The Perseverance rover has made an intriguing discovery at Jezero Crater, uncovering light-hued rocks that are rich in aluminum and contain kaolinite, a clay mineral known to form in water-rich environments on Earth.
Geologists suggest these rocks are "float rocks," which have separated from their original formations, likely due to water activity. This indicates that Jezero Crater was not merely a transient site for a brief flood; rather, it likely supported a persistent, wet environment.
"Finding kaolinite on a planet like Mars, which is currently cold and devoid of surface water, indicates that there was once a significant amount of water present," explained Adrian Broz, a postdoctoral researcher at Purdue University.
The White Stones of Jezero
Continuing its mission to uncover Mars' geological history, the Perseverance rover aims to identify signs of ancient life, with Jezero Crater being a focal point due to its believed ancient delta system.
Recently, Perseverance detected thousands of unusual light-toned rock fragments using its SuperCam and Mastcam-Z instruments, revealing the presence of clay minerals like kaolinite.
On Earth, kaolinite forms over millions of years as rocks are weathered by liquid water, stripping away elements like iron and magnesium while retaining aluminum.
Broz and his team sought to find Earth rocks similar to those on Mars, examining samples from the Eocene epoch, around 30-50 million years ago, and comparing them to a Martian sample known as "Chignik."
It is highly likely that the kaolinite on Mars originated from water, but researchers are investigating whether it was formed in a hydrothermal setting or through weathering. The latter appears more plausible, as the geochemical signatures align with Earth's paleosols formed during periods of heavy rainfall in ancient greenhouse climates.
The prevailing theory suggests these rocks developed as a weathering profile between 4.1 and 3.7 billion years ago, subsequently hardened by burial or impact heat, and eventually transported to the crater by ancient floods or impact ejecta.
Unraveling the Motherlode
This discovery also raises questions about Mars' historical water presence: where has it all gone? If Mars once had abundant surface water, it is largely absent today, leading to speculation about its fate.
Unlike Earth, Mars lacks plate tectonics, which means that once water is trapped within aluminum-rich clays, it remains there indefinitely. This process may have acted as a vast, irreversible sponge, sequestering the ancient Martian atmosphere deep within the planet's crust.
The origin of these "float rocks" remains uncertain; however, orbital data suggests they may have originated from the rim of Jezero Crater or the nearby Neretva Vallis meanders.
"Until we can reach these larger outcrops, these small rocks provide our only insight into their formation," stated Briony Horgan, a long-term planner for NASA's Perseverance rover mission and a planetary science professor at Purdue. "The evidence indicates these rocks are linked to ancient, warmer, and wetter environments."
Perseverance is currently moving toward the crater rim, and if it locates the original source of these aluminum-rich rocks, it will be stepping onto a Martian landscape that may have once resembled a rainy, tropical Earth.
The study titled Alteration history of aluminum-rich rocks at Jezero crater, Mars was published in Communications Earth, 2025; 6 (1).