The planet shows clear evidence for weathering and hydrothermal processes altering the surface in various areas, she adds. Indeed, Gale Crater and its looming Mt. Sharp, home to NASA's latest Mars rover, Curiosity, displays features that are hard to explain unless flowing or standing water were present on the surface.
The questions, she says are these: Which process dominated on a planetary scale at the time, "and what does that have to say about the overall climate?"
The team's eruptive proposition comes at a time when the notion of a persistent warm-wet-climate during the planet's first billion years has been taking hits in other ways.
For instance, a team of researchers from the United States and France has modeled Mars' early climate, when the sun would have been weaker. They conclude that the climate would have been too cold to sustain liquid water on the surface for very long. Instead, over time water ice would migrate to the poles, forming vast icecaps. Limited warming could occur from large impacts or volcanic eruptions, releasing enough water to carve features seen on the southern highlands.
Others have noted that water-carved features in widely spaced locations formed at vastly different times, weighing against a single span of wet climate as the source of flow that formed them.
Dr. Ehlmann and Dr. Meunier had teamed up on research last year that indicated clays could form through the interaction of water leaching through hot rocks. The clays resulting from hydrothermal activity would tend to show composition similar to clays formed at the surface.
But, Ehlmann recalls, Meunier mentioned the volcanic process, in which water trapped in lava interacted with the superhot material as it cooled and released its gases. Where water was involved, the interaction formed clays that filled nooks and crannies in the cooling rock. And its chemical signature was similar to the clays formed on the surface and via hydrothermal action. The evidence came from volcanic clays found in French Polynesia and in Brazil, which had experienced ancient eruptions where magma would flood the landscape from fissures in the rock.