Mid-infrared Spectra of Binary Asteroids With Spitzer/IRS

To date, 162 asteroids are known to be binary or multiple systems. Insights, such as the size and shape of their components, the nature of their surface, their bulk density are the key to understanding how these multiple asteroidal systems formed. We obtained 19.9h of observations using the Spitzer/IRS instrument to observe 26 known binary systems: 17 main-belt asteroids and 9 near-Earth asteroids. The low-resolution (R=100-600) thermal spectra (5-42 um) recorded by this instrument provide a powerful means of determining the asteroid size and albedo, as well as other physical properties, such as shape, spin state, thermal inertia, and surface roughness. Additionally, thermal emissivity spectra have strong silicate features (Si-O stretch and bend fundamentals at 10 and 20 um), which can be used to characterize the surfaces of asteroids and infer the best meteorite analog. To be able to recover the geometry of the binary system at the time of Spitzer observations, we initiated a large campaign of photometric observations with a worldwide network of observatories to measure lightcurves. We also recorded high S/N reflectance spectra in the visible and near-infrared using the IRTF and Lick telescopes. This survey is currently being conducted and the thermal infrared spectra of 23 binary asteroids have been successfully recorded. We will present their analysis emphasizing how these bulk density measurements are related to their taxonomic classes. Preliminary results suggest that C-type binary asteroids are composed of lower density material (bulk density of 1.2 g/cm3), than S-type asteroids ( 2.2 g/cm3), and the M-type Kalliope ( 3.3 g/cm3, see Descamps et al., Icarus, 2008). Their bulk density is systematically lower than their meteorite analogs suggesting a significant porosity of 30-50%. This research was supported by NASA PAST NNX07AP70G and it is based on Spitzer space telescope, which is operated by JPL under a contract with NASA.