HEAT FROM PLUTO

Submillimeter photometry from the James Clerk Maxwell Telescope on Mauna Kea is used to study thermal emission from Pluto. The brightness temperatures at 800 and 1300 mum are T(B)=42 +/- 5 K and TB=35 +/- 9 K, respectively, essentially confirming a prior measurement of T(B)=39 +/- 3 K at 1200 mum by Altenhoff et al. [A&A, 190, L15 (1988)]. These are substantially smaller than brightness temperatures obtained previously at 60 and 100 mum [Aumann & Walker, AJ, 94, 1088 (1987); Sykes et al., Science, 237, 1336 (1987)], showing that the surface of Pluto is nonisothermal, nongrey, or both. The data are incompatible with nitrogen-covered, isothermal T approximately 35 K Pluto models [Owen et al., Science, 261, 745 (1993)]. We suggest that the surface may be divided into cold regions coated by nitrogen ice plus warmer regions devoid of nitrogen, and we tentatively identify the latter with optically dark patches on Pluto's surface.