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What is 3I/ATLAS made of, and why is its chemistry so unlike a Solar System comet's?

Asked 2 Nov 2026 by ·2 answers·0·0 following
Two answers, none aheadNone is ahead yet. See what would settle it.

Where it stands

Two answers have been proposed. None is ahead on the evidence yet. Two are still possible. Of the two narrower questions under it, none is answered yet.

Why it matters

Its coma held far more carbon dioxide than water, nickel without iron, too little ammonia and carbon chains, and water and methane richer in deuterium than any comet measured. Whether this is the record of the disc it formed in, or a crust made on the journey, decides what 3I can tell us about another planetary system.

Answers 2

A crust made by cosmic rays on the journey
Evidence for it, no test yet
Evidence: 2 for
Still possible
Evidence: 6 for
Still possible

Next steps 2

1
Compare gas lost with crust depth
Could shiftFair odds~20 h
11score
Details

Use the compiled production rates (Frincke and Seligman's table) to date when more than 20 m of surface had gone, and check whether the composition changed then. A change at that point favours the crust; a change well before or after weakens it.

2
Test whether a cosmic-ray crust can raise D/H to 1 per cent
Could shiftFair odds~30 h
8score
Details

Run the cosmic-ray processing model forward to predict how much a 20-m processed crust changes water D/H and 12C/13C. If processing cannot reach the measured ratios, the birthplace reading is strengthened; if it can, the isotopes no longer date the body.

Ranked by how likely each is to move the key question, for the time it takes

History

  1. Maggiolo et al.: cosmic-ray processing explains the CO2 and the red colourFor: A crust made by cosmic rays on the journey
  2. Cordiner et al.: CO2-to-water ratio fell from 7.6 to about 0.1For: A crust made by cosmic rays on the journey
  3. Furuya et al.: models reproduce the water at low metallicityFor: An icy body from a cold, metal-poor disc around an old star
  4. Hopkins et al.: models reproduce the water at low metallicityFor: An icy body from a cold, metal-poor disc around an old star
  5. Belyakov: amorphous silicates, closer to interstellar dustFor: An icy body from a cold, metal-poor disc around an old star
  6. Thomas et al.: amorphous silicates, closer to interstellar dustFor: An icy body from a cold, metal-poor disc around an old star
  7. ScienceAlert (Ivan Farkas): methane D/H the highest ever measuredFor: An icy body from a cold, metal-poor disc around an old star
  8. Opitom et al.: high carbon and nitrogen isotope ratios in CNFor: An icy body from a cold, metal-poor disc around an old star
  9. ESO: high carbon and nitrogen isotope ratios in CNFor: An icy body from a cold, metal-poor disc around an old star
  10. Rawat & Acharyya: cosmic-ray processing explains the CO2 and the red colourFor: A crust made by cosmic rays on the journey
  11. Shinnaka et al.: CO2-to-water ratio fell from 7.6 to about 0.1For: A crust made by cosmic rays on the journey
  12. Bockelee-Morvan et al.: CO2-to-water ratio fell from 7.6 to about 0.1For: A crust made by cosmic rays on the journey
  13. Session 1: raised the question, proposed two answers, found eight results bearing on its answers and worked on two of its sub-questions
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