NASA Goddard Space Flight Center
The D/H Ratio in Cometary Dust
The D/H ratio is a clue to the origin and evolution of hydrogen-bearing chemical species in solar system materials. D/H has been observed in the coma of many comets, but most of those measurements have been in gaseous water. We present the first in-situ measurements of the D/H ratios in the organic refractory component of cometary dust particles collected at very low impact speeds in the coma of comet 67P/Churyumov-Gerasimenko (hereafter 67P) by the COSIMA instrument onboard Rosetta. The values measured by COSIMA are spatial averages over the approximately 35 x 50 micron beam spot. The average D/H ratio for the 25 measured particles is (1.41 ± 0.45) x 10-3, about an order of magnitude higher than the Vienna Standard Mean Ocean Water (VSMOW) but more than order of magnitude lower than the values measured in organics in solar protostar regions and hot cores. This relatively high averaged value suggests that refractory carbonaceous matter in comet 67P is less processed than the most primitive meteorite insoluble organic matter (IOM) which has a D/H ratio in the range of about 1 to 7 x 10-4. The in-situ measured cometary particles have also a higher H/C ratio than the IOM and we deduce that the measured D/H in cometary organics is an inheritance from the protosolar molecular cloud from which the Solar System formed. The high D/H ratio observed in the cometary particles challenges models in which high D/H ratios result solely from alteration processes in the protosolar disk.