NEAR Encounter with Asteroid 253 Mathilde: Overview

Icarus 140, 3–16 (1999) Article ID icar.1999.6120, available online at http://www.idealibrary.com on

J. Veverka, P. Thomas, A. Harch, B. Clark, J. F. Bell III, B. Carcich, and J. Joseph Department of Astronomy and CRSR, Cornell University, Space Sciences Building, Ithaca, New York 14853 E-mail: [email protected]

S. Murchie and N. Izenberg Applied Physics Laboratory, 11100 Johns Hopkins Road, Laurel, Maryland 20723-6099

C. Chapman and W. Merline Southwest Research Institute, 1050 Walnut Street, Suite 426, Boulder, Colorado 80302

M. Malin Malin Space Science Systems, Inc., P.O. Box 910148, San Diego, California 92191-0148

L. McFadden University of Maryland, Department of Astronomy, College Park, Maryland 20742

and

M. Robinson Northwestern University, Department of Geological Sciences, 309 Locy Hall, Evanston, Illinois 60208

Received May 14, 1998; revised January 28, 1999

is remarkably low: 1.3 0.3 g/cm3, a value consistent with a rub- On June 27, 1997, the NEAR spacecraft carried out the first- ble pile structure for the interior. Assuming that Mathilde’s rock ever encounter with a C-type asteroid, flying by 253 Mathilde at type is similar to that found in CM meteorites, the porosity of the a distance of 1212 km. We summarize findings derived from 330 interior must be some 50%. Shock and seismic disturbances associ- images obtained by NEAR’s MSI camera which cover about 60% ated with major impacts are expected to be transmitted very poorly of the surface of the asteroid. The highest resolution achieved was by Mathilde’s underdense interior, a fact which may explain the about 160 m/pixel. remarkable degree to which surface morphology and topography Mathilde is a low-reflectance object (geometric albedo = 0.047) have been preserved in spite of later major collisional events. with principal diameters of 66 48 44 km. The mean radius of Except for the lower geometric albedo (0.047 0.005), the photo- 26.4 1.3 km is somewhat smaller than the value of 30 km sug- metric properties of Mathilde are closely similar to those of Phobos. gested by previous telescopic data. Mathilde’s surface morphology The surface is extremely homogeneous in terms of both color or is dominated by large craters, at least four of which have diame- albedo: specifically, no color or albedo variations associated with ters comparable to the radius of Mathilde. The two largest, Ishikari craters have been identified. c 1999 Academic Press and Karoo, have diameters of 29.3 and 33.4 km, respectively. No Key Words: asteroids; asteroids, Eros; surfaces, asteroids. evidence of layering is exposed in the crater walls, but suggestions of downslope movement are present. The surface density of craters in the diameter range from 0.5 to 5 km is close to equilibrium satu- 1. INTRODUCTION ration, a situation in which as many craters are being destroyed as are being produced. Observed depth-to-diameter ratios for craters NEAR, the first mission of NASA’s new Discovery program, in this size range are close to those observed on the lunar surface. A disruption lifetime of about 4 billion years has been estimated for was launched on February 17, 1996, on a 3-year trajectory to Mathilde. the near-Earth asteroid 433 Eros. The spacecraft carries a com- Based on the mass determination obtained from Doppler track- plement of six science instruments: imager, near-infrared spec- ing (D. K. Yeomans et al., 1997, Science 278, 2106–2109) and the trometer, gamma-ray spectrometer, X-ray spectrometer, mag- volume derived from MSI images, the average density of Mathilde netometer, and laser rangefinder in addition to a radio science

