Kaolinite pebbles tip the scales: Early Mars looks more like a rainy Earth than a frozen world

Background

For decades, planetary scientists have argued over a deceptively simple question: What kind of planet was Mars when its river valleys, deltas, and lakes formed more than 3.5 billion years ago? Two end-member visions have dominated the debate:

• Warm and wet: a thicker atmosphere, higher temperatures, and sustained rainfall that carved valleys and fed lakes and groundwater.
• Cold and icy: a frigid world where glaciers, seasonal melting, and sporadic bursts of liquid water—perhaps triggered by volcanism or impacts—created riverlike landforms without a long-lived warm climate.

Both views have been kept alive by a mix of geomorphology (the shapes of ancient river systems), geochemistry (the minerals found on the surface), and climate modeling constrained by the faint young Sun paradox: the Sun was dimmer in the Noachian era, so keeping Mars above freezing for long periods requires potent greenhouse warming.

One of the most powerful arbiters in this debate is mineralogy. Different minerals form under different environmental conditions, serving as time capsules of climate and chemistry. Among them, kaolinite—a white, aluminum-rich clay common in tropical soils on Earth—stands out as a climate oracle. It forms when abundant, relatively warm, oxygenated water leaches away soluble elements (like sodium, potassium, calcium, and magnesium) from rocks such as feldspars and micas, leaving behind aluminum-silicate residues. That process thrives in high-rainfall settings and tends to purge iron and other cations, driving soils toward a distinctive chemistry.

In contrast, cold, icy environments with limited liquid water tend to preserve more of the original rock chemistry or favor different alteration products. Episodic meltwater that flashes through a landscape and then refreezes is generally not efficient at producing the deep, exhaustive leaching that kaolinite records.

What happened

A new study reports something Mars-watchers have been hoping to see in situ for years: pebbles dominated by kaolinite, found within an ancient river deposit. The clasts are rounded and sorted—classic signs of transport in flowing water—yet their internal mineralogy tells a more subtle story about the climate that fed those rivers.

The key pieces of evidence converge on the same conclusion:

1. The pebbles are kaolinite-rich, not merely dusted with clay. Spectral and compositional data indicate extensive alteration, consistent with parent rocks undergoing deep chemical weathering before they were broken up and carried downstream.

2. The textures and setting are fluvial. These clasts occur within conglomerates that resemble riverbed deposits on Earth—suggesting persistent flow rather than a singular catastrophic event.

3. Geochemical modeling and Earth analogs link kaolinite formation to sustained liquid water and temperatures that allow vigorous leaching. On Earth, kaolinite-rich pebbles are most common in regions with long-term, high rainfall—think tropical to subtropical climates—where soils and saprolites undergo prolonged flushing.

When you stitch these threads together, a clear narrative emerges. Before these pebbles were rounded and deposited in a river channel, their source rocks spent a long time in a wet environment that removed mobile elements and concentrated aluminum in a clay matrix. That prolonged weathering is very difficult to achieve with only brief melt seasons on an icy planet. Instead, it points to a hydrologic cycle robust enough to deliver precipitation, infiltration, and runoff over extended periods.

Why kaolinite is such a strong climate proxy

Not all clays make the same climate statement. Smectites (like nontronite and montmorillonite) are plentiful on Mars and can form under a range of conditions, including cooler temperatures and moderate water-rock interaction. Kaolinite, by contrast, typically requires:

• High water-to-rock ratios and repeated flushing
• Modest acidity (often mildly acidic to near-neutral pH)
• Temperatures often above freezing for much of the year
• Extended durations (thousands to millions of years) to fully leach cations

On Earth, kaolinite-rich profiles—sometimes called lateritic or kaolinitic saprolites—develop where rainfall is intense and persistent. They are the end-products of relentless weathering, not the calling card of sporadic thaws.

The Martian pebbles therefore encode a two-step history: long-lived weathering to create kaolinite-rich material, followed by physical erosion, transport, and deposition by flowing water. Both steps imply liquid water was common, and the first step especially requires a climate that was more rain-fed than melt-fed.

How the data link to rainfall

Rocks don’t come with built-in rain gauges. But there are multiple ways to infer rainfall from minerals and textures:

• Elemental depletion patterns: Strong leaching depletes alkalis and alkaline earths. The degree of depletion, measured against immobile elements like aluminum and titanium, can be matched to Earth analogs where the rainfall is known.
• Clay mineral assemblages: Assemblages dominated by kaolinite, with limited iron-rich clays or carbonates, point to intense flushing and acidic to neutral waters—conditions favored by frequent precipitation.
• Pebble rounding and sorting: Well-rounded pebbles in a graded riverbed imply sustained transport over meaningful distances and time, not just flash flows from a single event.

In combination with thermal-chemical models, the kaolinite signature allows researchers to reject the cold-and-icy endmember for the region sampled. While Mars likely experienced a range of climates over time and space, this deposit appears to record a chapter when the air was thicker, temperatures were milder, and water cycled through the atmosphere and back to the ground as rain.

Fitting the result into Mars’ broader story

Mars did not wear a single climate forever. Geological mapping shows that valley networks are densest in the older Noachian terrains, with signs of lakes and deltas persisting into the early Hesperian. Later, sulfate-rich deposits and evaporites indicate drying and oxidation as the atmosphere thinned. The new kaolinite pebbles reinforce the idea that at least part of early Mars supported a hydrologic cycle vigorous enough for soil-like weathering and sustained runoff.

