Mars Orbiter Images Suggest Water Flowed on Mars Far Later Than Once Thought

Scientists long thought these depressions where caused by surface ice that sublimated into vapor when much of Mars' atmosphere disappeared around 4 billion years ago. But note the tiny depression connecting the lower right of the center depression to the top of its neighbor to the southeast; researchers now believe liquid water carved channels like this some 800 million years after liquid water was thought to be unsustainable there. NASA

Spirit is still stuck in the sand, and Opportunity’s future beyond Mars’ next solstice is unclear, but the Mars Reconnaissance Orbiter continues to beam back groundbreaking images from high above the Red Planet. Using images taken from NASA’s MRO, researchers at Imperial and University Colleges London have determined that water-filled lakes existed on the Martian surface 800 million years later than previously thought.

Researchers have long thought Mars once sustained water on its surface, and discoveries by Spirit and Opportunity bolstered the idea, but astronomers generally agreed that between 4 billion and 3.8 billion years ago most of the Martian atmosphere vanished, rendering the surface cold and dry. This ushered in what’s known as the Hesperian Epoch, a period once thought too dry and cold to sustain liquid water at the surface.

But after studying some flat depressions above Ares Vallais, a giant canyon stretching some 1,250 miles along the Martian equator, researchers discovered what looked like drainage channels connecting them. Previous attempts to determine the origin of these depressions proved fruitless, but researchers theorized that nearby depressions were caused by sublimation, in which ice in the ground had changed directly into water vapor without passing to a liquid state. The evaporation of said ice would allow the soil to cave in, creating depressions.

The discovery of the interconnecting channels raises strong evidence for a new hypothesis: the depressions were not simply ice deposits bankrupted by sublimation, but lakes of liquid water, the channels running in between them resulting from periods when melting ice caused their banks to overrun. Counting crater impacts around the lakes, scientists then determined that they were about 3 billion years old, existing a full 800 million years after the Hesperian Epoch began.

Determining that Mars’ liquid-water heyday was still a good 3 billion years ago doesn’t shake what we know about our galaxy right to the core. But it does potentially correct a 1-billion-year accounting error in the Martian timeline and could dictate some good landing zones for future rover missions as well as alter the way we go about seeking former life forms on out big red neighbor. It also gives us a good excuse to collect some sweet aerial shots of the Martian surface into a gallery, which you can view here.