mercredi 14 juin 2017, par Darrell Strobel (Johns Hopkins University)
Lundi 26 juin 2017 à 16h00 , Lieu : Salle de confĂ©rence du bâtiment 17 Ă Meudon
On 14 July 2015, NASA’s New Horizons spacecraft observed an ultraviolet solar occultation of Pluto’s atmosphere with its ALICE ultraviolet spectrograph and performed a radio occultation that sounded Pluto’s atmosphere down to the surface with radio signals transmitted simultaneously by four antennas of the NASA Deep Space Network. From the solar occultation data line-of-sight (los) optical depths that yield los column densities for 5 molecular species, and extinction coefficients for haze. The radio occultation data yield N2 number density, pressure, and temperature profiles from the surface to about 110 km of altitude at two diametric points on the planet. This talk presents a synthesis of the results from these two occultations. We find a very stable, spherically symmetry, lower atmosphere, with well-mixed portion restricted to a planetary boundary layer (surface to 12 km ; Kzz 500-4000 cm2 s-1), peak temperature of 106 K at 25 km, cold isothermal temperature 65-68 K in Pluto’s upper atmosphere, and inferred CH4 surface mixing ratio (0.28-0.35)%. The inferred enhanced Jeans escape rates are 5-7 x 1022 N2 s-1 and 5-8 x 1025 CH4 s-1 at the exobase (r 2900 km, where the Kn = 0.7).
