Imagine looking up during a total lunar eclipse, expecting the moon to turn deep red—and then you notice a faint green glow from a mysterious visitor not from our solar system. That’s exactly what happened with 3I/ATLAS, and its unexpected color shift is opening a window into new comet chemistry and the nature of interstellar visitors.
What is 3I/ATLAS and What Has Been Seen So Far
3I/ATLAS is the third confirmed interstellar object (ISO) ever observed, and it’s behaving in ways that are surprising astronomers.
Discovered on July 1, 2025, by the survey system ATLAS in Chile, 3I/ATLAS has been confirmed as interstellar due to its hyperbolic trajectory. Observations by a range of ground-based and space telescopes (including Gemini South, James Webb Space Telescope (JWST), Hubble, SPHEREx and others) have shown it has a coma of gas and dust, a tail that is growing as it approaches the Sun, and a rich, unusual chemical composition. Because 3I/ATLAS comes from outside our solar system, whatever it does chemically or physically is especially interesting—it potentially reflects how comets form or evolve in other star systems. The fact that its tail is growing and its coma is active suggests it’s warming up and releasing volatiles (ices, gases) as it nears the Sun, much like comets from our system do.
So when something dramatic (like a color change) shows up in 3I/ATLAS, it’s not just a neat photo—it could reveal entirely new processes or materials we’ve never seen before.
The Discovery of the Green Glow During the Lunar Eclipse
During the lunar eclipse on September 7, 2025, astronomers captured images showing 3I/ATLAS glowing green, a surprising shift in color from earlier observations.
Amateur astronomers Michael Jäger and Gerald Rhemann in Namibia used the darkened skies during the eclipse to take deep images and noticed the green glow. Earlier, observations had shown 3I/ATLAS was poor in carbon-chain molecules (including diatomic carbon, or C₂), which are often the usual suspects for cometary green glows.
Under eclipse conditions, Earth’s sky becomes much darker, effectively removing the moonlight that normally drowns out faint features. This allowed astronomers to see subtler emissions from the comet, possibly emissions from gas molecules or freshly exposed materials. That earlier, the comet lacked certain carbon molecules, but now appears green, suggests a change has occurred—either from new volatile release or exposure of hidden layers.
This color change hints not only at physical erosion of the comet’s surface but perhaps also at previously unseen chemical processes tied to how the comet responds to solar heating—making this a key moment in observing comet evolution.
What the Chemistry So Far Tells Us and What’s Unusual
The chemical composition of 3I/ATLAS is already unusual in several ways, and the green glow adds even more mystery.
JWST infrared spectroscopy revealed that its coma is dominated by carbon dioxide (CO₂) gas, with water (H₂O), carbon monoxide (CO), carbonyl sulfide (OCS), dust and water ice also present. The CO₂/H₂O ratio in 3I/ATLAS is about 8:1, one of the highest ever seen in a comet. arXiv+2National Geographic+2 Also, water emission was confirmed at large distances from the Sun (around 3.5 AU) via ultraviolet measurements. arXiv Meanwhile, earlier in August, astronomers had not detected diatomic carbon. Most comets we know in our solar system have more water relative to carbon dioxide as they warm up; water sublimation becomes the dominant process. But in 3I/ATLAS, CO₂ seems to dominate early, which suggests either its formation environment was different—for example, formed where CO₂ ice lines were prominent—or the surface / structure of the nucleus is such that water is trapped or sublimates less readily. Furthermore, a sudden appearance of green suggests C₂ or similar molecules are now being released, perhaps because of internal changes (like heating, exposure, cracking) that expose hidden reservoirs.
All of this together paints a picture of a comet that both shares features with Solar System comets (dust tail, gas coma) and diverges significantly in volatile inventory and behavior. That divergence is what makes 3I/ATLAS scientifically thrilling—it challenges our models of comet formation and volatile evolution.
Why the Green Glow Is Especially Intriguing
The green glow isn’t just a pretty phenomenon—it could be evidence for new comet chemistry in action or unforeseen volatiles.
In space science, green cometary glows are usually tied to the presence of diatomic carbon (C₂) which fluoresces green when sunlight excites it. But in Kitt Peak observations just a month earlier, C₂ was found to be scarce. Other potential candidates are molecules like cyanide (CN), which has been detected in 3I/ATLAS’s coma. So, this green glow could indicate that as 3I/ATLAS warms, it’s releasing molecules that were previously frozen or trapped beneath its surface. Or it could signal the existence of molecules not usually considered in comet models. If the green is coming from CN or a mix of gases, the timing and quantity of outgassing will tell us about the internal structure of the comet and how volatiles are stored and released.
