Scientists have found that at least two separate reservoirs of ancient water with different chemical signatures have survived under the Martian surface.
This discovery shows that, unlike Earth, Mars probably did not have a single large global ocean of underground magma spanning the entire planet.
“A lot of people are trying to figure out the history of water on Mars,” explains planetary scientist Jessica Barnes of the University of Arizona.
'Where did the water come from? How long has it been in the crust (surface) of Mars? Where did the inner water of Mars come from? What can water tell us about how Mars formed and developed? '
Evidence has been found in the rocks of Mars. We can't hop on Mars and pick them up; in fact, to date we have not even conducted an automated mission to retrieve samples from Mars. But sometimes Mars itself comes to us.
Meteorites torn from the Martian crust fall to Earth from time to time. Here in the laboratories of the Earth, using the most modern methods, researchers have carefully studied two such meteorites – Allan Hills 84001, discovered in Antarctica in 1984, and Northwest Africa 7034, discovered in the Sahara Desert in 2011.
The team looked at the isotopes of hydrogen trapped in Martian meteorites. Isotopes are variants of an element with different numbers of neutrons; deuterium – also known as heavy hydrogen – has one proton and one neutron. Protium, or light hydrogen, has one proton and no neutrons.
Since hydrogen is one of the constituents of water, the ratio of these two isotopes trapped in the rock can help us understand the history of the water in which they were found in order to study the chemical processes to which it was subjected and its origins.
Barnes and her team are not the first to study hydrogen isotopes in Martian meteorites to try to learn about the planet's water.
On Mars, deuterium is the dominant hydrogen isotope in the atmosphere, probably due to solar radiation destroying protium.
Thus, Barnes and her team decided to take a closer look at the meteorites that originated in the Martian crust.
Allan Hills 84001, according to earlier dating methods for radioactive decay, interacted with liquid in the Martian crust about 3.9 billion years ago. A similar analysis determined that Northwest Africa 7034 interacted with liquid 1.5 billion years ago.
When Barnes and her team performed their isotope analysis, they found that both samples had the same isotopic ratios, conveniently located between the ratio found in Earth's water and the ratio found in the atmosphere of Mars. Even stranger, this ratio was similar to the younger rocks analyzed by the Curiosity rover on Mars.
This indicates that the chemical composition of this water has been unchanged for approximately 3.9 billion years – a completely unexpected result, given previous research.
But when the team compared their results to previous studies of hydrogen isotopes in meteorites from the Martian mantle, they found something truly amazing. Mantle meteorites fit into two distinct groups of igneous rocks called shergottite.
These two different chemical signatures indicate two different, unmixed reservoirs of water in the Martian mantle. Which could mean the global ocean of liquid magma beneath the mantle did not homogenize the layer above.
'This context is also important for understanding the past habitability and astrobiology of Mars.'
The study was published in the journal Nature Geoscience.
Sources: Photo: NASA / JPL-Caltech / University of Arizona
