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Bleached white rocks on Mars. On Earth, rocks like these only form after millions of years of hot, wet conditions. | Credit: NASA
Mars was once home to wet, humid areas that received heavy rainfall, similar to tropical regions on Earth, a new study of unusual bleached rocks suggests.
Researchers were intrigued by a particular light-colored rock that NASA’s Perseverance rover discovered on the Martian surface. Upon closer inspection, the rock turned out to be kaolinite, a type of aluminum-rich clay, the scientists reported in the study, which was published Dec. 1 in the journal Earth and Environment Communications.
On Earth, kaolinite almost always forms under very hot and steamy conditions, such as those found in tropical rainforests. It typically forms in rock that has been stripped of all other minerals by millions of years of regular rainfall. But today Mars it is famously cold and dry.
“So when you see kaolinite in a place like Mars, where it’s barren, cold and certainly without liquid water on the surface, it tells us that there was once a lot more water than there is today,” Adrian Brozsoil 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 looked strikingly similar, suggesting they formed in similar ways.
Satellite images of the Martian surface appear to show larger deposits of kaolinite elsewhere on the planet. However, Perseverance and other rovers have not yet explored those areas.
Three satellite images of probable kaolin clay minerals around the Jezero crater region (A – C). The graph shows the reflectance spectrum of the rocks of Mars compared to kaolinite on Earth (D). | Credit: NASA / Earth & Environment Communications
“Until we can get to these large outcroppings with the rover, these small rocks are our only evidence on the ground,” Briony Horganplanetary scientist at Purdue University and co-author of the study, said in the statement.
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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, although exactly when and how it dried up is still debated.
The main hypotheses suggest that the planet lost its water sometime between 3 billion and 4 billion years ago, when its magnetic field weakened enough for the solar wind to remove its atmosphere. But this process was probably complex and multifaceted. Studying this ancient clay could give scientists more insight into how and when Mars lost its water, the researchers said.
It could also provide clues about the potential habitability of Mars, Broz said, since “all life uses water.”