0019-1035/99 $30.00 Copyright c 1999 by Academic Press All rights of reproduction in any form reserved. 4 VEVERKA ET AL. investigation which analyzes the tracking signals from the space- et al. 1997). Developed for close-up studies of Eros from orbital craft (Farquhar et al. 1995). NEAR will arrive and begin its or- distances (expected spatial resolution at Eros is 3–4 m from a bital mission at Eros in early 1999. On its way to Eros the NEAR 30-km orbit), the instrument was not designed for imaging aster- spacecraft passed within 1212 km of asteroid 253 Mathilde on oids during distant fast flybys. Nevertheless, resolutions of some 27 June 1997. In this first spacecraft encounter with a C-type as- 160 m per pixel were achieved during the 1212-km encounter. teroid (the dark, probably carbon-rich type predominant in the The inflight performance of the MSI camera is reviewed by outer part of the belt), NEAR took over 500 images (Veverka Murchie et al. (1999). With the maximum exposure of 1 s, the et al. 1997a) and determined Mathilde’s mass to an accuracy of camera can detect objects as faint as +9.5 mag. Absolute cali- about 5% (Yeomans et al. 1997). bration, in particular the conversion of MSI measured fluxes to The purpose of this paper is twofold: V -mag, can be accomplished to 5–10%. (1) to provide an overview of the major results obtained during the encounter, results which are detailed in the six contribu- 2. THE FLYBY tions that make up this Special Issue of Icarus, and (2) to compare the Mathilde findings with those provided Carrying out the science objectives at Mathilde involved a earlier by Galileo for S-type asteroids 951 Gaspra and 243 Ida number of new challenges, in terms of both navigation and space- (Veverka et al. 1994a, Belton et al. 1996). craft operation (Yeomans et al. 1997, Harch et al. 1995, Harch and Heyler 1998). Because the spacecraft has fixed solar panels For convenience, Mathilde’s fundamental properties are sum- and no scan platform, the encounter had to be carried out in an marized in Table I. orientation which permitted enough sunlight to fall on the solar The Mathilde encounter took place 1.99 AU from the Sun, panels to provide the necessary power while slewing the space- shortly after the NEAR spacecraft reached the greatest heliocen- craft attitude to allow the field of view of the MSI camera to tric distance from the Sun on its 3-year journey to Eros. Because cover the region of the sky in which Mathilde was predicted to the solar panels on NEAR are sized to provide adequate power at be. To meet the power requirements the solar incidence angle the closer heliocentric distances of the Eros orbital phase, there on the solar panels had to be maintained at 50 or less, a factor was not enough power to operate all instruments during the which limited the total observing time around closest approach flyby. To conserve power and to avoid the risk of the spacecraft to approximately 25 min. As the approach phase angle was very aborting the flyby sequence if a power shortage developed, only large (140), it was decided to concentrate most of the imaging at one of the six instruments on NEAR, the Multispectral Imager and just after closest approach, when the illumination geometry (MSI), was turned on for the Mathilde encounter. This imager, was much more favorable. / an f 3.4 refractor, provided with a 244 537 pixel CCD, has The four goals of the imaging investigation at Mathilde were: . a 2.26 2 95 field of view (Veverka et al. 1997b, Hawkins (1) to obtain the highest resolution image possible to study surface morphology; (2) to provide a hemispheric view of the asteroid at a resolution of about 500 m per pixel to determine the aster- TABLE I oid’s size, shape, and volume; (3) to obtain hemispheric coverage Mathilde Parameters in color to look for evidence of compositional heterogeneity; and Groundbased (4) to search the vicinity of Mathilde for possible satellites. The spacecraft approached Mathilde looking close to the di- Orbit a = 2.6 AU e = 0.23 i = 6.9 rection of the Sun (approach phase angle 140 ). The actual Type C imaging sequence (Fig. 1) began some 5 min before closest ap- Geometric albedo 0.038 proach, when views of a crescent-illuminated Mathilde were ob- Mean radius 60 km Lightcurve tained at resolutions of about 500 m. The highest resolution data Period 17.4 days (160-m resolution) were obtained at closest approach (12l2 km), Amplitude 0.45 mag when the phase angle was close to 90 . The imaging sequence Probable axes (diameters) 70 50 50 km continued for another 20 min as the spacecraft receded from the asteroid and viewed Mathilde under good illumination at a phase NEAR angle of about 40. During this time multicolor global coverage Nominal axes (diameters) 66 48 46 km was obtained at 500-m resolution using the seven color filters on Mean radius 26.5 1.3km MSI which cover the spectral range from 400 to 1100 nm. The a 11,000 3 Volume 78,000 +12,000 km imaging sequence concluded with about 200 images devoted to Massb 1.033 0.044 1020 g a satellite search. The MSI camera is sensitive enough that ob- . . / 3 Mean density 1 3 0 2g cm jects as small as 40 m across could have been detected even if Geometric albedo 0.047 0.005 they were made of material as dark as Mathilde. a Thomas et al., this issue. On approach, six special sequences of images were obtained b Yeomans et al. 1997. beginning at 42 h before closest approach to perform optical MATHILDE OVERVIEW 5

FIG. 1. Schematic summary of the imaging sequence at Mathilde. navigation by detecting Mathilde against the star background. comparison, under much more favorable viewing conditions, Since NEAR approached Mathilde looking close to the direction Galileo detected Gaspra 53 days before encounter and Ida 33 of the Sun, the asteroid was detected ﬁrst as a faint dot almost days before! By the sixth and last sequence at 11 h before en- lost in the Sun’s glare, just 36 h before closest approach. For counter (Fig. 2), Mathilde had brightened to about +7.0(0.3)

FIG. 2. Optical navigation detection of Mathilde on approach. Ten MSI frames taken during OpNav Sequence 6 some 11 h before closest approach have been co-added and the strong background gradient due to solar glare (phase angle 140) has been removed. 6 VEVERKA ET AL.

