Mark V. Sykes

A refined “standard” thermal model for asteroids based on observations of 1 Ceres and 2 Pallas

Abstract We present ground-based thermal infrared observations of asteroids 1 Ceres and 2 Pallas made over a period of 2 years. By analysing these data in light of the recently determined occultation diameter of Ceres (933–945 km) and Pallas (538 km) and their known small-amplitude lightcurves, we have determined a new value for the infrared beaming parameter used in the “standard” thermal emission model for asteroids. The new value is significantly lower than that previously used, and when applied in the reduction of thermal infrared observations of other asteroids, should yield model diameters that are closer to actual diameters. In our formulation, we also incorporate the recently adopted IAU magnitude convention for asteroids, which uses the zero-phase magnitudes (including the opposition effect) the same as is used for satellites.

Systematic biases in radiometric diameter determinations

Abstract Radiometric diameters and albedos of asteroids and other Solar System bodies have generally been determined with the aid of a standard thermal model, which assumes a smooth nonrotating surface and relies on an empirical beaming factor to adjust model fluxes to the values observed for objects of known radius. It has been assumed that the need for a correction factor is generally due largely to the effects of roughness on the thermal emission from real surfaces. We show that in many cases the effects of rotation are also important and must be considered in radiometric diameter determinations. The thermal effect of rotation depends not only on the objects thermal inertia, rotation rate, and pole orientation, but also on its temperature: colder objects with constant rotation rate and thermal inertia radiate proportionally less of their heat on the day hemisphere and more on the night hemisphere. If the asteroids Ceres and Pallas have lunarlike thermal inertias the effect of rotation on their thermal emission is very important: in particular, large variations in thermal emission correlated with variations in subsolar latitude around their orbits are expected. Such variations are not observed, and we therefore suggest that these and other main-belt asteroids have thermal inertias ≤1.5 × 10 4 erg cm −2 sec −1 2 K −1 , ≤30% that of the Moon. We determine a disk-integrated beaming parameter of 0.72 for the Moon and use this value to correct empirically for roughness effects in our thermophysical models. As a result of the influence of temperature on the thermal effects of rotation, the standard thermal model systematically underestimates the diameters (and overestimates the albedos) of cold objects. For example, assuming a thermal inertia 20% of the lunar value and the Sun in the equatorial plane, a Trojan asteroid would be 11% larger than the diameter determined radiometrically at 10 μm, and if it had a lunar thermal inertia, it would be 67% larger than determined radiometrically at 10 μm. Errors at 20 μm are smaller. The error in radiometric diameter varies with the bodys albedo, thermal inertia, and subsolar latitude in ways that we describe.

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.

Vesta's shape and morphology

A New Dawn Since 17 July 2011, NASAs spacecraft Dawn has been orbiting the asteroid Vesta—the second most massive and the third largest asteroid in the solar system (see the cover). Russell et al. (p. 684) use Dawns observations to confirm that Vesta is a small differentiated planetary body with an inner core, and represents a surviving proto-planet from the earliest epoch of solar system formation; Vesta is also confirmed as the source of the howardite-eucrite-diogenite (HED) meteorites. Jaumann et al. (p. 687) report on the asteroids overall geometry and topography, based on global surface mapping. Vestas surface is dominated by numerous impact craters and large troughs around the equatorial region. Marchi et al. (p. 690) report on Vestas complex cratering history and constrain the age of some of its major regions based on crater counts. Schenk et al. (p. 694) describe two giant impact basins located at the asteroids south pole. Both basins are young and excavated enough amounts of material to form the Vestoids—a group of asteroids with a composition similar to that of Vesta—and HED meteorites. De Sanctis et al. (p. 697) present the mineralogical characterization of Vesta, based on data obtained by Dawns visual and infrared spectrometer, revealing that this asteroid underwent a complex magmatic evolution that led to a differentiated crust and mantle. The global color variations detailed by Reddy et al. (p. 700) are unlike those of any other asteroid observed so far and are also indicative of a preserved, differentiated proto-planet. Spacecraft data provide a detailed characterization of the second most massive asteroid in the solar system. Vesta’s surface is characterized by abundant impact craters, some with preserved ejecta blankets, large troughs extending around the equatorial region, enigmatic dark material, and widespread mass wasting, but as yet an absence of volcanic features. Abundant steep slopes indicate that impact-generated surface regolith is underlain by bedrock. Dawn observations confirm the large impact basin (Rheasilvia) at Vesta’s south pole and reveal evidence for an earlier, underlying large basin (Veneneia). Vesta’s geology displays morphological features characteristic of the Moon and terrestrial planets as well as those of other asteroids, underscoring Vesta’s unique role as a transitional solar system body.

