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ISSN 0038 0946, Solar System Research, 2011, Vol. 45, No. 1, pp. 43­52. © Pleiades Publishing, Inc., 2011. Original Russian Text © V.V. Busarev, 2011, published in Astronomicheskii Vestnik, 2011, Vol. 45, No. 1, pp. 45­54.

Asteroids 10 Hygiea, 135 Hertha, and 196 Philomela: Heterogeneity of the Material from the Reflectance Spectra
V. V. Busarev
Sternberg Astronomical Institute, Universitetskii pr. 13, Moscow, 119992 Russia
Received December 21, 2009

Abstract--The reflectance spectra of asteroids 10 Hygiea (C type), 135 Hertha (M type), and 196 Philomela (S type) are obtained in a range of 0.40­0.91 µm with different time intervals. In this paper, the technique of the spectral measurements of asteroids is analyzed and the reflectance spectra of Hygiea, Hertha, and Philo mela are interpreted. The main physical and chemical factors and processes influencing the spectral charac teristics of asteroids are considered. It is determined that the spectra of Hertha and Hygiea contain variations exceeding the measurement errors several times at different relative rotation phases, whereas spectral varia tions of Philomela caused by its rotation hardly exceed the error limits. Most probably, these variations are caused by local manifestations of the impact metamorphism of the material of asteroids in serious impact events. Results show that, to determine the prevailing spectral type and the corresponding mineralogy of each asteroid, one should estimate and take into account the changes in its spectral characteristics for a time inter val comparable to the rotation period. DOI: 10.1134/S0038094610061036

INTRODUCTION As we know from numerous optical laboratory studies of solids, the albedo, or reflectivity of a solid body in the visible range, is the averaged characteristic of its physical and chemical properties. The albedo variations of asteroids can be induced by several causes, the main of which are the changes in the ele ment composition of the material and/or its oxidation degree, as well as the physical state or structure of the material (the mean density, porosity, or granulometric composition). In the late 1970s, the results of approx imately 20 year studies of asteroids with the ground based optical telescopes, equipped with rather preci sion registering electronic instruments, showed that polarimetric and colorimetric parameters of these bodies change within the limits of measurement errors regardless of the observed side. This led to the conclu sion on the photometric homogeneity of the surfaces of asteroids; specifically, the absence of noticeable albedo variations was suggested (Burns and Tedesco, 1979; Degewij et al., 1979). It was even hypothesized that the homogeneity of the upper layer of the crushed material (regolith) of asteroids can be explained by impact processes, which should mix and uniformly distribute the surface material (Housen et al., 1979). However, Akimov et al. (1983) were the first to show that variations in the reflectance of asteroids caused by their rotation exceed the measurement errors. Com parison of the measured light curves of several best studied asteroids with their model curves calculated for different shape parameters of their bodies allowed the conclusion that their photometric heterogeneities may reach 0m,17 and are mainly determined by albedo
43

heterogeneities (Akimov et al., 1983). These authors note that there is no contradiction with the polarimet ric and colorimetric characteristics of asteroids obtained earlier. According to the works of the noted specialists (Bowell et al., 1979; Morrison and Zellner, 1979), if the range of albedo variations is assumed to be 100%, the ranges of variations in the polarimetric and colorimetric characteristics of asteroids are relatively small--not more than 1.5 and less than 30%, respec tively. The spectral dependence of the albedo is the reflec tance spectrum (in absolute units) of a solid body obtained at a zero phase angle. However, such depen dence can be easily measured only in a laboratory, whereas the main volume of the observational data on solid celestial bodies is obtained under phase angles differing from zero (they are sometimes varying or dif ferent). In this case, to describe the spectral properties of a solid celestial body, the normalized reflectance spectrum or the spectrum of the brightness coefficient (factor) is used. This simplified relative characteristic can be expressed as

( , ) = k ( pF ( , )) ( p0F ( , 0 )),

(1)

where is the monochromatic geometrical albedo accounting for the integral physical and chemical properties of the observed hemisphere of the celestial body (or, being more precise (by definition), its pro jection to the picture plane of the observer); F(, ) is the phase function (F(, ) = 1, when = 0), is the current value of the wavelength; 0 is the fixed wave length, at which the normalizing value of the bright ness coefficient 0(, 0) = 0F(, 0) is chosen (usu


