Continuum and spectroscopic observations of asteroid (21) Lutetia at millimeter and submillimeter wavelengths with the MIRO instrument on the Rosetta spacecraft

Abstract

The European Space Agency's Rosetta spacecraft made a close flyby of asteroid (21) Lutetia on July 10, 2010. The spacecraft carries a dual-band radiometer/spectrometer instrument, named MIRO, which operates at 190GHz (1.6mm) and 560GHz (0.5mm). During the flyby, the MIRO instrument measured the temperature of Lutetia in both the northern and southern hemispheres. At the time of the flyby, the northern hemisphere was seasonally sun-lit and warmer than the southern hemisphere. Subsurface (depths from ∼2mm to ∼2cm) temperatures ranged from ∼200K on the northern hemisphere to ∼60K on the southern hemisphere. A lunar-like regolith – very low thermal inertia<20J/(Km2s0.5) in the upper 1–3cm overlaying a layer of rapidly increasing density and thermal conductivity – is required to explain the observations. A spectroscopic search was made for H2O, CO, CH3OH, and NH3 in Lutetia's exosphere but none of the molecules were detected. An upper limit to the water column density was estimated to be <5×1011molecules/cm2 at the time of the flyby.