Early in martian history, the climate was warm enough for potentially habitable lakes and rivers of water to exist. However, roughly 3.6 billion years ago, the climate shifted from being habitable to inhabitable when liquid water disappeared from the surface. Although the cause of this transition remains unknown, new findings suggest that the loss of an important greenhouse gas transformed Mars into a dry planet.

The past martian climate can be analyzed using patterns and shifts in the spatial distribution of precipitation-fed rivers. A team of scientists led by Edwin Kite at the University of Chicago searched for water-influenced landforms using satellite images of the martian surface, including those from the 1972 Mariner 9 mission. By mapping overlapping riverbeds, the team reconstructed a timeline of paleo-river activity with respect to elevation and latitude over billions of years. Complementary to their climate reconstruction, they used a global climate model to simulate various atmospheric conditions and observed how these would affect rivers. They looked at several parameters in their models that would cause Mars to be temporarily warm enough for liquid water to exist. The results suggest that changing the amount of carbon dioxide (CO₂) in the atmosphere would not affect the stability of water. Instead, the loss of some important greenhouse gas must have caused the martian atmosphere to thin, forcing conditions to go from wet to dry.

Although the identity of this greenhouse gas remains unknown, these results show that non-CO₂ gases played a dominant role in changing the water flow distribution on Mars. The team recommends that future sites explored by Perseverance include ancient riverbeds to better understand Mars’ climate change. Since Mars is the only planet whose climate is known to have changed from habitable to inhabitable, understanding what caused this shift could be crucial, given the current state of climate change on Earth. READ MORE