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Surface Properties of (6) Hebe: A Possible Parent Body of Ordinary Chondrites
ICARUS
128, 104–113 (1997) IS975679
ARTICLE NO.
Surface Properties of (6) Hebe: A Possible Parent Body of Ordinary Chondrites1 F. MIGLIORINI Armagh Observatory, College Hill Armagh, BT61 9DG Northern Ireland, United Kingdom
A. MANARA Osservatorio Astronomico di Brera, I-20121 Milan (MI) Italy
F. SCALTRITI2 Osservatorio Astronomico di Torino, I-10025 Pino Torinese (TO), Italy
P. FARINELLA Dipartimento di Matematica, Universita` di Pisa, I-56127 Pisa (PI), Italy AND
A. CELLINO AND M. DI MARTINO Osservatorio Astronomico di Torino, 10025 Pino Torinese (TO), Italy E-mail: [email protected] Received March 28, 1996; revised December 9, 1996
We report for the first time rotationally resolved spectroscopic observations, as well as new photometric and polarimetric measurements, of the large S-type asteroid (6) Hebe, always at nearequatorial aspects. We have found evidence for only minor variations in Hebe’s visible reflectance spectrum over a rotational cycle, comparable to our measurement accuracy (a few percent) and consistent with an undifferentiated silicate assemblage on the surface. We have also confirmed previous results of the existence of small polarization changes, but they are not correlated with the (complex and asymmetric) photometric lightcurve of the asteroid. A plausible interpretation of these data is that Hebe’s surface is composed of undifferentiated materials, but presents texture, albedo, and spectral changes possibly related to the occurrence of energetic cratering events. Given the comparatively large size of Hebe, its proximity to resonance-related chaotic zones of the orbital element space (Farinella et al. 1993, Celest. Mech. 56, 287–305, and Icarus 101, 174–187), and its assignment by Gaffey et al. (1993, Icarus 106, 573–602) to the S(IV)
1 Partially based on observations carried out at the European Southern Observatory, La Silla, Chile 2 Visiting Astronomer, Complejo Astronomico El Leoncito operated under agreement between the Consejo National de Investigaciones Cientificas y Tecnicas de la Republica Argentina and the National Universities of La Plata, Cordoba and San Juan.
104 0019-1035/97 $25.00 Copyright 1997 by Academic Press All rights of reproduction in any form reserved.
taxonomic subclass, our results support the idea that this asteroid may be the source of a significant fraction of the ordinary chondrite meteorites. 1997 Academic Press
1. INTRODUCTION
(6) Hebe is a very interesting asteroid from the point of view of meteoritical science. With a size of about 200 km, it is one of the largest S-type asteroids, and belongs to the S(IV) subclass as defined by Gaffey et al. (1993). This subclass is characterized by reflectance spectra indicating a silicate surface material consistent with undifferentiated ordinary chondritic assemblages (although not excluding anomalous stony-irons and some primitive achondrite meteorites). It accounts for P30% of all S-type asteroids and has a significant concentration in the semimajor axis range between 2.3 and 2.6 AU, where the n6 (secular) and 3:1 (mean motion) resonances provide efficient transport routes from the main asteroid belt to the Earth (Farinella et al. 1994). Hebe’s orbital elements (semimajor axis 2.42 AU, eccentricity 0.20, inclination 158) place it not far from the 3:1 Kirkwood gap and very close to the n6 resonance. As a consequence, fragments ejected from it can easily reach
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either resonance, and over a 106 yr time scale their eccentricities can be pumped up to P0.6, corresponding to Earthcrossing orbits. Thus, owing to the combination of large size and favorable location in orbital element space, Hebe alone may account for a significant fraction of the overall meteorite flux onto the Earth (Farinella et al. 1993a,b, Morbidelli et al. 1994). An interesting possibility is that Hebe could be the source of one of the three main groups of ordinary chondrites (OCs), which together account for P80% of all meteorite falls and are believed to come from a small number of fairly large parent asteroids (Pellas and Fie´ni 1988, Lipschutz et al. 1989, Haack et al. 1996). What else do we know about Hebe’s physical properties? The spin period is 7.2745 hr (Lagerkvist et al. 1987), with an uncertainty of 60.00025 hr according to Gehrels and Taylor (1977). The period is close to the average value for large asteroids (Binzel et al. 1989), but the lightcurve morphology is complex, highly dependent upon aspect and phase, and has a comparatively small maximum amplitude (P0.2 mag). The asymmetric extrema suggest the presence of some albedo variegation on the surface, consistent with inferences from polarimetry data (Broglia et al. 1994). On the other hand, as shown by Cellino et al. (1989), shape effects alone can also cause complex photometric behavior, and indeed occultation data (see Magnusson 1986, p. 27) have indicated that a substantial part of Hebe’s lightcurve amplitude is due to shape effects (it is also intriguing to recall that a possible secondary occultation by a small satellite has been reported, see Dunham and Maley 1977). Critical to understanding Hebe’s properties are spectroscopic data: substantial spectral variations for different rotational phases would provide strong evidence for largescale chemical/mineralogical heterogeneity of the surface. In such a case, unless specific correlations are present (related to the functional relationships between mineral abundances and compositions in chondritic assemblages), the most plausible explanation would be that during its history the asteroid has experienced intense thermal processes, which have produced differentiated assemblages and surface mineralogical heterogeneity (Gaffey 1984). This may be due to ancient magmatic activity on the current surface, or to the exposure of different original stratigraphic units within a larger, differentiated precursor body disrupted by a catastrophic collision; there is also the possibility that some S-type asteroids might be partially differentiated, or only regionally melted (Taylor 1992). The first explanation probably applies to (4) Vesta, the second one to the two large S-type asteroids (8) Flora and (15) Eunomia (Gaffey 1984, Gaffey and Ostro 1987, Gaffey et al. 1989). Of course, all of these interpretations would not be consistent with an ordinary chondritic composition and therefore would rule Hebe out as a plausible OC parent body candidate. The available spectral data of Hebe so far have been quite limited, and their interpretation has been uncertain.
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While Wamsteker and Sather (1974) observed no significant color variation, Chapman et al. (1973) reported a prominent (B–V) color change (P0.05 mag) during a rotational cycle, and Gehrels and Taylor (1977) found a smaller (U–V) change (P0.02 mag), which they interpreted as due to the presence of reddened regions on the surface. However, these data do not rule out an undifferentiated composition for Hebe, because, even if real, such color variations might well be related to a combination of space weathering and impact effects, as recently inferred from the Galileo images of (243) Ida (Geissler et al. 1996, Chapman 1996), spectral observations of near-Earth asteroids (Binzel et al. 1996) and laboratory experiments on regolith processes (Moroz et al. 1996). In a recent abstract, Gaffey (1996) has reported that his own 1978–1979 and 1989 visible and infrared spectroscopic observations of Hebe confirmed the existence of small rotational spectral variations, but that they are consistent with an undifferentiated silicate assemblage on the surface. This conclusion is at odds with earlier ones of other researchers (e.g., Fanale et al. 1992), who argued that S-type spectra in general cannot be reconciled with those of OCs. Polarimetric changes over the rotational cycle have been also observed (Broglia et al. 1994). However, they are not necessarily diagnostic of a mineralogic surface heterogeneity, but could be caused by texture and/or albedo changes on a mineralogically homogeneous surface (Dollfus et al. 1989). Finally, we recall that the radar albedo of Hebe, as reported by Ostro et al. (1991, Table 2 and Fig. 4), is quite typical for large main-belt asteroids and does not suggest a high abundance of metals on its surface. Given the potential importance of Hebe as an OC parent body and the ongoing debate on this issue, we were motivated to carry out a dedicated campaign of spectroscopic observations of Hebe, in order to assess in some detail whether and/or which spectral changes appear for different rotational phases and whether Hebe may stand as a plausible OC parent body. We have also performed new photometric and polarimetric measurements of this asteroid, always at epochs when it was visible at a near-equatorial aspect. The remainder of this paper is organized as follows: in Section 2 we will report on our spectroscopic and photopolarimetric observation techniques. Seection 3 will be devoted to the description of the results and the interpretations of these observations. Finally, in Section 4 we will discuss the implications of these data for Hebe’s putative genetic relationship with OCs. 2. OBSERVATIONS AND DATA REDUCTION
2.1. Viewing Geometry and Rotational Phasing For practical reasons, spectroscopic and photopolarimetric observations were carried out during different observation runs, as described below. Since our goal was to
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detect possible spatial mineralogic variations, an essential requirement was a near-equatorial view of the asteroid during all the observations. From the pole coordinates of (6) Hebe reported by Magnusson (1989), the aspect angle (between the rotation axis and the direction of the Earth) at the times of the observations can be derived easily. Taking into account the existing uncertainty in the polar coordinates, for the spectroscopic run we have found that the aspect angle was in the interval between about 688 and 848; for the photopolarimetric observations, the corresponding interval was from about 798 to 898. Thus the viewing geometry of Hebe has always been favorable to detect possible changes phased with the rotational cycle, and the similar aspect angles at the epochs of different sets of observations imply that there was no large difference in surface covering and make it unlikely that some surface feature has shown up in one set and not in the other. The uncertainty in Hebe’s spin period, as quoted in Section 1, translates into an uncertainty of about 20 min (0.05 cycles) in the relative rotational phasing over the 15 months interval between the photopolarimetric and the spectroscopic observations. Thus we have been able to plot our data using, to a good accuracy, self-consistent relative phases.
