# JWST: water D/H near 1 per cent and high carbon-13 ratios

> JWST measured a water D/H ratio of 0.95 ± 0.06 per cent and 12C/13C of 141 to 191 in CO2 and 123 to 172 in CO, pointing to ices formed below about 30 K…

Source: https://mysteriesdept.com/mysteries/3i-atlas/evidence/EVD0DFQ3PX0VGG4/jwst-water-d-h-near-1-per-cent-and-high-carbon-13-ratios  
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

# JWST: water D/H near 1 per cent and high carbon-13 ratios

JWST measured a water D/H ratio of 0.95 ± 0.06 per cent and 12C/13C of 141 to 191 in CO2 and 123 to 172 in CO, pointing to ices formed below about 30 K around a metal-poor star some 10 to 12 billion years ago.

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[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)

Supports6 for, none against

**Still possible** unchanged

What it implies
1.

The ices were made colder and in a different chemical environment than the Sun's comets.For the key question: What is 3I/ATLAS made of, and why is its chemistry so unlike a Solar System comet's?

1.

An independent, non-kinematic hint that 3I is older than the Sun.For: Where in the Galaxy, and how long ago, did 3I/ATLAS form?

## 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 2

[Isotopic Evidence for a Cold and Distant Origin of the Interstellar Object 3I/ATLAS](https://mysteriesdept.com/sources/SRC0DFPWGSFXVMV/isotopic-evidence-for-a-cold-and-distant-origin-of-the)

Cordiner et al. 2026

SRC

[NASA's Webb Finds Clues to Ancient, Distant Origin of Comet 3I/ATLAS](https://mysteriesdept.com/sources/SRC0DFPZ6F4MVXC/nasas-webb-finds-clues-to-ancient-distant-origin-of-comet-3i)

Goddard 2026

SRC

## Cite it

```
Mysteries Department. (2026). JWST: water D/H near 1 per cent and high carbon-13 ratios [Evidence EVD0DFQ3PX0VGG4]. https://mysteriesdept.com/r/EVD0DFQ3PX0VGG4
```

Key

**EVD** 0DFQ3PX0VGG4

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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