematite, that the data was a "poor fit" for hematite, and the lichen-like formations of Eagle Crater are a different color from, and do not look anything like hematite, but instead resemble fungi or the lichen, Dibaeis baeomyces (Joseph et al. 2020a), and that the environment was never conducive to the formation of hematite.

Gale Crater Lichens: With few exceptions (Figure 8) the lichen-like formations observed in Gale Crater do not resemble the lichen Dibaeis baeomyces or any of the specimens photographed in Eagle Crater and Meridiani Planum. Many of the putative Gale Crater lichens (Figures 11-18) are similar to those growing on the west coast Ireland cliffs of Moher whereas others are similar to agaric lichens (Lichenomphalia umbellifera).

The lichens of Meridiani Planum and Gale Crater form vast colonies in the absence of any other biological features. By contrast many of the Gale Crater lichens were found in association with specimens resembling fungi and spherical ooids (Figures 11-12). On Earth, ooids (ooliths), are formed in shallow seas and generally consist of calcite, bacteria and residues of degraded mucous produced by filamentous cyanobacteria (Devaud and Girardclos, 2001). Specifically, ooids are fashioned by the binding action of exopolysaccharides (Kumar et al. 2018) secreted by algae on sedimentary grains followed by calcification (Farbrius, 1977).

There are numerous photographs taken by the rover Curiosity of ooid-like specimens in association with lichen-like surface features. The authors have also observed what appears to be colonies of filamentous cyanobacteria which have been photographed in association with what appear to be spherical ooids and "dimpled" lichens (Figures 11, 12).

The putative Gale Crater lichens are also found in close proximity to veins and deposits of what appears to be gypsum which has been identified in the Gale Crater (Rapin et al. 2016). Gypsum is a favored substrate for extremophile algae/cyanobacteria (Bothe, 2019). Terrestrial veins of gypsum are derived, in part, in association with water via the dissolution of surface or near surface sulfate-rich veins (Anthony et al. 2003a; Bock, 1961; Garcia-Ruiz et al. 2007; Van Driessche et al. 2012). Water plus gypsum also promotes biological activity. The green alga Closterium (and its various relatives), for example, accumulates gypsum and stores it as crystals (Sr and/or Ba sulfates) in vacuoles located in the apices of their spindle-shaped cells (He et al. 2014).

Alternatively, the veins of white-colored substances and veins found in association with these lichen-like formations (Figures 11, 13-15) may consist of quartz and hydrated calcium sulfates (Nachon et al. 2014; McLennan et al. 2013), all of which have been identified in the Gale Crater where these specimens were photographed. Quartz is also associated with biological activity, and quartz, dominated by lichens and biocrusts, has been identified in the Atacama Desert. Furthermore, cyanobacteria, green algae and microfungi have colonized the inner structures of small quartz gravel (Jung, et al. 2019). These putative Martian lichens have also been photographed in association with what appears to be ooids and calcium carbonate (Figure 11, 13-15), both of which are byproducts of cyanobacterial activity (Devaud and Girardclos, 2001). The Gale Crater lichens and the putative ooids are often densely packed together.

Many of the Gale Crater lichen-shaped formations have a central depression or nuclei giving them a "donut shape," as well as subsurface mycelium/hypa (Figures 11, 13-15, 17,18), similar to fungi and agaric lichens (Lichenomphalia umbellifera). Other specimens photographed in Gale Crater assume a diversity of forms similar to lichens and ooids discovered in Cambrian era deposits in northern China (Mei et al. 2019; Riaz et al. 2019), including "concentric and radial, rounded or elliptical, and with or without nuclei." By contrast, the lichen-like "Martian mushrooms" of Eagle Crater have one distinct form: a thin stem topped by a bulbous cap, colonies of which are oriented skyward as if engaged in photosynthesis, but without any evidence of ooids.

Therefore, hundreds of diverse lichen-like formations have been observed and photographed in various regions of Gale Crater which bear no resemblance to specimens photographed in Eagle Crater; whereas those of Eagle and Gale Crater have obvious counterparts on Earth. Eagle Crater is part of a vast plain (Meridiani Planum) just south of the Equator and is 5,200 miles (8,400 kilometers) from Gale Crater. There is no evidence that Eagle Crater was ever a lake or held large amounts of water. Therefore, differences in morphology may represent the on and off lake-like environment of Gale Crater vs the much dryer Eagle Crater. Or, like terrestrial lichens (Armstrong 2017, 2019), there may be hundreds of different varieties of Martian lichens. Photosynthesis is a trait that the putative lichens of Gale Crater may share in common with those of Eagle Crater (see Joseph et al. 2020a) and with their counterparts on Earth. This interpretation is supported by the discovery of what may be gas bubble apertures produced secondary to photosynthesis and oxygen release. These apertures are often found in association with the putative lichens of Gale Crater (Figures 17, 18).

7. Gas Bubbles and Photosynthesis

Lichens are photosynthesizing organisms which respire oxygen--activity associated with its photobiont; respiration being dominated by fungi and bacteria in the consortium. Photosynthesis generates oxygen. Numerous studies have demonstrated that lichens remain viable and maintain photosynthetic activity when exposed to simulated Martian temperatures, atmosphere, humidity, and UV radiation (de Vera 2012; De la Torre Noetzel, et al. 2017; Sanchez et al. 2012). For example, De la Torre Noetzel and colleagues (2017) exposed the lichen Xanthoria elegans to simulated Mars-analogue conditions for 1.5 years including direct exposure to ultraviolet (UV) irradiation, cosmic radiation, temperatures and vacuum conditions. It was found that the lichen photobiont showed an average viability rate of 71%, whereas 84% of the lichen mycobiont survived--although they were adapted to and evolved on Earth. Moreover, 50-80% of alga and 60-90% of the fungus symbiote demonstrated normal functioning (Brandt et al. 2015). This included the ability to engage in photosynthetic activity post-exposure to these harsh environments with minimal impairment (Meesen et al. 2014).

Likewise, the lichens of Gale Crater may be engaged in photosynthesis. Bubble-like open-cone apertures have also been observed in association with the lichen-like specimens of Gale Crater (Figures 17, 18). It is well established that photosynthesizing organisms, such as cyanobacteria, respire oxygen and release gas bubbles via the surrounding matrix (most noticeable in water). These gas-bubble-formations may become mineralized and fossilized (Bengtson et al. 2009; Sallstedt et al. 2018). Therefore, we hypothesize that the open-cone apertures observed in Gale Crater serve to ventilate oxygen respired during photosynthesis and constitute additional indications of biology. Evidence suggestive of photosynthesis was also observed in Eagle Crater (Joseph et al. 2020a). Moreover, gas bubble release is a function of green algae (cyanobacteria) mat-producing behavior. Formations similar to microbial mats have been identified in this report.

