WPC 2BV_Z#|,Scal) (MF)f\@N:,4L  P6oljw=ntv}}|xL?Ld4pznzr@zx66n7wyzzLm@wjgkeddd8xpppppȰnrrrr=6=6=6=6wyyyywwww|kpzyy|kpppnnnnzrrrrzzzzzzxx===6=nnt7t7t7t7t7wwwwyyLLLmmmmv@v@v@wwwwww|kxexexezt7wLmv@|k|kzyw//ddBoYYqwqT[qJYĤjd$dddxxxxdByuv`zuZy`yyyyyyyyyyyyyyyyyyy```````2 dZ1Tiempo Roman (PC,Scal) (MF)Geneva Roman (PC,Scal) (MF)Geneva Bold (PC,Scal) (MF)7A;XP2C8,:oRXP p5$A;X2C7,V0XP xQA;XX2C8,uXP xFvA;XdZQ(!,vhQP PH>A;hP"m ^8BBq4JJd8@8?qqqqqqqqqq==oǁzo>b}n|qqmowL?Ld4oyryt=zx56i6x{yzKg?x__fdddd8vooooorztztztzt>5>5>5>5x{{{{xxxxofoy{{ofooorrrryztztztztzzzzzzxx>>>5>b}in6n6n6n6n6xxxx{{KKK|g|g|g|gq?q?q?xxxxxxofwdwdwdyn6xK|gq?ofofy{x//ddBoYYqyqS\qJZĤjd$dddxxxxdByuv`zuZy`yyyyyyyyyyyyyyyyyyy```````2= RGH"m ^!&$\BeR((fDEGG(;"D:W9<:<<@@@@@VDVDVDVD\E\EO(O(O(M;M;M;M;D"D"D"UDUDUDUDUDUDlWH1 km may begin to depart below saturation equilibrium densities. In that case, the population of larger craters, yet to be revealed in the remaining images of Ida, may directly reflect the population of projectiles rather than the equilibrium size distribution that results from the preferential loss of smaller craters due to overlap and erosion by the saturation of still smaller craters.  S Cratering "Age."  At diameters of 1 km, the crater density is ~5 times greater on Ida than on Gaspra, whose surface age has been estimated at ~200 My  S (9). It is expected that Gaspra and Ida although they are in somewhat different parts of the asteroid belt are subject to impact by roughly the same  SN population of projectiles. Studies of the orbital distributions of larger asteroids  S  (15) show that over half of the objects that could intersect one body would intersect the other, and vice versa, with similar impact velocities. Since the smaller projectiles responsible for cratering are believed to be the widely dispersed collisional fragments of larger asteroids, the cratering flux hitting both bodies should be roughly the same (we note that this extrapolation is an assumption that bears critical evaluation). Assuming the impact rate on both bodies has been the same, and constant with time, then there are two possible (0*0*0*  S interpretations for the different crater populations: (i) the strength of the surface material is much greater on Gaspra and projectiles make smaller craters on  S Gaspra than on Ida, or (ii) projectiles make the same size craters on both bodies but Ida's surface is about 5 times older than Gaspra's. While  SH emphasizing that (i) cannot, at present, be ruled out, we prefer the second interpretation, which yields a rough estimate of the minimum age for exposure of Ida's surface to asteroidal projectiles of ~1 By. Since Ida is a member of the Koronis family, it must have been created as a fragment from the catastrophic disruption of the precursor body no earlier than that, so 1 By would seem to be a minimum age for the Koronis family as a whole. This is  S4 older than the expectations of Binzel (7)  and others based on considerations of the dynamics of the Koronis family.  S   Blocks, Chutes, Grooves, and Regolith %  SN % 4 <DL!T$4 <DL!T$ Blocks. Many isolated positive relief features are evident on the surface. Among them, we have confidently distinguished about 20 blocky features larger than 1 pixel across which distinctly contrast with their local background (most are indicated in Fig. 4). Six of the blocks are larger than 100 m across; the largest are ~150 m (~4 pixels) long, similar to the maximum sizes of  Sh$ blocks found on Phobos, Deimos, and the Moon (16). Generally it is not possible to say whether they are perched on the surface or partially buried larger objects. However, at least one object has an associated trail suggestive (0*0*0* of a low velocity impact by a block now sitting on the surface or of a dislodged boulder rolling across the surface. The spatial distribution of clearly identified blocks is nonuniform. There is a clustering of the majority in the eastern region, many within the two largest (~8 km diameter) craters that define Ida's shape in that region. There is a possible second cluster of blocks near the western end of Ida. In contrast, no blocks were identified with such high confidence in the wellimaged central region nor along Ida's limb, outside of the large craters. The spatial nonuniformity seems to be real, despite potential biases due to variable lighting geometry and resolution across the mosaic, foreshortening, smear, and local topographic effects. 