NEAR-IR SPECTROSCOPIC ANALYSIS OF MAINBELT X-ASTEROID 469 ARGENTINA

Observations of mainbelt X-type Asteroid 469 Argentina were obtained on the nights of August 19, 20, & 21, 2005 at the NASA IRTF on Mauna Kea, Hawaii. The reflectance spectra were obtained using the SpeX instrument in the low-resolution (or asteroid) mode. Up until the new millennium X-taxonomic class spectra were considered featureless. Within the past decade Clark et al. (2004) has shown the presence of mineralogically diagnostic absorption features in X asteroid spectra and made suggestions as to the nature of these features without any formal interpretation. The degenerate X taxonomy includes P-, M-, and E- type objects, but lacks the albedo information to distinguish between the types.

Now that it is known some X-type objects exhibit diagnostic mineralogical absorption features, we can better assess members of the X-asteroid population. By constraining the mineralogical composition of X-asteroids we can better understand conditions present in the late solar nebula and early solar system. In particular, we hope to obtain a better understanding of the nature and heliocentric distribution of the strong heating event that affected the early inner solar system.

The spectrum of Argentina exhibits no mineralogically diagnostic absorption features. The spectrum is slightly red sloped and has a very low albedo (~.039). Given the albedo information, Argentina falls within the P taxonomic class. Around 1.4 mm and 1.9 mm there are absorption features due to incomplete corrections for atmospheric water vapor. Argentina's surface is most likely composed of primitive dark material. The spectrum of Argentina matches quite well both the overall reflectance and slope of the laboratory reflectance spectrum of terrestrial coal tar residue obtained by Cloutis (1994). The coal tar residue is coal tar with solvent extractable fractions removed (Cloutis 1994). A slight difference is seen shortwards of 0.40 mm, in which the coal tar residue exhibits an absorption feature that is presumed to be due to the aromatic functional group (Cloutis 1994). We suggest the surface materials of 469 Argentina contain abundant carbon compounds with spectral properties similar to coal tar residue.

This work was supported by NASA Planetary Geology & Geophysics grant NNG04GJ86G.