Europa's atmosphere is thinner than previously thought
Europa is considered one of the most exciting destinations in the Solar System for future exploration because it shows strong indications of having an ocean beneath its icy crust.
Long, linear cracks and ridges crisscross Europa's surface, interrupted by regions of disrupted terrain where the surface ice crust has been broken up and re-frozen into new patterns.
Colour variations across the surface are associated with differences in geologic feature type and location. The polar regions are bluer than the more equatorial latitudes, which appear more white. This colour variation is thought to be due to differences in ice grain size in the two locations.
Europa has a crust made up of blocks, which are thought to have broken apart and 'rafted' into new positions, as shown in the image on the left. Image Credit: NASA/JPL/University of Arizona
Europa is surrounded by very tenuous hot, excited gas. Indications of possible plume activity were reported in 2013 by researchers using NASA's Hubble Space Telescope.
Data collected by Cassini's ultraviolet imaging spectrograph (UVIS) as Cassini sped through the Jupiter system en route to Saturn, shows that most of the plasma around Europa originates not from the moon itself, but from volcanoes on the nearby moon Io.
The researchers calculate that Europa contributes 40 times less oxygen than previously thought to its surrounding environment, making it less likely that the moon is regularly venting plumes of water vapour high into orbit.
"Our work shows that researchers have been overestimating the density of Europa's atmosphere by quite a bit," said Don Shemansky, a Cassini UVIS team member with Space Environment Technologies, who led the study.
The moon's tenuous atmosphere, which was already thought to be millions of times thinner than Earth's atmosphere, is actually about 100 times less dense than those previous estimates.
The data shows no evidence of plume activity occurring at the time of the flyby, so if there is plume activity, it is likely intermittent.
Ongoing plume activity at Europa, as Cassini has observed at Saturn's moon Enceladus, would inject large amounts of water vapour into the area around Europa's orbit if the plumes were large enough, but that is not what UVIS observed.
"It is certainly still possible that plume activity occurs, but that it is infrequent or the plumes are smaller than we see at Enceladus," said Amanda Hendrix, a Cassini UVIS team member with the Planetary Science Institute, who co-authored the new study.
Missions that visited Jupiter prior to Cassini provided strong indications that Io is the major contributor of material to the environment around Jupiter, and indicated a hot, low density plasma surrounding Europa. The new results confirm that. "Io is the real monster here," Shemansky said.
"Europa is a complex, amazing world, and understanding it is challenging given the limited observations we have," said Curt Niebur, Outer Planets program scientist at NASA Headquarters. "Studies like this make the most of the data we have and help guide the kinds of science investigations NASA should pursue in the future."
The Hubble Space Telescope is currently conducting an extensive six-month long survey looking for plume activity, and NASA is studying various possible Europa missions for future exploration.