White rocks found on Mars reveal the Red Planet was a tropical paradise 3 billion years ago.

An ancient Martian lake, a solitary robot and a scatter of unusually pale stones are turning Mars’s story on its head.

Fresh analysis of near-white rocks spotted by the Perseverance rover in Jezero Crater indicates that the Red Planet may once have hosted a warm, wet climate-remarkably similar to Earth’s tropical regions-more than 3 billion years ago.

Kaolinite in Jezero Crater: pale rocks in a red world

Since 2021, Perseverance has been exploring Jezero Crater, the basin of a former lake roughly 45 kilometres across. Amid dark boulders and rust-coloured dust, the science team picked out something that did not fit: small, very light fragments-almost white-scattered across the surface.

These “floating rocks”-loose stones with no obvious connection to the local bedrock-stood out for a particular reason: they are rich in kaolinite, a clay mineral common in tropical soils on Earth but only rarely observed directly on Mars.

"The presence of kaolinite at Jezero points to intense chemical alteration, with liquid water circulating for long periods across the Martian surface."

On-board instruments such as SuperCam and Mastcam-Z detected the characteristic infrared signal of hydroxyl groups bound to aluminium, supporting the clay-rich nature of these pale rocks. Put simply, the chemistry matches soils that have been heavily washed by abundant rainfall.

What kaolinite reveals about a Martian “tropical climate”

On Earth, kaolinite does not form by chance. It typically develops when rocks spend millions of years exposed to persistent rainfall in warm, humid conditions. Over time, the process “washes” the soil: many elements are leached out, leaving behind a white, aluminium-rich clay.

To test what they were seeing at Jezero, researchers compared the Martian material with two well-characterised Earth analogues: an Eocene palaeosol near San Diego and an ancient soil from Hekpoort in South Africa that is more than 2 billion years old. Their infrared spectra and chemical make-up showed striking similarities.

Several measurements strengthen the case for intense rainfall on ancient Mars:

• Titanium dioxide (TiO₂) levels of around 1.4% in certain samples, a value typical of strongly leached soils produced under heavy precipitation.
• Extremely low total iron content, consistent with iron being removed by water and carried elsewhere.
• No chemical signature characteristic of hydrothermal systems, which would produce a different elemental mix.

Weathering models suggest that generating this type of soil would require rainfall exceeding 1,000 millimetres per year for hundreds of thousands or millions of years, acting on volcanic or sedimentary terrain.

"This picture points to an ancient Mars with an active hydrological cycle: evaporation, cloud formation, repeated rainfall and stable lakes at the surface."

Where did the white rocks come from?

While the composition is becoming clearer, the geological source of these pale fragments remains an open question. They appear widely dispersed, yet there is no obvious nearby outcrop. So far, Perseverance has not identified a continuous in-place layer of kaolinite.

That has led the team to focus on two main scenarios.

Transport by ancient rivers

In the first explanation, rivers feeding Jezero’s lake-such as Neretva Vallis-could have eroded kaolinite-rich terrains at higher elevations and delivered blocks into the crater. Orbital images show signs of aluminium-bearing clays along fossilised meanders, lending weight to this idea.

Ejection by meteorite impacts

A second possibility involves impacts. A major collision could have excavated kaolinite-bearing rocks from distant locations and hurled fragments into Jezero Crater, scattering them across the surface like debris.

CRISM spectrometer data from the Mars Reconnaissance Orbiter have identified possible kaolinite zones to the south-west of Jezero, within less than 2 kilometres of the rover’s route, and also farther afield in places such as Nili Planum, where aluminium-rich clay layers overlie magnesium-rich clays.

"If these areas represent large kaolinite deposits, Mars may have gone through a phase of chemical weathering on a continental scale."

How the finding reshapes Mars’s water history

Kaolinite does not merely record water chemistry-it can also retain water within its structure. Some of that water is bound as hydroxyl; some is trapped as molecules within the clay’s internal spaces.

Certain samples, including a rock nicknamed Chignik, still show a hydration band near 1.9 micrometres. That feature implies the material was likely never heated above about 450 °C, the temperature at which kaolinite loses its structural water.

That detail feeds into a larger question: how much of Mars’s early water might have been locked away-effectively for the long term-inside clay minerals?

• If “kaolinisation” affected large regions, a meaningful fraction of the early atmospheric water could have been sequestered underground.
• Because Mars lacks active plate tectonics, this water would be unlikely to be efficiently recycled back to the surface, unlike the situation on Earth.

Such sequestration could help explain why a planet that once supported lakes, rivers and rainfall is now cold, dry and wrapped in a thin atmosphere.

A “tropical” Mars and lost habitability

Taken together, the evidence sketches an unusual scenario: a younger Mars with mild temperatures, frequent rain and heavily altered soils-conditions that are compatible with microbial life.

Environments with moderate pH, circulating water and dissolved oxygen can create favourable niches for microorganisms. In Earth’s tropical soils, kaolinite often occurs alongside ecosystems rich in organic matter, even though the clay itself tends to be poor in metallic nutrients.

"If Mars went through an ‘almost tropical’ phase, Jezero may preserve one of the most life-friendly eras the planet ever experienced."

For now, there has been no confirmed detection of complex organic compounds within these pale rocks. Even so, their scientific value is substantial. Kaolinite-bearing Martian samples returned to Earth by future sample-return missions would enable detailed laboratory work on isotopes, water content and potential biomarkers.

Terms and concepts that help explain the discovery

Term	What it means in plain language
Kaolinite	A white clay typical of tropical soils that have been strongly washed by rain; aluminium-rich and iron-poor.
Palaeosol	A fossil soil preserved in rock, recording the climate and chemistry of very ancient surfaces.
Chemical weathering	A process in which water and dissolved substances react with rocks, changing their original composition.
Kaolinisation	The transformation of rocks and soils into kaolinite-rich material caused by strong leaching by water.

To picture the setting, think of humid tropical regions on Earth where constant rain breaks down volcanic rocks and produces thick soils-reddish or pale-nearly stripped of metallic nutrients. Something comparable, on a planet-wide scale, appears to have occurred in parts of Mars billions of years ago.

From the perspective of future crewed missions, kaolinite-bearing rocks also raise practical considerations. Deposits like these could:

• Store water that might be released through controlled heating.
• Provide clays that could be useful for locally produced construction materials.
• Flag regions where the ancient climate was milder, making them promising for life-detection searches.

On the other hand, strongly leached soils are often poor in essential metallic minerals, potentially limiting the use of local resources for certain kinds of Martian mining. That creates a trade-off in which water-rich mineral deposits may not, at the same time, be the best sources of metals.