Elsevier

Icarus

Volume 300, 15 January 2018, Pages 174-199
Icarus

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

https://doi.org/10.1016/j.icarus.2017.09.006Get rights and content

Highlights

  • The Alice instrument on New Horizons measured a UV solar occultation by Pluto's atmosphere in 2015.

  • Densities were derived for N2, CH4, C2H2, C2H4, C2H6, and haze.

  • These imply low escape rates (CH4-dominated), a stable lower atmosphere, direct evidence for C2Hx photochemistry, and haze whose extinction coefficient is roughly proportional to N2 density.

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 (3–7) × 1022 N2 s−1 and (4–8) × 1025 CH4 s−1 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−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 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.

Introduction

We report here on the ultraviolet solar occultation by Pluto's atmosphere observed with the Alice spectrograph on NASA's New Horizons spacecraft. Ultraviolet occultations have proven invaluable for measuring the structure and composition of the other two N2-rich atmospheres in the outer solar system, Titan (Smith et al., 1982; Herbert et al., 1987; Koskinen et al., 2011; Kammer et al., 2013; Capalbo et al., 2015) and Triton (Broadfoot et al., 1989; Herbert and Sandel, 1991; Stevens et al., 1992; Krasnopolsky et al., 1992). By observing how the absorption by molecular species and extinction by haze particles vary with altitude as the Sun passes behind an atmosphere, it is possible to measure their vertical density profiles, and infer the pressure and temperature from the density of the majority species. Because pressure, temperature and composition are central to nearly every aspect of atmospheric science, the Pluto ultraviolet (UV) solar occultation was ranked as a Group 1 (required) observation for the New Horizons mission (Young et al., 2008). The UV solar occultation drove aspects of both the design of the Alice Ultraviolet Imaging Spectrograph (Stern et al., 2008) and the mission design of the New Horizons flyby past Pluto (Guo and Farquhar, 2008). We built the Alice instrument to observe the occulted solar flux from 52 to 187 nm, covering absorption by the N2 continuum on the short end and extinction by haze on the long end. We designed the spacecraft trajectory to pass through the Sun and Earth shadows of both Pluto and Charon, nearly diametrically for Pluto.

The UV solar occultation occurred from approximately 2015 July 14 12:15 to 13:32 UTC (spacecraft time). Roughly one terrestrial day later, at approximately 2015 July 15 12:38 UTC (ground receipt time), we received confirmation that the observations were successful and that the spacecraft successfully flew through Pluto's solar shadow. Downlink data volume constraints meant that this first “contingency download” of the UV solar occultation contained only the Alice housekeeping data, which included the total number of photons detected across all wavelengths each second; these data were discussed by Stern et al. (2015). The full downlink of the Pluto solar occultation (0.67 Gigabits) was completed on 2015 Oct 2. Initial analysis of Pluto's ultraviolet solar occultation (Gladstone et al., 2016) presented line-of-sight column abundances (aka light-of-sight column densities) of N2, CH4, C2H2, C2H4, and C2H6, and the local densities of N2, CH4, C2H2, and C2H4.

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 N2, CH4, C2H2, C2H4, C2H6 and haze, and (v) it includes a joint analysis with new results from the New Horizons radio occultation (Hinson et al., 2017).

Section snippets

Observations and reduction

We 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), and a previous Alice stellar occultation by Jupiter is described in Greathouse et al. (2010). Alice is an imaging spectrograph that has a bandpass from 52 to 187 nm, with a photocathode gap from 118 to 125 nm designed to decrease

Cross sections

For the ultraviolet solar occultation by Pluto observed by New Horizons, the refraction of Pluto's atmosphere can be ignored (Hinson et al., 2017), making the geometry of the occultation simple (Fig. 6). The ray connecting the Sun and the New Horizons spacecraft has a minimum distance to the body center, called the tangent radius, r', which can be defined by the surface radius, rs, and the height of the tangent point above the surface (the tangent height), h, by r' = rs + h. At a distance along