TABLE I Spectroscopic Observations of (6) Hebe. The Rotational Phase Has Been Estimated From a Period of 7h.274 (Lagerkvist et al. 1987)
2.2. Spectroscopy Observations were performed at ESO-La Silla on September 14–18, 1995, with the 1.52-m spectroscopic telescope equipped with a Boller and Chivens instrument. The CCD had 2048 3 2048 pixels, windowed at about 300 3 2048. The grating used was ruled at 225 gr/mm, so the ˚ /mm. The dispersion in the first order was of about 330 A CCD had a 15-em square pixel, yielding a dispersion of ˚ /pixel in the wavelength direction. The useful spectral 4.7 A ˚ , the instrumental range was from about 4800 to 9400 A ˚ FWHM being 9.8 A. Unfortunately, at the long wavelength ˚ ), a strong atmospheric absorption due end (beyond 9000 A 2 to OH degraded the signal-to-noise ratio of the images, making the spectrum of poorer quality. This is annoying, since in this spectral region variations due to mineralogical heterogeneity are likely to show up. The slit was 2 to 5 arcsec wide, aligned in the East–West direction. The slit width was much larger than the seeing conditions (typically about 1 arcsec), in order to minimize the possibility of losing the signal due to errors in pointing and guiding the target. An optical camera was used for target acquisition and guiding. The asteroid was very bright (mv P 8.5) and as a consequence it was quite difficult to center the asteroid image in the slit, due to the large amount of scattered light, resulting into a sizeable image in the camera screen. The target was confirmed by noting its motion relative to fixed stars. The observational circumstances are listed in detail in Table I.
In addition to the rotational spectra of Hebe, solar analog stars HD 20630, HD 1835, HD 144585, and 64 Hyades, flat fields, bias images, and lamp spectra (He–Ar) were taken during each night using the same instrumentation setup. Spectra were reduced with the Image Reduction and Analysis Facility (IRAF) package, installed on a SUNOS workstation at the Brera Observatory, according to the standard procedure described in Di Martino et al. (1995). This includes subtraction of the bias level, flattening
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of the data, elimination of the cosmic rays, subtraction of the sky, wavelength two-dimensional calibration, transformation from a two-dimensional spectrum to a uni-dimensional one, extinction correction, and division by the solar analog (Hardorp 1978). Finally, each spectrum was normal˚. ized to the asteroid/star flux at 7000 A 2.3. Photopolarimetry Simultaneous photometric and polarimetric observations of 6 Hebe were carried out on three consecutive nights (June 5 to 7, 1994). We used the Torino photopolarimeter attached at the 2.15-m telescope of Complejo Astronomico El Leoncito (Argentina). A full description of the instrument functioning can be found in Piirola (1988) and Scaltriti et al. (1989). Here we recall that the photopolarimeter allows the observer to perform linear and circular polarization measurements (by means of l/2 and l/4 plates), simultaneously in the five bands UBVRI. The ordinary and extraordinary beams produced by a calcite plate are alternately recorded by means of a rotating chopper. Dichroic filters, which reflect a desired spectral interval and transmit the other wavelengths, select the incoming beam according to the standard color bands UBVRI. In the dewar, cooled down to 2308C, five separate photomultipliers (three HAMAMATSU-type, for VRI, and two EMI-type, for UB) allow one to gather the signal of the object separately. The principle of operation of the photopolarimeter is that the polarization due to the sky background is directly eliminated; this has been found to be especially valuable in the observation of faint stars and when there is moonlight. In principle, if a considerable amount of background radiation is composed of scattered light, it can be highly polarized and small relative changes in the sky background could give rise to large errors in the measured polarization. Our photopolarimeter was especially designed to minimize this undesirable effect. The counts obtained by the equipment allow the observer to get the polarization parameters (amount of polarization, P, and position angle, PA) and the photometry of the object, in the five bands UBVRI. For both photometric and polarimetric curves, phases were computed according to the reference epoch (corresponding to the epoch of our first observation) JD 2449509.45, assuming a period of 7h.2745. As explained in Section 2.1, this was consistent with the phasing of our spectroscopic observations, as reported below. Unfortunately, the sky conditions during the three observing nights were not always good, so that we were not able to get a full-cycle coverage of the photometric lightcurve. However, the operational principle of the equipment allows one to obtain fairly good-quality polarimetric curves even when veiling or spread clouds are pres-
FIG 1. Averaged spectrum of Hebe, obtained by combining many different spectra calibrated with different solar analogs, and normalized ˚ . It matches very well the spectrum published by Xu et al. to 7000 A ˚ ) correspond (1995). The dots with 2-s error bars (normalized to 5500 A to data taken in the framework of the eight-color survey (ECAS) of Zellner et al. (1985).
ent in the sky. Thus, the overall quality of the polarimetric and photometric data reported below was not seriously degraded by the sky conditions, thanks also to the brightness of the object. A technical problem on the power supply of channel R prevented us from obtaining data at that wavelength. Therefore, we got results in the UBVI bands only. RESULTS
3.1. Spectroscopy In order to obtain a single low-noise reflectance spectrum of Hebe (Fig. 1), we have taken the average of all of our spectra. The procedure of adopting four different solar analog stars (see Section 2.2) allowed us to reduce the possible errors due to a calibration based on just one solar analog. Morover, spectra were taken on six different nights, minimizing uncertainties in the data due to variable sky conditions.
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spectra shown in Fig. 2 deviate in a consistent fashion above or below the averaged spectrum of Fig. 1 over a significant range of wavelengths; this is confirmed by the ratioed spectra plotted in Fig. 3. For example, the intensity of the 0.9 em feature (in the P0.8 to 0.92 em wavelength interval) is about 3% stronger than the average at phase 0.8, slightly less strong at phase 0.7, and about 2% weaker at phases 0.2 and 0.3 (that is, on the other side of the asteroid), whereas intermediate phases show ratios closer to 1. On the other hand, the small-wavelength portions of the spectra (between 0.5 and 0.7 em) indicate that Hebe is somewhat more blue than average at phases 0.8–0.9, and more red at phases 0.2–0.4. These changes are of the same order of magnitude as the U–V color variations reported by Gehrels and Taylor (1977, Fig. 28) and appear to have also the same rotational phasing, based on the comparison of lightcurves. These small spectral variations can be intepreted as due to different degrees of space weathering on Hebe’s surface, possibly related to largescale ejecta blankets resulting from relatively recent and energetic cratering events, similar to those observed in the
FIG 2. Rotationally resolved spectra of Hebe. Spectra obtained at different rotational phases were binned at intervals of 0.1 cycles (368) and then averaged and vertically offset for clarity. One step along the vertical scale corresponds to a 10% change (all the spectra are normalized ˚ ). The dashed curves correspond to the average spectrum of to 7000 A Fig. 1, after filtering out the small-scale wiggles due to observational noise.
The averaged reflectance spectrum shown in Fig. 1 can be compared with those available in the published literature. In particular we have compared our data to those obtained in the framework of the ECAS survey (Zellner et al. 1985) and to the spectrum published by Xu et al. (1995). In both cases, the agreement is very good (see Fig. 1 for the comparison with the ECAS data points). Our spectral range does not allow us to detect all the features typical of the S taxonomic type (e.g., the band at 2.2 em); on the other hand, the position of the olivine–pyroxene ˚ closely matches that corresponding to band near 9000 A the S(IV) subclass, as defined by Gaffey et al. (1994). In order to look for rotational spectral variations, we grouped our data into bins of 0.1 width in rotational phase and averaged the spectra available in each bin. As shown in Figs. 2 and 3, no differences exceeding a few percent have been detected, comparable to the noise in the data, and therefore no evidence is present for a large-scale spectral heterogeneity of Hebe’s surface. However, several
FIG 3. Ratios between the averaged spectrum of Hebe (Fig. 1) and the spectra taken at different rotational phases (Fig. 2), as specified by the labels on the right side. One step along the vertical scale corresponds to a 10% change.