8. Calcium Carbonate Encrusted Cyanobacteria and Nostoc Balls Evidence of calcite and calcium carbonate has been detected on Mars (Boynton et al. 2009; Sutter et al. 2012; Wray et al. 2016; Leshin et al. 2013; Archer et al. 2014). Calcite may be formed via geological weathering coupled with fluid evaporation (Anthony et al. 2003b); and this watery environment would promote biological activity, whereas calcium carbonate is almost exclusively a biological byproduct. However, the detection of large amounts of calcium carbonate have been elusive due most likely to the destruction of biomolecules by weathering and UV and other forms of ionizing radiation (Ertem et al. 2017; Bibring et al. 2006). Therefore, if large amounts of carbonate are being produced by algae currently or in the ancient past on Mars, much would likely be destroyed by abiogenic processes.

Yet, despite destructive UV rays, a number of published reports indicate the presence of calcium and calcium carbonate on Mars (Archer et al. 2013; Boynton et al 2009; Cannon et al. 2012; Krall et al. 2014). Wray et al. (2016), for example, found evidence of calcium-rich Martian carbonates as detected by the Compact Reconnaissance Imaging Spectrometer aboard the Mars Reconnaissance Orbiter. Likewise, the Mars Global Surveyor Thermal Emission Spectrometer detected carbonate in global Martian dust (Bandfield et al. 2003). Evidence of carbonate was also detected by the Thermal and Evolved Gas Analyzer on the Phoenix polar lander, which measured CO2 release consistent with breakdown of Ca‐rich carbonate in Martian soils (Boynton et al. 2009; Sutter et al. 2012), an interpretation supported by results from the Phoenix Chemistry Lab (Kounaves et al. 2010). Likewise, Curiosity's Mars Science Laboratory found evidence of CO2 release indicative of Fe/Mg-rich carbonates, albeit at minimal levels (Leshin et al. 2013; Archer et al. 2014; Ming et al. 2014).

Calcium carbonate is a biological byproduct of algae/cyanobacterial photosynthetic activity and their mucous secretions of polysaccharides which act as binding sites for Ca2+ thereby producing carbonate minerals and concentrations of calcium (Dittrich and Sibler, 2010; Kupriyanova et al. 2007; Samylina et al. 2016). Supporting the hypothesis that photosynthesizing, algae-like organisms have colonized Gale Crater, is evidence of what appear to be calcium biosignatures (Figure 19) and calcium encrusted cyanobacterial "Nostoc balls" (Figures 20-22). Calcium carbonate is precipitated in the mucous of cyanobacteria via photosynthetic CO2 or HCO3nostoc balls (Barnes and Chalker, 1990; Graham et al. 2014; Mei et al. 2020); evidence of which is presented here.

Figure 11. Sol 298: Specimens similar to dimpled spherical ooids and algae (cyanobacteria) on the surface of a Martian rock. On Earth, ooids (ooliths), are formed in shallow seas and generally consist of calcite, bacteria, and residues of degraded mucous produced by filamentous cyanobacteria. The white deposits may consist of calcium, gypsum or quartz. Cyanobacteria are found in association with gypsum. The green alga Closterium, for example, accumulates gypsum and stores it as crystals (Sr and/or Ba sulfates) in vacuoles located in the apices of their spindle-shaped cells. Calcium carbonate is precipitated in the mucous of cyanobacteria via photosynthetic CO2 or HCO3- uptake and which cements together microbial mats and ooids. It is also possible that the "dimpled" spheres may include lichens.

Figure 12. Sol 298: Specimens resembling spherical ooids and green algae on the surface of a Martian rock or mudstone.

Figure 13. Sol 298: Specimens resembling dimpled lichens with what may be hyphae along the surface/subsurface. Note hollow apertures in the upper right corner and lower center of photo, and which resembles an oxygen-gas vents typically produced by photosynthesizing organisms.

Figure 14. Sol 298: Specimens similar to spherical ooids and/or dimpled lichens. The thin, tube-like formations may consist of calcium and the green coloration may represent green or blue-green algae (cyanobacteria) which produce calcium via their mucous secretions.

Figure 15. Sol 298: Specimens similar to dimpled ooids and algae (cyanobacteria) on the surface and interior of a Martian rock. The white deposit may consist of calcium or gypsum, which are associated with cyanobacteria.

Figure 16. Sol 298: Specimens similar to dimpled spherical ooids, dimpled lichens and green or blue-green algae (cyanobacteria) on the surface and interior of a Martian rock and what appears to be a microbial mat (upper left) jutting out from the cavity. The micro-patterns embedded within and throughout the surface of this entire formation resemble colonies of mat-forming micro-organisms.

Figure 17. Sol 232: Specimens similar to dimpled lichens and gas-vent apertures for the release of oxygen secondary to photosynthesis. Photosynthesizing organisms, such as cyanobacteria, respire oxygen and release gas bubbles via the surrounding matrix (most noticeable in water). These gas bubble formations may become mineralized and fossilized. The white substance within the matrix may consist of calcium which is secreted via the mucous of cyanobacteria which, along with algae, comprise the lichen organism.

Figure 18. Sol 232 (Top): Specimens similar to gas-vent apertures for the release of oxygen secondary to photosynthesis. Photosynthesizing organisms respire oxygen and release gas bubbles via the surrounding matrix (most noticeable in water). (Bottom) Open globular structures, interpreted as formed by gas bubbles via cyanobacteria oxygen respiration within microbial mats (from Bengtson et al. 2009).

Figure 19. (Top) Sol 304: Possible calcium biosignature within a rock crevice. Alternate explanations include fungi. (Bottom) Sol 890: Possible calcium deposits within a specimen which resembles a microbial mat and thick bacterial crust.

Figure 20. CR0_473216607PRC_F0442414CCAM02853L1. Photographed by Curiosity's Micro-Imager Camera. These specimens resemble calcium incrustations and calcium carbonate encrusted cyanobacteria--perhaps similar to Nostoc flagelliforme, N. parmelioides, N. verrucosum, N. pruniforme as well as spherical "Nostoc balls," and vesiculous thalli (see Aboal et al. 2016). Each of these interconnected "balls" are estimated to be less than 0.1 mm in diameter, based on the microscopic camera specifications.

