Submillimeter observations of planets

A new program of ground-based observations at submillimeter (about 400-micron) wavelengths has yielded observations of Mercury, Venus, Mars, Jupiter, and Saturn. We report here observations near planetary conjunctions, which have minimal corrections for atmospheric extinction: Mercury, 361 + or - 65 K; Venus, 231 + or - 35 K; and Saturn, 205 + or - 15 K (based upon the area of the planetary disk), using Jupiter (150 K) and Mars (220 K) as photometric standards. The Mercury observations show that the brightness temperature does not decrease at the submillimeter wavelengths, relative to observations at 3 mm; for Venus, however, the brightness temperature appears appreciably lower than at millimeter wavelengths. The results for Saturn indicate a strong and possibly optically thick contribution from the rings.