Measurements of the 1.9 cm Thermal Radio Emission from Mercury

MEASUREMENTS of the thermal radio emission from Mercury at 11.3 cm have shown that the brightness temperature is approximately 300° K and does not vary significantly with phase angle 1 . This is not inconsistent with the observed rotation rate of Mercury of 58.4 ± 0.4 days 2 if the thermal inertia of the planet is sufficiently great for the temperature to remain reasonably constant at the depth at which the 11 cm emission originates. It might be expected, however, that a phase effect may exist at shorter wavelengths where the radio emission originates closer to the surface and where the temperature may be more closely related to the instantaneous value of the solar radiation. It is somewhat surprising therefore that Epstein 3 , who observed at 3.4 mm wavelength, claims no phase effect, with an anomalously low temperature of only ∼ 200° K.