Daniel D. Durda

Delivery of dark material to Vesta via carbonaceous chondritic impacts

NASA’s Dawn spacecraft observations of Asteroid (4) Vesta reveal a surface with the highest albedo and color variation of any asteroid we have observed so far. Terrains rich in low albedo dark material (DM) have been identified using Dawn Framing Camera (FC) 0.75 lm filter images in several geologic settings: associated with impact craters (in the ejecta blanket material and/or on the crater walls and rims); as flow-like deposits or rays commonly associated with topographic highs; and as dark spots (likely secondary impacts) nearby impact craters. This DM could be a relic of ancient volcanic activity or exogenic in origin. We report that the majority of the spectra of DM are similar to carbonaceous chondrite meteorites mixed with materials indigenous to Vesta. Using high-resolution seven color images we compared DM color properties (albedo, band depth) with laboratory measurements of possible analog materials. Band depth and albedo of DM are identical to those of carbonaceous chondrite xenolith-rich howardite Mt. Pratt (PRA) 04401. Laboratory mixtures of Murchison CM2 carbonaceous chondrite and basaltic eucrite Millbillillie also show band depth and albedo affinity to DM. Modeling of carbonaceous chondrite abundance in DM (1–6 vol.%) is consistent with howardite meteorites. We find no evidence for large-scale volcanism (exposed dikes/pyroclastic falls) as the source of DM. Our modeling efforts using impact crater scaling laws and numerical models of ejecta reaccretion suggest the delivery and emplacement of this DM on Vesta during the formation of the � 400 km Veneneia basin by a low-velocity (<2 km/s) carbonaceous impactor. This discovery is important because it strengthens the long-held idea that primitive bodies are the source of carbon and probably volatiles in the early Solar System.

Collision Rates in the Present-Day Kuiper Belt and Centaur Regions: Applications to Surface Activation and Modification on Comets, Kuiper Belt Objects, Centaurs, and Pluto–Charon

Abstract We present results from our model of collision rates in the present-day Edgeworth–Kuiper Belt and Centaur region. We have updated previous results to allow for new estimates of the total disk population in order to examine surface activation and modification time scales due to cratering impacts. We extend previous results showing that the surfaces of Edgeworth–Kuiper Belt objects are not primordial and have been moderately to heavily reworked by collisions. Objects smaller than about r =2.5 km have collisional disruption lifetimes less than 3.5 Gyr in the present-day collisional environment and have probably been heavily damaged in their interiors by large collisions. In the 30- to 50-AU region, impacts of 1-km-radius comets onto individual 100-km-radius objects occur on 7×10 7 –4×10 8 -year time scales, cratering the surfaces of the larger objects with ∼8–54 craters 6 km in diameter over a 3.5-Gyr period. Collision time scales for impacts of 4-m-radius projectiles onto 1-km-radius comets range from 3×10 7 , to 5×10 7 years. The cumulative fraction of the surface area of 1- and 100-km-radius objects cratered by projectiles with radii larger than 4 m ranges from a few to a few tens percent over 3.5 Gyr. The flux of Edgeworth–Kuiper Belt projectiles onto Pluto and Charon is also calculated and is found to be ∼3–5 times that of previous estimates. Our impact model is also applied to Centaur objects in the 5- to 30-AU region. We find that during their dynamical lifetimes within the Centaur region, objects undergo very little collisional evolution. Therefore, the collisional/cratering histories of Centaurs are dominated by the time spent in the Edgeworth–Kuiper Belt rather than the time spent on planet-crossing orbits. Further, we find that the predominant surface activity of Centaur objects like Chiron is most likely not impact-induced.

The size-frequency distribution of the zodiacal cloud: evidence from the solar system dust bands

Recent observations of the size–frequency distribution of zodiacal cloud particles obtained from the cratering record on the LDEF satellite are the latest evidence for a significant large particle population (100-μm diameter or greater) near 1 AU. Our previous modeling of the Solar System dust bands, features of the zodiacal cloud associated with the comminution of Hirayama family asteroids, has been limited by the fact that only small particles (25-μm diameter or smaller) have been considered. This was due to the prohibitively large amount of computing power required to numerically analyze the dynamics of larger particles. The recent availability of inexpensive, fast processors has finally made this work possible. Models of the dust bands are created, built from individual dust particle orbits, taking into account a size–frequency distribution of the material and the dynamical history of the constituent particles. These models are able to match both the shapes and amplitudes of the dust band structures observed by IRAS in multiple wavebands. The size–frequency index, q, that best matches the observations is approximately 1.4, a distribution in which the surface area (and hence the infrared emission) is dominated by large particles. However, in order to successfully model the “ten degree” band, which is usually associated with collisional activity within the Eos family, we find that the mean proper inclination of the dust particle orbits has to be approximately 9.35°, significantly different from the mean proper inclination of the Eos family (10.08°).

