New evidence about the early atmosphere on Mars points to a wet planet capable of supporting life

3D view of a wet blue planet. Credit: Planet Volumes / Anode on Unsplash

A new research has been published in Earth and Planetary Science Letters It indicates that Mars was born wet, with a dense atmosphere that allows warm to warm oceans for millions of years. To reach this conclusion, the researchers developed the first model of Martian atmospheric evolution that links the high temperatures associated with the formation of Mars in a molten state through the formation of the first oceans and atmospheres.

This model shows – as on modern Earth – that water vapor in the Martian atmosphere is concentrated in the lower atmosphere and that the upper atmosphere of Mars was “dry” because the water vapor would condense as clouds at lower levels in the atmosphere. Molecular hydrogen (H2), in contrast, did not condense and was carried into the upper atmosphere of Mars, where it was lost to space. This finding—that water vapor condensed and held on early Mars while molecular hydrogen neither condensed nor escaped—allows the model to relate directly to measurements made by the spacecraft, specifically the Curiosity spacecraft at the Mars Science Laboratory.

“We think we modeled an overlooked class in the earliest history of Mars in the time immediately following the formation of the planet. To explain the data, the atmosphere of primitive Mars must have been extremely dense (about 1,000 times as dense as the modern atmosphere). Mainly of molecular hydrogen. (H2), said Kaveh Pahlivan, a research scientist at the SETI Institute.

“This finding is significant because H.2 It is known to be a potent greenhouse gas in dense environments. This dense atmosphere would have produced a strong greenhouse effect, allowing very early warm-to-hot water oceans to settle on Mars for millions of years until H2 gradually into space. For this reason, we conclude that – sometime before the Earth itself formed – Mars was born wet.”

Data limiting the model is the ratio of deuterium to hydrogen (D/H) (deuterium is the heavy isotope of hydrogen) of various Martian samples, including Martian meteorites and those analyzed by Curiosity. Meteorites from Mars are mostly igneous rocks – formed when the interior of Mars melted, and magma rose towards the surface. The dissolved water in these inner igneous samples (derived from the mantle) has a deuterium to hydrogen ratio similar to that of Earth’s oceans, indicating that the planets started with similar D/H ratios and that their water came from the same source in the early solar system.

By contrast, Curiosity measured the D/H ratio of a 3-billion-year-old clay on Mars and found this value to be about 3 times that of Earth’s oceans. Apparently, by the time these ancient clays formed, the surface water reservoir on Mars – the hydrosphere – had a large concentration of deuterium relative to hydrogen. The only process known to produce this level of deuterium concentration (or “enrichment”) is the preferential loss of the lighter H isotope to space.

The model further shows that if the Martian atmosphere is H.2Richer at the time of its formation (and more than 1,000 times as dense as today), surface waters would naturally be enriched in deuterium by a factor of 2 – 3x relative to the interior, which would reproduce the observations. Deuterium prefers to split into a water molecule relative to molecular hydrogen (H .).2), which preferentially absorbs ordinary hydrogen and escapes from the upper atmosphere.

“This is the first published model that naturally reproduces this data, which gives us some confidence that the atmospheric evolution scenario we describe is consistent with early events on Mars,” Pahelvan said.

Aside from curiosity about early environments on planets, H2Rich atmospheres are important to SETI’s search for extraterrestrial life. Experiments dating back to the mid-20th century show that prebiotic molecules implicated in the origin of life readily form in H2Rich ambiance but not so easy in H.2Poor atmosphere (or more “oxidizing”). The implication is that early Mars was a warm version of modern Titan and at least as promising a site for the origin of life as the early Earth was, if not more promising.

NASA scientists helped find clouds on Mars

more information:
Kaveh Pahlivan et al., A primitive atmospheric origin of deuterium enrichment in the Martian hydrosphere, Earth and Planetary Science Letters (2022). DOI: 10.1016 / j.epsl.2022.117772

Provided by SETI Institute

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