Figure 21. (Top): CR0_473216607PRC_F0442414CCAM02853L1. Calcium carbonate encrusted cyanobacteria, perhaps similar to Nostoc flagelliforme as well as spherical "Nostoc balls," and vesiculous thalli (see Aboal et al. 2016). Species such as Subtifloria (a fossil member of the Girvanella group) also create spherical thrombolites (large and small micritic clots) via calcification of extracellular polymeric substances making up multiple biofilms and within which calcified sheaths of cyanobacteria are typically embedded (Mei et al. 2020). These specimens also resemble the bio-mineralization pattern of Candidatus Gloeomargarita lithophora, which form intracellular Ca-carbonates bio-mineralization and which may be scattered and/or arranged in one or several chains within its cells (Couradeau et al. 2012; Jinhua et al. 2016). Abiogenic calcium mineralization is an alternate explanation, except that mineral veins are planar and not bulged as seen here. (Bottom): Photomicrograph of a modern "Nostoc ball," stained with the DNA-binding fluorochrome DAPI and viewed in UV epifluorescence (photo by L. Graham).

Figure 22. CR0_464779288PRC_F0421020CCAM02758L1. Photographed by Curiosity's Micro-Imager Camera. These specimens resemble calcium incrustations and calcium carbonate encrusted cyanobacteria--perhaps similar to Nostoc flagelliforme, N. parmelioides, N. verrucosum, N. pruniforme, along with spherical "Nostoc balls," and vesiculous thalli.

"Nostoc balls" are formed by the calcification of extracellular polymeric substances making up multiple biofilms and reflect fundamental features of microbial growth and can form thrombolites by calcification (Graham et al. 2014; Mei et al. 2020). Fossils of calcified sheaths of filamentous cyanobacteria are typically found preserved within each clot (Mei et al. 2020). The specimens depicted in Figures 20-21, consist of what appear to be numerous calcium encrusted Nostoc balls, a calcium encrusted byproduct directly related to the photosynthetic activity of cyanobacteria.


9. The Geobiology of Weathering

In contrast to the putative lichens, ooids and gas vents which appear to be attached to mud stone, some of the algae-like specimens are associated with mafic and ultramafic rocks and soil which may have been subject to considerable weathering. Hence it could be argued that these specimens represent alteration surfaces for specific mineral assemblages that mimic algae. For example, Earthly chlorites, when weathered, may strain volcanic outcrops with a dark greenish to brownish color. The weathering and alteration of Fe and Ti-oxides may be responsible for the greenish appearance of rocks and soils as well. The bluish color of some specimens could be due to weathered surface of Fe-Mn rich rocks. However, it's been determined that the greenish-coloration associated with surface weathering is usually secondary to biology and colonies of green algae are usually found in association (Büdel et al. 2004; Jung et al. 2019; Wierzchos et al. 2011). On Earth, the weathering of rock and minerals is often due to biological activity (Cecchi et al. 2019; Gadd et al. 2011; Wilson 2004).

10. Martian Minerals Do Not Mimic Algae

The Mars rover Curiosity, via its multiple cameras and Mars Science Laboratory, has been exploring and investigating the Gale Crater since August 2012. Among Curiosity's suite of sampling and analysis instruments is the mineral-detecting CheMin X‐ray diffractometer (Bish et al. 2013; Vaniman et al. 2014). Trace amounts of chlorite, actinolite, epidote, olivine, and serpentine--which may appear green on Earth-- have also been detected on Mars via the analyses of spectral signals recorded from space (Schwenzer & Kring, 2009; Fairén et al. 2010; Ehlmann et al. 2009; Carter et al. 2010) and via the rover Curiosity in the Gale Crater (Michalsk and Niles, 2010; Bristow et al. 2015; Vaniman et al. 2014; Treiman et al. 2016). Those minerals detected in the Gale crater have been found in mixtures of clay and embedded in basaltic and phyllosilicate-bearing Martian rocks (Bristow et al. 2015; Vaniman et al. 2014). If large masses, sheets and clumps of these minerals crystalized and accumulated on the sides and tops of Martian rocks they could be misidentified as green algae. However, there is no evidence of crystallization or significant accumulation on the surface of rocks to support this possibility. And yet, the contribution of minerals should not be dismissed.

Terrestrial olivine is the primary mineral component in terrestrial basalt mantle rock and may be green in color when not weathered (Anthony et al. 2003c). Martian rocks in the Gale Crater have been subject to considerable weathering. Terrestrial pyroxene (a ferromagnesian mineral), is usually found in basalt and metamorphic rocks and although it can appear as a dark green, it is typically black. On Earth, serpentine may appear yellow or olive green in color but is usually black or brown. By contrast, actinolite may assume a fibrous, life-like appearance once it crystalizes (Anthony et al. 2003c). However, no crystalized actinolite has been detected in the Gale Crater.

Terrestrial chlorite, if crystalized, can also assume a micro-vegetative appearance, forming flaky or gray-green olive colored layers and clumps and is often found in a variety of rocks including slate, schist and phyllite. However, crystalized chlorite has not been detected. Moreover, in contrast to the wide surface areas that appear to be colonized by clumps and layers of green algae, with the exception of olivine, only trace amounts of these minerals have been detected. Martian actinolite, for example, could only be discerned by complex algorithm analyses of spectral and hydration features (Lin et al. 2016); whereas chlorite has been found primarily deep within crater walls and central peaks or mixed with clay, but not on the surface of rocks or mudstone (Schwenzer and Kring, 2009; Michalski & Niles, 2010; Milliken et al. 2010; Vaniman et al. 2014) where the algae-like masses have been photographed. Basaltic rocks are abundant in the Gale Crater, but "linear modeling," or rock spectra, indicates an average olivine composition of only ∼Fo45, which the authors of that study admit is an "overestimation" and which could be detected only indirectly because rocks containing olivine were covered with dust (Hamilton and Ruff, 2012). Moreover, there is no evidence of olivine crystallization on Mars, which, on Earth, gives this mineral a green coloration.

In fact, the relevant minerals in Gale Crater (i.e. chlorite, serpentine, olivine, actinolite, pyroxene) can only be detected via analysis of spectral absorption rates, or via chemical analyses of drill samples, and are typically embedded as a mixture in light- and dark bedrock and basalt (Michalsk and Niles, 2010; Bristow et al. 2015; Vaniman et al. 2014; Treiman et al. 2016) or volcanic and coarse grains of sand (Ehlmann, 2016). With the exception of olivine, the relevant mineral concentrations are often just above the detection thresholds and none have a crystalline shape (Bish et al. 2013). Even more importantly, these Martian minerals are not visible to the naked eye and were not detected on those rocks and sands where these algae-like specimens were photographed. By contrast to the lack of minerals visible on the surface, these algae-like specimens are quite extensive and form thin sheets, layers and clumps, and tend to grow in large patchy masses on the sides and tops of rocks and sand. However, although minerals cannot account for these observations, these same (and other) minerals may provide an energy source which sustains and promotes the growth of the putative biological specimens depicted in this report. For example, Bish et al. (2019) argued that XRD data from Mars revealed a rich mineralogy significantly similar to that found in the mineral-rich soils on the flanks of Mauna Kea volcano, Hawaii--an area in which algae, lichens, mosses and fungi proliferate (Jackson 1971). Terrestrial microorganisms, as well as fungi and algae-fungal symbiotes, feed on, metabolize, respire, excrete, or store these minerals internally or on the surface of their cell walls. Hence, when these organisms die, they often become mineralized fossils.

