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7h ago

Webb Detects Methane on Interstellar Comet 3I/ATLAS

The measurement reveals a carbon-dioxide–heavy chemistry that suggests the comet formed in an environment unlike most Solar System comets.

The top image shows interstellar comet 3I/ATLAS as seen with MIRI (Mid-Infrared Instrument) on NASA’s James Webb Space Telescope, along with contours that illustrate where different gases were located at the time the comet was viewed. Water vapor spreads far beyond the nucleus because much of it is released from icy grains in the coma, while carbon dioxide and methane are most concentrated near the comet’s nucleus. The bottom image shows the spectrum, with the labels indicating the features from the various gases that Webb found escaping from the comet. Credit: NASA, ESA, CSA, STScI, M. Belyakov (Caltech), I. Wong (STScI), Image Processing: A. Pagan (STScI)
Image
Hubble captured this image of 3I/ATLAS on July 21, 2025, when the comet was 446 million km (277 million miles) from Earth. Image credit: NASA / ESA / David Jewitt, UCLA / Joseph DePasquale, STScI.

Overview

  • The team published peer-reviewed results in The Astrophysical Journal Letters on April 8, 2026, and NASA, ESA and other agencies reported Webb’s direct methane detection in early June 2026.
  • Webb’s MIRI instrument took spatially resolved mid-infrared spectra on December 15–16 and December 27, 2025, when the comet was about 205 million miles and 236 million miles from the Sun.
  • Methane was detected concentrated near the nucleus and appeared after perihelion heating, which suggests the methane ice was buried under a surface layer and was released when solar heat reached deeper ice.
  • The observations confirm 3I/ATLAS is unusually rich in carbon dioxide and has a high methane-to-water ratio compared with typical Solar System comets, pointing to a different formation chemistry.
  • Webb recorded a sharp decline in overall gas output as the comet moved away from the Sun with water dropping fastest, and scientists say the volatile mix offers a rare comparative data point for how planetesimals form around other stars.