Interstellar Visitor 3I/ATLAS Carries Water Ice Mixed With Dust, Webb Finds

The third known interstellar object, 3I/ATLAS, does contain water ice, but the ice is blended with dark dust in tiny grains, according to a new analysis of James Webb Space Telescope data. The result, reported by Universe Today, may also change how astronomers read the light from icy bodies in our own solar system.

The paper was led by Dr. Silvia Protopapa of NASA's Goddard Space Flight Center and the Southwest Research Institute, and it appears in The Astrophysical Journal Letters.

Why did astronomers doubt there was water ice at all?

Water ice normally leaves clear fingerprints in infrared light, which is Webb's specialty. It absorbs light at 1.5, 2.0 and 3.0 microns. Astronomers have seen this pattern in comets from our solar system, such as Hartley 2.

In 3I/ATLAS, Webb saw the 3.0 micron signal clearly, but the 1.5 and 2.0 micron features were weak or missing. The object also had a blue slope in near-infrared light between 1.15 and 2.50 microns, unlike typical comets. According to Universe Today, those oddities led some observers to wonder whether any water ice was present.

How did the team explain the odd signal?

The researchers modeled how light scatters through particles that mix water ice with dust. That model matched the observed spectrum much more closely than pure, clean ice did. The team concluded the ice is locked inside microscopic grains that also contain dark minerals such as pyroxene and carbon.

In such grains, light can travel only a short distance before it hits dust. That mutes the 1.5 and 2.0 micron features, while the longer 3.0 micron light still gets through. The small grains also scatter shorter wavelengths more strongly, which accounts for the blue slope.

What changed after the object passed the Sun?

Webb observed 3I/ATLAS three times, from about 3.31 astronomical units before perihelion, its closest approach to the Sun, to about 5.65 AU after it. The spectra differed noticeably between observations.

After perihelion, the grains appeared to include a second population of larger, micron-sized aggregates rich in ice. The team also detected a Fresnel reflection near 3.1 microns, a sign of crystalline water ice. In that form, water molecules sit in an orderly lattice rather than a jumble.

The paper offers two possible reasons the crystalline ice showed up only after perihelion. Sunlight may have warmed the surface layers and rearranged them into crystals. Or solar energy may have blasted away outer dust and exposed cleaner crystalline ice that had been hidden.

How does 3I/ATLAS compare with objects in our solar system?

The infrared pattern of 3I/ATLAS matches certain trans-Neptunian objects, the icy and rocky leftovers that orbit beyond Neptune in the Kuiper belt. Webb has already observed many of them.

Researchers had previously seen the same 3.0 micron absorption without the 1.5 and 2.0 micron features in some of those objects. They had thought complex hydrocarbons or other organic compounds might explain it. According to the new paper, dusty ice may be enough.

Why does it matter?

The finding suggests that water ice behaves similarly in an object from another star system and in distant bodies near home. It also gives astronomers a possible alternative reading of spectra they had already collected from the outer solar system.

The conclusions rest on modeling of one object's spectra, and the explanation for the crystalline ice is still an open question in the paper. Universe Today noted that it is unclear whether Webb will get another chance like this during its operational life. Scientists are still analyzing the data collected as 3I/ATLAS passed through the solar system.

The object was discovered in July 2025. The previous known interstellar visitor, 2I/Borisov, appeared in 2019, before Webb's launch.

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Around Prague, based on reporting by Universe Today