Content area
Full text
The Galileo spacecraft performed six radio occultation observations of Jupiter's Galilean satellite Europa during its tour of the jovian system. In five of the six instances, these occultations revealed the presence of a tenuous ionosphere on Europa, with an average maximum electron density of nearly 104 per cubic centimeter near the surface and a plasma scale height of about 240 + 40 kilometers from the surface to 300 kilometers and of 440 60 kilometers above 300 kilometers. Such an ionosphere could be produced by solar photoionization and jovian magnetospheric particle impact in an atmosphere having a surface density of about 108 electrons per cubic centimeter. If this atmosphere is composed primarily of O2, then the principal ion is OZ and the neutral atmosphere temperature implied by the 240-kilometer scale height is about 600 kelvin. If it is composed of H20, the principal ion is H30+ and the neutral temperature is about 340 kelvin. In either case, these temperatures are much higher than those observed on Europa's surface, and an external heating source from the jovian magnetosphere is required.
The Galileo spacecraft is in orbit about Jupiter, studying Jupiter and the Galilean satellites (lo, Europa, Ganymede, and Callisto). Of these, only lo was known to have an atmosphere, which was observed by a radio occultation (1 ) of Pioneer 10 in 1973 (2). Observations and theoretical considerations suggest that Europa may have an atmosphere that originates from frozen surface water ice, most likely produced by particle impact (3, 4). Therefore, any atmosphere must consist of some mixture of H20, H, H2, OH, OZ, and O. Early Pioneer 10 observations suggested an atomic oxygen column density of around 1 x 1013 cm-2 (5), but these measurements may have been contaminated. Recent Hubble Space Telescope (HST) observations of the 13041356 A lines of atomic oxygen were used to deduce an OZ column density of about 1.5 x 1015 cm-2 (6). Recent Galileo measurements at Ganymede and Callisto indicated the presence of atomic hydrogen, with densities on the order of 1 x 104 cm-3 (7), which might suggest similar abundances at Europa.
A search for an atmosphere on Europa was carried out when Galileo was occulted by Europa three times: on 19 December 1996 (E4) and on 20...