Dust Activity Evolution and Constraints on Icy Grains in the Interstellar Comet 3I/ATLAS

We present Zwicky Transient Facility observations of the interstellar comet 3I/ATLAS to characterize its dust-activity evolution before and after perihelion. Before perihelion, the effective scattering cross section C e brightened across the inner 2 × 10 4 km as C e ∝ r H − γ , with γ increasing from 1.63 ± 0.14 to 1.88 ± 0.34 with aperture size. After perihelion, the heliocentric index decreased from 2.44 ± 0.09 in the 5 × 10 3 km aperture to 1.54 ± 0.09 in the 8 × 10 4 km aperture, indicating faster fading of the inner coma. Under adopted grain-size and residence-time assumptions, the implied dust mass-loss rates are ∼123–203 kg s −1 before perihelion and ∼596–1412 kg s −1 after perihelion. The morphology yields a sunward ejection-speed scale of v ej ∼ ( 190 ± 3 ) a μ m − 1 / 2 m s − 1 and an effective perpendicular velocity of v ⊥ = ( 79 ± 6 ) a μ m − 1 / 2 m s − 1 . Postperihelion surface-brightness profiles transiently steepened to k ∼ −2.7 in late November and early December, then became shallower and approached k = −1.5 by January. Matched g − r profiles show that this phase coincided with an anomalously blue outer coma that disappeared by January. We characterize the fading component with a projected destruction length of ℓ DE ∼ (6.4–7.2) × 10 4 km. Thermophysical sublimation calculations indicate that this scale, together with the order of magnitude of the water production near the main imaging epoch, is consistent with a 0 ∼ 5–10 μ m icy grains containing a small absorbing carbonaceous fraction of f C ∼ 0.1%. These results suggest that the early postperihelion coma likely contained a short-lived, volatile-bearing, icy-grain component whose fading contributed to the outer-coma brightness and color evolution.