44Blocks are likely to be impact ejecta and to be the largest components of a size distribution of particles that make up Ida's regolith. Their emplacement on the surface may have been affected by the rapid rotation of Ida, which substantially reduces the effective gravity at the two ends of the  S| asteroid (11). Preliminary calculations indicate that blocks launched randomly from the surface at velocities just below the escape velocity, and which achieve a temporary orbit around Ida, will impact preferentially on the leading rotational face (which coincides roughly with the eastern region), just as observed. Some blocks may have followed suborbital trajectories to their present positions. Alternatively, block distribution could reflect underlying lithology or regolith thickness. Some of the numerous blocks located near or within large impact craters may be directly associated with them (e.g. excavated by them or exposed by mass wasting).  (0*0*0*Ԍ44Because of their relatively small size, blocks have only a short lifetime  S against collisional disruption, about 30 80 My for the largest ones (17). Their presence on the surface indicates that one or more craters in the 1 10 km size range were formed during the last 30 80 My (only impacts that produce  SH craters in this size range can be expected to produce 100 m sized blocks).  %  S % Linear features.  Lineaments are relatively common on Ida, including systems of grooves, albedo stripes, and short crater chains. The most prominent  S system of grooves (i.e. linear depressions of uniform width), is located at the eastern limb; a second system stretches across the central region (Fig. 4). Grooves range up to 4 km in length and from 100 to 350 m in width, with depths probably no more than a few tens of meters. They range from having beaded outlines, suggesting a series of coalescing pits, to having sharply linear forms. Some grooves intersect craters, but they rarely intersect each other or bifurcate. They show no preferred orientation with respect to local slopes. Although comparable in morphology with grooves on Phobos and Gaspra, the global organization of Ida's grooves is less clearly defined than on these other  S objects (20). Since at least one groove terminates adjacent to a large block, a subset of grooves may simply be boulder tracks. In addition, grooves may be the surface expressions of massive planar fractures and joints in possible underlying bedrock of Ida, caused by the stress of large impacts. 44Albedo stripes and patches are seen within the large craters of the eastern region and are commonly oriented downslope, although it is difficult to ( 0*0*0* separate true albedo variations from abrupt changes in local slopes at projected ridge lines. Generally stripes are 50 to 100 m wide and range from 400 m to 4 km in length. These features may be the courses of optically immature regolith exposed during downslope movement, although some could be grooves or rays of ejecta from a hidden crater. There is a bright triangular feature 2.5 km long and 500 m wide in the eastern region (Fig. 4), which could be the result of downslope movement of surface materials. There is also at least one dark feature, besides the darkfloored craters.  Sb Chutes. Inside the rim of one 8 km crater we have found three examples of shallow depressions located upslope from small degraded craters. The largest is 1 km long and 400 m wide and they appear to be shallow. They probably are scars produced by failure of weak material on an oversteepened slope triggered by an impact. This again may imply the presence of a regolith.  SN Regolith.  