THE IONOSPHERE OF EUROPA FROM GALILEO RADIO OCCULTATIONS

The Galileo spacecraft performed six radio occultation observations of Jupiter's Galilean satellite Europa during its tour of the jovian sys tem, In five of the six instances, these occultations revealed the pre sence of a tenuous ionosphere on Europa, with an average maximum elect ron density of nearly 10(4) 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. S uch an ionosphere could be produced by solar photoionization and jovia n magnetospheric particle impact in an atmosphere having a surface den sity of about 10(8) electrons per cubic centimeter. If this atmosphere is composed primarily of O-2, then the principal ion is O-2(+) and th e neutral atmosphere temperature implied by the 240-kilometer scale he ight is about 600 kelvin. If it is composed of H2O, the principal ion is H3O+ and the neutral temperature is about 340 kelvin. In either cas e, these temperatures are much higher than those observed on Europa's surface, and an external heating source from the jovian magnetosphere is required.