Webb Reads Comet 3I/ATLAS's Alien Chemistry

The telescope's first chemical fingerprint of an interstellar visitor reveals methane levels rare in our Solar System—and a very foreign origin.

By The space-quantum Desk

An interstellar comet has handed astronomers their first mid-infrared chemical portrait of a visitor from another star system. The James Webb Space Telescope detected methane, water vapour, and carbon dioxide in the coma of Comet 3I/ATLAS—and what it found does not look like anything born in our Solar System. [1][2]

Webb observed 3I/ATLAS on two separate dates as the comet travelled back out of our Solar System after whipping around the Sun, when the comet was at heliocentric distances of 2.20 and 2.54 au. [2] Webb observed Comet 3I/ATLAS using MIRI's Medium Resolution Spectrometer, a powerful instrument designed to break infrared light into its component wavelengths, providing a spectrum at every point in a small patch of sky and allowing the team to measure what gases are present and visualise their distribution around the comet's nucleus. [1]

The headline result is methane. This marks the first direct detection of CH4 in an interstellar object. [2] The methane was concentrated near the nucleus, suggesting it was buried below the comet's surface and protected from evaporating until solar heat reached deeper parts of the icy outer shell. [1] The amount of methane relative to water is surprisingly high, and at a level that is rare in our Solar System. [1]

Carbon dioxide tells a similar story. [2] Both findings point to a very different formation environment and chemistry than the vast majority of comets that formed within our Solar System. [1] The comet's estimated nucleus diameter is 2.[2]

Each interstellar visitor is a data point in a sample size of three. The more exotic the chemistry, the sharper the question becomes: how much does planetary formation vary across the galaxy, and what does a comet built somewhere else actually tell us about the conditions that shaped it?

The results were published in The Astrophysical Journal Letters. The sharp rise in CH4 production relative to H2O suggests past CH4 depletion from the outermost layers, with the observed CH4 emerging from unprocessed subsurface material. [2]

The amount of methane relative to water found is surprisingly high, and at a level that is rare in our Solar System.NASA/ESA/CSA