Types of Outbursts From 67P/Churyumov-Gerasimenko, as Seen by Alice: Gas, Dust, Ice, and Hybrid
Résumé
During ESA's Rosetta mission, the Alice far-ultraviolet spectrograph (700-2050Å) monitored gas and dust emissions from the comet 67P/Churyumov-Gerasimenko. Several large outbursts of gas and dust were observed. We classify these outbursts into four types: gas, dust, ice, and hybrid. During a gas outburst, the brightness of coma emission lines can increase by factors of a few on short time scales. In the case of dust outbursts, the brightness of sunlight reflected from dust can increase by almost two orders of magnitude on timescales of a few tens of minutes. Typically, the reflectance spectrum of dust is very similar to that of the pre-perihelion nucleus: dark and relatively featureless with a blue spectral slope. However, a subset of the sedust outbursts show a markedly different character, with dust that is bright-in some cases brighter than the sunlit nucleus itself- and displaying a strong absorption feature around 1700Å, characteristic of water ice. In such outbursts, the "dust" is likely to consist primarily of icy grains. There are also a class of hybrid outbursts that show enhancements of gas, dust, and/or water ice. We present examples of each of these types and discuss possible creation mechanisms.