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BUSAREV

ally, the value corresponding to the middle of the V photometric band 0 = 0.55 µm, is chosen); and k is a constant factor. It is worth stressing that (, ) is a function of the geometric albedo and the phase func tion of the celestial body at different wavelengths, and it is independent of the changes in the shape of the body due to its rotation. Moreover, relation (1) is valid, if the observed object is a point source of radiation, and its spectrum is registered simultaneously in the whole spectral range. In the ground based optical studies, practically all of the asteroids are point sources (their angular sizes are less than several tenths of an arc second); therefore, the measurements yield their integral characteristics of the observed hemi sphere. Variations in the mean composition or the oxida tion degree of the observed portion of the surface of asteroids can be found in their successive in time (or in rotation phase) spectra of the diffuse reflec tion. Such variations appear as the changes in the slope and the shape of the continuum of the visible reflec tance spectra of asteroids. The continuum character izes the intensity and the width of the oxygen metal absorption band centered at 0.2 µm; it is induced by the electron transfer of a charge in silicate compounds containing oxygen (Platonov, 1976; Burns, 1993; Loeffler et al, 1994). Due to the changes in the com position of the material, the absorption bands, charac terizing the dominating minerals or their complexes, also appear (or disappear) in the successive reflectance spectra of asteroids. It is worth stressing that such min eralogical absorption bands are rather wide: from sev eral hundred to one­two thousand angstroms. Because of this, such bands, if their relative intensity is sufficient (more than 3­5%), can be reliably identified in the reflectance spectra of asteroids against the back ground of the high frequency noise component (see, e.g., Busarev et al., 2007). As we know, one of the most intense mineralogical absorption band in the near infrared reflectance spectra of asteroids and other solid atmosphereless celestial bodies with silicate composition is the pyroxene olivine band centered at 1 µm (Adams, 1975), which substantially influences the continuum shape of their reflectance spectra. One more absorption band, close to the one mentioned, is observed in the reflectance spectra of the hydrated or highly oxidized silicate material at 0.75­0.80 µm; it is appear due to the electron transfer of a charge Fe2+ Fe3+ (Burns et al., 1972; Platonov, 1976; Bakhtin, 1985; Burns, 1993). As our experience in the study of the terrestrial mineral samples containing iron forms of different valence shows, the intensity of the absorp tion band mentioned in the reflectance spectrum is also controlled by the total content of iron in the sili cate material rather than only by portions of two and three valent iron (Busarev et al., 2004). Interestingly, this band may even mask the 1 µm diagnostic absorp tion band of terrestrial pyroxenes and olivines if the Fe2O3 content is increased (Adams, 1975). Because of

a high content of hydrosilicates and their close phases in carbonaceous chondrites (Dodd, 1981; Jarosewich, 1990), the intense absorption band centered at 0.75­ 0.80 µm or at the close wavelengths imparts the char acteristic concave shape to their reflectance spectra in the whole range from 0.5 to 1.0 µm (see Busarev and Taran, 2002, for example). The important additional indicator of hydrosilicates and highly oxidized iron oxides in the visible range is the absorption band at 0.44­0.45 µm discovered in the spectra of diffuse reflection of the crushed samples of terrestrial serpen tines (Busarev et al., 2004). The equivalent width of this absorption band in the reflectance spectra of a set of serpentine samples turned out to correlate strongly with the Fe3+ content (Busarev et al., 2008). As indica tors of the oxidized material, the absorption bands centered at 0.60 and 0.67 µm can be used; they were found in the reflectance spectra of oxidized Fe and Fe­Ni compounds and minerals of the spinel group, which are complex oxides of Fe, Mg, Al, and Cr (Hiroi et al., 1996). In the work mentioned, it was shown that these weak absorption bands frequently occur in the reflectance spectra of S type asteroids. THE OBSERVATIONAL DATA AND THEIR DISCUSSION The spectra of asteroids 10 Hygiea, 135 Hertha, and 196 Philomela were acquired at different times from November 2004 to November 2008 with the 1.25 m telescope of the SAI (Sternberg Astronomical Institute) Crimean Observatory coupled with a charge coupled device spectrograph operating in the range from 0.40 to 0.91 µm with a spectral resolution of about 8 å. Each of the spectra of the asteroid was sequentially recorded by two portions (in the wave length intervals 0.40­0.67 and 0.65­0.91 µm or vice versa) and took about half an hour. Besides the aster oids, the star standards, simultaneously being the solar analogs by their spectrophotometric parameters (16 Cyg B and HD 10307) (Hardorp, 1980; Cayrel de Strobel, 1996, Glushneva, et al., 2000), were also observed. They were used to determine the spectral transparency of the terrestrial atmosphere and to approximate the reflectance spectra of asteroids. The mean moments, the conditions of observations of the asteroids and the star standards, and the errors in the reflectance spectra are listed in the table. The reflec tance spectra were approximated by the following for mula (Busarev, 1999):
( , ) = kI a ( , ) f ( )
-(M a (z )- M s (z ))

