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Detailed Images of Asteroid 25143 Itokawa from Hayabusa

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Science  02 Jun 2006:
Vol. 312, Issue 5778, pp. 1341-1344
DOI: 10.1126/science.1125722


Rendezvous of the Japanese spacecraft Hayabusa with the near-Earth asteroid 25143 Itokawa took place during the interval September through November 2005. The onboard camera imaged the solid surface of this tiny asteroid (535 meters by 294 meters by 209 meters) with a spatial resolution of 70 centimeters per pixel, revealing diverse surface morphologies. Unlike previously explored asteroids, the surface of Itokawa reveals both rough and smooth terrains. Craters generally show unclear morphologies. Numerous boulders on Itokawa's surface suggest a rubble-pile structure.

On 12 September 2005, the Hayabusa spacecraft arrived at the near-Earth asteroid 25143 Itokawa (1). Itokawa is categorized as an S (IV)– or Q-type asteroid, which are thought to be similar to ordinary chondrite meteorites through ground-based observations (2, 3). Hayabusa carries the telescopic Optical Navigation Camera (ONC-T), which is also called Asteroid Multiband Imaging CAmera (AMICA) when used for scientific observations (4). AMICA has both a wide-bandpass filter and seven narrowband filters, the central wavelengths of which are nearly equivalent to those of the Eight Color Asteroid Survey (ECAS) system (5) as follows: 380 (ul), 430 (b), 550 (v), 700 (w), 860 (x), 960 (p), and 1010 nm (zs). AMICA imaged the entire surface of Itokawa with a solar phase angle of ∼10° at the home position (HP), the altitude of which is ∼7 km (1). Because the angular resolution is 0.0057°/pixel (99.3 microrad/pixel), the nominal spatial resolution is 70 cm/pixel at the HP. Four position-angle glass polarizers were mounted on an edge of the 1024 pixel by 1024 pixel charge-coupled device chip (6). In addition to the nominal observations at the HP (Fig. 1), we obtained images for specific purposes (7), including images of the polar regions and some with very high resolution.

Fig. 1.

Global images of Itokawa. (A) Image no. ST2448357351 and (B) ST2426418029. “ST” denotes “science mode imaging” and “ONC-T image”; numbers are the time counter values. The surface of Itokawa is covered by numerous boulders, including the conspicuous ones indicated in the figure (Yoshinodai, Pencil, and Black boulder). Also shown is the Komaba crater. Smooth terrains (e.g., Muses Sea and Sagamihara) are distinctive because they lack boulders.

The size of Itokawa is 535 m by 294 m by 209 m, within a 1-m margin of error [see (1) for more details about the size and global characteristics]. Because the shape of Itokawa is somewhat reminiscent of a sea otter (Fig. 1), we use “head” (smaller part), “body” (larger part), and “neck” (junction between head and body) to denote position on the asteroid (1). The surface of Itokawa shows a variety of features that suggest a complex evolutional history. A notable example is the existence of “the black boulder,” which is a small boulder with extremely low albedo (Fig. 1). Facets have been identified at both ends of the elongated body of Itokawa; most of them are likely the results of impacts. Although several grooves exist, we have not been able to identify global-scale ones, such as those found on the martian moon Phobos and asteroid 433 Eros (8). This may suggest the lack of a global-scale internal structure, although surface manifestations of internal structures might be obscured by numerous boulders (1).

Two types of terrains (rough and smooth) are observed on the surface of Itokawa. The rough terrain occupies ∼80% of the surface. Numerous boulders exist on the rough terrains; these vary widely in size (from a few meters to ∼50 m) and shape (Fig. 1). Here, we loosely define boulders as (i) apparently rootless rocks and (ii) features with distinctive positive relief that are larger than a few meters in size. The largest boulder on Itokawa is named Yoshinodai (Fig. 1), with dimensions of ∼50 m by 30 m by 20 m. It is Embedded Image the size of Itokawa itself. Another conspicuous boulder is called Pencil, which shows distinctive positive relief as if it were embedded in the local terrain (Fig. 1). Boulders have a wide range of angularity and aspect ratio.

The cumulative size distribution of boulders larger than 5 m on the entire surface is shown in Fig. 2. The total number of boulders larger than 5 m exceeds 500. The existence of decameter-sized boulders on Itokawa, as well as the abundance of meter-sized boulders, cannot be explained by simple impact-cratering processes (1, 9). Thus, the boulders might have been produced when Itokawa was generated by a catastrophic disruption, which is consistent with the possible rubble-pile structure of Itokawa (1).

Fig. 2.

Cumulative boulder size distribution on the surface of Itokawa. Horizontal axis shows the width (in meters) of the boulder, where the width is defined as the maximum size of the boulder. Vertical axis shows the number of boulders larger than a particular width. Data were collected from the six images of ST2482160259, ST2484352917, ST2485860275, ST2492225173, ST2492513077, and ST2493031594 at an altitude of 3.779 to 4.913 km.

The log-log slope of the cumulative number of boulders on Itokawa'ssurface (10) is ∼ –2.8 (Fig. 2), which is shallower than that of previously studied small bodies, including Eros and Phobos (and even the Moon) (1113). For example, Eros shows a slope as steep as –3.2 for boulder sizes between 15 and 80 m (11). The shallower slope obtained for Itokawa implies that the boulders experienced less processing, including breaking, sorting, and transporting (11). Other probable reasons for the shallow slope may include mass dependencies both in boulder-ejection processes related to the lower gravity of Itokawa and in boulder-transport processes such as impact-induced shaking (14).

