It is a scientific consensus that Mars once had abundant sources of surface water, including groundwater, rivers, lakes, and a massive ocean that covered much of its northern hemisphere. And yet, orbiters and rovers have found only small amounts of carbonate rocks on its surface, far less than what scientists would expect from a planet that is believed to have once had abundant water flowing on its surface and a thick, carbon dioxide-rich atmosphere.
But a new geochemical study led by researchers from the Japan Aerospace Exploration Agency (JAXA) shows two things that could resolve the mystery. After comparing observational data to simulations, they found that feldspar-rich rocks found on the Martian surface show possible evidence of interaction with water and carbonates, and that ground groundwater could be burying carbonates underground.
This study was led by doctoral student Chang-Chin Wang with 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 (ESLI) at the Institute of Science Tokyo and the SETI Institute. The paper that describes their findings recently appeared in the journal JGR Planets.
Though carbonates are present on Mars, they’re not abundant. If Mars were wet for a long time, they should be abundant. Image Credit: ESA.
In addition to the scarcity of carbonate rocks on Mars’ surface, scientists have also puzzled over why the carbonates that have been found fall into two distinct groups: calcium- and iron-rich and magnesium-rich rocks. Previous research on water-rock interactions on Mars billions of years ago has assumed that water interacted with the planet’s most common rock types, which are composed of mafic, iron-, and magnesium-rich minerals.
Feldspar-rich rocks, however, have been increasingly detected on the planet’s surface by orbiters and rover missions. These rock-forming minerals are also the most abundant group on Earth, accounting for 60% of the Earth’s crust. On Mars, these rocks are distinguished by higher calcium, sodium, and aluminum content, with strong indications that they may have once been widespread.
In their study, Wang and his colleagues explored the possibility that these calcium/iron-rich carbonates could result from interaction with water. To this end, the team built one-dimensional thermochemical models that tracked how water dissolved minerals and created new ones (altering the chemistry of both) as it percolated through mafic or feldspar-rich rock under the conditions present on early Mars.
The simulations were based on the PHREEQC Version 3 geochemical code, which performs a wide variety of aqueous geochemical calculations. The simulations ranged from short bursts of alteration that lasted for a few years to longer episodes lasting up to 100,000 years. They also included two modes of water movement: diffusion through standing water and downward percolation through groundwater flow.
Artist’s impression of water under the Martian surface. Credit: ESA/Medialab
The results showed that feldspar-rich rock readily produces calcium/iron-rich carbonates under most conditions. Mafic rock, however, did so only during brief periods of alteration before enough magnesium-bearing minerals dissolved and the chemistry shifted toward magnesium-rich carbonates. This offers researchers a new way of examining what happened billions of years ago on Mars’ surface and how it resulted in what we see there today.
Similarly, the simulations also showed that percolating groundwater was far more efficient at forming carbonates than standing water. They further revealed that groundwater tended to dissolve these carbonates again near the surface and reprecipitate them deeper underground. This suggests that Mars may have substantial stores of carbonate rocks beneath the surface, which parallels theories about Mars “missing water.”
For future missions bound for Mars, these findings suggest that drilling operations into feldspar-rich terrain could reveal evidence of Mars’ warmer, wetter past. These operations could also find water that retreated underground as the Martian climate transitioned to a colder, drier climate billions of years ago. Since operations on Mars will center on water sources that will likely be underground aquifers, these finds will also enable continued exploration of the planet.
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Source: Universe Today
