Deciphering the Water Sources in Hyperactive Comet 103P/Hartley 2: Insights from Deep Impact

Detailed studies of the hyperactive comet 103P/Hartley 2 enabled by Deep Impact’s extended mission (DIXI), allow, for the first time, for multiple sources of water in the comet’s coma to be disentangled and quantified. The spacecraft’s high spatial resolution spectral observations acquired at a dedicated science cadence will not readily be repeated for other comets. With this unique dataset, it is possible to distinguish three sources of water, which occur at different spatial scales. Direct sublimation from the illuminated hemisphere of the nucleus contributes an ambient sunward coma component, while a narrow and more columnated source originates from the illuminated waist of the nucleus where icy fallback material is hypothesized to accumulate. Beyond ~5 km, the waist source is not discernible in the data and thus would not be detectable telescopically. In addition, an anti-sunward coma source beyond ~40 km is identified, which persists over the full nucleus rotation period. This source is attributed to icy grains that have been pushed by solar radiation pressure in the anti-sunward direction before they sublimate. The water production maps and asymmetry of the coma density enhancements due to the three sources are qualitatively consistent with a simple model of sublimation from a combination of the sunward nucleus sources and anti-sunward icy grains. By examining the water sources separately, the waist accounts for <5% of the comet’s water production, the combined nucleus sources supply ~60% of the total water production, and anti-sunward icy grains contribute 25-40% of the water production during Hartley 2’s 2010 apparition. The significant fraction of water production from icy grains in the coma not only explains Hartley 2’s hyperactivity but has also revealed a process that is likely common. The proportion of icy grains and the composition of those ices will vary, but similar effects will occur in many comets.

This work was supported by NASA through Discovery Data Analysis Programs 80NSSC18K1041 and 80NSSC18K1280.