During the Spirit rover’s journey across the Columbia Hills of Gusev Crater, Mars it discovered some peculiar features around a geological feature called Home Plate (Fig 1). Jutting out from the dusty surface surrounding Home Plate were patchy outcrops of fist-sized nodules up to fifteen centimetres each in size. Closer imaging revealed that many of the nodules in these outcrops were digitate, or finger-like, structures and spectral analysis showed that they were composed of opaline silica with traces of halite (salt, or NaCl). Although there are many sources of opaline silica here on Earth, the nodular opaline silica digitate structures at Columbia Hills were found as a layer within a succession of erupted volcanic rocks that lie on top of rocks that have been affected by acid-sulfate alteration and covered by rocks that are unaltered. Our team have interpreted these deposits as originating from past hot spring activity in the area. Hot spring silica is important on Earth for not only providing a habitat for a variety of microbial life (i.e. those that make their living from chemical energy or from photosynthesis), but also being an excellent medium for preserving traces of microbial life: the hot spring fluids immediately entomb the microbes in silica that is dissolved in the fluids. Could the opaline silica deposits of Columbia Hills be evidence of life on Mars? How can we know for certain?
The Spirit rover’s view of the eastern edge of Home Plate. The yellow box lies within an area of nodular opaline silica outcrop and highlights a false-colour close up of the nodular silica with digitate structures. Modified from Ruff et al., 2020.
To better understand the environment in which the silica outcrops in the Columbia Hills formed, planetary scientists have pieced together clues from the surrounding area and made comparisons to hydrothermal settings on Earth.
The Champagne Pool hot spring at Wai-O-Tapu, New Zealand. Photo by: Martin Van Kranendonk.
One hypothesis for the origin of the opaline silica at Home Plate is that sulfuric acid steam from volcanic fumaroles (gas vents) condensed on basaltic rocks, which caused leaching of metals from the basalts and a thin residue of opaline silica. However, the morphology, or shape, and texture of the Home Plate nodules and their digitate structures are challenging to explain by this process, as is the distribution of the deposits as a relatively thick layer within a succession of volcanic rocks.
Another view of the digitate structures of Home Plate, Mars . Credit: Mars Guy on YouTube, 2022.
A second hypothesis is that the digitate structures on the nodules were formed by wind erosion. However, tests at the University of Arizona have demonstrated that wind erosion produces very different patterns to those found at Home Plate.
An alternative hypothesis arose from the discovery of hot spring deposits on Earth with features that look remarkably similar to those associated with the opaline silica deposits found on Mars. High in the Atacama Desert of Chile is El Tatio, the world’s third largest field of hot springs, in one of the most Mars-like environments on our planet. It has high UV, low temperatures, and dry, highly evaporative conditions. The El Tatio field is home to more than 100 geysers, hot springs, pools, mud pots and fumaroles. Within the outflow channels of many El Tatio hot springs, Steve Ruff and Jack Farmer of Arizona State University found digitate structures on nodular deposits that have a comparable morphology and composition to those discovered on Mars. Locations such as these are called analogue environments and provide planetary scientists with an opportunity to gain further insight into the off-world environments they study beyond the imaging and data returned by robotic missions. The nodular deposits of opaline silica with digitate structures found at El Tatio formed in shallow, near-neutral pH water. These warm waters were observed to host a thriving ecosystem of microbial biofilms and thick mats populated with various species of bacteria and diatoms.
A portion of the volcanic hydrothermal system at El Tatio, Chile: Note the nodular deposits in the foreground near the hammer. From Ruff & Farmer, 2016.
As with the nodular deposits on Mars, the nodular deposits with digitate structures at El Tatio are composed of opaline silica. The branching, digitate structures on the nodules form via accumulation of silica on microbial biofilms, creating finely laminated structures known as stromatolites, which grow over the course of many years. Thin sections of El Tatio digitate structures reveal the finely laminated internal layering of opaline silica alternating with thicker laminae that are littered with tiny cavities containing filamentous microbes. The microbial intervals observed within the digitate structures are the entombed remnants of once living microbial communities. Similar microtextural features and patterns caused by silica precipitation on microbial biofilms have been observed in siliceous microstromatolites throughout New Zealand and in many hot spring deposits elsewhere.
Home Plate opaline silica nodules with digitate structures (left image) compared with the digitate structures of El Tatio nodular opaline silica deposits (right image). The white bar indicates a scale of 5cm in each image. Credit: Ruff & Farmer, 2016.
The El Tatio field observations and analysis of the digitate structures discovered in hot springs around the world raises the question: Could the digitate structures on the nodular deposits of opaline silica at Home Plate represent a potential biosignature in the form of fossilised microstromatolites?
NASA’s Mars Science Definition Team defines a potential biosignature as ‘an object, substance, and/or pattern that might have a biological origin and thus compels investigators to gather more data before reaching a conclusion as to the presence or absence of life’. Although it is still possible that the Mars digitate structures were formed by abiotic processes, the digitate structures on the opaline silica nodules at Home Plate on Mars certainly share a strong morphological resemblance with those at El Tatio, and elsewhere on Earth and thus warrant further investigation.
