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To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2008.09.004 Byline: Alan W. Harris (a), Michael Mueller (a), Carey M. Lisse (b), Andrew F. Cheng (b) Keywords: Asteroids; Infrared observations; Photometry; Spectrophotometry Abstract: The Karin cluster is one of the youngest known families of main-belt asteroids, dating back to a collisional event only 5.8[+ or -]0.2 Myr ago. Using the Spitzer Space Telescope we have photometrically sampled the thermal continua (3.5-22 [mu]m) of 17 Karin cluster asteroids of different sizes, down to the smallest members discovered so far, in order to make the first direct measurements of their sizes and albedos and study the physical properties of their surfaces. Our targets are also amongst the smallest main-belt asteroids observed to date in the mid-infrared. The derived diameters range from 17.3 km for 832 Karin to 1.5 km for 75176, with typical uncertainties of 10%. The mean albedo is p.sub.v=0.215[+ or -]0.015, compared to 0.20[+ or -]0.07 for 832 Karin itself (for H=11.2[+ or -]0.3), consistent with the view that the Karin asteroids are closely related physically as well as dynamically. The albedo distribution (0.12a[c]1/2p.sub.va[c]1/20.32) is consistent with the range associated with S-type asteroids but the variation from one object to another appears to be significant. Contrary to the case for near-Earth asteroids, our data show no evidence of an albedo dependence on size. However, the mean albedo is lower than expected for young, fresh 'S-type' surfaces, suggesting that space weathering can darken main-belt asteroid surfaces on very short timescales. Our data are also suggestive of a connection between surface roughness and albedo, which may reflect rejuvenation of weathered surfaces by impact gardening. While the available data allow only estimates of lower limits for thermal inertia, we find no evidence for the relatively high values of thermal inertia reported for some similarly sized near-Earth asteroids. Our results constitute the first observational confirmation of the legitimacy of assumptions made in recent modeling of the formation of the Karin cluster via a single catastrophic collision 5.8[+ or -]0.2 Myr ago. Author Affiliation: (a) DLR Institute of Planetary Research, Rutherfordstrasse 2, 12489 Berlin, Germany (b) Planetary Exploration Group, Space Department, Johns Hopkins University Applied Physics Laboratory, 11100 Johns Hopkins Rd, Laurel, MD 20723, USA Article History: Received 6 August 2008; Revised 25 September 2008; Accepted 26 September 2008

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2006.06.015 Byline: Andrew F. Cheng, Andrew J. Dombard Keywords: Comets; nucleus; Geological processes; Geophysics Abstract: Three Jupiter family comets have now been observed by spacecraft with the surprising result that these comets lack unambiguous impact craters. Large-scale topography generally appears to be softened on these comets, although sharp topography is preserved at small scales. We find that viscous relaxation of water ice may explain these observations, given reasonable assumptions about ice grain size and temperatures attained in the interiors. We suggest that both the shapes and the cratering records of Jupiter family comets may be substantially modified and no longer reflect cometary formation processes or collisional evolution in the Kuiper Belt. Author Affiliation: The Johns Hopkins University, Applied Physics Laboratory, Laurel, MD 20723, USA Article History: Received 11 May 2006

A new synthesis of asteroid collisional evolution is motivated by the question of whether most asteroids larger than ~ 1 km size are strengthless gravitational aggregates (rubble piles). NEAR found Eros not to be a rubble pile, but a shattered collisional fragment, with a through-going fracture system, and an average of about 20 m regolith cover. Of four asteroids visited by spacecraft, none appears likely to be a rubble pile, except perhaps Mathilde. Nevertheless, current understanding of asteroid collisions and size-dependent strength, and the observed distribution of rotation rates versus size, have led to a theoretical consensus that many or most asteroids larger than 1 km should be rubble piles. Is Eros, the best-observed asteroid, highly unusual because it is not a rubble pile? Is Mathilde, if it is a rubble pile, like most asteroids? What would be expected for the small asteroid Itokawa, the MUSES-C sample return target? An asteroid size distribution is synthesized from the Minor Planet Center listing and results of the Sloan Digital Sky Survey, an Infrared Space Observatory survey, the Small Main-belt Asteroid Spectroscopic Survey and the Infrared Astronomical Satellite survey. A new picture emerges of asteroid collisional evolution, in which the well-known Dohnanyi result, that the size distribution tends toward a self-similar form with a 2.5-index power law, is overturned because of scale-dependent collision physics. Survival of a basaltic crust on Vesta can be accommodated, together with formation of many exposed metal cores. The lifetimes against destruction are estimated as 3 Gyr at the size of Eros, 10 Gyr at ten times that size, and 40 Gyr at the size of Vesta. Eros as a shattered collisional fragment is not highly unusual. The new picture reveals the new possibility of a transition size in the collisional state, where asteroids below 5 km size would be primarily collisional breakup fragments whereas much larger asteroids are mostly eroded or shattered survivors of collisions. In this case, well-defined families would be found in asteroids larger than about 5 km size, but for smaller asteroids, families may no longer be readily separated from a background population. Moreover, the measured boulder size distribution on Eros is re-interpreted as a sample of impactor size distributions in the asteroid belt. The regolith on Eros may result largely from the last giant impact, and the same may be true of Itokawa, in which case about a meter of regolith would be expected there. Even a small asteroid like Itokawa may be a shattered object with regolith cover. Keywords: Asteroids; Asteroids, collisions; Asteroids, size distribution; Reqolith

