New clues about Mars’ early atmosphere point to a wet planet capable of supporting life

New research published in Earth and Planetary Science Letters indicates that Mars was born humid, with a dense atmosphere that allowed 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.

By contrast, molecular hydrogen (H2) did not condense and moved 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 spacecraft, specifically the Curiosity spacecraft at the Mars Science Laboratory.

“We think we modeled a class that was overlooked in the earliest history of Mars in the time immediately following the planet’s formation. To explain the data, the atmosphere of primordial Mars must have been very dense (about 1,000 times as dense as the modern atmosphere) and composed primarily of molecular hydrogen ( H2),” said Kaveh Pahlivan, SETI Institute research scientist.

“This discovery is important because H2 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 the surface of Mars for millions of years until H2 is gradually being lost to space. For this reason, we conclude that – sometime before 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 similar deuterium to hydrogen ratio 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 largely concentrated 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 also shows that if the Martian atmosphere was rich in H at the time of its formation (and about 1,000 times more dense as it is today), then naturally surface waters would be enriched in deuterium by a factor of 2-3x relative to the interior, reproducing observations. Deuterium prefers splitting into a water molecule relative to molecular hydrogen (H2), which preferentially takes up 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, the H2-rich atmosphere is important to the SETI Institute’s research on extraterrestrial life. Experiments dating back to the mid-20th century show that prebiotic molecules implicated in the origin of life readily form in such H2-rich atmospheres but not so readily in H2-poor (or more “oxidative”) atmospheres. 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.

About SETI Institute

Founded in 1984, the SETI Institute is a not-for-profit, interdisciplinary research and educational organization whose mission is to lead humanity’s quest to understand the origins and spread of life and intelligence in the universe and to share that knowledge with the world. Our research spans the physical and biological sciences and leverages data analytics, machine learning, and advanced signal detection techniques. The SETI Institute is a distinguished research partner for industry, academia, and government agencies, including NASA and the National Science Foundation.

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Rebecca MacDonald
Communications Manager
SETI Institute
mcdonald@seti.org.

astrobiology