Researchers used Webb's MIRI (Mid-Infrared Instrument) during two observing sessions after the comet passed its closest point to the Sun. The first set of observations took place on December 15 and 16, when 3I/ATLAS was about 205 million miles (329 million kilometers) from the Sun. A second round followed on December 27, with the comet about 236 million miles (379 million kilometers) away.

Webb Detects Methane on an Interstellar Comet

For the first time, scientists directly identified methane gas on an interstellar visitor.

Methane is an extremely volatile substance that can quickly change from solid ice to gas. Its appearance only after the comet had already passed close to the Sun suggests the methane was buried beneath the surface. According to the research team, the comet's upper layers likely shielded the methane ice until solar heating penetrated deeper into the icy interior.

The amount of methane compared to water also surprised researchers. The ratio is much higher than what is typically seen in comets from our own solar system, with only a handful of known examples showing similar characteristics.

Unusually Rich in Carbon Dioxide

The observations also confirmed another unusual feature of 3I/ATLAS. The comet releases exceptionally large amounts of carbon dioxide relative to water, far exceeding the levels commonly measured in solar system comets.

Together, the methane and carbon dioxide measurements point to a formation history that differs significantly from that of most comets that originated around our Sun. The results suggest that 3I/ATLAS formed in a very different chemical environment before beginning its journey through interstellar space.

Gas Production Drops as the Comet Moves Away

Webb also tracked how the comet's activity changed as it moved farther from the Sun.

Scientists observed a sharp decline in the production of gases, with water showing the steepest decrease. This behavior is expected as the comet receives less solar energy. As temperatures fall, less ice vaporizes from the surface and near-surface layers.

Water is less volatile than methane or carbon dioxide, which means its gas production shuts down more quickly as the comet cools.

How Webb Measured the Comet's Chemistry

The observations were carried out using MIRI's Medium Resolution Spectrometer, an instrument that separates infrared light into its individual wavelengths. By analyzing those wavelengths, researchers can determine which gases are present.

The spectrometer also functions as an integral field unit, allowing scientists to obtain a spectrum at every location across a small region of sky. This capability enabled the team not only to identify gases surrounding the comet's nucleus, but also to map how those gases were distributed around the object.