I s ( ) ,

(2)

where (, ) is the spectral distribution of the bright ness coefficient (or factor) of the asteroid; I(, ), and Is() are the spectral distributions of the intensity of the light flux from the asteroid and the star (solar ana log), respectively; f() is the function of the spectral transparency of the atmosphere for the specified observatory determined for each of the observational
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Time, coordinates, and conditions of the spectral observations of asteroids and solar analog stars UT h, min, s , h, min, s , r, AU ­ 2.115 2.115 2.115 2.115 ­ 3.367 3.367 6.2 2.163 1.308 ­ 1.307 ­ 1.306 1.306 ­ 2.516 2.516 2.279 ­ 2.304 ­ +25 01 25 2.507 2.274 ­ 3.494 3.494 3.156 ­ 3.157 ­ 3.492 2.273 ­ ­ 7.4 7.4 ­ 2.6 2.6 9.7 ­ 10.5 ­ 0.9 2.271 7.9 ­ ­ 2.269 8.4 11.2 6.2 11.2 6.2 11.1 11.1 4.9 10.4 10.4 11.5 4.9 11.5 4.9 10.3 3.151 2.3 11.0 ­ 0.000 0.451 ­ 0.243 ­ 0.135 0.219 ­ 0.340 0.377 0.887 ­ 0.457 ­ 0.556 1.7 10.2 0.021 1.7 10.2 0.000 ­ 6.2 ­ 1.394 1.307 1.390 1.123 1.604 1.689 1.425 1.447 1.460 1.562 1.861 1.432 1.628 2.170 1.344 1.268 1.304 1.422 1.937 0.080 0.018 0.049 0.058 0.026 0.120 16.3 11.2 0.042 1.311 16.3 11.2 0.030 1.306 0.021 0.057 ­ 0.046 0.094 ­ 0.041 0.060 ­ 0.034 ­ 0.043 0.081 ­ 0.024 0.023 0.030 16.3 11.2 0.012 1.301 0.048 16.3 11.2 0.000 1.298 0.036 0.017 0.017 0.008 0.012 ­ 0.012 0.019 ­ 0.010 0.010 ­ 0.007 ­ 0.012 0.019 ­ 0.006 0.008 0.007 ­ 6.2 ­ 1.223 ­ ­ M(z) 17 49 39 19 17 21 19 23 30 19 32 38 19 38 40 21 03 04 22 42 11 23 16 58 16 37 30 23 47 28 01 28 43 18 29 30 00 56 04 18 35 30 01 13 22 01 55 43 23 25 00 02 09 35 03 11 02 21 06 36 22 25 30 20 34 48 22 59 15 02 13 08 04 39 24 01 41 47 02 14 47 01 41 47 +42 36 48 +08 35 56 +42 36 48 02 16 22 +08 35 53 04 44 34 +25 14 19 04 44 36 +25 14 24 01 41 47 +42 36 48 03 25 09 +22 24 10 03 25 11 +22 24 16 19 41 52 +50 31 00 03 26 14 +22 26 57 19 41 52 +50 31 00 03 27 13 +22 29 25 02 39 26 +09 21 10 19 41 52 +50 31 00 00 27 00 +08 51 17 2.370 00 27 01 +08 51 24 2.370 19 41 52 +50 31 00 ­ 00 28 31 +05 54 40 1.237 00 28 31 +05 54 40 1.237 00 28 31 +05 54 41 1.237 00 28 31 +05 54 41 1.237 19 41 52 +50 31 00 ­ , degree, arc min, arc s , AU , deg V, mag nitude 1 2 ­ 0.081 0.081 0.060 0.056 ­ 0.047 0.072 ­ 0.115 0.059 ­ 0.100 ­ 0.065 0.071 ­ 0.100 0.065 0.062 3

