Publications of the Astronomical Society of the Pacific 101:122-125, January 1989 ON THE WAVELENGTH DEPENDENCE OF APPARENT EMISSIVITY OF ASTEROID MICROWAVE EMISSIONS: CERES AND VESTA

The standard model procedure employed for the determination of asteroid photometric properties from single-wavelength infrared flux-density measurements has long been suggested for use with single-wavelength microwave flux-density measurements. Microwave observers, however, have usually assumed that the radio continuum spectra of the asteroids should closely follow a rapidly rotating blackbody curve. Recent observations of the microwave continuum spectrum of asteroids show that the cm-wavelength emissivity is nearly 25% lower than both the mm-wavelength and infrared emissivity (which is nearly identical to the blackbody value) and constant between 2 and 20 cm. Clearly, cm-wavelength flux-density measurements are not consistent with the infrared-based albedo data used in past analyses of the centimeter fluxes. In the mm-wavelength range the apparent emissivity properties of the surface are close to what is observed in the infrared. Thus, mm-wavelength measurements give photometric properties which closely resemble those derived from the infrared. It is therefore necessary to make some major modifications to the conventional physical parameters used in treating single-wavelength observations (even over an extended range of phase angles) when the observing wavelength is longer than about 1 cm. The major modification required is the adoption of an emissivity of 0.8 for wavelengths longer than 1 cm. These effects are evident in the apparent emissivities determined from recently published observations of Ceres and Vesta.