Interstellar comet 3I/ATLAS has sparked intrigue among scientists after images captured during the total lunar eclipse on September 7, 2025, revealed a striking green glow. While green emissions are not uncommon for comets as they approach the Sun, the cause behind this particular hue remains a mystery. Standard sources of the green fluorescence, namely dicarbon (C2), appear to be scarce in the chemical composition of 3I/ATLAS, indicating that the comet may possess unique characteristics yet to be uncovered.
Astrophotographers Gerald Rhemann and Michael Jäger documented the comet’s vibrant blue-green coma, which measured approximately 2.5 arcminutes across. Their observations, taken from Namibia, also captured a short tail extending from the comet on September 9. As comets near the Sun, the heat causes the ices surrounding their rocky nuclei to sublimate, forming a gaseous atmosphere known as a coma. The solar radiation subsequently energizes these gas molecules, leading to fluorescence across various wavelengths.
Unique Chemical Composition of 3I/ATLAS
Data obtained from the James Webb Space Telescope (JWST) indicates that 3I/ATLAS has an atypical chemical makeup, featuring higher-than-normal levels of carbon dioxide. Other elements detected include nickel and cyanogen; however, these compounds do not typically induce green fluorescence. The absence of the expected C2 further complicates the scenario, suggesting that scientists may need to explore alternative explanations for the comet’s glow.
A preprint study led by Luis Salazar Manzano from the University of Michigan, Ann Arbor, highlights another layer to this enigma. The early detection of cyanogen implies a significant depletion of carbon-chain molecules, including both C2 and C3. The researchers noted, “Our upper limit on the C2-to-CN ratio places 3I/ATLAS among the most carbon-chain depleted comets known.” This finding raises fascinating questions about the comet’s origins and composition.
Upcoming Closest Approach to Earth
As 3I/ATLAS continues its journey, scientists are keenly awaiting its closest approach to Earth, scheduled for December 2025. With ongoing observations, there is hope that researchers will gather additional data to unravel the comet’s mysteries. The unusual green glow and its underlying chemistry not only captivate amateur astronomers but also represent an opportunity for the scientific community to deepen its understanding of interstellar visitors.
The case of 3I/ATLAS underscores the complexities involved in studying celestial objects. Each discovery adds a layer to our comprehension of the universe and the processes that govern it. As we move closer to the comet’s next pass, the excitement within the astronomical community continues to build, promising more revelations from this enigmatic interstellar traveler.