It is not likely that the algae-like specimens presented here reflect the accumulation of green-colored minerals (i.e. chlorite, actinolite, epidote, olivine, serpentine) which somehow massed together on the surface of Martian rocks and sand and crystalized together to take vegetative-like forms.

11. Martian Mineral Microbiology

Algae, fungi, lichens, and a variety of microorganisms have played a major role in the genesis of numerous minerals, including chlorite, serpentine, olivine, actinolite, and pyroxene (Bender et al. 2002; Maixner et al. 2008; Gadd et al. 2011; Gazze, et al. 2012; Jones et al. 1981; Templeton et al. 2011; Wilson & Jones 1984, Wilson 2004). Microorganisms have been active in the formation and decomposition of minerals in the earth’s crust since soon after life appeared on this planet, accomplished via their well-developed ability to bind metallic ions. Ion binding allows their cells to serve as nucleation sites for initiating and mediating biomineralization, as well as respiration, expiration, decomposition and secretion, and to accumulate mineral and metal byproducts along the cell wall (Polgari et al. 2006; Gadd et al. 2011; Bender et al. 2002; Ayupova and Maslennikov 2012).

Chemolithotrophs are well adapted for life on Mars and function at optimal oxidizing and decomposing capacity in high CO 2 anoxic environments by autotrophically fixing CO2 from the atmosphere (Rawlings et al. 2003; Todaro and Vogel, 2014). Thus, Mars, with its mineral deposits and atmosphere that consists of 95% CO2, provides an ideal environment for these species.

Although biomineralizing species may also obtain energy via photosynthesis, chemolithotrophs generally employ ferrous iron or reduced inorganic sulfur compounds or chlorite, as an electron donor (Gadd et al. 2011; Bender et al. 2002)--substances widespread on Mars. Microbes that grow in mineral-rich environments may utilize a wide range of metal ions (Leduc and Ferroni 1994) and they may interact with algae, fungi, and lichens thereby forming mutualistic mineral-dependent relationships (Gadd et al. 2011).

Generally, it is within terrestrial (vs aquatic) environments where these organisms exert their most profound influence, including bioweathering, mineral transformations and mycogenic biomineral formation (Cecchi et al. 2019; Gadd et al. 2011). For example, algae, fungi and lichens can affect changes in metal speciation, toxicity and mobility, as well as induce mineral formation or mineral dissolution or deterioration (Cecchi et al. 2019; Rawlings et al. 2003; Wilson 2004).

These organisms are also responsible for the biochemical weathering and decomposition of rocks and minerals which they may transform into calcium or other minerals (Ascaso et al. 1990; Duchafour 1979; Jones and Wilson 1985, 1986). This includes the formation of calcium in the interface between basalt and other rocks (Jones and Wilson 1986), for which there is evidence on Mars as shown in Figures 11, 13-15, 17-22. Moreover, cyanobacteria also trap fine minerals which are incorporated into their crusts and networks (Rahmonov and Piatek 2007).

12. Perchlorates and the Geobiology of Chlorite

Chlorite is one of two potentially green-colored minerals so far detected on Mars which, on Earth, may take a clumpy, vegetative shape when crystalized or bio-weathered. Terrestrial perchlorate-reducing bacteria, dwelling in high CO2 anaerobic environments (Chaudhuri et al. 2002; Maixner et al. 2008) feed on and metabolize chlorite, which is transformed into chloride and O2, which provide energy to the cell and is respired. There is evidence, as presented in this report, of biologically produced apertures serving to ventilate oxygen respiration during photosynthesis (Figures 5, 13, 15).

The synthesis of oxygen plays a significant role in the reductive pathway of perchlorate--mediated by the enzyme chlorite dismutase and a chlorite dismutase gene cld (Bender et al. 2002; Maixner et al. 2008). Perchlorate also appears to be widespread in Martian soils with concentration ranging from 0.5 to 1% (Bishop et al. 2014; Kounaves, et al 2014; Wilson et al. 2016). Dismutase and the chlorite dismutase gene appear to have an ancient phylogeny, appearing very early in evolution, and are widely distributed among bacteria and archaea (Maixner et al. 2008). By analogy, Martian organisms may have acquired or evolved the same or similar genes, thereby providing a biochemical means of synthesizing oxygen billions of years ago, as evidenced by putative Martian stromatolites dated to 3.7 bya (Noffke 2015). Many species reduce perchlorate and chlorate into chlorite, whereas yet others transform chlorite into chloride and O2 (Coates et al. 1999; Kengen et al. 1999; Bender et al. 2002)--a transformational process which not only serves as an energy source but which can prevent self-toxification due to the accumulation of chlorite (Maixner et al. 2008). Thus, some microbes may be metabolizing perchlorates and secrete chlorates as a waste product. Whereas other species produce chlorite and reduce chlorite or chlorates--depending on the availability of mutualistic relations with other species and the presence of and concentrations of chlorate or perchlorate in the surrounding environment. In so doing they produce molecular oxygen (Hagedoorn et al. 2002) which is a metabolic energy source. The availability of these minerals determines the biomass of these organisms and the extent to which they are able to chemically and physically alter these and other minerals (Leake et al. 2008).

Thus, a variety of microorganisms, including algae, fungi and algae-fungal-lichen symbiotes, dwelling in an anoxic, high CO2 environments, are dependent on chlorite and perchlorates which they may transform, reduce, and recycle. The same is true of serpentinite (Wilson et al. 1981), olivine (Wilson 2004) and actinolite (Gablina et al. 2016). The minerals detected on Mars would provide oxygen and the energy needs for a variety of organisms. Likewise, the detection of these Martian minerals may reflect biological activity.

By contrast, although perchlorates and related salts have been detected, their surface quantities are insufficient and they have not been observed to assume clumps or shapes, much less formations that might be mistaken for algae, lichens, and other organisms.

