Langley solid particles that strike the spacecraft. Celestial mechanics J. D. Anderson, JPL Measurement of the masses of Jupiter and the Galilean satellites with high precision. Ultraviolet photometry D. L. Judge, U. Measurement of ultraviolet Southern California emissions of the Jovian atmosphere and from circumsatellite gas clouds. Imaging T. Gehrels, U. Arizona Reconnaissance imaging of photopolarimetry Jupiter and its satellites; study of atmospheric dynamics. Jovian infrared G. Münch, Caltech Measurement of Jovian thermal structure temperature and heat budget; determination of helium to hydrogen ratio. S-Band occultation A. J. Kliore, JPL Probes of structure of Jovian atmosphere and ionosphere. Atmospheric Structure. Several Pioneer investigations yielded information on the variation of atmospheric temperature and pressure in the regions above the ammonia clouds. Near the equator, at a level where the atmospheric pressure is the same as that on the surface of the Earth (1 bar), the temperature is -108° C. About 150 kilometers higher, where the pressure drops to 0.1 bar, is the minimum atmospheric temperature of about -165° C. Above this point the temperature rises again, reaching about -123° C near a pressure level of 0.03 bar. Presumably this temperature rise is due to absorption of sunlight by a thin haze of dust particles in the upper atmosphere of Jupiter. Internal Structure. The measurements of the amount of helium, of the gravitational field, and of the size of the internal heat source on Jupiter greatly clarified scientists’ understanding of the deep interior of the planet. Calculations showed that the core of Jupiter must be so hot that hydrogen cannot become solid, but must remain a fluid throughout the interior. Even at great depths, therefore, Jupiter does not have a solid surface. The theory that the Great Red Spot was the result of interactions with a surface feature below the clouds thus became untenable. Whatever its exact nature, the Red Spot must be a strictly atmospheric phenomenon.