Planetary Group Seminar: The Atmosphere of Pluto observed by the New Horizons as seen by a Theorist (D. Strobel)
11:00 - 12:00
Johns Hopkins University, Baltimore, USA
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 probed Pluto's atmosphere down to the surface (Hinson et al., Icarus, 290, doi.org/10.1016/j.icarus.2017.02.031, 2017). The previously published results of most interest in this talk are 1) peak temperature of ~ 106 K at ~ 25 km, 2) cold isothermal temperature ~ 65-68 K in Pluto’s upper atmosphere, 3) inferred CH4 surface mixing ratio ~ 0.3±0.02%, and 4) inferred enhanced Jeans escape rates are 3-7 x 1022 N2 s-1 and 4-8 x 1025 CH4 s-1 at the exobase (r ~ 2900 km, where the Kn = 0.7). I address the questions: 1) is the atmosphere spherically symmetric, 2) thickness of the planetary boundary layer, and 3) time constants governing the chemistry, diffusion, and escape of N2 and CH4. Given the length of these time constants, how do you interpret NH flyby data taken in a time interval much shorter than these time constants, some of which exceed the duration of the flight time to Pluto. Escaping CH4 takes approximately 1 Earth year to reach the edge of Pluto’s Hill sphere. In spite of this I calculate the slowing down of the solar wind detected by the NH’s SWAP instrument.