“It’s hard to form rocks like these without a lot of water,” said Dr. Briony Horgan, a planetary scientist and member of NASA’s Perseverance rover team. That single observation sums up the extraordinary implications of a recent find on Mars-pale, aluminum-rich stones scattered across Jezero Crater that could be relics of a climate far warmer and wetter than the Red Planet’s current frozen desert.
The find focuses on the clay mineral called kaolinite, which, on Earth, forms only in very hot and humid conditions-most commonly in tropical rainforests. In its genesis, millions of years of rainfall have to persistently leach away other minerals until the pure aluminum silicate is left behind. Perseverance’s SuperCam and Mastcam-Z instruments identified the mineral’s spectral signature in thousands of small rock fragments, ranging from pebbles to boulders, lying along the rover’s traverse. The chemical profile shows enrichment in aluminum and titanium but with low iron and magnesium, matching deeply weathered paleosols on Earth rather than hydrothermal deposits, ruling out formation by hot water circulation.
This mineralogical fingerprint points to one of the wettest intervals in Mars’ history. In climate models simulating the Noachian epoch about four billion years ago, precipitation-driven erosion produces valley networks across a wide range of elevations, unlike ice-melt scenarios that restrict valley formation to highlands. Such widely distributed rainfall is in concert with the widespread detection of kaolinite, supporting the hypothesis that ancient Mars sustained rain-and possibly snow-over geologically significant timescales.
Putting the discovery in the Jezero Crater fills out the intrigue: once a lake twice as big as Lake Tahoe, its delta was fed by a powerful river that could carry meter-scale boulders. And yet, no large outcrop of kaolinite exists nearby, so where these scattered fragments have come from is an open question. They might have washed in from distant uplands, delivered by catastrophic flooding, or ejected by an impact event. Orbital imagery reveals extensive kaolinite deposits elsewhere on Mars, but until a rover reaches them, these small fragments remain the only ground-truth samples of such extreme chemical weathering.
The instrumentation carried aboard Perseverance was important for deciphering the history contained within the rocks. SuperCam provided laser-induced breakdown spectroscopy for determining elemental composition, while Mastcam-Z supplied high-resolution color imaging for correlating mineral signatures with visual textures. Comparisons to terrestrial analogs from San Diego, California, and South Africa showed essentially identical structures, reinforcing similar formation processes despite the planet’s very disparate modern climates.
Kaolinite formation under rainfall-driven leaching is a slow, cumulative process on Earth and generally takes place in thick weathering profiles tens of meters deep, as in temperate-tropical analogs. These profiles take 0.2 to 8 million years of warm humid conditions-or longer periods of oscillating climates. It is reasonable that the Martian samples represent a comparable timescale, implying the early Mars might have sustained habitable surface environments long enough for microbial life to emerge and persist.
This find also connects to more general hydrological evidence. Inverted channel studies-fluvial sinuous ridges-across the vast areas of Noachis Terra suggest precipitation-fed rivers extending for more than 15,000 kilometers. Combined with Jezero’s delta and kaolinite-rich soils, these features challenge the long-standing view of Mars as predominantly cold and dry, instead pointing out a complex, active water cycle on the planet.
Any finally supportive planetary engineering approach desires to understand the evolution of the climate on Mars for further explorations. The atmosphere of ancient Mars would have to be thicker and warmer if it had tropical-like rains, possibly because of outgassing by volcanoes or greenhouse gases. Determining how such a planet actually reached its thin, CO₂-rich atmosphere today likely through magnetic field loss and solar wind stripping-can, therefore, inform models of the retaining atmosphere and guide the design of life-support systems for human missions. “All life uses water,” said Dr. Adrian Broz, lead author of the study. “So when we think about the possibility of these rocks on Mars representing a rainfall-driven environment, that is a really incredible, habitable place where life could have thrived if it were ever on Mars.” The fragments of kaolinite, mute witnesses of an ancient tropical Mars, are now among the most exciting clues in the search for its lost habitability.
