O. S. Barnouin-Jha

The Rubble-Pile Asteroid Itokawa as Observed by Hayabusa

During the interval from September through early December 2005, the Hayabusa spacecraft was in close proximity to near-Earth asteroid 25143 Itokawa, and a variety of data were taken on its shape, mass, and surface topography as well as its mineralogic and elemental abundances. The asteroids orthogonal axes are 535, 294, and 209 meters, the mass is 3.51 × 1010 kilograms, and the estimated bulk density is 1.9 ± 0.13 grams per cubic centimeter. The correspondence between the smooth areas on the surface (Muses Sea and Sagamihara) and the gravitationally low regions suggests mass movement and an effective resurfacing process by impact jolting. Itokawa is considered to be a rubble-pile body because of its low bulk density, high porosity, boulder-rich appearance, and shape. The existence of very large boulders and pillars suggests an early collisional breakup of a preexisting parent asteroid followed by a re-agglomeration into a rubble-pile object.

Touchdown of the Hayabusa Spacecraft at the Muses Sea on Itokawa

After global observations of asteroid 25143 Itokawa by the Hayabusa spacecraft, we selected the smooth terrain of the Muses Sea for two touchdowns carried out on 19 and 25 November 2005 UTC for the first asteroid sample collection with an impact sampling mechanism. Here, we report initial findings about geological features, surface condition, regolith grain size, compositional variation, and constraints on the physical properties of this site by using both scientific and housekeeping data during the descent sequence of the first touchdown. Close-up images revealed the first touchdown site as a regolith field densely filled with size-sorted, millimeter- to centimeter-sized grains.

Regolith migration and sorting on asteroid Itokawa.

High-resolution images of the surface of asteroid Itokawa from the Hayabusa mission reveal it to be covered with unconsolidated millimeter-sized and larger gravels. Locations and morphologic characteristics of this gravel indicate that Itokawa has experienced considerable vibrations, which have triggered global-scale granular processes in its dry, vacuum, microgravity environment. These processes likely include granular convection, landslide-like granular migrations, and particle sorting, resulting in the segregation of the fine gravels into areas of potential lows. Granular processes become major resurfacing processes because of Itokawas small size, implying that they can occur on other small asteroids should those have regolith.

Mass and Local Topography Measurements of Itokawa by Hayabusa

The ranging instrument aboard the Hayabusa spacecraft measured the surface topography of asteroid 25143 Itokawa and its mass. A typical rough area is similar in roughness to debris located on the interior wall of a large crater on asteroid 433 Eros, which suggests a surface structure on Itokawa similar to crater ejecta on Eros. The mass of Itokawa was estimated as (3.58 ± 0.18) × 1010 kilograms, implying a bulk density of (1.95 ± 0.14) grams per cubic centimeter for a volume of (1.84 ± 0.09) × 107 cubic meters and a bulk porosity of ∼40%, which is similar to that of angular sands, when assuming an LL (low iron chondritic) meteorite composition. Combined with surface observations, these data indicate that Itokawa is the first subkilometer-sized small asteroid showing a rubble-pile body rather than a solid monolithic asteroid.

Lobateness of impact ejecta deposits from atmospheric interactions

As an ejecta curtain advances through an atmosphere, it creates a vortex ring. By analogy with smoke rings, the curtain-driven vortex ring develops instabilities that result in waves. The number of these waves depends upon the aspect ratio of the vortex ring (i.e., the ratio of the core vortex radius to the vortex radius) and the Reynolds number (or strength) of the flow in the vortex ring. In laboratory experiments the number of sinuous features at the edges of contiguous ejecta ramparts is consistent with the theoretical expectations for the origin of waves created in a curtain-driven vortex ring. Observing the formation of these sinuous features provides direct evidence that they indeed result from instabilities in the curtain-driven vortex ring. Scaling relations for curtain velocity, curtain size, and time of crater formation permit testing whether or not such instabilities explain the lobateness or sinuosity of distal ejecta facies at broad scales on planets with atmospheres. Scaling relationships predict that the number of flow lobes observed for craters on both Venus and Mars should increase with increasing transient crater radius to the three-fourths power, a prediction that is consistent with observation. Consequently, the curtain-driven vortex may play an important role in controlling the morphology of ejecta on planets with atmospheres. Variations in the number of flow lobes for a given crater size probably reflect different impact conditions either in target properties (grain size, volatile content) and/or ambient atmospheric conditions.

