Catherine B. Olkin

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.

The geology of Pluto and Charon through the eyes of 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 and Charon display a complex geology, including evidence for tectonics and cryovolcanoes. NASA’s New Horizons spacecraft has revealed the complex geology of Pluto and Charon. Pluto’s encounter hemisphere shows ongoing surface geological activity centered on a vast basin containing a thick layer of volatile ices that appears to be involved in convection and advection, with a crater retention age no greater than ~10 million years. Surrounding terrains show active glacial flow, apparent transport and rotation of large buoyant water-ice crustal blocks, and pitting, the latter likely caused by sublimation erosion and/or collapse. More enigmatic features include tall mounds with central depressions that are conceivably cryovolcanic and ridges with complex bladed textures. Pluto also has ancient cratered terrains up to ~4 billion years old that are extensionally faulted and extensively mantled and perhaps eroded by glacial or other processes. Charon does not appear to be currently active, but experienced major extensional tectonism and resurfacing (probably cryovolcanic) nearly 4 billion years ago. Impact crater populations on Pluto and Charon are not consistent with the steepest impactor size-frequency distributions proposed for the Kuiper belt.

The recent expansion of Pluto's atmosphere.

Stellar occultations—the passing of a relatively nearby body in front of a background star—can be used to probe the atmosphere of the closer body with a spatial resolution of a few kilometres (ref. 1). Such observations can yield the scale height, temperature profile, and other information about the structure of the occulting atmosphere. Occultation data acquired for Plutos atmosphere in 1988 revealed a nearly isothermal atmosphere above a radius of ∼1,215 km. Below this level, the data could be interpreted as indicating either an extinction layer or the onset of a large thermal gradient, calling into question the fundamental structure of this atmosphere. Another question is to what extent Plutos atmosphere might be collapsing as it recedes from the Sun (passing perihelion in 1989 in its 248-year orbital period), owing to the extreme sensitivity of the equilibrium surface pressure to the surface temperature. Here we report observations at a variety of visible and infrared wavelengths of an occultation of a star by Pluto in August 2002. These data reveal evidence for extinction in Plutos atmosphere and show that it has indeed changed, having expanded rather than collapsed, since 1988.

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.

Ralph: A Visible/Infrared Imager for the New Horizons Pluto/Kuiper Belt Mission

The New Horizons instrument named Ralph is a visible/near infrared multi-spectral imager and a short wavelength infrared spectral imager. It is one of the core instruments on New Horizons, NASA’s first mission to the Pluto/Charon system and the Kuiper Belt. Ralph combines panchromatic and color imaging capabilities with SWIR imaging spectroscopy. Its primary purpose is to map the surface geology and composition of these objects, but it will also be used for atmospheric studies and to map the surface temperature. It is a compact, low-mass (10.5 kg) power efficient (7.1 W peak), and robust instrument with good sensitivity and excellent imaging characteristics. Other than a door opened once in flight, it has no moving parts. These characteristics and its high degree of redundancy make Ralph ideally suited to this long-duration flyby reconnaissance mission.

New Horizons: Anticipated Scientific Investigations at the Pluto System

The New Horizons spacecraft will achieve a wide range of measurement objectives at the Pluto system, including color and panchromatic maps, 1.25–2.50 micron spectral images for studying surface compositions, and measurements of Pluto’s atmosphere (temperatures, composition, hazes, and the escape rate). Additional measurement objectives include topography, surface temperatures, and the solar wind interaction. The fulfillment of these measurement objectives will broaden our understanding of the Pluto system, such as the origin of the Pluto system, the processes operating on the surface, the volatile transport cycle, and the energetics and chemistry of the atmosphere. The mission, payload, and strawman observing sequences have been designed to achieve the NASA-specified measurement objectives and maximize the science return. The planned observations at the Pluto system will extend our knowledge of other objects formed by giant impact (such as the Earth–moon), other objects formed in the outer solar system (such as comets and other icy dwarf planets), other bodies with surfaces in vapor-pressure equilibrium (such as Triton and Mars), and other bodies with N2:CH4 atmospheres (such as Titan, Triton, and the early Earth).