13. Microbiological Tunneling of Martian Rocks and Minerals

Various organisms burrow within olivine, chlorite and basalt rocks (Aspandiar and Eggleton 2002; Delvigne et al. 1979; Fisk et al. 1006; Haggart & Bustin 1999; Jones et al. 1981, Wilson 2004), creating etchings or tunnels and channels within which, for example, fungal hyphae (Gazze et al. 2012) or filamentous and tubular cellular organisms may be embedded (Sanz-Montero and Rodriguez-Aranda, 2009; Polgari et al. 2006; Ayupova et al. 2016; Ayupova and Maslennikov, 2012; Kilias et al. 2007; Sakakibara et al. 2014). Cyanobacteria and lichens burrow into rocks through the secretion of oxalic acid and cause weathering. They may derive their carbon source from the dissolution of limestone rocks. The tunneling depicted in Figure 23 (and others like it), photographed in the Gale Crater, has all the characteristics of a tunneling biosignature.

Biological vs abiogenic tunneling can be distinguished by asymmetrical vs symmetrical apertures (McLaughlin et al. 2010). For example, if due entirely to a chemical process, crystalline millstones, or pebbles driven against a relatively soft rock surface by wind or fluid pressure, produce a smooth and rounded cavity and with numerous adjacent cavities which are nearly identical in shape. These abiogenic tunnels are often linked together by longitudinal interconnected surface channels and overlapping holes (Tyler & Barghoorn, 1963; Knoll & Barghoorn, 1974; Wacey et al. 2008).

By contrast, biological tunneling is linked to watery environments, typified by jagged asymmetrical entry holes, with each tunnel, or hole, largely separate from one another. The tunneling and surface hole is due to microbial dissolution, demineralization, and removal of substrate, the fluids providing a transport system for the chemicals needed by these organisms for dissolution and as nutrients (Staudigel et al. 2008; McLaughlin et al. 2010). However, biological tunneling eventually ceases in the absence of water (McLaughlin et al. 2010). Thus, as water levels wax and wane--as appears to be the case for the Gale Crater-- biological tunneling would be affected.

Martian minerals and biological tunneling (Velbel, 2012; Fisk et al. 2006; White et al. 2014), as well as biological activity (McKay et al. 2009; Thomas-Keprta et al. 2002, 2009), have been detected in several Martian meteorites. For example, channels and tunnels with similar biosignatures, and in association with olivine, have been discovered in Martian meteorite Nakhla (Fisk et al. 2006) and Yamato (White et al. 2014).

Likewise, numerous substrates in the Gale Crater also show evidence of biological tunneling. As exemplified by Figure 23 (which is just one example of many) almost all the perforations on the surface of this rock meet the criteria for biological tunneling. And this perforated substrate was photographed in the Gale Crater which was inundated at various times by water.


14. Hypotheses and Speculation: Trace Fossils?

It can be argued that Martian minerals have been subject to considerable biological activity, bio-weathering and tunneling, and serve as an energy source for a variety of chemolithotrophic and biomineralizing organisms including those which engage in photosynthesis, i.e. algae, fungi, and lichens (Cecchi et al. 2019; Rawlings et al. 2003; Wilson, 2004).

A variety of organisms accumulate minerals internally and externally (Polgari et al. 2006; Gadd et al. 2011; Bender et al. 2002; Ayupova and Maslennikov, 2012), including chlorite (Ayupova et al. 2016; Haggart & Bustin, 1999). Many Martian minerals have been formed, hydrated, in recessive bodies of water (Xue & Jin, 2013; Schwenzer & Kring, 2009; Carter et al. 2010; Michalski & Niles, 2010; Lin et al. 2016), which, on Earth, are host to innumerable organisms.

Terrestrial microorganisms not only feed on minerals but bind metallic ions and act as nucleation sites for initiating and mediating biomineralization. This causes minerals to accumulate within and on the exterior of these organisms (Polgari et al. 2006; Gadd et al. 2011; Bender et al. 2002; Ayupova and Maslennikov, 2012). In consequence, when fossilized in regressive bodies of water, only the accumulated minerals may be detected (see Ayupova et al. 2016; Haggart & Bustin, 1999; Ran et al. 1999; Sanz-Montero and Rodriguez-Aranda, 2009; Sakakibara et al. 2014).

Gale Crater is believed to have been a water‐rich environment that underwent early mineralization, conditions which would have preserved the denizens of this ancient habitat (Grotzinger et al. 2012, 2015). Therefore, the presence of various minerals on Mars may serve as a food and energy source and byproduct of biological activity and could represent the biomineralized, fossilized remains of these organisms as is true on Earth (Ayupova et al. 2016; Haggart and Bustin, 1999; Ran et al. 1999; Sanz-Montero and Rodriguez-Aranda, 2009; Sakakibara et al. 2014). In addition, Eigenbrode and colleagues (2018) reported organic matter preserved in 3-billion-year-old mudstones at Gale crater, which may represent the residue of Martian organisms. This hypothesis is supported by Figures 28-33 which depict fossil-like specimens, many similar to metazoans and which may have formed in regressive bodies of water.

15. Speculations: Evolution, Trace Fossils, Tube Worms, Metazoans?

The Gale Crater is marked by potassium-rich fluvial valleys and water pathways (Grotzinger et al. 2015b). The Gale Crater appears to have repeatedly filled with water (Bibring et al. 2006; Cabrol et al. 1999; Fairen et al. 2014; Murchie et al. 2009; Siebach and Grotzinger, 2014; Buz et al. 2017) and could have sustained a variety of species and promoted their evolutionary development due to major changes in their watery environment and availability of liquids. Wet followed by dry spells may also account for what could be fossilized impressions of water dwelling and subsurface Martian organisms including metazoans (Figures 27-33).

To speculate, some of the fossil-like specimens are similar to complex metazoans and burrowing worms, which evolved on Earth 500 million years ago. These include formations which resemble Cambrian fauna that first appeared during the Ordovician (e.g. Calymene callicephala, Flexicalymene meeki, Homotelus bromidensis, Isotelus sp., Pseudogygites canadensis, Streptelasma sp.). Our speculative interpretation of the organisms represented (pro and con) are detailed in the captions of Figures 33-37. For example, Foraminfera live under the effects of water currents and if they accumulate, and die, they are disposed isoriented. In fact, what appears to be "fossilized" Foraminfera-like specimens (Figures 33-34) could also be explained as an accumulation of plagioclase crystals.

Unfortunately, it is impossible to make precise determinations as to the identity or exact nature of these mineralized, fossil-like specimens, and this might be true if these same "fossils" were discovered on Earth (Graham 2019). Microfossil-like shapes are ubiquitous in Earth’s geological record and despite careful extraction, preparation and microscopic examination there is great debate as to their authenticity (Knoll 2015, Brazier et al. 2003, De Gregorio et al. 2011; Marshall et al. 2011; Wacey et al. 2016). Samples have been repeatedly challenged as abiotic and a consequence of mineralization.

