more cratering events. It is perhaps signifi- cant that morphologically similar smooth, flat-floored areas of presumably ponded ma- terials are seen in some other (noncrater) depressions on Eros (Fig. 2). Evidently there is an effective process on Eros that separates fine-grained materials from coarser regolith. Either this same process or another mecha- nism is able to transport the fine-grained materials over considerable lateral distances (16 ). Some linear features, mostly grooves, pre- viously identified in lower resolution images, can be found in the high-resolution LAF cov- erage as elongated depressions some tens of meters in width (Fig. 6). They are subtle depressions up to 25 m in depth (measured from shadows) with varying widths and amounts of asymmetry in profiles. Many have v-shaped cross sections indicative of the collapse of loose materials. Although some have superposed craters, indicating consider- able age, other sections have sharp slope intersections and well-defined crests that may be younger. As with grooves on other bodies (17 ), their ultimate origin may be related to fractures in a more solid interior, but their surface expressions are controlled by the properties of loose materials, which may have been disturbed and in effect partially refreshed, multiple times while some other crater-induced degradation has occurred. The LAF images provide evidence that Eros has a widespread regolith, typically sev- eral tens of meters in thickness. Exceptions may occur, especially locally on steep slopes (18). Similar indications of thick regoliths on small bodies have been deduced from space- craft investigations of the tiny moons of Mars, Phobos and Deimos (5), and of aster- oids 243 Ida (19) and 253 Mathilde (20). References and Notes 1. For general characteristics of Eros, see J. Veverka et al., Science 289, 1993 (2000). 2. “Regolith” is used here in the general sense of un- consolidated fragmental material, regardless of for- mation or transport mechanisms. 3. The NEAR camera, the MSI, covers the spectral range from 400 to 1000 nm and has a 2.25°-by-2.90° field of view. The 244-by-537–pixel Thomson charge-cou- pled device has rectangular pixels that subtend 9.5 m by 16.1 m at 100 km. For details, see S. E. Hawkins et al., Space Sci. Rev. 82, 31 (1997) and J. Veverka et al., J. Geophys. Res. 102, 23709 (1997). 4. The solar incidence angle was 59° and the viewing (or emission) angle was 39°. 5. For Phobos, see P. C. Thomas et al., J. Geophys. Res. 105, 15,091 (2000). For the moon, see P. H. Schultz, Moon Morphology (Univ. of Texas Press, Austin, TX, 1976). Deimos, the outer satellite of Mars, was viewed by Viking at a resolution of 3 to 4 m [see P. C. Thomas, J. Veverka, Icarus 42, 234 (1980)]. 6. S. Murchie et al., Icarus, in press. 7. For detailed passbands of filters and for reduction procedures used in analyzing MSI color data, see S. Murchie et al., Icarus 140, 66 (1999). 8. This calculation was done using the scaling given by P. Lee et al., Icarus 120, 87 (1996). If L is the size in meters of the largest block produced during the excavation of a crater of diameter D (in meters), then L  0.25 D 0.7 . The largest blocks in the LAF area are about 60 to 70 m across, corresponding to a crater 3 km in diameter or larger (that is, the size of Selene or bigger). 9. For a good discussion of various possible erosional and degradational processes on airless bodies, see J. F. Lindsay, Lunar Stratigraphy and Sedimentology (Elsevier Scientific, New York, 1976). 10. W. L. Quaide, V. R. Oberbeck, J. Geophys. Res. 73 5247 (1968). 11. Mars crossers such as Eros are in unstable orbits and must have spent most of the time since their forma- tion in the main asteroid belt where collisions could have been relatively frequent during the earliest ep- ochs [see P. Michel et al., Astron. J. 116, 2023 (1998)]. 12. R. J. Pike, in Impact and Explosion Cratering, D. J. Roddy, R. O. Pepin, R. B. Merill, Eds. (Pergamon, New York, 1977), p. 489 –509. 13. G. B. Neukum et al., Moon 12, 201 (1975). 14. P. H. Schultz, D. E. Gault, Proc. Lunar Sci. Conf. 6, 2862 (1975). 