Europa, emission from oxygen and hydrogen, and inferred jets of water

In the blue areas near the south pole of Europa, a satellite of Jupiter,
aural emissions from oxygen and hydrogen have been
detected by the Hubble Space Telescope. NASA image.

Just as we have aurora at the north pole because charged particles enter Earth's magnetic field, Europa has an aurora at its south pole because it is in the intense magnetic field of Jupiter. When atomic oxygen and hydrogen are excited by the magnetic field, they produce an aural glow that can be detected spectroscopically. The oxygen and hydrogen have been interpreted as being the products of water molecules torn apart by electrons along the magnetic field lines.

Top row: Images of the hemispheres
of Europa; other rows: combined images of
the hydrogen and oxygen emissions. This is
Figure 1 in the Science Express paper

   











The measurements were made from Hubble Space Telescope in December 2012, nearly a year before reported this week in Science Express and summarized in this NASA press release. The scientists involved (Lorenz Roth et al.) have stressed the need for caution because the Hubble Space Telescope was "pushed to its limits to see this very faint emission." Scientists are excited about this for two reasons. First, if confirmed, it would mean that Europa becomes the second moon spewing out water plumes (Enceladus is the other). Second, because there is good evidence that an ocean of liquid water exists under the surface of Europa, the plumes would be a way to sample its composition without having to drill through a thick crust. (The evidence for subsurface water is in the surface morphology and magnetometer measurements.)
     The plumes vary in intensity with the orbital position of Europa, but not in a way that is easily explained. They are active only when the moon is the farthest from Jupiter, instead of the more logical position closest to Jupiter.  The scientists postulate that the cracks that emit the water are closed when Europa is closest to Jupiter, and open when it is farthest away. The plumes extend up to about 125 miles altitude and the erupted "water" falls back onto the surface rather than escaping into space.  In the figure to the right, the detected "jets" in December 2012 are compared with 1999 and November 2012 images when the particles were not detected. In December 2012 the plume was near apocenter, and the other two times, close to pericenter, lending support to predictions of tidal modeling.