Strangely bleached rocks on Mars suggest the planet was once a tropical oasis

A new study of unusually bleached rocks suggests Mars once had humid regions with heavy rainfall, similar to Earth’s tropical regions.

Researchers were intrigued by a unique, brightly colored rock that NASA’s Perseverance rover discovered on the surface of Mars. Upon closer examination, the rock was found to be kaolinite, a type of clay rich in aluminum, the scientists reported in the study, published Dec. 1 in the journal Communication Earth & Environment.

On Earth, kaolinite is most often formed under very warm, steamy conditions, such as those found in tropical rainforests. They usually form in rocks that have been stripped of all other minerals by millions of years of regular rainfall. However, modern Mars is famous for being cold and dry.

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“So when you look at kaolinite in a place like Mars, which is barren and cold and has no surface liquid water, you know that there was once much 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 Martian kaolinite, studied by Perseverance’s instruments, with Earth-based samples taken from South Africa and San Diego. These rocks were strikingly similar, suggesting that they were formed in a similar way.

Satellite images of the surface of Mars appear to show larger deposits of kaolinite elsewhere on Earth. But Perseverance and other rovers have not yet explored those areas.

“Until we can actually reach these large outcrops with rovers, these small rocks are the only evidence that they are on the ground,” Bryony Hogan, a planetary scientist at Purdue University and a co-author of the study, said in a statement.

The presence of kaolinite on Mars lends weight to the hypothesis that Mars was a wet oasis at some point in its distant past, although exactly when and how it became dry is still debated.

A leading hypothesis suggests that at some point between 3 and 4 billion years ago, the planet lost its water because its magnetic field weakened enough that the solar wind stripped away its atmosphere. However, 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.”