The NEAR laser rangefinder (NLR) obtained more than 16 million range returns from asteroid 433 Eros. We present the first results from analyses of topographic profiles interpreted with the aid of simultaneous, boresighted images obtained by the NEAR multispectral imager (MSI). The location of the NLR boresight relative to that of MSI is determined by detailed correlations of ranging data and simultaneous images, including cases where the laser boresight slewed off and on the limb of the asteroid and cases where the laser illuminated a boulder close to the time of an image. In the data presented, the precision of the range measurements is about 1 m, with the minimum spot diameter under 5 m, and successive spots are contiguous or overlapping. Elevation on the irregular object Eros is given with respect to the gravitational and centrifugal potential. Landslides in craters are characterized. Possible crater benches are identified. Examples of infilled craters are presented. These observations suggest a depth of unconsolidated regolith, which is subject to sliding, of typically a few tens of meters. An example of structurally controlled cratering is presented. Examples of tectonic features are described. Surface roughness on Eros is approximately self-affine from scales of a few meters to hundreds of meters. A comparison of fractal statistics shows that Eros is extremely rough on observed scales, when compared to terrestrial a'a lava on submeter scales and undisturbed lunar regolith on subcentimeter scales. Key Words: asteroid surfaces; asteroids; Eros; regoliths; tectonics.

Preliminary measurements of craters and boulders have been made in various locations on Eros from images acquired during the first nine months of NEAR Shoemaker's orbital mission, including the October 2000 low altitude flyover. (We offer some very preliminary, qualitative analysis of later LAF images and very high-resolution images obtained during NEAR's landing on 12 February 2001). Craters on Eros >100 m diameter closely resemble the saturated crater population of Ida; Eros is more heavily cratered than Gaspra but lacks the saturated giant craters of Mathilde. These craters and the other large-scale geological features were formed over a duration of very roughly 2 Gyr while Eros was in the main asteroid belt, between the time when its parent body was disrupted and Eros was injected into an Earth-approaching orbit (probably tens of Myr ago). Saturation equilibrium had been expected to shape Eros' crater population down to very small sizes, as on the lunar maria. However, craters Key Words: asteroids, Eros; cratering; impact processes; geological processes; surfaces, asteroids.

NEAR Shoemaker imaging of the asteroid Eros has revealed a variety of surface forms that probably reflect underlying structures. They range from global to local in scale. The most prominent positive topographic feature is a long ridge, Rahe Dorsum, which extends over 18 km and has the morphology of a thrust fault. Two large, ancient trough systems are also present. Grooves are ubiquitous, commonly occur in suborthogonal sets, and have linear or scalloped outlines. The grooves on Eros are similar in morphology and scale to those on other small bodies such as Gaspra, Ida, and Phobos and are likely to have a similar mode of origin, namely drainage of regolith into underlying fractures. Models that use groove dimensions as indicators of regolith thickness imply depths of up to several tens of meters. Many craters have been subjected to structural control, resulting in squared outlines. We find no evidence that specific groove sets are associated with individual impact events; however, evidence exists that preexisting structures have been reactivated by later impact activity. The large scale of Rahe Dorsum and many grooves on Eros's surface suggests that the asteroid is a largely coherent, but fractured body. Key Words: asteroids; Eros; impact processes; regoliths; asteroid surfaces; tectonics.

We model the time- and longitude-dependent charged particle absorption signatures caused by solid body absorbers, such as icy satellites, in Saturn's magnetosphere. The reduction in particle density caused by passage of such an absorber through a magnetic flux tube is calculated numerically. We then solve numerically for the time- and space-dependent distribution of particles in phase space assuming transport by radial diffusion and longitudinal particle drifts. The fully time- and longitude-averaged solution of the lossy radial diffusion equation is assumed to be the initial state, and the calculation is performed for up to a few complete satellite orbital periods. In this way, we obtain a numerical solution for the effects of particle absorption and transport during the most recent orbit(s) of the absorbing body around Saturn, while we approximate the result of all previous orbits of the absorber with the fully relaxed, steady state solution. We define 'high' and 'low' diffusion rates, depending on whether the diffusive fill-in of the absorption wake is fast or slow compared to the time before the satellite re-encounters its wake. For two cases, using parameters appropriate for the low energy charged particle (LECP) instruments on the Voyager spacecraft, we simulate count rate profiles where microsignatures have been observed. In the case of an ion microsignature near Enceladus, we cannot reproduce the data using radial diffusion rates inferred by other studies. For an electron microsignature very close to Tethys, we do find that the simulated count rate profile has a deep enough minimum that it could be detected by the Voyager instrument. In some cases, observed ion microsignatures are deeper than the maximum possible depth predicted by our model for a fresh absorption signature immediately downstream of a moon, assuming high diffusion. In the low diffusion case, our model predicts deeper signatures, sufficient to account for observed microsignatures. However, in that case, the fully relaxed solution contains dramatic changes in the initial phase space density and furthermore we would then expect microsignatures to be observed at every satellite L-shell crossing, which is not the case.

Ion space phase densities for Neptune at fixed first and second adiabatic invariant values for charged particle motion are examined using Voyager 2 data. Inward radial diffusion with near L=10 energetic ion source is indicated. Inbound and outbound phase space density agreement at similar invariant values suggest radiation belts that are quasistationary and axisymmetric. Consistency with relatively weak L-dependence of D(sub)LL suggests interchange diffusion. If ion loss into Neptunian atmosphere is significant, such diffusion could power the planet's aurora.