The long-dead Galileo spacecraft may have flown right through a plume of water spurting up from the surface of Jupiter’s moon Europa. A new analysis of 20-year-old data shows just that, adding evidence that the much-contested fountains actually exist there.
In the late 1990s, the Galileo probe detected an unusually warm area on Europa’s surface. Astronomers connected that anomaly to a potential plume of water emerging from the moon’s subsurface ocean. In recent years, images from the Hubble Space Telescope have also shown what appear to be plumes around that same spot.
But those observations were low-resolution and inconclusive, and other analyses and observations have seen no trace of these geysers. “The evidence for the plumes has always been on the intriguing level, but not definitive,” says at the California Institute of Technology in Pasadena.
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Flipping magnetic fields
A new analysis of magnetic field data from Galileo strengthens the case Europa is indeed spewing out liquid water. at the University of Michigan in Ann Arbor and his colleagues examined data from the probe’s closest flyby in more detail than ever before.
The data was collected in 1997, but we didn’t know what to look for within it until last year, when the Cassini spacecraft flew through a similar plume on Saturn’s moon Enceladus.
“It’s hard to notice unless you’re looking for it. These plumes are very diffuse – it’s not like you’re flying through a firehose and you didn’t notice it,” says at NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, California.
As Galileo flew just 206 kilometres above Europa’s surface, the magnetic field it was monitoring flipped in direction and decreased sharply in strength. When a magnetic field sweeps past a jet of water, it can create an electrical current in the jet. That current, in turn, generates another magnetic field that counters the first. Cassini saw this process in action at Enceladus, and now the researchers think Galileo saw it on Europa.
Galileo also encountered a sudden spike the density of the plasma surrounding Europa right at the beginning of the strange magnetic signal. The researchers suggest this is because the plasma was piling up at the edge of the plume, unable to flow through or around it. This is some of the strongest evidence yet, says at JPL.
Searching for life
Plumes on this icy moon could help us find signs of life in its subsurface ocean. “Looking at plume material is a way to access the ocean chemistry that might help us reveal whether or not Europa’s ocean is habitable, or even inhabited,” says Hand.
But because Europa is so small and distant, we may not know for sure if the plumes are real until we go there. “We’re not gonna believe it for sure until we go back with a spacecraft and take a picture,” says Phillips.
Both NASA and the European Space Agencies have missions in the works to visit Europa in the 2020s, so we may be able to take that picture soon.
Nature Astronomy
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