One of the special features of Saturn’s moon, Enceladus, magnificently photographed and analyzed by the Cassini probe, are its geysers: close to its south pole, large fountains made from ice crystals burst from the ocean under the crust and leap into space. Like Enceladus, we know that Jupiter’s moon Europa is also an icy moon. Whether there are also geysers on Europa, however, is anything but clear. If there are geysers on Europa, this would be an advantage because then we could find out very simply what is in the ocean believed to be under Europa’s crust. Since Europa will receive two visitors in the 2020s (the ESA probe JUICE and the NASA probe Europa Clipper will travel to the moon), curiosity is growing from year to year.

The Hubble space telescope has already twice photographed something that has been interpreted as ice fountains. However, the evidence hasn’t convinced everyone, because these images are at the very edge of Hubble’s detection limits and do not always support this conclusion. Therefore, all that currently can be said is that “Europa probably also has geysers.”

A research team has now added new evidence to these images. Experts have looked at old data again, which was delivered by the Galileo probe that was crashed into Jupiter in 2003. Galileo approached up to 400 kilometers from Europa and measured the moon’s weak magnetic field, as well as the plasma density, that is, the number of particles in the area of space Galileo was passing through. The researchers’ conclusion: taken together, the measured values only make sense if there had simultaneously been a geyser eruption from the area that Hubble had already identified before as the source. Thus, during its 1997 flyby, Galileo did not see or photograph the fountains, but did sense them with its measuring instruments.