A hydrogen-rich first atmosphere for Mars inferred from clays on its surface
According to new research, Mars may have been born a blue and water-covered world, long before the Earth had even finished forming. The discovery could open a window for scientists on an overlooked 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’s earliest atmosphere was much denser than today, and primarily composed of molecular hydrogen, very different from the thin, carbon dioxide atmosphere it retains today.
Even though it is the lightest molecule, hydrogen would have had big implications for Mars’ earliest climate. Molecular hydrogen, it turns out, is a powerful greenhouse gas.
“It’s a paradox that so many observations suggest liquid water on early Mars, even though water freezes on present-day Mars, and the ancient sun was 30% dimmer than today,” said Steve Desch, professor of astrophysics in ASU's School of Earth and Space Exploration and one of the team scientists. “Traditionally considered greenhouse gases like 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 strong enough greenhouse gas to have allowed very early warm-to-hot water oceans to be stable on the Martian surface for many millions of years, until the hydrogen was gradually lost to space.
A different type of atmosphere
To determine the composition of the ancient atmosphere on Mars, team scientists developed the first evolutionary models that include high-temperature processes associated with Mars' formation in a 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 mix of molecular hydrogen and water vapor.
The results from the models revealed water vapor in the Martian atmosphere behaved like water vapor in our modern-day Earth's atmosphere: it condensed in the lower atmosphere as clouds, creating a “drier” upper atmosphere. Molecular hydrogen, by contrast, did not condense anywhere, and was the main constituent of the upper atmosphere of Mars. From there, this light molecule was lost to space.
"This key insight — that water vapor condenses and is retained on early Mars whereas molecular hydrogen does not condense and can escape — allows the model to be linked directly to measurements made by spacecraft, specifically, the Mars Science Laboratory rover Curiosity," said Kaveh Pahlevan, a research scientist at the SETI Institute and lead author of the study.
Martian hydrogen, then and now
The new model has allowed new interpretations of deuterium-to-hydrogen (D/H) data from Mars samples analyzed in laboratories on Earth and by NASA rovers on Mars.
Hydrogen atoms in molecules can either be normal hydrogen (a nucleus with one proton) or "heavy" hydrogen, called deuterium (a nucleus with one proton and one neutron). The number of deuterium atoms in a sample divided by the number of normal hydrogen atoms is called the deuterium-to-hydrogen, or D/H ratio.
Meteorites from Mars are mostly igneous rocks, basically solidified magmas. They formed when the interior of Mars melted, and the magma ascended toward the surface. The water dissolved in these interior (mantle-derived) samples contain hydrogen with a D/H ratio similar to that of the Earth's oceans, indicating that the two planets started with very similar D/H ratios, and their water came from the same source in the early solar system.
In contrast, when the Mars Science Laboratory measured the isotopes of hydrogen in an ancient 3-billion-year-old clay on the Martian surface, it found a D/H ratio value about three times that of Earth's oceans. Therefore, the hydrosphere of Mars — the surface water reservoir that reacted with rocks to form these clays — must have had a high concentration of deuterium relative 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 as quickly.
The research from this comprehensive model shows that if the Martian atmosphere were 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, relative to the interior, which is precisely what the Mars Science Laboratory observed.
“This is the first model that naturally reproduces these observations, giving us some confidence that the evolutionary scenario we have described corresponds to the earliest events on Mars,” Pahlevan said.
A boost for life on early Mars?
Hydrogen atmospheres may even be favorable for the origin of life. The Stanley-Miller experiments dating back to the middle of the 20th century have shown that prebiotic molecules implicated in the origin of life form readily in such hydrogen-rich, "reducing" atmospheres, but not so readily in hydrogen-poor, "oxidizing" atmospheres like those of modern-day Earth or Mars.
The team's research findings 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. Earth as we know it did not finish forming until after the moon-forming impact, after tens of millions of years of solar system evolution. Long before that, Mars may have had a thick, hydrogen-rich atmosphere, clement temperatures and a surface covered in blue oceans.
In addition to Desch and Pahlevan, authors of the paper include Lindy Elkins-Tanton and Peter Buseck, both of whom are affiliated with ASU's School of Earth and Space Exploration (Buseck is also affiliated with ASU's School of Molecular Sciences), and Laura Schaefer, who is affiliated with the Department of Geological Sciences at Stanford University.