David Jewitt

Crystalline water ice on the Kuiper belt object (50000) Quaoar

The Kuiper belt is a disk-like structure consisting of solid bodies orbiting the Sun beyond Neptune. It is the source of the short-period comets and the likely repository of the Solar Systems most primitive materials. Surface temperatures in the belt are low (∼ 50 K), suggesting that ices trapped at formation should have been preserved over the age of the Solar System. Unfortunately, most Kuiper belt objects are too faint for meaningful compositional study, even with the largest available telescopes. Water ice has been reported in a handful of objects, but most appear spectrally featureless. Here we report near-infrared observations of the large Kuiper belt object (50000) Quaoar, which reveal the presence of crystalline water ice and ammonia hydrate. Crystallinity indicates that the ice has been heated to at least 110 K. Both ammonia hydrate and crystalline water ice should be destroyed by energetic particle irradiation on a timescale of about 107 yr. We conclude that Quaoar has been recently resurfaced, either by impact exposure of previously buried (shielded) ices or by cryovolcanic outgassing, or by a combination of these processes.

Properties of the Trans-Neptunian Belt: Statistics from the Canada-France-Hawaii Telescope Survey*

We present the results of a wide-field survey designed to measure the size, inclination, and radial distributions of Kuiper Belt objects (KBOs). The survey found 86 KBOs in 73 deg2 observed to limiting red magnitude of 23.7 using the Canada-France-Hawaii Telescope and the 12K × 8K CCD mosaic camera. For the first time, both ecliptic and off-ecliptic fields were examined to more accurately constrain the inclination distribution of the KBOs. The survey data were processed using an automatic moving-object detection algorithm, allowing a careful characterization of the biases involved. In this work, we quantify fundamental parameters of the classical KBOs (CKBOs), the most numerous objects found in our sample, using the new data and a maximum likelihood simulation. Deriving results from our best-fit model, we find that the size distribution follows a differential power law with exponent q = 4.0 (1 σ, or 68.27% confidence). In addition, the CKBOs inhabit a very thick disk consistent with a Gaussian distribution of inclinations with a half-width of i1/2 = 20 deg (1 σ). We estimate that there are NCKBO(D > 100 km) = 3.8 × 104 (1 σ) CKBOs larger than 100 km in diameter. We also find compelling evidence for an outer edge to the CKBOs at heliocentric distances R = 50 AU.

THE ACTIVE ASTEROIDS

Some asteroids eject dust, unexpectedly producing transient, comet-like comae and tails. First ascribed to the sublimation of near-surface water ice, mass-losing asteroids (also called main-belt comets) can in fact be driven by a surprising diversity of mechanisms. In this paper, we consider 11 dynamical asteroids losing mass, in nine of which the ejected material is spatially resolved. We address mechanisms for producing mass loss including rotational instability, impact ejection, electrostatic repulsion, radiation pressure sweeping, dehydration stresses, and thermal fracture, in addition to the sublimation of ice. In two objects (133P and 238P) the repetitive nature of the observed activity leaves ice sublimation as the only reasonable explanation, while in a third ((596) Scheila), a recent impact is the cause. Another impact may account for activity in P/2010 A2, but this tiny object can also be explained as having shed mass after reaching rotational instability. Mass loss from (3200) Phaethon is probably due to cracking or dehydration at extreme (~1000?K) perihelion temperatures, perhaps aided by radiation pressure sweeping. For the other bodies, the mass-loss mechanisms remain unidentified, pending the acquisition of more and better data. While the active asteroid sample size remains small, the evidence for an astonishing diversity of mass-loss processes in these bodies is clear.

