Perseverance Detects Tropical Clay on Mars, Pointing to Ancient Rainfall

NASA’s Perseverance rover has identified an unusual set of light-colored, spotty rocks in Jezero Crater that researchers interpret as aluminum-rich kaolinite — a clay typically formed by prolonged, rain-driven weathering in tropical environments on Earth.

A new study published in Communications Earth & Environment reports that the rocks, which range from pebble to boulder size, show spectral and textural features consistent with kaolinite. On Earth, kaolinite commonly forms after millions of years of sustained wet weather that leaches other minerals away, so its presence on Mars would imply prolonged surface water and a warmer, wetter climate in the planet’s past.

Comparisons and Context. The research team compared the Martian observations with terrestrial kaolinite from sites including San Diego, California, and locations in South Africa, finding notable similarities in appearance and spectral signature. While rover instruments cannot yet provide the same level of lab analysis as returned samples, the in-situ data are strong enough to make kaolinite a leading interpretation.

“You need so much water that we think these could be evidence of an ancient warmer and wetter climate where there was rain falling for millions of years,” said Briony Horgan, a Purdue planetary scientist and member of the Perseverance team.

Where Did the Rocks Come From? Fragments of the clay-like material appear at multiple locations along Perseverance’s traverse through Jezero, a basin long suspected to be a former lake. That distribution raises a key question: were the fragments eroded and carried into Jezero by rivers that formed the delta, or were they deposited by an impact that scattered material across the basin? The answer remains uncertain.

Next Steps. Larger outcrops of the light-colored material could settle the question, but reaching and studying those targets requires Perseverance to drive to them and examine them up close. Future work will include targeted rover observations and, if possible, caching samples for eventual laboratory analysis on Earth via planned sample return missions.

“All life uses water,” said lead author Adrian Broz. “If these rocks record a rainfall-driven environment, that would represent a potentially habitable place where life could have thrived if it ever existed on Mars.”

Why It Matters. If confirmed, kaolinite in Jezero would deepen evidence that parts of Mars experienced complex, long-lived, water-rich environments. Those findings will help scientists constrain how Mars transformed from a wetter world into the cold, thin-atmosphere planet we see today — a change likely tied to atmospheric loss following major shifts in the planet’s magnetic field.

The discovery is an important step but not a definitive proof: confirming a rainfall-driven, tropical-like climate on ancient Mars will require more in-situ observations and returned laboratory samples.