15. The amount of material can be estimated by as- suming a crater profile somewhat reduced from a fresh crater parabolic form with a depth/diameter (d/D) ratio of 0.2 (12). For a crater depth of 0.15 D, a central one-third of the crater would have a maximum depth of one-ninth the rim depth, or 0.017 D. For a linear depth/radius model, the de- posit thickness would be one-third of the crater depth, or 0.05 D. These end-member estimates give a maximum depth of fill in a 100-m crater of 1.7 to 5 m, and a depth of fill of less than 1 m for a 20-m crater. It cannot be argued that the ob- served relationship between pond depth and crater diameter is explained because larger craters are on average older (form less frequently) than smaller craters and thus have had more time to accumu- late ejecta from successive impact events. In such a case, at least some of the smaller craters would be expected to have as much fill as the larger ones, something that is not observed. 16. It has been suggested that electrostatic effects can both levitate and transport fine dust on airless bodies such as asteroids and the moon [see P. Lee, Icarus, 124, 181 (1996) and T. Gold, Mon. Not. R. Astron. Soc. 115, 585 (1955)]. 17. J. Veverka et al., Icarus 107, 72 (1994); P. C. Thomas, J. Veverka, Icarus 40, 394 (1979). 18. M. T. Zuber et al., Science 289, 2097; A. F. Cheng et al., Icarus, in press. 19. R. Sullivan et al., Icarus 120, 119 (1996). 20. J. Veverka et al., Science 285, 562 (1999). 21. We thank the Mission Design, Mission Operations, and Spacecraft teams of the NEAR Project at the Applied Physics Laboratory of Johns Hopkins Univer- sity for their dedicated and successful efforts that resulted in achieving the closest ever flyover of a solar system body by an orbiting spacecraft. 27 December 2000; accepted 7 March 2001 Laser Altimetry of Small-Scale Features on 433 Eros from NEAR-Shoemaker Andrew F. Cheng, 1 * Olivier Barnouin-Jha, 1 Maria T. Zuber, 2,3 Joseph Veverka, 4 David E. Smith, 3 Gregory A. Neumann, 2 Mark Robinson, 5 Peter Thomas, 4 James B. Garvin, 3 Scott Murchie, 1 Clark Chapman, 6 Louise Prockter 1 During the Near Earth Asteroid Rendezvous (NEAR)–Shoemaker’s low-altitude flyover of asteroid 433 Eros, observations by the NEAR Laser Rangefinder (NLR) have helped to characterize small-scale surface features. On scales from meters to hundreds of meters, the surface has a fractal structure with roughness dominated by blocks, structural features, and walls of small craters. This fractal structure suggests that a single process, possibly impacts, dominates surface morphology on these scales. The NEAR-Shoemaker mission (1) has mea- sured the shape of asteroid 433 Eros from orbit with a laser altimeter (2), enabling quan- titative assessments of the asteroid’s surface morphology at scales of hundreds of meters to kilometers (3). Previous results from the NLR (4 ) suggested that Eros is a consolidat- ed object whose shape is dominated by col- lisions. Clustered steep slopes, beyond ex- pected angles of repose, are present over 2% of the surface area (4 ). During the low-altitude flyover of Eros on 26 October 2000, simultaneous observations with the NLR and the multispectral imager (MSI) were obtained at a spatial resolution of 1 m, which is at least three times the resolution achieved previously (5). During the flyover, the NLR was operated continuously at a 2-Hz pulse repetition frequen- cy. The NLR range precision is 1 m, and the NLR boresight direction, which is illuminated by the laser, is close to the center of the MSI image field of view (3, 6–8). As the surface moves past the instrument boresight (owing to orbital motion, asteroid rotation, and spacecraft maneuvers), the laser spots trace out a track 1 The Johns Hopkins University Applied Physics Labo- ratory, Laurel, MD 20723– 6099, USA. 2 Department of Earth, Atmospheric, and Planetary Sciences, Massa- chusetts Institute of Technology, Cambridge, MA 02139, USA. 3 Earth Sciences Directorate, NASA/God- dard Space Flight Center, Greenbelt, MD 20771, USA. 4 Space Sciences Building, Cornell University, Ithaca, NY 14853, USA. 5 Department of Geological Sciences, 309 Locy Hall, Northwestern University, Evanston, IL 60208, USA. 6 Southwest Research Institute, 1050 Walnut Street, Suite 426, Boulder, CO 80302, USA. *To whom correspondence should be addressed. E- mail: andrew.cheng@jhuapl.edu R EPORTS 20 APRIL 2001 VOL 292 SCIENCE www.sciencemag.org 488