Mars, Jezero and Kaolinite: Pale Rocks Rewrite the Red Planet’s History

An ancient Martian lake, a lone robot and several unusually pale rocks are overturning the history of Mars.

Fresh analysis of nearly white rocks spotted by the Perseverance rover in Jezero Crater suggests that the Red Planet had a warm, wet climate more than 3 billion years ago-one surprisingly similar to tropical regions on Earth.

Pale rocks on a red world

Since 2021, Perseverance has been travelling across Jezero Crater, an ancient lake basin roughly 45 kilometres wide. Amid dark boulders and reddish dust, the science team noticed something unusual: small, exceptionally light-coloured fragments, almost white, scattered across the landscape.

These “float rocks”-loose stones with no obvious connection to the local bedrock-attracted attention for a particular reason: they contain abundant kaolinite, a clay commonly found in tropical soils on Earth but seldom observed directly on Mars.

The presence of kaolinite in Jezero indicates an intense process of chemical weathering, with liquid water circulating across the Martian surface for long periods.

Instruments carried by the rover, including SuperCam and Mastcam-Z, detected the infrared signature characteristic of aluminium-bound hydroxyl groups, confirming that these pale rocks are clay-rich. Put simply, their chemistry matches that of soils heavily leached by plentiful rainfall.

What kaolinite reveals about Mars’s “tropical climate”

On Earth, kaolinite does not form by chance. It develops when rocks remain exposed to persistent rainfall in warm, humid conditions for millions of years. This process “washes” the soil, removing a range of elements and leaving behind a white, aluminium-rich clay.

Researchers compared the rocks from Jezero with two well-documented examples from Earth: 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 compositions showed striking similarities.

Several figures reinforce the case for intense rainfall on Mars:

• Titanium dioxide (TiO₂) levels of around 1.4% in certain samples, a value typical of soils extensively leached by heavy precipitation.
• Extremely low total iron content, suggesting that water removed the element and carried it elsewhere.
• No chemical signature typical of hydrothermal systems, which would have produced a different combination of elements.

Weathering models indicate that a climate capable of producing this kind of soil would require rainfall exceeding 1,000 millimetres a year for hundreds of thousands or millions of years, acting on volcanic or sedimentary ground.

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

Where did these white rocks originate?

Although their composition is now reasonably well understood, the geological source of these rocks still puzzles scientists. The fragments are widely scattered, with no apparent nearby outcrop. So far, Perseverance has not found a continuous in-situ layer of kaolinite.

This has led the team to propose two principal possibilities:

Transport by ancient rivers

The first possibility is that rivers feeding Jezero Lake, such as Neretva Vallis, eroded kaolinite-rich terrain at higher elevations and carried the blocks into the crater. Orbital images reveal signs of aluminium clays in fossilised meanders, strengthening this explanation.

Ejection by meteorite impacts

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

Data from the CRISM spectrometer aboard the Mars Reconnaissance Orbiter have identified potential kaolinite zones south-west of Jezero, less than 2 kilometres from the rover’s route, as well as in more distant regions such as Nili Planum, where layers of aluminium-rich clays overlie magnesium clays.

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

How this finding changes the history of water on Mars

Kaolinite does more than preserve water’s chemical record: it physically holds water in its structure. Some of that water is bound as hydroxyl, while some consists of molecules trapped within spaces in the clay.

Certain samples, including the rock named Chignik, also show a hydration band near 1.9 micrometres. This signature indicates that the material was probably never heated beyond approximately 450 °C, the temperature at which kaolinite loses its structural water.

That detail raises a broader question: how much of Mars’s ancient water may have become almost permanently locked inside clay minerals?

• If vast parts of the planet underwent “kaolinisation”, a substantial portion of the early atmosphere’s water may have been sequestered underground.
• Because Mars lacks active plate tectonics, this water is unlikely to be recycled back to the surface, unlike on Earth.

This process could help explain why a planet that once had lakes, rivers and rainfall now presents a cold, dry environment with a thin atmosphere.

Tropical Mars and lost habitability

Taken together, the data portray an intriguing young Mars with a mild climate, frequent rain and intensely weathered soils-conditions compatible with microbial life.

Settings with moderate pH, circulating water and dissolved oxygen provide favourable niches for microorganisms. In Earth’s tropical soils, kaolinite is often associated with ecosystems rich in organic matter, even though the clay itself contains few metallic nutrients.

If Mars had an “almost tropical” phase, Jezero may preserve one of the most life-friendly eras ever to exist on the planet.

For now, no complex organic compounds have been conclusively detected in these pale rocks. Yet their scientific potential is considerable. Samples of Martian kaolinite returned to Earth laboratories by future sample-return missions would allow detailed measurements of isotopes, water content and possible biomarkers.

Terms and concepts that help explain the discovery

Term	What it means in plain language
Kaolinite	A white clay typical of tropical soils heavily washed by rain, rich in aluminium and low in iron.
Palaeosol	Fossilised soil preserved in rock that records 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 conversion of rocks and soils into kaolinite-rich materials through strong water-driven leaching.

To picture this setting more clearly, consider humid tropical parts of Earth, where constant rainfall erodes volcanic rocks and produces deep reddish or pale soils that contain almost no metallic nutrients. Something comparable, on a planetary 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 questions. Deposits of this type may:

• Store water that could be released through controlled heating.
• Supply clays useful for locally produced construction materials.
• Mark regions where the ancient climate was milder and where the search for signs of life may be especially promising.

On the other hand, heavily leached soils are generally poor in essential metallic minerals, which may restrict the use of local resources for certain forms of Martian mining. This creates a situation in which areas rich in mineral-bound water may not also be the best sources of metals.