Colleen Anne McGhee

Observations of Saturn's Ring-Plane Crossings in August and November 1995

Observations of Saturns ring system with the Hubble Space Telescope during the 10 August 1995 Earth crossing and the 17 to 21 November 1995 solar crossing indicate that the F ring dominates their apparent edge-on thickness of 1.2 to 1.5 kilometers. The F ring is slightly inclined with respect to the A ring, which may explain the approximately 50-minute difference in apparent crossing times for the east and west ring ansae in August. Prometheus lags its predicted position by about 19 degrees in longitude. The faint G ring is neutral or reddish in color and is confined to a radial range of 2.72 to 2.85 Saturn radii. The broad, distinctly blue E ring flares outward to a maximum thickness of about 15,000 kilometers at 7.5 Saturn radii and appears to have a spatially uniform particle size distribution.

Palomar observations of the R impact of comet Shoemaker-Levy 9: II. Spectra

We present mid-infrared spectroscopic observations from Palomar observatory of the impact of fragment R of comet P/Shoemaker-Levy 9 with Jupiter on 21 July 1994. Low-resolution 8–13 µm spectra taken near the peak of the lightcurve show a broad emission feature that resembles the silicate feature commonly seen in comets and the interstellar medium. We use this feature to estimate the dust content of the impact plume. The overall infrared spectral energy distribution at the time of peak brightness is consistent with emission from an optically-thin layer of small particles at ∼600 K. Integrating over the spectrum and the lightcurve, we obtain a total radiated energy from the R impact of ≥ 2 × 10^(25) ergs and a plume mass of ≥ 3 × 10^(13) g.

Palomar observations of the R impact of comet Shoemaker‐Levy 9: I. Light curves

We present near-infrared observations from Palomar observatory of the impact of fragment R of comet Shoemaker-Levy 9 with Jupiter on 21 July 1994. Two instruments were used to image the event at 3.2 and 4.5 microns simultaneously. The lightcurves from these image sequences both show two faint precursor flashes, a bright main peak, and several oscillations over the following hour. We identify the precursor flashes with the entry of the bolide into Jupiters upper atmosphere, and with the post-impact ejecta plume rising above the planets limb. The main peak is due to the re-entry of the collapsing plume into Jupiters atmosphere and the resultant shock heating.