“The fact that these carbon and oxygen isotope values are higher than anything else measured on Earth or Mars points towards a process (or processes) being taken to an extreme,” stated NASA’s Goddard Space Flight Center’s David Burtt, commenting on the most recent observations from the Curiosity rover’s exploration of Gale Crater. For those watching the quest for life on Mars, this declaration is a landmark moment in the discovery of the planet’s early climate and its potential for life.
The tale starts in the rock layers of Gale Crater, a location selected for its geologically diverse record of Mars history. Since landing, Curiosity has drilled with precision, sampled with accuracy, and analyzed with finesse the sediments of this former water-saturated basin. The Sample Analysis at Mars (SAM) instrument on the rover, a bench-top laboratory with a Tunable Laser Spectrometer (TLS), has been instrumental. By heating solid rock powder to almost 900°C, SAM releases gases from minerals particularly carbonates enabling the accurate measurement of their isotopic content. These measurements, published in the Proceedings of the National Academy of Sciences, have shown the most massive carbon (δ13C) and oxygen (δ18O) isotope values ever measured for Martian material (δ13C = 72–110‰; δ18O = 59–91‰).
Isotopes are atoms of the same element with varying masses, and their mineral ratios serve as time capsules, locking in information about the environment in which they were formed. On Mars, like on Earth, preferential removal of the lighter isotopes through evaporation leaves behind the heavier isotopes. In Gale Crater, the heavy isotope enrichment in carbonates indicates heavy evaporation much greater than occurs on terrestrial planets (two to three times greater than terrestrial variations). In accordance with Burtt, “While evaporation can cause significant oxygen isotope changes on Earth, the changes measured in this study were two to three times larger. This means two things: 1) there was an extreme degree of evaporation driving these isotope values to be so heavy, and 2) these heavier values were preserved so any processes that would create lighter isotope values must have been significantly smaller in magnitude.”
How did these conditions come to be? The research team suggests two climate situations for ancient Gale Crater, each as dire in its own right for surface life. The first is one of episodic wet–dry cycles, where temporary lakes or ponds would fill and periodically dry out, depositing increasingly heavy isotopic and concentrated brines behind them. The second is a vision of cold, salty-ice (cryogenic) conditions, with carbonates forming from brines at freezing temperatures. Co-author Jennifer Stern continues, “Wet-dry cycling would indicate alternation between more habitable and less habitable environments, while cryogenic temperatures in the mid-latitudes of Mars would indicate a less habitable environment where most water is locked up in ice and not available for chemistry or biology, and what is there is extremely salty and unpleasant for life”.
These are not speculations. Orbital and in-situ mineralogical information, complemented by stratigraphic data from Curiosity’s journey up Mount Sharp, record a shift from older lake beds to wind-deposited (aeolian) material, documenting the planet’s transition from wet to drier conditions (wet/dry cycles). The occurrence of hydrated sulfates, clay minerals, and carbonates in different levels of stratigraphy is validating the evidence of changing availability and chemistry of water over millions of years (ancient lakes at Gale Crater).
Technically, the isotopic analysis conducted by SAM’s TLS instrument is a planetary science triumph. The technique analyses the heavy-to-light ratios of isotopes in CO2 expelled from carbonates and thus directly records evidence for the processes operating in the environment. Rayleigh-type evaporation and cryogenic precipitation must act together to generate the observed isotopic enrichments no one process can cause the data alone (detailed isotopic mechanisms). The findings reinforce the way Martian carbon and water cycles have deviated from those of Earth, particularly in the absence of a strong biosphere.
In spite of such hostile surface conditions suggested by these results, hunting for Martian life is a long way from being over. Burtt observes, “Our samples are not consistent with an ancient environment with life (biosphere) on the surface of Mars, although this does not rule out the possibility of an underground biosphere or a surface biosphere that began and ended before these carbonates formed.” Mars has been found to have deep caves carved out by ancient volcanism, and such subsurface conditions may be havens for extremophile microbes life forms on Earth that live in darkness, cold, and high salinity. Mars caves could potentially still contain liquid water hidden from the extreme surface and could be top candidates for future astrobiological research.
Its implications go beyond habitability. Isotopic history of Martian carbonates informs models of atmospheric loss, volatile cycling, and the planet’s overall climate history. With Curiosity and Perseverance continuing their campaigns, isotopic, mineralogical, and sedimentological complementarity will be key in assembling the intricate puzzle of how Mars evolved from a lake- and river-rich world to the frozen desert it is today.
For space and planetary scientists, the message is clear: Mars is a planet of extremes, and its rocks are hiding secrets that only the most sensitive instruments and the most tenacious questions can elicit.
