The formation of Mars and its geological evolution is not accompanied by active mixing of the mantle as previously assumed because it includes several separate tanks of water. To such conclusion scientists have come, having analysed the isotopic composition of hydrogen in the Martian meteorites. Article published in Nature Geoscience.
Mars, like Earth, is composed of core, mantle, crust and atmosphere. These planets were formed at about the same time, passing through stages of accretion, its differentiation into different structural components and further thermochemical evolution. It is believed that after the accretion of a planetary surface for some time, was covered with a continuous magma ocean, in which a constant bombardment of large bodies and the active substance were mixed, followed by the formation of a crust, mantle and core. As was initially distributed the water to the structural components of Mars, which was its isotopic composition, can be understood by analyzing Martian meteorites, data of astronomical observations and Mars missions, from space station to the landing modules and Rovers.
The information obtained from all these sources, tells us that Mars has at least two tanks of water, differing in their isotopic composition. The tank in this case involves no reservoir, and all water contained in a certain environment. The first of them concentrated in the atmosphere and polar caps of the planet, the second, presumably, refers to the primary Martian mantle. The ratio of deuterium — a heavy isotope of hydrogen and ordinary hydrogen (D/H) in the two tanks differs by about five times: from 9,3×10-4 to 1.99 ×10-4. Deuterium in the atmosphere more, as light isotopes of hydrogen are fast disappearing.
Analysis of hydrogen isotope composition of Martian meteorites and rocks on the surface of Mars shows different ratios of D/H that lie between these two extremes. Still this variation was explained by the mutual mixing of any earthly contamination. However, there is no unambiguous evidence that Mars once had the same plate tectonics, as on Earth, and in her absence, the crust acts as a physical and chemical barrier between the atmosphere and the mantle. Therefore, it is unclear whether once in the history of Mars active mixing breeds.
Jessica Barnes (Jessica Barnes) and her colleagues from the UK and Germany was investigated using secondary ion mass spectrometry samples of two Martian meteorites Allan Hills (ALH) 84001 and Northwest Africa (NWA) 7034, which presumably experienced the effects of liquid and gaseous components of the crust of Mars — the first 3.9 billion years ago, the second — 1.5 billion years ago. The only thing common to these samples of the mineral that contains water, was Apatite. His analysis showed that within the experimental error of the isotope ratio was the same in both meteorites — between of 3.12 × 10-4 and about 4.67 ×10-4. Deuterium in the ancient Martian crust was less than in the modern atmosphere of the planet, and thus the isotopic composition of hydrogen in it remained stable at least for the 2.4 billion years that separates two of the meteorite.
These data are compared with the isotope composition of hydrogen in other Martian meteorites — shergottites, or basaltic meteorites, which are believed to represent the results of partial melting of the mantle. While it was already known that the Martian basalts are from at least two different geochemical regions of the mantle and are divided into depleted and enriched.
Studies have shown that the water content and the ratio D/H isotopic composition of hydrogen in the enriched shergottite higher than in lean (36-72 ppm of water and D/H (8,03 ± 0,52) × 10-4. vs 14-23 ppm of water and D/H of 1.99 ± 0,02 × 10-4.), contrary to the hypothesis that the only source of water in the mantle.