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

The European Space Agencyʹs 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 190 GHz (1.6 mm) and 560 GHz (0.5 mm). 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 ∼2 mm to ∼2 cm) temperatures ranged from ∼200 K on the northern hemisphere to ∼60 K on the southern hemisphere. A lunar-like regolith – very low thermal inertia<20 J/(K m2 s0.5) in the upper 1–3 cm 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 Lutetiaʹs exosphere but none of the molecules were detected. An upper limit to the water column density was estimated to be <5×1011 molecules/cm2 at the time of the flyby.