Pluto's interaction with the solar wind

This study provides the first observations of Plutogenic ions and their unique interaction with the solar wind. We find ~20% solar wind slowing that maps to a point only ~4.5 RP upstream of Pluto and a bow shock most likely produced by comet-like mass loading. The Pluto obstacle is a region of dense heavy ions bounded by a “Plutopause” where the solar wind is largely excluded and which extends back >100 RP into a heavy ion tail. The upstream standoff distance is at only ~2.5 RP. The heavy ion tail contains considerable structure, may still be partially threaded by the interplanetary magnetic field (IMF), and is surrounded by a light ion sheath. The heavy ions (presumably CH4+) have average speed, density, and temperature of ~90 km s−1, ~0.009 cm−3, and ~7 × 105 K, with significant variability, slightly increasing speed/temperature with distance, and are N-S asymmetric. Density and temperature are roughly anticorrelated yielding a pressure ~2 × 10−2 pPa, roughly in balance with the interstellar pickup ions at ~33 AU. We set an upper bound of <30 nT surface field at Pluto and argue that the obstacle is largely produced by atmospheric thermal pressure like Venus and Mars; we also show that the loss rate down the tail (~5 × 1023 s−1) is only ~1% of the expected total CH4 loss rate from Pluto. Finally, we observe a burst of heavy ions upstream from the bow shock as they are becoming picked up and tentatively identify an IMF outward sector at the time of the NH flyby.

VERTICAL STRUCTURE IN PLUTO'S ATMOSPHERE FROM THE 2006 JUNE 12 STELLAR OCCULTATION

Pluto occultations are historically rare events, having been observed in 1988, 2002, 2006, and, as Pluto moves into the crowded Galactic plane, on several occasions in 2007. Here we present six results from our observations of the 2006 June 12 event from several sites in Australia and New Zealand. First, we show that Plutos 2006 bulk atmospheric column abundance, as in 2002, is over twice the value measured in 1988, implying that nitrogen frost on Plutos surface is 1.2-1.7 K warmer in 2006 than 1988 despite a 9% drop in incident solar flux. We measure a half-light shadow radius of 1216 ± 8.6 km in 2006, nominally larger than published values of 1213 ± 16 km measured in 2002. Given the current error bars, this latest half-light radius cannot discriminate between continued atmospheric growth or shrinkage, but it rules out several of the volatile transport scenarios modeled by Hansen & Paige. Second, we resolve spikes in the occultation light curve that are similar to those seen in 2002 and model the vertical temperature fluctuations that cause them. Third, we show that Plutos upper atmosphere appears to hold a steady temperature of ~100 K, as predicted from the methane thermostat model, even at latitudes where the methane thermostat is inoperative. This implies that energy transport rates are faster than radiational cooling rates. Fourth, this occultation has provided the first significant detection of a non-isothermal temperature gradient in Plutos upper atmosphere also reported by Elliot et al., possibly the result of CO gas in Plutos upper atmosphere. Fifth, we show that a haze-only explanation for Plutos light curve is extremely unlikely; a thermal inversion is necessary to explain the observed light curve. And sixth, we derive an upper limit for the haze optical depth of 0.0023 in the zenith direction at average CCD wavelengths.

Global warming on Triton

Triton, Neptunes largest moon, has been predicted to undergo significant seasonal changes that would reveal themselves as changes in its mean frost temperature. But whether this temperature should at the present time be increasing, decreasing or constant depends on a number of parameters (such as the thermal properties of the surface, and frost migration patterns) that are unknown. Here we report observations of a recent stellar occultation by Triton which, when combined with earlier results, show that Triton has undergone a period of global warming since 1989. Our most conservative estimates of the rate of temperature and surface-pressure increase during this period imply that the atmosphere is doubling in bulk every 10 years—significantly faster than predicted by any published frost model for Triton,. Our result suggests that permanent polar caps on Triton play a dominant role in regulating seasonal atmospheric changes. Similar processes should also be active on Pluto.

Pluto's Spectrum from 1.0 to 4.2 μm: Implications for Surface Properties

We present spectra of Plutos anti-Charon hemisphere obtained from the Keck and Subaru telescopes from 2.8 to 4.2 μm. Combined with 1-2.5 μm spectra from the Infrared Telescope Facility, this collective data set lets us constrain several surface frost properties. The surface area of pure nitrogen frost (as opposed to nitrogen with dissolved methane) is constrained to be 6% or less. The areal fractions of pure methane and methane dissolved in nitrogen are almost equal. The grain size of pure methane is constrained to be near 200 μm. An additional surface component with spectral properties similar to Titan tholin was necessary to fit the entire 1-4.2 μm spectrum; our best-fit model requires 21% of Plutos anti-Charon hemisphere (by area) to be this Titan tholin component. Contrary to Sasaki et al.s spectra of Plutos sub-Charon hemisphere, we find no evidence for other hydrocarbons on this face of Pluto from data in the 3-3.3 μm region. We were not able to constrain the temperature of pure methane.