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View from the Stardust Spacecraft S TARDUST AT C OMET W ILD 2 RESEARCH ARTICLE ECTION S Surface of Young Jupiter Family Comet 81 P/Wild 2: View from the Stardust Spacecraft PECIAL 1 2 3 2 3 4 S Donald E. Brownlee, * Friedrich Horz, Ray L. Newburn, Michael Zolensky, Thomas C. Duxbury, Scott Sandford, Zdenek Sekanina,2 Peter Tsou,3 Martha S. Hanner,5 Benton C. Clark,6 Simon F. Green,7 Jochen Kissel8 Images taken by the Stardust mission during its flyby of 81P/Wild 2 show the comet lar and rough region of partially excavated to be a 5-kilometer oblate body covered with remarkable topographic features, material, whereas flat-floor features do not including unusual circular features that appear to be impact craters. The presence of have halo regions and are bounded by steep high-angle slopes shows that the surface is cohesive and self-supporting. The comet cliffs. We identified three pit-halo features, of does not appear to be a rubble pile, and its rounded shape is not directly consistent which Rahe (5) is imaged with the best res- with the comet being a fragment of a larger body. The surface is active and yet it olution and illumination conditions (Figs. 1 retains ancient terrain. Wild 2 appears to be in the early stages of its degradation and 3). Rahe consists of a central circular pit phase as a small volatile-rich body in the inner solar system. 0.5 km in diameter with a rounded bottom and modestly elevated rim. This pit is surrounded Jupiter family comets (JFCs) such as 81 the 6.1 km sϪ1 flyby, the mission used its by a relatively flat annulus (halo) of excavated P/Wild 2 typically range from the orbit of optical navigation camera (3) to take 72 material roughly 1.2 km in diameter that is Jupiter to inner regions of the solar system. close-up images of the nucleus. These images bounded on its outer edge by sharp ragged cliffs These subliming-disintegrating bodies have were taken every 10 s, and exposure times up to 200 m in height. Some of the halo regions inner solar system lifetimes that are typi- were toggled between 10 and 100 ms to are at least as deep as the central pit. The halo cally limited to only ϳ10,000 years be- alternate between proper exposure of the depression surrounding the central pit is not cause of the effects of both solar heating comet’s surface and faint jet features and to complete, and there is a ϳ45° sector where the and gravity-driven orbital change (1). Al- enable the camera to autonomously track the original surface remains, forming a mesa ex- though short-lived as JFCs, they are ancient nucleus while moving at maximum rates ex- tending to the edge of the central pit. bodies whose history is believed to typical- ceeding 1° s–1. The imaging sequence yielded The second category, flat-floor features, ly include formation at the edge of the solar a series of stereopairs covering a range of has flat floors encircled by steep, nearly ver- nebula, long-term storage beyond the orbit solar phase angles from –72° to 103°. The tical cliffs. Two examples are Left Foot (LF) of Neptune, transfer into the inner solar closest image has a minimum image scale of and a crater 400 m in diameter 2 km west of system (Ͻ5 million years ago), and even- 14 m per pixel. LF (Fig. 3). Although LF is a double depres- tual loss by sublimation, disintegration, im- Shape and size. Wild 2 was found to be sion feature, it displays the salient features of pact onto the Sun or a planet, or ejection oblate, unlike the prolate shapes of the two flat-floor craters. Its northern lobe is 650 m from the solar system (1). On 2 January previously imaged comet nuclei, Halley and across with a stereometrically measured 2004, the Stardust spacecraft flew 236 km Borrelly. The shape can be modeled as a depth of ϳ140 m. The walls are nearly ver- from Wild 2, a body that was captured into tri-axial ellipsoid having radii of 1.65 by 2.00 tical (Ͼ70°) in places and make sharp contact a JFC orbit only 30 years ago as the result by 2.75 Ϯ 0.05 km (4). The shortest axis, with the crater floor. There is some rubble at of a close encounter with Jupiter (2). This inferred to be the axis of rotation, has a right the base of cliffs that is detectable in the flyby provided a brief but intimate view of ascension of 110° and a declination of –13°. highest resolution images. The flat floors a body whose surface should contain The longest axis, used to define the prime seem to be inert at the present time and records of processes that occurred in inner meridian, has an ephemeris position angle of resistant to sublimation because none of them and outer solar system environments sepa- 155°. At kilometer