Cometary dust trails: I. Survey

Abstract Cometary dust trails were first observed by the Infrared Astronomical Satellite and consist of large refractory particles ejected from short-period comets at low velocities. Consequently, they tend to be found near the orbital paths of their parent bodies, their long and narrow appearance reminescent of airplane contrails. An examination of the entire sky as seen by IRAS has resulted in the detection of a total of eight trails associated with known short-period comets (Churyumov-Gerasimenko, Encke, Gunn, Kopff, Pons-Winnecke, Schwassmann-Wachmann 1, Tempel 1, and Tempel 2) as well as many faint trails having no known parents. Trails tended to be associated with objects having low perihelion distances that were observed near perihelion. We infer that the trail phenomenon is general to all short-period comets, and that future spacebased infrared detectors (e.g., ISO) will observe a different ensemble of trails as other comets pass through perihelion. Trail comets are found to lose the bulk of their mass in the large refractory trail particles, and are found to have a median refractory/volatile mass ratio of ≈3. This suggests that comets in general may be more like “frozen mudballs” than the canonical “dirty snowballs.”

The discovery of dust trails in the orbits of periodic comets

Analysis of data from the Infrared Astronomical Satellite has yielded evidence for narrow trails of dust coincident with the orbits of periodic comets Tempel 2, Encke, and Gunn. Dust was found both ahead of and behind the orbital positions of these comets. This dust was produced by the low-velocity ejection of large particles during perihelion passage. More than 100 additional dust trails are suggested by the data, almost all near the detection limits of the satellite. Many of these dust trails may be derived from previously unobserved comets.

Sublimation in bright spots on (1) Ceres.

The dwarf planet (1) Ceres, the largest object in the main asteroid belt with a mean diameter of about 950 kilometres, is located at a mean distance from the Sun of about 2.8 astronomical units (one astronomical unit is the Earth–Sun distance). Thermal evolution models suggest that it is a differentiated body with potential geological activity. Unlike on the icy satellites of Jupiter and Saturn, where tidal forces are responsible for spewing briny water into space, no tidal forces are acting on Ceres. In the absence of such forces, most objects in the main asteroid belt are expected to be geologically inert. The recent discovery of water vapour absorption near Ceres and previous detection of bound water and OH near and on Ceres (refs 5, 6, 7) have raised interest in the possible presence of surface ice. Here we report the presence of localized bright areas on Ceres from an orbiting imager. These unusual areas are consistent with hydrated magnesium sulfates mixed with dark background material, although other compositions are possible. Of particular interest is a bright pit on the floor of crater Occator that exhibits probable sublimation of water ice, producing haze clouds inside the crater that appear and disappear with a diurnal rhythm. Slow-moving condensed-ice or dust particles may explain this haze. We conclude that Ceres must have accreted material from beyond the ‘snow line’, which is the distance from the Sun at which water molecules condense.

The formation and origin of the IRAS zodiacal dust bands as a consequence of single collisions between asteroids

Abstract The zodiacal dust bands discovered by IRAS can be explained as products of single collisions between asteroids. Debris from such a collision is distributed about the plane of the ecliptic as particles experience differential precession of their ascending nodes due to dispersion of their semimajor axes. For each collision, two bands, one on each side of the ecliptic, are formed on time scales of 10 5 to 10 6 years. The band pairs observed by IRAS are most likely the result of collisions between asteroids ∼15 km in diameter that occured within the last several million years. Further analysis of the IRAS sky survey data and of any future, more sensitive surveys should reveal additional, fainter band pairs. Our model suggests that asteroid collisions are sufficient to account for the bulk of the observed zodiacal thermal emission.

