Io: An Intense Brightening Near 5 Micrometers

Science  16 Feb 1979:
Vol. 203, Issue 4381, pp. 643-646
DOI: 10.1126/science.203.4381.643


Spectrophotometric observations of the jovian satellite Io on 20 and 21 February 1978 (Universal Time) were made from 1.2 to 5.4 micrometers. Io's brightness at 4.7 to 5.4 micrometers was found to be three to five times greater at an orbital phase angle of 68° than at orbital phase angles of 23° (5.5 hours before the brightening) and 240° (20 hours after the brightening). Since the 5-micrometer albedo of Io is near unity under ordinary conditions, the observed transient phenomenon must have been the result of an emission mechanism. Although several such mechanisms were examined, the actual choice is not clear.