scales, the nucleus is a are detectably associated with observed jets. rated by a distance of 40 AU and times of rounded body, evidence that it is not a colli- Although neither of these two features billions of years. sional fragment of a larger object unless have raised rims or concentric ejecta aprons, Stardust is a sample return mission, and rounding occurs by postcollisional processes. typical hallmark signs for impact, we believe its primary goal is the collection of submilli- Its prolate ellipsoid shape spinning around that the pit-halo craters are impact features. meter particles for laboratory analysis. As the short axis maintains a fairly constant They differ from conventional craters, such thousands of particles were collected during cross section with time, explaining why a as those found on asteroids, probably because rotational light curve has not been detected of two factors: (i) a moderately cohesive 1Department of Astronomy, University of Washing- by ground-based observations. Like other target material that is also porous and (ii) Ϫ4 ton, Seattle, WA 98195, USA. 2National Aeronautics primitive solar system bodies, Wild 2 is dark, extremely low gravity, ϳ10 g. A volatile- and Space Administration (NASA) Johnson Space and its preliminary geometric albedo was de- rich target is potentially important as well, Center, Houston, TX 77058, USA. 3Jet Propulsion Lab- termined to be 0.03 Ϯ 0.015. An overview of but the effects of volatiles on crater morphol- oratory, California Institute of Technology, Pasadena, the closest images is shown in Fig. 1, and the ogy are unknown. A surface composed of CA 91109–8099, USA. 4NASA Ames Research Center, Moffet Field, CA 94035–1000, USA. 5University of naming terminology used by the research cohesive porous material is probably a con- Massachusetts, Amherst, MA 01003–9291, USA. team is shown in Fig. 2. sequence of the comet being a mix of fine 6Lockheed Martin Astronautics, Denver, CO, 80201, The surface con- dust and volatiles. Sublimation and the mo- 7 Surface depressions. USA. Planetary and Space Sciences Research Insti- tains depressions ranging in size up to nearly bilization of volatiles leads to porosity and tute, Open University, Milton Keynes MK7 6AA, UK. 8Max-Planck-Institut fu¨r Aeronomie, 37191 Katlen- 2 km across. We distinguish two primary sintering or other processes that produce burg-Lindau, Germany. morphologies for circular depressions: pit moderately bound, rigid materials (6–8). *To whom correspondence should addressed. E-mail: halo and flat floor. Pit-halo features have a A possible origin of the pit-halo morphol- [email protected] rounded central pit surrounded by an irregu- ogy is that they are the result of impact into 1764 18 JUNE 2004 VOL 304 SCIENCE www.sciencemag.org S TARDUST AT C OMET W ILD 2 S PECIAL homogeneous, cohesive and brittle material or a flat floor may exceed the escape velocity of steep walls like the flat-floored craters, in a microgravity environment. They may be ϳ1msϪ1. It is also possible that ejecta, and whereas others have rounded edges that may analogs to typical microcraters seen on lunar even landslide materials, are so porous, fine- have been modified by ablation or the move- rocks (9). The cm size and smaller lunar grained, weak, and charged with volatiles that ment of loose materials down the slopes of S craters have central, often glass-lined, pits they disintegrate into fine powder and are the depressions. Such irregular depressions ECTION surrounded by ragged regions of spalled and blown away by escaping vapor. Most of the have no apparent link to impacts and might ejected material (Fig. 4). The sector-shaped, small features on Wild 2 are subdued, as if be related to sublimation processes. unejected region around Rahe is a feature modestly degraded or eroded. There are no Some of the largest depressions are also commonly seen in lunar microcraters. unambiguous small impact structures Ͻ0.5 km, complex structures with features that could An origin of the excavated annulus by spal- and their absence may be because of loss via have formed by impact, sublimation, mass lation processes does, however, require sub- surface erosion or alternatively the impactor wasting, ablation or a combination of all or stantial target strength, which is the reason population encountered by Wild 2 had an un- some of these processes. The largest de- why we also offer an alternative explanation usually restricted size range, resulting in impact pressional features are Right Foot (ϳ1 km), related to lunar regolith craters some 50 to craters Ͼ0.5 km. Shoemaker basin (1.6 km), Mayo (1.2 km), 100 m across.
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