Explaining persistent hydrogen in Mars’ atmosphere

The fact that the cold, dry Mars of today had flowing rivers and lakes several billion years ago has puzzled scientists for decades. Now, Harvard researchers think they have a good explanation for a warmer, wetter ancient Mars.

Building on prior theories describing the Mars of yore as a hot again, cold again place, a team led by researchers at the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) have determined the chemical mechanisms by which ancient Mars was able to sustain enough warmth in its early days to host water, and possibly life.

“It’s been such a puzzle that there was liquid water on Mars, because Mars is further from the sun, and also, the sun was fainter early on,” said Danica Adams, NASA Sagan Postdoctoral Fellow and lead author of the new paper in Nature Geoscience.

Hydrogen was previously theorized as the magic ingredient, mixed with carbon dioxide in the Martian atmosphere to trigger episodes of greenhouse warming. But the lifetime of atmospheric hydrogen is short, so a more detailed analysis was required.

Now, Adams, Robin Wordsworth, the Gordon McKay Professor of Environmental Science and Engineering at SEAS, and team have performed photochemical modeling—similar to methods used today to track air pollutants—to fill in details of the early Martian atmosphere’s relationship to hydrogen, and how that relationship changed over time.

“Early Mars is a lost world, but it can be reconstructed in great detail if we ask the right questions,” Wordsworth said. “This study synthesizes atmospheric chemistry and climate for the first time, to make some striking new predictions—which are testable once we bring Mars rocks back to Earth.”

Adams modified a model called KINETICS to simulate how a combination of hydrogen and other gases reacting with both the ground and the air controlled the early Martian climate.

She found that during Mars’ Noachian and Hesperian periods, between 4 and 3 billion years ago, Mars experienced episodic warm spells over about 40 million years, with each event lasting 100,000 or more years. These estimates are consistent with geologic features on Mars today. The warm, wet periods were driven by crustal hydration, or water being lost to the ground, which supplied enough hydrogen to build up in the atmosphere over millions of years.

During the fluctuations between warm and cold climates, the chemistry of Mars’ atmosphere was also fluctuating. CO₂ is constantly hit by sunlight and converted to CO. In warm periods, the CO could recycle back into CO₂, making CO₂ and hydrogen dominant. But if it was cold for long enough, the recycling would slow down, build up CO, and bring about a more reduced state, a.k.a. less oxygen. The redox states of the atmosphere thus changed dramatically over time.

“We’ve identified time scales for all of these alternations,” Adams said. “And we’ve described all the pieces in the same photochemical model.”

The modeling work lends potential new insight into conditions that supported prebiotic chemistry—the underpinnings of later life as we know it—during warm periods, and challenges for the persistence of that life during intervals of cold and oxidation. Adams and others are starting to work on finding evidence of those alternations using isotope chemical modeling, and they plan to compare those results to rocks from the upcoming Mars Sample Return mission.

Because Mars lacks plate tectonics, unlike Earth, the surface seen today is similar to that of long ago, making its history of lakes and rivers that much more intriguing. “It makes a really great case study for how planets can evolve over time,” Adams said.