Structure and composition of Pluto's atmosphere from the New Horizons solar ultraviolet occultation

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 N 2 , CH 4 , C 2 H 2 , C 2 H 4 , C 2 H 6 , 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 (3-7) x 10 22 N 2 s -1 and (4-8) x 10 25 CH 4 s -1 at the exobase (at a radius of ~ 2900 km, or an altitude of ~1710 km). (2) We measured CH 4 abundances from 80 to 1200 km above the surface. A joint analysis of the Alice CH 4 and Alice and REX N 2 measurements implied a very stable lower atmosphere with a small eddy diffusion coefficient, most likely between 550 and 4000 cm 2 s -1 . Such a small eddy diffusion coefficient placed the homopause within 12 km of the surface, giving Pluto a small planetary boundary layer. The inferred CH 4 surface mixing ratio was ~ 0.28-0.35%. (3) The abundance profiles of the "C 2 H x hydrocarbons" (C 2 H 2 , C 2 H 4 , C 2 H 6 ) were not simply exponential with altitude. We detected local maxima in line-of-sight abundance near 410 km altitude for C 2 H 4 , near 320 km for C 2 H 2 , and an inflection point or the suggestion of a local maximum at 260 km for C 2 H 6 . We also detected local minima near 200 km altitude for C 2 H 4 , near 170 km for C 2 H 2 , and an inflection point or minimum near 170-200 km for C 2 H 6 . These compared favorably with models for hydrocarbon production near 300-400 km and haze condensation near 200 km, especially for C 2 H 2 and C 2 H 4 (Wong et al. 2017). (4) We found haze that had an extinction coefficient approximately proportional to N 2 density. This paper extends the analysis of Gladstone et al. (2016) in the following ways: (i) it uses an improved reduction of the raw observations, and includes more details about the observation and reduction process, (ii) it presents error analysis, including correlations between the measurements of various species, (iii) it includes analysis of extinction by haze at the long-wavelength end of the Alice range, (iv) it improves or extends the density retrievals of N 2 , CH 4 , C 2 H 2 , C 2 H 4 , C 2 H 6 and haze, and (v) it includes a joint analysis with new results from the New Horizons radio occultation (Hinson et al. 2017). Observations and ReductionWe recap here the salient features of the Alice ultraviolet spectrograph on the New Horizons spacecraft and its observation of Pluto's atmosphere during the solar occultation. Alice (which is a name, not an acronym) is described in more detail in Stern et al. (2008), FIGURE 1. The Alice Solar Occultation Channel (SOCC), Pluto, and the Sun at the time of solar ingress at 2015 Jul 14 12:44 UT (left) an...

Photon-induced desorption of larger species in UV-irradiated methane ice

“…Our experiments show that photoninduced processes, as discussed in Wong et al (2017), can contribute to the processes enriching the gas-phase abundance of C 2 H 6 and C 2 H 2 species, while C 2 H 4 was not detected in the gas-phase. These results qualitatively agree with Pluto observations reported by Young et al (2018), which measured C 2 H 6 to be the most abundant hydrocarbon (apart from CH 4 ), followed by C 2 H 2 and C 2 H 4 .…”

“…3.3. C 2 H 4 , which was not identified by IR or QMS, could be responsible for the bump appearing between 160-180 nm (7.75-6.89 eV), according to its UV absorption spectrum (Lu et al 2004;Young et al 2018).…”

“…On the other hand, pure CH 4 is abundant in transneptunian objects (TNOs) such as Triton or Pluto , and references therein) and may be a common ice component in outer regions of protoplanetary disks. The composition of Pluto's atmosphere contains not only N 2 and CH 4 , but also C 2 H 2 , C 2 H 4 , and C 2 H 6 (Young et al 2018). The surface temperature is high enough to allow thermal desorption of CH 4 , explaining its presence in the gas-phase, while other species require the presence of alternative pathways.…”

“…This photoproduct can desorb if the excess energy of the parent photofragments is sufficient to overcome the binding energy of the ice, due to the exothermicity of the reaction (Andersson & van Dishoeck 2008;Fayolle et al 2013;Fillion et al 2014;Bertin et al 2016). Photochemical desorption or photochemidesorption only applies to species that are formed on the ice surface and are immediately ejected to the gas, leading to a constant photodesorption yield during the irradiation , 2018Cruz-Diaz et al 2016).…”

Volatile evolution and atmospheres of Trans-Neptunian objects

“…Prior to the flyby, the expected escape rate was ~[0.4-4] x 10 27 N 2 s -1 (Zhu et al 2014), consistent with energy-limited escape. The escape rate that is derived from the observed density and temperature profile is much lower: (3-7) × 10 22 N 2 s −1 and (4-8) × 10 25 CH 4 s −1 (Young et al 2018). Applying these new observations has been confounded by the fact that the principal cooling agent in Pluto's upper atmosphere, which also compressed the extent of Pluto's atmosphere, is still uncertain (Gladstone & Young 2019).…”