However, as to the controversy over the authenticity of the earliest terrestrial microfossils, it may also be that accumulated minerals are all that remains of these fossilized organisms. This would explain their fossil-like biogenic shape and why all that remains are the accumulated minerals. Although the fossil-like impression presented in Figures 28-33 may represent the biosignatures of various organisms including metazoans, at best, we can only speculate and offer hypothesis and arguments both pro and con. We therefore offer up these fossil-like formations as targeted examples for additional investigation by the scientific community, as well as extraction and analyses by future robotic missions to Mars.

Figure 23. CR0_425453645PRC_F0060704CCAM01315L1: Microscopic detail of rock with tunnels/holes possibly secondary to biological activity. Photographed by Curiosity's Micro- Chemistry Camera. Biological (vs abiogenic) tunnels and perforations are typically asymmetrical in shape, with each hole separate from one another, and are due to microbial dissolution, demineralization, and removal of substrate (McLaughlin et al. 2010). The overall pattern is consistent with a biological interpretation.

Figure 24. Sol 298: Algae and lichen-like-mushroom-shaped specimens atop what appears to be microbial mats and, surrounded by ovoid and tubular specimens.

Figure 25. Sol 298: Dark colored specimens with crevice which resemble colonies of tubular organisms. See also Figures 26-28. These and the light-colored specimens embedded in the matric, adjacent to what may be ooids and dimpled lichens, are also similar to fossilized cyanobacteria which have been found in association with ooids within Cambrian strata in Northern China (Latif, 2019).

Figure 26. (Clockwise from top) Sol 890, 298, 1905: Specimens resembling colonies of micro-organisms and tube worms (bottom right and see Figure 27 below).

Figure 27. Sol 1905: Specimens resembling colonies of tube worms (see Figure 26 above) and mushroom-lichens (bottom right). Contrast with Figure 28 the colors of which have been desaturated by NASA, thereby giving these same specimens (top photo) a gray-rock-like fossilized appearance (see Figure 28).

Figure 28. Sol 1905: Specimens resembling mineralized trace fossils. These specimens are approximately 1 to 5 mm in length on average. In Figures 26, 27, and 28, one of these "worm-like" specimens has an open aperture at one end, suggesting either an orifice (Figures 26, 27) or that they are hollow (Figure 28). Whereas the specimens depicted in Figure 27 have colorization suggesting biology, the gray specimens depicted here are similar to trace fossils of burrowing tube worms and the tunneling and burrowing systems made by worms in Cambrian sedimentary rocks (see Vannier, 2010), with the tunnels filling with sediment then mineralizing and fossilizing after waters receded in the Gale Crater. As depicted in this NASA desaturated photo, these specimens also resemble the trace fossils of treptichnids, priapulid worms, and their burrowing curved branching beneath and along the mud of the seafloor on Earth, circa 545 mya and which filled with sediment and mineralized (Vannier, 2010). As the NASA image curator did not reply to inquiries, it is unknown if NASA/JPL desaturated the colors of these "worm-like" specimens and their surroundings, or if NASA applied false colors to these same specimens which are much darker in color in Figure 27 and have other biological-features as also depicted in other photos taken on Sol 1905.

Figure 29. Sol 809: (Bottom enlarged, desaturated). These specimens resemble mineralized trace fossils formed, on Earth, during the Cambrian Ordovician transition, by microbes (Girvanella) and siliceous filamentous sponges, with lime, mud and biocasts between them (see Lee et al. 2014). Specimens similar to these have been found on the sediment surface of a relatively flat seashore in northern China during the late Cambrian extinction and the Great Odovician Biodiversification Event (Lee et al. 2014, 2016; Lou and Reitner 2015). Specimens similar to a variety of metazoans can also be viewed in Figures 30, 31. That these represent a pure mineralogical argumentation appears implausible as the orientation of the spindle-like, whitish structures are not organized like mineral structures (Graham, 2019; Graham et al 2016).

Figure 30. Sol 880: Specimens resembling trace fossils of metazoans (e.g. Calymene callicephala, Flexicalymene meeki, Homotelus bromidensis, Isotelus sp., Pseudogygites canadensis, Streptelasma sp.) including filamentous silica sponges upon and impressed into the surface of this Martian rock lying face-up on the floor of the Gale Crater. These formations are approximately 2 -3 mm in length on average. They are similar to metazoan fossils formed at the end of Cambrian (Lee et al. 2016). Although some resemble foraminifera in shape, their patterns of organizational preservation do not. On Earth, dead foraminifera rain down on the sea floor and their mineralized tests often become preserved as fossils in the accumulating sediment. Foraminifera are believed to have evolved around 1.15 bya, and have left a rich biomineralized, fossilized assembly of their multiple forms which include globular, spiral, spherical, and tubular species (Tappen and Loeblich, 1998; McIlroy et al. 2001) with all these diverse forms appearing together in a single species (Pawlowski et al. 2002). It is noteworthy that nearly identical complex metazoan-like forms have been photographed in other nearby locations within Gale Crater, including and especially the "ice-cream-cone" specimen at center right (see Figures 31, 32).

Figure 31. (Top) Sol 809. (Bottom) Sol 809. Similar specimens in two different locations, photographed alongside tubular, curved, and other fossil-like structures which resemble a variety of metazoans.

Figure 32. Sol 809, 880, 869: A variety of specimens, resembling the fossilized remains of metazoans (Calymene callicephala, Flexicalymene meeki, Homotelus bromidensis, Isotelus sp., Pseudogygites canadensis, Streptelasma sp.), photographed on the floor of the Gale Crater.

Figure 33. (Top) Sol 869: Specimens resembling mineralized fossils of worms and metazoans, approximately 1 to 2 mm in length. (Bottom): Sol 1905 compared with Sol 869.


16. Fossilized Stromatolites and Microbial Mats

Sedimentary microstructures similar to the microstructure of stromatolites, microbial mats and microbialites, have been identified on Mars (Noffke, 2015; Joseph et al. 2019; Rizzo and Cantasano, 2016; Ruffi and Farmer, 2016; Small, 2015). If these Martian specimens are biological in origin is not known with certainty as not all terrestrial stromatolites are biological (Buick et al. 1995; Lowe, 1994; Riding, 2008; Wacy, 2010). The same could be said for what appear to be microbialites observed on Mars as the interpretation of their identity has been based on microstructure and not overall morphology. In terms of gross morphology, and until this report, there have been no reports of specimens which resemble terrestrial stromatolites in overall morphology. Presented here are six Martian specimens, photographed in the dried lake bed of the Gale Crater, and three of which (Figures 36-45) closely resemble concentric-domical stromatolites growing in Lake Thetis, Australia.

