This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: Carbonate minerals are among the best records of past liquid water on Mars: They form when water carrying dissolved carbon dioxide reacts with rock, and their composition reflects the chemistry of both. Yet Mars has surprisingly little carbonate on its surface compared with what climate models predict, given that the planet likely had a (at least transiently) thick, CO2-rich atmosphere more than 3 billion years ago.
Where that missing carbon dioxide went and why the carbonates that have been found come in two compositionally distinct groups—calcium/iron-rich and magnesium-rich—remain open questions. Most previous studies of Martian water-rock reactions assumed water interacted with mafic, iron- and magnesium-rich rock, the most common rock type on the planet. But feldspar-rich rocks, distinguished by their higher calcium, sodium and aluminum content, have increasingly been detected by orbiters and rovers and may have once been widespread.
Chang-Chin Wang, a doctoral student at the University of Tokyo, led a study with associate professor Mohit Melwani Daswani at ELSI and professor Tomohiro Usui at the University of Tokyo and JAXA that asked whether protolith composition—the starting rock that water alters—could explain the calcium/iron-rich carbonates. The team built one-dimensional thermochemical models using a geochemical code to simulate water percolating through columns of either mafic or feldspar-rich rock under cold, carbon dioxide-rich conditions resembling those on ancient Mars. The models tracked how water chemistry evolved as it dissolved primary minerals and precipitated new ones across both short bursts of alteration lasting a few years and longer episodes lasting up to 100,000 years.
They also examined two modes of water movement: diffusion through standing water and downward percolation mimicking groundwater flow. The findings have been published in the Journal of Geophysical Research: Planets. The simulations show that feldspar-rich rock readily produces calcium/iron-rich carbonates under most conditions tested, while mafic rock does so only during brief alteration, before enough magnesium-bearing minerals have dissolved to shift the chemistry toward magnesium-rich carbonates.
This gives researchers a new way to interpret what is found at the Martian surface: Where calcium/iron-rich carbonates appear without traces of olivine, a magnesium-rich mineral that survives only limited water-rock reaction, a feldspar-rich source rock becomes the more likely explanation. The team also found that percolating groundwater, as opposed to standing water, was far more efficient at forming carbonates but tended to dissolve them again near the surface and reprecipitate them deeper underground. This offers a mechanism for why Mars may hold a substantial reservoir of carbonates, and the carbon dioxide locked within them, hidden below a surface that appears comparatively carbonate-poor.
The findings suggest that the dichotomy in Martian carbonate composition is not only a record of climate and hydrology but also a fingerprint of the crust that those waters once flowed through. They point toward feldspar-rich terrain and subsurface drilling as promising targets for future missions seeking buried evidence of the planet's wetter past. Wang et al, Alteration of Feldspar‐Rich Rocks on Ancient Mars and Its Possible Link to Ca/Fe‐Rich Carbonates, Journal of Geophysical Research: Planets (2026).
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