Strange Green Glow From Interstellar Comet 3I/ATLAS Has Scientists Puzzled
Images of interstellar comet 3I/ATLAS captured during the September 7 total lunar eclipse hint that the latest visitor to the Solar System may be turning green. That color is not unusual for a comet, as many glow green when heated and vaporize. But for 3I/ATLAS the signal is odd: observations of its chemistry show very few signs of the dicarbon (C2) molecules usually responsible for green fluorescence. This could mean that C2 is present but not yet detected, or that another molecule is producing the glow. Either way, the chemistry of 3I/ATLAS still holds secrets that scientists have yet to uncover.
In This Article:
Green Glow Under Scrutiny: The Usual Suspect C2 Is Scarce
The images were taken by astrophotographers Gerald Rhemann and Michael Jäger from Namibia during the total lunar eclipse on the night of 7 September 2025. As a comet grows closer to the Sun, the ices bound around its rocky nucleus sublime, turning into a gas atmosphere, or coma. Molecules in this gas, excited by solar radiation, fluoresce and glow across visible, near-infrared, ultraviolet, and radio wavelengths. We know from JWST observations that 3I/ATLAS has a peculiar chemical composition with larger-than-usual proportions of carbon dioxide. Other observations also show the presence of nickel and cyanogen. But these do not normally make comets emit green fluorescence, and the molecule responsible for the glow has not been found.
Unusual Chemistry Emerges: CO2, Nickel, Cyanogen
The study notes that 3I/ATLAS contains larger-than-usual amounts of carbon dioxide. Other observations reveal nickel and cyanogen. However, these compounds do not typically produce green fluorescence, and the molecule behind the glow has not yet been identified.
Two Explanations, One Big Mystery
One possibility is that C2 is present but below the detection threshold of current observations. A second possibility is that a different molecule is responsible for the green glow. According to a preprint led by astronomer Luis Salazar Manzano of the University of Michigan, Ann Arbor, the early detection of cyanogen implies a strong depletion of carbon-chain molecules—including both C2 and C3. "Our upper limit on the C2-to-CN ratio places 3I/ATLAS among the most carbon-chain depleted comets known."
December Close Approach: The Data Could Finally Solve It
When 3I/ATLAS makes its closest pass to Earth in December, scientists plan to collect more data with greater precision. The findings could reveal new aspects of interstellar comet chemistry and perhaps rewrite what we know about what makes a comet glow. Until then, the mystery remains: a green glow whose source has yet to be found.
