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Mars’s Earliest History Shows The Red Planet Was Born Wet

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Earth and Planetary Science Letters reports that Mars’ rich atmosphere allowed warm-to-hot waters for millions of years. To reach this result, astronomers built the first model of the history of the Martian atmosphere that ties Mars’ molten origin to the formation of the earliest oceans and atmosphere.

This model indicates that, like on Earth, water vapour in the Martian atmosphere was concentrated in the lower atmosphere, and that the high atmosphere of Mars was “dry” because the water vapour condensed out as clouds at lower altitudes in the atmosphere. Molecular hydrogen (H2), on the other hand, did not condense.

Instead, it moved up into Mars’s upper atmosphere and was lost to space. This finding – that water vapour condensed and was retained on early Mars – permits the model to be linked directly to spacecraft measurements, notably the Mars Science Laboratory rover Curiosity.

The researchers think they have modelled a neglected period in the early history of Mars, just after the planet formed. Kaveh Pahlevan, a research scientist at the SETI Institute, said, “to explain the data, the primordial Martian atmosphere must have been very dense (more than ~1000x as dense as the modern atmosphere) and composed primarily of molecular hydrogen (H2).”

This finding is significant since it is well established that H2 is a powerful greenhouse gas in surroundings that are somewhat dense. Very early warm-to-hot water oceans would have been able to remain stable on the Martian surface for millions of years until the H2 was progressively lost to space thanks to this atmosphere’s high greenhouse effect. 

Very early warm-to-hot water oceans would have been able to remain stable on the Martian surface for millions of years until the H2 was progressively lost to space thanks to this atmosphere’s high greenhouse effect. 

The model is limited by the deuterium-to-hydrogen (D/H) ratio of different Martian samples, such as meteorites and those analysed by Curiosity. Deuterium is a heavy form of hydrogen. Mars meteorites are primarily igneous rocks, having been created as magma rose from deep inside the Martian crust to the planet’s outer layers.

The deuterium-to-hydrogen ratio of the water dissolved in these inner (mantle-derived) igneous rocks is comparable to that of the oceans on Earth, suggesting that the two planets initially had identical D/H ratios and that their water originated from the same source in the early Solar System.

On the other hand, Curiosity measured the D/H ratio of 3-billion-year-old clay on the surface of Mars and found that it is about three times higher than that of the oceans on Earth.

It appears that the hydrosphere, Mars’ surface water reservoir, had significantly concentrated deuterium in relation to hydrogen by the time these ancient clays formed. This amount of deuterium enrichment (or concentration) can only be achieved by preferentially losing the lighter H isotope to space.

The model also demonstrates that if the Martian atmosphere was H2-rich during its origin (and more than 1000x as dense as it is today), the surface waters would naturally be enriched in deuterium by a factor of 2-3x relative to the interior, reproducing the results.

In contrast to molecular hydrogen (H2), which preferentially absorbs regular hydrogen and escapes from the top of the atmosphere, deuterium prefers partitioning into the water molecule.

“This is the first published model that naturally reproduces these data, giving us some confidence that the atmospheric evolutionary scenario we have described corresponds to early events on Mars,” added Pahlevan.

H2-rich atmospheres play a crucial role in the SETI Institute’s quest for extraterrestrial life, apart from human interest in the first habitats on planets.

Prebiotic molecules linked to the start of life can form rapidly in such H2-rich atmospheres, according to experiments dating back to the middle of the 20th century, but not as readily in H2-poor (or more “oxidising”) atmospheres.

The conclusion is that early Mars was a heated counterpart of present Titan and a site for the birth of life that was at least as favourable as early Earth, if not more promising.

Source: doi.org/10.1016/j.epsl.2022.117772

Image Credit: Getty

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