The 2009 Wesley impact: Asteroidal or Cometary?

We present Earth-based observations of Jupiter from 1994 and 2009, which respectively capture the effects on Jupiter’s atmosphere by the impacts of Comet D/Shoemaker-Levy 9 (SL9) and the impact by an unknown object whose visible impression on Jupiter’s appearance was discovered by Anthony Wesley.  Previous studies have suggested the 2009 impactor was by an asteroid on the basis of differences in Jupiter’s atmospheric response compared to the 1994 impact by SL9.  These differences include detections of 9.1-μm silicate features in the 2009 impact site (Orton et al., 2010, Icarus 211, 587-602) and the fact the 2009 debris field shrank faster (Hammel et al., 2010, ApJL 715, L150-L154), both of which suggest the 2009 impactor was more rocky/refractory in composition.  However, Schenk et al. 2004 (Jupiter: The Planet, Satellites and Magnetosphere, Bagenal, Dowling, McKinnon, 427-456) state that comets are orders of magnitude more likely to impact Jupiter than asteroids since Jupiter should have cleared its orbit a long time ago. Thus, either (1) the 2009 impact was caused by an asteroid and therefore a statistical fluke, (2) Jupiter-Family Comets (JFCs) are a highly heterogeneous population, with some containing rocky/refractory interiors hidden from remote-sensing, or (3) there is a population of asteroids among bodies classified as JFCs. In order to explore these hypotheses, we performed a comparative spectral re-analysis of broadband imaging and low-resolution spectra measured during/after the 1994 and 2009 impacts. The comparison used consistent procedures for reduction and calibration of the data, atmospheric models, radiative-transfer software and spectroscopic line data in order to facilitate direct comparisons between 1994 and 2009 events.