The smooth terrains are distinguished from the surrounding rough terrains (Fig. 1) by both a lack of boulders and the featureless smooth surfaces with apparently similar brightness. These characteristics are consistent with the view that the smooth terrain is covered by finer materials (1). The origin of the boulders on the smooth terrains seems to be complicated: Some boulders are surrounded by shallow depressions, whereas most others are not. The depressions (Fig. 3A) may indicate that these boulders were softly landed after the formation of the terrain; however, dynamical interactions between the boulder and fine particles during the resurfacing processes, such as seismic shaking, remain a possible origin.

Fig. 3.

(A) Muses Sea, with the Shirakami area composing the southern part of the “neck” region (ST2474731509). Small white arrows near Yatsugatake indicate the thin, boulder-rich layer similar to landslide deposits. Depressions in Muses Sea are marked by long gray arrows. The Komaba crater and some crater candidates are also indicated. (B) The darker depressed region in Little Woomera (ST2498167622), which is one of the largest facets, is surrounded by a brighter rim. (Inset) A high-resolution image of the northern rim of Little Woomera (ST2516321279). Note the stronger brightness contrast in the inset.

As noted by Fujiwara et al. (1), the low–gravitational potential regions coincide with the smooth terrains (Muses Sea and Sagamihara) (Fig. 1), which suggests gravitational movement of the finer materials after the formation of the asteroid. The evolution of the smooth terrains likely involves processes for grain-size sorting and dynamical interactions between regolith and boulders, although the transport and deposition mechanisms are not well understood at present. The movements of the regolith materials on the surface of the asteroid are also suggested by a range of surface morphologies; for example, some craters are almost perfectly covered by regolith, which implies the emplacements of deposited materials.

In contrast to the asteroids previously explored by spacecraft, craters on Itokawa are generally indistinct. Several circular depressions, which are interpreted as craters, are seen (Figs. 1 and 3). Some of the circular depressions on rough terrains have floors apparently filled with finer particles (for example, Komaba in Fig. 3A), which are surrounded by brighter rims. As discussed below, the brighter albedo could be related to the fresh materials, which were exposed to the surface by the recent impact and/or downslope motions of possibly weathered materials on a steep slope. The circular and depressed shape of the Little Woomera region (Fig. 3B) suggests that it is the remnant of a large impact, which might be true for some of the other large circular depressions and facets on Itokawa.

The total number of craters on Itokawa is small: Even if we include all of the indefinite crater candidates, the total number of possible craters is less than 100. Because we can detect boulders as small as a few meters in the 70-cm-resolution images obtained at the HP, similar-sized craters could be recognized under proper illumination conditions. Therefore, future detailed studies of crater statistics will not drastically increase the total number of craters. The limited total number of craters and their generally obscure morphologies might be attributed to resurfacing processes, such as seismic shaking (14), armoring by numerous boulders (15), or the paucity of smaller impactors (15); alternatively, Itokawa could have been generated by a relatively recent impact in the main belt and then transferred to its current orbit.

Among the most intriguing characteristics of Itokawa are the heterogeneities in color (Fig. 4) and albedo, which are unusual because no previously observed asteroid bodies show large variations in both of these characteristics (1619). We found a variation of more than 30% in v-band albedo, as compared with only ∼15% variation in the w-band/b-band color ratio. There appears to be a correlation between color and albedo on Itokawa; in general, brighter areas are bluer, whereas darker areas are redder. These variations might be due to the space weathering process (2023), but other possibilities, such as heterogeneities in mineralogical composition and grain size of surface materials (24), cannot be ruled out.

Fig. 4.

Composite color images constructed from b-, v-, and w-band data. The contrast adjustment was done in each image to enhance the color variation.

Brighter areas are mostly found in (i) areas with steeper slopes (Fig. 3A); (ii) areas of local high terrain, including edges of facets; and (iii) apparently eroded areas, such as crater rims. The brighter area in Shirakami (Fig. 3A) has a slope angle that is steeper than a typical angle of repose of granular materials. This situation indicates that most boulders in this area could be gravitationally unstable, which may explain the limited number of boulders there. The notable difference in the number of boulders between the brighter area in Shirakami and the proximal darker area implies the rearrangements of boulders. Thus, the brighter area may have been formed by the removal of the superposed boulders, which exposed the inner bright materials (25). Both the boulders and eroded material may have been deposited at the break of the slope and formed a thin, boulder-rich layer at the base of Shirakami and Yatsugatake. Although this process can explain the consistencies in both sizes and angularities of the materials in the layer, the lack of flow front and talus features may imply further complexities.

Another example of the brighter area is in the Little Woomera region on the edge of the Otter's body (Fig. 3B). Little Woomera is a relatively darker depressed region surrounded by rimlike features, which are about 10% brighter when viewed from HP. However, images with higher spatial resolution reveal that the 10% brighter features are composed of the combinations of both darker and 20 to 30% brighter materials (Fig. 3B); that is, the real brightness contrasts are not resolved by lower spatial resolution images. A few darker areas apparently overlap the brighter area, showing relatively sharp boundaries. This dichotomy may indicate that a part of the dark and boulder-rich surfaces was removed by shaking caused by impacts or planetary encounters, leading to exposure of the underlying relatively bright area.

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