Developing space weathering on the asteroid 25143 Itokawa

Puzzlingly, the parent bodies of ordinary chondrites (the most abundant type of meteorites) do not seem to be abundant among asteroids. One possible explanation is that surfaces of the parent bodies become optically altered, to become the S-type asteroids which are abundant in the main asteroid belt. The process is called ‘space weathering’—it makes the visible and near-infrared reflectance spectrum of a body darker and redder. A recent survey of small, near-Earth asteroids suggests that the surfaces of small S asteroids may have developing stages of space weathering. Here we report that a dark region on a small (550-metre) asteroid—25143 Itokawa—is significantly more space-weathered than a nearby bright region. Spectra of both regions are consistent with those of LL5-6 chondrites after continuum removal. A simple calculation suggests that the dark area has a shorter mean optical path length and about 0.04 per cent by volume more nanophase metallic iron particles than the bright area. This clearly shows that space-weathered materials accumulate on small asteroids, which are likely to be the parent bodies of LL chondrites. We conclude that, because LL meteorites are the least abundant of ordinary (H, L, and LL) chondrites, there must be many asteroids with ordinary-chondrite compositions in near-Earth orbits.

Ejecta entrainment by impact-generated ring vortices : Theory and experiments

Laboratory experiments indicate that an advancing ejecta curtain displaces the atmosphere and creates strong winds that entrain large amounts of fine-grained ejecta. A theoretical model describing the interaction between an atmosphere and the outward moving ejecta curtain allows one to estimate the magnitude and velocity of the winds in the induced ring vortex in order to establish the importance of such winds at planetary scales. Model estimates for the initial magnitude of the flow within the basal vortex match experimental results within observational uncertainty and reveal that the flow in the ring vortex is turbulent. Successful comparisons of the vortex generation model with experiments allow preliminary applications to be made at much broader scales. Curtain generated winds by a 30-km-diameter crater should entrain ejecta particle diameters smaller than 5 mm on Mars, 30 cm on Earth, and 8 m on Venus.

Laser altimeter observations from MESSENGER's first Mercury flyby.

A 3200-kilometers-long profile of Mercury by the Mercury Laser Altimeter on the MESSENGER spacecraft spans ∼20% of the near-equatorial region of the planet. Topography along the profile is characterized by a 5.2-kilometer dynamic range and 930-meter root-mean-square roughness. At long wavelengths, topography slopes eastward by 0.02°, implying a variation of equatorial shape that is at least partially compensated. Sampled craters on Mercury are shallower than their counterparts on the Moon, at least in part the result of Mercurys higher gravity. Crater floors vary in roughness and slope, implying complex modification over a range of length scales.

The Actual Dynamical Environment About Itokawa

The dynamical environment about and on Asteroid 25143 Itokawa is studied using the shape and rotation state model estimated during the close proximity phase of the Hayabusa mission to that asteroid. We first discuss the general gravitational properties of the shape model assuming a constant density. Next we discuss the actual dynamical environment about this body, both on the surface and in orbit, and consider the orbital dynamics of a Hayabusa-like spacecraft. Then we detail one of the approaches used to estimate the mass of the body, using optical and lidar imaging, during the close proximity phase.

Investigating the interactions between an atmosphere and an ejecta curtain 2. Numerical experiments

The locus of ejecta excavated during an impact generates a debris curtain that expands outward. In an atmosphere this advancing curtain acts like a semipermeable barrier that displaces the surrounding gas. The generated flow separates near the top of the curtain to form a vortex ring whose strong winds entrain, transport, and deposit fine-grained ejecta, affecting the morphology of distal ejecta deposited on planets with atmospheres. We have investigated how the curtain width and velocity, particle concentration, size distribution and velocity parallel to the curtain, and the density, viscosity, and compressibility of the surrounding atmosphere controls the flow strength of these winds. Wind tunnel tests (Part 1 [Barnouin-Jha et al., this issue]) show that for an ejecta-like porous plate, the hydraulic resistance, a measure of energy losses for one-dimensional porous flow, governs the position along the curtain where it becomes effectively impermeable. Combined with suitable cratering models and published hydraulic resistance data, this information allows estimating the flow strength or circulation generated by an advancing curtain. The present study assesses the influence of atmospheric compressibility and particle motion parallel to the curtain surface on the curtains circulation in order to improve these estimates. Numerical experiments indicate that atmospheric compressibility has little effect on the circulation at Mach number below 0.5, consistent with analytical solutions. Analytical solutions show, however, that this flow circulation should increase significantly at higher Mach numbers. The numerical experiments also show that individual ejecta traveling parallel to the surface of the curtain enhance the induced circulation by 9% to 33%.