Eric Schindhelm

The atmosphere of Pluto as observed by New Horizons

New Horizons unveils the Pluto system In July 2015, the New Horizons spacecraft flew through the Pluto system at high speed, humanitys first close look at this enigmatic system on the outskirts of our solar system. In a series of papers, the New Horizons team present their analysis of the encounter data downloaded so far: Moore et al. present the complex surface features and geology of Pluto and its large moon Charon, including evidence of tectonics, glacial flow, and possible cryovolcanoes. Grundy et al. analyzed the colors and chemical compositions of their surfaces, with ices of H2O, CH4, CO, N2, and NH3 and a reddish material which may be tholins. Gladstone et al. investigated the atmosphere of Pluto, which is colder and more compact than expected and hosts numerous extensive layers of haze. Weaver et al. examined the small moons Styx, Nix, Kerberos, and Hydra, which are irregularly shaped, fast-rotating, and have bright surfaces. Bagenal et al. report how Pluto modifies its space environment, including interactions with the solar wind and a lack of dust in the system. Together, these findings massively increase our understanding of the bodies in the outer solar system. They will underpin the analysis of New Horizons data, which will continue for years to come. Science, this issue pp. 1284, 10.1126/science.aad9189, 10.1126/science.aad8866, 10.1126/science.aae0030, & 10.1126/science.aad9045 Pluto’s atmosphere is cold, rarefied, and made mostly of nitrogen and methane, with layers of haze. INTRODUCTION For several decades, telescopic observations have shown that Pluto has a complex and intriguing atmosphere. But too little has been known to allow a complete understanding of its global structure and evolution. Major goals of the New Horizons mission included the characterization of the structure and composition of Pluto’s atmosphere, as well as its escape rate, and to determine whether Charon has a measurable atmosphere. RATIONALE The New Horizons spacecraft included several instruments that observed Pluto’s atmosphere, primarily (i) the Radio Experiment (REX) instrument, which produced near-surface pressure and temperature profiles; (ii) the Alice ultraviolet spectrograph, which gave information on atmospheric composition; and (iii) the Long Range Reconnaissance Imager (LORRI) and Multispectral Visible Imaging Camera (MVIC), which provided images of Pluto’s hazes. Together, these instruments have provided data that allow an understanding of the current state of Pluto’s atmosphere and its evolution. RESULTS The REX radio occultation determined Pluto’s surface pressure and found a strong temperature inversion, both of which are generally consistent with atmospheric profiles retrieved from Earth-based stellar occultation measurements. The REX data showed near-symmetry between the structure at ingress and egress, as expected from sublimation driven dynamics, so horizontal winds are expected to be weak. The shallow near-surface boundary layer observed at ingress may arise directly from sublimation. The Alice solar occultation showed absorption by methane and nitrogen and revealed the presence of the photochemical products acetylene and ethylene. The observed nitrogen opacity at high altitudes was lower than expected, which is consistent with a cold upper atmosphere. Such low temperatures imply an additional, but as yet unidentified, cooling agent. A globally extensive haze extending to high altitudes, and with numerous embedded thin layers, is seen in the New Horizons images. The haze has a bluish color, suggesting a composition of very small particles. The observed scattering properties of the haze are consistent with a tholin-like composition. Buoyancy waves generated by winds flowing over orography can produce vertically propagating compression and rarefaction waves that may be related to the narrow haze layers. Pluto’s cold upper atmosphere means atmospheric escape must occur via slow thermal Jeans’ escape. The inferred escape rate of nitrogen is ~10,000 times slower than predicted, whereas that of methane is about the same as predicted. The low nitrogen loss rate is consistent with an undetected Charon atmosphere but possibly inconsistent with sublimation/erosional features seen on Pluto’s surface, so that past escape rates may have been much larger at times. Capture of escaping methane and photochemical products by Charon, and subsequent surface chemical reactions, may contribute to the reddish color of its north pole. CONCLUSION New Horizons observations have revolutionized our understanding of Pluto’s atmosphere. The observations revealed major surprises, such as the unexpectedly cold upper atmosphere and the globally extensive haze layers. The cold upper atmosphere implies much lower escape rates of volatiles from Pluto than predicted and so has important implications for the volatile recycling and the long-term evolution of Pluto’s atmosphere. MVIC image of haze layers above Pluto’s limb. About 20 haze layers are seen from a phase angle of 147°. The layers typically extend horizontally over hundreds of kilometers but are not exactly horizontal. For example, white arrows on the left indicate a layer ~5 km above the surface, which has descended to the surface at the right. Observations made during the New Horizons flyby provide a detailed snapshot of the current state of Pluto’s atmosphere. Whereas the lower atmosphere (at altitudes of less than 200 kilometers) is consistent with ground-based stellar occultations, the upper atmosphere is much colder and more compact than indicated by pre-encounter models. Molecular nitrogen (N2) dominates the atmosphere (at altitudes of less than 1800 kilometers or so), whereas methane (CH4), acetylene (C2H2), ethylene (C2H4), and ethane (C2H6) are abundant minor species and likely feed the production of an extensive haze that encompasses Pluto. The cold upper atmosphere shuts off the anticipated enhanced-Jeans, hydrodynamic-like escape of Pluto’s atmosphere to space. It is unclear whether the current state of Pluto’s atmosphere is representative of its average state—over seasonal or geologic time scales.

