Interstellar Comet 3I/ATLAS is believed to be much older than the whole Solar System. It traveled across the galaxy for at least 10 million years without passing another star, and probably a lot longer. It was to be expected that its closest approach to the Sun (perihelion), even though it was farther away than Earth gets, would be an exciting moment. Now, scientists know just how exciting it got.
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Researchers compared observations of the comet by NASA’s Spectro-Photometer for the History of the Universe, Epoch of Reionization, and Ices Explorer (SPHEREx) in August and December. They saw a dramatic increase in activity, as was expected for a comet, and gained some fascinating insights into where this frozen cosmic shard might have formed.
The international team of astronomers reports that all of the types of ice in the comet appear to be sublimating. This is the phase change where a solid turns into a gas, skipping the liquid phase. The ices of the comet are vaporized by sunlight, forming the coma, the fuzzy atmosphere of the comet.
The scientists report the presence of a variety of molecules, from water to carbon dioxide, monoxide, and nitride. All of these are seen to have a rather circular distribution around the comet, apart from the hydrocarbons, which are a tad more squishy, and the dust, whose distribution is asymmetrical. The dust is what forms the anti-tail, a rare extra tail formed by the comet releasing larger, heavier dust particles. These molecules are not immediately pushed away by solar radiation, creating the solar-pointing tail.
The team found that the light reflection of water molecules got 20 times brighter post-perihelion. The increase in water ice sublimation might have had another important consequence. The lack of hydrocarbons and carbon nitrides in the previous observations, and their novel appearance in the December ones, suggests that they were either trapped in the water ice or buried underneath it.
The team also reports the large extension of carbon dioxide gas around the comet, confirming that this is an exceptional object when it comes to its composition. The amount of carbon dioxide points to intriguing formation pathways for this incredible object.
“The ratio measured for the amount of CO2 gas relative to H2O is among the highest ever observed in a Solar System comet, demonstrating that the coma of 3I/ATLAS is very CO2-rich,” NASA explained, months ago before this work. “This may indicate that 3I/ATLAS was exposed to higher levels of radiation than comets from inside the Solar System or that it formed in a region of its original planetary disk where CO2 ice naturally freezes out from the gas.”
Comet 3I/ATLAS is now traveling away from the Solar System, and it is getting too dim to be seen with amateur telescopes. It will pass relatively close to Jupiter in March, so there’s a chance for some extra deep-space observations.
The study has been submitted for publication in Research Notes of the AAS and a preprint is available on the arXiv.
