The interstellar object 3I/ATLAS grazed the habitable zone of the Solar System on a path that was aligned to within 4.88 degrees with the orbital plane of Earth around the Sun. 3I/ATLAS also exhibited a prominent sunward jet, likely made of large fragments of water ice or rock that were able to penetrate through the Solar wind and radiation (as I discussed in a paper with Eric Keto, published here).
The SPHEREx space observatory detected organic molecules, such as CH3OH, H2CO, CH4, and C2H6 with a production rate of 5x10^{26} molecules per second, of order a tenth of the simultaneous production of water molecules (as reported here).
The robust spectroscopic detection of methane (CH4) was confirmed by the Webb telescope here. Interestingly, methane was only detected after the passage of 3I/ATLAS near the Sun. Its delayed production is puzzling because methane ice is hyper-volatile, with a significantly lower sublimation temperature than carbon dioxide (CO2), having a value of -220 compared to -97 degrees Celsius, respectively. This implies that methane ice near the surface of 3I/ATLAS would have been vigorously sublimating at the time of the first reports of outgassing from 3I/ATLAS before perihelion. However, neither the Webb spectroscopy nor the SPHEREx spectrophotometry from August 2025, detected methane. This suggests that methane was depleted in the outermost layers of 3I/ATLAS and was released as a result of the warming by sunlight only close to the Sun. Within this scenario, the early detection of carbon-monoxide (CO) outgassing on 3I/ATLAS is surprising, as carbon monoxide is more volatile than methane and should therefore be even more depleted from the surface, yet it was detected prior to methane. Why did methane appear only close to the Sun?
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