Measurements of the near-nucleus coma of comet 67P/Churyumov-Gerasimenko with the Alice far-ultraviolet spectrograph on Rosetta

The Alice far-ultraviolet spectrograph onboard Rosetta is designed to observe emissions from various atomic and molecular species from within the coma of comet 67P/ Churyumov-Gerasimenko and to determine their spatial distribution and evolution with time and heliocentric distance. Methods. Following orbit insertion in August 2014, Alice made observations of the inner coma above the limbs of the nucleus of the comet from cometocentric distances varying between 10 and 80 km. Depending on the position and orientation of the slit relative to the nucleus, emissions of atomic hydrogen and oxygen were initially detected. These emissions are spatially localized close to the nucleus and spatially variable with a strong enhancement above the comet’s neck at northern latitudes. Weaker emission from atomic carbon and CO were subsequently detected. Results. Analysis of the relative line intensities suggests photoelectron impact dissociation of H2O vapor as the source of the observed H i and O i emissions. The electrons are produced by photoionization of H2 O. The observed C i emissions are also attributed to electron impact dissociation, of CO2 , and their relative brightness to H i reflects the variation of CO2 to H2 O column abundance in the coma.

Hot Gas Lines IN T Tauri Stars

For Classical T Tauri Stars (CTTSs), the resonance doublets of N V, Si IV, and C IV, as well as the He II 1640 A line, trace hot gas flows and act as diagnostics of the accretion process. In this paper we assemble a large high-resolution, high-sensitivity data set of these lines in CTTSs and Weak T Tauri Stars (WTTSs). The sample comprises 35 stars: 1 Herbig Ae star, 28 CTTSs, and 6 WTTSs. We find that the C IV, Si IV, and N V lines in CTTSs all have similar shapes. We decompose the C IV and He II lines into broad and narrow Gaussian components (BC and NC). The most common (50%) C IV line morphology in CTTSs is that of a low-velocity NC together with a redshifted BC. For CTTSs, a strong BC is the result of the accretion process. The contribution fraction of the NC to the C IV line flux in CTTSs increases with accretion rate, from ~20% to up to ~80%. The velocity centroids of the BCs and NCs are such that V_(BC) ≳4 V_(NC), consistent with the predictions of the accretion shock model, in at most 12 out of 22 CTTSs. We do not find evidence of the post-shock becoming buried in the stellar photosphere due to the pressure of the accretion flow. The He II CTTSs lines are generally symmetric and narrow, with FWHM and redshifts comparable to those of WTTSs. They are less redshifted than the CTTSs C IV lines, by ~10 km s^(–1). The amount of flux in the BC of the He II line is small compared to that of the C IV line, and we show that this is consistent with models of the pre-shock column emission. Overall, the observations are consistent with the presence of multiple accretion columns with different densities or with accretion models that predict a slow-moving, low-density region in the periphery of the accretion column. For HN Tau A and RW Aur A, most of the C IV line is blueshifted suggesting that the C IV emission is produced by shocks within outflow jets. In our sample, the Herbig Ae star DX Cha is the only object for which we find a P-Cygni profile in the C IV line, which argues for the presence of a hot (10^5 K) wind. For the overall sample, the Si IV and N V line luminosities are correlated with the C IV line luminosities, although the relationship between Si IV and C IV shows large scatter about a linear relationship and suggests that TW Hya, V4046 Sgr, AA Tau, DF Tau, GM Aur, and V1190 Sco are silicon-poor, while CV Cha, DX Cha, RU Lup, and RW Aur may be silicon-rich.

