NASA's Perseverance rover has discovered several thousand small, light-colored rock fragments, or floating rocks, some of which show the spectral signature of an aluminum-rich clay mineral called kaolinite. To interpret their formation, planetary researchers used data from Perseverance's SuperCam and Mastcam-Z instruments to compare the chemical composition and reflectance spectra of the floating rocks with deeply weathered paleosols (ancient soils) and hydrothermal kaolin deposits from Earth's geological record. The enrichment in aluminum and titanium coupled with the depletion of iron and magnesium does not resemble hydrothermal deposits and is instead comparable to bleached paleosol horizons that formed under heavy rainfall during Earth's past greenhouse climates. Thus, these rocks likely represent some of the wettest periods in Mars' history.
Mastcam-Z landscape and multispectral images of light-colored floating rocks atop the rim of Jezero Crater near the Hans Amundson Memorial workspace (Sol 912), demonstrating the spectral diversity of this material. Image credit: Broz etc.., two: 10.1038/s43247-025-02856-3.
“In other parts of Mars, rocks like these are probably some of the most important outcrops we've seen from orbit because they are so difficult to form,” said Dr. Bryony Horgan, long-term planner for the Perseverance mission and a postdoctoral researcher at Purdue University.
“You need so much water that we think it may be evidence of an ancient warmer, wetter climate that had rain for millions of years.”
“Tropical climates, such as rainforests, are the most common environments for kaolinite clay to be found on Earth,” added Dr. Adrian Broz, postdoctoral fellow at Purdue University.
“So when you see kaolinite in a place like Mars, where it's barren, cold and definitely doesn't have liquid water on the surface, it tells us that there was once a lot more water than there is today.”
The kaolinite fragments, which range from pebbles to boulders, are the latest pieces of a larger debate about the climate of Mars that occurred billions of years ago.
Initial studies with SuperCam and Mastcam-Z instruments were used to compare kaolinite with similar rocks found on Earth.
The Martian fragments could provide significant insight not only into the planet's past ecological stages, but also into how Mars arrived at its current barren state.
“Kaolinite also has its own mystery,” Dr. Horgan said.
“There are no major outcrops nearby that could have produced the light-colored rocks, despite them being scattered throughout the mission route that Perseverance has followed since landing at Jezero Crater in February 2021.”
“The crater used to contain a lake about twice the size of Lake Tahoe.”
“They are clearly recording an incredible water event, but where did they come from?”
“Perhaps they were washed into Jezero Lake by a river that created a delta, or maybe they were thrown into Jezero Lake by an impact and they're just scattered there. We're not really sure.”
Satellite images have revealed large exposures of kaolinite in other areas of Mars.
“But until we can get to these large outcrops with a rover, these small rocks will be our only ground-based evidence of how these rocks may have formed,” Dr Horgan said.
“And right now, the evidence in these rocks really points to an ancient warmer, wetter environment.”
Mastcam-Z and SuperCam observations of hydrated aluminum-rich floating rocks in Jezero Crater on Mars. Image credit: Broz etc.., two: 10.1038/s43247-025-02856-3.
The researchers compared samples of Martian kaolinite examined by Perseverance with rock samples found in the vicinity of San Diego, California, and South Africa. The rocks from the two planets are very similar to each other.
In addition to tropical climates with heavy rainfall, kaolinite on Earth also forms in hydrothermal systems when hot water leaches the rock.
But this process creates a different chemical signature in the rock than leaching by rain at lower temperatures over thousands or millions of years.
The scientists used data sets from three different sites to compare the hydrothermal leaching scenario with that of Martian rocks.
Rocks on Mars such as kaolinite provide a similar time capsule, potentially containing billions of years ago information about the history of environmental conditions on the planet.
“All life uses water. So when we think about these rocks on Mars being a rain-driven environment, it really is an incredible, habitable place where life could thrive if it ever existed on Mars,” Dr Brose said.
teams paper was published in the magazine Connection Earth and environment.
_____
AP Brothers etc.. 2025. Alteration history of aluminum-rich rocks at Jezero Crater, Mars. Commune Earth Environment 6, 935; two: 10.1038/s43247-025-02856-3
