Meteorites tell story of Red Planet’s early history.
Mars’s mantle may contain multiple distinct reservoirs of mineral-bound water preserved from the planet’s early history, according to a new study.
Writing in the journal Nature Geoscience, researchers from the US, the UK and Germany say their findings suggest the mantle has a diverse chemical composition and might not have been formed from a global magma ocean.
The isotopic composition of hydrogen on Mars provides insights into different water reservoirs on the planet, and existing models of the planet’s formation have assumed the mantle has a homogenous hydrogen isotope composition.
However, the varied isotopic compositions of the planet’s rocks and atmosphere have complicated these models and the identification of distinct water reservoirs in and on Mars.
In the recent work, a team led by Jessica Barnes from the Lunar and Planetary Laboratory at the University of Arizona, US, chemically analysed the hydrogen isotope composition of minerals from two meteorites from the planet’s crust, where they interacted with water.
And not just any meteorites. Their subjects were the famous Allan Hills 84001 (controversial in the 1990s for allegedly containing Martian microbes) and Black Beauty (aka Northwest Africa 7034), which formed when a huge impact combined various pieces of Martian crust.
They found that the two have a similar hydrogen isotope composition, and that this composition is also similar to more recent material from the crust, indicating that the composition of water has been consistent for the past 3.9 billion years.
They then compared this to data from other Martian meteorites and found that Mars’s mantle contains at least two reservoirs of mineral-bound water that have distinct hydrogen isotope compositions.
"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," Barnes says.
Two distinct planetesimals with vastly different water contents could have collided and never fully mixed, she adds, noting that this context “is also important for understanding the past habitability and astrobiology of Mars".
Water locked in Earth rocks is unfractionated, meaning it doesn't deviate much from the standard reference value of ocean water. However, Mars's atmosphere is heavily fractionated: it is mostly populated by deuterium, or heavy hydrogen.
Measurements from Martian meteorites – many excavated from deep within the planet by impact events – run the gamut between Earth and Mars' atmosphere measurements, Barnes says.
She and colleagues set out to investigate the hydrogen isotope composition of the Martian crust by studying samples they knew originated there. Black Beauty was especially helpful, because it's a mashup of surface material from many different points in the planet’s history.
The isotopic ratios of the meteorite samples fell about midway between the value for Earth rocks and Mars' atmosphere. When the findings were compared with previous studies, including results from the Curiosity Rover, it seems this was the case for most of Mars' four-billion-year history.
"We thought, ok this is interesting, but also kind of weird," Barnes says. "How do we explain this dichotomy where the Martian atmosphere is being fractionated, but the crust is basically staying the same over geological time?"
They also grappled, they say, with trying to explain why the crust seemed so different from the Martian mantle, the rock layer which lies below.
"Martian meteorites basically plot all over the place, and so trying to figure out what these samples are actually telling us about water in the mantle of Mars has historically been a challenge,” Barnes says. “The fact that our data for the crust was so different prompted us to go back through the scientific literature and scrutinise the data."
They found that two geochemically different types of volcanic rocks – enriched shergottites and depleted shergottites – contain water with different hydrogen isotope ratios. Enriched shergottites contain more deuterium than the depleted shergottites, which are more Earth-like.
"It turns out that if you mix different proportions of hydrogen from these two kinds of shergottites, you can get the crustal value," Barnes says.
She and her colleagues believe the shergottites are recording the signatures of two different hydrogen – and, by extension, water – reservoirs within Mars. The stark difference suggests more than one source might have contributed water to Mars and that Mars did not have a global magma ocean.