High-resolution Ultraviolet Radiation Fields of Classical T Tauri Stars

The far-ultraviolet (FUV; 912-1700 ?) radiation field from accreting central stars in classical T Tauri systems influences the disk chemistry during the period of giant planet formation. The FUV field may also play a critical role in determining the evolution of the inner disk (r < 10?AU), from a gas- and dust-rich primordial disk to a transitional system where the optically thick warm dust distribution has been depleted. Previous efforts to measure the true stellar+accretion-generated FUV luminosity (both hot gas emission lines and continua) have been complicated by a combination of low-sensitivity and/or low-spectral resolution and did not include the contribution from the bright Ly? emission line. In this work, we present a high-resolution spectroscopic study of the FUV radiation fields of 16 T Tauri stars whose dust disks display a range of evolutionary states. We include reconstructed Ly? line profiles and remove atomic and molecular disk emission (from H2 and CO fluorescence) to provide robust measurements of both the FUV continuum and hot gas lines (e.g., Ly?, N V, C IV, He II) for an appreciable sample of T Tauri stars for the first time. We find that the flux of the typical classical T Tauri star FUV radiation field at 1?AU from the central star is ~107 times the average interstellar radiation field. The Ly? emission line contributes an average of 88% of the total FUV flux, with the FUV continuum accounting for an average of 8%. Both the FUV continuum and Ly? flux are strongly correlated with C IV flux, suggesting that accretion processes dominate the production of both of these components. On average, only ~0.5% of the total FUV flux is emitted between the Lyman limit (912 ?) and the H2 (0-0) absorption band at 1110 ?. The total and component-level high-resolution radiation fields are made publicly available in machine-readable format.

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

The Alice instrument on NASAs New Horizons spacecraft observed an ultraviolet solar occultation by Plutos atmosphere on 2015 July 14. The transmission vs. altitude was sensitive to the presence of N_2, CH_4, C_2H_2, C_2H_4, C_2H_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 Plutos 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) × 10^(22) N_2 s^(−1) and (4–8) × 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_2H_x hydrocarbons” (C_2H_2, C_2H_4, C_2H_6) were not simply exponential with altitude. We detected local maxima in line-of-sight abundance near 410 km altitude for C_2H_4, near 320 km for C_2H_2, and an inflection point or the suggestion of a local maximum at 260 km for C_2H_6. We also detected local minima near 200 km altitude for C_2H_4, near 170 km for C_2H_2, and an inflection point or minimum near 170–200 km for C_2H_6. These compared favorably with models for hydrocarbon production near 300–400 km and haze condensation near 200 km, especially for C_2H_2 and C_2H_4 (Wong et al., 2017). (4) We found haze that had an extinction coefficient approximately proportional to N_2 density.

DIRECT MEASUREMENT OF INTERSTELLAR EXTINCTION TOWARD YOUNG STARS USING ATOMIC HYDROGEN Lyα ABSORPTION

Interstellar reddening corrections are necessary to reconstruct the intrinsic spectral energy distributions (SEDs) of accreting protostellar systems. The stellar SED determines the heating and chemical processes that can occur in circumstellar disks. Measurement of neutral hydrogen absorption against broad Lyα emission profiles in young stars can be used to obtain the total H i column density (N(H i)) along the line of sight. We measure N(H i) with new and archival ultraviolet observations from the Hubble Space Telescope (HST) of 31 classical T Tauri and Herbig Ae/Be stars. The H i column densities range from log_10(N(H i)) ≈19.6–21.1, with corresponding visual extinctions of A_V =0.02–0.72 mag, assuming an RV of 3.1. We find that the majority of the H i absorption along the line of sight likely comes from interstellar rather than circumstellar material. Extinctions derived from new HST blue-optical spectral analyses, previous IR and optical measurements, and new X-ray column densities on average overestimate the interstellar extinction toward young stars compared to the N(H i) values by ~0.6 mag. We discuss possible explanations for this discrepancy in the context of a protoplanetary disk geometry.

Probing the Inner Regions of Protoplanetary Disks with CO Absorption Line Spectroscopy

Carbon monoxide (CO) is the most commonly used tracer of molecular gas in the inner regions of protoplanetary disks. CO can be used to constrain the excitation and structure of the circumstellar environment. Absorption line spectroscopy provides an accurate assessment of a single line of sight through the protoplanetary disk system, giving more straightforward estimates of column densities and temperatures than CO and molecular hydrogen (H2) emission line studies. We analyze new observations of ultraviolet CO absorption from the Hubble Space Telescope along the sightlines to six classical T Tauri stars. Gas velocities consistent with the stellar velocities, combined with the moderate-to-high disk inclinations, argue against the absorbing CO gas originating in a fast-moving disk wind. We conclude that the far-ultraviolet observations provide a direct measure of the disk atmosphere or possibly a slow disk wind. The CO absorption lines are reproduced by model spectra with column densities in the range N(12CO) ~ 1016-1018 cm–2 and N(13CO) ~ 1015-1017 cm–2, rotational temperatures T rot(CO) ~ 300-700 K, and Doppler b-values, b ~ 0.5-1.5 km s–1. We use these results to constrain the line-of-sight density of the warm molecular gas (n CO ~ 70-4000 cm–3) and put these observations in context with protoplanetary disk models.