TABLE II TABLE III Mathilde OPNAVs Mathilde Encounter

OPNAV Time (h) Range (million km) Predicted MAG Actual MAG Delta Date of NEAR flyby 27 June 1997 Time of closest approach 12h 55m 54.5s UT 2 37 1.32 7.7–8.7 9.6 +0.9 Flyby distance 1212 km 3 30 1.07 7.4–8.4 9.5 +1.1 Flyby speed 9.93 km/s 4 24 0.88 6.9–7.9 9.0 +1.1 Approach phase angle 140 5 18 0.66 6.3–7.3 8.2 +0.9 Departure phase angle 40 6 11 0.40 5.3–6.3 7.0 +0.7 Duration of imaging sequence 25 min Total No. of frames taken 534 Note. Approach phase 140. Total No. of frames of Mathilde 330 visual magnitude, about a magnitude fainter than our preen- counter estimate of +5.3 to +6.3 (Table II). Based on a rapid mosaic designed to capture the highest possible resolution image analysis of these data, the pointing was updated to ensure that of the asteroid (Fig. 3). the camera would be looking at Mathilde at the time of the flyby. Although the sequence executed precisely as planned, only The last update was sent to the spacecraft only 5 h before the en- about 330 of the 534 MSI frames exposed contain the asteroid counter. As already noted, unlike Galileo NEAR does not have a (Table IV). Some were intentionally pointed at the region around scan platform and the whole spacecraft must be slewed to keep Mathilde to search for satellites. Others were used as insurance the camera pointed at the asteroid. to cover the uncertainty region of Mathilde’s position at the The flyby was executed flawlessly. The spacecraft passed the time of closest approach, the linear extent of which was about asteroid at 1212 km, very close to the planned 1200-km miss 10 times the camera field of view. A representative sequence of distance (Table III). All images were obtained as planned: the MSI images is shown in Fig. 4. final optical navigation solution was so accurate that Mathilde Although MSI has an autoexposure capability (Hawkins et al. was captured within the central frame of the closest-approach 1997), this feature was not used during the Mathilde flyby. All

FIG. 3. Mathilde’s actual position relative to the sequence of MSI frames taken to capture the highest resolution view of the asteroid. Shown is the 2 uncertainty ellipsoid in Mathilde’s position. Dimensions 460 by 160 km. MATHILDE OVERVIEW 7

FIG. 4. Sample of consecutive MSI frames. See Table IV. exposures were preplanned using model calculations, assuming was limited to about 20 radii around Mathilde: no satellite down a Phobos-like photometric function (Simonelli et al. 1998), with to a limiting size of 40 m was detected. The duration of the the geometric albedo lowered to 0.035 (by changing the value of post closest approach satellite search was limited by spacecraft the single scattering albedo) and assuming a triaxial model for constraints. Given that the spacecraft had to be maintained in Mathilde with a mean diameter of 61 km. Generally, three sets an attitude which provided sufficient illumination to the solar of exposures were obtained, with the middle exposure chosen to panels to power the spacecraft and the MSI camera without re- yield a maximum DN value of 1000 in the nominal albedo case. course to the battery, the entire data-taking sequence during the (The maximum possible DN value is 210 or 4096.) The other two encounter was limited to about 25 min. exposures of each set were chosen to bracket the middle exposure by a factor of 2. This strategy proved successful, yielding 3. SHAPE, SIZE, AND MEAN DENSITY properly exposed images during all phases of the encounter. The fact that on approach Mathilde turned out to be fainter On the basis of control point and stereo measurements, as than expected in the optical navigation images (see Table II) can well as from constraints provided by observed limb profiles and be attributed to a slight overestimation of Mathilde’s size (see terminator locations, Thomas et al. (1999) derive a model which below) and to a characteristic of Mathilde’s surface: the signif- reproduces accurately the observed shape of Mathilde. The fi- icant effect of large shadows at extreme phase angles produced delity of the model can be judged from Fig. 6, which includes by the huge crater concavities that define Mathilde’s shape. two synthetic images of Mathilde derived from the shape model A major goal of the Mathilde encounter was a search for satel- interpolated to fill a gap in our data coverage between the high = lites, motivated in part by speculations that a large satellite might phase view ( 126 ) at left and the closest approach view = help explain the asteroid’s unusually slow rotation period. On ( 90 ) at right. . approach our ability to search for satellites was limited by sev- The mean radius of Mathilde derived by Thomas et al.,2641 eral factors, but principally by the large approach phase angle. A 1.3 km, is somewhat smaller than the value of 30 km sug- 1 search of the approach images which cover the whole so-called gested by averaging several telescopic estimates. Some 60% of Hill sphere estimated to extend some 100 radii around Mathilde, Mathilde’s surface was observed during the flyby. The NEAR revealed no satellite larger than 10 km in radius, assuming the data, obtained over an interval of 25 min, do not place a useful same albedo as Mathilde. Approximately 200 images were obtained to search for satellites after closest approach under a much 1 The difference is consistent with the fact that we find a slightly higher value more favorable phase angle of about 39 (Fig. 5). Our coverage for the geometric albedo (see Table I). 8 VEVERKA ET AL.

