Tropical Storms Drenched Mars for Millions of Years, Strange Pale Rocks Suggest

That Mars once hosted water is now old news, but have you ever imagined the Red Planet drenched in tropical storms?

NASA’s Perseverance rover has discovered light-colored rocks tossed about its mission path. On Earth, this white, aluminum-rich kaolinite clay forms over millions of years as rainwater gradually leaches other minerals from rocks and sediments. The discovery bolsters the theory that Mars may have once featured wet oases, humid climates, and rainfall similar to those seen in our planet’s tropical environments.

Ancient rain indicators

On Earth, kaolinite clay is most often found in tropical climates like rainforests, “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 postdoctoral collaborator on the Perseverance rover, said in a Purdue University statement. He is the lead author of a study published yesterday in the journal Communications Earth & Environment.

The kaolinite fragments in question are anywhere from the size of a pebble to as big as a boulder, and early examinations with Perseverance’s instruments found that they are similar to terrestrial rocks near San Diego, California, and in South Africa. Kaolinite also forms in hydrothermal systems in which hot water does the leaching, but that results in a different chemical signature than the one caused by millions of years of rain at lower temperatures.

Mysterious origin

The rocks, however, raise an interesting question. How did they get there? There isn’t any major outcropping in the vicinity where they may have originated from. However, Perseverance landed next to Mars’ Jezero crater in 2021, which used to host a lake around twice the size of Lake Tahoe.

“They’re clearly recording an incredible water event, but where did they come from?†said Briony Horgan, co-author of the study and a professor of planetary science at Purdue University. “Maybe they were washed into Jezero’s lake by the river that formed the delta, or maybe they were thrown into Jezero by an impact and they’re just scattered there. We’re not totally sure.†Broz is a member of Horgan’s Planetary Surface Processes Research Group.

Satellite images have revealed large kaolinite outcroppings elsewhere on Mars, “but until we can actually get to these large outcroppings with the rover, these small rocks are our only on-the-ground evidence for how these rocks could have formed,†Horgan said. “And right now the evidence in these rocks really points toward these kinds of ancient warmer and wetter environments.â€

Water and life

If you’re wondering why researchers are spending so much time investigating water on Mars instead of solely focusing on what everyone is really interested in—extraterrestrial life—it’s worth remembering that the two things are closely connected. On Earth, anyway, and that’s our only point of reference.

“All life uses water,†Broz said. “So when we think about the possibility of these rocks on Mars representing a rainfall-driven environment, that is a really incredible, habitable place where life could have thrived if it were ever on Mars.â€