Carbon Rocks Deep Underground Could Hold the Evidence of Mars' Watery Past
It is a matter of scientific consensus that Mars once harbored abundant sources of surface water — including expansive groundwater systems, winding river channels, ancient lake basins, and even a massive primordial ocean that may have covered much of its northern hemisphere billions of years ago. This vision of an early, wet Mars is supported by decades of orbital imagery, geochemical analyses, and surface exploration by rovers. And yet, a stubborn geological puzzle has long confounded planetary scientists: if early Mars was truly awash in liquid water beneath a thick, carbon dioxide-rich atmosphere, where are all the carbonate rocks that should have formed as a result?
On Earth, the reaction between liquid water, atmospheric CO₂, and silicate rocks reliably produces carbonate minerals such as calcite and dolomite in enormous quantities. Mars, however, has yielded only trace amounts of carbonates on its surface — far less than geochemical models would predict for a planet once believed to have sustained liquid water for millions, or even hundreds of millions, of years. This discrepancy has stood as one of the most nagging open questions in Mars planetary science.
Now, a groundbreaking new geochemical study led by researchers from the Japan Aerospace Exploration Agency (JAXA) proposes a compelling two-part resolution to this mystery. By comparing observational data from Mars orbiters and rovers to sophisticated geochemical simulations, the team found that feldspar-rich rocks on the Martian surface show previously underappreciated evidence of water-rock interaction and carbonate formation — and, crucially, that ancient groundwater systems may have systematically driven these carbonates to depths far below the surface, effectively hiding a major portion of Mars' geological carbonate record from view.
The Research Team and the Study
This landmark investigation was led by doctoral student Chang-Chin Wang, working alongside Professor Tomohiro Usui of the Institute of Space and Astronautical Science at JAXA and the University of Tokyo, and Associate Professor Mohit Melwani Daswani from the Earth-Life Science Institute (ELSI) at the Institute of Science Tokyo and the SETI Institute. Their findings were recently published in the prestigious peer-reviewed journal JGR Planets, a leading venue for research in planetary geoscience.
"If Mars were wet for a long time, carbonates should be abundant on its surface. Their scarcity is one of the most important unsolved problems in Mars science — and these new simulations suggest the answer may lie beneath our feet, or rather, beneath Mars' surface."
The study builds on a growing body of work examining water-rock interactions on ancient Mars, but distinguishes itself by incorporating a rock type that previous models largely ignored: feldspar-rich lithologies, which recent orbital and rover data suggest may have been far more widespread on early Mars than previously assumed.
The Carbonate Paradox: A Long-Standing Mystery
To appreciate the significance of this new research, it helps to understand why the carbonate scarcity on Mars is so scientifically troubling. On Earth, carbonate rocks — primarily limestone and dolostone — make up a substantial fraction of the sedimentary record. They form when atmospheric CO₂ dissolves in water to form carbonic acid, which then reacts with calcium, magnesium, or iron-bearing silicate minerals to precipitate stable carbonate compounds. Given that early Mars is thought to have possessed a much thicker CO₂ atmosphere than it does today, and that liquid water is believed to have been present at or near the surface, the expectation has been that Mars should be littered with carbonate deposits.
Missions including NASA's Mars Exploration Rovers, the Curiosity rover, and multiple orbital platforms including the CRISM spectrometer aboard NASA's Mars Reconnaissance Orbiter have identified carbonates in only limited quantities and locations. Furthermore, the carbonates that have been detected fall into two chemically distinct categories:
- Calcium- and iron-rich carbonates, such as those detected in certain ancient terrains and crater exposures
- Magnesium-rich carbonates, detected notably in regions such as the Nili Fossae and in Martian meteorites
This bimodal distribution has itself been difficult to explain using existing models, which have typically assumed that water on ancient Mars interacted primarily with mafic rocks — dark, iron- and magnesium-rich igneous rocks similar to basalt. While mafic rocks are indeed widespread on Mars, they alone do not easily account for the calcium- and iron-rich carbonate group.
Feldspar: An Overlooked Piece of the Puzzle
Feldspar minerals are the single most abundant group of minerals in Earth's continental crust, accounting for roughly 60% of crustal composition. They are characterized by their content of calcium, sodium, potassium, and aluminum, giving them a distinct chemical signature compared to mafic minerals. On Mars, feldspar-rich rocks — sometimes referred to as felsic or intermediate lithologies — have been increasingly detected by both orbiting spectrometers and surface rovers in recent years.
Notably, feldspar-bearing rocks have been identified in ancient Noachian-era terrains, which date to more than 3.7 billion years ago — precisely the epoch when Mars is believed to have been warmest and wettest. The presence of these rocks in the ancient geological record strongly implies they were a significant component of the early Martian crust, yet their role in carbonate formation had remained largely unexplored until now.
Wang and colleagues hypothesized that water-rock interactions involving feldspar-rich lithologies could explain the formation of calcium- and iron-rich carbonates on ancient Mars, thereby accounting for one of the two observed carbonate groups. To test this hypothesis rigorously, the team developed a series of sophisticated one-dimensional thermochemical models that tracked how water dissolves minerals and precipitates new ones as it percolates through rock under conditions representative of early Mars.
