Scientists have long been perplexed by the presence of flowing rivers and lakes on the cold, dry Mars billions of years ago. Researchers at the Harvard John A. Paulson School of Engineering and Applied Sciences have now put forward a novel explanation for the warmer, wetter ancient Mars. Led by Danica Adams, the team delved into the chemical mechanisms that may have enabled early Mars to maintain enough warmth to sustain liquid water, potentially supporting life. Previous theories suggested hydrogen in the Martian atmosphere played a crucial role in triggering greenhouse warming episodes. Through detailed photochemical modeling, the team demonstrated how interactions between hydrogen, carbon dioxide, and other gases may have influenced the climate on ancient Mars, leading to periodic warm spells lasting tens of thousands of years. These warm, wet periods were fueled by crustal hydration, which released hydrogen into the atmosphere over millions of years. The fluctuating climate on Mars between warm and cold phases also influenced the atmospheric chemistry, affecting the redox states over time. The study, which integrates atmospheric chemistry and climate modeling, offers new predictions that can be tested once Mars rocks are brought back to Earth during the Mars Sample Return mission. Insights from this research could provide valuable information on the conditions that supported prebiotic chemistry and the evolution of planets like Mars. Due to Mars' lack of plate tectonics, its surface today resembles that of ancient times, offering a unique opportunity to study planetary evolution. The work by Adams and her team highlights the importance of understanding the historical climate of Mars and its implications for the potential existence of life on the planet.