Astronomers are observing a final burst of activity from interstellar comet 3I/ATLAS, the third confirmed object from beyond our solar system to pass through. Initial observations in 2025 revealed this comet as the fastest interstellar visitor yet discovered, and now, new data reveals its surface is experiencing delayed but significant sublimation events: essentially, ice volcanoes erupting long after its closest approach to the sun.
Delayed Sublimation Explained
The comet’s dramatic brightening in December 2025 – observed by NASA’s SPHEREx space observatory – occurred two months after its closest solar flyby. This delay isn’t counterintuitive: energy from the sun takes time to penetrate deep into the comet’s icy layers. As astrophysicist Carey Lisse explains, the comet is “full-on erupting into space,” releasing carbon-rich materials and water ice that have been locked beneath the surface for billions of years.
Earlier observations in August 2025 showed high concentrations of carbon dioxide, with smaller amounts of carbon monoxide and water. By December, SPHEREx detected a diverse mix of organic compounds, rocky debris, and other chemicals. The delayed sublimation suggests that 3I/ATLAS has spent eons in interstellar space, accumulating a radiation-processed crust that finally gave way to the sun’s energy penetrating its depths.
Why This Matters
The timing of these eruptions is crucial. Comets are thought to be pristine samples of the early solar system, but 3I/ATLAS is not from our solar system. Its composition provides insights into the building blocks of planetary systems elsewhere. The fact that sublimation occurred so late in its journey raises questions about how long such materials remain frozen within interstellar objects and how cosmic radiation alters them over time.
Caltech mission scientist Phil Korngut points out that the comet’s ancient origins mean these chemicals haven’t been exposed to space for billions of years, making this a unique opportunity to study their original state. The data collected by SPHEREx will continue to be analyzed long after the comet fades from view, offering astronomers a wealth of information about interstellar materials.
This comet represents a rare snapshot of materials from another star system, helping scientists understand the diversity of planetary formation beyond our own.
The next interstellar object may not visit for decades or centuries, but 3I/ATLAS leaves behind a legacy of data that will shape our understanding of the cosmos for years to come.