This dovetails with several independent lines of evidence:

• Orbital detections of aluminum-rich clays scattered across old terrains
• Ancient deltaic deposits that look strikingly Earth-like in planform and stratigraphy
• Lakebed mudstones revealing multiple wetting and drying cycles
• Atmospheric escape measurements showing Mars lost substantial volatile inventory over time

Kaolinite, in this context, is the missing puzzle piece that directly connects rainfall to deep chemical alteration of crustal rocks.

Key takeaways

• Kaolinite-rich pebbles discovered in an ancient river deposit on Mars indicate prolonged chemical weathering before fluvial transport.
• Kaolinite typically forms under high rainfall and sustained liquid-water conditions; it is difficult to generate in cold, episodically melting climates.
• The textures of the deposit—rounded pebbles within a conglomerate—argue for persistent river flow, not a single catastrophic flood.
• Together, the mineralogy and geomorphology strongly favor a warm, wet phase on early Mars, at least regionally and for extended periods.
• The finding complements prior evidence from deltas, lake sediments, and clay-rich terrains, while challenging purely “cold and icy” models for valley network formation.

What to watch next

• Refining climate constraints: Future work will quantify temperatures, rainfall rates, and durations by comparing Martian chemical indices of alteration to Earth analogs and by running weathering models under plausible ancient atmospheres (CO2, H2, and CH4 mixtures; cloud feedbacks; volcanic inputs).
• Spatial context: Are kaolinite-rich pebbles localized or widespread? Broader mapping can reveal whether intense chemical weathering was regional or global.
• Layer-by-layer histories: If similar pebbles occur at multiple stratigraphic levels, they could outline repeated wet intervals rather than a single climatic episode.
• Organic preservation potential: Kaolinite and associated fine-grained sediments can trap and protect organic molecules. That makes such deposits intriguing targets for biosignature searches.
• Sample return prioritization: If any kaolinite-bearing clasts have been cached for return to Earth, they could become star samples. Laboratory techniques—X-ray diffraction, micro-CT, nanoscale secondary ion mass spectrometry, and isotopic analyses—would pin down weathering pathways, pH, redox state, and the timing of alteration with far greater precision than in situ tools allow.
• Model reconciliation: Some regions of Mars record glacial landforms, while others now show clear rainfall signatures. Expect a new generation of climate models that allow both to be true at different times or latitudes—perhaps with oscillations driven by volcanic outgassing, impacts, or orbital cycles.

FAQ

Why is kaolinite such a big deal compared to other Martian clays?

Kaolinite is a hallmark of deep chemical weathering under abundant, relatively warm, oxygenated water with high rainfall. Many Martian clays (especially smectites) can form under a broader range of conditions, including cooler climates or hydrothermal settings. Finding pebbles dominated by kaolinite strongly implies a persistent hydrologic cycle, not just episodic melts.

Could volcanic or hydrothermal alteration make kaolinite without rainfall?

Local hydrothermal systems can generate kaolinite, but they usually leave distinctive chemical and textural fingerprints (e.g., associated silica sinter, alteration zoning, or proximity to vents). In contrast, kaolinite-rich pebbles derived from regional weathering profiles and found in river deposits point to surface climate processes—repeated leaching across a landscape—rather than isolated hydrothermal events.

How warm are we talking for early Mars?

“Warm” here doesn’t mean tropical beaches. It means average conditions above the freezing point often enough to sustain rainfall and soil-like weathering over long periods. That could be temperatures in the single digits Celsius on average, with seasonal variations. The crucial point is not the exact number but the sustained presence of unfrozen water cycling through the atmosphere.

Does this mean all of early Mars was warm and wet?

Not necessarily. Mars likely saw regional and temporal variability. The new result shows at least one environment experienced extended wet, leaching conditions consistent with frequent rainfall. Other regions may record colder, icier episodes. Planetary climates can be patchy in space and time—Earth’s own ancient record includes both warm intervals and global glaciations.

How do scientists estimate rainfall from rocks?

They use multiple proxies: the types and proportions of clays, the degree of leaching inferred from elemental ratios (e.g., loss of Na, K, Ca relative to Al and Ti), the textures indicating transport energy and duration, and comparisons to weathering profiles in well-studied Earth environments. Combined with geochemical modeling, these proxies can bracket plausible rainfall rates and durations.

What about the faint young Sun paradox—how could Mars stay warm?

Researchers are exploring several mechanisms: thicker CO2 atmospheres with collision-induced absorption, additional greenhouse gases like H2 and CH4 from volcanism, high-altitude cloud feedbacks, and transient warming from impacts. The kaolinite evidence adds incentive to find sustained warming solutions rather than relying solely on short-lived spikes.

Does kaolinite improve the odds for past life?

Indirectly, yes. A climate capable of rainfall and long-lived surface waters improves habitability. Kaolinite and related fine-grained sediments can trap organic molecules and preserve microenvironments. While not proof of life, such settings are prime targets for biosignature exploration.

Could these pebbles have formed under ice and later altered?

Deep, exhaustive leaching to form kaolinite generally requires pervasive liquid water flow through soils or regolith over long spans—conditions that are difficult to sustain beneath thick, persistent ice. While brief subglacial flows occur, they typically don’t achieve the level of leaching recorded by kaolinite-rich clasts found in fluvial deposits.

Source & original reading

Ars Technica Science: https://arstechnica.com/science/2026/02/ancient-mars-was-warm-and-wet-not-cold-and-icy/