Hubble's Faint Object Spectrograph (FOS) recorded dramatic changes at the magnetosphere crossing that provided a rare opportunity to gather more clues on the comet's true composition. During a two minute period on July 14, HST detected strong emission from ionized magnesium (Mg II), an important component of both comet dust and asteroids. However, if the nuclei were ice-laden - as expected of a comet nucleus - astronomers expected to detect the hydroxyl radical (OH). The HST did not see OH, casting some doubt on the cometary nature of P/Shoemaker-Levy 9. Eighteen minutes after comet P/Shoemaker- Levy 9 displayed the flare-up in Mg II emissions, there was also a dramatic change in the light reflected from the dust particles in the comet.