SPHEREx spots water in 3I/ATLAS

An interstellar comet is basically a sample-return mission that nobody had to launch. The catch is that the sample never slows down, never turns around, and only gives astronomers a short look before it disappears back into deep space. That is why NASA’s SPHEREx observations of 3I/ATLAS matter. In August 2025, the infrared telescope caught the comet early enough to separate what looked like water ice in the dust from a broad cloud of carbon dioxide gas around it. (irsa.ipac.caltech.edu) ### What is 3I/ATLAS? 3I/ATLAS is the third confirmed object known to have come from outside our solar system, after 1I/ʻOumuamua and 2I/Borisov. The ATLAS survey in Chile first reported it on July 1, 2025, and NASA quickly put multiple spacecraft and telescopes on the case because these things do not stick around. (science.nasa.gov)ed sky-mapping mission. Instead of taking one pretty picture at one color, it breaks incoming light into many wavelengths from roughly 0.75 to 5 microns. That matters because different molecules and ices leave different fingerprints there. For a fast-moving comet, that means SPHEREx can do chemistry at a distance — not just brightness. (irsa.ipac.caltech.edu) ### So what did it see in August? The early data were dominated by two things — water ice absorption and carbon-dioxide gas emission. The team also resolved a very extended CO2 coma, about 3 arcminutes in radius, which is huge on the sky for this kind of target. But the surprising part is what SPHEREx did not firmly see then: water vapor and carbon mo(irsa.ipac.caltech.edu)arxiv.org) ### Wait — water ice or water gas? Water ice and water gas are different clues. Ice absorption means frozen water was present in the dust or on grains in the coma. Water gas emission means active sublimation — sunlight heating ice until it escapes as vapor. In the August observations, the strong signal was the ice signature, while gas-phase water was at best barely detectable. That makes the early result (arxiv.org)apor.” (arxiv.org) ### Why is the CO2 number important? Because it says the comet was already active before perihelion, and not in a generic way. The estimated CO2 production rate was about 9.4 × 10^26 molecules per second. Water and CO had much lower limits — about 1.5 × 10^26 and 2.8 × 10^26 molecules per second. So the early coma seems to have been driven much more by carbon dioxide than by the usual water-dominated pict(arxiv.org 1)(arxiv.org 2) ### Did the story change later? Yes — a lot. SPHEREx looked again in December 2025, after perihelion, and NASA says the comet had brightened dramatically. That later campaign picked up stronger emissions from water, CO, CN, and several organics such as methanol, cyanide-bearing species, and methane. In other words, the August snapshot was the opening act, not the full chemistry set. (science.nasa.gov)/nasas-spherex-mission-tracks-brightening-of-interstellar-comet/)) ### Why should anyone care? Because this is the first time SPHEREx showed, in practice, that it can chemically map an interstellar visitor rather than just notice one. 3I/ATLAS gave astronomers a moving target from another planetary system, and SPHEREx managed to sort ice, dust, and gas into separate pieces of the stor(science.nasa.gov)bearing volatiles, organics — look familiar elsewhere. (irsa.ipac.caltech.edu) ### Bottom line The clean version is this: SPHEREx did not just “spot water” in 3I/ATLAS. It caught an interstellar comet in the act of waking up, with strong water-ice signatures and a carbon-dioxide-rich coma first, then a much richer gas chemistry later. That is exactly the kind of layered, time-dependent picture you want if you are trying to read the history of a comet that formed around some other star. (irsa.ipac.caltech.edu)