One of the most promising locations to search for life in our solar system isn’t on a planet — it’s on a moon, specifically Jupiter’s moon Europa. Now, new evidence from the Hubble Space Telescope shows that there seems to be persistent water vapor spread across one of Europa’s hemispheres, where it is being sublimated from ice on the surface. But why this vapor is only present on one side of the moon remains unclear.

Europa is particularly promising as a location to look for life because it is thought to host a liquid ocean beneath the icy crust of its surface. Previously, scientists had detected plumes of water vapor on Europa which seem to be erupting through the ice, similar to how geysers throw up water on Earth. But this recent discovery suggests there is a second source of water vapor on Europa as well.

The recent research looked at Hubble data collected between 1999 and 2015 and found long-term presence of water vapor spread over a large area of the moon. It is present only in Europa’s trailing hemisphere, which is the half of the moon which is opposite the direction in which it is moving.

A similar analysis to the Hubble data about Europa was also recently used to find water vapor in the atmosphere of Jupiter’s moon Ganymede by the same researcher, Lorenz Roth of the KTH Royal Institute of Technology, Space and Plasma Physics, Sweden.

“The observation of water vapor on Ganymede, and on the trailing side of Europa, advances our understanding of the atmospheres of icy moons,” said Roth in a statement. “However, the detection of a stable water abundance on Europa is a bit more surprising than on Ganymede because Europa’s surface temperatures are lower than Ganymede’s.”

Roth found that, even in the chilly temperatures of -260 degrees Fahrenheit on Europa, the ice was sublimating (changing from solid to gas without becoming a liquid) in the sunlight. However, the strangest part of this phenomenon — why it is happening only on one hemisphere — remains a mystery.

To learn more about this intriguing moon, the European Space Agency’s Jupiter Icy Moons Explorer (JUICE) mission will be traveling there after its launch next year, along with NASA’s Europa Clipper mission, set to launch in 2024.

The research is published in the journal Geophysical Research Letters.

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