From Kuiper Belt Object to Cometary Nucleus: The Missing Ultrared Matter

We combine new and published data to show that the optical color distributions of cometary nuclei and Kuiper belt objects (KBOs) are significantly different. The nuclei are, as a group, bluer than the KBOs, indicating that the surface chemical and/or physical properties of the two types of bodies are different. Objects in the dynamically intermediate Centaur class have optical colors like those of KBOs, while the color distribution of candidate dead comets is indistinguishable from that of the cometary nuclei. We infer that the surfaces of KBOs are modified upon entry to the inner solar system. We consider several mechanisms and conclude that the color change is most likely caused by the rapid burial of ancient surface materials exposed in the Kuiper belt. The distinctive, ultrared material that is present on the surfaces of some KBOs is absent on the cometary nuclei.

LARGE KUIPER BELT OBJECTS: THE MAUNA KEA 8K CCD SURVEY

We describe a large-area ecliptic survey designed to assess the sky-plane surface density of bright Kuiper Belt objects. We used an 8192 ) 8192 pixel CCD mosaic to image 51.5 deg2 to a 50% detection threshold red magnitude Thirteen new Kuiper Belt objects were identi—ed in the survey, m R \ 22.5. including some of the brightest and, presumably, largest known examples. We use Monte Carlo models to assess the eUects of observational bias in our survey and to examine (1) the size distribution of bright objects in the Kuiper Belt, (2) the possible existence of a cutoU in the size distribution at large radii, (3) the intrinsic ratio of Plutinos to nonresonant (ii classical ˇˇ) Kuiper Belt objects, (4) the intrinsic ratio of populations in the 3:2 and 2:1 mean motion resonances, and (5) the radial extent of the Kuiper Belt.

Colors and Spectra of Kuiper Belt Objects

We present new measurements of the optical colors of Kuiper belt objects, principally from the Keck 10 m telescope. The measurements confirm the existence of a wide spread in the B-V, V-R, and R-I color indices as we found previously. Relative to the sun, the Kuiper belt objects exhibit reflected colors from nearly neutral to very red. The optical and optical-infrared (V-J) color indices are mutually correlated, showing the presence of a single reddening agent from 0.45 to 1.2 μm. On the other hand, we find no evidence for linear correlations between the color and absolute magnitude (a proxy for size), instantaneous heliocentric distance, semimajor axis, or with any other orbital property. In this regard, the Kuiper belt objects differ from the main-belt asteroids, in which strong radial color gradients exist. We find no statistically significant evidence for bimodal or other nonuniform color distributions, either in our data or in data previously reported to show such evidence. The impact resurfacing hypothesis is reexamined in the light of the new color data and is rejected as the primary cause of the observed color dispersion. We also present new near-infrared reflection spectra of 1993 SC, 1996 TS66, 1999 DE9 and 2000 EB173, taken at the Keck and Subaru telescopes. These spectra, combined with others from the published literature, provide independent evidence for compositional diversity in the Kuiper belt. Objects 2000 EB173, 1993 SC, and 1996 TS66 are spectrally bland, while 1999 DE9 shows solid-state absorption bands.

Cometary grain scattering versus wavelength, or, what color is comet dust?

Optical and near-infrared observations of comets are combined in a systematic study of the wavelength dependence of the scattering from cometary grains. The normalized rate of change of the reflectivity of cometary grains with respect to the wavelength of observations decreases as the wavelength increases. The observed wavelength dependence of the reflectivity gradient is consistent with an origin by scattering from micron-sized or larger, slightly absorbing spheres. The optically important comet grains are about an order of magnitude larger than the optically important interstellar grains. This size difference is a probable result of grain growth in the cloud from which the comets condensed. Grain properties differ so widely among the comets that any phase angle or heliocentric distance dependences of the continuum color are hidden. 42 references.