The Formation of Encke Meteoroids and Dust Trail

Abstract We observed Comet 2P/Encke with the Infrared Space Observatory ISOCAM on July 14, 1997, from a particularly favorable viewing geometry above the comets orbital plane and at a distance of 0.25 AU. A structured coma was observed, along with a long, straight dust trail. For the first time, we are able to observe the path of particles as they evolve from the nucleus to the trail. The particles that produce the infrared coma are large, with a radiation to gravitational force ratio β mm-sized particles). The dust trail follows the orbit of the comet across our image, with a central core that is 2×104 km wide, composed of particles with β 10−3, in marked contrast to other comets like P/Halley and C/Hale-Bopp. The structure of the coma requires anisotropic emission and requires that the spin axis of the nucleus be nearly parallel to the orbital plane, resulting in strong seasonal variations of the particle emission. While most of the infrared coma emission is due to dust produced during the 1997 apparition, the core of the dust trail requires emissions from previous apparitions. The total mass lost during the 1997 apparition is estimated to be 2–6×1013 g. Compared to the gas mass loss from ultraviolet observations, the dust-to-gas mass ratio is 10–30, much higher than has ever been suggested from visible light observations. Using the recently measured nuclear diameter, we find that Encke can last only 3000–10,000 ρN yr (where ρN is the nuclear density in g cm−3) at its present mass loss rate.

Cryovolcanism on Ceres

INTRODUCTION Classic volcanism prevalent on terrestrial planets and volatile-poor protoplanets, such as asteroid Vesta, is based on silicate chemistry and is often expressed by volcanic edifices (unless erased by impact bombardment). In ice-rich bodies with sufficiently warm interiors, cryovolcanism involving liquid brines can occur. Smooth plains on some icy satellites of the outer solar system have been suggested as possibly cryovolcanic in origin. However, evidence for cryovolcanic edifices has proven elusive. Ceres is a volatile-rich dwarf planet with an average equatorial surface temperature of ~160 K. Whether this small (~940 km diameter) body without tidal dissipation could sustain cryovolcanism has been an open question because the surface landforms and relation to internal activity were unknown. RATIONALE The Framing Camera onboard the Dawn spacecraft has observed >99% of Ceres’ surface at a resolution of 35 m/pixel at visible wavelengths. This wide coverage and resolution were exploited for geologic mapping and age determination. Observations with a resolution of 135 m/pixel were obtained under several different viewing geometries. The stereo-photogrammetric method applied to this data set allowed the calculation of a digital terrain model, from which morphometry was investigated. The observations revealed a 4-km-high topographic relief, named Ahuna Mons, that is consistent with a cryovolcanic dome emplacement. RESULTS The ~17-km-wide and 4-km-high Ahuna Mons has a distinct size, shape, and morphology. Its summit topography is concave downward, and its flanks are at the angle of repose. The morphology is characterized by (i) troughs, ridges, and hummocky areas at the summit, indicating multiple phases of activity, such as extensional fracturing, and (ii) downslope lineations on the flanks, indicating rockfalls and accumulation of slope debris. These morphometric and morphologic observations are explained by the formation of a cryovolcanic dome, which is analogous to a high-viscosity silicic dome on terrestrial planets. Models indicate that extrusions of a highly viscous melt-bearing material can lead to the buildup of a brittle carapace at the summit, enclosing a ductile core. Partial fracturing and disintegration of the carapace generates slope debris, and relaxation of the dome’s ductile core due to gravity shapes the topographic profile of the summit. Modeling of this final phase of dome relaxation and reproduction of the topographic profile requires an extruded material of high viscosity, which is consistent with the mountain’s morphology. We constrained the age of the most recent activity on Ahuna Mons to be within the past 210 ± 30 million years. CONCLUSION Cryovolcanic activity during the geologically recent past of Ceres constrains its thermal and chemical history. We propose that hydrated salts with low eutectic temperatures and low thermal conductivities enabled the presence of cryomagmatic liquids within Ceres. These salts are the product of global aqueous alteration, a key process for Ceres’ evolution as recorded by the aqueously altered, secondary minerals observed on the surface. Perspective view of Ahuna Mons on Ceres from Dawn Framing Camera data (no vertical exaggeration). The mountain is 4 km high and 17 km wide in this south-looking view. Fracturing is observed on the mountain’s top, whereas streaks from rockfalls dominate the flanks. Volcanic edifices are abundant on rocky bodies of the inner solar system. In the cold outer solar system, volcanism can occur on solid bodies with a water-ice shell, but derived cryovolcanic constructs have proved elusive. We report the discovery, using Dawn Framing Camera images, of a landform on dwarf planet Ceres that we argue represents a viscous cryovolcanic dome. Parent material of the cryomagma is a mixture of secondary minerals, including salts and water ice. Absolute model ages from impact craters reveal that extrusion of the dome has occurred recently. Ceres’ evolution must have been able to sustain recent interior activity and associated surface expressions. We propose salts with low eutectic temperatures and thermal conductivities as key drivers for Ceres’ long-term internal evolution.