Object

Date

16 Cyg B

2004 11 07

135 Hertha (1)

2004 11 07

135 Hertha (2)

2004 11 07

135 Hertha (3)

2004 11 07

SOLAR SYSTEM RESEARCH

135 Hertha (4)

2004 11 07

16 Cyg B

2007 10 04

ASTEROIDS 10 HYGIEA, 135 HERTHA, AND 196 PHILOMELA

Vol. 45

10 Hygiea (1)

2007 10 04

10 Hygiea (2)

2007 10 04

No. 1

16 Cyg B

2008 10 28

196 Philomela

2008 10 28

2011

135 Hertha

2008 10 29

16 Cyg B

2008 10 29

135 Hertha

2008 10 30

16 Cyg B

2008 10 30

135 Hertha

2008 10 31

135 Hertha

2008 10 31

HD 10307

2008 11 25

10 Hygiea (1)

2008 11 26

10 Hygiea (2)

2008 11 26

196 Philomela

2008 11 28

HD 10307

2008 11 28

196 Philomela

2008 12 01

HD 10307

2008 12 01

10 Hygiea

2008 12 02

Note: UT is universal time; and are right ascension and declination, respectively; and r is the geocentric and heliocentric distances, respectively; is the phase angle; V is the visible magnitude; is the relative rotation phase; M(z) is the air mass; and the errors in the reflectance spectra of the asteroids 1, 2, and 3 are the standard deviations in 0.44­0.45, 0.59­ 0.60, and 0.84­0.85 m, respectively the numbers in brackets next to the names of some asteroids indicate the order numbers of their spectra obtained on the same date.

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46 1.6 Normalized reflectance spectrum 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0 0.35 0.45

BUSAREV 135 Hertha (07/08 11 04) 1 (0.000)

2 (0.012) 3 (0.030)

4 (0.042) 0.55 0.65 0.75 Wavelength, µm 0.85 0.95

Fig. 1. The normalized (to the value at a wavelength of 0.55 µm) reflectance spectra of asteroid 135 Hertha obtained on November 7­8, 2004. Spectra 1­4 are shown in chronological order (bottom up) and shifted relative to each other for convenience. The relative phase of the asteroid's rotation is given in brackets. The rotation phase of the very first spectrum is assumed to be zero.

nights; (M(z)­Ms(z)) is the air mass difference depending on the zenith distances z of the asteroid and the star analog at the moments of their observation; and k is a factor. We see from Eq. (1) that (, ) and I(, ) depend on both a wavelength and a phase angle of the asteroid . It is worth noting that, at 0°, the spectral distribution of the brightness coeffi cient of the observed hemisphere of the asteroid trans forms to the spectral distribution of its geometrical albedo p(). From the calculated reflectance spectra of the asteroids, their relative mean root square errors (the standard deviations from the continuum line) in a range of 0.44­0.85 µm were determined. They amount to less than 1­2% in the middle of the speci fied range and increases to approximately 5­7% at its ends (see table). Then, the calculated reflectance spectra of the asteroids were smoothed with the "run ning average" method and normalized to the value at a wavelength of 0.55 µm. In some cases, after such smoothing, the spectra near 0.40­0.44 and 0.85­0.91 µm were additionally extrapolated with a polynomial to eliminate the residual noise component in the reflec tance spectra of the asteroids beyond a range of 0.44­ 0.85 µm. The normalized reflectance spectra of the asteroids are shown in Figs. 1­5. The corresponding values of the relative rotation phase of the asteroids are specified in the plots near the labeling of the spectra (in brackets) and in the table. For each of the aster oids, the zero phase was assumed to be the rotation phase (RPh) corresponding to the first of the obtained spectra. The data in the table and in the plots are arranged chronologically. Now, we describe the reflec tance spectra of each of the asteroids in detail. The M type asteroid 135 Hertha (Tholen, 1989) has a rotation period P = 8.40h (Batrakov et al., 2000); its IRAS diameter (the diameter determined from the Infrared Astronomical Satellite observations) is

