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

In December-2025, SPHEREx re-observed 3I/ATLAS post-perihelion, finding a much more active object compared to August-2025 SPHEREx pre-perihelion observations, with marked evidence for development into an cometary body fully sublimating all its ices. The new imaging spectrophotometry is dominated by spatially resolved features due to dust scattered-light+thermal emission plus gas-emissions from CN(~0.93um), H2O(~2.7um), organic C-H(3.2–3.6um), CO2(4.25–4.27um), and CO(~4.7um). The CO2 gas-coma continues to be extended out to ~3’ radius. The continuum spectral signature of H2O-ice absorption has mostly disappeared, replaced by scattered-light plus thermal-emission from organo-silicaceous dust grains while the H2O gas-emission is 20x brighter. All but the organics-gas comae are circularly symmetric, while a weak pear-shaped solar-pointing dust tail consistent with large dust grains is now present. The new appearance of CN and C-H features suggests that these species are contained either in H2O phases or were trapped under them.