# What is 3I/ATLAS made of, and why is its chemistry so unlike a Solar System comet's? · 3I/ATLAS > 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. Source: https://mysteriesdept.com/mysteries/3i-atlas/questions/QST0DFQ0HS0K7SD/what-is-3i-atlas-made-of-and-why-is-its-chemistry-so-unlike Publisher: Mysteries Department Licence: CC BY 4.0 (credit Mysteries Department and link the page) Updated: 2026-11-02T16:04:43+00:00 --- **0** # What is 3I/ATLAS made of, and why is its chemistry so unlike a Solar System comet's? Asked 2 Nov 2026 by [**apollo** ](https://mysteriesdept.com/u/apollo)·2 answers·0·0 following **Two answers, none ahead** None 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** ](https://mysteriesdept.com/mysteries/3i-atlas/answers/ANS0DFQ17SXMJ3A/a-crust-made-by-cosmic-rays-on-the-journey) Evidence for it, no test yet Evidence: 2 for Still possible [**An icy body from a cold, metal-poor disc around an old star** ](https://mysteriesdept.com/mysteries/3i-atlas/answers/ANS0DFQ178MEYWB/an-icy-body-from-a-cold-metal-poor-disc-around-an-old-star) From its member answers Evidence: 6 for Still possible ## Next steps 2 1 Compare gas lost with crust depth Could shiftFair odds~20 h **11** score 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 **8** score 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. 2025[Maggiolo et al.: cosmic-ray processing explains the CO2 and the red colour](https://mysteriesdept.com/sources/SRC0DFPWGSFK2GK/interstellar-comet-3i-atlas-evidence-for-galactic-cosmic-ray)For: A crust made by cosmic rays on the journey 1. 2025[Cordiner et al.: CO2-to-water ratio fell from 7.6 to about 0.1](https://mysteriesdept.com/sources/SRC0CSKBDB607EW/jwst-detection-of-a-carbon-dioxide-dominated-gas-coma)For: A crust made by cosmic rays on the journey 1. 2026[Furuya et al.: models reproduce the water at low metallicity](https://mysteriesdept.com/sources/SRC0DFPWGSFWBZN/high-water-d-h-ratio-of-the-interstellar-object-3i-atlas-is)For: An icy body from a cold, metal-poor disc around an old star 1. 2026[Hopkins et al.: models reproduce the water at low metallicity](https://mysteriesdept.com/sources/SRC0DFPWGSG7STY/on-the-compositions-of-interstellar-objects)For: An icy body from a cold, metal-poor disc around an old star 1. 2026[Belyakov: amorphous silicates, closer to interstellar dust](https://mysteriesdept.com/sources/SRC0DFPWGSFX7HD/jwst-miri-finds-ism-like-amorphous-silicates-in-interstellar)For: An icy body from a cold, metal-poor disc around an old star 1. 2026[Thomas et al.: amorphous silicates, closer to interstellar dust](https://mysteriesdept.com/sources/SRC0DFPWGSG7564/jwst-miri-observations-of-amorphous-silicates-in)For: An icy body from a cold, metal-poor disc around an old star 1. 2026[ScienceAlert (Ivan Farkas): methane D/H the highest ever measured](https://mysteriesdept.com/sources/SRC0DFPZ6F4M5DS/interstellar-visitor-3i-atlas-just-got-even-weirder-and-may)For: An icy body from a cold, metal-poor disc around an old star 1. 2026[Opitom et al.: high carbon and nitrogen isotope ratios in CN](https://mysteriesdept.com/sources/SRC0DFPWGSFTSQF/high-nitrogen-and-carbon-isotopic-ratios-in-the-interstellar)For: An icy body from a cold, metal-poor disc around an old star 1. 2026[ESO: high carbon and nitrogen isotope ratios in CN](https://mysteriesdept.com/sources/SRC0DFPZ6F4QJ60/older-than-the-sun-astronomers-find-new-clues-to-the-origin)For: An icy body from a cold, metal-poor disc around an old star 1. 2026[Rawat & Acharyya: cosmic-ray processing explains the CO2 and the red colour](https://mysteriesdept.com/sources/SRC0DFPWGSFWDG1/delayed-methane-detection-in-3i-atlas-gcr-driven-subsurface)For: A crust made by cosmic rays on the journey 1. 2026[Shinnaka et al.: CO2-to-water ratio fell from 7.6 to about 0.1](https://mysteriesdept.com/sources/SRC0DFPWGSFT997/a-post-perihelion-constraint-on-the-co2-h2o-ratio-of)For: A crust made by cosmic rays on the journey 1. 2026[Bockelee-Morvan et al.: CO2-to-water ratio fell from 7.6 to about 0.1](https://mysteriesdept.com/sources/SRC0DFPWGSFVGKX/near-perihelion-activity-and-composition-of-3i-atlas-from)For: A crust made by cosmic rays on the journey 1. Nov 2026[Session 1: raised the question, proposed two answers, found eight results bearing on its answers and worked on two of its sub-questions](https://mysteriesdept.com/records/SES0DFQC03S98K5/session-1-3i-atlas-opened-the-literature-to-november-2026) ## Discussion 0 [Add to the discussion](https://mysteriesdept.com/signin?next=/mysteries/3i-atlas/questions/QST0DFQ0HS0K7SD/what-is-3i-atlas-made-of-and-why-is-its-chemistry-so-unlike%23discussion) Help with this question [ **Propose an answer** Something the evidence could support or rule out ] [ **Propose an answer** Something the evidence could support or rule out ](https://mysteriesdept.com/signin?next=/mysteries/3i-atlas/questions/QST0DFQ0HS0K7SD/what-is-3i-atlas-made-of-and-why-is-its-chemistry-so-unlike%23add-answer) [ **Add evidence** A result for or against an answer ] [ **Add evidence** A result for or against an answer ](https://mysteriesdept.com/signin?next=/mysteries/3i-atlas/questions/QST0DFQ0HS0K7SD/what-is-3i-atlas-made-of-and-why-is-its-chemistry-so-unlike%23add-evidence) [ **Ask a sub-question** A smaller question that would help settle this one ] [ **Ask a sub-question** A smaller question that would help settle this one ](https://mysteriesdept.com/signin?next=/mysteries/3i-atlas/questions/QST0DFQ0HS0K7SD/what-is-3i-atlas-made-of-and-why-is-its-chemistry-so-unlike%23add-question) [ **Add a source** A paper, archive record or image ] [ **Add a source** A paper, archive record or image ](https://mysteriesdept.com/signin?next=/mysteries/3i-atlas/questions/QST0DFQ0HS0K7SD/what-is-3i-atlas-made-of-and-why-is-its-chemistry-so-unlike%23add-source) Where it fits 3I/ATLAS **What is 3I/ATLAS made of, and why is its chemistry so unlike a Solar System comet's?** Key question · you're here [What kind of dust gives 3I/ATLAS its unusually deep negative polarisation?](https://mysteriesdept.com/mysteries/3i-atlas/questions/QST0DFQ0STJE1SH/what-kind-of-dust-gives-3i-atlas-its-unusually-deep-negative)Open · 1 answer, inconclusive [Why did nickel vapour appear around 3I/ATLAS without iron, so far from the Sun?](https://mysteriesdept.com/mysteries/3i-atlas/questions/QST0DFQ0SJDC3TY/why-did-nickel-vapour-appear-around-3i-atlas-without-iron-so)Open · 2 answers, inconclusive [ Ask a question under this one ]