![ZTF image sequences of 3I before and after perihelion. Panel (a) shows the pre-perihelion images, and panel (b) shows the post-perihelion images. The observation time is marked in each panel. The white scale bars indicate angular distances of 30′′ in panel (a) and 60′′ in panel (b). All images are oriented with north up and east to the left. The cyan arrows labeled ⊙, −⊙, and −v indicate the projected solar, anti-solar, and anti-velocity directions, respectively, and the cyan cross marks the photometric center. -- [astro-ph.EP]](https://i0.wp.com/astrobiology.com/wp-content/uploads/2026/10/Dust-Activity.png?fit=1200%2C674&ssl=1)
[astro-ph.EP] 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 Ce brightened across the inner 2×104 km as Ce∝r−γH, 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 5×103 km aperture to 1.54±0.09 in the 8×104 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 vej∼(190±3)a−1/2μm m s−1 and an effective perpendicular velocity of v⊥=(79±6)a−1/2μm m s−1. Post-perihelion 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 ℓDE∼(6.4–7.2)×104 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 a0∼5–10 μm icy grains containing a small absorbing carbonaceous fraction of fC∼0.1%.
These results suggest that the early post-perihelion coma likely contained a short-lived, volatile-bearing icy-grain component whose fading contributed to the outer-coma brightness and color evolution
Comments: 19 pages. Published in The Astrophysical Journal Letters
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Astrophysics of Galaxies (astro-ph.GA)
Cite as: arXiv:2610.21206 [astro-ph.EP] (or arXiv:2610.21206v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2610.21206
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Journal reference: ApJL, 1008, L26 (2026)
Related DOI:
https://doi.org/10.3847/2041-8213/ae9a89
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Submission history
From: Bin Liu
[v1] Thu, 15 Oct 2026 04:02:27 UTC (918 KB)
https://arxiv.org/abs/2610.21206
Astrobiology,
