The current cold and arid conditions on Mars contrast sharply with its past when it had rivers and lakes billions of years ago, a mystery that has intrigued scientists for many years. Researchers from Harvard now believe they have a solid explanation for the existence of a warmer, wetter ancient Mars.
Expanding on previous theories that depicted ancient Mars as experiencing cycles of heating and cooling, a team from the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) has identified the chemical processes that allowed Mars to maintain sufficient warmth in its early history to support water, and potentially life.
Danica Adams, a NASA Sagan Postdoctoral Fellow and the leading author of the new study published in Nature Geoscience, stated, “It has been quite a puzzle to understand how liquid water existed on Mars, considering it is farther from the sun and that early on the sun was less bright.”
Initially, scientists thought hydrogen was the key element mixed with carbon dioxide in the Martian atmosphere, triggering greenhouse warming events. However, because atmospheric hydrogen has a short lifespan, a more thorough analysis was essential.
Adams, alongside Robin Wordsworth, the Gordon McKay Professor of Environmental Science and Engineering at SEAS, and their team conducted photochemical modeling, similar to modern methods that track air pollutants, to explore the ancient Martian atmosphere’s interactions with hydrogen and how these interactions evolved over time.
According to Wordsworth, “Early Mars is like a lost world, but we can piece it together in great detail if we approach the right questions. This study merges atmospheric chemistry and climate science for the first time, offering some remarkable new predictions that can be tested once we retrieve Mars rocks to Earth.”
Adams utilized a model named KINETICS to simulate how various gases, including hydrogen, reacted with both the ground and the atmosphere to determine Mars’ early climate.
Her research indicated that during Mars’ Noachian and Hesperian periods, approximately 4 to 3 billion years ago, the planet experienced warm intervals lasting around 40 million years each, with individual warm spells extending over 100,000 years. These findings align with the geologic features observed on Mars today. The warm, wet phases were driven by crustal hydration, where water absorbed by the ground released enough hydrogen to accumulate in the atmosphere over millions of years.
As Mars cycled between warm and cold climates, its atmospheric chemistry also changed. Carbon dioxide (CO2) is continuously broken down by sunlight into carbon monoxide (CO). During warmer periods, CO could convert back into CO2, making CO2 and hydrogen the primary components. However, prolonged cold would slow this recycling process, causing CO to accumulate and create a reduced atmospheric state, or less oxygen. Thus, the redox states of the Martian atmosphere varied drastically over time.
“We’ve pinpointed the timelines for all these changes,” Adams remarked. “And we’ve integrated all these components into a single photochemical model.”
This modeling research offers new insights into the conditions favorable for prebiotic chemistry—the foundations for life as we know it—during warm periods, while also highlighting challenges that life may have faced during cold and oxidizing phases. Adams and her team are beginning to investigate evidence of these shifts via isotope chemical modeling, intending to compare their findings with samples from the upcoming Mars Sample Return mission.
Unlike Earth, which has plate tectonics, Mars’ unchanging surface provides a direct link to its distant past, making its history of lakes and rivers even more fascinating. “It serves as an excellent case study on how planets can evolve over time,” Adams explained.
Adams initiated this project while pursuing her Ph.D. at the California Institute of Technology, which provided the photochemical model she utilized. This research received support from NASA and the Jet Propulsion Laboratory.
