Methane has been seen spewing from comet 3I/ATLAS, marking the first time that the gas has been identified on an interstellar object. The amount of methane, relative to water, is also larger than is typically seen in comets from our solar system, further highlighting how different this interstellar visitor is than objects from our own cosmic neighborhood.
The James Webb Space Telescope (JWST) initially observed the interstellar comet with its Mid-Infrared Instrument (MIRI) on Dec. 15-16 2025, when 3I/ATLAS was 205 million miles (330 million kilometers or 2.20 astronomical units) from the sun. However, there was a problem with two of the observations, which failed to acquire a guide-star in order for the telescope to point accurately. This meant that those two observations had to be repeated later, on Dec. 27, when 3I/ATLAS was 236 million miles (380 million km or 2.54 AU) from the sun.
As it happened, these repeat observations turned out to be quite fortuitous.
Both sets of observations came less than two months after 3I/ATLAS had reached perihelion – its closest point to the sun – on Oct. 29, 2025. The extra heating from the sun had warmed the comet’s surface, increasing the amount of outgassing from the comet, but the level of outgassing was beginning to decrease as the comet moved away.
MIRI detected water vapor streaming at large distances from the comet’s nucleus as icy grains in the coma (the gaseous ‘atmosphere’ surrounding the comet’s solid nucleus) vaporized.
However, this is where the observations on the Dec. 27 came into play: JWST noted that the production of water vapor had dropped sharply between Dec. 16 and 27, indicating that solar heating was fading and more water-ice was staying frozen, especially since by then 3I/ATLAS had crossed the ‘snow line’, which is the distance from the sun beyond which temperatures are low enough for water vapor to freeze to ice.
“The water-ice line in the solar system is located around 2.5AU, and as 3I ATLAS approached those heliocentric distances … water production from the coldest regions of 3I’s surface and coma was starting to shut down,” wrote the research team, led by Caltech’s Matthew Belyakov, in their science paper. “Meanwhile, because of their much lower vapor pressures, carbon dioxide and methane are expected to have remained fully activated.”
JWST also detected carbon-dioxide gas and even nickel vapor, matching previous observations and confirming that 3I/ATLAS has a surprisingly large abundance of carbon dioxide relative to water vapor.
Most intriguing, though, was the detection of methane for the first time. Though not a rare gas, it hadn’t been detected on either of the previous two interstellar objects seen to pass through the solar system, and only became apparent on 3I/ATLAS after perihelion.
The reason for its delayed appearance is probably because the methane is buried deeper into the cometary nucleus and it took time for the Sun’s heat to reach those depths and warm the methane enough for it to sublimate and burst out. Once upon a time the comet probably did have methane closer to, or even on, the surface, but it was lost to space long ago.
“This could imply that 3I previously underwent a period of significant heating within its natal planetary system prior to its ejection into the cold interstellar medium that depleted the methane in the outermost layers,” wrote the researchers. “Consequently, the surviving reservoir of primordial methane-ice resides at depth and was only fully activated after the thermal wave induced by 3I’s perihelion passage propagated into the interior.”
They also point out that the delayed production of methane is mirrored by a similar delayed production of carbon monoxide outgassing, which became notable in December by undergoing a 40-fold increase relative to carbon dioxide.
Another fascinating facet is that, like carbon dioxide, methane also has a surprisingly high abundance relative to water on 3I/ATLAS.
Yet while the ratio of methane and carbon dioxide to water seems unusually high for our solar system, it could be quite ordinary for the star system in which 3I/ATLAS formed, possibly as long ago as 11–12 billion years. Those ratios indicate that 3I/ATLAS must have formed in a somewhat different environment compared to the one in which the comets native to our solar system formed, with different physical conditions and a different chemistry.
These findings illustrate the importance of studying interstellar objects, in that they give us a view into other planet-forming environments that we wouldn’t otherwise get. In turn, these insights can provide comparisons that can teach us more about how our own solar system and Earth formed.
The results were reported in The Astrophysical Journal Letters.
