An analysis of Martian meteorites shows that there are at least two different types of water contained in reservoirs on Mars, suggesting that the planet did not form in a similar way to Earth and may have obtained its hydrogen from outside forces.
The Red Planet is composed of a core, mantle, crust, and atmosphere, which means that it evolved through geological processes much like Earth. Previous theories suggested that Mars also may have once had a global magma ocean, however, the planet lacks evidence of ever having plate tectonics and, without them, the crust would have served as a “physicochemical barrier” between the atmosphere and geological fluids contained within the mantle. Ancient river valleys and deltas suggest that Mars has a watery past, but today the atmosphere is too thin for liquid water to exist on the surface, according to NASA. Rather, it is held in ice form below the planet’s surface in the polar regions.
But where this water came from – and how Mars came to be – has long been a mystery to scientists.
That’s where researchers at the University of Arizona come in. To reconstruct the water history and the planet’s origin, scientists chemically analyzed two Martian meteorites known to have interacted with fluids contained below the surface of the planet: Northwest Africa 7034, or Black Beauty, and Allan Hills 84001. Specifically, two types, or isotypes, of hydrogen were found to exist within their contains. One form contained only a proton in its nucleus, which is known as “light hydrogen,” and the second both a proton and a neutron, also known as deuterium or “heavy hydrogen.”
On Earth, water held within rocks is unfractionated, meaning that it does not deviate much from ocean water. On the other hand, previous meteorite analyses have suggested that Mars is heavily fractioned and varies greatly.
"The prevailing hypothesis before we started this work was that the interior of Mars was more Earthlike and unfractionated, and so the variability in hydrogen isotope ratios within Martian samples was due to either terrestrial contamination or atmospheric implantation as it made its way off Mars," said Jessica Barns, assistant professor of planetary sciences at the University of Arizona Lunar and Planetary Laboratory, in a statement.
Isotopic ratios from the meteorites fell halfway between those found on Earth and others seen in Mars’ atmosphere, an interesting discovery considering that Mars' atmosphere is being fractionated but the crust is staying roughly the same throughout the planet’s geological history. Further analysis found that there are also two different types of Martian volcanic rocks – enriched shergottites and depleted shergottites – that both contain water with different hydrogen isotope ratios.
The two different types of hydrogen, which are described in Nature Geoscience, suggest that Mars has at least two different types of water that likely came from different sources. Over the planet's several-billion-year-history, two celestial objects with different water contents may have collided and never have mixed.
"These two different sources of water in Mars' interior might be telling us something about the kinds of objects that were available to coalesce into the inner, rocky planets," said Barns.