Line-of-sight abundances: hydrocarbons and haze

The retrieval of N2, CH4, C2H6, C2H2, C2H4, and haze line-of-sight abundances presented in this paper was performed individually at each altitude, in a method very similar to that used in Gladstone et al. (2016). We began by fitting for the hazes and hydrocarbons using only the wavelengths 100–180 nm, where the signal-to-noise of the occultation data was highest, and N2 did not contribute. In a later step (Section 5), we included N2 to analyze the wavelengths below 65 nm. As discussed below,

Line-of-sight abundances: nitrogen

As described in Section 3, the N2 cross section is dominated by continuum absorption shortward of 66.123 nm. At longer wavelengths, the retrieval of N2 is complicated by the interaction of the very narrow ro-vibrational lines of N2 electronic states, the solar spectrum, and the instrumental line-spread function. For this paper, we analyzed the N2 continuum, and deferred analysis of the discrete ro-vibrational N2 absorption spectral region to a later study.

The Alice bandpass that contained the N2

Local number density

The line-of-sight abundance, N, is the integral of the local number density, n, along the line of sight (Fig. 6 and Eq. (6)). Under certain assumptions, one can invert this relationship to derive n given N. By far, the most common assumption is that of local spherical symmetry (that is, within the region where the ray path intersects Pluto's atmosphere). For Pluto's upper atmosphere, spherical symmetry appeared to be a very good assumption. In the Alice Pluto solar occultation data itself,

Temperatures and mixing ratios

If the density of N2 were directly measured from Pluto's surface to 1200 km altitude, then pressures could be derived from hydrostatic equilibrium, temperatures could be directly measured from the scale height, and mixing ratios could be derived by simple ratios of densities. However, as shown in Fig. 17, this was not possible without some modeling to interpolate between the N2 density measured by the Alice solar occultation and that measured by the REX radio occultation.

The altitude of unit

Summary of observations

The present work supersedes Gladstone et al. (2016) by including new and more rigorous reductions of the Alice solar UV occultation, with improved analysis and error propagation. Additionally, the temperature and mixing ratio analysis presented here incorporated newer analysis of the REX radio occultation (Hinson et al., 2017), which used the complete REX dataset — something that had not yet been downlinked for the Gladstone et al. (2016) report. Fig. 22 demonstrates the differences between the

Future work

This analysis used C2H2, C2H4, and C2H6 cross-sections measured in the laboratory at 150 K, 140 K, and 150 K respectively, which was warmer than the ∼65–70 K in much of Pluto's atmosphere. We estimated that the current cross sections might lead to systematic errors in derived abundances of ∼10–20%. New laboratory measurements can improve the accuracy of the retrieval.

Other species were predicted to be present, from Pluto photochemical models (e.g., Summers et al., 1997, Krasnopolsky and

Acknowledgment

This work was supported, in part, by funding from NASA's New Horizons mission to the Pluto system. The New Horizons Mission Design and Navigation teams enabled us to watch this glorious sunset and sunrise. Werner Curdt provided the high spectral-resolution solar models. We gratefully acknowledge the publicly available solar data and spectroscopic data: LISIRD Lyman-alpha data from http://lasp.colorado.edu/lisird/lya/, GOES15 soft X-ray flux from http://www.swpc.noaa.gov/; and the Titan

References (90)

  • K. Kameta et al.

    Photoabsorption, photoionization, and neutral-dissociation cross sections of C2H6 and C3H8 in the extreme-UV region

    J. Electron Spectrosc. Related Phenom.

    (1996)
  • K. Kameta et al.

    Photoabsorption, photoionization, and neutral-dissociation cross sections of simple hydrocarbons in the vacuum ultraviolet range

    J. Electron Spectrosc. Related Phenom.

    (2002)
  • J.A. Kammer et al.

    Composition of Titan's upper atmosphere from Cassini UVIS EUV stellar occultations

    Planet. Space Sci.

    (2013)
  • B.N. Khare et al.

    Optical constants of organic tholins produced in a simulated Titanian atmosphere – From soft X-ray to microwave frequencies

    Icarus

    (1984)
  • T.T. Koskinen et al.

    The mesosphere and lower thermosphere of Titan revealed by Cassini/UVIS stellar occultations

    Icarus

    (2011)
  • P. Lavvas et al.