79.2 km, and its albedo is 0.14 (Tedesco et al., 2004). Variations in its reflectance spectra measured in the RPh interval 0.000­0.042 during the night of Novem ber 7­8, 2004, were relatively small (Fig. 1). However, their general concave shape is typical of hydrated sili cate material or the material of carbonaceous chon drite composition, which may compose the C­F type asteroids, but this is not typical of the M type aster oids, as the latter should contain metallic compounds and high temperature minerals of the pyroxene and olivine type (Gaffey et al., 1989; 2002). At an RPh of 0.012­0.042, the reflectance of the asteroid gently decreased by approximately 20­40% in a range of 0.70­0.91 µm (Fig. 1, cures 2­4), which is probably connected with the appearance of the absorption band at 1.0 µm caused by the growth of the mean content of pyroxene and olivine in the material of the observed hemisphere of the asteroid. A similar shape of the reflectance spectra was also demonstrated by Hertha in October 29­31, 2008, when the RPh values were different, and their range was wider. Observations of the asteroid in the period mentioned showed that the shape of its spectra was more noticeably changed (Fig. 2): on October 31, it was concave (RPh = 0.135) and concave convex, probably due to the appearance of the pyroxene olivine absorption band at 1.0 µm (RPh = 0.219); on October 20, it was slightly convex, which is typical of M or S type asteroids (RPh = 0.243); and on October 29, it was again concave (RPh = 0.451). It is worth noting that, on October 29 and 31, the absorption band at 0.44­0.45 µm and the weak combined absorption band of olivine and pyrox ene at 0.50 µm (Fig. 2) were registered more clearly in the reflectance spectra. The first one is connected with the presence of Fe3+ in the asteroid material, and the second one is induced by the forbidden by spin elec tron transition in Fe2+ in the crystalline fields of these minerals (Platonov, 1976; Bahktin, 1985). Thus, Her tha's reflectance spectra obtained at different rotation
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ASTEROIDS 10 HYGIEA, 135 HERTHA, AND 196 PHILOMELA 1.8 1.6 Normalized reflectance spectrum 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0 0.35 0.45 0.55 0.65 0.75 Wavelength, µm 0.85 0.95 29 10 08 (0.451) 31 10 08 (0.135) 30 10 08 (0.243) 135 Hertha 31 10 08 (0.219)

47

Fig. 2. The normalized (to the value at a wavelength of 0.55 µm) reflectance spectra of asteroid 135 Hertha obtained on October 29­31, 2008. The spectra are shifted relative to each other for convenience.

1.4 Normalized reflectance spectrum 1.2 1.0 0.8 0.6 0.4 0.2 0 0.35 0.45

10 Hygiea (04/05 10 07)

1 (0.000)

2 (0.021)

0.55

0.65 0.75 Wavelength, µm

0.85

0.95

Fig. 3. Normalized (to the value at a wavelength of 0.55 µm) reflectance spectra of asteroid 10 Hygiea obtained on October 4­5, 2007. The spectra 1­4 are shifted relative to each other for convenience. The relative phase of the asteroid's rotation is given in brackets. The rotation phase of the very first spectrum is assumed to be zero.

phases testify to the changeability of its observed spec tral type (from C­F to M­S) and, consequently, to the substantial heterogeneity of the composition of its surface material. The C type asteroid 10 Hygiea (Tholen, 1989) has a rotation period P =27.62h (Batrakov et al., 2000); its IRAS diameter is 407.1 km, and its albedo is 0.07 (Tedesco et al., 2004). Its two first spectra were mea sured at night on November 4­5, 2007 (Fig. 3) with a small time interval (about half an hour). The spectra differ slightly, within the limits of measurement errors (see table), which could be expected for a rather slowly rotating asteroid. However, the shape of these reflec
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tance spectra (see Fig. 3) does not agree with the determined spectral type of Hygiea, type C, to which the low temperature mineralogy is assigned (Gaffey et al., 1989; 2002). This shape most likely resembles the reflectance spectrum of the high temperature mineral--olivine (Platonov, 1976)--typical of S type asteroids (Gaffey et al., 1989). At the other values of RPh, the spectra of Hygiea were also registered with a small time difference (about an hour) on November 25­26, 2008 (the two upper curves in Fig. 4). Though some differences (~10%­20%) are observed in them in a range of 0.65­0.91 µm, they generally agree with the spectral type C. In these reflectance spectra, there is a


48 1.6 1.4 Normalized reflectance spectrum 1.2 1.0 0.8 0.6 0.4 0.2 0 0.35 0.45

BUSAREV 10 Hygiea 26 11 08 (0.340)

26 11 08 (0.377)

26 12 08 (0.556)

0.55

0.65 0.75 Wavelength, µm

0.85

0.95

Fig. 4. The normalized (to the value at a wavelength of 0.55 µm) reflectance spectra of asteroid 10 Hygiea obtained on November 26­ December 2, 2008. The spectra are shown in chronological order (bottom up) and shifted relative to each other for convenience.

