John Arthur Stansberry

Surface compositions across Pluto and Charon.

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 and Charon have surfaces dominated by volatile ices, with large variations in color and albedo. INTRODUCTION The Kuiper Belt hosts a swarm of distant, icy objects ranging in size from small, primordial planetesimals to much larger, highly evolved objects, representing a whole new class of previously unexplored cryogenic worlds. Pluto, the largest among them, along with its system of five satellites, has been revealed by NASA’s New Horizons spacecraft flight through the system in July 2015, nearly a decade after its launch. RATIONALE Landforms expressed on the surface of a world are the product of the available materials and of the action of the suite of processes that are enabled by the local physical and chemical conditions. They provide observable clues about what processes have been at work over the course of time, the understanding of which is a prerequisite to reconstructing the world’s history. Materials known to exist at Pluto’s surface from ground-based spectroscopic observations include highly volatile cryogenic ices of N2 and CO, along with somewhat less volatile CH4 ice, as well as H2O and C2H6 ices and more complex tholins that are inert at Pluto surface temperatures. Ices of H2O and NH3 are inert components known to exist on Pluto’s large satellite Charon. New Horizons’ Ralph instrument was designed to map colors and compositions in the Pluto system. It consists of a charge-coupled device camera with four color filters spanning wavelengths from 400 to 970 nm plus a near-infrared imaging spectrometer covering wavelengths from 1.25 to 2.5 μm, where the various cryogenic ices are distinguishable via their characteristic vibrational absorption features. RESULTS New Horizons made its closest approach to the system on 14 July 2015. Observations of Pluto and Charon obtained that day reveal regionally diverse colors and compositions. On Pluto, the color images show nonvolatile tholins coating an ancient, heavily cratered equatorial belt. A smooth, thousand-kilometer plain must be able to refresh its surface rapidly enough to erase all impact craters. Infrared observations of this region show volatile ices including N2 and CO. H2O ice is not detected there, but it does appear in neighboring regions. CH4 ice appears on crater rims and mountain ridges at low latitudes and is abundant at Pluto’s high northern latitudes. Pluto’s regional albedo contrasts are among the most extreme for solar system objects. Pluto’s large moon Charon offers its own surprises. Its H2O ice–rich surface is unlike other outer solar system icy satellites in exhibiting distinctly reddish tholin coloration around its northern pole as well as a few highly localized patches rich in NH3 ice. CONCLUSION Pluto exhibits evidence for a variety of processes that act to modify its surface over time scales ranging from seasonal to geological. Much of this activity is enabled by the existence of volatile ices such as N2 and CO that are easily mobilized even at the extremely low temperatures prevalent on Pluto’s surface, around 40 K. These ices sublimate and condense on seasonal time scales and flow glacially. As they move about Pluto’s surface environment, they interact with materials such as H2O ice that are sufficiently rigid to support rugged topography. Although Pluto’s durable H2O ice is probably not active on its own, it appears to be sculpted in a variety of ways through the action of volatile ices of N2 and CO. CH4 ice plays a distinct role of its own, enabled by its intermediate volatility. CH4 ice condenses at high altitudes and on the winter hemisphere, contributing to the construction of some of Pluto’s more unusual and distinctive landforms. The latitudinal distribution of Charon’s polar reddening suggests a thermally controlled production process, and the existence of highly localized patches rich in NH3 ice on its surface implies relatively recent emplacement. Enhanced color view of Pluto’s surface diversity This mosaic was created by merging Multispectral Visible Imaging Camera color imagery (650 m per pixel) with Long Range Reconnaissance Imager panchromatic imagery (230 m per pixel). At lower right, ancient, heavily cratered terrain is coated with dark, reddish tholins. At upper right, volatile ices filling the informally named Sputnik Planum have modified the surface, creating a chaos-like array of blocky mountains. Volatile ice occupies a few nearby deep craters, and in some areas the volatile ice is pocked with arrays of small sublimation pits. At left, and across the bottom of the scene, gray-white CH4 ice deposits modify tectonic ridges, the rims of craters, and north-facing slopes. The New Horizons spacecraft mapped colors and infrared spectra across the encounter hemispheres of Pluto and Charon. The volatile methane, carbon monoxide, and nitrogen ices that dominate Pluto’s surface have complicated spatial distributions resulting from sublimation, condensation, and glacial flow acting over seasonal and geological time scales. Pluto’s water ice “bedrock” was also mapped, with isolated outcrops occurring in a variety of settings. Pluto’s surface exhibits complex regional color diversity associated with its distinct provinces. Charon’s color pattern is simpler, dominated by neutral low latitudes and a reddish northern polar region. Charon’s near-infrared spectra reveal highly localized areas with strong ammonia absorption tied to small craters with relatively fresh-appearing impact ejecta.

