Bleached Martian Rocks Point to Tropical-Clay Evidence — Mars May Have Once Been Rainy and Humid

Unusually pale, bleached rocks examined by NASA's Perseverance rover have been identified as kaolinite, an aluminum-rich clay that on Earth almost exclusively forms under hot, wet, tropical conditions. A study published Dec. 1 in the journal Communications Earth & Environment reports that measurements from multiple rover instruments show the Martian specimens closely match terrestrial kaolinite from South Africa and San Diego.

What The Researchers Found

Kaolinite typically forms when prolonged, intense rainfall leaches other minerals out of rock over long timescales, leaving behind the characteristic light-colored clay. That formation pathway is consistent with warm, humid, rainforest-like environments on Earth — a striking contrast to present-day Mars, which is cold, dry and lacks stable surface water.

“So when you see kaolinite on a place like Mars, where it's barren, cold and with certainly no liquid water at the surface, it tells us that there was once a lot more water than there is today,” said Adrian Broz, a soil scientist at Purdue University and the study’s lead author.

Broz and colleagues used several instruments aboard Perseverance to characterize the mineralogy of the bleached rocks and then compared those spectra and structural traits with terrestrial kaolinite samples. The close match in composition and texture suggests similar formation processes — prolonged chemical weathering under warm, wet conditions.

Broader Context and Implications

Orbital observations indicate there may be larger kaolinite-rich outcrops elsewhere on Mars, though rovers have not yet visited those sites to confirm them at ground level. As Briony Horgan, a planetary scientist and co-author of the study, noted:

“Until we can actually get to these large outcroppings with the rover, these small rocks are our only on-the-ground evidence.”

The discovery strengthens the idea that parts of ancient Mars were significantly wetter and more humid — perhaps even oasis-like environments — before the planet became the cold desert we see today. Leading scenarios propose Mars lost much of its water between about 3 billion and 4 billion years ago as its magnetic field weakened and solar wind eroded the atmosphere, although the process was likely complex and occurred in stages.

Why This Matters

Studying ancient clay deposits such as these kaolinite-bearing rocks can help scientists constrain when and how Mars dried out and evaluate the planet’s past habitability. Because liquid water is a key ingredient for life as we know it, evidence for sustained warm, wet conditions raises the importance of targeting these deposits in future missions and sample-return efforts.

Bottom line: Kaolinite on Mars provides tangible mineralogical evidence that some regions experienced prolonged, warm, rainy conditions in the deep past — making those regions prime targets for understanding the planet’s climate history and its potential for past life.