How early Mars stayed warm enough for water and possibly life

For decades, scientists have puzzled over the contradiction between Mars’ present-day barren landscape and its ancient history of flowing rivers and lakes. The planet is now cold and dry, yet geological evidence suggests that liquid water once existed in warm abundance.

Researchers at Harvard have taken a significant step toward explaining how early Mars could have sustained a climate warm enough to support water – and potentially life.

Mars’ climate shifts

Mars orbits farther from the Sun than Earth, and the Sun itself was fainter billions of years ago. This makes it difficult to explain how the planet ever reached temperatures high enough for liquid water to exist on its surface.

Scientists have long speculated that greenhouse gases, such as carbon dioxide and hydrogen, played a role in warming Mars intermittently. However, the exact mechanisms behind these warming periods remained unclear.

A team at the Harvard John A. Paulson School of Engineering and Applied Sciences has now explored the chemical processes that may have contributed to early Mars’ climate.

The study expands on the idea that Mars experienced alternating warm and cold periods. By modeling atmospheric chemistry, the researchers were able to identify key factors that influenced these shifts.

Danica Adams, a NASA Sagan Postdoctoral Fellow and lead author of the study, emphasized the long-standing mystery surrounding Mars’ climate.

“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,” she said.

Hydrogen’s role in warming Mars

Previous studies suggested that hydrogen, when mixed with carbon dioxide, could have triggered greenhouse warming episodes on Mars. However, a major issue with this hypothesis was that atmospheric hydrogen does not last long, making it difficult to sustain warming over extended periods.

To address this problem, Adams and her colleagues, including Professor Robin Wordsworth of Harvard, conducted photochemical modeling.

These techniques, commonly used today to study air pollution on Earth, provided deeper insight into the interactions between hydrogen and the early Martian atmosphere.

“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.”

Simulating the climate of early Mars

Adams used a model called KINETICS to analyze how a combination of hydrogen and other gases reacted with both the ground and the air, influencing Mars’ climate billions of years ago.

The modeling suggested that during the Noachian and Hesperian periods – between 4 and 3 billion years ago – Mars experienced episodic warm spells. These warming events lasted for over 100,000 years each and continued in cycles for around 40 million years.

The findings align with geological features on Mars that indicate past water flow. According to the study, these warm periods were largely driven by a process known as crustal hydration, where water was absorbed into the ground.

This process supplied the atmosphere with hydrogen over millions of years, leading to greenhouse effects that warmed the planet temporarily.

Shifting atmospheric chemistry

While Mars alternated between warm and cold phases, its atmospheric chemistry was also constantly evolving. Carbon dioxide, a dominant gas in the Martian atmosphere, interacts with sunlight, converting into carbon monoxide.

In warmer periods, the carbon monoxide recycled back into carbon dioxide, reinforcing greenhouse conditions. However, during prolonged cold spells, this recycling slowed, allowing carbon monoxide to accumulate and shift the atmosphere into a more reduced state, meaning it had less oxygen.

These atmospheric changes influenced the planet’s redox states, or its ability to support oxidation-reduction reactions over time. Understanding these shifts is essential for piecing together Mars’ climatic history and its potential to support life.

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

Early life on Mars

The study offers new insights into the environmental conditions that may have supported prebiotic chemistry – the chemical processes that precede the emergence of life.

During warm periods, conditions could have allowed for complex organic reactions to occur. However, prolonged cold spells and oxidation events would have posed significant challenges for the persistence of life.

Adams and her colleagues are now focused on finding physical evidence of these atmospheric transitions through isotope chemical modeling. Their goal is to compare their predictions with actual Martian rock samples.

The upcoming Mars Sample Return mission is expected to provide crucial data that could validate or refine their findings.

A window into planetary evolution

Unlike Earth, Mars lacks plate tectonics, which means its surface has remained largely unchanged for billions of years. This unique characteristic makes the planet a valuable case study for understanding how planetary climates evolve over time.

Studying Mars’ atmospheric history not only helps scientists reconstruct its warm past but also provides insights into the broader processes that shape planetary environments throughout the universe.

“It makes a really great case study for how planets can evolve over time,” said Adams, who began this research as a Ph.D. student at California Institute of Technology.

The study received funding from the NASA Jet Propulsion Laboratory. The team’s findings mark a significant advancement in understanding Mars’ climatic past and pave the way for future discoveries about the planet’s ability to support life.

The study is published in the journal Nature Geoscience.

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