Mars has an extensive sedimentary record, dating back at least 4.3 billion years, which shows that early Mars had a very difficult climate from today, with permanently wet environments, and possibly even the conditions for life. Whether-or-not this early climate also included areas with episodic or periodic wet intervals has to date, however, been unclear. Such fluctuations in hydrodynamic conditions leave distinctive traces in the sedimentary record, such as cracks, however they are also easily eroded away, and models of the early Martian climate have been ambivalent about the existence of such conditions.
In a paper published in the jornal Nature on 9 August 2023, William Rapin of the Institut de Recherche en Astrophysique et Planétologie at the Université de Toulouse 3, Gilles Dromart of the Laboratoire de Géologie de Lyon Terre, Planètes, Environnement at the École normale supérieure de Lyon, Ben Clark of the Space Science Institute, Juergen Schieber of Indiana University, Edwin Kite of the University of Chicago, Linda Kah of the University of Tennessee, Knoxville, Lucy Thompson of the University of New Brunswick, Olivier Gasnault, Jeremie Lasue, and Pierre-Yves Meslin, also of the Institut de Recherche en Astrophysique et Planétologie at the Université de Toulouse 3, and Patrick Gasda and Nina Lanza of the Los Alamos National Laboratory, report the presence of a well-preserved polygonal mud-crack pattern on strata dated to about 3.6 billion years ago (dating to the Hesperian Eon of Mars), which they believe to be evidence of a wet-dry cycling system, and therefore to provide useful new insight into the early climate of Mars.
NASA's Curiosity Rover has documented hundreds of metres of sediments deposited in lakes., rivers, intermittent lakes, and lake-margin settings within Gale Crater on Mars. The vast majority of these have been smectite (silicone and aluminium rich clay) mudstones, but the rover recently encountered a sulphate-bearing unit, apparently marking a major environmental transition foumd in stratified terrains across Mars. At this horizon the rover found an apparent sulphate evaporite deposit.
The unit at the base of the sulphate-bearing strata has widespread centimetre-scale polygonal patterns, formed of straight lines intersecting at triple junctions. All observations of this were made within an 18 m interval, with some variations apparently caused by subsequent alteration of the same basic pattern. These polygons appear to persist for tens of centimetres, and can be seen on stepped blocks of sediment. Where visible on bedding plains the polygons are made of raised ridges are about 1 cm high, with the polygons having an average diameter of 4 cm. Junctions have an average angle of 120°. The ridges are made up of aligned nodules, of variable size, and juxtaposed in a variety of ways, and apparently made of calcium sulphate and magnesium sulphate minerals. The sediment in which these ridges are emplaced is generally sulphate-poor, though with some patches with raised calcium sulphate levels.
In situ observations of polygonal ridges. (a) General view of bedrock surrounding the rover on sols 3154 to 3156 showing widespread polygonal ridges. (b) Close-up showing ‘stepped’ exposure of polygons within large bedrock blocks. (c) View of bedrock with polygons and locations of ChemCam analysis on ridge (red rectangle) and Alpha Particle X-Ray Spectrometer analysis on smooth host bedrock (dotted circle). (d) Remote micro-image of cemented ridge with spots analysed by ChemCam (reticles 1 to 5), highlighting details of nodular texture. (e), (f) Bedrock with polygonal pattern (e) and interpretative overlay (f) that shows prominent ridges (solid red lines), less certain ridges (dotted red lines) and cross-cutting later-stage calcium-sulphate-filled veins (white areas). Rapin et al. (2023).On Earth, polygonal ridge patterns can form on evaporite deposits as a result of subsurface salinity convection, but Rapin et al. believe this to be unlikely, as this generally only occurs on pure salt crusts, and produces polygon patterns with much larger diameters (which would presumably be larger still if they formed under Mars's low gravity conditions). They instead suggest that the more likely explanation is that the ridges formed originally as cracks within a drying sediment, which was then infilled by salt-rich water, which evaporated leaving the nodules, which are more resilient to erosion than the surrounding sediment, leading to raised ridges as this is eroded away. Newly formed desiccation cracks generally have T-junctions between blocks, but where they undergo repeated cycles of wetting and drying, these tend to reform into more even 120° Y-junctions, with about 10 cycles of wetting and frying typically needed on Earth for 120° to be reached, giving a pattern of even hexagons.