Orbital Evolution of Interplanetary Dust

The two most important dynamical features of the zodiacal cloud are: (i) t he dust bands associated with t he major Hirayama asteroid families, and (ii) the circumsolar ring of dust particles in resonant lock with th e Eart h. Oth er important dynamical features include the offset of th e center of symmetry of th e cloud from the Sun, the radial gradient of the ecliptic polar brightness at th e Earth, and th e warp of th e cloud. The dust bands provide th e st rongest evidence th at a substantial and possibly dominant fraction of the cloud originate s from aster oids. However, the characteristic diameter of these asteroidal particles is probably several hundred microns and the migration of th ese large particles towards th e inner Solar System due to Poynting Robert son light drag and their slow passage through secular resonances at the inner edge of the asteroid belt result s in large increases in th eir eccent ricities and inclinations. Because of these orbital changes, the dividing line between asteroidal and comet ary type orbits in the inner Solar System is probably not sharp, and it may be difficult to distinguish clearly between ast eroidal and cometary particles on dynamical grounds alone.

Zodiacal Dust Bands

One of the outstanding problems in solar system science is the source of the particles that constitute the zodiacal cloud. The zodiacal dust bands discovered by IRAS have a pivotal role in this debate because, without doubt, they are the small, tail end products of asteroidal collisions. Geometrical arguments are probably the strongest and the plane of symmetry of the dust bands places their source firmly in the asteroid belt. A cometary source, Comet Encke for example, could exist at the distance of the mainbelt, but the dynamics of cometary orbits makes the formation of cometary dust bands impossible, unless, of course, there is a significant (comparable in volume to the asteroidal families) source of comets interior to the orbit of Jupiter with low (asteroidal) orbital eccentricities. We have suggested that the dust bands are associated with the prominent asteroidal families. The link with the Themis and Koronis families is good but the link with Eos remains to be proved. We show here by detailed modeling that even though the filtered infrared flux in the 25µm waveband associated with the dust bands is only ~1% of the total signal, this is only the “tip of the iceberg” and that asteroidal dust associated with the bands constitutes ~10% of the zodiacal cloud. This result, plus the observed size-frequency distribution of mainbelt asteroids and the observed ratio of the number of family to non-family asteroids allows us to estimate that asteroidal dust accounts for about one third of the zodiacal cloud. The discovery of the “leading-trailing” asymmetry of the zodiacal cloud in the IRAS data and our interpretation of this asymmetry in terms of a ring of asteroidal particles in resonant lock with the Earth is important for two reasons. (1) The existence of the ring strongly suggests that large (diameter ≥ 12µm) asteroidal particles (or particles with low orbital eccentricities) are transported to the inner solar system by drag forces. (2) The observed ratio of the trailing-leading asymmetry allows an independent estimate of the contribution of asteroidal particles to the zodiacal cloud. These new results have important implications for the source of the interplanetary dust particles (IDPs) collected at the Earth. Because asteroidal particles constitute about one third of the zodiacal cloud and are transported to the inner solar system by drag forces, gravitational focussing by the Earth that results in the preferential capture of particles from orbits with low inclinations and low eccentricities and the possible “funneling” effect of the ring itself, imply that nearly all of the unmelted IDPs collected at the Earth are asteroidal.

Collisional Evolution in the Vulcanoid Region: Implications for Present-Day Population Constraints

Abstract We explore the effects of collisional evolution on putative Vulcanoid ensembles in the region between 0.06 and 0.21 AU from the Sun in order to constrain the probable population density and population structure of this region today. Dynamical studies have shown that the Vulcanoid Zone (VZ) could be populated. However, we find that the frequency and energetics of collisional evolution this close to the Sun, coupled with the efficient radiation transport of small debris out of this region, together conspire to create an active and highly intensive collisional environment that depletes any very significant population of rocky bodies placed in it, unless the bodies exhibit orbits that are circular to ∼10 −3 or less or highly lossy mechanical properties that correspond to a fraction of impact energy significantly less than 10% being imparted to ejecta. The most favorable locale for residual bodies to survive in this region is in highly circular orbits near the outer edge of the dynamically stable Vulcanoid Zone (i.e., near 0.2 AU), where collisional evolution and radiation transport of small bodies and debris proceed most slowly. If the mean random orbital eccentricity in this region exceeds ∼10 −3 , then our work suggests it is unlikely that more than a few hundred objects with radii larger than 1 km will be found in the entire VZ; assuming the largest objects have a radius of 30 km, then the total mass of bodies in the VZ down to 0.1 km radii is likely to be no more than ∼10 −6 M ⊕ , −3 the mass of the asteroid belt. A 0.01-AU-wide ring near the outer stability boundary of the VZ at 0.2 AU would likely not contain over a few tens of objects with radii larger than 1 km. Despite the dynamical stability of large objects in this region (Evans, N. W., and S. Tabachnik, 1999, Nature 399, 41–43), it is plausible that the entire region is virtually empty of kilometer-scale and larger objects.