Observations of Transiting Exoplanets with the James Webb Space Telescope (JWST)

This article summarizes a workshop held on March, 2014, on the potential of the James Webb Space Telescope (JWST) to revolutionize our knowledge of the physical properties of exoplanets through transit observations. JWSTs unique combination of high sensitivity and broad wavelength coverage will enable the accurate measurement of transits with high signal-to-noise. Most importantly, JWST spectroscopy will investigate planetary atmospheres to determine atomic and molecular compositions, to probe vertical and horizontal structure, and to follow dynamical evolution, i.e. exoplanet weather. JWST will sample a diverse population of planets of varying masses and densities in a wide variety of environments characterized by a range of host star masses and metallicities, orbital semi-major axes and eccentricities. A broad program of exoplanet science could use a substantial fraction of the overall JWST mission.

“TNOs are Cool”: A survey of the trans-Neptunian region - IX. Thermal properties of Kuiper belt objects and Centaurs from combined Herschel and Spitzer observations

Aims. The goal of this work is to characterize the ensemble thermal properties of the Centaurs/trans-Neptunian population. Methods. Thermal flux measurements obtained with Herschel/PACS and Spitzer/MIPS provide size, albedo, and beaming factors for 85 objects (13 of which are presented here for the first time) by means of standard radiometric techniques. The measured beaming factors are influenced by the combination of surface roughness and thermal inertia effects. They are interpreted within a thermophysical model to constrain, in a statistical sense, the thermal inertia in the population and to study its dependence on several parameters. We use in particular a Monte-Carlo modeling approach to the data whereby synthetic datasets of beaming factors are created using random distributions of spin orientation and surface roughness. Results. Beaming factors eta range fromvalues textless1 to similar to 2.5, but high eta values (textgreater2) are lacking at low heliocentric distances (r(h) textless 30 AU). Beaming factors lower than 1 occur frequently (39% of the objects), indicating that surface roughness effects are important. We determine a mean thermal inertia for Centaurs/TNO of Gamma = (2.5 +/- 0.5) J m(-2) s(-1/2) K-1, with evidence of a trend toward decreasing Gamma with increasing heliocentric (by a factor similar to 2.5 from 8-25 AU to 41-53 AU). These thermal inertias are 2-3 orders of magnitude lower than expected for compact ices, and generally lower than on Saturns satellites or in the Pluto/Charon system. Most high-albedo objects are found to have unusually low thermal inertias. Our results suggest highly porous surfaces, in which the heat transfer is affected by radiative conductivity within pores and increases with depth in the subsurface.

THE ALBEDO-COLOR DIVERSITY OF TRANSNEPTUNIAN OBJECTS

We analyze albedo data obtained using the Herschel Space Observatory that reveal the existence of two distinct types of surface among midsized trans-Neptunian objects. A color-albedo diagram shows two large clusters of objects, one redder and higher albedo and another darker and more neutrally colored. Crucially, all objects in our sample located in dynamically stable orbits within the classical Kuiper Belt region and beyond are confined to the bright red group, implying a compositional link. Those objects are believed to have formed further from the Sun than the dark neutral bodies. This color-albedo separation is evidence for a compositional discontinuity in the young solar system.

“TNOs are Cool”: A survey of the trans-Neptunian region XI. A Herschel-PACS view of 16 Centaurs

