The D/H ratio in cometary water has been shown to vary between 1 and 3 times the Earth’s oceans value , in both Oort cloud comets and Jupiter-family comets originating from the Kuiper belt .
This has been taken as evidence that comets contributed a relatively small fraction of the terrestrial water .
We present new sensitive spectroscopic observations of water isotopologues in the Jupiter-family comet 46P/Wirtanen carried out using the GREAT spectrometer aboard the Stratospheric Observatory for Infrared Astronomy ( SOFIA ) .
The derived D/H ratio of ( 1.61 \pm 0.65 ) \times 10 ^ { -4 } is the same as in the Earth’s oceans .
Although the statistics are limited , we show that interesting trends are already becoming apparent in the existing data .
A clear anti-correlation is seen between the D/H ratio and the active fraction , defined as the ratio of the active surface area to the total nucleus surface .
Comets with an active fraction above 0.5 typically have D/H ratios in water consistent with the terrestrial value .
These hyperactive comets , such as 46P/Wirtanen , require an additional source of water vapor in their coma , explained by the presence of subliming icy grains expelled from the nucleus .
The observed correlation may suggest that hyperactive comets belong to a population of ice-rich objects that formed just outside the snow line , or in the outermost regions of the solar nebula , from water thermally reprocessed in the inner disk that was transported outward during the early disk evolution .
The observed anti-correlation between the active fraction and the nucleus size seems to argue against the first interpretation , as planetesimals near the snow line are expected to undergo rapid growth .
Alternatively , isotopic properties of water outgassed from the nucleus and icy grains may be different due to fractionation effects at sublimation .
In this case , all comets may share the same Earth-like D/H ratio in water , with profound implications for the early solar system and the origin of Earth’s oceans .