Modeling Ancient Mars: Geochemical Simulations in Detail
The team's simulations were built upon the widely-used PHREEQC Version 3 geochemical modeling code, developed by the United States Geological Survey. PHREEQC is capable of performing a broad range of aqueous geochemical calculations, including speciation modeling, reaction-path calculations, and advective-reactive transport simulations — making it an ideal tool for reconstructing the complex chemistry of ancient Martian water systems.
The simulations explored a wide parameter space, including:
- Duration of water-rock interaction, ranging from brief alteration episodes lasting only a few years to extended interactions spanning up to 100,000 years
- Two distinct modes of water movement: diffusion through standing, stagnant water bodies (such as lakes or ponded groundwater), and active downward percolation through flowing groundwater systems
- Both mafic and feldspar-rich rock compositions, allowing direct comparison of their respective geochemical behaviors under identical conditions
- Temperature and pressure conditions consistent with estimates for early Noachian Mars
The results were striking in their clarity. Feldspar-rich rocks readily and efficiently produced calcium- and iron-rich carbonates across most simulated conditions, regardless of the duration of alteration or the mode of water movement. Mafic rocks, by contrast, initially produced small amounts of calcium-bearing carbonates, but as magnesium-bearing minerals dissolved into solution, the geochemical equilibrium shifted rapidly toward the production of magnesium-rich carbonates instead. This elegant differentiation neatly explains why two distinct carbonate groups exist on Mars today: they are, in essence, the geochemical fingerprints of two different rock types interacting with ancient water.
Groundwater as the Key: Burying the Evidence
Perhaps the most consequential finding of the study concerns not what types of carbonates formed, but where they ultimately ended up. The simulations revealed a profound asymmetry between the two water movement regimes: percolating groundwater was dramatically more efficient at forming carbonates than standing water. More importantly, the models showed that flowing groundwater tended to dissolve carbonates near the surface and then reprecipitate them at greater depths as the chemistry of the water changed with increasing pressure, temperature, and mineral contact.
This process — known in geochemistry as dissolution-reprecipitation cycling — would effectively act as a pump, continuously stripping carbonate minerals from near-surface environments and depositing them in the subsurface. Over geological timescales, this mechanism could account for a significant fraction of Mars' "missing" carbonate inventory, relocating it to depths that current surface missions simply cannot access.
This finding resonates powerfully with existing theories about Mars' "missing water" problem — the observation that the volume of water inferred from ancient valley networks, lake sediments, and ocean shorelines far exceeds the amount of water detected in present-day surface ice deposits. Many researchers have proposed that a substantial portion of this water migrated underground as Mars cooled and its climate dried. The new JAXA study suggests that not only water, but its associated carbonate mineral products, may have undergone the same deep migration.
Implications for Future Mars Exploration
The findings carry significant implications for the planning and scientific objectives of future Mars exploration missions, both robotic and, eventually, human. If substantial carbonate deposits are buried in the Martian subsurface — particularly in regions underlain by ancient feldspar-rich terrain — then drilling operations represent a high-priority strategy for uncovering Mars' geological and climatic history.
Specifically, the study suggests several important directions for future investigation:
- Targeted drilling into feldspar-rich ancient terrains, which are prime candidates for hosting buried carbonate repositories that could preserve a record of Mars' early climate
- The search for subsurface water ice and liquid brines, which may coexist with carbonate deposits in the deep subsurface and could represent accessible water resources for future human missions
- Enhanced spectroscopic mapping from orbit to better characterize the global distribution of feldspar-rich lithologies and prioritize exploration targets
- Integration of these geochemical models with broader climate and hydrological models of early Mars to refine our understanding of how long liquid water persisted and under what conditions
The NASA Mars Exploration Program and ESA's ExoMars initiative have both identified subsurface exploration as a critical frontier. The Rosalind Franklin rover, equipped with a drill capable of reaching depths of up to two meters, is specifically designed to access material that has been shielded from the harsh radiation environment at Mars' surface — conditions under which organic molecules and, potentially, carbonate minerals would be far better preserved.
A Broader Picture: Mars as a Geochemically Complex World
Beyond the immediate implications for the carbonate paradox, this study underscores a broader and increasingly appreciated truth about Mars: it is a geochemically complex and diverse world whose history cannot be adequately described by simple, single-rock-type models. Just as the diversity of Earth's crust — with its granites, basalts, sedimentary sequences, and metamorphic terrains — is essential to understanding our planet's geological and biological evolution, the heterogeneity of Martian lithologies must be accounted for in any complete reconstruction of Mars' past.
The recognition that feldspar-rich rocks played a significant role in shaping Mars' geochemical evolution opens up new lines of inquiry across multiple disciplines, from mineralogy and hydrology to astrobiology. Carbonate minerals, after all, are not merely passive recorders of past water activity — they are also potential repositories for biosignatures, preserved organic molecules, and isotopic records that could speak directly to the question of whether Mars ever hosted microbial life.
"The subsurface of Mars may represent a vast, largely unexplored archive of the planet's watery past — one that has been protected from billions of years of radiation, impacts, and atmospheric loss. Unlocking that archive is one of the great scientific challenges of our time."
As robotic prospectors continue to survey the Martian surface and as plans for crewed missions take shape, studies like this one from the JAXA team provide an indispensable roadmap — pointing scientists not just toward the rocks we can see, but toward the deeper, hidden layers where the true story of Mars may yet be waiting to be told.
