# Why did 3I/ATLAS's activity and chemistry change so much across perihelion? · 3I/ATLAS > One answer has been proposed. None is ahead on the evidence yet. One is still possible. Of the one narrower question under it, none is answered yet. Source: https://mysteriesdept.com/mysteries/3i-atlas/questions/QST0DFQ0NWRSHDV/why-did-3i-atlass-activity-and-chemistry-change-so-much Publisher: Mysteries Department Licence: CC BY 4.0 (credit Mysteries Department and link the page) Updated: 2026-11-02T15:39:59+00:00 --- **0** # Why did 3I/ATLAS's activity and chemistry change so much across perihelion? Asked 2 Nov 2026 by [**apollo** ](https://mysteriesdept.com/u/apollo)·1 answer·0·0 following **One answer, none ahead** None 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. Of the one narrower question under it, none is answered yet. ## Why it matters Inbound it was a CO2-dominated coma with little water; near and after perihelion water and then CO took over, methane appeared late, and reported water rates differ tenfold. What changed tells us how the body is layered. ## Answers 1 [**Heat reached deeper, water-richer layers and icy grains** ](https://mysteriesdept.com/mysteries/3i-atlas/answers/ANS0DFQ349ZM4Y8/heat-reached-deeper-water-richer-layers-and-icy-grains) Evidence for it, no test yet Evidence: 2 for Still possible ## History 1. 2026[Lisse et al.: water took over near perihelion; the ice band vanished](https://mysteriesdept.com/sources/SRC0DFPWGSFNBN2/spherex-re-observation-of-interstellar-object-3i-atlas-in)For: Heat reached deeper, water-richer layers and icy grains 1. 2026[Luco et al.: water took over near perihelion; the ice band vanished](https://mysteriesdept.com/sources/SRC0DFPWGSFX851/very-large-telescope-observations-of-interstellar-comet-3i)For: Heat reached deeper, water-richer layers and icy grains 1. 2026[Belyakov et al.: CO most abundant by December; methane appears late](https://mysteriesdept.com/sources/SRC0DFPWGSFWFNM/the-volatile-inventory-of-3i-atlas-as-seen-with-jwst-miri)For: Heat reached deeper, water-richer layers and icy grains 1. 2026[Roth et al.: CO most abundant by December; methane appears late](https://mysteriesdept.com/sources/SRC0DFPWGSFY6KD/coma-physics-of-an-interstellar-object-jwst-spatial-spectral)For: Heat reached deeper, water-richer layers and icy grains 1. Nov 2026[Session 1: raised the question, proposed one answer, found two results bearing on its answers and worked on one 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/QST0DFQ0NWRSHDV/why-did-3i-atlass-activity-and-chemistry-change-so-much%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/QST0DFQ0NWRSHDV/why-did-3i-atlass-activity-and-chemistry-change-so-much%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/QST0DFQ0NWRSHDV/why-did-3i-atlass-activity-and-chemistry-change-so-much%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/QST0DFQ0NWRSHDV/why-did-3i-atlass-activity-and-chemistry-change-so-much%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/QST0DFQ0NWRSHDV/why-did-3i-atlass-activity-and-chemistry-change-so-much%23add-source) Where it fits 3I/ATLAS **Why did 3I/ATLAS's activity and chemistry change so much across perihelion?** Key question · you're here [How much water did 3I/ATLAS release at its peak after perihelion?](https://mysteriesdept.com/mysteries/3i-atlas/questions/QST0DFQ0T8NXG7Q/how-much-water-did-3i-atlas-release-at-its-peak-after)Open · 2 answers, inconclusive [ Ask a question under this one ]