# CO2-to-water ratio fell from 7.6 to about 0.1

> The coma's CO2-to-water ratio fell from 7.6 ± 0.3 at 3.3 au inbound to about 0.1 near perihelion and stayed far below the inbound value at 2.9 au outbound.

Source: https://mysteriesdept.com/mysteries/3i-atlas/evidence/EVD0DFQ3VTKN1HK/co2-to-water-ratio-fell-from-7-6-to-about-0-1  
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

# CO2-to-water ratio fell from 7.6 to about 0.1

The coma's CO2-to-water ratio fell from 7.6 ± 0.3 at 3.3 au inbound to about 0.1 near perihelion and stayed far below the inbound value at 2.9 au outbound.

Supported0

## What it means

Key question[What is 3I/ATLAS made of, and why is its chemistry so unlike a Solar System comet's?](https://mysteriesdept.com/mysteries/3i-atlas/questions/QST0DFQ0HS0K7SD/what-is-3i-atlas-made-of-and-why-is-its-chemistry-so-unlike)

No answer ahead yet

**Open** unchanged

Answer this evidence bears on[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)

Supports2 for, none against

**Still possible** unchanged

What it implies
1.

The surface gas inbound was not representative of the body as a whole.For the key question: What is 3I/ATLAS made of, and why is its chemistry so unlike a Solar System comet's?

1.

Composition changed with depth or heating, not just with distance.For: Why did 3I/ATLAS's activity and chemistry change so much across perihelion?

## Next steps 2

1

Compare gas lost with crust depth

Could shiftFair odds~20 h

[On: What is 3I/ATLAS made of, and why is its chemistry so unlike a Solar System comet's?](https://mysteriesdept.com/mysteries/3i-atlas/questions/QST0DFQ0HS0K7SD/what-is-3i-atlas-made-of-and-why-is-its-chemistry-so-unlike)

**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

[On: What is 3I/ATLAS made of, and why is its chemistry so unlike a Solar System comet's?](https://mysteriesdept.com/mysteries/3i-atlas/questions/QST0DFQ0HS0K7SD/what-is-3i-atlas-made-of-and-why-is-its-chemistry-so-unlike)

**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

## Sources 3

[A post-perihelion constraint on the CO2/H2O ratio of interstellar comet 3I/ATLAS from [O I] forbidden lines](https://mysteriesdept.com/sources/SRC0DFPWGSFT997/a-post-perihelion-constraint-on-the-co2-h2o-ratio-of)

Shinnaka et al. 2026

SRC

[JWST detection of a carbon dioxide dominated gas coma surrounding interstellar object 3I/ATLAS](https://mysteriesdept.com/sources/SRC0CSKBDB607EW/jwst-detection-of-a-carbon-dioxide-dominated-gas-coma)

Cordiner et al. 2025

SRC

[Near-perihelion activity and composition of 3I/ATLAS from JUICE/MAJIS observations](https://mysteriesdept.com/sources/SRC0DFPWGSFVGKX/near-perihelion-activity-and-composition-of-3i-atlas-from)

Bockelee-Morvan et al. 2026

SRC

## Cite it

```
Mysteries Department. (2026). CO2-to-water ratio fell from 7.6 to about 0.1 [Evidence EVD0DFQ3VTKN1HK]. https://mysteriesdept.com/r/EVD0DFQ3VTKN1HK
```

Key

**EVD** 0DFQ3VTKN1HK

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

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