Identifying near-Earth object families

Abstract The study of asteroid families has provided tremendous insight into the forces that sculpted the main belt and continue to drive the collisional and dynamical evolution of asteroids. The identification of asteroid families within the NEO population could provide a similar boon to studies of their formation and interiors. In this study we examine the purported identification of NEO families by Drummond [Drummond, J.D., 2000. Icarus 146, 453–475] and conclude that it is unlikely that they are anything more than random fluctuations in the distribution of NEO osculating orbital elements. We arrive at this conclusion after examining the expected formation rate of NEO families, the identification of NEO groups in synthetic populations that contain no genetically related NEOs, the orbital evolution of the largest association identified by Drummond [Drummond, J.D., 2000. Icarus 146, 453–475], and the decoherence of synthetic NEO families intended to reproduce the observed members of the same association. These studies allowed us to identify a new criterion that can be used to select real NEO families for further study in future analyses, based on the ratio of the number of pairs and the size of strings to the number of objects in an identified association.

Impacts into porous foam targets: possible implications for the disruption of comet nuclei

Abstract We have conducted a series of impact experiments to examine the response of very porous foam targets to various impacts. Under near-vacuum conditions, closed-pore and open-pore foam targets were subjected to ∼1 km s −1 impacts from aluminum and foam projectiles. We found that open-pore targets absorbed the impacts with little or no global fragmentation or noticeable cratering, exhibiting only local damage along the path of the projectile, which tunneled through the target. Closed-pore targets exhibited nearly explosive disruption, apparently resulting from stresses built up within the target due to internal pressurization from air that could not escape the target interior during evacuation of the impact chamber. These results suggest that build-up of internal volatile pressure within the nuclei of collisionally or dynamically unevolved comets could allow comparatively small impacts onto their surfaces to result in disproportionately disruptive outcomes.

THE B612 MISSION DESIGN

This paper describes a mission proposed by the B612 Foundation to demonstrate the feasibility of docking a spacecraft with a small asteroid and applying a contro lled, steady thrust to it in order to measurably alter the asteroids orbit and rotation pole by the year 2015. The target would be a rocky 200 -meter asteroid with a mass of about 10 billion kilograms that does not pose any impact threat to the Earth. The technology goal of the mission is to demonstrate a measurable change in the orbital velocity of the asteroid, say 0.2 cm/sec, minimum. In addition, in situ science would also be performed to determine materials and structural properties of the surface. Sec ondary goals could include technology demonstrations for mining the natural resources found on the asteroid. The spacecraft would have liftoff mass of less than 20 metric tons, including fuel for the trip to the asteroid and fuel to push the asteroid once it has arrived, and it would be launched on a single heavy lift rocket such as the Proton, Ariane 5 or Titan 4. The spacecraft borrows heavily from NASAs Jupiter Icy Moons Orbiter (JIMO) concept vehicle in that it would rely on nuclear reactor power and i on -propulsion systems. A major departure, however, would be the use of a promising new propulsion engine known as the VASIMR (Variable Specific Impulse Magnetoplasma Rocket) which uses radio waves to excite fuel into a plasma and magnetic fields to direct the expanding stream of

Low-cost airborne astronomy imager to begin research phase

For decades, airborne astronomy and geophysical observations have proven useful adjuncts to ground-based and space-based instrumentation, particularly for optical and infrared studies [e.g., Larson, 1995].Compared to ground-based instruments, airborne research platforms offer superior atmospheric transmission, the ability to reach remote and often otherwise inaccessible locations over the Earth, and virtually guaranteed good weather for observing the sky.Airborne platforms also offer substantial cost advantages over space-based instruments.With funding from Southwest Research Institute (SwRI) and NASA, we have developed the hardware and tech-niques to routinely conduct valuable astronomical and aeronomical observations from high-performance, two-seater military-type aire raft. These platforms cost far less than larger, more conventional airborne platforms, offering savings that are often of an order of 10:1 per flight hour. Smaller platforms based throughout the world eliminate the need for expensive, campaignstyle movement of specialized large aircraft and logistics support teams, and can react faster to transient events. The Southwest Ultraviolet Imaging System-Airborne (SWUIS-A) imager has been flight tested in 14 airborne missions since 1997. With initial systems development and operational trials completed, a vigorous operational research phase is beginning.