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variations have a relatively small amplitude, but a complex behavior. As remarked in Section 1, such behavior in principle may be associated both with a globally irregular (nonellipsoidal) shape and with large-scale albedo variegation of the surface (related for instance to impact shock-darkening processes of chondritic materials, as suggested by Britt and Pieters 1991). Moreover, the different shapes and amplitudes of the lightcurves at different wavelengths suggest the existence of color variations, in agreement with the conclusions reported in Section 3.1. Note that for most known processes occurring on asteroid surfaces (grain size alteration, melting and crystallization), albedo variations are accompanied by spectral changes (Fanale et al. 1992, Moroz et al. 1996). 3.3. Polarimetry
FIG 4. Photometric lightcurves of Hebe in U, B, V, and I colors. The data were collected during three different nights, on June 5, 6, and 7, 1994, at El Leoncito Observatory (Argentina).
Galileo spacecraft images of (243) Ida (see discussion in Chapman 1996) or to subtle mineralogical variations, consistent with an undifferentiated silicate assemblage (as concluded by Gaffey 1996). In either case, our spectral data are consistent with a genetic relationship between Hebe and OCs. 3.2. Photometry Our lightcurves are shown in Fig. 4; most but not all of the rotational cycle was covered. The general trend is in agreement with previous results (e.g., Lagerkvist et al. 1987). In spite of the partial coverage of the lightcurve, three maxima (at P0.1, 0.3, and 0.6 phases) and three minima (P0.18, 0.45, and 0.9 phases) can be distinguished in it. Owing to the unfavorable sky conditions, we were not able to perform a transformation to the standard UBVRI system; the Dm’s given in Fig. 4 refer to a frequently observed nearby star which differed from night to night. As for the accuracy of these observations, typical errors are of the order of 0.02–0.03 mag (but twice as large for the rotational phases between 0.8 and 1.0, covered during the worst night). These (limited) data confirm that Hebe’s photometric
It is well known that asteroids, like other atmosphereless Solar System bodies, show a well-defined variation of polarization as a function of the Sun–asteroid–Earth phase angle a; the corresponding curve is often used to infer the albedo (see, e.g., Lupishko and Mohamed 1996). In particular, polarization is usually expressed through the Pr parameter, that is the ratio (I' 2 Ii)/(I' 1 Ii), I' and Ii being the intensities of the reflected light polarized perpendicular and parallel to the Earth–Sun–scattering plane, respectively. The relationship between the coefficient of linear polarization P (which is the quantity directly obtained from the observations) and Pr is simply Pr 5 P cos (2u), where u is the angle between the position angle (PA) of the observed polarized radiation and the plane perpendicular to the plane of scattering. Figure 5 shows the coefficient of linear polarization P (in percent) and the corresponding polarization angle PA plotted versus rotational phase. In making these plots, polarimetric data were grouped in bins of 0.1 in rotational phase and averaged in each bin. A simple first-order Fourier fitting of the P vs rotational phase curve is also shown and gives a satisfactory fit to the data. The amplitude of the P curves decreases from the U to the I band. Especially in the U, B, and V bands, the positions of the extrema of the curve are quite close to each other. On the other hand, except for the U band, the PA curves do not show any well-defined pattern, and there is no clear correlation between the trends observed in different colors. In the U band, where a definite trend is visible, the positions of the maximum and the minimum do not coincide with the corresponding positions in the P curve. All these findings are consistent with previous observations of Broglia et al. (1994). It is interesting to note that the trends apparent in the polarimetric and photometric lightcurves are different, and there is no clear correlation between them. We will discuss the possible interpretation of these observations below.
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FIG 5. Polarimetric lightcurves of Hebe in U, B, V, I colors. Left, the coefficient of linear polarization P (in percent) vs rotational phase, and a first-order Fourier fit. Right, the PA polarization angle (in degrees) vs rotational phase. Data collected at El Leoncito Observatory (Argentina) on June 5, 6, and 7, 1994.
As for Pr vs a curve, we recall that for asteroids it always shows a characteristic behavior, with the presence of a negative branch of polarization, whose origin has not yet been fully understood (see Muinonen 1994 and references therein). Figure 6 shows our observed values of Pr vs a (our observations were performed at a 5 128.5), together with other Pr measurements reported in the literature for Hebe (Zellner and Gradie 1976). In this figure, nightly averages of Pr for each of our three observing nights have been plotted for each color with different symbols. Our values agree very well with the general trend indicated by previous observations. This can be considered as further confirmation of the accuracy of our polarimetric measurements. We note that the lack of a correlation between the polarimetric and photometric lightcurves as shown in Figs. 4 and 5 is quite unusual for asteroids. For instance, in the wellknown case of (4) Vesta, a clear correlation between the photometric and polarimetric behaviors is present and has been used to unequivocally derive the correct rotation period of the asteroid (see e.g., Dollfus et al. 1989 and references therein). Such a correlation has been generally explained as diagnostic of large-scale albedo variegation of the surface. In particular, since Vesta’s polarimetric
variation is roughly sinusoidal over a rotational cycle, the presence of a hemispheric albedo asymmetry has been proposed as a plausible explanation for both the polarimetric and the photometric lightcurves and has been used to derive the orientation of the spin axis as well as the polar flattening of this sizeable asteroid (Cellino et al. 1987). In the case of Hebe the polarimetric behavior is generally similar to that of Vesta, but the photometric lightcurve is much more peculiar. There are two possible explanations for Hebe’s behavior. First, we may interpret the lack of correlation between photometric and polarimetric data as suggesting that the polarimetric variation in this case is due to large-scale changes in the surface texture, rather than to significant albedo features. We stress that a detailed theoretical understanding of the relationship between polarimetric behavior and microscopic surface texture is still lacking for atmosphereless solar system bodies, so for the time being we cannot test this possibility in a quantitative way. Alternatively, large-scale albedo patterns resulting in significant polarization changes may be present, without playing an important role in molding the lightcurve morphology, in which global shape effects would be predominant. Although Hebe’s size (almost 200 km in diameter) implies that it is not likely to be an irregularly shaped fragment from a larger parent asteroid (as is the case for most smaller asteroids, see Catullo et al. 1984, Davis et al. 1989), its relatively small lightcurve amplitude does not require large deviations from an axisymmetric shape, especially if sur-
FIG 6. Nightly averages of Pr vs Sun-asteroid-Earth phase angle for our UBVI polarimetric observations of Hebe (closed symbols). Open symbols correspond to the data reported by Zellner and Gradie (1976).