Figure 34. Stromatolites of Lake Thetis, winter (top) summer (bottom).

Figure 35. Sol 122, 654. 309: Concentric-domical specimens, Gale Crater.

Figure 36. (Top) Submerged Lake Thetis stromatolite (permission to reproduce photo granted by Lyn Lindfield and (Bottom) Sol 122: Martian specimen with collapsed dome and evidence of fossilized fenestrae within the upper portion of the walls. This specimen appears to be fossilized and displays the vertical and inward orientation typically caused by upward-migrating microbial colonies at the sediment-water interface. Several "peanut-brittle" specimens resembling thrombolite mats appear in the bottom portion of the photo (see Figure 36).

Figure 37. Sol 122 (A Top). "Peanut-brittle" specimens resembling microbial mats Sol 309 (Bottom): Domical-concentric-shaped specimen with concentric laminae (center right), which sits adjacent to three overlapping "peanut-brittle" specimens resembling thrombolite bacterial mats. (B): Terrestrial microbial Mat from Damer (2016) (permission to reproduce granted by Dr. Bruce Damer). (C) Algae-bacterial mat. (Photo Credit, University of Waterloo).

Figure 38. (Top): Lake Thetis under water stromatolite (Photo Credit: Government of Western Australia Department of Mines and Petroleum, see also Grey and Planavsky 2009). (Bottom) Sol 529: Martian specimen with evidence of concentric lamination and fossilized fenestrae and "peanut-brittle" specimens resembling thrombolite bacterial mats.

Figure 39. Sol 308 (Top): Water pathways leading down and curving around a Martian Specimen resembling (Bottom): tthe remains of a Lake Thetis underwater stromatolite (Photo Credit: Government of Western Australia Department of Mines and Petroleum, see also Grey and Planavsky 2009).

That the concentric Martian specimens presented here are similar to those in Lake Thetis is of great significance as the lakes of Western Australia have been identified as ideal analogs for Mars when it was presumably flush with rivers, lakes and streams (Baldridge et al. 2009; Bridges et al. 2015; Graham et al. 2016; Nguyen et al. 2014).

A detailed examination indicates that two of the six domincal-concentric Martian specimens meet most and one meets all of the criteria for a stromatolite which is biological in origin as defined and detailed by Allwood et al. (2006), Buick et al. (1981, 1995), Lowe (1994), Riding (1999, 2008), and Wacy (2010). Specimens which resemble microbial mats, and which are adjacent or attached to these concentric structures, were also compared to and found to have a morphology similar to terrestrial mats and those in Lake Thetis. The evidence and observations reported here, therefore, meet the criteria for biology.

Scale Bars, Size, and Magnification: Curiosity's Mast Camera is not a microscope and has only telescopic capabilities, thereby capturing images of numerous specimens, as depicted here, all of which are at varying and unknown distances from the camera and each other. Because of depth of field, it is impossible to apply accurate scale bars to any of these specimens and the same is true of those photographed at Lake Thetis, each of which are approximately 0.9 m (3 ft) in diameter and 0.4 m (1.2 ft) in height.

The estimated diameter (D) and height (H) of the putative Martian stromatolites are as follows: Sol 528 (Figure 44-45) D= 60 cm, H=25 cm; Sol 309 (Figure 37) D=5 cm, H=2 cm; Sol 122/529 (Figure 36,44-45), D=30 cm, H=20 cm; Sol 173 (Figures 40-43), D=30 cm, H=25 cm. The domical specimens depicted in Sol 173 were subject to 300% to 400% magnification to emphasize their micro- and macro-structure, including laminae, fenestrae (gas bubbles) and concentric organizational geometry.

17. The Biology and Stromatolites of Lake Thetis

Lake Thetis is located 1.5 km inland from the Indian Ocean shoreline in Western Australia and has a total shore line of approximately 1000 m (Grey et al. 1990). The lake has a maximum water depth of 2.25 m and total alkalinity is slightly higher than 0.5% meq/L and slightly more basic than seawater with pH ranges from 8.28 – 8.6 (Grey and Planavsky, 2009). As such, the alkalinity may be similar to the waters which are presumed to have filled the Gale Crater (see Baldridge et al. 2009; Schwenzer et al. 2016; Grotzinger et al. 2014).

If "Gale Crater Lake" was slightly alkaline, similar to Lake Thetis, is unknown (see Balta and McSween, 2013; Filiberto et al. 2014). However, alkali igneous rocks and basalts are abundant in the Gale Crater (Payre et al. 2017; Schmidt et al. 2014; Trieman et al. 2016; Anderson et al. 2015; Sautter et al. 2014; Stolper et al. 2013). There is also evidence that these alkali Martian rocks were metasomatized by large bodies of water (Stolper et al. 2013), which may have caused alkali to leach into these waters (Schmidt et al. 2014).

Therefore, the waters of "Gale Crater Lake" may have been (and may still be) somewhat alkaline (Schwenzer et al. 2016; Grotzinger et al. 2014), not like sea-water, and may have been similar to the lakes of Western Australia, including the slightly alkaline Lake Thetis. Other investigators have come to similar conclusions (Baldridge et al. 2009; Benison and LaClair, 2003; Bridges et al. 2015; Cocks et al. 2014; Graham et al. 2012, 2016; Nguyen et al. 2014).

Lake Thetis is host to a variety of microoganisms, including green algae and cyanobacteria and several types of living microbial mats, thrombolites and concentric-domed stromatolites (Grey et al. 1990; Grey and Planavsky, 2009). Carbon isotopes indicate that photosynthesis is driving the formation of stromatolites in Lake Thetis (Grey et al. 1990). The microbial mats consist of a variety of bacteria and several types of cyanobacteria are the dominant primary producers (Grey and Planavsky, 2009; Latif et al. 2019). Microbialites with laminae (i.e. stromatolites), and nodular bacterial mats and thrombolites which have a non-laminted clotted "peanut brittle" structure, coexist often within the same matrix (Grey et al. 1990; Grey and Planavsky, 2009; Reitner et al. 1996). Because of the ebb and flow of summer vs winter water levels, the stromatolite domes of Lake Thetis are submerged in winter, whereas in summer most of these structures are above water and which can cause collapse of the domes (Figure 34).

18. Analysis of Concentric-Domical Specimens Sol 173

In the present study three concentric specimens which are morphologically similar to stromatolites growing in Lake Thetis were identified (Figures 36, 38-45). As only one of the three (Sol 173) was suitable for detailed comparative analyses, it was enlarged to various magnifications and compared with living and fossilized stromatolites from Lake Thetis.

