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Structure and Composition of Pluto's atmosphere from the New Horizons Solar Ultraviolet Occultation | Leslie A. Young
; Joshua A. Kammer
; Andrew J. Steffl
; G. Randall Gladstone
; Michael E. Summers
; Darrell F. Strobel
; David P. Hinson
; S. Alan Stern
; Harold A. Weaver
; Catherine B. Olkin
; Kimberly Ennico
; David J. McComas
; Andrew F. Cheng
; Peter Gao
; Panayotis Lavvas
; Ivan R. Linscott
; Michael L. Wong
; Yuk L. Yung
; Nathanial Cunningham
; Michael Davis
; Joel Wm. Parker
; Eric Schindhelm
; Oswald H.W. Siegmund
; John Stone
; Kurt Retherford
; Maarten Versteeg
; | Date: |
5 Apr 2017 | Abstract: | The Alice instrument on NASA’s New Horizons spacecraft observed an
ultraviolet solar occultation by Pluto’s atmosphere on 2015 July 14. The
transmission vs. altitude was sensitive to the presence of N2, CH4, C2H2, C2H4,
C2H6, and haze. We derived line-of-sight abundances and local number densities
for the 5 molecular species, and line-of-sight optical depth and extinction
coefficients for the haze. We found the following major conclusions: 1) We
confirmed temperatures in Pluto’s upper atmosphere that were colder than
expected before the New Horizons flyby, with upper atmospheric temperatures
near 65-68 K. The inferred enhanced Jeans escape rates were (3e22-7e22) N2/s
and (4e25-8e25) CH4/s at the exobase (at a radius of ~2900 km, or an altitude
of ~1710 km). 2) We measured CH4 abundances from 80 to 1200 km above the
surface. A joint analysis of the Alice CH4 and Alice and REX N2 measurements
implied a very stable lower atmosphere with a small eddy diffusion coefficient,
most likely between 550 and 4000 cm2/s. Such a small eddy diffusion coefficient
placed the homopause within 12 km of the surface, giving Pluto a small
planetary boundary layer. The inferred CH4 surface mixing ratio was
~0.28-0.35%. 3) The abundance profiles of the C2Hx hydrocarbons (C2H2, C2H4,
C2H6) were not simply exponential with altitude. We detected local maxima in
line-of-sight abundance near 410 km altitude for C2H4, near 320 km for C2H2,
and an inflection point or the suggestion of a local maximum at 260 km for
C2H6. We also detected local minima near 200 km altitude for C2H4, near 170 km
for C2H2, and an inflection point or minimum near 170-200 km for C2H6. These
compared favorably with models for hydrocarbon production near 300-400 km and
haze condensation near 200 km, especially for C2H2 and C2H4 (Wong et al. 2017).
4) We found haze that had an extinction coefficient approximately proportional
to N2 density. | Source: | arXiv, 1704.1511 | Services: | Forum | Review | PDF | Favorites |
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