Observing this glow gives scientists a real-time experiment: watching how an interstellar comet “turns on” when warmed, observing its gas release, dust ejection, and color changes. That helps refine our broader understanding of cometary physics and chemistry.
What We Still Don’t Know – The Unanswered Questions
While the green glow is exciting, many key questions remain unanswered.
We don’t yet have spectroscopic confirmation that C₂ is present in sufficient amounts to explain the glow. The timing and spatial distribution (where exactly in the coma or which orientational sector) of the glow is not yet well mapped. Also, the interplay of CO₂ vs H₂O vs other volatiles (how heat penetrates, how dust cover insulates) is still under study. These gaps matter because they influence our interpretation. If the green glow turns out to be mostly CN or some exotic molecule, that changes the chemistry story. If it’s C₂, then it suggests hidden reservoirs being exposed. Understanding the spatial structure (sunward vs anti-sunward sides) can tell us about how the comet’s surface and interior are arranged. And knowing the ratio of volatiles over time helps test theories of how comets evolve under solar heating and how they may differ when formed in different star systems.
The path forward is clear: more detailed spectroscopy, multi-wavelength monitoring, and tracking how the glow evolves (intensity, color, spatial distribution) as the comet passes perihelion (closest approach to the Sun) and beyond.
Why This All Matters: Implications for Astronomy & Beyond
Studying 3I/ATLAS and its green glow does more than satisfy curiosity—it informs our understanding of planetary system formation, interstellar chemistry, and how universal or varied these processes are.
The chemical makeup (especially the high CO₂/H₂O ratio) of 3I/ATLAS is unlike most comets in our solar system. That suggests formation in a different protoplanetary disk, under different temperature, radiation, or chemical environments. Its trajectory and origins may connect it to older, metal-poor regions of our galaxy. If comets forming around other stars can end up with very different volatile compositions, that can affect everything from how planets in those systems develop atmospheres, to how comet impacts deliver water and organics. It also challenges assumptions used in our models—e.g., that water ice is always abundant or that behavior seen in solar system comets is generic.
So, 3I/ATLAS offers a rare chance: an interstellar probe made purely of ice, dust, and gas, giving us physical samples (via light and spectra) of a system we can’t visit. It helps frame our solar system in a broader context – are we typical, or just lucky neighbours?
What to Watch Next and How This Will Be Done
The next few weeks and months are critical in how much we’ll learn from 3I/ATLAS.
The comet is set to reach perihelion around October 29, 2025. Observatories are planning extensive spectroscopic work, imaging in multiple wavelengths, and observing the evolution of its tail and coma. JWST, Hubble, ground-based telescopes, and space observatories continue to be on the case. There are also observations tracking the comet’s brightness, tail length, gas emissions (CN, C₂, CO, CO₂, water), and polarimetric properties. As 3I/ATLAS gets closest to the Sun, heating will increase, possibly releasing more volatiles, creating more visible tails, and potentially changing color even more. The best time to catch subtle emissions or shifts (e.g. from C₂) is when the comet is active but not overwhelmed by sunlight or dust scattering. Also, measuring polarization of light and mapping where emissions come from (e.g., sunward side vs tail) gives geometric clues about the comet’s structure.
These observations will let scientists test hypotheses (Is the green from C₂? From CN? From something else? Is the volatile mix changing over time?). And once the comet moves away from Earth’s view, we’ll lose that chance for a while—making these coming observations especially precious.
Conclusion
3I/ATLAS is more than just a transient visitor. It’s a messenger from beyond, bearing clues about places that planets and comets form under different cosmic conditions. This green glow—unexpected, breathtaking, scientifically provocative—isn’t just pretty; it’s a direct signal that we may not yet understand the full diversity of cometary chemistry and behavior.
If the green comes from hidden carbon molecules being exposed, or from CN or other volatile gases behaving differently at a distance, it could reshape how we think about the building blocks of planetary systems. Even more, it underscores the fact that our Solar System is just one example among many, and that the universe may have a much wider range of chemical recipes than we’ve assumed. Explore the Cosmos with Us — Join NSN Today
