Mars may have been born into a blue, water-covered world, long before Earth even finished forming, according to new research. This discovery could open a window for scientists on a little-known chapter in Martian history.
In a recent study published in Earth and Planetary Science Letters, a team of researchers, including several from Arizona State University, found that Mars’ early atmosphere was much denser than today and composed mostly of molecular hydrogen. , very different from the thin carbon dioxide. atmosphere that it retains today.
Even though it is the lightest molecule, hydrogen would have had big implications for the early climate of Mars. It turns out that molecular hydrogen is a powerful greenhouse gas.
“It’s a paradox that so many observations suggest liquid water in early Mars, even though water freezes on today’s Mars, and the ancient sun was 30% dimmer than today. ‘ today,” said Steve Desch, professor of astrophysics at the School of Earth and ASU. Space Exploration and one of the team’s scientists. “Greenhouse gases traditionally thought of as CO2 would freeze on an early Mars. (Hydrogen) in the atmosphere is an unexpected way to stabilize liquid water.
According to the team’s calculations, molecular hydrogen is a greenhouse gas potent enough to have allowed the first warm-to-warm water oceans to be stable on the surface of Mars for several million years, until until the hydrogen is gradually lost to space.
A different atmosphere
To determine the composition of the ancient atmosphere on Mars, team scientists developed the first evolutionary models that include the high-temperature processes associated with the formation of Mars in the molten state and the formation of the first oceans and atmosphere. These models showed that the main gases emerging from the molten rock would be a mixture of molecular hydrogen and water vapour.
Model results revealed that water vapor in the Martian atmosphere behaved like water vapor in Earth’s atmosphere today: it condensed in the lower atmosphere as clouds, creating a “more dry”. Molecular hydrogen, on the other hand, did not condense anywhere and was the main constituent of Mars’ upper atmosphere. From there, this molecule of light was lost to space.
“This key idea – that water vapor condenses and is retained in early Mars whereas molecular hydrogen does not condense and can escape – allows the model to be directly linked to measurements made by spacecraft. spacecraft, especially the Mars Science Laboratory’s Curiosity rover,” Kaveh said. Pahlevan, researcher at the SETI Institute and lead author of the study.
Martian hydrogen, yesterday and today
The new model has enabled new interpretations of deuterium hydrogen (D/H) data from Mars samples analyzed in laboratories on Earth and by NASA rovers on Mars.
Hydrogen atoms in molecules can be either normal hydrogen (a nucleus with a proton) or “heavy” hydrogen, called deuterium (a nucleus with a proton and a neutron). The number of deuterium atoms in a sample divided by the number of normal hydrogen atoms is called the deuterium to hydrogen ratio, or D/H ratio.
Mars meteorites are mostly igneous rocks, basically solidified lava. They formed when the interior of Mars melted and magma rose to the surface. The dissolved water in these interior samples (derived from the mantle) contains hydrogen with a D/H ratio similar to that of Earth’s oceans, indicating that the two planets started out with very similar D/H ratios. and that their water came from the same source. in the early solar system.
In contrast, when the Mars Science Laboratory measured hydrogen isotopes in 3 billion-year-old ancient clay on the surface of Mars, it found a D/H ratio about three times that of Earth’s oceans. Earth. Therefore, the hydrosphere of Mars – the reservoir of surface water that reacted with rocks to form these clays – must have had a high concentration of deuterium compared to hydrogen. The only plausible way to have this level of deuterium enrichment is to lose most of the hydrogen gas to space: normal hydrogen is lost, but deuterium, being slightly heavier, is not lost so quickly.
Research from this global model shows that if the Martian atmosphere was dense and hydrogen-rich at the time of its formation, then the surface waters would naturally be enriched in deuterium by a factor of two to three, compared to the interior. , which is precisely what the Mars Science Laboratory observed.
“This is the first model that naturally replicates these observations, giving us some confidence that the evolutionary scenario we described matches early events on Mars,” Pahlevan said.
A boost for life at the beginning of March?
Hydrogen atmospheres can even be favorable to the origin of life. Stanley-Miller experiments from the mid-twentieth century showed that prebiotic molecules implicated in the origin of life form readily in such “reducing” hydrogen-rich atmospheres, but not so readily in “oxidizing” atmospheres. low in hydrogen. atmospheres like those of today’s Earth or Mars.
The team’s research results imply that early Mars was at least as promising a site for the origin of life as early Earth was, if not more promising – long before Earth existed. The Earth as we know it only finished forming after the impact of the formation of the Moon, after tens of millions of years of evolution of the solar system. Long before that, Mars may have had a thick, hydrogen-rich atmosphere, warm temperatures, and a surface covered in blue oceans.
In addition to Desch and Pahlevan, the paper’s authors include Lindy Elkins-Tanton and Peter Buseck, both affiliated with ASU’s School of Earth and Space Exploration (Buseck is also affiliated with the ASU School of Molecular Sciences), and Laura Schaefer, who is affiliated with the Department of Geological Sciences at Stanford University.
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