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face features such as craters or ridges produce complex (and variable) shadowing effects. Despite these interpretation problems, our work shows that in the absence of close-up spacecraft observations, unravelling the main physical and mineralogical properties of asteroid surfaces requires the simultaneous consideration of data from several different observing techniques. Rotationally resolved data are especially important when the detection of strong, large-scale heterogeneity is a crucial test for inferences about the asteroid’s thermal history. In the case of Hebe, we have not carried out rotationally resolved spectral measurements at infrared wavelengths, which could have provided additional constraints on the nature of its surface variegation. The recently reported observational results of Gaffey (1996) appear to confirm the main results of our work. 4. CONCLUSIONS
The main finding from this work is that Hebe’s visible reflectance spectrum shows only minor variations over the course of the asteroid’s rotation, and that these variations do not require that differentiated materials are exposed on the surface. In this respect, Hebe appears to be different from other large S-type asteroids such as (8) Flora and (15) Eunomia, whose rotationally resolved spectra show clear variations, which do not appear to be consistent with undifferentiated (ordinary chondritic) assemblages (Gaffey et al. 1989). Although photometric and polarimetric data suggest that the surface of Hebe is optically heterogeneous, probably due to changes in the microscopic surface texture and/or albedo, this may be the outcome of a variety of processes related to impact events. We thus propose our most plausible interpretation of the data: (i) Hebe’s surface is composed of an undifferentiated mineral assemblage. (ii) Since its formation, the surface has undergone a number of energetic impact cratering events, causing some changes in its large-scale albedo/polarization properties and possibly minor spectral variations related to the exposure of ‘‘fresh’’ (i.e., unaffected by space weathering processes) subsurface material. According to this interpretation, Hebe can be considered as a plausible parent body for OCs. Following Farinella et al. (1993a), we suggest that a substantial fraction of these meteorites may have been generated by large impact craters on this asteroid. This is consistent with a simple orderof-magnitude estimate on the meteorite delivery efficiency of such events. If we consider craters formed by projectile asteroids .1 km in diameter, the average frequency of these events for a target as large as Hebe can be estimated at one every 2 3 107 yr (assuming an intrinsic collision probability of 2.8 3 10218 km22 yr21 and a projectile popula-
tion of about 2 3 106; see Farinella and Davis 1992, 1994). This is comparable with the typical cosmic-ray exposure (CRE) ages of OCs—H chondrites in particular display a strong CRE peak at about 8 Myr (Anders 1964, Marti and Graf 1992), which may well correspond to the last such cratering event. The evidence for different orbital pathways followed by different subgroups of these H chondrites (Graf and Marti 1995) may be explained by the availability of both the n6 and the 3:1 resonant routes near Hebe. Larger impact events have also occurred on Hebe, of course, but may have been separated on average by longer time intervals; since the dynamical/collisional lifetime of fragments injected into the n6 resonance is shorter than 108 yr (Morbidelli et al. 1994, Bottke et al. 1995), it is unlikely that current meteorite falls record such larger and rarer impacts. If we assume that the mass ejected from Hebe’s craters is typically some 900 times that of the projectile, consistent with Holsapple’s (1993) scaling of crater dimensions for rocky surfaces in the gravity regime and a mean impact velocity of 6.6 km/sec (Farinella and Davis 1992), we get an ejecta production rate of P6 3 107 kg/yr. Conservatively assuming that: (i) only P 10% of this material is injected into the resonances and reaches an Earth-crossing orbit (according to the results of Farinella et al. 1993b and Morbidelli et al. 1994); (ii) P10% of the latter material has meteorite-like sizes (smaller than a few meters; a plausible estimate for crater ejecta); and (iii) another factor of P10 is lost during the transfer process due to collisional disruption, impact with other planets and the Sun, or ejection from the solar system (this is consistent with the typical exposure ages of ordinary chondrites, which are about a factor 10 shorter than the Earth-impact lifetimes of Earth-crossing bodies), we eventually get an Earth influx of some 6 3 104 kg/yr. This is about 20% of the overall meteorite influx, as estimated (in order of magnitude) by Halliday et al. (1984) and Wetherill (1985). Of course this kind of comparison has only a semi-quantitative value, but is important to show how different kinds of arguments and data can be linked together in a selfconsistent scenario. Impact cratering models, meteoritic evidence, the observational results reported in this paper, and the facts that Hebe is the largest S(IV) object and lies very close to the resonant chaotic zones (see Section 1) all point to the conclusion that this asteroid should be considered as one of the most promising candidate OC parent bodies and should become the target of further observational efforts. ACKNOWLEDGMENTS We are grateful to R.P. Binzel for making his spectrum of (6) Hebe available to us. Constructive reviews by M. Gaffey and B. Clark have led to substantial improvements with respect to a previous version of the paper. This work has received partial financial support from the Italian
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Space Agency (ASI), the Italian Ministry for University and Scientific Research (MURST), and the European Union Human Capital and Mobility Programme (Contract CHRX-CT94–0445).
REFERENCES ANDERS, J. B. 1964. Origin, age, and composition of meteorites. Space Sci. Rev. 3, 583–714. BINZEL, R. P., S. J. BUS, T. H. BURBINE, AND J. M. SUNSHINE 1996. Spectral properties of near-Earth asteroids: Evidence for sources of ordinary chondrite meteorites. Science 273, 946–948. BINZEL, R. P., P. FARINELLA, V. ZAPPALA`, AND A. CELLINO 1989. Asteroid rotation rates: Distributions and statistics. In Asteroids II (R. P. Binzel, T. Gehrels, and M. S. Matthews, Eds.), pp. 416–441. Univ. of Arizona Press, Tucson. BOTTKE, W. F. JR., M. C. NOLAN, R. GREENBERG, AND R. A. KOLVOORD 1995. Collisional lifetimes and impact statistics of near-Earth asteroids. In Hazards Due to Comets and Asteroids (T. Gehrels, Ed.), pp. 337–357. Univ. of Arizona Press, Tucson. BRITT, D. T., AND C. M. PIETERS 1991. Darkening in gas-rich ordinary chondrites: Spectral modelling and implications for the regoliths of ordinary chondrite parent bodies. Lunar Planet. Sci. Conf. XXII, 141–142. BROGLIA, P., A. MANARA, AND P. FARINELLA 1994. Polarimetric observations of (6) Hebe. Icarus 109, 204–209. CATULLO, V., V. ZAPPALA`, P. FARINELLA, AND P. PAOLICCHI 1984. Analysis of the shape distribution of asteroids. Astron. Astrophys. 138, 464–468. CELLINO, A., V. ZAPPALA`, M. DI MARTINO, P. FARINELLA, AND P. PAOLICCHI 1987. Flattening, pole, and albedo features of 4 Vesta from photometric data. Icarus 70, 546–565. CELLINO, A., V. ZAPPALA`, AND P. FARINELLA 1989. Asteroid shapes and lightcurve morphology. Icarus 78, 298–310. CHAPMAN, C. R. 1996. S-type asteroids, ordinary chondrites, and space weathering: The evidence from Galileo’s fly-bys of Gaspra and Ida. Meteoritics Planet. Sci. 31, 699–726. CHAPMAN, C. R., T. B. MCCORD, AND T. V. JOHNSON 1973. Asteroid spectral reflectivities. Astron. J. 78, 126–140. DAVIS, D. R., S. J. WEIDENSCHILLING, P. FARINELLA, P. PAOLICCHI, AND R. P. BINZEL 1989. Asteroid collisional history: Effects on sizes and spins. In Asteroids II (R. P. Binzel, T. Gehrels, and M. S. Matthews, Eds.), pp. 805–826. Univ. of Arizona Press, Tucson. DI MARTINO M., A. MANARA, AND F. MIGLIORINI 1995. 1993 VW: An ordinary chondrite-like near-Earth asteroid. Astron. Astrophys. 302, 609–612. DOLLFUS, A., M. WOLFF, J. E. GEAKE, D. F. LUPISHKO, AND L. M. DOUGHERTY 1989. Photopolarimetry of Asteroids. In Asteroids II (R. P. Binzel, T. Gehrels, and M. S. Matthews, Eds.), pp. 594–616. Univ. of Arizona Press, Tucson. DUNHAM, D. W., AND P. D. MALEY 1977. Possible observation of a satellite of a minor planet. Occult. Newslett. 1, 115–117. FANALE. F. P., B. E. CLARK, AND J. F. BELL 1992. A spectral analysis of ordinary chondrites, S-type asteroids, and their component minerals: Genetic implications. J. Geophys. Res. 97, 20,863–20,874. FARINELLA, P., AND D. R. DAVIS 1992. Collision rates and impact velocities in the main asteroid belt. Icarus 97, 111–123. FARINELLA, P., AND D. R. DAVIS 1994. Will the real asteroid size distribution please step forward? Lunar Planet. Sci. Conf. XXV, 365–366. FARINELLA, P., CH. FROESCHLE´, AND R. GONCZI 1993a. Meteorites from the asteroid 6 Hebe. Celest. Mech. 56, 287–305.
