A retrospective analysis of mid-infrared observations of the Comet D/Shoemaker-Levy 9 and Wesley impacts on Jupiter

Abstract

We present a retrospective analysis of Earth-based mid-infrared observations of Jupiter capturing the aftermath of the impacts by Comet D/Shoemaker-Levy 9 (henceforth SL9) in July 1994 and the unknown object previously termed the “Wesley impactor” in July 2009. While the effects of both impacts on Jupiter’s atmosphere have been reported previously, we were motivated to re-examine both events using consistent data reduction and analysis methods to enable robust, quantitative comparisons. To study the aftermath of the SL9 impacts, we examined infrared (7.8–20.5μm) spectrophotometry of Jupiter measured by the MIRAC (Mid-Infrared Camera) on NASA’s Infrared Telescope Facility on July 20, 21 1994 and low-resolution (R = 100) N-band (7–13μm) spectroscopy measured by SpectroCam-10 on the Palomar Telescope on July 22 1994. To study the aftermath of the Wesley impact, we examined low-resolution (R = 100) N- and Q-band (R = 80, 17–25μm) spectroscopy recorded by Gemini South Telescope’s T-ReCS (Thermal-Region Camera Spectrograph). We analyzed the observations with two independent analyses: (1) a least-squares search over a grid of candidate mineral species to determine the composition of impact residue and (2) a radiative transfer analysis to derive atmospheric information. We observe that the SL9 impact sites are enhanced in stratospheric CH4 emissions at 7.9μm, due to shock heating and adiabatic compression from plume re-entry, and from 8.5–11.5μm due to stratospheric NH3 emission and non-gaseous cometary material, in agreement with previous work. In the G impact site, we derive NH3 concentrations of 5.7−2.8+4.5 ppmv at 30 mbar. In new findings, we find that the SL9 impact sites also exhibit a non-gaseous emission feature at 18–19μm. The non-gaseous emission at 8.5–11.5μm and 18–19μm emission result is best reproduced by predominantly amorphous olivine and (obsidian) silica at similar abundances. The Wesley impact site exhibits enhanced emissions from 8.8–11.5μm and 18–19μm. We found this could be reproduced by predominantly amorphous olivine and stratospheric gaseous NH3 at concentrations of 150−121+338 ppbv retrieved at 30 mbar. Stratospheric abundances of NH3 are a factor of ∼40 higher in the SL9 impacts compared to the Wesley impact, which confirms the former reached deeper, NH3-richer altitudes of Jupiter’s atmosphere compared to the latter. The absence of silicas in the Wesley impact would place an upper limit of 10 km/s on the incident angle and ∼9∘ on the entry angle of the impactor such that shock heating associated with the impact did not reach temperatures required for silicates to be converted.