TABLE IV Mathilde Flyby Picture List TABLE IV—Continued MATHILDE OVERVIEW 9

TABLE IV—Continued TABLE IV—Continued

Note. MET, mission event time, a unique picture identifier; lat, latitude (); lon, west longitude (); range, range in km; phase, solar phase angle (); exp, exposure time (s); filter, MSI filter; hires, resolution (pixel width) km/pxl; lores, resolution (pixel length) km/pxl. Filter designations are as follows (Veverka et al. 1997b) (filter No., spectral coverage (nm)): 0, 700 100; 1, 550 15; 2, 450 25; 3, 760 10; 4, 950 20; 5, 900 20; 6, 1000 25; 7, 1050 40. MSI has rectangular pixels (27 16 m), corresponding to 162 96 rad/pixel.

constraint on the spin period of Mathilde, which according to Mottola et al. (1995) is 417 h (with a lightcurve amplitude of 0.45 mag). No changes in shadow positions were noted over a period of some 10 min when the highest resolution views are available, from which Thomas et al. set a lower limit of 1.8 days on Mathilde’s spin period. Neither NEAR nor telescopic observations place useful constraints on Mathilde’s pole position. The nominal shape model has dimensions of 66 48 44 km, for a volume of 77200 km3. Thomas et al. estimate that the volume is uncertain by about 14%, a result which yields a mean density of 1.34 0.2 g/cm3, using the mass measurement of 1.033(0.044) 1020 g by Yeomans et al. (1997). Thomas et al. stress that the observational constraints make it extremely unlikely that the true volume of Mathilde is smaller than their minimum estimate, implying that the density of Mathilde must be less than 1.5 g/cm3.

4. SURFACE FEATURES AND PROPOSED NAMES

Thomas et al. produced a sketch map (Fig. 7), which includes provisional designations (yet to be approved by the IAU) for the principal features, predominantly large craters. The locations and names are summarized in Table V. As already noted, the lo- cation of Mathilde’s spin axis remains undetermined; therefore, the coordinate system shown is arbitrary and uses the J2000 ref- erence frame as basis (Thomas et al. 1999). Given Mathilde’s very low albedo and possible carbonaceous composition, the proposed names are those of coal-mining regions and coal mines 10 VEVERKA ET AL.

FIG. 5. Representative sequence of images from NEAR’s satellite search. on Earth. A series of Mathilde views, with the names of promi- The geology of Mathilde is discussed in detail by Thomas nent craters superimposed, is provided in Fig. 8. et al. (1999), while Chapman et al. (1999) focus on the cratering record preserved on the asteroid’s surface. Craters down to 5. THE GEOLOGY OF MATHILDE diameters of some 500 m can be detected reliably in the NEAR images: nothing unusual has been noted about their morphology. Mathilde’s surface morphology is dominated by large craters, Thomas et al. ﬁnd depth-to-diameter ratios for intermediate- at least four of which have diameters comparable to the radius sized craters (diameters about 1 to 5 km) to range from 0.12 of Mathilde. The largest craters are Ishikari and Karoo, with to 0.25, compared to the well-determined value of 0.2 for fresh diameters of 29.3 and 33.4 km, respectively (Fig. 8). craters on the Moon (Pike 1977) and to values slightly below MATHILDE OVERVIEW 11

FIG. 6. Demonstration of the ﬁdelity of the shape and photometric models derived from the Mathilde data. At the two ends are two real views of Mathilde (phase 138 at left; phase 90 at right). In between are two “interpolated” synthesized views showing intermediate aspects of Mathilde not imaged by NEAR. Such modeling provides a useful visualization of the relative positions of major surface features. Prominent craters are identiﬁed by letters: A, Jixi; B, Damodar; C, Karoo.

