Strangely bleached rocks on Mars hint that the Red Planet was once a tropical oasis

Mars was once home to wet, humid areas that received heavy rainfall, similar to tropical regions on Earth, a new study of unusually bleached rocks suggests.

Researchers were intrigued by peculiar light-colored rocks that NASA's Perseverance rover discovered on the Martian surface. Upon closer inspection, the rocks turned out to be kaolinite, an aluminum-rich type of clay, the scientists reported in the study, which was published Dec. 1 in the journal Communications Earth & Environment.

On Earth, kaolinite almost always forms under very warm, steamy conditions, such as those found in tropical rainforests. It typically forms in rocks that have been stripped of all other minerals by millions of years of regular rainfall. But present-day Mars is famously cold and dry.

"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," Adrian Broz, a soil scientist at Purdue University and lead author of the study, said in a statement.

Broz and his team compared the structure of the Martian kaolinite, which had been examined by multiple instruments on Perseverance, with terrestrial samples taken from South Africa and San Diego. The rocks appeared strikingly similar, suggesting they formed in similar ways.

Satellite images of the Martian surface appear to show larger kaolinite deposits elsewhere on the planet. However, Perseverance and other rovers have not explored those areas yet.

"Until we can actually get to these large outcroppings with the rover, these small rocks are our only on-the-ground evidence," Briony Horgan, a planetary scientist at Purdue University and co-author of the study, said in the statement.

The presence of kaolinite on Mars adds weight to the hypothesis that the Red Planet was a wet oasis at some point in the distant past, though exactly when and how it dried up are still debated.

The leading hypotheses suggest that the planet lost its water sometime between 3 billion and 4 billion years ago, when its magnetic field weakened enough for solar winds to strip away its atmosphere. But this process was likely complex and multifaceted. Studying these ancient clays could give scientists more insight into how and when Mars lost its water, the researchers said.

It could also provide clues about Mars' potential habitability, Broz said, since "all life uses water."