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Half of Europa lies beneath a thin veil of water vapor.

And NASA’s Hubble Space Telescope data suggests Europa has a long-term and persistent presence of water vapor above its surface, according to a recent study published in the journal Geophysical Research Letters.

Bizarrely, it’s only present on one hemisphere. But, in addition to bringing us a step closer to finding life on Europa, this information could have a significant influence on forthcoming NASA missions to Europa, and the search for life on alien worlds.

The level of water vapor on Europa surprised scientists

Europa houses a colossal ocean beneath its icy shell, within which there may be signs of indigenous life. If it’s there, we’ve yet to see it, but we have really good reasons for believing that it’s likely. But even if there’s nothing living in Europa’s oceans, knowing of them has advanced astronomers’ grasp of the atmospheric structure of icy moons in the universe, and paves the way for future science missions to Jupiter, designed to look for signs of life in the Jovian system, among other things. Water vapor has been spotted on Europa before, but these were typically associated with plumes erupting extravagantly through the moon’s ice, which Hubble captured in a 2013 photo. Earth has geysers a lot like this, but the ones on Europa extend more than 60 miles high. And these geysers create short-term blobs of water vapor in the moon’s atmosphere, which generate only one-billionth of the surface pressure on Earth’s atmosphere. But the new results reveal similar amounts of water vapor, but spread across a much more vast area of Europa, according to observations from Hubble from 1999 to 2015. This means the Jovian moon supports a long-term water vapor atmosphere, but only on Europa’s trailing hemisphere, or the half of it that is on the opposite side of the direction of the icy body’s orbit of the host planet, Jupiter.

The discovery came from a new analysis of Hubble’s database of archival spectra and images, courtesy of a novel technique that also recently detected water vapor in the atmosphere of Jupiter’s other moon, Ganymede. The study came from Lorenz Roth, at Sweden’s KTH Royal Institute of Technology, of the space and plasma physics department. “The observation of water vapor on Ganymede, and on the trailing side of Europa, advances our understanding of the atmospheres of icy moons,” Roth said, in a press release. “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.”

Persistent water vapor on Europa could influence upcoming NASA missions

Europa is a comparably light shade because of the ice abundant in its surface, and this reflects far more sunlight than Ganymede, which is why Europa’s surface is 60°F cooler than its darker sister moon. On Europa, days can reach high temperatures that are unconscionably low, at -260°F. But even at such brain-numbing temperatures, recent observations show that the water is sublimating, or undergoing a phase change directly from solid to vapor, without passing through the liquid state. And this is happening directly from the surface of Europa, just like on Ganymede.

Roth had to dive deep into Hubble datasets to make this discovery, separating ultraviolet observations of Europa from the years 1999, 2012, 2014, and 2015, which is when the moon was positioned in several orbital positions. These observations were snapped by Hubble’s Imaging Spectrograph (STIS). And the ultraviolet STIS observations enabled Roth to explore the abundance of oxygen on the icy body. This is significant because oxygen is one of the crucial components of water, and Roth found its consistency in the atmosphere by interpolating the strength of emissions at various wavelengths, which enabled him to infer, inductively, that there was a long-term presence of water vapor on Europa. This could have substantial implications for future missions to Europa, including probes like NASA’s Europa Clipper, the European Space Agency’s (ESA’s) Jupiter Icy Moons Explorer (JUICE), and more. Whether or not these probes discover signs of life on Jupiter’s moons, we will certainly learn more about the evolution of the Jovian system, which will teach astronomers more about other Jupiter-like planets orbiting faraway stars.

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