Preliminary Infrared Radiometry of the Night Side of Mercury from Mariner 10

Abstract

The infrared radiometer on Mariner 10 measured the thermal emission from the planet with a spatial resolution element as small as 40 kilometers in a broad wavelength band centered at 45 micrometers. The minimum brightness temperature (near local midnight) in these near-equatorial scans was 100°K. Along the track observed, the temperature declined steadily from local sunset to near midnight, behaving as would be expected for a homogeneous, porous material with a thermal inertia of 0.0017 cal cm^(-2) sec^(-½) °K^(-1), a value only slightly larger than that of the moon. From near midnight to dawn, however, the temperature fluctuated over a range of about 10°K, implying the presence of regions having thermal inertia as high as 0.003 cal cm^(-2) sec^(-½) °K^(-1).