Comets and Asteroids

A D/H Ratio Consistent With Earth’s Water In Halley-type Comet 12P From ALMA HDO Mapping

By Keith Cowing
Press Release
August 11, 2025
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A D/H Ratio Consistent With Earth’s Water In Halley-type Comet 12P From ALMA HDO Mapping
ALMA emission maps of H2O and HDO in comet 12P/Pons-Brooks. The image cubes have been spectrally integrated, continuum-subtracted, and are centered on the thermal continuum peak. The contour separation is 5 for H2O, and 1 for HDO, where is the RMS noise of each map. Inset panels (upper right) show the spectral line profiles extracted at the origin, as a function of cometocentric velocity (v). The spatial resolution (beam FWHM) is shown lower left, and the directions of the (sky-projected) comet-Sun and orbital trail vectors are shown lower right. The coordinate axes are sky-projected offsets (in kilometers) from the continuum peak, aligned with the celestial RA (right ascension) and Dec. (declination) grid. — astro-ph.EP

The D/H ratio in particular, helps reveal the relationship between (and heritage of) different H2O reservoirs within the Solar System. Here we present interferometric maps of water (H2O) and semiheavy water (HDO) in the gas-phase coma of a comet (Halley-type comet 12P/Pons-Brooks), obtained using the Atacama Large Millimeter/submillimeter Array (ALMA).

The maps are consistent with outgassing of both H2O and HDO directly from the nucleus, and imply a coma D/H ratio (for water) of (1.71±0.44)×10−4.

This is at the lower end of the range of previously-observed values in comets, and is consistent with D/H in Earth’s ocean water. Our results suggest a possible common heritage between a component of the Oort cloud’s water ice reservoir, and the water that was delivered to the young Earth during the early history of the Solar System.

M. A. Cordiner, E. L. Gibb, Z. Kisiel, N. X. Roth, N. Biver, D. Bockelée-Morvan, J. Boissier, B. P. Bonev, S. B. Charnley, I. M. Coulson, J. Crovisier, M. N. Drozdovskaya, K. Furuya, M. Jin, Y.-J. Kuan, M. Lippi, D. C. Lis, S. N. Milam, C. Opitom, C. Qi, A. J. Remijan

Comments: Published in Nature Astronomy, 8 August 2025
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2508.05925 [astro-ph.EP] (or arXiv:2508.05925v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2508.05925
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Related DOI:
https://doi.org/10.1038/s41550-025-02614-7
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Submission history
From: Martin Cordiner PhD
[v1] Fri, 8 Aug 2025 01:15:34 UTC (1,521 KB)
https://arxiv.org/abs/2508.05925

Astrobiology,

Keith Cowing

Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp veteran, (he/him) 🖖🏻

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