Context. Centaurs are the transitional population between trans-Neptunian objects (TNOs) and Jupiter-family comets. Their physical properties provide an insight into TNO properties, but only under restricted conditions since Centaurs are closer to the Sun and Earth. For this reason it is possible to access the smaller ones, which is more difficult to do with the TNO population. Aims. The goal of this work is to characterize a set of 16 Centaurs in terms of their size, albedo, and thermal properties. We study the correlations, for a more extended sample obtained from the literature, of diameter, albedo, orbital parameters, and spectral slopes. Methods. We performed three-band photometric observations using Herschel-PACS and used a consistent method for the data reduction and aperture photometry of this sample to obtain monochromatic flux densities at 70, 100, and 160 mu m. Additionally, we used Spitzer-MIPS flux densities at 24 and 70 pm when available. We also included in our Centaur sample scattered disk objects, a dynamical family of TNOs, using results previously published by our team, and some Centaurs observed only with the Spitzer-MIPS instrument. Results. We have determined new radiometric sizes and albedos of 16 Centaurs. The first conclusion is that the albedos of Centaur objects are not correlated with their orbital parameters. Similarly, there is no correlation between diameter and orbital parameters. Most of the objects in our sample are dark (pv textless 7%) and most of them are small (D textless 120 km). However, there is no correlation between albedo and diameter, in particular for the group of small objects as albedo values are homogeneously distributed between 4 to 16%. The correlation with the color of the objects showed that red objects are all small (mean diameter 65 km), while the gray ones span a wide range of sizes (mean diameter 120 km). Moreover, the gray objects tend to be darker, with a mean albedo of 5.6%, compared with a mean of 8.5% (ranging from 5 to 15%) for the red objects.

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

Abstract Observations of the Pluto-Charon system, acquired with the ALMA interferometer on June 12–13, 2015, have led to the detection of the CO(3-2) and HCN(4-3) rotational transitions from Pluto (including the hyperfine structure of HCN), providing a strong confirmation of the presence of CO, and the first observation of HCN in Pluto’s atmosphere. The CO and HCN lines probe Pluto’s atmosphere up to ∼450xa0km and ∼900xa0km altitude, respectively, with a large contribution due to limb emission. The CO detection yields (i) a much improved determination of the CO mole fraction, as 515xa0±xa040xa0ppm for a 12 μbar surface pressure (ii) strong constraints on Pluto’s mean atmospheric dayside temperature profile over ∼50–400xa0km, with clear evidence for a well-marked temperature decrease (i.e., mesosphere) above the 30–50xa0km stratopause and a best-determined temperature of 70xa0±xa02xa0K at 300xa0km, somewhat lower than previously estimated from stellar occultations (81u2009± xa06xa0K), and in agreement with recent inferences from New Horizons / Alice solar occultation data. The HCN line shape implies a high abundance of this species in the upper atmosphere, with a mole fraction >1.5xa0×xa010 − 5 above 450xa0km and a value of 4xa0×xa010 − 5 near 800xa0km. Assuming HCN at saturation, this would require a warm (>92xa0K) upper atmosphere layer; while this is not ruled out by the CO emission, it is inconsistent with the Alice-measured CH4 and N2 line-of-sight column densities. Taken together, the large HCN abundance and the cold upper atmosphere imply supersaturation of HCN to a degree (7–8 orders of magnitude) hitherto unseen in planetary atmospheres, probably due to a lack of condensation nuclei above the haze region and the slow kinetics of condensation at the low pressure and temperature conditions of Pluto’s upper atmosphere. HCN is also present in the bottom ∼100xa0km of the atmosphere, with a 10 − 8 –10 − 7 mole fraction; this implies either HCN saturation or undersaturation there, depending on the precise stratopause temperature. The HCN column is (1.6xa0±xa00.4)× 1014xa0cm − 2 , suggesting a surface-referred vertically-integrated net production rate of ∼2xa0×xa0107xa0cm − 2 xa0s − 1 . Although HCN rotational line cooling affects Pluto’s atmosphere heat budget, the amounts determined in this study are insufficient to explain the well-marked mesosphere and upper atmosphere’s ∼70xa0K temperature, which if controlled by HCN cooling would require HCN mole fractions of (3–7) × 10 − 4 over 400–800xa0km. We finally report an upper limit on the HC3N column density ( − 2 ) and on the HC15N / HC14N ratio (

The Size and Shape of the Oblong Dwarf Planet Haumea

AbstractnWe use thermal radiometry and visible photometry to constrain the size, shape, and albedo of the large Kuiper belt object Haumea. The correlation between the visible and thermal photometry demonstrates that Haumea’s high amplitude and quickly varying optical light curve is indeed due to Haumea’s extreme shape, rather than large scale albedo variations. However, the well-sampled high precision visible data we present does require longitudinal surface heterogeneity to account for the shape of lightcurve. The thermal emission from Haumea is consistent with the expected Jacobi ellipsoid shape of a rapidly rotating body in hydrostatic equilibrium. The best Jacobi ellipsoid fit to the visible photometry implies a triaxial ellipsoid with axes of length 1,920xa0×xa01,540xa0×xa0990xa0km and density

The mutual orbit, mass, and density of the large transneptunian binary system Varda and Ilmarë

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