New Horizons Alice ultraviolet observations of a stellar occultation by Jupiter's atmosphere

Abstract The Alice ultraviolet spectrograph onboard the New Horizons spacecraft observed two occultations of the bright star χ Ophiucus by Jupiter’s atmosphere on February 22 and 23, 2007 during the approach phase of the Jupiter flyby. The ingress occultation probed the atmosphere at 32°N latitude near the dawn terminator, while egress probed 18°N latitude near the dusk terminator. A detailed analysis of both the ingress and egress occultations, including the effects of molecular hydrogen, methane, acetylene, ethylene, and ethane absorptions in the far ultraviolet (FUV), constrains the eddy diffusion coefficient at the homopause level to be 3.4 - 2.8 + 9.0 × 10 6  cm 2  s −1 , consistent with Voyager measurements and other analyses (Festou, M.C., Atreya, S.K., Donahue, T.M., Sandel, B.R., Shemansky, D.E., Broadfoot, A.L. [1981]. J. Geophys. Res. 86, 5717–5725; Vervack Jr., R.J., Sandel, B.R., Gladstone, G.R., McConnell, J.C., Parkinson, C.D. [1995]. Icarus 114, 163–173; Yelle, R.V., Young, L.A., Vervack Jr., R.J., Young, R., Pfister, L., Sandel, B.R. [1996]. J. Geophys. Res. 101 (E1), 2149–2162). However, the actual derived pressure level of the methane homopause for both occultations differs from that derived by Festou et al., 1981 , Yelle et al., 1996 from the Voyager ultraviolet occultations, suggesting possible changes in the strength of atmospheric mixing with time. We find that at 32°N latitude, the methane concentration is 3.1 - 0.5 + 0.5 × 10 8  cm −3 at 70,397 km, the methane concentration is 1.2 - 0.3 + 0.3 × 10 9  cm −3 at 70,383 km, the acetylene concentration is 1.4 - 0.2 + 0.4 × 10 8  cm −3 at 70,364 km, and the ethane concentration is 6.8 - 0.8 + 1.1 × 10 8  cm −3 at 70,360 km. At 18°N latitude, the methane concentration is 3.2 - 0.7 + 0.7 × 10 8  cm −3 at 71,345 km, the methane concentration is 1.2 - 0.2 + 0.6 × 10 9  cm −3 at 71,332 km, the acetylene concentration is 1.6 - 0.6 + 0.3 × 10 8 cm −3 at 71,318 km, and the ethane concentration is 7.0 - 2.5 + 2.4 × 10 8  cm −3 at 71,315 km. We also find that the H 2 occultation light curve is best reproduced if the atmosphere remains cold in the microbar region such that the base of the thermosphere is located at a lower pressure level than that determined by in situ instruments aboard the Galileo probe (Seiff, A., Kirk, D.B., Knight, T.C.D., Young, R.E., Mihalov, J.D., Young, L.A., Milos, F.S., Schubert, G., Blanchard, R.C., Atkinson, D. [1998]. J. Geophys. Res. 103 (E10), 22857–22889) – the Sieff et al. temperature profile leads to too much absorption from H 2 at high altitudes. However, this result is highly model dependent and non-unique. The observations and analysis help constrain photochemical models of Jupiter’s atmosphere.