New data from the Mars orbiter MAVEN has found that the water on Mars moves into the upper atmosphere where it is lost to space much faster than expected.

It had previously been believed that Mars’ water loss only occurred in the lower atmosphere, which is a much slower process. Scientists had also believed that water on Mars would behave as it does on Earth, where temperatures and the atmosphere act to block it from reaching the upper atmosphere where it can easily and quickly be lost to space. Instead, MAVEN found a lot of water in the upper atmosphere.

When the team extrapolated their findings back 1 billion years, they found that this process can account for the loss of a global ocean about 17 inches deep. “If we took water and spread it evenly over the entire surface of Mars, that ocean of water lost to space due to the new process we describe would be over 17 inches deep,” Stone said. “An additional 6.7 inches would be lost due solely to the effects of global dust storms.”

During global dust storms, 20 times more water can be transported to the upper atmosphere. For example, one global dust storm lasting 45 days releases the same amount of water to space as Mars would lose during a calm Martian year, or 687 Earth days.

This data reinforces the theories that Mars once had liquid water on its surface, either as intermittent oceans or as lakes and rivers. Or it suggests that Mars once had a lot more glaciers than it does now, reinforcing a competing theory that glaciers formed the Martian features we on Earth routinely associate with flowing water.