J. Wm. Parker

Differentiation of the asteroid Ceres as revealed by its shape

The accretion of bodies in the asteroid belt was halted nearly 4.6 billion years ago by the gravitational influence of the newly formed giant planet Jupiter. The asteroid belt therefore preserves a record of both this earliest epoch of Solar System formation and variation of conditions within the solar nebula. Spectral features in reflected sunlight indicate that some asteroids have experienced sufficient thermal evolution to differentiate into layered structures. The second most massive asteroid—4 Vesta—has differentiated to a crust, mantle and core. 1 Ceres, the largest and most massive asteroid, has in contrast been presumed to be homogeneous, in part because of its low density, low albedo and relatively featureless visible reflectance spectrum, similar to carbonaceous meteorites that have suffered minimal thermal processing. Here we show that Ceres has a shape and smoothness indicative of a gravitationally relaxed object. Its shape is significantly less flattened than that expected for a homogeneous object, but is consistent with a central mass concentration indicative of differentiation. Possible interior configurations include water-ice-rich mantles over a rocky core.

THE CANADA-FRANCE ECLIPTIC PLANE SURVEY—FULL DATA RELEASE: THE ORBITAL STRUCTURE OF THE KUIPER BELT*

We report the orbital distribution of the trans-Neptunian objects (TNOs) discovered during the Canada–France Ecliptic Plane Survey (CFEPS), whose discovery phase ran from early 2003 until early 2007. The follow-up observations started just after the first discoveries and extended until late 2009. We obtained characterized observations of 321 deg 2 of sky to depths in the range g ∼ 23.5–24.4 AB mag. We provide a database of 169 TNOs with high-precision dynamical classification and known discovery efficiency. Using this database, we find that the classical belt is a complex region with sub-structures that go beyond the usual splitting of inner (interior to 3:2 mean-motion resonance [MMR]), main (between 3:2 and 2:1 MMR), and outer (exterior to 2:1 MMR). The main classical belt (a = 40–47 AU) needs to be modeled with at least three components: the “hot” component with a wide inclination distribution and two “cold” components (stirred and kernel) with much narrower inclination distributions. The hot component must have a significantly shallower absolute magnitude (Hg) distribution than the other two components. With 95% confidence, there are 8000 +18001600 objects in the main belt with Hg 8.0, of which 50% are from the hot component, 40% from the stirred component, and 10% from the kernel; the hot component’s fraction drops rapidly with increasing Hg. Because of this, the apparent population fractions depend on the depth and ecliptic latitude of a trans-Neptunian survey. The stirred and kernel components are limited to only a portion of the main belt, while we find that the hot component is consistent with a smooth extension throughout the inner, main, and outer regions of the classical belt; in fact, the inner and outer belts are consistent with containing only hot-component objects. The Hg 8.0 TNO population estimates are 400 for the inner belt and 10,000 for the outer belt to within a factor of two (95% confidence). We show how the CFEPS Survey Simulator can be used to compare a cosmogonic model for the orbital element distribution to the real Kuiper Belt.

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.

Massive field stars and the stellar clustering law

The distribution of N*, the number of OB stars per association or cluster, appears to follow a universal power-law form N in the local universe. We evaluate the distribution of N* in the Small Magellanic Cloud using recent broadband optical and space-ultraviolet data, with special attention to the lowest values of N*. We find that the power-law distribution in N* continues smoothly down to N* = 1. This strongly suggests that the formation of field massive stars is a continuous process with those in associations and that the field stars do not originate from a different star formation mode. Our results are consistent with the model that field massive stars represent the most massive members in groups of smaller stars, as expected if the clustering law applies to much lower masses as is expected from the stellar initial mass function (IMF). These results are consistent with the simultaneous existence of a universal IMF and a universal clustering law. Jointly, these laws imply that the fraction of field OB stars typically ranges from about 35% to 7% for most astrophysical situations, with an inverse logarithmic dependence on the most populous cluster, and hence on galaxy size and/or star formation rate. There are important consequences for global feedback effects in galaxies: field stars should therefore contribute proportionately to the volume of the warm ionized medium, and equal relative contributions by superbubbles of all sizes to the interstellar porosity are expected.

The Discovery of Argon in Comet C/1995 O1 (Hale-Bopp)

On 1997 March 30.14, we observed the EUV spectrum of the bright comet C/1995 O1 (Hale-Bopp) at the time of its perihelion, using our Extreme Ultraviolet Spectrograph sounding-rocket telescope/spectrometer. The spectra reveal the presence H Lyβ, O+, and, most notably, argon. Modeling of the retrieved Ar production rates indicates that comet Hale-Bopp is enriched in Ar relative to cosmogonic expectations. This in turn indicates that Hale-Bopps deep interior has never been exposed to the 35-40 K temperatures necessary to deplete the comets primordial argon supply.

