Interstellar Object 3I/ATLAS Turns Green as Outgassing Rises Near Sun

The interstellar object 3I/ATLAS has shifted from red to green in recent weeks as it moves closer to the Sun, driven by a sharp, apparently threshold-driven rise in cyanide and nickel outgassing, scientists monitoring the object said.

Researchers using the European Southern Observatory’s Very Large Telescope in Chile reported a rapid, "super-linear" increase in emissions of cyanide and nickel and a change in the material ejected from the object's surface. The changes, documented over the last two weeks, have altered the object’s color and the opacity of its plume, and the team said the behavior appears inconsistent with a simple, linear response to sunlight or temperature.

In a paper accompanying the observations, the team described the rise in emissions as "super-linear" and said the rate-limited increase was not explained by the availability of photons or by a proportional temperature response. Instead, they wrote, the pattern is "more likely temperature-activated and/or a threshold process that is being switched on," meaning a minimum temperature must be reached before the new outgassing regime begins.

Harvard astrophysicist Avi Loeb, who tracks 3I/ATLAS, said the threshold description is apt. "Imagine that you want to buy a ticket to see a show. As long as you have a small amount of money in your pocket, you will not get in, but once you have more than some minimum, you will be able to see the show," he wrote in a blog post explaining the effect. Loeb also noted that the ATLAS team interprets the change as a shift from sunlight scattering by dust lifted from a reddened surface to the production of small, highly reflective icy grains that alter the plume's opacity.

Earlier observations of 3I/ATLAS showed no classic cometary tail, even though many scientists initially labeled it a comet. The object has displayed extensions of scattered light toward the Sun rather than away from it, a geometry that differs from typical comet behavior. Some measurements have also suggested the object is producing light of its own, beyond reflected sunlight, a phenomenon under continued investigation.

The object is roughly Manhattan-sized, and it has provided an unusually close look at an interstellar body. Deep Random Survey telescopes captured images in June, and teams using the Gemini North telescope and other facilities have taken follow-up observations during July and August to trace its changing appearance. The European Space Agency and NASA shared an image of 3I/ATLAS taken on July 21 as part of the observing campaign.

Scientists emphasized that 3I/ATLAS offers a rare opportunity to study material from beyond the solar system. It is only the third confirmed interstellar object discovered passing through the solar system and the first to take a trajectory through the inner solar system that brought it close enough to multiple planets to enable detailed observations. Loeb calculated the particular path it followed had a likelihood of about 0.02 percent.

The ATLAS team suggested the anomalous evolution — the rapid color change and the composition shift of the ejected grains — may reflect volatile constituents becoming active once exposed to sufficient heat, releasing gases and lofting icy particles. Those icy grains could scatter sunlight differently and also change the spectral signature seen by telescopes, producing the observed green hue.

Researchers continue spectroscopic monitoring to quantify the chemical species involved and to constrain the physical processes on the object's surface. Future observations aim to measure temporal changes in emission lines, particle sizes in the coma, and any anisotropies in the material being released. The data will help determine whether the activity is driven by sublimation of exotic ices, chemical reactions triggered at a threshold temperature, or other processes.

The rapid evolution of 3I/ATLAS underscores the challenges and scientific value of tracking transient interstellar visitors. Each new measurement refines models of how interstellar material responds to solar heating and provides a rare sample of the diversity of objects produced in other stellar systems. Observatories worldwide remain on alert for further changes as 3I/ATLAS continues its passage through the inner solar system.