0.2 estimated for Gaspra (Carr et al. 1994) and for Ida (Sullivan discussed by Clark et al. (1999) makes it very difficult to detect et al. 1996). possible ejecta blankets. No blocks of ejecta have been identi- The fresher intermediate size craters on Mathilde appear fied; if any exist, they cannot be larger than 200 to 300 m across. bowl-shaped; no examples of complex craters with flat or ringed No evidence of layering is seen in the walls of the very large floors are observed. Various stages of crater degradation are craters imaged at high resolution, but at least two or three ex- present, consistent with the observation that Mathilde’s surface amples of downslope movement have been identified (Veverka is at the saturation equilibrium level in terms of crater density et al. 1997a, Thomas et al. 1999). (see below and Chapman et al. 1999). A few craters have definite No strong evidence of a pervasive global fabric such as the raised rims, but no striking examples of ejecta blankets are noted. grooves and facets on Gaspra (Veverka et al. 1994b), is ob- The extreme color and albedo blandness of Mathilde’s surface served. There are, however, three suggestions of at least local structure or fabric: the polygonal outlines of some craters, a sin- uous marking with the characteristics of an exposed layer, and TABLE V a few scarps or ridges. One of these scarps, of 20 km extent, Crater Names, Locations, and Diameters and an associated relief of some 200 km, is the largest structural feature on Mathilde noted by Thomas et al. While the absence Latitude Longitude Diameter Name ()() (km) Image of grooves and facets may not be surprising on a body having the presumed structure of a low-density rubble pile, the existence Benham 19.0 247.2 2.2 42826360 of some evidence of fabric, of which the polygonality of some Maritsa 44.6 151.5 2.4 42826370 craters is most widespread, is interesting. Mulgildie 57.7 176.1 2.5 42826360 Both Thomas et al. and Chapman et al. point out the re- Jerada 42.1 177.3 2.5 42826360 Kalimantan 7.7 123.7 2.7 42826488 markable degree to which surface morphology and topography Clackmannan 18.9 260.8 2.8 42826360 have been preserved on Mathilde in spite of later major colli- Matanuska 27.3 217.3 2.9 42826360 sional events. Large impact events can destroy distal topography Quetta 45.6 165.5 3.2 42826360 through seismic shaking (including spallation) or by blanketing Similkameen 13.5 104.7 3.4 42826622 it with ejecta. There are no obvious clues on Mathilde to sug- Lorraine 48.1 142.5 4.1 42826370 Aachen 9.2 60.9 4.8 42826360 gest that either process has been effective. Clearly, Mathilde’s Oaxaca 38.7 186.3 5.2 42826360 underdense interior can be expected to transmit shock and seis- Enugu 15.3 151.6 5.9 42826488 mic energy poorly (Thomas et al. 1999, Chapman et al. 1999, Lublin 55.3 156.8 6.5 42826360 Davis 1999). The apparent absence of evidence that surface mor- Teruel 28.1 142.7 7.6 42826488 phology has been modified by ejecta is more surprising. For ex- Otago 23.7 164.5 7.9 42826488 Zulia 39.5 30.9 12.3 42826686 ample, on Ida, a body with weaker gravity than Mathilde, there Baganur 14.6 191.6 16.4 42826488 is strong evidence of impact ejecta on the surface (Geissler et al. Jixi 12.3 256.5 19.9 42826112 1996), as there is even on the surface of Mars’ tiny satellite Damodar 73.0 263.8 28.7 42826112 Deimos (Thomas et al. 1996a). Kuznetsk 45.9 88.9 28.5 42827050 An analysis of the cratering record preserved on Mathilde’s Ishikari 66.2 186.9 29.3 42827050 Karoo 33.5 98.4 33.4 42826622 surface is presented by Chapman et al. (1999), who find that for craters in the range from 0.5 to 5 km the surface is close 12 VEVERKA ET AL.

