This image from the Mid-Infrared Instrument (MIRI) shows the interstellar comet in three different light wavelengths and illustrates where different gases were located at the time the comet was viewed.

Water vapour spreads far beyond the nucleus because much of it is released from icy grains in the surrounding coma, while carbon dioxide and methane are more concentrated near the comet’s nucleus.

Webb made the observations on two separate dates as the comet travelled back out of our Solar System after whipping around the Sun. The first observation occurred 15 to 16 December, when the comet was about 330 million km from the Sun. This was followed by a second observation on 27 December, when the comet was about 380 million km from the Sun.

For the first time on an interstellar visitor, Webb directly detected methane gas. Methane was observed only now in Comet 3I/ATLAS, suggesting that it was buried below the comet’s surface. In this way it remained protected from evaporating until heat from the comet’s close pass to the Sun reached deeper parts of the icy outer shell. The amount of methane relative to water found is surprisingly high, and at a level that is rare in our Solar System.

Webb’s observations also confirmed that Comet 3I/ATLAS remains unusually rich in carbon dioxide, releasing far more carbon dioxide relative to water when compared to typical Solar System comets.

Both these findings point to a very different formation environment and chemistry than the vast majority of comets that formed within our Solar System.

Webb observed Comet 3I/ATLAS using MIRI’s Medium Resolution Spectrometer, a powerful instrument designed to break infrared light into its component wavelengths. This spectrometer provides a spectrum at every point in a small patch of sky, allowing the team to measure what gases are present and visualise their distribution around the comet’s nucleus. 

[Image description: The composite image shows three side-by-side maps of different chemical species. The maps are mostly red with a bright orange centre, resembling pixelated clouds glowing on a black background. From left to right they are labelled “water”, “carbon dioxide” and “methane”.]

The results were published recently in The Astrophysical Journal Letters.

See also NASA 3I/ATLAS blog.