SPHEREx Reobservation of Interstellar Object 3I/ATLAS in 2025 December: Detection of Increased Post-perihelion Activity, Refractory Coma Dust, and New Coma Gas Species

Abstract In 2025 December, SPHEREx re-observed 3I/ATLAS post-perihelion, finding a much more active object compared to 2025 August SPHEREx pre-perihelion observations, with marked evidence for development into an cometary body fully sublimating all its ices. The new imaging spectrophotometry was dominated by spatially resolved features due to light scattered by dust, along with thermal emission, plus gas-line-emissions from CN(∼0.93 μ m), H 2 O(∼2.7 μ m), organic C–H(3.2–3.6 μ m), CO 2 (4.25–4.27 μ m), and CO(∼4.7 μ m). The CO 2 gas-coma continued to be extended out to ∼3′ radius. The continuum spectral signature of H 2 O-ice absorption had mostly disappeared, replaced by scattered-light plus thermal-emission from organo-silicaceous dust grains while the H 2 O gas-emission is 40× higher. The CO- and CO 2 -gas comae were circularly symmetric, while the other comae appear morphologically similar to the dust-coma with its pear-shaped, solar-pointing, large icy dust grains dust tail. The new appearance of CN and C–H features suggests that these species were contained either in H 2 O phases or were trapped under them.