The Nature and Frequency of the Gas Outbursts in Comet 67P/Churyumov–Gerasimenko Observed by the Alice Far-ultraviolet Spectrograph on Rosetta

Alice is a far-ultraviolet imaging spectrograph onboard Rosetta that, amongst multiple objectives, is designed to observe emissions from various atomic and molecular species from within the coma of comet 67P/Churyumov-Gerasimenko. The initial observations, made following orbit insertion in August 2014, showed emissions of atomic hydrogen and oxygen spatially localized close to the nucleus and attributed to photoelectron impact dissociation of H2O vapor. Weaker emissions from atomic carbon were subsequently detected and also attributed to electron impact dissociation, of CO2, the relative H I and C I line intensities reflecting the variation of CO2 to H2O column abundance along the line-of-sight through the coma. Beginning in mid-April 2015, Alice sporadically observed a number of outbursts above the sunward limb characterized by sudden increases in the atomic emissions, particularly the semi-forbidden O I 1356 multiplet, over a period of 10-30 minutes, without a corresponding enhancement in long wavelength solar reflected light characteristic of dust production. A large increase in the brightness ratio O I 1356/O I 1304 suggests O2 as the principal source of the additional gas. These outbursts do not correlate with any of the visible images of outbursts taken with either OSIRIS or the navigation camera. Beginning in June 2015 the nature of the Alice spectrum changed considerably with CO Fourth Positive band emission observed continuously, varying with pointing but otherwise fairly constant in time. However, CO does not appear to be a major driver of any of the observed outbursts.

On the possible noble gas deficiency of Pluto’s atmosphere

Abstract We use a statistical–thermodynamic model to investigate the formation and composition of noble-gas-rich clathrates on Pluto’s surface. By considering an atmospheric composition close to that of today’s Pluto and a broad range of surface pressures, we find that Ar, Kr and Xe can be efficiently trapped in clathrates if they formed at the surface, in a way similar to what has been proposed for Titan. The formation on Pluto of clathrates rich in noble gases could then induce a strong decrease in their atmospheric abundances relative to their initial values. A clathrate thickness of order of a few centimeters globally averaged on the planet is enough to trap all Ar, Kr and Xe if these noble gases were in protosolar proportions in Pluto’s early atmosphere. Because atmospheric escape over an extended period of time (millions of years) should lead to a noble gas abundance that either remains constant or increases with time, we find that a potential depletion of Ar, Kr and Xe in the atmosphere would best be explained by their trapping in clathrates. A key observational test is the measurement of Ar since the Alice UV spectrometer aboard the New Horizons spacecraft will be sensitive enough to detect its abundance ∼10 times smaller than in the case considered here.

Far-UV phase dependence and surface characteristics of comet 67P/Churyumov-Gerasimenko as observed with Rosetta Alice

The Alice far-ultraviolet (FUV) spectrograph onboard Rosetta has, for the first time, imaged the surface of a comet, 67P/Churyumov-Gerasimenko (67P), in the FUV. With spatially resolved data, the nucleus properties are characterized in the FUV, including phase dependence, albedo, and spectral slope. Regional measurements across the nucleus are compared to discern any compositional variations. Methods. Hapke theory was utilized to model the phase dependence of the material on the surface of 67P. The phase dependence of 67P was derived from a subset of data acquired at various phase angles in November 2014, within 50 km of the comet such that the nucleus was spatially resolved. The derived photometric correction was then applied to a different subset of spatially resolved data sampling several distinct geographical regions on the nucleus acquired in August−November 2014 under similar viewing geometries. Results. In the FUV, the surface of 67P is dark, blue sloped, has an average geometric albedo of 0.054±0.008 at 1475 A near the center of the Alice bandpass, and is mostly uniform from region to region, with the exception of the Hatmehit region, which is slightly more reflective. These results are consistent with the suggestion made by the Rosetta OSIRIS and VIRTIS teams that the surface of 67P is covered with a homogeneous layer of material and that surface ice is not ubiquitous in large abundances. The modeled Hapke parameters, specifically the single scattering albedo (w) and the asymmetry factor (ζ), are determined to be 0.031 ± 0.003 and −0.530 ± 0.025 near the center of the Alice bandpass at 1475 A. These parameters are consistent with measurements of other comet nuclei that have been observed by flyby missions in the visible and the near-infrared regimes.