Where to find biosignatures on Mars? A Case for Clays

Michael Phillips is a planetary geoscience Ph.D. student at the University of Tennessee, Knoxville. His dissertation covers a wide range of topics including strange sublimation pits on Mercury called “hollows”, microbial habitats in the Atacama Desert of Chile (the world’s driest desert), and the composition and origin of the most ancient rocks on Mars as seen from orbit. His work is furthering our understanding of the planets, what happens on their surfaces, how they formed, and whether any of them besides Earth ever harbored life. He’s also a rock climber, on-again-off-again runner, and very mediocre stringed instrument player.

Title: Inhabited subsurface wet smectites in the hyperarid core of the Atacama Desert as an  analog for the search for life on Mars 

Authors: Armando Azua‑Bustos, Alberto G. Fairén, Carlos González Silva, Daniel Carrizo,  Miguel Ángel Fernández‑Martínez, Cristián Arenas‑Fajardo, Maite Fernández‑Sampedro,  Carolina Gil‑Lozano, Laura Sánchez‑García, Carmen Ascaso, Jacek Wierzchos, & Elizabeth B.  Rampe. 

First Author’s Institution: Centro de Astrobiología (CSIC-INTA), 28850 Madrid Spain

Status: Published on November 5, 2020 in Nature: Scientific Reports, open access available. 

Introduction

Humans have been searching for life on the surface of Mars with robots since the 1970s. Besides addressing that fundamental question “are we alone?”, finding and studying martian life would constitute a paradigm-shifting advance in science. But without any obvious signs of life that we can see with Earth-based telescopes or from spacecraft orbiting Mars, how do we decide where to send our explorative landers and rovers? This decision-making process is aided by studying Earth’s most Mars-like places – “terrestrial Mars analogs”. By understanding where life resides within terrestrial Mars analogs, we can make more informed decisions about where to send our science-robots. Today’s paper reports on a clay-rich layer 30 cm below the surface of the driest desert in the world. Within this layer are preserved signs of life, or biosignatures, as well as actively metabolizing microbes. The authors propose that subsurface clay minerals might host biosignatures on Mars and could be sought with ESA’s Rosalind Franklin rover, NASA’s Mars 2020 Perseverance rover, and CNSA’s Tianwen-1 rover.

The authors of today’s study dug 60 cm-deep pits in the hyperarid Yungay region of the Chilean  Atacama Desert (AD), and found a water-laden, smectite-rich layer at ~30 cm-depth in all pits (Fig. 1). This is the first time such a water-laden layer has been found in the AD. The AD, specifically the Yungay region in the hyperarid core, is the oldest and driest (non-polar) desert on  Earth. Its antiquity of aridity along with relatively high exposure to UV radiation, extreme  diurnal (day/night) temperature swings, very low soil organic content, and high salinity are qualities that make the AD one of the best terrestrial Mars-analog environments. The Yungay  region rarely experiences rain events (~2 mm/yr), but two rainfalls in 2015 delivered a combined ~45 mm(!) of rain to the area. In 2017, an additional rain event delivered ~19 mm of rain and gave rise to lagoons. Historical records in this region stretch back beyond 500 years, and no recorded rainfall events of this frequency and magnitude exist. The rainfall events significantly impacted microbial populations, and  produced some amazing, rarely-seen events, such as desert bloom. These rainfalls were also the likely source for water in the smectite-rich layer that is the topic of today’s study. 

FigFigure 1:. a) Diagram showing typical profile view  of the soil horizons with mineralogy and water activity. b) context view of the field site. Red arrow marks the pit shown in (c). (c,  d) example of pits (1m-long by 60 cm-wide by 60 cm-deep) dug in the study. Note the dark bands in both pits that are the water laden, smectite-rich layer. Figure 1 from the paper.

Vessels of clay

What are smectites and why are smectite-rich terrains a good place to find signs of life? First,  smectites are a type of clay mineral. Clay minerals are structurally and chemically diverse  hydrous aluminum phyllosilicates. Hydrous simply means that these minerals have water (H2O  or OH) bound in their crystal structure. Aluminum indicates that these minerals contain the  element aluminum. Phyllosilicates, or sheet silicates, are a class of minerals made up of stacked layers (sheets) of metal-oxygen tetrahedra and octahedra (Fig. 2a). Not all clay minerals are created equal, and the smectites are a special class of clay minerals that swell when hydrated. This property makes smectites especially good at preserving organic molecules between their sheets (Fig. 2b). So, a smectite-rich terrain on Mars would be a good candidate for biosignature detection missions because: 1) smectites require liquid water to form and liquid water is a requirement for life as we know it, and 2) smectites have the capacity to preserve biosignatures within their crystal structures.

Figure 2: The structure of clays taken from Schulze, 2005. a) Generic structure of 1:1 clays (top) and 2:1 clays (bottom). Layers  in 1:1 clays consist of one tetrahedral sheet bonded to one octahedral sheet. Layers in 2:1 clays consist of two tetrahedral sheets  bonded above and below an octahedral sheet. b) Smectites are 2:1 clays with the largest gap between their layers of any clay minerals. Organics can be preserved between the layers of smectite clays.

But what does finding biosignatures and metabolically active microbes in a wet subsurface layer  on Earth – just after it rained – have to do with finding life on Mars? Well, the conditions in the  AD might be analogous to the conditions present on Mars ~4.5 – 3.5 billion years ago during the Noachian and early- to middle-Hesperian Periods.  Although water-ice clouds have been observed on present-day Mars, and water-ice frost was observed by the Phoenix and Viking II landers, it doesn’t rain on Mars today and liquid water is not stable on the martian surface. However, early Mars likely experienced wet climatic intervals conducive to river formation with long dry periods in between, analogous to conditions in the AD.

Tomorrow never knows

In February 2021 NASA’s Perseverance rover landed at Jezero Crater, once host to an open basin lake in the Noachian Period, and in 2023 ESA’s Rosalind Franklin rover will land at Oxia Planum, a shallow basin rich in clay minerals. The hope is that Perseverance and Rosalind Franklin may finally reveal ever elusive evidence for life on Mars. Smectites have been identified from orbit at both sites, and today’s study bolsters the hope that smectite bearing outcrops might preserve biosignatures from the distant past. The authors even suggest that smectite samples should be cached for return because many of the analyses required to rigorously test the biogenicity of organic molecules in smectites need to be done in a laboratory on Earth. Perhaps these upcoming missions will discover paradigm-shifting evidence for life on Mars, or, perhaps, we will need to keep looking for signs of a martian biome elsewhere. Other possibilities besides smectite-rich terrains exist, including salt-rich areas, or hydrothermal vents that host silica sinter deposits evocative of terrestrial microbialites. Whatever our future rovers find, it seems that missions to Mars will continue to pique our interest and that Mars will remain an attractive target for answering some of humanity’s most fundamental questions.

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1 Comment

  1. Really liked the article. On point, short but informative in a way that even a non-profession space enthusiast can understand, and informative about potentially important subject.

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