FIG. 7. Sketch map of Mathilde from Thomas et al. (this issue), showing provisional names for major surface features. to equilibrium saturation, a condition in which as many craters 6. PHOTOMETRIC AND COLOR PROPERTIES are being destroyed as are being produced. This situation is like that observed on asteroid 243 Ida (Chapman et al. 1996a), but From a combination of telescopic data between 1.5 and 16.5 is unlike that on 951 Gaspra (Chapman et al. 1996b), where a phase obtained by Mottola et al. (1995) and NEAR observations relatively younger surface was observed. at phase angles ranging from 39 to 136, Clark et al. (1999) de- To derive a lower limit (because the surface is saturated, only a rive the first-ever detailed photometric properties of a C-type as- lower limit can be derived) on the age of Mathilde’s surface, one teroid. The resulting geometric albedo of 0.047 0.005 agrees must know the size distribution of impacting objects, the flux well with that estimated by telescopic techniques, once the mean of this population, and the mechanical properties of Mathilde’s radius is adjusted downward as discussed above. Albedo varia- surface. Chapman et al. suggest that the latter are so uncertain tions on Mathilde are very subdued. Allowing for the fact that that no precise lower bound on Mathilde’s age can be derived. A some of the remaining brightness variations apparent in photo- somewhat more optimistic approach is adopted by Davis (1999). metrically corrected data may be due to inadequately modeled In a previous study Farinella and Davis (1991) estimated the life- small-scale topography in the Mathilde shape model, albedo time against collisional disruption for the S-type asteroid 951 variations on this asteroid are restricted to about 10% of the Gaspra of some 200 myr, while Greenberg et al. (1994) derived mean, a smaller range than is observed on two other well stud- a value a factor of 5 greater. Analogous calculations are carried ied low-albedo small bodies: Phobos (Simonelli et al. 1998) out for Mathilde by Davis (1999), making reasonable assump- and Deimos (Thomas et al. 1996a). Albedo variations associ- tions about the size distribution and flux of the impacting popu- ated with crater rims and with ejecta have been noted on Phobos lation (believed to be essentially the same as those which affect and to varying degrees on S asteroids Gaspra and Ida. No such Gaspra and Ida), but making allowance for the likely weaker variations are evident on Mathilde. This lack of albedo variation mechanical strength of C-type objects relative to S-type aster- related to surface morphology is consistent with the concomi- oids. It is concluded that a C-type asteroid the size of Mathilde tant lack of variations in color (Veverka et al. 1997a). Mathilde’s has a nominal disruption lifetime of about 4 billion years and homogeneity in albedo and color is interesting in that the only that the time interval needed to form the four or five giant craters somewhat brighter surface of Phobos does show measurable is noticeably less, probably 400 to 2000 million years. In other variations, as do different size fractions of pulverized carbona- words, it is not surprising that bodies large enough to form the ceous meterorites and carbonaceous meteorite analogs measured huge craters have hit Mathilde, while at the same time Mathilde in the laboratory (e.g., Johnson and Fanale 1973). One implica- has not been demolished by a catastrophic impact. Davis sug- tion is that not only globally, but even locally in the proximity gests that Mathilde is likely to be the remnant of a larger body of crater rims, Mathilde’s regolith has a uniform texture/particle which lost 50% or more of its initial mass by cratering. size distribution. MATHILDE OVERVIEW 13

FIG. 8. Four views of Mathilde with provisional names of prominent craters indicated. Locations of named features are given in Table V.

