Herschel measurements of the D/H and 16O/18O ratios in water in the Oort-cloud comet C/2009 P1 (Garradd)

TL;DR

This study measures the D/H and 16O/18O ratios in water from the Oort-cloud comet C/2009 P1 (Garradd) using Herschel, revealing diversity in cometary water isotopic compositions and challenging previous assumptions of uniformity.

Contribution

First measurement of D/H ratio in water of an Oort-cloud comet C/2009 P1 (Garradd) using Herschel, showing variability in cometary isotopic ratios.

Findings

D/H ratio in C/2009 P1 (Garradd) is 2.06±0.22×10^-4

H2^16O/H2^18O ratio is 523±32, consistent with terrestrial value

Oort-cloud comets exhibit diverse D/H ratios, not a single archetype

Abstract

The D/H ratio in cometary water is believed to be an important indicator of the conditions under which icy planetesimals formed and can provide clues to the contribution of comets to the delivery of water and other volatiles to Earth. Available measurements suggest that there is isotopic diversity in the comet population. The Herschel Space Observatory revealed an ocean-like ratio in the Jupiter-family comet 103P/Hartley 2, whereas most values measured in Oort-cloud comets are twice as high as the ocean D/H ratio. We present here a new measurement of the D/H ratio in the water of an Oort-cloud comet. HDO, H_2O, and H_2^18O lines were observed with high signal-to-noise ratio in comet C/2009 P1 (Garradd) using the Herschel HIFI instrument. Spectral maps of two water lines were obtained to constrain the water excitation. The D/H ratio derived from the measured H_2^16O and HDO production rates is 2.06+/-0.22 X 10**-4. This result shows that the D/H in the water of Oort-cloud comets is not as high as previously thought, at least for a fraction of the population, hence the paradigm of a single, archetypal D/H ratio for all Oort-cloud comets is no longer tenable. Nevertheless, the value measured in C/2009 P1 (Garradd) is significantly higher than the Earth's ocean value of 1.558 X 10**-4. The measured H_2^16O/H_2^18O ratio of 523+/-32 is, however, consistent with the terrestrial value.