In December 2013, NASA’s Hubble Space Telescope spotted evidence of water vapor venting off the south polar region of Jupiter’s icy moon Europa. The best explanation, researchers thought at the time, would be water plumes erupting off the moon’s surface; previous work has indicated the existence of an ocean under the icy crust.
Now, astronomers have made the startling announcement that those giant geysers -- which may have spouted to an altitude of 201 kilometers -- have mysteriously vanished. Follow-up observations with Hubble earlier this year showed no signs of the plumes, Space.com reports.
Researchers aren’t sure why they’ve vanished, though there are several possible ways to explain it. Perhaps like volcanoes on Earth, Europa’s water vapor geysers are sporadic; or perhaps the plumes are sometimes simply too small to see.
Or maybe the plumes are only visible to Hubble at certain times. "It could be just the way that we use the auroral emissions coming from those plumes at the wavelengths of light that we use with Hubble," Southwest Research Institute’s Kurt Retherford tells Space.com. These events depend on Jupiter’s plasma environment, and maybe Hubble spotted the plumes just as they were being lit up excited particles.
Still... perhaps the geysers never existed in the first place, and Hubble’s 2012 observations were an artifact of some sort. Retherford thinks that’s unlikely, and his team will be looking for plumes on Europa again from November through April.
Meanwhile, as far back as 2005, jets of water vapor have been spotted spewing off the surface of Saturn’s moon Enceladus by NASA's Cassini orbiter. The plumes on these two moons have remarkably similar abundances of water vapor. But because Europa has a roughly 12 times stronger gravitational pull than Enceladus, the minus-40-degree-Celsius vapor for the most part doesn’t escape into space as it does on Enceladus. Rather, it falls back onto the surface.