Scientists have made a surprising discovery on Mars, namely that water-ice appears to be present under the surface in some regions near its equator.

The discovery, led by Jack Wilson from the Johns Hopkins University Applied Physics Laboratory in Maryland, was made by looking at old data from NASA’s Mars Odyssey spacecraft. The findings are published in the journal Icarus.

They studied a region on Mars called the Medusae Fossae Formation, which is very nearly slap-bang on the equator. The spacecraft had taken images of this and other regions on Mars, but by using image-reconstruction techniques, the team were able to essentially cut the spacecraft’s altitude in half to get a better view.

While Odyssey can’t directly detect water, it can detect hydrogen, and that’s what the researchers found. It was way more hydrogen than would be expected at the equator, suggesting that water-ice is buried under the surface.

Such a finding can’t definitively be identified as water-ice yet, as there are possible other sources. These include possible deposits of hydrated salts, or the hydrogen could come from other compounds like chlorine.

But elsewhere on Mars, we’ve seen large amounts of hydrogen where we do expect to find water-ice. In 2002, for example, Odyssey found abundant hydrogen at high latitudes, which was confirmed as being from water in 2008 by NASA’s Phoenix lander.

However, while this makes sense at the poles, it does not at the equator. Ice here should not be able to last more than about 125,000 years. This is because it should sublimate (turn from solid to gas) into the atmosphere, as the pressure here is just 0.6 percent Earth’s.

If ice is confirmed to be present at the equator, it would suggest the tilt of Mars has changed. At the moment, it’s at an angle of 25 degrees, but it was about 45 degrees a few million years ago, which could explain the presence of ice.

Water-ice here would have a number of benefits. There’s a small chance it could rise to the surface and melt, providing a habitat for microbes. Equally enticingly, it could be used as fuel by future human explorers, reducing the amount of fuel they need to take with them.

“So for now, the signature remains a mystery worthy of further study, and Mars continues to surprise us,” Wilson said in a statement.