The Strange Case of 133P/Elst-Pizarro: A Comet among the Asteroids*

We present a new investigation of the comet-asteroid transition object 133P/(7968) Elst-Pizarro. We find mean optical colors (BV =0 .69� 0.02, VR =0 .42� 0.03, RI =0 .27� 0.03) and a phase-darkening coefficient (� =0 .044� 0.007 mag deg � 1 ) that are comparable both to other comet nuclei and to C-type asteroids. As in 1996, when this objects comet-like activity was first noted, data from 2002 show a long, narrow dust trail in the projected orbit of the object. Observations over several months reveal changes in the structure and brightness of this trail, showing that it is actively generated over long periods of time. Finson-Probstein modeling is used to constrain the parameters of the dust trail. We find optically dominant dust particle sizes of ad � 10 � mr eleased with low ejection velocities (vg � 1.5 m s � 1 ) over a period of activity lasting at least 5 months in 2002. The double- peaked light curve of the nucleus indicates an aspherical shape (axis ratio a/b � 1.45 � 0.07) and rapid rotation (periodProt = 3.471 � 0.001 hr). The practical identification of 133P/Elst-Pizarro as a comet (i.e., a mass-losing body) is not in doubt, but the origin of the mass loss is unclear. The 1996 trail has been previously explained as debris released by a chance impact, but our discovery of recurrent activity renders this interpretation implausible. We consider two hypotheses for the activity in 133P/Elst-Pizarro. The ejection of particles is naturally explained if the object is a barely active Jupiter-family comet that has evolved into an asteroid-like orbit, perhaps under the prolonged action of nongravitational forces due to asymmetric mass loss. In this case, the orbital similarity to the Themis family must be considered coincidental. Alternatively, 133P/Elst-Pizarro could be a true member of the Themis family on which buried ice has been recently excavated by impact.

Spectroscopic Monitoring of Comet C/1996 B2 (Hyakutake) with the JCMT and IRAM Radio Telescopes

Molecular radio lines were monitored in comet C/1996 B2 (Hyakutake) from 1996 February 10 to June 23, using the James Clerk Maxwell Telescope and the 30 m telescope and the Plateau de Bure interferometer of the Institut de Radioastronomie Millimetrique. We report on observations of HCN, CH3OH, CO, H2CO, CS, and H2S and on the evolution of their production rates with heliocentric distance (rh), from 1.86 down to 0.24 AU at perihelion. Most production rates increased roughly as r down to 0.6 AU preperihelion. Closer to the Sun, they stalled before decreasing beyond 0.6 AU postperihelion, when observations resumed. The CS/HCN ratio varied as r from 1.2 to 0.24 AU. A rapid increase of the mean gas temperature in the coma is measured, and the gas expansion velocity increased from 0.55 to 1.6 km s-1, as the comet approached the Sun from 1.6 to 0.3 AU. Molecular abundances of the minor species around 1 AU are similar to those observed in other comets, while the CO abundance relative to water is high (≈22%). Coarse mapping was used to check the comets position and to investigate the density distribution of the molecules within the coma. It provides constraints on the size of the extended source of formaldehyde, found to be between 1.2 and 2 times the scale length of H2CO itself. The density distribution of CS is compatible with its production from the photodissociation of a short-lived molecule such as CS2. The density distribution observed for CO can be mostly explained by a nuclear source.

A recent disruption of the main-belt asteroid P/2010 A2.

Most inner main-belt asteroids are primitive rock and metal bodies in orbit about the Sun between Mars and Jupiter. Disruption, through high-velocity collisions or rotational spin-up, is believed to be the primary mechanism for the production and destruction of small asteroids and a contributor to dust in the Sun’s zodiacal cloud, while analogous collisions around other stars feed dust to their debris disks. Unfortunately, direct evidence about the mechanism or rate of disruption is lacking, owing to the rarity of the events. Here we report observations of P/2010 A2, a previously unknown inner-belt asteroid with a peculiar, comet-like morphology. The data reveal a nucleus of diameter approximately 120 metres with an associated tail of millimetre-sized dust particles. We conclude that it is most probably the remnant of a recent asteroidal disruption in February/March 2009, evolving slowly under the action of solar radiation pressure, in agreement with independent work.