The CFEPS Kuiper Belt Survey: Strategy and presurvey results

Abstract We present the data acquisition strategy and characterization procedures for the Canada–France Ecliptic Plane Survey (CFEPS), a sub-component of the Canada–France–Hawaii Telescope Legacy Survey. The survey began in early 2003 and as of summer 2005 has covered 430 square degrees of sky within a few degrees of the ecliptic. Moving objects beyond the orbit of Uranus are detected to a magnitude limit of m R = 23 – 24 (depending on the image quality). To track as large a sample as possible and avoid introducing followup bias, we have developed a multi-epoch observing strategy that is spread over several years. We present the evolution of the uncertainties in ephemeris position and orbital elements of a small 10-object sample of objects tracked through these epochs as part of a preliminary presurvey starting a year before the main CFEPS project. We describe the CFEPS survey simulator, to be released in 2006, which allows theoretical models of the Kuiper belt to be compared with the survey discoveries. The simulator utilizes the well-documented pointing history of CFEPS, with characterized detection efficiencies as a function of magnitude and rate of motion on the sky. Using the presurvey objects we illustrate the usage of the simulator in modeling the classical Kuiper belt. The primary purpose of this paper is to allow a user to immediately exploit the CFEPS data set and releases as they become available in the coming months.

The Small Satellites 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 rapidly rotating small moons have bright icy surfaces with impact craters. INTRODUCTION The Pluto system is surprisingly complex, comprising six objects that orbit their common center of mass in approximately a single plane and in nearly circular orbits. When the New Horizons mission was selected for flight by NASA in 2001, only the two largest objects were known: the binary dwarf planets Pluto and Charon. Two much smaller moons, Nix and Hydra, were discovered in May 2005, just 8 months before the launch of the New Horizons spacecraft, and two even smaller moons, Kerberos and Styx, were discovered in 2011 and 2012, respectively. The entire Pluto system was likely produced in the aftermath of a giant impact between two Pluto-sized bodies approximately 4 to 4.5 billion years ago, with the small moons forming within the resulting debris disk. But many details remain unconfirmed, and the New Horizons results on Pluto’s small moons help to elucidate the conditions under which the Pluto system formed and evolved. RATIONALE Pluto’s small moons are difficult to observe from Earth-based facilities, with only the most basic visible and near-infrared photometric measurements possible to date. The New Horizons flyby enabled a whole new category of measurements of Pluto’s small moons. The Long Range Reconnaissance Imager (LORRI) provided high–spatial resolution panchromatic imaging, with thousands of pixels across the surfaces of Nix and Hydra and the first resolved images of Kerberos and Styx. In addition, LORRI was used to conduct systematic monitoring of the brightness of all four small moons over several months, from which the detailed rotational properties could be deduced. The Multispectral Visible Imaging Camera (MVIC) provided resolved color measurements of the surfaces of Nix and Hydra. The Linear Etalon Imaging Spectral Array (LEISA) captured near-infrared spectra (in the wavelength range 1.25 to 2.5 μm) of all the small moons for compositional studies, but those data have not yet been sent to Earth. RESULTS All four of Pluto’s small moons are highly elongated objects with surprisingly high surface reflectances (albedos) suggestive of a water-ice surface composition. Kerberos appears to have a double-lobed shape, possibly formed by the merger of two smaller bodies. Crater counts for Nix and Hydra imply surface ages of at least 4 billion years. Nix and Hydra have mostly neutral (i.e., gray) colors, but an apparent crater on Nix’s surface is redder than the rest of the surface; this finding suggests either that the impacting body had a different composition or that material with a different composition was excavated from below Nix’s surface. All four small moons have rotational periods much shorter than their orbital periods, and their rotational poles are clustered nearly orthogonal to the direction of the common rotational poles of Pluto and Charon. CONCLUSION Pluto’s small moons exhibit rapid rotation and large rotational obliquities, indicating that tidal despinning has not played the dominant role in their rotational evolution. Collisional processes are implicated in determining the shapes of the small moons, but collisional evolution was probably limited to the first several hundred million years after the system’s formation. The bright surfaces of Pluto’s small moons suggest that if the Pluto-Charon binary was produced during a giant collision, the two precursor bodies were at least partially differentiated with icy surface layers. Pluto’s family of satellites. NASA’s New Horizons mission has resolved Pluto’s four small moons, shown in order of their orbital distance from Pluto (from left to right). Nix and Hydra have comparable sizes (with equivalent spherical diameters of ~40 km) and are much larger than Styx and Kerberos (both of which have equivalent spherical diameters of ~10 km). All four of these moons are highly elongated and are dwarfed in size by Charon, which is nearly spherical with a diameter of 1210 km. The scale bars apply to all images. The New Horizons mission has provided resolved measurements of Pluto’s moons Styx, Nix, Kerberos, and Hydra. All four are small, with equivalent spherical diameters of ~40 kilometers for Nix and Hydra and ~10 kilometers for Styx and Kerberos. They are also highly elongated, with maximum to minimum axis ratios of ~2. All four moons have high albedos (~50 to 90%) suggestive of a water-ice surface composition. Crater densities on Nix and Hydra imply surface ages of at least 4 billion years. The small moons rotate much faster than synchronous, with rotational poles clustered nearly orthogonal to the common pole directions of Pluto and Charon. These results reinforce the hypothesis that the small moons formed in the aftermath of a collision that produced the Pluto-Charon binary.

