Might the secret to Mars’ habitability be hidden in the chemistry of its rocks? New high-resolution data from NASA’s Perseverance rover indicate that Jezero Crater, long occupied by an ancient lake, experienced not only one but several episodes of watery activity each with unique chemical conditions that might have favored life at different periods.
Results are from the rover’s Planetary Instrument for X-ray Lithochemistry (PIXL), which fires a focused X-ray beam at rock surfaces to quantify elemental composition at sub-millimeter resolution. The method, a type of micro–X-ray fluorescence spectroscopy, enables scientists to reconstruct the mineralogy of Martian rocks with unprecedented accuracy. Applying Rice University-created Mineral Identification by Stoichiometry (MIST) software, scientists detected 24 unique mineral species within Jezero’s bedrock and sedimentary deposits. MIST not only compares PIXL chemical measurements to established mineral formulas but also employs uncertainty modeling, executing multiple statistical tests in repeated runs to assign confidence levels to every identification.
“The minerals we find in Jezero using MIST support multiple, temporally distinct episodes of fluid alteration,” she said. “Which indicates there were several times in Mars’ history when these particular volcanic rocks interacted with liquid water and therefore more than one time when this location hosted environments potentially suitable for life.”
The mineral record also indicates a chemical sequence. The first planet alteration event, captured in the oldest volcanic rocks of the crater floor, is indicated by minerals like greenalite, hisingerite and ferroaluminoceladonite markers of acidic high-temperature fluids. On our planet, such an environment, the analog to Yellowstone’s hot, acidic springs, can be inhospitable to most life, but extremophiles thrive in niches. Co-author Kirsten Siebach said, “These hot, acidic conditions would be the most challenging for life. But on Earth, life can persist even in extreme environments like the acidic pools of water at Yellowstone.”
A subsequent episode introduced cooler, neutral-pH waters, generating minerals such as minnesotaite and clinoptilolite. These precipitated at lower temperatures and across a larger area, including crater floor and delta deposits, indicative of a more stable, possibly hospitable environment. The most extensive and prevalent alteration phase was low-temperature, alkaline fluids that produced sepiolite a fibrous clay mineral prevalent in terrestrial soils that is capable of adsorbing and sheltering organic molecules. Sepiolite’s ubiquity across all units drilled so far indicates a basin-wide groundwater or lake environment that may have harbored microbial life.
These large geochemical trends correlate with more localized, fine-scale ones from Perseverance’s other instruments. In the Bright Angel formation, SHERLOC deep-UV Raman and fluorescence spectroscopy identified organic-associated minerals such as vivianite and greigite in mudstone. Vivianite, which is an iron phosphate, only generally forms at low temperature, in anoxic, neutral-PH phosphorus-rich waters that are conducive to preserving biosignatures. Greigite, an iron sulfide, is produced by sulfate-reducing reactions that can be abiotic or microbially driven. Their spatial correlation with organic fluorescence signals suggests intricate redox chemistry that could have cycled nutrients and energy.
Such phosphate minerals have been found within the Onahu outcrop at the fan top, where PIXL diffraction data corresponded to metavivianite, ferrolaueite, and ferroberaunite oxidation products of vivianite. These phases probably precipitated in pore space with Fe-Mg carbonates, implying precipitation from reducing, Fe²⁺-rich fluids to be followed by later oxidation. On Earth, similar mineral assemblages can be formed by microbially mediated iron reduction coupled to organic matter oxidation, making them of astrobiological interest.
The combination of PIXL’s quantitative chemistry and MIST’s stoichiometric modeling is essential for this work. Mars samples cannot be obtained as powders or thin sections in situ, and so the capacity of MIST to treat mixed-pixel data and to propagate measurement uncertainties is important in ensuring robust mineral identification. The ensuing mineralogical archive of Jezero Crater will inform sample selection for eventual return to Earth, where laboratory equipment can interrogate isotopic compositions, trace organics, and microtextures at nanometer scales.
The changing water chemistry preserved in Jezero’s rocks from acidic hydrothermal environments to neutral groundwater to alkaline lake bottoms gives a sense of a dynamic world in which habitability went in and out of fashion over geologic time. To planetary geologists and astrobiologists, such discoveries are not a list of minerals, but a chronology of shifting conditions, each with its own promise of supporting life.