Mathilde’s light scattering properties (or “photometric func- Mathilde are very similar to those of Phobos. Macroscopic sur- tion”) show that Mathilde’s surface is not only much darker face roughness, as measured by Hapke’s parameter 2, does not (lower single scattering albedo), but also more backscattering seem to differ significantly among the three asteroids (Mathilde, than the surfaces of S-type asteroids. Except for a significantly Gaspra, and Ida) studied by spacecraft to date. In addition to lower single scattering albedo, the photometric properties of the mean geometric albedo of 0.047, Clark et al. derive a 14 VEVERKA ET AL. phase integral of q = 0.28 and a Bond albedo of 0.013 for TABLE VI Mathilde. How Porous Is Mathilde? Since the NIS spectrometer on NEAR could not be utilized Rock type CI CM CO/CV during the Mathilde flyby (due to insufficient spacecraft power, as discussed above), very little new information about Mathilde’s Rock Density (g/cm3) 2.25 2.75 3.45 spectral properties can be added to the knowledge derived from Mathilde porosity (%) for mean density 40 6518619 telescopic data. The one exception is the new insight that at least of 1.34 0.2 g/cm3 on this C-asteroid, large scale and local color variations of any significance are absent. Binzel et al. (1996) found that Mathilde’s spectrum is es- to explain Mathilde’s mean density assuming that the asteroid sentially gray, with a shallow absorption shortward of 500 nm consists of carbonaceous-chondrite-like rock. The values range and a suggestion of a dip near 900 nm. Based on these char- from 30 to 70%, with a most likely value of about 50%. acteristics, it was concluded that Mathilde’s spectrum does not An unlikely possibility is that subsurface ice contributes to the 3 match well those of CM or CV chondrites. Gorlovka, a shock- low density. Given the 0.9 g/cm density of water ice, the pres- darkened ordinary chondrite, and Y-826162, an anomalous CI ence of water ice cannot be the major explanation for Mathilde’s chondrite, were proposed as better matches. Observations below density. As summarized in Table VII, such an explanation tween 1.2 and 3.3 m by Rivkin et al. (1997) show that unlike requires that some 67 to 83% of Mathilde’s volume to be ice, the anomalous CI chondrite, Mathilde does not display a 3.0-m a very unlikely proposition in our view, given Mathilde’s geo- water of hydration absorption in its spectrum. Strong arguments logic evolution and current understanding of C-type asteroids. have also been advanced (e.g., Chapman 1996) to the effect that For example, it is difficult to understand why, if C-type as- C-type asteroids are unlikely to be related to shocked ordinary teroids are made up largely of water ice, no evidence of out- chondrites. gassing or comet-like activity has ever been reported in asso- While an unaltered sample of the hydrated CM meteorite ciation with a C-type asteroid. It seems equally unlikely that Murchison fails to match Mathilde’s spectral properties, as al- significant quantities of ice would be preserved inside an ob- ready noted by Binzel et al. (1996), a heated sample provides ject as heavily affected by severe collisions as Mathilde has an excellent match (Hiroi et al. 1993): heating drives off the been (Chapman et al. 1999, Davis 1999), especially if this ob- water of hydration, removes the 3.0-m absorption, and flattens ject has a low-albedo surface and orbits at 2.6 AU from the the slope of the near-infrared continuum. As a useful working Sun. It is certainly true that rather small amounts of pulver- hypothesis, we will assume in our discussion that Mathilde ma- ized carbonaceous material can lower the albedo of water ice terial is similar to dehydrated CM material. significantly and perhaps even mask completely the characteristic near-infrared absorption bands (e.g., Clark 1981). However, 7. DISCUSSION in the case of Mathilde, several facts suggest that water ice is not an important constituent: (a) no spectral evidence of wa- The determination of Mathilde’s mean density is based on the ter ice or of hydrated materials has been observed, and (b) no first direct measurement of an asteroid mass from perturbations exposures of higher albedo materials have been seen. Jupiter’s on a passing spacecraft. The mass of 243 Ida was estimated by satellite Callisto provides an instructive example of how diffi- Belton et al. (1995) based on constraints on the orbit of Ida’s cult it is to mask all surface evidence of water ice, even in the satellite Dactyl. Neither Ida nor Gaspra produced measurable presence of appreciable amounts of a dark, nonicy component. perturbations on the path of Galileo. Finally, we stress that no morphologic evidence of a surface The strong constraints on Mathilde’s volume (Thomas et al. rich in subsurface ice occurs in the NEAR images. Features that 1999) and the precise mass determination (Yeomans et al. 1997) might be attributed to “sapping” or “solifluction” do not occur on yield a surprisingly low mean density of 1.3 0.2 g/cm3,a Mathilde’s steeper slopes, nor are there any regions of collapsed value which argues for significant void space inside of Mathilde terrain within craters. All of these considerations argue against (Veverka et al. 1997a). As stressed by Thomas et al., it is difficult water ice being a significant constituent of either Mathilde’s to develop a shape model for Mathilde which is consistent with surface or interior. observations and for which the asteroid’s mean density would 3 exceed 1.5 g/cm . TABLE VII Theideathatcollisionallyevolvedasteroidscouldhaveporous, Is Ice the Answer? underdense structures is not new (Barks 1960). Unfortunately, to estimate the fraction of void space within Mathilde necessitates CI CM CO/CV an assumption about the composition of Mathilde rock. As discussed above, we adopt the working hypothesis that this rock is Rock density 2.25 2.75 3.45 Ice density 0.90 0.90 0.90 similar to carbonaceous chondrite material, most likely CM-type Ice fraction (%) needed (1.34 g/cm3)677683 rock. Table VI summarizes the fraction of void space required MATHILDE OVERVIEW 15