FARINELLA, P., C. FROESCHLE´, AND R. GONCZI 1994. Meteorite delivery and transport. In Asteroids Comets Meteors 1993 (A. Milani, M. Di Martino, and A. Cellino, Eds.), pp. 205–222. Kluwer, Dordrecht. FARINELLA, P., R. GONCZI, CH. FROESCHLE´, AND C. FROESCHLE´ 1993b. The injection of asteroid fragments into resonances. Icarus 101, 174–187. GAFFEY, M. J. 1984. Rotational spectral variations of asteroid (8) Flora: Implications for the nature of the S-type asteroids and for the parent bodies of the ordinary chondrites. Icarus 60, 83–114. GAFFEY, M. J. 1996. Asteroid 6 Hebe: Spectral evaluation of the prime large mainbelt ordinary chondrite parent body candidate, with implications from space weathering of Gaspra and the Ida-Dactyl system. Lunar Planet Sci. XXVII, 391–392. GAFFEY, M. J., AND S. J. OSTRO 1987. Surface lithologic heterogeneity and body shape for asteroid (15) Eunomia: Evidence from rotational spectral variations and multi-color lightcurve inversions. Lunar Planet Sci. XVIII, 310–311. GAFFEY, M. J., J. F. BELL, AND D. P. CRUIKSHANK 1989. Reflectance spectroscopy and asteroid surface mineralogy. In Asteroids II (R. P. Binzel, T. Gehrels, and M. S. Matthews, Eds.), pp. 98–127. Univ. of Arizona Press, Tucson. GAFFEY, M. J., J. F. BELL, R. HAMILTON BROWN, T. H. BURBINE, J. L. PIATEK, K. L. REED, AND D. A. CHAKY 1993. Mineralogical variations within the S-type asteroid class. Icarus 106, 573–602. GEHRELS, T., AND R. C. TAYLOR 1977. Minor planets and related objects. XXII. Phase functions for (6) Hebe. Astron. J. 82, 229–237. GEISSLER, P., J.-M. PETIT, D. DURDA, R. GREENBERG, W. BOTTKE, AND M. NOLAN 1996. Erosion and ejecta reaccretion on 243 Ida and its moon. Icarus 120, 140–157. GRAF, T., AND K. MARTI 1995. Collisional history of H chondrites. J. Geophys. Res. 100, 21,247–21,263. HAACK, H., P. FARINELLA, E. R. D. SCOTT, AND K. KEIL 1996. Meteoritic, asteroidal, and theoretical constraints on the 500 Ma disruption of the L chondrite parent body. Icarus 119, 182–191. HALLIDAY, I., A. T. BLACKWELL, AND A. A. GRIFFIN 1984. The frequency of meteorite falls on the Earth. Science 223, 1405–1407. HARDORP, J. 1978. The Sun among the Stars. I. A search for solar spectral analogs. Astron. Astrophys. 63, 383–390. HOLSAPPLE, K. 1993. The scaling of impact processes in planetary sciences. Ann. Rev. Earth Planet. Sci. 21, 333–373. LAGERKVIST, C.-I., M. A. BARUCCI, M. T. CAPRIA, M. FULCHIGNONI, L. GUERRIERO, E. PEROZZI, AND V. ZAPPALA` 1987. Asteroid Photometric Catalogue. Ed. Consiglio Nazionale delle Ricerche, Rome. LIPSCHUTZ, M. E., M. J. GAFFEY, AND P. PELLAS 1989. Meteoritic parent bodies: Nature, number, size and relation to present-day asteroids. In Asteroids II (R. P. Binzel, T. Gehrels, and M. S. Matthews, Eds.), pp. 740–777. Univ. of Arizona Press, Tucson. LUPISHKO, D. F., AND R. A. MOHAMED 1996. A new calibration of the polarimetric albedo scale of asteroids. Icarus 119, 209–213. MAGNUSSON, P. 1986. Distribution of spin axes and senses of rotation for 20 large asteroids. Icarus 68, 1–39. MAGNUSSON, P. 1989. Pole determinations of asteroids. In Asteroids II (R. P. Binzel, T. Gehrels, and M. S. Matthews, Eds.), pp. 1180–1190. Univ. of Arizona Press, Tucson. MARTI, K., AND T. GRAF 1992. Cosmic-ray exposure history of ordinary chondrites. Annu. Rev. Earth Planet. Sci. 20, 221–243. MORBIDELLI, A., R. GONCZI, CH. FROESCHLE´, AND P. FARINELLA 1994. Delivery of meteorites through the n6 secular resonance. Astron. Astrophys. 282, 955–979.
SURFACE PROPERTIES OF (6) HEBE MOROZ, L. V., A. V. FISENKO, L. F. SEMJONOVA, C. M. PIETERS, AND N. N. KOROTAEVA 1996. Optical effects of regolith processes on S-asteroids as simulated by laser shots on ordinary chondrite and other mafic materials. Icarus 122, 366–382. MUINONEN, K. 1994. Coherent backscattering by Solar System dust particles. In Asteroids Comets Meteors 1993 (A. Milani, M. Di Martino, and A. Cellino, Eds.), pp. 271–296, Kluwer, Dordrecht. OSTRO, S. J., D. B. CAMPBELL, J. F. CHANDLER, A. A. HINE, R. S. HUDSON, K. D. ROSEMA, AND I. I. SHAPIRO 1991. Asteroid 1986 DA: Radar evidence for a metallic composition. Science 252, 1399–1404. PELLAS, P., AND C. FIE´NI 1988. Thermal histories of ordinary chondrite parent asteroids. Lunar Planet. Sci. Conf. XIX, 915–916. PIIROLA, V. 1988. Simultaneous five-colour (UBVRI) photopolarimeter. In Polarized Radiation of Circumstellar Origin (G. V. Coyne et al., Eds.), pp. 735–746. Univ. of Arizona Press, Tucson. SCALTRITI, F., V. PIIROLA, A. CELLINO, E. ANDERLUCCI, L. CORCIONE, G. MASSONE, F. RACIOPPI, AND F. PORCU 1989. The UBVRI photopo-
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larimeter of the Torino Astronomical Observatory. Mem. Soc. Astron. Ital. 60, 243–246. TAYLOR, G. J. 1992. Core formation in asteroids. J. Geophys. Res. 97, 14,717–14,726. WAMSTEKER, W., AND R. E. SATHER 1974. Minor planets and related objects. XVII. Five-color photometry of four asteroids. Astron. J. 79, 1465–1470. WETHERILL, G. W. 1985. Asteroidal source of ordinary chondrites. Meteoritics 20, 1–21. XU, S., R. P. BINZEL, T. H. BURBINE, AND S. J. BUS 1995. Small mainbelt asteroid spectroscopic survey: Initial results. Icarus 115, 1–35. ZELLNER, B., AND J. GRADIE 1976. Minor planets and related objects. XX. Polarimetric evidence for the albedos and compositions of 94 asteroids. Astron. J. 81, 262–280. ZELLNER, B., D. J. THOLEN, AND E. F. TEDESCO 1985. The eightcolor asteroid survey: Results for 589 minor planets. Icarus 61, 355–416.
128, 104–113 (1997) IS975679
ARTICLE NO.
Surface Properties of (6) Hebe: A Possible Parent Body of Ordinary Chondrites1 F. MIGLIORINI Armagh Observatory, College Hill Armagh, BT61 9DG Northern Ireland, United Kingdom
A. MANARA Osservatorio Astronomico di Brera, I-20121 Milan (MI) Italy
F. SCALTRITI2 Osservatorio Astronomico di Torino, I-10025 Pino Torinese (TO), Italy
P. FARINELLA Dipartimento di Matematica, Universita` di Pisa, I-56127 Pisa (PI), Italy AND
A. CELLINO AND M. DI MARTINO Osservatorio Astronomico di Torino, 10025 Pino Torinese (TO), Italy E-mail: [email protected] Received March 28, 1996; revised December 9, 1996
We report for the first time rotationally resolved spectroscopic observations, as well as new photometric and polarimetric measurements, of the large S-type asteroid (6) Hebe, always at nearequatorial aspects. We have found evidence for only minor variations in Hebe’s visible reflectance spectrum over a rotational cycle, comparable to our measurement accuracy (a few percent) and consistent with an undifferentiated silicate assemblage on the surface. We have also confirmed previous results of the existence of small polarization changes, but they are not correlated with the (complex and asymmetric) photometric lightcurve of the asteroid. A plausible interpretation of these data is that Hebe’s surface is composed of undifferentiated materials, but presents texture, albedo, and spectral changes possibly related to the occurrence of energetic cratering events. Given the comparatively large size of Hebe, its proximity to resonance-related chaotic zones of the orbital element space (Farinella et al. 1993, Celest. Mech. 56, 287–305, and Icarus 101, 174–187), and its assignment by Gaffey et al. (1993, Icarus 106, 573–602) to the S(IV)
1 Partially based on observations carried out at the European Southern Observatory, La Silla, Chile 2 Visiting Astronomer, Complejo Astronomico El Leoncito operated under agreement between the Consejo National de Investigaciones Cientificas y Tecnicas de la Republica Argentina and the National Universities of La Plata, Cordoba and San Juan.
