A D/H Ratio Consistent with Earth's Water in Halley-type Comet 12P from ALMA HDO Mapping

TL;DR

This study uses ALMA to map water and semiheavy water in a comet's coma, revealing a D/H ratio similar to Earth's water, suggesting a shared origin of water in the Solar System.

Contribution

First interferometric HDO mapping of a Halley-type comet's coma, providing new insights into its D/H ratio and its implications for Earth's water origin.

Findings

D/H ratio in comet 12P is consistent with Earth's ocean water.

Comet's water likely originates from the same reservoir as Earth's water.

Results support a common heritage of water in the Solar System.

Abstract

Isotopic measurements of Solar System bodies provide a primary paradigm within which to understand the origins and histories of planetary materials. The D/H ratio in particular, helps reveal the relationship between (and heritage of) different H$_2$O reservoirs within the Solar System. Here we present interferometric maps of water (H$_2$O) 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 H$_2$O and HDO directly from the nucleus, and imply a coma D/H ratio (for water) of $(1.71 \pm 0.44)\times10^{-4}$. 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.