2011 Annual Science Report
University of Hawaii, Manoa Reporting | SEP 2010 – AUG 2011
Surfaces of Trojan Asteroids
With a total mass similar to the main asteroid belt, the Trojan asteroids are a major feature in the solar system. Thermal infrared (TIR) emission spectra of Trojan asteroids obtained with the Spitzer space telescope exhibit a 10-µm emissivity plateau that closely resembles the emission feature of active comets. This result is very puzzling because the light scattering properties of a regolith aster-oid surface is significantly different from a diffuse dust cloud of comet. It has been suggested that the Trojan surfaces may consist of fine-grained silicates suspended in a transparent matrix.
To understand the surface structure of Jovian Trojans, team member Bin Yang worked with Paul Lucey (University of Hawaii) to construct a modified Mie/Hapke hybrid model to explain the Trojan spectra. In addition to the archival thermal IR spectra of the large Trojans, NIR spectra of these Trojans were also obtained during Yang’s previous survey. Their goal was to determine whether the radiative transfer model could simultaneously fit the Trojan spectra both in the NIR and TIR spectral regions. Also, Yang collaborated with Timothy Glotch (Stony Brook University) to acquire a series of laboratory infrared emissivity spectra of olivinesaltcarbon mixtures to examine the potential weaknesses in the thermal spectral model. Their results show that the Trojan spectra over a wide wavelength range can be consistently explained by ~1wt% silicates and 2-10 wt% highly absorbing material (such as carbon or iron) suspended in a transparent matrix if the particles are ~1 μm in size and smaller. Finegrained silicates, carbon, and iron (the latter found in abundance in Stardust samples from Wild 2) are common products of sublimation of comets and asteroid collisions and are expected contaminants on outer solar system objects. The transparent matrix is consistent with a deposit of salt on the surface of the large Trojans. They suggest that early in the solar system history shortlived radionuclides could have heated these icerich objects causing melting, internal circulation of water, dissolution of soluble material, briny water volcanism driven by internal volatiles, and finally sublimation of the frozen brines leaving a lag deposit of salt. Over solar system history fine dust from comets and asteroids contaminated and colored these salty surfaces to produce the spectral properties observed today.