2024 December 27

MISSION 29P and James Webb Space Telescope data

Richard Miles

The 6.5-m James Webb Space Telescope (JWST) is the pre-eminent space observatory of our times, and just 7 months into its science mission, on 2023 February 20, it was directed to observe what could be considered the most bizarre object in our solar system known as Comet 29P/Schwassmann-Wachmann, a likely newcomer from the trans-Neptunian region that currently exhibits frequent outbursts. JWST’s observing capabilities go far beyond anything that preceded it, being able to observe in the infrared at high spectral resolution, not only using a slit but also through a 30×30 shuttered array provided 0.1 arcsecond spatial resolution too. Twelve months on, the JWST data became available to publicly download.

The BAA Comet Section has targeted a special effort by way of MISSION 29P to greatly improve our understanding of 29P, and in the hope that new findings will help to attract the attention of the astronomical community and show it to be a worthy destination for a future space probe. A series of three papers led by Richard Miles and published in the professional journal, Icarus in 2016 provided new insights into its strange behaviour, pointed to an exceptionally slow spin rate (P ~58 days), and put forward a new mechanism for heat-transfer within comet nuclei that can create a strong crust and trap reservoirs of molten cryomagma that occasionally erupt at the surface forming strong outbursts. Exceptionally strong outbursts are thought to be a result of explosive ex-solution of CO from the gas-laden cryomagma. It has since been disappointing that the professional community has largely overlooked these papers and is still on record trying to explain 29P’s behaviour by means of a somewhat discredited mechanism based on an unrealistic amorphous-to-crystalline H2O ice phase change as the energy source for its outbursts.

Downloading the JWST data and comparing them with our own ground-based observations has proven enlightening. An analysis of both the BAA observations of 29P and the JWST data has enabled us to compare and contrast the two sets of observations, the findings from which support much of what was written back in 2016 and further elucidates the nature of this bizarre object. It happened that a very strong outburst took place on 2023 February 21, i.e. just 30 hours or so after JWST had stared at 29P for several hours. The region of the nucleus the outburst originated from showed little evidence of carbon monoxide (CO) gas leakage in the JWST data, indicative of a sealed crust under pressure.  Where CO outgassing was seen by JWST, we show by calibration with ground-based photometry that the associated coma exhibited an extremely low dust-to-gas mass ratio of about 0.01 or less, showing little dust entrainment in the outflowing gas, and again indicative of a strong crust.

A key aspect of the proposed outburst mechanism involves the solution of CO and other hypervolatile gases in a hydrocarbon-rich cryomagma, which can freeze and seal the crust on the night-time hemisphere of the nucleus. Although 29P is always surrounded by a significant dust coma, it is possible to tease out just the light reflected by the nucleus by deriving the photometric profile of the coma. We have done that both for the ground-based observations and those from JWST, identifying the detailed spectrum of the nucleus separately from the coma contribution. An absorption band in the former case strongly suggests the presence of a solid ice mixture of CO and hydrocarbons such as methane and ethane, which if true also supports the proposed outburst mechanism.

Beware – the article is a rather long one and goes into the science involved revealing the new findings about the nucleus and coma. Further updates will be included in due course.

Click here to see the results of our analysis

Richard Miles

2024 December 30