    Titan's vertical aerosol structure at the Huygens landing site: constraints on particle size, density, charge, and refractive in- dex

    Icarus

    (2010)
  • LeeL.C. et al.

    The absorption cross sections of N2, O2, CO, NO, CO2, N2O, CH4, C2H4, C2H6, and C4H10 from 180 to 700 Å

    J. Quant. Spectrosc. Radiat. Transfer

    (1973)
  • E. Lellouch et al.

    Exploring the spatial, temporal, and vertical distribution of methane in Pluto's atmosphere

    Icarus

    (2015)
  • E. Lellouch

    Detection of CO and HCN in Pluto's atmosphere with ALMA

    Icarus

    (2017)
  • F. Nimmo

    Mean radius and shape of Pluto and Charon from New Horizons images

    Icarus

    (2017)
  • R.G. Roble et al.

    A technique for recovering the vertical number density profile of atmospheric gases from planetary occultation data

    Planet. Space Sci.

    (1972)
  • D.A. Shaw et al.

    A study of the absolute photoabsorption cross section and the photionization quantum efficiency of nitrogen from the ionization threshold to 485 Å

    Chem. Phys.

    (1992)
  • J.A. Stansberry et al.

    A model for the overabundance of methane in the atmospheres of Pluto and Triton

    Planet. Space Sci.

    (1996)
  • D.F. Strobel

    Titan's hydrodynamically escaping atmosphere: escape rates and the structure of the exobase region

    Icarus

    (2009)
  • D.F. Strobel et al.

    Comparative planetary nitrogen atmospheres: density and thermal structures of Pluto and Triton

    Icarus

    (2017)
  • A.D. Toigo et al.

    General circulation models of the dynamics of Pluto's volatile transport on the eve of the New Horizons encounter

    Icarus

    (2015)
  • WongM.L et al.

    The photochemistry of Pluto's atmosphere as illuminated by New Horizons

    Icarus

    (2017)
  • ZhuX. et al.

    The density and thermal structure of Pluto's atmosphere and associated escape processes and rates

    Icarus

    (2014)
  • A.L. Broadfoot

    Ultraviolet spectrometer observations of Neptune and Triton

    Science

    (1989)
  • F.J. Capalbo et al.

    Titan's upper atmosphere from Cassini/UVIS solar occultations

    Astrophys. J.

    (2015)
  • J.W. Chamberlain et al.

    Theory of Planetary atmospheres: An Introduction to Their Physics and Chemistry

    (1987)
  • W. Curdt et al.

    The SUMER spectral atlas of solar-disk features

    Astron. Astrophys.

    (2001)
  • D. Despois et al.

    Observations of molecules in comets

    Astrochem.: Recent Success. Curr. Chall.

    (2005)
  • A. Dias-Oliveira

    Pluto’s atmosphere from stellar occultations in 2012 and 2013

    Astrophys. J.

    (2015)
  • G.R. Gladstone et al.

    Ly α @Pluto

    Icarus

    (2015)
  • G.R. Gladstone

    The atmosphere of Pluto as observed by New Horizons

    Science

    (2016)
  • GrundyW.M.

    Surface compositions across Pluto and Charon

    Science

    (2016)
  • GuoY. et al.

    New Horizons mission design

    Space Sci. Rev.

    (2008)
  • A.N. Heays

    Photoabsorption and photodissociation in molecular nitrogen

    (2011)
  • F. Herbert et al.

    The upper atmosphere of Uranus - EUV occultations observed by Voyager 2

    J. Geophys. Res.

    (1987)
  • F. Herbert et al.

    CH4 and haze in Triton's lower atmosphere

    J. Geophys. Res. Suppl.

    (1991)
  • L. Heroux et al.

    Summary of full-disk solar fluxes between 250 and 1940 A

    J. Geophys. Res.

    (1977)
  • M. Horanyi et al.

    Dust ablation in Pluto's atmosphere

  • W.F. Huebner et al.

    Solar photo rates for planetary atmospheres and atmospheric pollutants

    Astrophys. Space Sci

    (1992)
  • R.E. Huffman et al.

    Absorption coefficients of Nitrogen in the 1000–580 Å wavelength region

    J. Chem. Phys.

    (1963)
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