Images captured during the total lunar eclipse on September 7, 2025, reveal that the interstellar comet 3I/ATLAS may be emitting a strange green glow. This phenomenon is not entirely unusual for comets, which often appear green as their ices sublimate into gas when they approach the Sun. However, the chemistry of 3I/ATLAS raises intriguing questions, as current observations show a scarcity of the dicarbon (C2) molecules typically responsible for this coloration.
Astrophotographers Gerald Rhemann and Michael Jäger in Namibia documented the comet’s blue-green coma, which spanned approximately 2.5 arcminutes. They also noted a short tail on September 9, captured with a 12″/f-3.6 astrograph. As comets near the Sun, the heat causes the ices surrounding their rocky nuclei to transform into gas, creating a surrounding atmosphere known as a coma. Solar radiation excites the molecules within this gas, causing them to fluoresce and emit light across a spectrum of wavelengths.
Observations from the James Webb Space Telescope (JWST) indicate that 3I/ATLAS possesses an unusual chemical composition, with a higher-than-normal concentration of carbon dioxide. Alongside this, there are traces of nickel and cyanogen. Despite these findings, none of these compounds typically induce the green fluorescence observed in comets, and the key molecule responsible for the glow, C2, has not been detected.
The situation is further complicated by research led by Luis Salazar Manzano from the University of Michigan, which suggests a significant depletion of carbon-chain molecules, including both C2 and C3. According to their findings, the early detection of cyanogen hints at a startling lack of these carbon compounds. “Our upper limit on the C2-to-CN ratio,” they note, “places 3I/ATLAS among the most carbon-chain depleted comets known.”
This ongoing mystery raises questions about the comet’s unique chemistry and the processes at play. Scientists are hopeful that as 3I/ATLAS makes its closest approach to Earth in December, they will gather enough data to unravel the secrets it holds. The anticipation surrounding this event continues to grow as researchers aim to deepen their understanding of this interstellar visitor and the complexities of its chemical makeup.
