Small, bright rocks scattered across Mars’ reddish surface offer fresh clues that parts of the planet may have once been much wetter than today. These light-colored patches stand out sharply against the surrounding terrain and suggest that some regions of Mars once hosted a humid environment with frequent rainfall, similar to tropical regions on Earth.
NASA’s Perseverance rover identified the rocks as kaolinite clay, a white material rich in aluminum. On Earth, kaolinite only forms after rocks and sediments have been stripped of most other minerals by long-term exposure to water. This process usually requires millions of years of persistent precipitation in a warm, humid climate.
Study links rare clay to long-term rainfall
The research was published in a peer-reviewed journal Earth and environment communication. The study was led by Adrian Broz, a postdoctoral researcher at Purdue University working in Briony Horgan’s lab. Horgan serves as a long-term planner on NASA’s Perseverance rover mission and is a professor of planetary sciences in Purdue’s Department of Earth, Atmospheric and Planetary Sciences.
“Elsewhere on Mars, rocks like these are probably some of the most important outcrops we’ve seen from orbit because they’re so hard to form,” Horgan said. “You need so much water that we think it could be evidence of an ancient warmer and wetter climate where it rained for millions of years.”
Broz explained that on Earth, kaolinite is most commonly found in tropical environments, such as rainforests, where heavy rainfall causes intense chemical weathering.
“So when you see kaolinite in a place like Mars, where it’s barren, it’s cold, and there’s absolutely no liquid water on the surface, it tells us that there was once a lot more water than there is today,” said Broz, a postdoctoral fellow on the Perseverance rover.
Rover Instruments Reveal Clues to Martian Climate History
The pieces of kaolinite observed by Perseverance range in size from small pebbles to large boulders. Although modest, they add important evidence to the ongoing debate about what Mars was like billions of years ago. The rover’s SuperCam and Mastcam-Z instruments were used to analyze the rocks and compare them to similar materials found on Earth.
These Martian samples could help scientists better understand the environmental changes Mars has experienced over time and how the planet transitioned from a wetter world to the dry landscape we see today.
A geological mystery in the Jezero crater
Despite their importance, the origin of these light-colored rocks remains unclear. Horgan noted that there is no obvious nearby source where the kaolinite could have formed, although debris appears scattered along the rover’s path since landing at Crater Lake in February 2021. Scientists believe the crater once contained a lake roughly twice the size of Lake Tahoe.
“They clearly record an incredible water event, but where did they come from?” Horgan said. “Maybe they were washed into Lake Jezero by the river that formed the delta, or maybe the impact dropped them into the lake and they’re just scattered there. We’re not entirely sure.”
Satellite data has revealed large deposits of kaolinite elsewhere on Mars, but Perseverance has yet to reach those locations.
“But until we actually get to these large outcrops with a rover, these small rocks are our only evidence on the ground of how these rocks could have formed,” Horgan said. “And right now, the evidence in these rocks really points to these kinds of ancient warmer, wetter environments.”
Earth comparisons strengthen the case
To better understand how Martian kaolinite formed, Broz compared data from the rover with rock samples collected near San Diego, California, and in South Africa. The chemical signatures of the Earth and Mars samples were very similar.
Broz noted that kaolinite can also form through hydrothermal processes on Earth, where hot water changes rock underground. However, this method leaves a distinct chemical pattern that differs from the signature created by long-term exposure to rain at lower temperatures. Data from three separate sites were used to evaluate whether hydrothermal activity could explain the samples from Mars, and the results favored precipitation as the most likely cause.
A time capsule for habitability
Kaolinite and similar rocks on Mars act as geologic records that store information about environmental conditions billions of years ago. These materials offer a rare glimpse into whether Mars once had an environment capable of supporting life.
“All life uses water,” Broz said. “So when we think about the possibility that these rocks on Mars represent a rain-driven environment, it’s a really incredible, habitable place where life could thrive if it ever was on Mars.”
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