There is abundant evidence for a regolith on Ida. The downslope orientation of chutes and bright stripes, the morphology of pitted grooves, the range of crater morphologies, dark floored craters, and craters with bright rims are all most simply explained in terms of a ubiquitous regolith. The size of craters showing flatbottomed topography suggests a regolith depth, in those localities, of a few hundred meters. Photometric studies, which may clarify the relationship of all these features to the properties of the regolith, must await a:& 0*0*0* more accurate shape model that can be derived only after images with complete rotational coverage are returned later in 1994.  Sv  Comparisons with Gaspra %  S %  Both objects share an elongated, irregular shape, apparently with at least some  S large planar facets. Grooves exist on both asteroids. The two asteroids show major differences in their cratering records and in the nature of their surface layers. Ida's surface has been exposed to sufficient cratering to have reached  Sb equilibrium, at least for craters up to diameters of ~1 km. Unlike Gaspra (9),  S4 Ida has many large craters up to a sizable fraction (~0.7) of Ida's mean radius. As a recorder of cratering, Gaspra's surface appears to have been "reset" at least once (i.e., all preexisting craters destroyed, perhaps by shaking due to a large impact) and the present population of resolved craters (>100 m diameter) is so sparse that it represents the production function. Based on the reasonable assumptions that the cratering rate and size distribution of  S  impacting objects should be similar at Ida and Gaspra (15) and that both bodies have similar impact strengths, Ida's surface must be much older (perhaps 5 to 10 times) than Gaspra's. Ida's largest impacts failed to have the resetting effects observed on Gaspra, possibly reflecting a dependence on body size of response to such events.  S:& 44Although they share many geologic features (e.g. craters, grooves, regolith), Ida shows a greater range of geological features than Gaspra. This ( 0*0*0* may be due to the presence of deeper regolith on Ida, whose stronger gravity will naturally retain a greater fraction of ejecta; an indication of this may be the presence of a few possible rayed craters on Ida but not on Gaspra. A second factor is Ida's longer lifetime against catastrophic disruption, which we estimate  SH at 1.5 By vs. 500 My for Gaspra (21). The asteroid has survived long enough to both generate and accumulate more ejecta on its surface. 44It remains to be seen whether the two asteroids have similar photometric and spectral properties when compared at high spatial resolution. Globally integrated groundbased spectrophotometric data indicate that intriguing albedo  Sb and spectral variations may be present on the largest scale on Ida (4). This suggests that compositional contrasts may also be present on this object that are even more extreme than on Gaspra, despite the homogeneity already  S reported for this face of Ida by the Galileo NIMS investigators (22). Images that provide data on color and on the true shape of the object have already been returned in the period February through June 1994, and are undergoing  SN analysis.  % % % % % % %ԌReferences and Notes %   S  1. Ida is an Stype (2) mainbelt asteroid and a member of the Koronis family  Sv (3).  Groundbased observations (4) indicate an elongated object (axial ratios a/b=1.82, b/c=1.15), with mean diameter ~28 km, and geometric albedo 0.25. It has retrograde spin with a period of 4.633 h about an axis that points in one  S of two possible directions (4), indistinguishable in telescopic data. Variations  S observed during a rotation period indicate possible albedo (4)  and compositional variations (5) across its surface. The brighter members of the  Sb Koronis family are taxonomically homogeneous (6), and the observed distribution of spin states has been interpreted in terms of a young age for the  S Koronis family relative to the age of the solar system (7).   S 2. D.J. Tholen, In Asteroids II, (Eds. R.P. Binzel et al., Univ. of Arizona Press, Tucson), 11391150 (1989).  S  3. K. Hirayama, Astron. J., 31 , 185188 (1918); A. Carusi and G.B. Valsecchi,  S Astron. Astrophys., 115 , 327335 (1982); V. Zappala et al., Astron. J., 100 , 2030 S 2046 (1990); J.G. Williams, Icarus, 96 , 251280 (1992).  Sh$ 4. R.P. Binzel et al., Icarus, 105 , 310325 (1993); M. GonanoBeurer et al.,  S:& Astron. Astrophys., 254 , 393396 (1992); D.J. Tholen, private comm. (1992):& 0*0*0* provided the Galileo project with an estimate of the mean diameter and geometric albedo.  