Interstellar Visitor 3I/ATLAS shows renewed activity, releasing water and organics

NASA's SPHEREx observed that interstellar comet 3I/ATLAS had low activity in August but by December its water and carbon dioxide output rose dramatically and spectra showed organic molecules and a cyanide line; SPHEREx will observe the object again in April.

Interstellar object 3I/ATLAS brightened and began releasing volatile material after its close passage by the Sun, and new pre-print results describe those changes. The paper led by Carey Lisse reports infrared observations from NASA's SPHEREx mission, which maps the sky and follows targets of opportunity. SPHEREx observed the object in August and again in December, allowing a direct comparison before and after perihelion. Researchers are using these data to examine how an interstellar comet responds when solar heating reaches previously processed material. Key observations: - In August, SPHEREx recorded very little activity from 3I/ATLAS, with only minor outgassing and almost no detectable water; the authors suggest a cosmic-ray–processed crust had depleted near-surface ices. - By December, water production had risen to nearly 40 times the August level, while carbon dioxide production increased by about 80 times. - The ratio of carbon monoxide to carbon dioxide changed to about 2.5, a value more typical of carbon-monoxide–dominant comets seen in the solar system. - Spectra in December indicate a mix of organic-bearing volatiles that likely include methanol, formaldehyde, methane, and ethane, and a new spectral line at 0.925 μm consistent with cyanide outgassing. - The outgassing had a "pear-shaped" appearance with the stem pointing toward the Sun and no clear anti-sunward fine-particle tail; the authors infer the halo particles are relatively large, on the order of centimeters to decimeters. - The authors report the object is actively losing material as it passes near the Sun but do not expect it to completely disintegrate before leaving the solar system; SPHEREx has a final observation window planned for April. Summary: SPHEREx infrared monitoring shows 3I/ATLAS shifted from low activity in August to strong outgassing in December, with large increases in water and carbon dioxide and the appearance of organic signatures and cyanide. Authors interpret the change as solar heating penetrating a processed crust and exposing volatile ices. The object is moving away from the Sun and SPHEREx is scheduled for one more observing pass in April, and additional analyses and papers are expected.