Chapman et al. stress that very large craters, five of which Chapman, C. R., E. V.Ryan, W.J. Merline, G. Neukum, R. Wagner, P.C. Thomas, have diameters between 20 and 33 km, occur with uniquely J. Veverka, and R. A. Sullivan 1996a. Cratering on Ida. Icarus 120, 77–86. high spatial density on Mathilde. The formation of such large Chapman, C. R., J. Veverka, M. J. S. Belton, G. Neukum, and D. Morrison craters is not unexpected, assuming (as is likely) a cratering pop- 1996b. Cratering on Gaspra. Icarus 120, 231–245. ulation similar to that which has impacted the Moon. It is the Chapman, C. R., W. J. Merline, and P. Thomas 1999. Cratering in Mathilde. Icarus 140, 28–33. preservation of so many of these large craters which is notewor- Clark, Beth E., J. Veverka, P. Helfenstein, P. C. Thomas, J. F. Bell III, A. Harch, thy. Remarkably, the last of the major cratering events did not M. S. Robinson, S. L. Murchie, L. A. McFadden, and C. R. Chapman 1999. destroy or even noticeably modify some of the earlier ones. This NEAR photometry of Asteroid 253 Mathilde. Icarus 140, 53–65. apparent ineffectiveness of major impacts to modify preexist- Clark, R. N. 1981. The spectral reflectance of water-mineral mixtures at low ing topography except in the immediate vicinity of the event is temperatures. J. Geophys. Res. 86, 3074–3086. probably explained by Mathilde’s underdense interior and likely Davis, D. 1999. The collisional history of Asteroid 253 Mathilde. Icarus 140, weak mechanical strength: it might be relatively easy to produce 49–52. a crater cavity, but difficult to transmit the energy effectively to Farinella, A. P., and D. R. Davis 1991. The collisional lifetime of 951 Gaspra. ejecta or to distant parts of the asteroid. Proc. Lunar Planet. Sci. Conf. 22nd, 363-364. Structurally, the three asteroids visited by spacecraft to date Farquhar, R. W., D. W. Dunham, and J. V. McAdams 1995. NEAR mission overview and trajectory design. J. Astron. Sci. 43, 353–371. seem to differ significantly from one another. Gaspra has a Geissler, P., J.-M. Pettit, D. Durda, R. Greenberg, W. F. Bottke, M. C. Nolan, strongly faceted shape and other features suggestive of a mono- and J. Moore 1996. Erosion and ejecta reaccretion on 243 Ida and its moon. lithic body with a global fabric and perhaps considerable me- Icarus 120, 140–157. chanical strength (Veverka et al. 1994a). Ida has a very elon- Greenberg, R., M. Noland, W. Bottke, R. Kolvoord, and J. Veverka 1994. gated, almost bifurcated shape, possibly a somewhat underdense Collisional history of Gaspra. Icarus 107, 84–97. interior, and at least a hint of different mechanical properties Harch, A., and G. A. Heyler 1998. Design and execution of the asteroid at the two ends (Belton et al. 1996, Thomas et al. 1996b). Mathilde flyby. J. Astron. Sci., in press. Mathilde is a monolithic, very underdense body, with a generally Harch, A., D. Dunham, R. Farquhar, J. Veverka, B. Williams, and D. Yeomans spheroidal shape modified by large crater cavities. Clearly, not 1995. Encounter strategy for a flyby of the main-belt asteroid 253 Mathilde. J. Astron. Sci. 43, 399–415. all asteroids are alike, and their continued detailed investigation Hawkins, S. E., E. Darlington, S. Murchie, K. Peacock, T. Harris, C. Hersman, by spacecraft is essential to obtaining a firm understanding of M. Elko, D. Prendergast, B. Ballard, R. Gold, J. Veverka, and M. Robinson their nature and evolution. 1997. Multispectral imager on the Near-Earth Rendezvous Mission. Space Sci. Rev. 82, 31–100. ACKNOWLEDGMENTS Hiroi, T., C. M. Pieters, M. E. Zolensky, and M. E. Lipschutz 1993. Evidence of thermal metamorphism on the C,G,B, and F asteroids. Science 261, This contribution is dedicated to the memory of Dr. Jurgen Rahe, who played 1016–1018. a key role in making the NEAR mission and the exploration of Mathilde a reality. Johnson, T. V., and F. P. 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