SPHEREx Detects Increased Sublimation Activity on Interstellar Object 3I/ATLAS

The interstellar object, 3I/ATLAS, was first discovered in July 2025, and made its closest approach to the sun (perihelion) in late October. New observations of 3I/ATLAS were taken in December from the SPHEREx observatory—a near-infrared space observatory used for spectrophotometry. The analysis of these observations was recently discussed by a team of scientists in a paper on arXiv, and reveals some dramatic differences from the data taken before 3I/ATLAS reached perihelion. Pre-perihelion 3I/ATLAS observations SPHEREx first analyzed spectrographic data from 3I/ATLAS in August, shortly after its discovery. At the time, the interstellar object was moving inward, getting closer to the sun, but still between Jupiter and Mars. At this time, spectrographic analysis showed "barely detectable" H2O-gas, according to the study authors, along with a CO2 coma. The CO2 gas production was estimated in a prior study to be about 9.4 x 1026 molecules/sec, with upper limits for H2O and CO being much lower. Carbon-based organic compounds (collectively referred to as C-H), like methanol (CH3OH), formaldehyde (H2CO), methane (CH4), and ethane (C2H6) were not detectable at the time. Post-perihelion 3I/ATLAS observations Post-perihelion observations revealed a much more active comet, with analysis indicative of a full ice sublimation. In contrast with the pre-perihelion observations, strong emissions were detected from CN, H2O, organic C-H, CO2, and CO gases. The team found that both CO and H2O gas emissions had increased 20-fold. "The CO2 emitted flux is only about 33% larger than in August, which verifies that this species was fully active in August pre-perihelion. The CO emitted flux has increased ~20x, meaning that the CO/CO2 abundance ratio has also increased ~15-fold. Along with the greatly enhanced H2O flux and new C-H species emission, this implies that a new ice reservoir is now active along with the one supporting the CO2-coma," the study authors explain. Imaging also showed changes in 3I/ATLAS's comae, with all comae aside from the CN and C-H organics, exhibiting a round shape, while the dust and organics comae are markedly pear-shaped, pointing toward the sun. The team says that these differences suggest that CN and C-H species are coming from the dust, and the H2O, CO2, and CO-gas are from a symmetric region around 3I/ATLAS'S nucleus. A journey past the ice line The researchers say the changes seen in 3I/ATLAS reflect the change in temperature as it traveled past the solar system's ice line—the distance from the sun where the temperature is low enough for volatile compounds such as water, ammonia, methane, carbon dioxide and carbon monoxide to condense into solid grains. Going inward would have the reverse effect, resulting in the sublimation of more of the comet's ice. "This change from pre-perihelion observations makes sense because, in August 2025, 3I's behavior was dominated by large icy dust grain emission, with the icy grains too cold to sublimate anything more volatile than CO2 fully. By December-2025, though, the ISO had spent ~3.5 months inside the solar system's ice line, and all of the cometary constituents, not just the highly volatile CO2 and CO ice portions, were active. I.e., bulk matrix comet material was evaporating, releasing everything the comet contains," the study authors write. According to the researchers, the composition of 3I/ATLAS is in line with other typical solar system comets. These similarities between interstellar and solar system comets suggest similar processes. More detailed analysis of the SPHEREx data is still to come, and can be expected before April 2026, when the next SPHEREx survey will take place. © 2026 Science X Network