Scientists at the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) have uncovered the chemical processes that allowed Mars to sustain warm conditions long enough to host liquid water - and possibly primitive life. Their findings build on earlier theories suggesting that ancient Mars experienced cycles of warming and cooling.
"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 study published in Nature Geoscience.
Previous theories posited that hydrogen mixed with carbon dioxide in the Martian atmosphere caused greenhouse warming. However, since atmospheric hydrogen dissipates quickly, researchers needed a more detailed analysis to explain its persistence.
Adams, alongside Robin Wordsworth, the Gordon McKay Professor of Environmental Science and Engineering at SEAS, used advanced photochemical modeling to unravel the dynamics of Mars' early atmosphere. These methods, which are similar to those used for monitoring Earth's air pollutants, revealed how hydrogen interacted with other gases and the Martian surface to influence the planet's climate.
"Early Mars is a lost world, but it can be reconstructed in great detail if we ask the right questions," Wordsworth explained. "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 adapted the KINETICS model to simulate how a mix of hydrogen and other gases reacted with both the atmosphere and the ground. The results showed that during Mars' Noachian and Hesperian periods, between 4 and 3 billion years ago, the planet experienced episodic warm spells spanning approximately 40 million years. Each warming episode, lasting 100,000 years or more, was driven by crustal hydration, which released hydrogen into the atmosphere over extended periods. These estimates align with Mars' geological features observed today.
As Mars alternated between warm and cold climates, its atmospheric chemistry also shifted. Sunlight constantly converted carbon dioxide (CO2) into carbon monoxide (CO). During warm periods, CO could recycle into CO2, maintaining a balance with hydrogen. However, prolonged cold spells slowed this recycling process, leading to a buildup of CO and a more reduced atmospheric state with less oxygen. These redox fluctuations significantly influenced the planet's climate dynamics.
"We've identified time scales for all of these alternations," Adams noted. "And we've described all the pieces in the same photochemical model."
The study's findings offer potential clues about prebiotic chemistry - the early conditions that could have supported life during warm periods - and the challenges such life would face during colder, oxidized phases. Adams and her team are now exploring isotope chemical modeling to verify these processes, using data from Mars rocks to be returned by upcoming NASA missions.
Mars' lack of plate tectonics preserves its ancient surface, making it an ideal site for studying planetary evolution. "It makes a really great case study for how planets can evolve over time," Adams said.
Adams initiated this research as a Ph.D. student at the California Institute of Technology, where the KINETICS model is hosted. The study received support from NASA and the Jet Propulsion Laboratory.
Research Report:Episodic warm climates on early Mars primed by crustal hydration
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