Close-up of a thin section of a digitate structure from El Tatio (left image: scale bar = 500 μm), and an enlarged view (right) from the boxed area showing silicified sheaths of a filamentous cyanobacteria (scale bar = 50 μm). Credit: Ruff & Farmer, 2016.
When discerning the origins of microscopically small features, the frontier between biotic and abiotic signatures is a fine one. The challenge is compounded by the fact that biological information held within fossils and minerals is altered over time by weathering and other environmental processes. To confirm the origin of the ancient Mars digitate structures, direct microscopic and geochemical analyses will be required, specifically of their internal features. The analytical tools required to perform these tests are large, heavy, and expensive to be sent to Mars, and community confirmation of any possible sign of life in Martian rocks would have to be confirmed by research groups across the world. The LifeSpringsMars mission will eschew scientific analysis on the surface of Mars and simply collect samples for return to Earth where the most sophisticated, thorough analyses can be completed. Until that happens, we cannot know for certain if these intriguing, and geologically important, rocks once held Martian life.
We are currently investigating three aspects relating to the formation of nodular opaline silica deposits with digitate structures.
First, we are undertaking laboratory experiments that will simulate the growth experiments of the microstromatolites identified by Handley et al. (2005) in the waters of Champagne Pool. Our intent here is to recreate the digitate growth features in the lab, using the actual fluid and microbial consortium present in Champagne Pool under varying physical conditions (airspeed, evaporation rate, splashing, etc.). In a second step, we will perform exactly the same experiments but using sterilised hot spring fluids to eliminate the effect of biology to determine whether similar structures can be formed abiogenically. (i.e., in the absence of life).
Second, we are performing experiments with supersaturated hot spring analogue fluids plus organic molecules, but without life. In these experiments, we want to investigate whether the addition of simple organic compounds – such as those that are known to occur in meteorites, for example, will have an effect on silica precipitation and whether these might also form digitate-like features.
And third, we are undertaking high-resolution imaging and trace element analysis of digitate nodules from El Tatio in order to characterise more completely the growth mechanisms of these structures and determine whether there may be a distinct geochemical signature of life. See the research tab “How do geology and biology interact in a hot spring?” for more details.
Want to learn more?
You can delve into the science of ancient hot spring deposits on Earth and on Mars and their potential for preserving ancient signs of life in these key papers:
Guido, D., Campbell, K., Foucher, F., & Westall, F., 2019. Life is everywhere in sinters: examples from Jurassic hot spring environments of Argentine Patagonia. Geological Magazine, v. 156, pp. 1631–1638.
Handley, K.M., & Campbell, K.A., 2011. Character, analysis, and preservation of biogenicity in terrestrial siliceous stromatolites from geothermal settings. In: V.C. Tewari and J. Seckbach (eds.), STROMATOLITES: Interaction of Microbes with Sediments. Cellular Origin, Life in Extreme Habitats and Astrobiology, v. 18, pp. 359–381. Springer, Dordrecht.
Handley, K. M., Campbell, K. A., Mountain, B. W., & Browne, P. R. L., 2005. Abiotic-biotic controls on the origin and development of spicular sinter: In situ growth experiments, Champagne Pool, Waiotapu, New Zealand. Geobiology, v. 3(2), pp. 93-114.
Ruff, S. W., Farmer, J. D., Calvin, W. M., Herkenhoff, K. E., Johnson, J. R., Morris, R. V. et al., 2011. Characteristics, distribution, origin, and significance of opaline silica observed by the Spirit rover in Gusev crater, Mars. Journal of Geophysical Research E: Planets, v. 116(4) doi:10.1029/2010JE003767
Ruff, S.W, & Farmer, J.D., 2016. ‘Silica deposits on Mars with features resembling hot spring biosignatures at El Tatio in Chile’ Nature Communications, v. 7: 13554.
Ruff, S.W., Campbell, K.A., Van Kranendonk, M.J., Rice, M.S., & Farmer, J.D., 2020. ‘The Case for Ancient Hot Springs in Gusev Crater, Mars’. Astrobiology, v. 20, pp. 475-499..
Squyres, S. W., Arvidson, R. E., Ruff, S., Gellert, R., Morris, R. V., Ming, D. W., et al., 2008. Detection of silica-rich deposits on Mars. Science, v. 320, pp. 1063-1067.
Sriaporn, C., Campbell, K.A., Millan, M., Ruff, S.W., Van Kranendonk, M.J., & Handley, K.M., 2020. Stromatolitic digitate sinters form under wide-ranging physicochemical conditions with diverse hot spring microbial communities. Geobiology, v. 18, pp. 619-640.Teece, B.L., George, S.C., Djokic, T., Campbell, K.A., Ruff, S.W., & Van Kranendonk, M.J., 2020. Biomolecules from fossilised hot-spring sinters: implications for the search for life on Mars. Astrobiology, v. 20, pp. 537-551.