Marc William Buie

The Deep Ecliptic Survey: A Search for Kuiper Belt Objects and Centaurs. II. Dynamical Classification, the Kuiper Belt Plane, and the Core Population

The Deep Ecliptic Survey (DES)?a search optimized for the discovery of Kuiper belt objects (KBOs) with the Blanco and Mayall 4 m telescopes at the Cerro Tololo Inter-American Observatory and Kitt Peak National Observatory?has covered 550 deg2 from its inception in 1998 through the end of 2003. This survey has a mean 50% sensitivity at VR magnitude 22.5. We report here the discoveries of 320 designated KBOs and Centaurs for the period 2000 March through 2003 December and describe improvements to our discovery and recovery procedures. Our data and the data products needed to reproduce our analyses in this paper are available through the NOAO survey database. Here we present a dynamical classification scheme, based on the behavior of orbital integrations over 10 Myr. The dynamical classes, in order of testing, are Resonant, Centaur, Scattered-Near, Scattered-Extended, and Classical. (These terms are capitalized when referring to our rigorous definitions.) Of the 382 total designated KBOs discovered by the DES, a subset of 196 objects have sufficiently accurate orbits for dynamical classification. Summary information is given for an additional 240 undesignated objects also discovered by the DES from its inception through the end of 2003. The number of classified DES objects (uncorrected for observational bias) are Classical, 96; Resonant, 54; Scattered-Near, 24; Scattered-Extended, 9; and Centaur, 13. We use subsets of the DES objects (which can have observational biases removed) and larger samples to perform dynamical analyses on the Kuiper belt. The first of these is a determination of the Kuiper belt plane (KBP), for which the Classical objects with inclinations less than 5? from the mean orbit pole yield a pole at R.A. = 27392 ? 062 and decl. = 6670 ? 020 (J2000), consistent with the invariable plane of the solar system. A general method for removing observational biases from the DES data set is presented and used to find a provisional magnitude distribution and the distribution of orbital inclinations relative to the KBP. A power-law model fit to the cumulative magnitude distribution of all KBOs discovered by the DES in the VR filter yields an index of 0.86 ? 0.10 (with the efficiency parameters for the DES fitted simultaneously with the population power law). With the DES sensitivity parameters fixed, we derive power-law indices of 0.74 ? 0.05, 0.52 ? 0.08, and 0.74 ? 0.15, respectively, for the Classical, Resonant, and Scattered classes. Plans for calibration of the DES detection efficiency function and DES magnitudes are discussed. The inclination distribution confirms the presence of hot and cold populations; when the geometric sin i factor is removed from the inclination distribution function, the cold population shows a concentrated core with a full width at half-maximum of approximately 46, while the hot population appears as a halo, extending beyond 30?. The inclination distribution is used to infer the KBO distribution in the sky, as a function of latitude relative to the KBP. This inferred latitude distribution is reasonably consistent with the latitude distribution derived from direct observation, but the agreement is not perfect. We find no clear boundary between the Classical and Scattered classes either in their orbital inclinations with respect to the KBP or in their power-law indices in their respective magnitude distributions. This leaves open the possibility that common processes have shaped the distribution of orbital parameters for the two classes.

Discovery of two new satellites of Pluto

Plutos first known satellite, Charon, was discovered in 1978. It has a diameter (∼1,200 km) about half that of Pluto, which makes it larger, relative to its primary, than any other moon in the Solar System. Previous searches for other satellites around Pluto have been unsuccessful, but they were not sensitive to objects ≲150 km in diameter and there are no fundamental reasons why Pluto should not have more satellites. Here we report the discovery of two additional moons around Pluto, provisionally designated S/2005 P 1 (hereafter P1) and S/2005 P 2 (hereafter P2), which makes Pluto the first Kuiper belt object known to have multiple satellites. These new satellites are much smaller than Charon, with estimates of P1s diameter ranging from 60 km to 165 km, depending on the surface reflectivity; P2 is about 20 per cent smaller than P1. Although definitive orbits cannot be derived, both new satellites appear to be moving in circular orbits in the same orbital plane as Charon, with orbital periods of ∼38 days (P1) and ∼25 days (P2).

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.

High albedos of low inclination Classical Kuiper belt objects

Abstract We present observations of thermal emission from fifteen transneptunian objects (TNOs) made using the Spitzer Space Telescope . Thirteen of the targets are members of the Classical population: six dynamically hot Classicals, five dynamically cold Classicals, and two dynamically cold inner Classical Kuiper belt objects (KBOs). We fit our observations using thermal models to determine the sizes and albedos of our targets finding that the cold Classical KBOs have distinctly higher visual albedos than the hot Classicals and other TNO dynamical classes. The cold Classicals are known to be distinct from other TNOs in terms of their color distribution, size distribution, and binarity fraction. The Classical objects in our sample all have red colors yet they show a diversity of albedos which suggests that there is not a simple relationship between albedo and color. As a consequence of high albedos, the mass estimate of the cold Classical Kuiper belt is reduced from approximately 0.01 M ⊕ to approximately 0.001 M ⊕ . Our results also increase significantly the sample of small Classical KBOs with known albedos and sizes from 21 to 32 such objects.

