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