Sv 5. M.A. Barucci et al., Planet. Space Sci., in press (1994); S. Mottola et al.,  SH submitted to Planet. Space Sci. (1993).  S 6. J.C. Gradie et al., In Asteroids, (Ed T. Gehrels, Univ. of Arizona Press,  S Tucson), 359390 (1979); R.P. Binzel and S. Xu, Icarus, 106 , 608611 (1993).  Sb 7. R.P. Binzel, Icarus, 100 , 274289 (1992); R.P. Binzel, Ph.D. Dissertation,  S4 University of Texas (1986); R.P. Binzel, Icarus, 73 , 303313 (1988). 8. Galileo encountered Ida on UT 28.703 Aug., 1993, at a heliocentric distance  S of 2.95 au. The spacecraft flew south of the asteroid (75o S ecliptic latitude) at a speed of 12.4 km/sec, and at a range of 2400 km.  S  9. M.J.S. Belton et al., Science, 257 , 16471652 (1992); J. Veverka et al.,  S Icarus,  107 , 217 (1994a); J. Veverka et al., Icarus,  107 , 7283 (1994b); M.H.  S Carr et al., Icarus,  107 , 6171 (1994); P.C. Thomas et al., Icarus,  107 , 2336  S" (1994); C.R. Chapman et al., Icarus, in press (1994); M.E. Davies et al., Icarus,  Sh$  107 , 1822 (1994); P. Helfenstein et al., Icarus,  107 , 3760 (1994); R. Greenberg  S:& et al., Icarus,  107 , 8497 (1994).  (0*0*0*Ԍ S 10. M.J.S. Belton et al., Space Sci. Rev, 60 , 413455 (1992). 11. Because of its shape and rotation, Ida's effective surface gravity must vary  Sv by as much as a factor of 2 across its surface. For a density of 3.5 g.cmé3, we  SH estimate mean surface gravity of ~1.4 cm/sec2.  Centrifugal accelerations of  S ~0.4 cm/sec2 apply at the ends of the asteroid. 4 <DL!T$4 <DL!T$12. Craters >500 m diameter were identified, classified, and measured in a region of ~320 sq. km., mostly in the central region where the lighting is good and the slant angle not too great. Craters down to about one third that size were studied in a 25 sq. km. subregion (craters <100 m diameter are visible in the image but have not been examined in this preliminary study). A larger area (723 sq. km.) was used to obtain better statistics for craters 1 to 4 km in diameter. About 470 craters are in the total sample; the image scale (used for crater diameters and area calculation) was known to about 5% when these studies were done. 4 <DL!T$4 <DL!T$  S 13. Chapman, C. R., Mosher, J. A., and Simmons, G., J. Geophys. Res., 75 , 14451466 (1970).  S:& 4 <DL!T$4 <DL!T$14. Hartmann, W. K. Icarus, 60 , 5674 (1984). 4 <DL!T$4 <DL!T$Ќ S 15. W.F. Bottke et al., submitted to Icarus (1993).  S 16. S.W. Lee et al., Icarus,  68 , 7786 (1986). 4 <DL!T$4 <DL!T$17. Using the rate dependent strengthscaling law from Housen and Holsapple  S (18) and an RMS collision velocity of 3.92 km/s (15) and assuming half of the energy is partitioned into disruption of the target, we find that blocks of 30 100 m diameter can be disrupted by impactors which are 1.3 4 m in diameter. The  S adopted collision probability (15) is 3.8 x 10é18 kmé2 yré1. Assuming a power law exponent of 4.0 for small projectiles yields 30 80 m.y. for block lifetime against disruption (possibly younger if there are enhanced collisions by Koronis family  S asteroids). These results are in accord with meteorite exposure ages (19), which suggest that meter sized objects survive a few tens of millions of years.  S| 4 <DL!T$4 <DL!T$18. K.R. Housen and K.A Holsapple, Icarus, 84 , 226253 (1990).  S  19. D. Bogard, in Asteroids, (Ed. T. Gehrels, Univ. of Arizona Press, Tucson), 558578 (1979).  S" 20. P. Thomas et al., J. Geophys. Res., 84 , 84578477 (1979); J. Veverka  et  Sh$ al., Icarus,  107 , 7283 (1994). Because recognizability of lineaments can be strongly influenced by the single azimuth of illumination geometry, our knowledge of lineament orientations could be strongly biased. (0*0*0*Ԍ S ԙ21. Using the Holsapple scaling law [K. A. Holsapple, Ann. Rev. Earth Planet.  