104 0019-1035/97 $25.00 Copyright 1997 by Academic Press All rights of reproduction in any form reserved.
taxonomic subclass, our results support the idea that this asteroid may be the source of a significant fraction of the ordinary chondrite meteorites. 1997 Academic Press
1. INTRODUCTION
(6) Hebe is a very interesting asteroid from the point of view of meteoritical science. With a size of about 200 km, it is one of the largest S-type asteroids, and belongs to the S(IV) subclass as defined by Gaffey et al. (1993). This subclass is characterized by reflectance spectra indicating a silicate surface material consistent with undifferentiated ordinary chondritic assemblages (although not excluding anomalous stony-irons and some primitive achondrite meteorites). It accounts for P30% of all S-type asteroids and has a significant concentration in the semimajor axis range between 2.3 and 2.6 AU, where the n6 (secular) and 3:1 (mean motion) resonances provide efficient transport routes from the main asteroid belt to the Earth (Farinella et al. 1994). Hebe’s orbital elements (semimajor axis 2.42 AU, eccentricity 0.20, inclination 158) place it not far from the 3:1 Kirkwood gap and very close to the n6 resonance. As a consequence, fragments ejected from it can easily reach
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either resonance, and over a 106 yr time scale their eccentricities can be pumped up to P0.6, corresponding to Earthcrossing orbits. Thus, owing to the combination of large size and favorable location in orbital element space, Hebe alone may account for a significant fraction of the overall meteorite flux onto the Earth (Farinella et al. 1993a,b, Morbidelli et al. 1994). An interesting possibility is that Hebe could be the source of one of the three main groups of ordinary chondrites (OCs), which together account for P80% of all meteorite falls and are believed to come from a small number of fairly large parent asteroids (Pellas and Fie´ni 1988, Lipschutz et al. 1989, Haack et al. 1996). What else do we know about Hebe’s physical properties? The spin period is 7.2745 hr (Lagerkvist et al. 1987), with an uncertainty of 60.00025 hr according to Gehrels and Taylor (1977). The period is close to the average value for large asteroids (Binzel et al. 1989), but the lightcurve morphology is complex, highly dependent upon aspect and phase, and has a comparatively small maximum amplitude (P0.2 mag). The asymmetric extrema suggest the presence of some albedo variegation on the surface, consistent with inferences from polarimetry data (Broglia et al. 1994). On the other hand, as shown by Cellino et al. (1989), shape effects alone can also cause complex photometric behavior, and indeed occultation data (see Magnusson 1986, p. 27) have indicated that a substantial part of Hebe’s lightcurve amplitude is due to shape effects (it is also intriguing to recall that a possible secondary occultation by a small satellite has been reported, see Dunham and Maley 1977). Critical to understanding Hebe’s properties are spectroscopic data: substantial spectral variations for different rotational phases would provide strong evidence for largescale chemical/mineralogical heterogeneity of the surface. In such a case, unless specific correlations are present (related to the functional relationships between mineral abundances and compositions in chondritic assemblages), the most plausible explanation would be that during its history the asteroid has experienced intense thermal processes, which have produced differentiated assemblages and surface mineralogical heterogeneity (Gaffey 1984). This may be due to ancient magmatic activity on the current surface, or to the exposure of different original stratigraphic units within a larger, differentiated precursor body disrupted by a catastrophic collision; there is also the possibility that some S-type asteroids might be partially differentiated, or only regionally melted (Taylor 1992). The first explanation probably applies to (4) Vesta, the second one to the two large S-type asteroids (8) Flora and (15) Eunomia (Gaffey 1984, Gaffey and Ostro 1987, Gaffey et al. 1989). Of course, all of these interpretations would not be consistent with an ordinary chondritic composition and therefore would rule Hebe out as a plausible OC parent body candidate. The available spectral data of Hebe so far have been quite limited, and their interpretation has been uncertain.
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While Wamsteker and Sather (1974) observed no significant color variation, Chapman et al. (1973) reported a prominent (B–V) color change (P0.05 mag) during a rotational cycle, and Gehrels and Taylor (1977) found a smaller (U–V) change (P0.02 mag), which they interpreted as due to the presence of reddened regions on the surface. However, these data do not rule out an undifferentiated composition for Hebe, because, even if real, such color variations might well be related to a combination of space weathering and impact effects, as recently inferred from the Galileo images of (243) Ida (Geissler et al. 1996, Chapman 1996), spectral observations of near-Earth asteroids (Binzel et al. 1996) and laboratory experiments on regolith processes (Moroz et al. 1996). In a recent abstract, Gaffey (1996) has reported that his own 1978–1979 and 1989 visible and infrared spectroscopic observations of Hebe confirmed the existence of small rotational spectral variations, but that they are consistent with an undifferentiated silicate assemblage on the surface. This conclusion is at odds with earlier ones of other researchers (e.g., Fanale et al. 1992), who argued that S-type spectra in general cannot be reconciled with those of OCs. Polarimetric changes over the rotational cycle have been also observed (Broglia et al. 1994). However, they are not necessarily diagnostic of a mineralogic surface heterogeneity, but could be caused by texture and/or albedo changes on a mineralogically homogeneous surface (Dollfus et al. 1989). Finally, we recall that the radar albedo of Hebe, as reported by Ostro et al. (1991, Table 2 and Fig. 4), is quite typical for large main-belt asteroids and does not suggest a high abundance of metals on its surface. Given the potential importance of Hebe as an OC parent body and the ongoing debate on this issue, we were motivated to carry out a dedicated campaign of spectroscopic observations of Hebe, in order to assess in some detail whether and/or which spectral changes appear for different rotational phases and whether Hebe may stand as a plausible OC parent body. We have also performed new photometric and polarimetric measurements of this asteroid, always at epochs when it was visible at a near-equatorial aspect. The remainder of this paper is organized as follows: in Section 2 we will report on our spectroscopic and photopolarimetric observation techniques. Seection 3 will be devoted to the description of the results and the interpretations of these observations. Finally, in Section 4 we will discuss the implications of these data for Hebe’s putative genetic relationship with OCs. 2. OBSERVATIONS AND DATA REDUCTION
2.1. Viewing Geometry and Rotational Phasing For practical reasons, spectroscopic and photopolarimetric observations were carried out during different observation runs, as described below. Since our goal was to
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detect possible spatial mineralogic variations, an essential requirement was a near-equatorial view of the asteroid during all the observations. From the pole coordinates of (6) Hebe reported by Magnusson (1989), the aspect angle (between the rotation axis and the direction of the Earth) at the times of the observations can be derived easily. Taking into account the existing uncertainty in the polar coordinates, for the spectroscopic run we have found that the aspect angle was in the interval between about 688 and 848; for the photopolarimetric observations, the corresponding interval was from about 798 to 898. Thus the viewing geometry of Hebe has always been favorable to detect possible changes phased with the rotational cycle, and the similar aspect angles at the epochs of different sets of observations imply that there was no large difference in surface covering and make it unlikely that some surface feature has shown up in one set and not in the other. The uncertainty in Hebe’s spin period, as quoted in Section 1, translates into an uncertainty of about 20 min (0.05 cycles) in the relative rotational phasing over the 15 months interval between the photopolarimetric and the spectroscopic observations. Thus we have been able to plot our data using, to a good accuracy, self-consistent relative phases.
TABLE I Spectroscopic Observations of (6) Hebe. The Rotational Phase Has Been Estimated From a Period of 7h.274 (Lagerkvist et al. 1987)
2.2. Spectroscopy Observations were performed at ESO-La Silla on September 14–18, 1995, with the 1.52-m spectroscopic telescope equipped with a Boller and Chivens instrument. The CCD had 2048 3 2048 pixels, windowed at about 300 3 2048. The grating used was ruled at 225 gr/mm, so the ˚ /mm. The dispersion in the first order was of about 330 A CCD had a 15-em square pixel, yielding a dispersion of ˚ /pixel in the wavelength direction. The useful spectral 4.7 A ˚ , the instrumental range was from about 4800 to 9400 A ˚ FWHM being 9.8 A. Unfortunately, at the long wavelength ˚ ), a strong atmospheric absorption due end (beyond 9000 A 2 to OH degraded the signal-to-noise ratio of the images, making the spectrum of poorer quality. This is annoying, since in this spectral region variations due to mineralogical heterogeneity are likely to show up. The slit was 2 to 5 arcsec wide, aligned in the East–West direction. The slit width was much larger than the seeing conditions (typically about 1 arcsec), in order to minimize the possibility of losing the signal due to errors in pointing and guiding the target. An optical camera was used for target acquisition and guiding. The asteroid was very bright (mv P 8.5) and as a consequence it was quite difficult to center the asteroid image in the slit, due to the large amount of scattered light, resulting into a sizeable image in the camera screen. The target was confirmed by noting its motion relative to fixed stars. The observational circumstances are listed in detail in Table I.
