What does it signify when a comet from another star system suddenly and mysteriously changes color in our skies? For astronomers monitoring 3I/ATLAS just the third confirmed interstellar object ever seen the transformation from a reddish color to a brilliant green in early September has become one of its most fascinating developments so far.
The shift was captured by comet hunter Michael Jäger, who, under Namibia’s dark skies, photographed deep images showing the shifted glow. Harvard astronomer Avi Loeb added that the shift may be attributed to a “steep rise in the production of cyanide,” a molecule that is known to fluoresce green when energized by ultraviolet sunlight. Cyanide, plus diatomic carbon, is a major source of the emerald coma in certain comets. Laboratory studies of astrochemistry have demonstrated that these molecules are unstable in the extreme radiation of interstellar space and dissociate before they can get very far from the nucleus which is why the green color is only found in the coma, and not in the tail.
Spectroscopic measurements by NASA’s SPHEREx mission and the James Webb Space Telescope indicate that 3I/ATLAS is chemically distinct from most comets in the solar system. Its coma is composed primarily of carbon dioxide, accounting for 87 percent of its mass, with much smaller percentages of carbon monoxide and water. This is one of the most CO₂-heavy-to-water ratios ever measured in a comet, interstellar or otherwise. Such composition indicates it was created near the CO₂ ice line of its original protoplanetary disk or experienced long exposure to radiation before being expelled into interstellar space. The lack of much carbon monoxide indicates that it was “well baked and boiled” prior to departing its home system, according to astronomer Carey Lisse.
The object’s behavior has been peculiar from the beginning. It started outgassing outside of Jupiter’s orbit earlier than most comets possibly due to the fact that CO₂ sublimates at lower temperatures than ice. Its orbit is also atypical, passing close to Mars, Jupiter, and Venus relatively, in a path that is unusually flat and linear for a comet. Travelling at more than 130,000 mph, 3I/ATLAS takes a one-way trip through the solar system, having probably taken billions of years to travel the Milky Way’s dense disk on the way here.
Higher-quality imaging has also uncovered an “anti-tail” a feature pointing toward the Sun and not away from it seen with the Hubble Space Telescope. This is thought to be sunlight glinting off massive icy grains that are expelled from the nucleus and last longer than smaller dust particles. These grains may change the opacity of the surrounding plume, adding to the changing brightness and color profile.
The green coma appearance is more than just a sight to behold; it signals a period of increased activity as the comet approaches perihelion on October 29. By then, it will be on the opposite side of the Sun from Earth and hence observation directly from our planet virtually impossible. But other spacecraft, which are stationed elsewhere such as ESA’s JUICE mission in the vicinity of Venus and Mars orbiters will have first-class views to observe its busiest time. Instruments on these platforms may take high-resolution spectra of the coma and tail, providing unique information about the chemistry of a body that was born in another stellar cradle.
The chance is temporary. Following its tight flyby just outside Mars’ orbit, 3I/ATLAS will retreat back into interstellar space, never to be seen again. For planetary scientists, every data point from the strange CO₂ prevalence to the cyanide spike responsible for its greenish hue contributes to an expanding comparative context for the formation and evolution of planetary systems throughout the galaxy. As Loeb noted, its “interesting anomalies” render it an exception to icy wanderers, an ambassador of a remote past whose fleeting presence tests and refines our theories of cometary science.
