Images and caption contributed by Bill Mahaney.
Asteroids, especially Near Earth Objects (NEOs), are considered to be dry, barren piles of rock, either C-, S- or M-type bodies (Asphaug 2007). A recent (2005) engineering feat carried out by the Japanese Aerospace Agency (JAXA) resulted in the successful rendezvous of the Hayabusa Spacecraft with Asteroid 25143-Itokawa, located some 320 million km from Earth.
Image 1: Annotated rubble-strewn surface of Asteroid 25143-Itokawa. Circles outline debris-flow sources, arrows indicate debris flow channels, white/black circle indicates a possible hydrological sink. Talus accumulations can be seen between the two sets of arrows with southeasterly orientation on an arbitrary grid. Anastomosing shallow channels to right of the Muses-C fine debris area, outlined in white tone, may indicate release of meltwater from permafrost and emplacement of slope wash. Parabolic lines outline stone-banked lobe ridges. Two fairly recent impacts, judging from tonal contrast, are labelled A and B on Image 1. Image courtesy of the Japan Aerospace Agency (JAXA). [Full resolution image]
Itokawa measures 558 m by 288 m and rotates every 12 hrs (see also the August 2007 featured image) with a bulk density of 1.84 g/cc and porosity of ~40% (Abe et al. 2006). Itokawa is the first sub-kilometer size asteroid to be imaged close up by spacecraft. Microgravity variations of the asteroid range from near zero to a few millionths of Earth's gravity (Miyamoto 2007; Fujiwara et al. 2006), producing a low friction component to the retention of clastic surficial material, and a low escape velocity. Itokawa, with an orbital path of 556 days, crosses Earth's orbit, producing a wide variation in perihelion and aphelion (NEODys; Lederer et al., 2005; Lowry, S.C., et al., 2005). It has a low albedo (0.24), and is composed largely of pyroxene, olivine and Ca-plagioclase, as determined by on-board instruments on the Hayabusa space craft (Abell et al. 2007).
Recently released images taken provides close up photography of the surface. Close scrutiny shows a global regolith of rubble. The surface is composed of fine gravel and sand (e.g. Muses-C, Image 1) contrasted with clast-strewn surfaces of rubble. The coarse debris ranges from pebble size to boulders tens of meters in diameter (Image 2). This is thought to be rearranged infrequently by seismic disturbances from meteorite impacts which episodically vibrate the surface, thereby creating a mobile regolith of superficial materials organized into sheets of imbricated gravels (Miyamoto et al. 2007).
Image 2: An undesignated clast-covered slope on Itokawa-25143. Image, courtesy of the Planetary Society. After Fig 1c in Mahaney and Kapran, 1999. [Full resolution image]
Close observations of various networks of surficial materials also reveal deposits closely related to periglacial landforms on Earth (Mahaney et al, 2009; Mahaney and Kapran, 1999), similar in kind to talus (Perez 1989), stone-banked lobes (Mahaney 1987; Vieira and Ramos 2003), polygonal ground and debris flows (Mahaney 1990).
Deposits on Itokawa are marked on Image 1, showing inclinations of surface gravels in talus accumulations as well as debris flows originating in raised rubble mounds resembling frost boils or polygonal features. High-resolution images to a scale of ~6mm/pixel (Fig. 2B and D in Miyamoto et al., 2007) clearly show poorly organized lobate-shaped forms similar to stone-banked lobes on Earth, two of the most prominent ones annotated on Image 1.
Debris flows on Earth require meltwater for rapid movement, producing levee ridges astride narrow troughs. On Itokawa, these systems terminate in a depression or hydrological sink, as indicated on Image 1. Given the low resolution (2 mb) imagery released by JAXA, the best examples of several possible debris flows are indicated with enclosed black circles marking the source on Image 1, the troughs marked with arrows leading to a sink (marked with a black/white circle). To the right, East on an arbitrary N-S grid, a series of parabolic ridges and troughs suggests stone-banked lobes (Mahaney 1987; Vieira and Ramos 2003), morphologically similar to periglacial deposits on Earth, found on gentle slopes of 10-20o, and usually associated with an active layer of permafrost lubricating the rubble/matrix mass of material, thereby assisting in friction release and movement.
To the right of the Muses-C, a region of fine clasts, an irregular white-colored apron of debris, oriented N-S on an arbitrary grid, outlines an area that could be slope wash deposited from meltwater drainage originating in a rubble cluster at the top of the image. Fabric analysis in one of the large rubble accumulated zones (Image 3) shows clasts behaving exactly on the asteroid as they would on Earth, responding to the underlying change in slope to generate talus accumulations (Mahaney et al, 2009).
Image 3. Clast orientation (density above; rose below) from area in the upper middle ground above (north) in Image 1 indicates a movement to the southeast on an arbitrary horizon with a steep inclination (b27° off the horizon).
Examination of other areas on the image show darker to lighter areas of random to rearranged rubble reflecting zones of debris, some with segregated clasts similar to polygonal ground on Earth (Image 4) (Mahaney and Kapran, 2009).
Image 4. Close-up of rubble-strewn surface of Itokawa showing sorted nets and polygonal structures (arrows) partly merging into shallow stone-banked lobes as indicated. White areas to right in the image are interpreted to be recently overturned clasts unaffected to any degree by solar weathering. In contrast, the nearly uniform grey tone across the rest of the surface is the result of release of nanophase Fe from solar weathering. After Fig 2 from Mahaney and Kapran, 2009. [Full Resolution Image]
The presence of periglacial deposits, some requiring meltwater from permafrost, raises questions about the composition of the frozen surface and subsurface mass of Itokawa, and the radiation balance/hydrologic change required to release meltwater. Moreover, the release of meltwater within a matrix of material known to have coatings of nannophase (secondary) Fe provides two of the basic building blocks which would allow for weathering processes on what is otherwise considered to be a cold, near-waterless planetary body with low gravity. If debris flows and stone-banked lobes on Itokawa behave in a similar fashion as those on Earth, and if there is permafrost and episodic meltwater release, it may be necessary to reassess commonly-held ideas about the surficial processes and landform genesis on asteroids
The prevailing theory that asteroids are dry, sterile bodies, some as large as small planets, devoid of water, may have to accommodate new observations that indicate the presence of meltwater as well as landforms similar to talus operating under microgravity.
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