Far-Ultraviolet Spectroscopy of Venus and Mars at 4 Å Resolution with the Hopkins Ultraviolet Telescope on Astro-2

Far-ultraviolet spectra of Venus and Mars in the range 820-1840 Å at ~4 Å resolution were obtained on 1995 March 13 and 12, respectively, by the Hopkins Ultraviolet Telescope (HUT), which was part of the Astro-2 observatory on the space shuttle Endeavour. Longward of 1250 Å, the spectra of both planets are dominated by emission of the CO fourth positive (A¹Π-X¹Σ⁺) band system and strong O I and C I multiplets. In addition, CO Hopfield-Birge bands, B¹Σ⁺-X¹Σ⁺ (0, 0) at 1151 Å and C¹Σ⁺-X¹Σ⁺ (0, 0) at 1088 Å, are detected for the first time, and there is a weak indication of the E¹Π-X¹Σ⁺ (0, 0) band at 1076 Å in the spectrum of Venus. The B-X band is blended with emission from O I λ1152. Modeling the relative intensities of these bands suggests that resonance fluorescence of CO is the dominant source of the emission, as it is for the fourth positive system. Shortward of Lyα, other emission features detected include O II λ834, O I λ989, H I Lyβ, and N I λλ1134 and 1200. For Venus, the derived disk brightnesses of the O I, O II, and H I features are about one-half of those reported by Hord et al. from Galileo EUV measurements made in 1990 February. This result is consistent with the expected variation from solar maximum to solar minimum. The Ar I λλ1048, 1066 doublet is detected only in the spectrum of Mars, and the derived mixing ratio of Ar is of the order of 2%, consistent with previous determinations.