The fact that the cold, dry Mars of today once had flowing rivers and lakes several billion years ago has puzzled scientists for decades. Now, Harvard researchers believe they have a compelling explanation for a warmer, wetter ancient Mars.
How ancient Mars sustained warmth
Building on prior theories describing Mars as a planet that alternated between hot and cold periods, a team led by researchers at the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) has identified the chemical processes that may have allowed early Mars to remain warm enough to support liquid water—and possibly life.
“It’s been such a puzzle that there was liquid water on Mars because Mars is farther from the Sun, and also, the Sun was fainter early on,” said Danica Adams, NASA Sagan Postdoctoral Fellow and lead author of the new study published in Nature Geoscience.
The role of hydrogen in Mars’ early atmosphere
Previous theories suggested that hydrogen mixed with carbon dioxide in the Martian atmosphere could have triggered greenhouse warming. However, hydrogen has a short atmospheric lifespan, necessitating a more detailed investigation.
Now, Adams, along with Robin Wordsworth, the Gordon McKay Professor of Environmental Science and Engineering at SEAS, and their team, have performed photochemical modeling—similar to techniques used today to track air pollutants—to uncover how Mars’ atmosphere interacted with hydrogen and evolved 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—ones that can be tested when Mars rocks are returned to Earth.”
Simulating Mars’ climate through photochemical modeling
To investigate these atmospheric changes, Adams modified a model called KINETICS to simulate how interactions between hydrogen, other gases, and the planet’s surface influenced early Martian climate.
She discovered that during Mars’ Noachian and Hesperian periods, between 4 and 3 billion years ago, the planet experienced episodic warm spells over approximately 40 million years. Each warm period lasted 100,000 years or more—consistent with the geologic features seen on Mars today. These warm and wet phases were driven by crustal hydration, where water lost to the ground supplied enough hydrogen to accumulate in the atmosphere over millions of years.
As Mars cycled between warm and cold periods, its atmospheric chemistry also changed. Sunlight constantly broke down CO₂ into CO. During warm periods, CO could recycle back into CO₂, making CO₂ and hydrogen the dominant gases. However, if the cold persisted for long enough, the recycling process would slow down, leading to a buildup of CO and creating a more oxygen-deficient (reduced) atmosphere. “We’ve identified time scales for all of these alternations,” Adams said. “And we’ve described all the pieces in the same photochemical model.”
Implications for future missions
This modeling work provides new insights into the conditions that may have supported prebiotic chemistry—the foundation for life—during warm periods while highlighting the challenges that early life would have faced during colder, more oxidized intervals. To further explore these atmospheric changes, Adams and her colleagues are conducting isotope chemical modeling to compare their predictions with data from upcoming Mars Sample Return missions.
Unlike Earth, Mars lacks plate tectonics, meaning that its surface has remained largely unchanged over billions of years. This makes Mars an exceptional natural laboratory for studying planetary evolution. “It makes a really great case study for how planets can evolve over time,” Adams said.
Adams began this research as a Ph.D. student at the California Institute of Technology, which hosts the photochemical model she used. The study was supported by NASA and the Jet Propulsion Laboratory.
Source: Harvard School of Engineering and Applied Sciences (SEAS).
Image credit: NASA/JPL/University of Arizona
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