# CO most abundant by December; methane appears late

> By 22-23 December 2025 CO was the most abundant gas, with water and CO2 each about 40 per cent of it, and methane was detected for the first time in an…

Source: https://mysteriesdept.com/mysteries/3i-atlas/evidence/EVD0DFQ54XAFGSW/co-most-abundant-by-december-methane-appears-late  
Publisher: Mysteries Department  
Licence: CC BY 4.0 (credit Mysteries Department and link the page)  

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[3I/ATLAS](https://mysteriesdept.com/mysteries/3i-atlas)Evidence

Evidence · 3I/ATLAS

# CO most abundant by December; methane appears late

By 22-23 December 2025 CO was the most abundant gas, with water and CO2 each about 40 per cent of it, and methane was detected for the first time in an interstellar object only in mid-December.

Supported0

## What it means

Key question[Why did 3I/ATLAS's activity and chemistry change so much across perihelion?](https://mysteriesdept.com/mysteries/3i-atlas/questions/QST0DFQ0NWRSHDV/why-did-3i-atlass-activity-and-chemistry-change-so-much)

No answer ahead yet

**Open** unchanged

Answer this evidence bears on[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)

Supports2 for, none against

**Still possible** unchanged

What it implies
1.

Volatile ices that were not active inbound became active after perihelion.For the key question: Why did 3I/ATLAS's activity and chemistry change so much across perihelion?

1.

The bulk composition differs from the inbound surface.For: What is 3I/ATLAS made of, and why is its chemistry so unlike a Solar System comet's?

## Sources 2

[Coma Physics of an Interstellar Object: JWST Spatial-Spectral Mapping of 3I/ATLAS](https://mysteriesdept.com/sources/SRC0DFPWGSFY6KD/coma-physics-of-an-interstellar-object-jwst-spatial-spectral)

Roth et al. 2026

SRC

[The Volatile Inventory of 3I/ATLAS as seen with JWST/MIRI](https://mysteriesdept.com/sources/SRC0DFPWGSFWFNM/the-volatile-inventory-of-3i-atlas-as-seen-with-jwst-miri)

Belyakov et al. 2026

SRC

## Cite it

```
Mysteries Department. (2026). CO most abundant by December; methane appears late [Evidence EVD0DFQ54XAFGSW]. https://mysteriesdept.com/r/EVD0DFQ54XAFGSW
```

Key

**EVD** 0DFQ54XAFGSW

Kind

Evidence

Tier

Supported

Found in

[Session 1](https://mysteriesdept.com/mysteries/3i-atlas/sessions/SES0DFQC03S98K5/session-1-3i-atlas-opened-the-literature-to-november-2026) · 2 Nov 2026

Version

1

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