A new image of the elusive interstellar visitor 3I/ATLAS shows the object bathed in a vivid green glow. Astronomers attribute this phenomenon to molecular reactions triggered by intense solar ultraviolet radiation.
The image was captured by astronomer Qicheng Zhang at the Lowell Observatory in Arizona. Zhang used the observatory's powerful Discovery Telescope to make fresh observations of 3I/ATLAS as it sped past our solar system on November 5.
Taken using a filter tuned to detect diatomic carbon, the image reveals that as 3I/ATLAS approached its closest point to the Sun on 29 October, ultraviolet light began to break down complex hydrocarbons in the comet’s coma.
This formed free-floating two-carbon molecules in the celestial body that turn fluorescent green when activated.
"It's sort of for the same reason that if we stay out in the sun too long without sunscreen, we get sunburnt," Zhang told Live Science.
While the green colouring may appear dramatic, it does not necessarily indicate the comet has permanently changed colour. The hue could simply reflect a moment-in-time reaction to solar exposure rather than a lasting shift in composition.
What is 3I/ATLAS?
3I/ATLAS, which is only the third ever visible comet that originated beyond our Solar System, was discovered on 1 July 2025 and made its closest approach to the Sun on 29 October.
Earlier studies showed that it looked unusually blue and bright in the weeks before this point, suggesting rapid gas release and behaviour not seen in most comets from our Solar System.
Now that it is moving away from the Sun, astronomers are closely studying its green glow and the absence of a visible tail to learn more about its composition and where it came from.
