ULTRAVIOLET DISCOVERIES AT ASTEROID (21) LUTETIA BY THEROSETTAALICE ULTRAVIOLET SPECTROGRAPH

Abstract

The NASA Alice ultraviolet (UV) imaging spectrograph on board the ESA Rosetta comet orbiter successfully conducted a series of flyby observations of the large asteroid (21) Lutetia in the days surrounding Rosetta's closest approach on 2010 July 10. Observations included a search for emission lines from gas, and spectral observations of the Lutetia's surface reflectance. No emissions from gas around Lutetia were observed. Regarding the surface reflectance, we found that Lutetia has a distinctly different albedo and slope than both the asteroid (2867) Steins and Earth's moon, the two most analogous objects studied in the far ultraviolet (FUV). Further, Lutetia's ~10% geometric albedo near 1800 A is significantly lower than its 16%-19% albedo near 5500 A. Moreover, the FUV albedo shows a precipitous drop (to ~4%) between 1800 A and 1600 A, representing the strongest spectral absorption feature observed in Lutetia's spectrum at any observed wavelength. Our surface reflectance fits are not unique but are consistent with a surface dominated by an EH5 chondrite, combined with multiple other possible surface constituents, including anorthite, water frost, and SO2 frost or a similar mid-UV absorber. The water frost identification is consistent with some data sets but inconsistent with others. The anorthite (feldspar) identification suggests that Lutetia is a differentiated body.