  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. 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.
