Detection of HD on Neptune and Determination of D/H Ratio from ISO/SWS Measurements
H. Feuchtgruber (ISO SOC, Villafranca), E. Lellouch (Obs. Meudon), Th. de Graauw (SRON Groningen), Th. Encrenaz (Obs. Meudon), M. Griffin (QMWC London)
The deuterium abundance in Jupiter and Saturn probably reflects the D/H ratio in the protosolar nebula. In contrast, Uranus and Neptune may have been enriched in deuterium by the mixing of their H envelopes with volatiles originating from D-enriched ices. Observational support for this plausible scenario is still needed, however, as current D/H determinations on Uranus and Neptune come from still relatively uncertain near-IR measurements of CH D.
Observations with the Short Wavelength Spectrometer (SWS) of the Infrared Space Observatory (ISO), performed on Oct. 9, 1996 and March 26, 1997, have allowed the detection of the rotational R(2) line of HD at 37.7 micron (265.3 cm ). The line, observed at a resolving power of about 1400 with the grating mode of SWS, shows up as an emission feature of about 10 Jy over a 95 Jy continuum. This observation represents in fact the first ever detection of HD in Neptune. An upper limit on the HD R(3) line at 28.5 m was also obtained.
Modelling of these observations and a preliminary estimate of the D/H ratio in Neptune will be given. As the HD mixing ratio inferred from the observations is sensitive to the Neptune lower stratosphere temperature profile, we use the 28.2 micron S(0) line H , detected simultaneously with HD, as a modelling constraint.