Discrete sources of cryovolcanism on the nucleus of Comet 29P/Schwassmann–Wachmann and their origin

Abstract

Evidence for long-lived sources of cryovolcanism on the nucleus of the Comet 29P/Schwassmann–Wachmann has been found from a study of its times of outburst (t0) and the morphological development of inner coma structures. Analysis of data from the Minor Planet Center observations archive spanning 2002–2014 and other observations have yielded 64 outburst times of mainly well-observed events with a median timing uncertainty of 0.40d. Outbursts comprise largely (i) isolated explosive events; or (ii) multiple outbursts occurring typically within 5–15d of each other. On rare occasions, a form of continuous or gradually increasing activity is manifest, appearing to be the result of a series of mini-outbursts. Quasi-periodicity in t0 is manifested as an excess of outbursts every 52–60d, along with a paucity of events every ∼30d and ∼90d. Seasonal changes in activity are evident from the temporal analysis of the outburst data. An unambiguous periodicity of 57.6±0.4d has been found in the times of 26 outbursts during 2010–2014, with all active sources at that time localised within a longitude span of ∼135–150°. Cluster analysis of t0 data for 2002–2010 and 2010–2014, and HST imaging from 1996 confirm and refine the apparent periodicity, indicating that outbursts appear to be grouped in longitude centred on at least 6 circumferential locations. Sources of activity generally persist for at least 10–20yr, and some appear discrete in nature, able to re-outburst after a single day–night cycle. Given that outbursts are triggered by solar heating, the analysis yields a value for the mean solar day of 57.71±0.06d, equivalent to a sidereal rotation period of 57.09±0.06d, assuming the more probable prograde direction of spin. A novel outburst mechanism is outlined in which some cometary ices, principally solid CH4, confined under pressure (>12kPa) beneath a stabilisation crust, begin to melt and absorb supervolatile gases, mainly CO and N2. These gases liberate considerable heat (5–7kJmol−1) via their enthalpy of solution inducing further melting deep within the nucleus where direct insolation heating is absent. This gas-solute process is most active near the solid–liquid interface, where the solvent temperature is lowest and gas solubility is highest. An outburst occurs when insolation heating of the crust above a gas-laden subsurface reservoir softens paraffinic hydrocarbons and causes a crustal plate to dislodge under the accumulated gas pressure, the sudden release of which provokes the explosive ex-solution of dissolved gases, principally CO, propelling entrained dust and debris into space. Fissures reseal as the plate sinks back under the gravitational influence of the large nucleus and the adhesive, waxy hydrocarbon fraction solidifies, permitting a new outburst cycle to begin. A detailed account of the gas ex-solution mechanism is the subject of a partner paper (Miles, R. [2015]. Icarus).