The hexagons are located in a sulphate-poor sediment overlaying a sulphate-rich nodular bedrock, making the precipitation of sulphate minerals plausible, although it is unlikely that they originally formed in their current configuration. Most likely, the salts were precipitated in a slurry with sediment particles in original cracks with a T-junction formation, but that then redissolved with each cycle of wetting and re-precipitated with each cycle of evaporation, eventually forming the resilient hexagon-shaped patterns seen today.
Larger colour image of bedrock with polygonal ridges for context. MastCam image and close-ups (a), (b) and (c) with rectangle locations of close-up view. Close-ups (b), (c) show bedrock 10 to 20 meters away where regularly spaced ridges and nodules can be observed supporting lateral extension of the same polygonal pattern although camera resolution prevents detailed geometrical analysis at this distance. Rapin et al. (2023).The water that caused these patterns was probably brought in by periodic flooding and groundwater recharge, which would have added sediment to the deposits (leading to the depth of strata we see today), while dissolving any surface salts, so that each drying event is likely to have precipitated more salt than its predecessor. This dessication-water recharge cycle is likely to have affected only the upper few centimetres of sediment at any one time, with the hexagons therefore able to move up through the sediment column with repeated cycles. The consistency of the size and shape of the polygons implies a repeated cycle of regular intensity, while the variable size and shape of the nodules within the ridges implies multiple generations of growth.
The repetitive nature of the floods and the limited amount of depth penetration makes it likely that these cycles were seasonal, although a shorter-term cycle cannot be ruled out. The time period over which this cycle occurred is unclear, although the ridges have been identified at multiple points within an 18 m succession, which on a typical Earth floodplain with a sedimentation rate of about 0.01 mm per year, would imply a period of thousands to millions of years, although this might not have been a continuous interval of seasonal flooding; possibly the occurrence of seasonal interludes was itself part of a larger cycle. Individual blocks of polygon propagation are often more than 2 m thick, with no visible boundaries to other polygon-bearing or polygon-free strata, which suggests that if there were longer dry intervals without sedimentation, then little erosion took place in these intervals either.
The mature hexagon shapes of the polygons indicate they were formed by repeated wet-dry cycles, and the thickness of the strata in which they are found implies that this cycle occurred at least episodically for a very long period of time. Mud crack polygons have also been seen in the underlying Murray Formation, but these typically have a T-junction structure, suggestive of a single drying event, whereas the polygons reported by Rapin et al. appear to be indicative of a repeated wet-dry cycle, which occurred for a sustained period of time on the early surface of Mars. This agrees with models of the early climate of the planet which have suggested that a single event, such as a meteor impact or supervolcano, forming all the water-related features on Mars (one a popular hypothesis) is unlikely. Instead, Mars appears to have had a longer period of Earth-like climate with seasonal flooding events, and evaporite deposits forming in seasonal lakes.
A climate with wet and dry cycles is considered to be conducive to, and perhaps essential for, the type of prebiotic chemical evolution needed to form the precursors for life. Dessication lowers the amount of water available, thereby increasing the proportion of soluble ingredients within the remaining liquid, raising the rate at which reactions can occur. In particular, nucleotides form more readily from nucleobases in a concentration, and also polymerise to form larger molecules such as DNA or RNA more readily, and amino acids more readily form proteins under such conditions. Dioctahedral smectites, which appear to be ubiquitous clay minerals on the surface of Mars, are capable of tightly adsorbing nucleotides through cation exchange, and have been suggested as having been vital for the formation of the first pre-biotic organic polymers. Thus, seasonal pools on the surface of Mars could have reasonably formed the 'warm little ponds' proposed by Darwin as a location for the appearance of life.
Sediments in Gale Crater have been shown to contain about 500 g of organic material per cubic metre, as well as a variety of other soluble elements. The discovery that this site also once underwent seasonally wet and dry cycles supports the idea that this area was once suitable for prebiotic chemistry, but it is highly unlikely to have been the only place on Mars where such conditions were found, although the discovery does re-enforce the importance of Gale Crater as a site of global importance for understanding the early history of Mars.
The discovery of evidence of wet-dry cycling from a time when organics and volatiles are known to have been accumulating on Mars for over a billion years supports the idea that conditions on Mars during the Noachian–Hesperian transition period may have been favourable for the emergence of life on Mars, possibly more so than the earlier, and apparently wetter, Noachian Eon.
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