The Extreme Kuiper Belt Binary 2001 QW322

The study of binary Kuiper Belt objects helps to probe the dynamic conditions present during planet formation in the solar system. We report on the mutual-orbit determination of 2001 QW322, a Kuiper Belt binary with a very large separation whose properties challenge binary-formation and -evolution theories. Six years of tracking indicate that the binarys mutual-orbit period is ≈25 to 30 years, that the orbit pole is retrograde and inclined 50° to 62° from the ecliptic plane, and, most surprisingly, that the mutual orbital eccentricity is <0.4. The semimajor axis of 105,000 to 135,000 kilometers is 10 times that of other near-equal-mass binaries. Because this weakly bound binary is prone to orbital disruption by interlopers, its lifetime in its present state is probably less than 1 billion years.

The Shape and Surface Variation of 2 Pallas from the Hubble Space Telescope

Protoplanet 2 Pallas With a diameter of 265 kilometers, 2 Pallas is one of the largest bodies in the main asteroid belt. Now Schmidt et al. (p. 275) have characterized its surface and shape using images from the Hubble Space Telescope. Color variations and topography were revealed that are possibly linked to the asteroids thermal evolution and to the formation of its orbital family—the population of asteroids that share the same properties as 2 Pallas and are thought to be the fragments of a collision. In particular, a large-impact crater was observed that could represent the source of the Pallas family. 2 Pallas represents the third intact protoplanet in the main asteroid belt, joining asteroids 1 Ceres and 4 Vesta. Like the asteroids Ceres and Vesta, 2 Pallas is a protoplanet that has remained intact since its formation. We obtained Hubble Space Telescope images of 2 Pallas in September 2007 that reveal distinct color and albedo variations across the surface of this large asteroid. Pallas’s shape is an ellipsoid with radii of 291 (±9), 278 (±9), and 250 (±9) kilometers, implying a density of 2400 (±250) kilograms per cubic meter—a value consistent with a body that formed from water-rich material. Our observations are consistent with the presence of an impact feature, 240 (±25) kilometers in diameter, within Pallas’s ultraviolet-dark terrain. Our observations imply that Pallas is an intact protoplanet that has undergone impact excavation and probable internal alteration.

Hubble Space Telescope STIS Observations of Comet 19P/Borrelly during the Deep Space 1 Encounter

In support of the NASA Deep Space 1 (DS1) mission to comet 19P/Borrelly, we obtained Hubble Space Telescope (HST) images and ultraviolet (UV) spectra of the comet near the time of the DS1 flyby in 2001 September. The HST data provide context information on 19P/Borrellys circumnuclear dust environment, the rotational period and rotational phase of its nucleus, the H2O and CS2 production rates, the dust production rate, the dust reflectivity in the visible and mid-UV, and the time variability of these quantities around the time of the DS1 encounter. We derive average values of Q = (3.0 ± 0.6) × 1028 molecules s-1, [CS2/H2O] = (1.0 ± 0.3) × 10-3, and Qdust ≈ 240 kg s-1. The corresponding dust-to-gas mass ratio is 0.24, but this is only a rough estimate because the dust production rate is uncertain by about an order of magnitude. The dust continuum was strongly reddened between 2400 and 3200 A, and the Afρ value of 745 ± 15 cm near 6500 A was ~2.5 times larger than the value near 2900 A. The observed coma morphology consisted of a strong jet centered ~6° from the projected solar vector, one broad fan centered ~23° from the sunward direction, and another broad fan centered ~18° from the antisunward direction. The light curve of the optical continuum, as measured in target acquisition images, has an amplitude of ~40% in a square aperture that subtends 160 km at the comet; the rotational period could not be independently derived from the HST images but is consistent with the value of ~26 hr derived from HST observations in 1994 and ground-based images in 2000. The new HST data reveal a prominent offset in the emission peak of neutral gas molecules, and therefore in the peak column densities of gas in the coma, relative to the position of the cometary nucleus, which may be related to the offset in ion densities observed in situ by the DS1 Plasma Experiment for Planetary Exploration (PEPE) plasma spectrometer.