Resonance Occupation in the Kuiper Belt: Case Examples of the 5:2 and Trojan Resonances

As part of our ongoing Deep Ecliptic Survey (DES) of the Kuiper belt, we report on the occupation of the 1 : 1 (Trojan), 4 : 3, 3 : 2, 7 : 4, 2 : 1, and 5 : 2 Neptunian mean motion resonances (MMRs). The previously unrecognized occupation of the 1 : 1 and 5 : 2 MMRs is not easily understood within the standard model of resonance sweeping by a migratory Neptune over an initially dynamically cold belt. Among all resonant Kuiper belt objects (KBOs), the three observed members of the 5 : 2 MMR discovered by DES possess the largest semimajor axes (a ≈ 55.4 AU), the highest eccentricities (e ≈ 0.4), and substantial orbital inclinations (i ≈ 10°). Objects (38084) 1999HB12 and possibly 2001KC77 can librate with modest amplitudes of ~90° within the 5 : 2 MMR for at least 1 Gyr. Their trajectories cannot be explained by close encounters with Neptune alone, given the latters current orbit. The dynamically hot orbits of such 5 : 2 resonant KBOs, unlike hot orbits of previously known resonant KBOs, may imply that these objects were preheated to large inclination and large eccentricity prior to resonance capture by a migratory Neptune. Our first discovered Neptunian Trojan, 2001QR322, may not owe its existence to Neptunes migration at all. The trajectory of 2001QR322 is remarkably stable; the object can undergo tadpole-type libration about Neptunes leading Lagrange (L4) point for at least 1 Gyr with a libration amplitude of 24°. Trojan capture probably occurred while Neptune accreted the bulk of its mass. For an assumed albedo of 12%–4%, our Trojan is ~130–230 km in diameter. Model-dependent estimates place the total number of Neptune Trojans resembling 2001QR322 at ~20–60. Their existence helps to rule out violent orbital histories for Neptune.

The binary Kuiper-belt object 1998 WW31

The recent discovery of a binary asteroid during a spacecraft fly-by generated keen interest, because the orbital parameters of binaries can provide measures of the masses, and mutual eclipses could allow us to determine individual sizes and bulk densities. Several binary near-Earth, main-belt and Trojan asteroids have subsequently been discovered. The Kuiper belt—the region of space extending from Neptune (at 30 astronomical units) to well over 100 AU and believed to be the source of new short-period comets—has become a fascinating new window onto the formation of our Solar System since the first member object, not counting Pluto, was discovered in 1992 (ref. 13). Here we report that the Kuiper-belt object 1998 WW31 is binary with a highly eccentric orbit (eccentricity e ≈ 0.8) and a long period (about 570 days), very different from the Pluto/Charon system, which was hitherto the only previously known binary in the Kuiper belt. Assuming a density in the range of 1 to 2 g cm-3, the albedo of the binary components is between 0.05 and 0.08, close to the value of 0.04 generally assumed for Kuiper-belt objects.

A giant impact origin for Pluto's small moons and satellite multiplicity in the Kuiper belt

The two newly discovered satellites of Pluto (P1 and P2) have masses that are small compared to both Pluto and Charon—that is, between 5 × 10-4 and 1 × 10-5 of Plutos mass, and between 5 × 10-3 and 1 × 10-4 of Charons mass. This discovery, combined with the constraints on the absence of more distant satellites of Pluto, reveal that Pluto and its moons comprise an unusual, highly compact, quadruple system. These facts naturally raise the question of how this puzzling satellite system came to be. Here we show that P1 and P2s proximity to Pluto and Charon, the fact that P1 and P2 are on near-circular orbits in the same plane as Plutos large satellite Charon, along with their apparent locations in or near high-order mean-motion resonances, all probably result from their being constructed from collisional ejecta that originated from the Pluto–Charon formation event. We also argue that dust–ice rings of variable optical depths form sporadically in the Pluto system, and that rich satellite systems may be found—perhaps frequently—around other large Kuiper belt objects.

Albedo maps of Pluto and Charon: Initial mutual event results

Abstract We present single-scattering albedo maps of the surfaces of Pluto and Charon based primarily on mutual event observations. The dataset contains 3374 photometric observations that cover 15 different satellite transit events, 14 satellite eclipse events, and other out-of-eclipse photometry spanning 1954 to 1986. The maps consist of a 59 × 29 grid of tiles for each body. We applied the technique of maximum entropy image reconstruction to invert the lightcurves, thus revealing surface maps of single-scattering albedo. The surface of Pluto is seen to have albedo features similar to our previous spot model maps (Buie and Tholen 1989). In particular, a south polar cap is evident in the map of Pluto. The north polar region is brighter than the equatorial regions but is not as bright as the south pole. Single-scattering albedos range from 0.98 in the south polar cap to a low near 0.2 at longitudes corresponding to the lightcurve minimum. The map of Charon is somewhat darker with single-scattering albedos as low as 0.03.