Terrestrial deuterium-to-hydrogen ratio in water in hyperactive comets

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 cometsand Jupiter-family cometsoriginating from the Kuiper belt. We present new sensitive spectroscopic observations of water isotopologuesin the Jupiter-family comet46P/Wirtanen carried out using the GREAT spectrometer aboard the Stratospheric Observatoryfor 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. Cometswith 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 cometsbelong 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 planetesimalsnear the snow lineare expected to undergo rapid growth. Alternatively, isotopic propertiesof water outgassedfrom the nucleus and icy grains may be different due to fractionation effects at sublimation. In this case, all cometsmay 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.