Upper limits for an atmosphere on Io.

The May 14, 1971, occultation of beta Scorpii C by Io was successfully observed in ultraviolet light near Kingston, Jamaica. Within the limits established by time resolution and the signal-to-noise ratio, both the disappearance and reappearance were found to be instantaneous. Upper limits for the surface pressure for N2, CH4, and H2 atmospheres are 0.09, 0.13, and 44. microbar, respectively. The corresponding number densities are 6.2 x 10 to the 12th power, 9.4 x 10 to the 12th power, and 3.2 x 10 to the 15th power per cu cm. An isothermal atmosphere at a temperature of 100 K was assumed.