An analysis of two Martian meteorites — Northwest Africa (NWA) 7034 and Allan Hills (ALH) 84001 — shows that Mars likely received water from at least two vastly different sources early in its history; the variability implies that Mars, unlike Earth and the Moon, never had a global ocean of magma.
“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 Dr. Jessica Barnes, a researcher at NASA’s Johnson Space Center and the Lunar and Planetary Laboratory at the University of Arizona.
“Two distinct planetesimals with vastly different water contents could have collided and never fully mixed. This context is also important for understanding the past habitability and astrobiology of Mars.”
Dr. Barnes and colleagues were able to piece together Mars’ water history by looking for clues in two isotopes of hydrogen: light hydrogen and deuterium.
The researchers analyzed samples they knew were originated from the Martian crust: NWA 7034 (also known as the Black Beauty meteorite) and ALH 84001 meteorites.
Black Beauty was especially helpful because it’s a mashup of surface material from many different points in Mars’ history.
“This allowed us to form an idea of what Mars’ crust looked like over several billions of years,” Dr. Barnes said.
The isotopic ratios of the meteorite samples fell about midway between the value for Earth rocks and Mars’ atmosphere.
When the team’s findings were compared with previous studies, including results from NASA’s Curiosity rover, it seems that this was the case for most of Mars’ 4 billion-plus-year history.
“We thought, ok this is interesting, but also kind of weird. How do we explain this dichotomy where the Martian atmosphere is being fractionated, but the crust is basically staying the same over geological time?” Dr. Barnes said.
The scientists also grappled with trying to explain why the crust seemed so different from the Martian mantle, the rock later which lies below.
“If you try and explain this fairly constant isotopic ratio of Mars’ crust, you really can’t use the atmosphere to do that. But we know how crusts are formed. They’re formed from molten material from the interior that solidifies on the surface,” Dr. Barnes said.
“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.”
The idea that Mars’ interior was Earthlike in composition came from one study of a Martian meteorite thought to have originated from the mantle — the interior between the planet’s core and its surface crust.
“However, 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,” Dr. Barnes said.
“The fact that our data for the crust was so different prompted us to go back through the scientific literature and scrutinize the data.”
The study authors found that two geochemically different types of Martian 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, they found.
“It turns out that if you mix different proportions of hydrogen from these two kinds of shergottites, you can get the crustal value,” Dr. Barnes said.
“We think that the shergottites are recording the signatures of two different hydrogen — and by extension, water — reservoirs within Mars. The stark difference hints to them that more than one source might have contributed water to Mars and that Mars did not have a global magma ocean.”
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| A close view of the surface of Mars. What lies below continues to intrigue. |
“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 Dr. Jessica Barnes, a researcher at NASA’s Johnson Space Center and the Lunar and Planetary Laboratory at the University of Arizona.
“Two distinct planetesimals with vastly different water contents could have collided and never fully mixed. This context is also important for understanding the past habitability and astrobiology of Mars.”
Dr. Barnes and colleagues were able to piece together Mars’ water history by looking for clues in two isotopes of hydrogen: light hydrogen and deuterium.
The researchers analyzed samples they knew were originated from the Martian crust: NWA 7034 (also known as the Black Beauty meteorite) and ALH 84001 meteorites.
Black Beauty was especially helpful because it’s a mashup of surface material from many different points in Mars’ history.
“This allowed us to form an idea of what Mars’ crust looked like over several billions of years,” Dr. Barnes said.
The isotopic ratios of the meteorite samples fell about midway between the value for Earth rocks and Mars’ atmosphere.
When the team’s findings were compared with previous studies, including results from NASA’s Curiosity rover, it seems that this was the case for most of Mars’ 4 billion-plus-year history.
“We thought, ok this is interesting, but also kind of weird. How do we explain this dichotomy where the Martian atmosphere is being fractionated, but the crust is basically staying the same over geological time?” Dr. Barnes said.
The scientists also grappled with trying to explain why the crust seemed so different from the Martian mantle, the rock later which lies below.
“If you try and explain this fairly constant isotopic ratio of Mars’ crust, you really can’t use the atmosphere to do that. But we know how crusts are formed. They’re formed from molten material from the interior that solidifies on the surface,” Dr. Barnes said.
“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.”
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| Illustration showing the present-day hydrogen reservoirs in and on Mars. The mass fractions (pie chart) of Martian water are based on the mass of water in the bulk crust (C), the mantle (M) and the combined inventory (A) of the atmosphere and polar ice deposits (PID). The mantle mass fraction is the combination of depleted shergottites (DS) and enriched shergottites (ES). Image credit: Barnes et al, doi: 10.1038/s41561-020-0552-y. |
The idea that Mars’ interior was Earthlike in composition came from one study of a Martian meteorite thought to have originated from the mantle — the interior between the planet’s core and its surface crust.
“However, 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,” Dr. Barnes said.
“The fact that our data for the crust was so different prompted us to go back through the scientific literature and scrutinize the data.”
The study authors found that two geochemically different types of Martian 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, they found.
“It turns out that if you mix different proportions of hydrogen from these two kinds of shergottites, you can get the crustal value,” Dr. Barnes said.
“We think that the shergottites are recording the signatures of two different hydrogen — and by extension, water — reservoirs within Mars. The stark difference hints to them that more than one source might have contributed water to Mars and that Mars did not have a global magma ocean.”
| Bibliography: Multiple early-formed water reservoirs in the interior of Mars Jessica J. Barnes, Francis M. McCubbin, Alison R. Santos, James M. D. Day, Jeremy W. Boyce, Susanne P. Schwenzer, Ulrich Ott, Ian A. Franchi, Scott Messenger, Mahesh Anand & Carl B. Agee Nat. Geosci. (2020). https://doi.org/10.1038/s41561-020-0552-y |
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Space & Astrophysics