In addition to the rotational spectra of Hebe, solar analog stars HD 20630, HD 1835, HD 144585, and 64 Hyades, flat fields, bias images, and lamp spectra (He–Ar) were taken during each night using the same instrumentation setup. Spectra were reduced with the Image Reduction and Analysis Facility (IRAF) package, installed on a SUNOS workstation at the Brera Observatory, according to the standard procedure described in Di Martino et al. (1995). This includes subtraction of the bias level, flattening
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of the data, elimination of the cosmic rays, subtraction of the sky, wavelength two-dimensional calibration, transformation from a two-dimensional spectrum to a uni-dimensional one, extinction correction, and division by the solar analog (Hardorp 1978). Finally, each spectrum was normal˚. ized to the asteroid/star flux at 7000 A 2.3. Photopolarimetry Simultaneous photometric and polarimetric observations of 6 Hebe were carried out on three consecutive nights (June 5 to 7, 1994). We used the Torino photopolarimeter attached at the 2.15-m telescope of Complejo Astronomico El Leoncito (Argentina). A full description of the instrument functioning can be found in Piirola (1988) and Scaltriti et al. (1989). Here we recall that the photopolarimeter allows the observer to perform linear and circular polarization measurements (by means of l/2 and l/4 plates), simultaneously in the five bands UBVRI. The ordinary and extraordinary beams produced by a calcite plate are alternately recorded by means of a rotating chopper. Dichroic filters, which reflect a desired spectral interval and transmit the other wavelengths, select the incoming beam according to the standard color bands UBVRI. In the dewar, cooled down to 2308C, five separate photomultipliers (three HAMAMATSU-type, for VRI, and two EMI-type, for UB) allow one to gather the signal of the object separately. The principle of operation of the photopolarimeter is that the polarization due to the sky background is directly eliminated; this has been found to be especially valuable in the observation of faint stars and when there is moonlight. In principle, if a considerable amount of background radiation is composed of scattered light, it can be highly polarized and small relative changes in the sky background could give rise to large errors in the measured polarization. Our photopolarimeter was especially designed to minimize this undesirable effect. The counts obtained by the equipment allow the observer to get the polarization parameters (amount of polarization, P, and position angle, PA) and the photometry of the object, in the five bands UBVRI. For both photometric and polarimetric curves, phases were computed according to the reference epoch (corresponding to the epoch of our first observation) JD 2449509.45, assuming a period of 7h.2745. As explained in Section 2.1, this was consistent with the phasing of our spectroscopic observations, as reported below. Unfortunately, the sky conditions during the three observing nights were not always good, so that we were not able to get a full-cycle coverage of the photometric lightcurve. However, the operational principle of the equipment allows one to obtain fairly good-quality polarimetric curves even when veiling or spread clouds are pres-
FIG 1. Averaged spectrum of Hebe, obtained by combining many different spectra calibrated with different solar analogs, and normalized ˚ . It matches very well the spectrum published by Xu et al. to 7000 A ˚ ) correspond (1995). The dots with 2-s error bars (normalized to 5500 A to data taken in the framework of the eight-color survey (ECAS) of Zellner et al. (1985).
ent in the sky. Thus, the overall quality of the polarimetric and photometric data reported below was not seriously degraded by the sky conditions, thanks also to the brightness of the object. A technical problem on the power supply of channel R prevented us from obtaining data at that wavelength. Therefore, we got results in the UBVI bands only. RESULTS
3.1. Spectroscopy In order to obtain a single low-noise reflectance spectrum of Hebe (Fig. 1), we have taken the average of all of our spectra. The procedure of adopting four different solar analog stars (see Section 2.2) allowed us to reduce the possible errors due to a calibration based on just one solar analog. Morover, spectra were taken on six different nights, minimizing uncertainties in the data due to variable sky conditions.
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spectra shown in Fig. 2 deviate in a consistent fashion above or below the averaged spectrum of Fig. 1 over a significant range of wavelengths; this is confirmed by the ratioed spectra plotted in Fig. 3. For example, the intensity of the 0.9 em feature (in the P0.8 to 0.92 em wavelength interval) is about 3% stronger than the average at phase 0.8, slightly less strong at phase 0.7, and about 2% weaker at phases 0.2 and 0.3 (that is, on the other side of the asteroid), whereas intermediate phases show ratios closer to 1. On the other hand, the small-wavelength portions of the spectra (between 0.5 and 0.7 em) indicate that Hebe is somewhat more blue than average at phases 0.8–0.9, and more red at phases 0.2–0.4. These changes are of the same order of magnitude as the U–V color variations reported by Gehrels and Taylor (1977, Fig. 28) and appear to have also the same rotational phasing, based on the comparison of lightcurves. These small spectral variations can be intepreted as due to different degrees of space weathering on Hebe’s surface, possibly related to largescale ejecta blankets resulting from relatively recent and energetic cratering events, similar to those observed in the
FIG 2. Rotationally resolved spectra of Hebe. Spectra obtained at different rotational phases were binned at intervals of 0.1 cycles (368) and then averaged and vertically offset for clarity. One step along the vertical scale corresponds to a 10% change (all the spectra are normalized ˚ ). The dashed curves correspond to the average spectrum of to 7000 A Fig. 1, after filtering out the small-scale wiggles due to observational noise.
The averaged reflectance spectrum shown in Fig. 1 can be compared with those available in the published literature. In particular we have compared our data to those obtained in the framework of the ECAS survey (Zellner et al. 1985) and to the spectrum published by Xu et al. (1995). In both cases, the agreement is very good (see Fig. 1 for the comparison with the ECAS data points). Our spectral range does not allow us to detect all the features typical of the S taxonomic type (e.g., the band at 2.2 em); on the other hand, the position of the olivine–pyroxene ˚ closely matches that corresponding to band near 9000 A the S(IV) subclass, as defined by Gaffey et al. (1994). In order to look for rotational spectral variations, we grouped our data into bins of 0.1 width in rotational phase and averaged the spectra available in each bin. As shown in Figs. 2 and 3, no differences exceeding a few percent have been detected, comparable to the noise in the data, and therefore no evidence is present for a large-scale spectral heterogeneity of Hebe’s surface. However, several
FIG 3. Ratios between the averaged spectrum of Hebe (Fig. 1) and the spectra taken at different rotational phases (Fig. 2), as specified by the labels on the right side. One step along the vertical scale corresponds to a 10% change.
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variations have a relatively small amplitude, but a complex behavior. As remarked in Section 1, such behavior in principle may be associated both with a globally irregular (nonellipsoidal) shape and with large-scale albedo variegation of the surface (related for instance to impact shock-darkening processes of chondritic materials, as suggested by Britt and Pieters 1991). Moreover, the different shapes and amplitudes of the lightcurves at different wavelengths suggest the existence of color variations, in agreement with the conclusions reported in Section 3.1. Note that for most known processes occurring on asteroid surfaces (grain size alteration, melting and crystallization), albedo variations are accompanied by spectral changes (Fanale et al. 1992, Moroz et al. 1996). 3.3. Polarimetry
FIG 4. Photometric lightcurves of Hebe in U, B, V, and I colors. The data were collected during three different nights, on June 5, 6, and 7, 1994, at El Leoncito Observatory (Argentina).
Galileo spacecraft images of (243) Ida (see discussion in Chapman 1996) or to subtle mineralogical variations, consistent with an undifferentiated silicate assemblage (as concluded by Gaffey 1996). In either case, our spectral data are consistent with a genetic relationship between Hebe and OCs. 3.2. Photometry Our lightcurves are shown in Fig. 4; most but not all of the rotational cycle was covered. The general trend is in agreement with previous results (e.g., Lagerkvist et al. 1987). In spite of the partial coverage of the lightcurve, three maxima (at P0.1, 0.3, and 0.6 phases) and three minima (P0.18, 0.45, and 0.9 phases) can be distinguished in it. Owing to the unfavorable sky conditions, we were not able to perform a transformation to the standard UBVRI system; the Dm’s given in Fig. 4 refer to a frequently observed nearby star which differed from night to night. As for the accuracy of these observations, typical errors are of the order of 0.02–0.03 mag (but twice as large for the rotational phases between 0.8 and 1.0, covered during the worst night). These (limited) data confirm that Hebe’s photometric
It is well known that asteroids, like other atmosphereless Solar System bodies, show a well-defined variation of polarization as a function of the Sun–asteroid–Earth phase angle a; the corresponding curve is often used to infer the albedo (see, e.g., Lupishko and Mohamed 1996). In particular, polarization is usually expressed through the Pr parameter, that is the ratio (I' 2 Ii)/(I' 1 Ii), I' and Ii being the intensities of the reflected light polarized perpendicular and parallel to the Earth–Sun–scattering plane, respectively. The relationship between the coefficient of linear polarization P (which is the quantity directly obtained from the observations) and Pr is simply Pr 5 P cos (2u), where u is the angle between the position angle (PA) of the observed polarized radiation and the plane perpendicular to the plane of scattering. Figure 5 shows the coefficient of linear polarization P (in percent) and the corresponding polarization angle PA plotted versus rotational phase. In making these plots, polarimetric data were grouped in bins of 0.1 in rotational phase and averaged in each bin. A simple first-order Fourier fitting of the P vs rotational phase curve is also shown and gives a satisfactory fit to the data. The amplitude of the P curves decreases from the U to the I band. Especially in the U, B, and V bands, the positions of the extrema of the curve are quite close to each other. On the other hand, except for the U band, the PA curves do not show any well-defined pattern, and there is no clear correlation between the trends observed in different colors. In the U band, where a definite trend is visible, the positions of the maximum and the minimum do not coincide with the corresponding positions in the P curve. All these findings are consistent with previous observations of Broglia et al. (1994). It is interesting to note that the trends apparent in the polarimetric and photometric lightcurves are different, and there is no clear correlation between them. We will discuss the possible interpretation of these observations below.
