THE ULTRAVIOLET REFLECTIVITY OF JUPITER AT 3.5 ANGSTROM RESOLUTION FROM ASTRO-1 AND ASTRO-2

The reflectivity of Jupiter in the wavelength band from 1450 to 1850 A ngstrom at 3.5 Angstrom resolution has been determined from measuremen ts of Jovian equatorial spectra obtained by the Hopkins Ultraviolet Te lescope, which was flown on the Space Shuttle Columbia during the 1990 December Astro-l mission and then on the Endeavour during the 1995 Ma rch Astro-2 mission. Good model fits to the data were obtained with on ly three hydrocarbon absorbers and an electron-impacted H-2 dayglow so urce. Small changes (similar to 10%) in the reflectivity at the limit of the mission-to-mission instrument calibrations were observed; these resulted in somewhat different hydrocarbon abundances in 1990 and 199 5. We have determined that the abundances of acetylene, ethane, and et hylene were 39 +/- 3 ppb, 3 +/- 1 ppm, and less than 0.4 ppb, respecti vely, in 1990, and 28 +/- 3 ppb, 3 +/- 1 ppm, and 0.4 +/- 0.2 ppb in 1 995. These mixing ratios are consistent with results from recent IUE s pectra at lower resolution. The integrated (1450-1850 Angstrom) contri bution of the H-2 dayglow was 800 +/- 400 R in 1990 and less than 400 R in 1995, which was a period spanning solar maximum and solar minimum .