Detection of HD in the atmospheres of Uranus and Neptune: a new determination of the D/H ratio

Observations with the Short Wavelength Spectrometer (SWS) onboard the Infra red Space Observatory (ISO) have led to the first unambiguous detection of HD in the atmospheres of Uranus and Neptune, from its R(2) rotational line at 37.7 mu m Using S(0) and S(1) quadrupolar lines of H-2 at 28.2 and 17.0 mu m as atmospheric thermometers, we derive D/H ratios of 5.5((+3.5)(-1.5)) x 10(-5) and 6.5((+2.5)(-1.5)) x 10(-5) on Uranus and Neptune respectively . This confirms that compared to Jupiter and Saturn, Uranus and Neptune hav e been enriched in deuterium by the mixing of their atmospheres with D-rich protoplanetary ices. Assuming complete mixing of the planets, the D/H rati o in these ices is however found to be lower than in cometary ices.