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FIG 5. Polarimetric lightcurves of Hebe in U, B, V, I colors. Left, the coefficient of linear polarization P (in percent) vs rotational phase, and a first-order Fourier fit. Right, the PA polarization angle (in degrees) vs rotational phase. Data collected at El Leoncito Observatory (Argentina) on June 5, 6, and 7, 1994.
As for Pr vs a curve, we recall that for asteroids it always shows a characteristic behavior, with the presence of a negative branch of polarization, whose origin has not yet been fully understood (see Muinonen 1994 and references therein). Figure 6 shows our observed values of Pr vs a (our observations were performed at a 5 128.5), together with other Pr measurements reported in the literature for Hebe (Zellner and Gradie 1976). In this figure, nightly averages of Pr for each of our three observing nights have been plotted for each color with different symbols. Our values agree very well with the general trend indicated by previous observations. This can be considered as further confirmation of the accuracy of our polarimetric measurements. We note that the lack of a correlation between the polarimetric and photometric lightcurves as shown in Figs. 4 and 5 is quite unusual for asteroids. For instance, in the wellknown case of (4) Vesta, a clear correlation between the photometric and polarimetric behaviors is present and has been used to unequivocally derive the correct rotation period of the asteroid (see e.g., Dollfus et al. 1989 and references therein). Such a correlation has been generally explained as diagnostic of large-scale albedo variegation of the surface. In particular, since Vesta’s polarimetric
variation is roughly sinusoidal over a rotational cycle, the presence of a hemispheric albedo asymmetry has been proposed as a plausible explanation for both the polarimetric and the photometric lightcurves and has been used to derive the orientation of the spin axis as well as the polar flattening of this sizeable asteroid (Cellino et al. 1987). In the case of Hebe the polarimetric behavior is generally similar to that of Vesta, but the photometric lightcurve is much more peculiar. There are two possible explanations for Hebe’s behavior. First, we may interpret the lack of correlation between photometric and polarimetric data as suggesting that the polarimetric variation in this case is due to large-scale changes in the surface texture, rather than to significant albedo features. We stress that a detailed theoretical understanding of the relationship between polarimetric behavior and microscopic surface texture is still lacking for atmosphereless solar system bodies, so for the time being we cannot test this possibility in a quantitative way. Alternatively, large-scale albedo patterns resulting in significant polarization changes may be present, without playing an important role in molding the lightcurve morphology, in which global shape effects would be predominant. Although Hebe’s size (almost 200 km in diameter) implies that it is not likely to be an irregularly shaped fragment from a larger parent asteroid (as is the case for most smaller asteroids, see Catullo et al. 1984, Davis et al. 1989), its relatively small lightcurve amplitude does not require large deviations from an axisymmetric shape, especially if sur-
FIG 6. Nightly averages of Pr vs Sun-asteroid-Earth phase angle for our UBVI polarimetric observations of Hebe (closed symbols). Open symbols correspond to the data reported by Zellner and Gradie (1976).
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face features such as craters or ridges produce complex (and variable) shadowing effects. Despite these interpretation problems, our work shows that in the absence of close-up spacecraft observations, unravelling the main physical and mineralogical properties of asteroid surfaces requires the simultaneous consideration of data from several different observing techniques. Rotationally resolved data are especially important when the detection of strong, large-scale heterogeneity is a crucial test for inferences about the asteroid’s thermal history. In the case of Hebe, we have not carried out rotationally resolved spectral measurements at infrared wavelengths, which could have provided additional constraints on the nature of its surface variegation. The recently reported observational results of Gaffey (1996) appear to confirm the main results of our work. 4. CONCLUSIONS
The main finding from this work is that Hebe’s visible reflectance spectrum shows only minor variations over the course of the asteroid’s rotation, and that these variations do not require that differentiated materials are exposed on the surface. In this respect, Hebe appears to be different from other large S-type asteroids such as (8) Flora and (15) Eunomia, whose rotationally resolved spectra show clear variations, which do not appear to be consistent with undifferentiated (ordinary chondritic) assemblages (Gaffey et al. 1989). Although photometric and polarimetric data suggest that the surface of Hebe is optically heterogeneous, probably due to changes in the microscopic surface texture and/or albedo, this may be the outcome of a variety of processes related to impact events. We thus propose our most plausible interpretation of the data: (i) Hebe’s surface is composed of an undifferentiated mineral assemblage. (ii) Since its formation, the surface has undergone a number of energetic impact cratering events, causing some changes in its large-scale albedo/polarization properties and possibly minor spectral variations related to the exposure of ‘‘fresh’’ (i.e., unaffected by space weathering processes) subsurface material. According to this interpretation, Hebe can be considered as a plausible parent body for OCs. Following Farinella et al. (1993a), we suggest that a substantial fraction of these meteorites may have been generated by large impact craters on this asteroid. This is consistent with a simple orderof-magnitude estimate on the meteorite delivery efficiency of such events. If we consider craters formed by projectile asteroids .1 km in diameter, the average frequency of these events for a target as large as Hebe can be estimated at one every 2 3 107 yr (assuming an intrinsic collision probability of 2.8 3 10218 km22 yr21 and a projectile popula-
tion of about 2 3 106; see Farinella and Davis 1992, 1994). This is comparable with the typical cosmic-ray exposure (CRE) ages of OCs—H chondrites in particular display a strong CRE peak at about 8 Myr (Anders 1964, Marti and Graf 1992), which may well correspond to the last such cratering event. The evidence for different orbital pathways followed by different subgroups of these H chondrites (Graf and Marti 1995) may be explained by the availability of both the n6 and the 3:1 resonant routes near Hebe. Larger impact events have also occurred on Hebe, of course, but may have been separated on average by longer time intervals; since the dynamical/collisional lifetime of fragments injected into the n6 resonance is shorter than 108 yr (Morbidelli et al. 1994, Bottke et al. 1995), it is unlikely that current meteorite falls record such larger and rarer impacts. If we assume that the mass ejected from Hebe’s craters is typically some 900 times that of the projectile, consistent with Holsapple’s (1993) scaling of crater dimensions for rocky surfaces in the gravity regime and a mean impact velocity of 6.6 km/sec (Farinella and Davis 1992), we get an ejecta production rate of P6 3 107 kg/yr. Conservatively assuming that: (i) only P 10% of this material is injected into the resonances and reaches an Earth-crossing orbit (according to the results of Farinella et al. 1993b and Morbidelli et al. 1994); (ii) P10% of the latter material has meteorite-like sizes (smaller than a few meters; a plausible estimate for crater ejecta); and (iii) another factor of P10 is lost during the transfer process due to collisional disruption, impact with other planets and the Sun, or ejection from the solar system (this is consistent with the typical exposure ages of ordinary chondrites, which are about a factor 10 shorter than the Earth-impact lifetimes of Earth-crossing bodies), we eventually get an Earth influx of some 6 3 104 kg/yr. This is about 20% of the overall meteorite influx, as estimated (in order of magnitude) by Halliday et al. (1984) and Wetherill (1985). Of course this kind of comparison has only a semi-quantitative value, but is important to show how different kinds of arguments and data can be linked together in a selfconsistent scenario. Impact cratering models, meteoritic evidence, the observational results reported in this paper, and the facts that Hebe is the largest S(IV) object and lies very close to the resonant chaotic zones (see Section 1) all point to the conclusion that this asteroid should be considered as one of the most promising candidate OC parent bodies and should become the target of further observational efforts. ACKNOWLEDGMENTS We are grateful to R.P. Binzel for making his spectrum of (6) Hebe available to us. Constructive reviews by M. Gaffey and B. Clark have led to substantial improvements with respect to a previous version of the paper. This work has received partial financial support from the Italian
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Space Agency (ASI), the Italian Ministry for University and Scientific Research (MURST), and the European Union Human Capital and Mobility Programme (Contract CHRX-CT94–0445).
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