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Mechanism behind low dust speeds and faint anti-solar tail in 3I/ATLAS

Determine which mechanism—dominance of large particles, weak gas flow due to subsurface sublimation through a porous mantle, limited source region extent, or a combination of these factors—best explains the observed dust-gas poor coupling in 3I/ATLAS, evidenced by dust ejection speeds far below the thermal gas speed and a faint radiation-pressure tail at heliocentric distance r_H ≈ 3.8 au.

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Background

The HST observations indicate dust ejection that is anisotropic and characterized by very low speeds compared to the thermal speed of sublimating gas molecules. The authors infer sunward and perpendicular dust speeds proportional to β{1/2} that are much smaller than the gas thermal speed, implying poor dust-gas coupling.

They identify several plausible causes: the dominance of large grains, weak gas flow (potentially due to sublimation through a porous mantle beneath the physical surface), a limited source region extent, or a combination of these. Although the faintness of the anti-solar tail hints at large-particle dominance, the current dataset cannot discriminate among these possibilities.

References

Dust speeds much lower than V_th indicate poor coupling, either because the particles are large, or because of weak gas flow (perhaps because sublimation occurs from beneath the physical surface of the nucleus through a porous mantle), or a source region of limited spatial extent or some combination of these reasons. While we do not currently possess sufficient data to decide between these possibilities, the faintness of the tail relative to the sunward ejected dust most simply suggests the dominance of large particles.

— Hubble Space Telescope Observations of the Interstellar Interloper 3I/ATLAS (2508.02934 - Jewitt et al., 4 Aug 2025) in Subsection Morphology