Space environment of an asteroid preserved on micrograins returned by the Hayabusa spacecraft

Abstract

Records of micrometeorite collisions at down to submicron scales were discovered on dust grains recovered from near-Earth asteroid 25143 (Itokawa). Because the grains were sampled from very near the surface of the asteroid, by the Hayabusa spacecraft, their surfaces reflect the low-gravity space environment influencing the physical nature of the asteroid exterior. The space environment was examined by description of grain surfaces and asteroidal scenes were reconstructed. Chemical and O isotope compositions of five lithic grains, with diameters near 50 μm, indicate that the uppermost layer of the rubble-pile-textured Itokawa is largely composed of equilibrated LL-ordinary-chondrite-like material with superimposed effects of collisions. The surfaces of the grains are dominated by fractures, and the fracture planes contain not only sub-μm-sized craters but also a large number of sub-μm- to several-μm-sized adhered particles, some of the latter composed of glass. The size distribution and chemical compositions of the adhered particles, together with the occurrences of the sub-μm-sized craters, suggest formation by hypervelocity collisions of micrometeorites at down to nm scales, a process expected in the physically hostile environment at an asteroid's surface. We describe impact-related phenomena, ranging in scale from 10(-9) to 10(4) meters, demonstrating the central role played by impact processes in the long-term evolution of planetary bodies. Impact appears to be an important process shaping the exteriors of not only large planetary bodies, such as the moon, but also low-gravity bodies such as asteroids.

Fig. 1.

Surface images obtained by SEM of Grain A with shock-induced textures. (A) The entire Grain A, indicating the locations for the higher-magnification images. (B) Shock lamellae developed on olivine (Ol). (C) Quenched melt on and along a crack crosscutting the shock lamellae.

Fig. 2.

Surface images (SEM unless otherwise specified) of Grain B with sub-μm-sized craters. (A) A part of Grain B, with the location on the complete grain indicated in the inserted BSE (back-scattered electron) image located in the SE part of the photograph. F1, F2, and F3 represent fracture planes, numbered in order of the relative timing of their formation (with F1 the earliest-form plane). (B) A BSE image of the region of (A), showing a barred-olivine texture. (C, D, E) Sub-μm-sized craters on Ol substrate. A pit rim with chains of seven or eight spheres. (F) An intersection of F1 and F3 and a scaly fabric on F1. (G) A cross section of the scaly fabric showing the sawtooth morphology on F1.

Fig. 3.

Surface images of Grain C and its adhered objects, CAPs (common adhered particles) and MSGs (molten splash-shape glasses). (A) Entire Grain C. The grain was cut into three pieces by FIB as shown in the insert in the SE of this image. Di and Pl* indicate diopside and diaplectic plagioclase. (B) Polarized-reflected-light image obtained for the middle slab with thickness of 15 μm. Troilite (Tro) are included in Di. An occurrence of K-feldspar (Kfs) is shown in the inserted BSE image for the SE part of the slab. (C) Microbowls on Pl* substrate partially filled with Tro. Some of these microbowls contain considerable void space. (D) Sub-μm-scale crater on a Di substrate. (E) CAPs on a Pl* substrate. (F) Composite CAP consisting of Ol, plagioclase (Pl), and Di. (G) MSG wrapping around CAPs. (H) Flattened and convex-disk-shaped MSG with degassing vesicles. (i) MSG covering an edge on the surface.

Fig. 4.

SEM images of Grain E and adhered objects, DOO (dome-outline objects) and PIC (particularly irregularly-shaped clots). (A) The entire Grain E. (B) DOO and its spherical morphology. Numerous Fe-rich Ol grains are located at the interface between this DOO and its low-Ca pyroxene (low-Ca Px) substrate. (C) PIC consisting of multiple phases, but dominantly sub-μm- to μm-sized merrillite and glass domains and interstitial low-Ca Px and Ol.

Fig. 5.

Oxygen isotope and major element composition. (A) Oxygen three-isotope diagram showing silicate minerals collected by the Hayabusa spacecraft and representative compositions of major primary components of solar system matter. This plot includes bulk-rock data for refractory Ca-Al-rich inclusions (CAIs), carbonaceous, ordinary, enstatite, and R chondrites, and Mars. Ion microprobe data for Ol, Pl, Di, and low-Ca Px are indicated by the colored symbols (blue, light blue, green, and orange, respectively). Overlapping data for multiple phases are shown by the pie charts. The relative dimensions of the probed areas are expressed in the charts accordingly. The data for the minerals from Itokawa grains scatter around the range for whole-rock ordinary-chondrites and form an elongated cluster with a slope of approximately 0.5. The means of the analyses for each phase fall into the region for LL-ordinary chondrites defined by Clayton (15). Variation observed in O-isotope compositions is consistent with that observed by Yurimoto, et al. (5) although they interpreted the variation as possibly reflecting analytical ambiguity. (B) A diagram to show relative abundance of Mg-Mn-Fe and Na-K-Ca of silicate minerals from Itokawa grains plotted with those in equilibrated (–6) ordinary chondrites (L and LL).

Fig. 6.

Cumulative size distribution of solid fragments and quenched melt droplets. Mean of major- and minor-axes of adhered objects randomly chosen was evaluated (Dataset S1). On the log-log plot where grain size is larger than 0.7 μm, quenched melt droplets (MSGs and DOOs, n = 16) yield a slope shallower than that estimated from solid fragments (CAPs, n = 898). The distribution could be obtained by the splash of melt droplets associated with the shock-induced melting on Itokawa. In the same range, a best-fit slope for solid fragments yields −2.31 (red line). Particles smaller than approximately 1 μm are underrepresented because of their size, therefore, the obtained slope would be somewhat underestimated. The slope suggests catastrophic fragmentation processes by collision cascades might play important role for the evolution of Itokawa’s surface.

Fig. P1.

Landscapes of an asteroid’s surface exposed to space. All images were obtained by electron microscopy. (A) A representative grain recovered by the Hayabusa spacecraft. (B) A sub-micrometer-scale crater with chains of spheres slightly overhanging the pit edge. (C) A micrometer-sized material that was molten, quenched, and became glued onto the grain surface. (D) A plane with a scaly fabric eroded on the asteroidal surface.