Eclipse Measurements of Io's Sodium Atmosphere
NICHOLAS M. SCHNEIDER 1,
DONALD M. HUNTEN 1,
WILLIAM K. WELLS 1, and
LAURENCE M. TRAFTON 2
1 Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ 85721.
2 Astronomy Department, University of Texas, Austin, TX 78712.
The satellites of Jupiter eclipsed each other in 1985, and these events allowed an unusual measurement of the sodium in Io's extended atmosphere. Europa was used as a mirror to look back through the Io atmosphere at the sun. The measured column abundances suggest that the atmosphere is collisionally thin above 700 kilometers and may be collisionally thin to the surface. The sodium radial profile above 700 kilometers resembles a 1500 kelvin exosphere with a surface density near 2 x 104 sodium atoms per cubic centimeter, but a complete explanation of the dynamics requires a more complex nonthermal model: the calculated loss rates suggest that the atmosphere is being replaced on a time scale of hours.
Submitted on February 27, 1987
Accepted on July 7, 1987
