The Occultation of Beta Scorpii by Jupiter. 11. the Hydrogen-Helium Abundance in the Jovian Atmosphere

The helium abundance in the Jovian atmosphere has been determined by a new method from our observations of the occultation of fl Scorpii by Jupiter. The occultation light curves, consisting of simultaneous records at three wavelengths with a time resolution of 0.01 5, are dominated by sharp, well-defined intensity peaks or spikes. Because of the wavelength-dependence of the refractivity of the Jovian atmosphere, the arrival time of a given spike at the Earth is also wavelength dependent. A new method for determining the composition of a hydrogenhelium atmosphere from such data is presented, and the principal sources of error are discussed and evaluated. The derived helium fraction is f(He) = 0.16 (+0.19, -0.16) by number, and probably applies to a region below the turbopause, where the atmosphere is well mixed. In this case, a Jovian atmosphere composed mainly of helium can be ruled out. Our derived helium fraction is consistent with the solar abundance [f(He) 0.10]. Subject headings: abundances, planetary - atmospheres, planetary - Jupiter