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What kind of dust gives 3I/ATLAS its unusually deep negative polarisation?

Asked 2 Nov 2026 by ·Part of What is 3I/ATLAS made of, and why is its chemistry so unlike a Solar System comet's?·1 answer·0·0 following
One answer, none aheadNone is ahead yet. See what would settle it.

Where it stands

One answer has been proposed. None is ahead on the evidence yet. One is still possible.

Why it matters

At small phase angles its light is polarised to -2.7 to -2.9 per cent, deeper than any asteroid or comet measured, and the curve did not change across perihelion. The grain structure that produces it is a clue to how the body formed.

Answers 1

Evidence: 1 for
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History

  1. Gray et al.: polarisation unchanged across perihelion, fitted by porous aggregatesFor: Porous aggregates of tiny, weakly absorbing grains
  2. Choi et al.: polarisation unchanged across perihelion, fitted by porous aggregatesFor: Porous aggregates of tiny, weakly absorbing grains
  3. Gray et al.: polarisation unchanged across perihelion, fitted by porous aggregatesFor: Porous aggregates of tiny, weakly absorbing grains
  4. Session 1: raised the question, proposed one answer and found one result bearing on its answers
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