S Sci., 21 , 333373 (1993)] with an experimentally measured gravitational  S self-compression term (plus other assumptions as in (17), we estimate that Ida can be shattered by a 1.7 km diameter projectile. Ida is in the size range at which the collisional energy needed to shatter it is comparable to that needed for fragments to escape from Ida's gravity. Thus the lifetime against dispersal is about the same as the lifetime against shattering. Here we assume that Ida responds to collisions as if it were a strong rocky body; the uncertainties in the lifetime of 1.5 By are at least a factor of 2, due mostly to the uncertainties in (a) the effective strength and (b) the asteroid size distribution at sizes between 1 and 25 km.  S 22. R. W. Carlson et al., EOS (Trans. A.G.U.), 74 (43), 384. 23. Manuscript in preparation for submission. 24. We thank the Galileo project and the National Aeronautics and Space Administration for support; in particular we wish to recognize the success of the Galileo Flight Team lead by W. O'Neil, N. Ausman and M. Landano in the face of many difficulties. In particular, the playback tape management work of O. Adams was critical to the acquisition of the images discussed here. The (beautiful) Ida mosaic used in this work was constructed within L. Wainio's group at MIPS. In this respect we particularly wish to recognize the (0*0*0* professionalism of Ms. Helen Mortensen. We also acknowledge the substantial inputs that have been made to this work by the following colleagues: D.J. Tholen, R.P. Binzel, P. Magnusson, A. Barucci, S. Mottola, R. Sullivan, R. Pappalardo, P. Geissler, JM Petit, J. Moore, W. F. Bottke, M. Nolan, E. Ryan, W. Merline, B.E.A. Mueller, E. Asphaug, B. Carcich, P. Lee, D. Simonelli, R. Wagner, P. J. Guske, J. Yoshimizu and R. Hasegawa. A portion of this research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under contract with NASA. The National Optical Astronomy Observatories are operated by the Association of Universities for Research in Astronomy, Inc. (AURA), under cooperative agreement with the National Science foundation. % % % Figure Captions %  S %  Figure 1. Mosaic of Ida with Gaspra included, at the same scale, for comparison.  Sh$  Figure 2. Ida with a model ellipsoid superposed and oriented according to the  S:& two possible rotation poles (4). The solution in the top panel, with the north  S ( pole in the direction of RA = 50o.25, Dec(deg.): = 76o.32 (J2000), is clearly (0*0*0* preferred. The retrograde rotation is in the sense that the righthand (eastern) part of Ida is swinging outofthepaper, toward the viewer, while the lefthand part is receding.  SH  Figure 3. Examples of a groove, boulders, and a range of crater degradation states. (a) Enlargement of the eastern end of Ida showing the location of several prominent boulders (arrows at left) on the inner wall of a large crater. Arrows at the top point to a prominent groove, extending for at least 3 km. The image is ~12 km in height; north is approximately toward the top. (b) Range of crater degradation states in the western central region of Ida. The large central crater is ~4.5 km in diameter and has been modified by many subsequent impacts on its rim and in its interior, as has the smaller crater on its eastern rim. Several relatively fresh craters with linear rim crests are along or beyond its southern rim. The image is ~8.5 km wide; north is toward the top.  S   Figure 4. Map of geological features on Ida. Locations of the five frames which make up the Ida picture (UTC [hh:mm:ss] command times, phase [deg.], range [km]): (A) 16:47:56.266, 48.3, 3821; (B) 16:48:04.933, 49.3, 3738; (C) 16:48:13.600, 50.3, 3656; (D) 16:48:22.266, 51.3, 3575; (E) 16:49:22.933, 60.0, 3057. The major regions used in the text are identified.  S  Figure 5. (a) Differential sizefrequency (per sq. km per km diameter increment) relationships were sampled in 3 different counting regions. Incomplete data at small diameters have been omitted. The line is a least squares fit to the solid  Sv data points and has a slope of 3.3. (b) Cumulative sizefrequency (per sq. km) relationships are plotted for all craters and for the three morphological classes  S noted in the text. (c) Rplot of crater densities (differential crater densities from  S the two smaller counting areas divided by diameteré3). Ida data are shown as open circles. The horizontal line has been set at a density of 0.3 typical of the most heavily cratered terrains known on planetary satellites.