Hyperactivity in 103P/Hartley 2: Chunks from the sub-surface in Type IIa jet regions

Abstract

We analyze the observed radial distribution of column densities of water-ice particulates embedded in the primary jet region (J1) of 103P's inner coma at altitudes between 439 and 1967m (Protopapa et al., 2014, Icarus 238, 191–204) and determine the speed and acceleration of particles and their mass-flow within the filaments of the jet. This is done by applying a CO2 driven (Type IIa) jet model proposed by Belton (2010, Icarus 210, 881–897). The model utilizes water-ice particles dislodged in the source regions of the jet filaments and accelerated by CO2 to explain the radial distribution of water-ice particulates. We provide an explanation for the remarkably different radial distribution of refractory dust particles by hypothesizing that the majority of the dust originates directly from the nucleus surface in inter-filament regions of the jet complex and is accelerated by H2O. Our model provides a mass-flow of water from the J1 jet complex that is ∼40 times greater than the constant speed sublimation model discussed by Protopapa et al. but is still too small to explain the hyperactivity of the comet. Speeds in the flow are increased by a factor up to ∼20 over those found by Protopapa et al.To account for the hyperactivity, most of the mass dislodged in the filament source regions must be in weakly accelerated large chunks that achieve only low speeds en route to the region of observation. These chunks soon leave the filamentary jet structure due to the rotation of the nucleus and do not contribute to the column densities observed at higher altitudes in the jet filaments. Employing the results of Kelley et al. (2015. Icarus 262, 187–189)) on total cross-section and mass-flow in the coma we find that large chunks with the same bulk properties as the nucleus can increase the active fraction of the comet by two or three times. With the exception that the chunks do not need to be “nearly pure water ice”, these results support the hypothesis that hyperactivity in comet 103P/Hartley 2 is the result of super-volatiles that “… drag out chunks of nearly pure water ice that then sublime to provide a large fraction of the total H2O gaseous output of the comet (A'Hearn et al., 2011. Science 332, 1396–1400). We find that the largest chunks (effective radius ∼4m) are probably dislodged at an average rate less than 1 chunk/filament/1–3 rotation periods near perihelion and we estimate that the filamentary source regions, if 50–100m in diameter, could be excavated to a depth of 44–171m in a single perihelion passage. We note similarities with groups of active pits discovered on 67P (Vincent et al., 2015. Nature 523, 63–66) and suggest that these features may have, in the past, supported Type IIa super-volatile jet outflows that are now essentially exhausted.