EVIDENCE FOR A LOW SURFACE-TEMPERATURE ON PLUTO FROM MILLIMETER-WAVE THERMAL EMISSION MEASUREMENTS

Thermal continuum emission from the Pluto-Charon system has been detec ted at wavelengths of 800 and 1300 micrometers, and significant upper limits have been obtained at 450 and 1100 micrometers. After the subtr action of emission from Charon, the deduced surface temperature of muc h of Pluto is between 30 and 44 kelvin, probably near 35 to 37 kelvin. This range is significantly cooler than what radiative equilibrium mo dels have suggested and cooler than the surface temperature derived by the Infrared Astronomy Satellite. The low temperature indicates that methane cannot be present at the microbar pressure levels indicated by the 1988 stellar occultation measurements and that the methane featur es in Pluto's spectrum are from solid, not gas-phase, absorptions. Thi s result is evidence that Pluto's atmosphere is dominated by nitrogen or carbon monoxide rather than menthane.