We used to think of Mars as a bone dry husk of a world. While there is some evidence of occasional flowing water on the surface, most of the planet’s water is long gone. We can still reconstruct some of Mars’ liquid past with the help of rock samples in the form of Martian meteorites. A team from the University of Arizona analyzed a pair of such objects and determined that Mars may have had two distinct water reservoirs, and that could change how we understand planetary formation.
Scientists may one day be able to send samples from Mars to Earth — the Perseverance rover will collect some material and store it for a possible future sample return mission. Until then, the best we can do is study the handful of meteorites known to come from Mars. The Arizona team led by Jessica Barnes analyzed two Martian meteorites known as Allan Hills 84001 (of Maritan microbe fame) and Northwest Africa 7034. The former was discovered in Antarctica in 1984 and the latter came from the Sahara Desert in 2011.
The key to understanding past water on Mars is the way hydrogen isotopes are distributed in the samples. Water contains hydrogen, but not all hydrogen is the same. Hydrogen with one neutron is known as deuterium or “heavy hydrogen.” If there are no neutrons, that’s called protium or “light hydrogen.” On Earth, the vast majority of hydrogen is protium with a ratio of 1:6,420. However, deuterium is more common on Mars today.
When evaluating the meteorites, the mix of materials allowed the team to track hydrogen isotope levels over about 4 billion years. They found the hydrogen ratio fell between that of Earth and modern Mars consistently over that time. But that didn’t match previous research, which found hydrogen isotopes in Martian meteorites vary considerably.
The key to understanding the results was the geochemistry of two different types of volcanic rocks on Mars. So-called “enriched shergottites” have more deuterium and “depleted shergottites” have more protium, as you’d find on Earth. If you mix those two materials together, you get close to the values observed in this study.
According to Barnes, enriched and depleted shergottites may be a record of two different water reservoirs in the planet’s past. That doesn’t happen on Earth because our planet has a global ocean of subsurface magma that homogenizes the crust. If Mars never had that, it could tell us a lot about the kinds of materials that were available to merge into planets in the early solar system.
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