As is evident upon examining Figures 40-43, this concentric Martian specimen consists of and includes five layers of crinkly and wavy nodular laminae with several orders of curvature, an abundance of detrital material, the presence of what appears to be numerous fenestrae/gas bubbles, a central (albeit collapsed) axial zone, and preferential vertical, upward and inward growth which is typically caused by upward-migrating microbial colonies at the sediment-water interface (Planavsky and Grey, 2008; Reid et al. 2000; Reitner et al. 1996). Moreover, extensive nodular biological mats and thrombolites were identified on and adjacent to this specimen. Moreover, the specimens depicted in Sol 173, do not have the appearance of fossils and appear to be moist.

The concentric specimen depicted in Sol 173 (Figures 40-43) meets the criteria for a biological vs an abiogenic formation (Allwood et al. 2006; Buick et al. 1981, 1995; Lowe, 1994) and should therefore be considered evidence of a Martian stromatolite. Based on geometry, it appears that this and the other putative Martian stromatolites underwent preferential vertical, upward and inward growth which is typically caused by upward-migrating microbial colonies at the sediment-water interface (Planavsky and Grey, 2008; Reid et al. 2000; Reitner et al. 1996). However, of the six formations presented in this report, only the specimen depicted in Sol 173 meets all the criteria for biology vs abiotic-mechanical activity (see Allwood et al. 2006). By contrast, Sol 308, 528, 529 appear to be fossilized and meet some but not all the criteria for a living stromatolite.

To speculate: Because the aquatic environment of "Gale Crater Lake" may have been similar to Lake Thetis, this may explain why the specimens resembling Martian stromatolites are morphologically similar to the stromatolites of Lake Thetis.

Figure 40. Sol 173: Concentric specimens with collapsed domes displaying the vertical and inward orientation which is typically caused by upward-migrating microbial colonies. Specimen at center (bottom) is adjacent to specimens covered with numerous nodular-mat like formations and fenestrae.

Figure 41. Sol 173: This specimen has all the features of a terrestrial stromatolite from Lake Thetis, including microbialites with concentric lamina, nodular bacterial mats and thrombolites which have a non-laminated, clotted "peanut brittle" structure, as well as what appears to be numerous fenestrae/gas bubbles. These specimens also appear to be moist and may represent living organisms.

Figure 42. Sol 173 (A-top): This concentric specimen has all the features of a terrestrial stromatolite from Lake Thetis (B-bottom), including microbialites with concentric lamina, nodular bacterial mats and thrombolites which have a non-laminted clotted "peanut brittle" structure, as well as what appears to be numerous fenestrae/gas bubbles.

Figure 43. Sol 173: Five layers of concentric lamina and numerous fenestrae/gas bubbles, thrombolites, and other features typical of stromatolites from Lake Thetis.

19. Visual Analysis of Concentric-Domical Specimens Sol 122, and 529

Martian specimen Sol 529, as depicted in Figure 37), is marked by concentric thrombolitic lamination, and fenestrae and surrounded by "peanut-brittle" specimens resembling fossilized bacterial mats and is nearly identical to the stromatolites of Lake Thetis.

This similarity to terrestrial stromatolites is even more evident upon examination of Sol 122 (see Figures 35, 44 and 45). The fenestrae within the upper portion of the walls appears to be fossilized, though this is unknown. When enlarged and portions are selectively examined, Sol 122 (Sol 528) displays most of the features of a biological formation which was constructed in a body of water, and thus, like Sol 173 and Sol 122/528, shares many of the characteristics of Lake Thetis stromatolites, including being surrounded by what appears to be fossilized bacterial mats and layers of yellowish masses which could be dried algae (see Figure 44).

Figure 44. Sol 528 (see Fig 45). Fossilized bacterial mats, thrombolitic fenestrae, and layers of dried algae?

Figure 45. Sol 528: This concentric specimen has all the features of a terrestrial stromatolite from Lake Thetis, including microbialites with concentric and wavy nodular laminae with several orders of curvature, an abundance of detrital material, and the presence of what appears to be numerous fenestrae/gas bubbles.


The authors, established experts in astrobiology, astrophysics, biophysics, geobiology, microbiology, lichenology, phycology, botany, and mycology, have presented a sample of NASA photos depicting numerous specimens similar to terrestrial algae, microbial mats, stromatolites, filamentous tangles, algae-fungal-lichen symbiotes, colonies of what may be tubular-shaped organisms, calcium carbonate bio-secretions, ooids and metazoan-like fossils. It also appears that some specimens may be alive, moist, or covered by a thin sheet of ice. These impressions are speculative.

Arguments and data, both pro and con, have been presented and abiogenic explanations detailed, particularly in regard to mineralization. We cannot rule out the possibility that some of the specimens presented are unusual formations of salts and minerals. However, terrestrial salts, sand, minerals and rocks are infested with cyanobacteria, and these and other organisms often act to bind these substrates together and may give them a green coloration. The various shades of green on the Martian surface are strongly suggestive of biology.

Nevertheless, it is possible that some of the evidence presented may be due to unusual volcanic formations, geological weathering and the presence of green chlorite and other Martian minerals. What appears to be trace fossils of metazoans may represent abiotic mineralization. However, bio-weathering, the microbiology of mineralization and mineralized fossils may also be partly responsible for the existence of the minerals detected on Mars.

Our purpose in conducting this study and presenting this evidence was not to prove there is life on Mars. Our mission was to survey the landscape and identify specimens for future investigation and robotic examination, evaluation, extraction, analyses; and to identify specific features that may be targeted, collected and returned to Earth for study, or, preferably to the International Space Station, thus minimizing the dangers of exposing our planet to Martian organisms.

In conclusion: specimens photographed in Gale Crater were found to resemble algae, lichens, calcium-encrusted cyanobacteria, stromatolites, microbial mats, gas domes constructed by oxygen-respiring organisms, calcium biosignatures, fossilized metazoans, and colonies of micro-organisms. It is not likely that minerals, salts or other abiogenic features can account for all this evidence. The findings and observations detailed in this report, coupled with fact-based speculation and theorizing by the authors, support the hypotheses that, beginning billions of years ago, algae-like organisms and algae-fungal symbiotes may have colonized Gale Crater.

Manuscript Received: January 19, 2020
Manuscript Revised: February 21, 2020
Second Revision Received: February 28, 2020
Total Referees/Reviewers: Six, five accepting pending revision.
Manuscript Accepted for publication by three Senior Editors (F.H., D.D., A.E.).

Compliance with Ethical Standards: The authors have complied with all ethical standards and report no conflicts of interest, financial or non-financial, which may have influenced the writing of this report. There are no funding sources to report. All authors have contributed time and effort to this article.
Acknowledgments: The authors wish to thank the two editors and five of the six referees for their very helpful comments and which led to major revisions, thereby greatly improving this article.


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