How Meteorites Helped Reveal Mars’ Watery Past

Multiple "water reservoirs" within the red planet are responsible for Mars' water.

A group of scientists analyzed meteorite fragments in order to reconstruct Mars’ chaotic history, revealing along the way Mars’ watery past. The new discovery published in Nature Geoscience suggests that Mars may not have had a global magma ocean like Earth, and that its geological past is very different.

Martian meteorites studied to reconstruct its chaotic history suggest that the red planet may not have had an ocean of global magma and likely obtained its water from at least two different sources early in its history.

“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,” explained Jessica Barnes, an assistant professor of planetary sciences in the University of Arizona Lunar and Planetary.

The new study reveals that at least two distinct planetesimals with vastly distinctive water contents could have collided and never fully mixed. “This context is also important for understanding the past habitability and astrobiology of Mars.”

Barnes and her team analyzed the Black Beauty meteorite and the infamous Allan Hills 84001 meteorite, controversial in the 1990s for allegedly containing Martian microbes, to reconstruct the history and planetary origins of Mars.

People have been trying to understand the history of Martian water for decades. Many questions have remained unanswered: Where did the water come from? How long did it remain in a liquid state on the surface?

Where did the inner water of Mars come from, and what can water tell us about how Mars formed and evolved? Barnes and her team were able to reconstruct the history of Mars’ water by looking for clues to two types—isotopes—of hydrogen. A hydrogen isotope contains a proton in its nucleus – this is sometimes called “light hydrogen.” The other isotope is called deuterium, which contains a proton and a neutron in the nucleus; this is sometimes called “heavy hydrogen.” The ratio of these two hydrogen isotopes tells planetary scientists the processes and possible origins of water in the rocks, minerals, and glass in which they are found.

For more than two decades have researchers recorded the isotopic proportions of Martian meteorites, and their data was everywhere, “and there wasn’t really a trend,” Barnes added.

An image of the Martian surface taken by the HiRISE camera on board the MRO. Image Credit: NASA / JPL / University of Arizona.
An image of the Martian surface taken by the HiRISE camera onboard the MRO. Image Credit: NASA / JPL / University of Arizona.

The water enclosed in Earth’s rocks is what is described as unfractionated, which means it doesn’t deviate much from the standard reference value of ocean water – a 1: 6,420 ratio of heavy to light hydrogen.

The atmosphere of Mars, on the other hand, is highly fractional: it is populated mainly by deuterium or heavy hydrogen, probably because the solar wind removed light hydrogen in the distant past when the red planet was stripped of most of its atmosphere.

Measurements of Martian meteorites, many of which were excavated from the depths of Mars by impact events, spanned the entire gamut between measurements of Earth and Mars’ atmosphere.

Barnes’s team set out to investigate the hydrogen isotope composition of the Martian crust, specifically by studying samples they knew originated from the crust – the Black Beauty and Allan Hills meteorites. Black Beauty was especially useful because it is a mixture of surface material from many different points in the history of Mars.

“This gave us an idea of ​​what the crust of Mars looked like for several billion years,” adds Barnes.

The scientists discovered that two geochemically different types of Martian volcanic rocks -enriched shergottites and depleted shergottites – carry water with different hydrogen isotope ratios. Enriched shergottites include 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 said.

Barnes and her colleagues believe that the shergottites are registering the signatures of two different hydrogen – and by extension, water – reservoirs within the red planet. The stark discrepancy hints that more than one source might have contributed water to Mars and that Mars did not have a global magma ocean, as previously believed.

The study is published in Nature Geoscience.

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