SPHEREx detects water on 3I/ATLAS

The interesting part here is that the headline version of this story is a little wrong. SPHEREx absolutely saw water tied to 3I/ATLAS, but the August 2025 result was mainly water ice, not a robust water-gas detection. What stood out in those first observations was a strong ice absorption signature and a surprisingly large cloud of carbon dioxide around the object. That matters because 3I/ATLAS is only the third known interstellar object ever caught passing through our solar system — so every decent spectrum is a rare look at chemistry from another star system. (irsa.ipac.caltech.edu) ### What did SPHEREx actually see in August? SPHEREx and NASA’s IRTF took low-resolution infrared observations of 3I/ATLAS in mid-August 2025, covering roughly 0.75 to 5.0 microns. The spectrum was dominated by two things — water-ice absorption and carbon-dioxide emission. The team also said the object looked spatially extended, which is a comet clue: you are not just seeing a bare rock, you are seeing material around it. (arxiv.org) ### Why is “water ice” different from “water gas”? Because they tell you different physical stories. Water ice absorption means sunlight passed through or bounced off icy grains and lost light at specific wavelengths. Water gas emission means molecules in the coma are actively radiating after sublimating off the object. In August, the SPHEREx team said the evidence pointed strongly to ice, while the gas side for H2O(arxiv.org)imit rather than a firm detection. (arxiv.org) ### What was the strongest August result? Carbon dioxide. The team resolved a bright CO2 coma out to about 3 arcminutes in radius and estimated a CO2 production rate of 9.4 × 10^26 molecules per second. By contrast, the August paper gave conservative 3-sigma upper limits for water and carbon monoxide gas — 1.5 × 10^26 molecules per second for H2O and 2.8 × 10^26 for CO. So the cleanest early message was not “water g(arxiv.org)envelope.” (arxiv.org) ### Was 3I/ATLAS already acting like a comet? Yes, but in a restrained way. The August images did not show an obvious jet, tail, or trail, which made the object look less dramatic than many familiar comets. Even so, the infrared data implied that most of the measured continuum light was coming from coma dust, not from the nucleus itself. Basically, it was active — just not yet in full showy mode. (arxiv.org)ater gas show up clearly? Later. SPHEREx re-observed 3I/ATLAS in December 2025, after perihelion, and that follow-up looked much more active. The spectrum then included H2O gas emission at 2.7 microns, along with CN, organics in the 3.2 to 3.6 micron range, CO2, and CO. The team said the water-gas emission was about 40 times brighter than in the August pre-perihelion data, while the earlier water-ice absorption signature had mostly faded. (arxiv.org) ### Why would the chemistry change that much? Heating. As 3I/ATLAS moved closer to and then past the Sun, more of its frozen material started sublimating. That can flip the spectrum from looking ice-dominated to gas-and-dust dominated. It is a bit like seeing frost on a window first, then steam in the room later — same water, different phase, different signal. The December brightening suggests the object’s most reve(arxiv.org)rlier August look. (science.nasa.gov) ### Why does this matter beyond one comet? Because interstellar objects are samples from elsewhere, and we almost never get to inspect them this closely. SPHEREx is giving scientists phase-by-phase snapshots of how one of these visitors behaves as solar heating ramps up. The August result says 3I/ATLAS arrived carrying abundant i(science.nasa.gov)That is a much more useful story than the simplified “SPHEREx found water” version. (irsa.ipac.caltech.edu) ### Bottom line? SPHEREx did not really “detect water on 3I/ATLAS” in just one simple way. In August 2025, it saw strong water ice and a big CO2 coma. Clear water gas came later, in December 2025, when the interstellar comet became much more active. (arxiv.org)