Europa: Have We Been Overestimating Jupiter's Moon?

The moons of our solar system always seem to boast more amazing phenomena than they initially let on, revealing more mysteries and wonders even as we gain a semblance of understanding. With this in mind, astronomers would be hard-pressed to overestimate the wonders of a moon. However, that seems to be exactly what they have done concerning Europa, one of Jupiter's many moons.

The largest planet in our solar system, the gas giant that is Jupiter plays host to a stunning 67 natural satellites - and those are just the ones that we know about. Four of these satellites, called the Galilean moons, are far larger than the rest, and can be seen even with a simple stargazer's telescope.

That is, in fact, how Galileo Galilei identified the massive moons back in 1610, sighting Io, Europa, Ganymede, and Callisto with his 30x magnification telescope.

Europa, the smallest of this quartet, is also often referred to as "Jupiter II" thanks to coloration similar to its host planet. However, modern inspection has revealed that Europa is a dry terrestrial moon colored by what are likely irons and clays. That's far different than the gaseous material causing Jupiter's oranges and whites.

However, it was also suspected that the moon was surrounded by a thick and hot atmosphere of excited gas - an atmosphere that may expel plumes of fine particulate water from time-to-time - much like Saturn's sixth largest, Enceladus.

However, a new analysis of data and imagery collected by the spacecraft Cassini as it flew by Jupiter in 2001 has revealed that most of the hot, excited gas around Europa originates not from the moon itself, but from volcanoes on the nearby moon Io.

"Our work shows that researchers have been overestimating the density of Europa's atmosphere by quite a bit," Don Shemansky, a Cassini UVIS team member who led the study, said in a statement.

According to this latest research, which has since been published in The Astrophysical Journal, Europa may actually be a relatively dull hunk of rock, boasting an exceptionally thin atmosphere that contributes 40 times less oxygen than previously thought to its surrounding environment. The atmosphere, which was already thought to be millions of times thinner than Earth's own atmosphere, is actually about 100 times less dense than previous estimates, and an unlikely source for expelling water vapor.

"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, who co-authored the study. "If eruptive activity was occurring at the time of Cassini's flyby, it was at a level too low to be detectable by UVIS."

"Io is the real monster here," added Shemansky, who explained that the larger Galilean moon could even be contributing to Jupiter's gaseous atmosphere with its violent surface activity.

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