1.6 1.4 Normalized reflectance spectrum 1.2 1.0 0.8 0.6 0.4 0.2 0 0.35 0.45 0.55

196 Philomela 01 12 08 (0.457)

28 11 08 (0.887)

28 10 08 (0.000)

0.65 0.75 Wavelength, µm

0.85

0.95

Fig. 5. Normalized (to the value at a wavelength of 0.55 µm) reflectance spectra of asteroid 196 Philomela obtained on October 28­December 2, 2008. The spectra are shown in chronological order (bottom up) and shifted relative to each other for convenience. The relative phase of the asteroid's rotation is given in brackets. The rotation phase of the very first spectrum is assumed to be zero.

weak absorption band of Fe3+ at 0.44­0.45 µm, con firming the low temperature mineralogy of the mate rial. Hygiea's last reflectance spectrum, measured on December 1­2, 2008, at RPh of 0.556 (the lower curve of Fig. 4), characterizes the asteroid's side that is opposite to the side corresponding to the two spectra (in Fig. 3). The shape of this reflectance spectrum is rather unusual and differs noticeably from the spectra of Hygiea acquired at the other rotation phases. In the whole spectral range that we use, the slope of this spec trum is negative (Fig. 4). Such a shape is typical of the reflectance spectra of the B and F type asteroids, close to the C type (Tholen, 1989). Thus, the reflec tance spectra of Hygiea obtained at its different rota

tion phases suggest the variability of its spectral type from C to B­F and even S. Finally, three spectra of asteroid 196 Philomela were measured. It is an S type asteroid (Tholen, 1989); its rotation period is P = 8.34h (Batrakov et al., 2000), the IRAS diameter is 136.4 km, and the albedo is 0.23 (Tedesco et al., 2004). The spectra were acquired on October 28­29, 2008, November 28­29, 2008, and December 1­2, 2008, at different rotation phases; but they are more or less uniformly distributed through the rotation period of the asteroid (table). By their shape, the reflectance spectra of Philomela cor respond to the spectral class S and differ by ~6%­9% only at the boundaries of the used spectral range,
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ASTEROIDS 10 HYGIEA, 135 HERTHA, AND 196 PHILOMELA

49

which is within the limits of measurement errors (Fig. 5). Such results of the measurements testify to the high temperature mineralogy (Galley et al., 1989) and rel atively homogeneous composition of Philomela's sur face material. The weak absorption bands at 0.44­ 0.45, 0.60, and 0.67 µm in the second and third (by time) spectra indicate the presence of small surface formations composed of oxidized and/or hydrated material on Philomela or to the admixture of this material in the main material of the asteroid. It is worth noting that the substantial variation in the reflectance spectra of Hygiea and Hertha are reg istered in the period of the rather stable spectral trans parency of the atmosphere. In the normalizing proce dure of the reflectance spectra, the brightness varia tions connected with the irregular shape of the asteroids considered were eliminated. During observa tions of the asteroids, the light phase angles were small, and they change in relatively narrow limits (0.9°­2.6°, 2.3°­10.5°, and 7.4°­16.3° for Hygiea, Philomela, and Hertha, respectively). Consequently, the phase function could not influence their spectral reflectance much. Because of this, we can assert that the spectral differences found during the rotation of the asteroids are connected with the changes of the mean spectral reflectivity or the albedo of their observed hemisphere and, consequently, with the mean chemical and mineralogical composition of their material. DISCUSSION The most probable causes of the local heterogene ities of the material on asteroids are the consequences of their mutual collisions or falls of smaller bodies onto their surfaces. The frequency and energy of such impacts were very high in the past (~3­4 Ma), which was caused by the resonance and gravitational distur bances from growing Jupiter and large preplanetary bodies coming to the main belt of asteroids (Safronov, 1969; Safronov and Ziglina, 1991; O'Brien et al., 2007). However, smaller bodies may also fall onto asteroids now (see, for example Petit et al., 2001; Bot tke et al., 2005). The most noticeable traces of the impact events on the surfaces of asteroids are impact craters and ejecta of the material which are well seen in the high resolution images of asteroids taken during spacecraft encounters. As we know from the literature on studies of impact craters on the Earth (see, for example, Melosh, 1989; Grieve, 1991), at the impact of meteoritic bodies with the Earth's surface with velocities of about 1­10 km/s, the pressure in the epi center may reach tens of gigapascals, and the temper ature, several thousand degrees. The result of the influence of such pressures and temperatures on the silicate material is its complete melting and partial evaporation, at least in the crater's bottom. However, for asteroids, as studies of meteorites and the model calculations show, even at very strong impacts, the
SOLAR SYSTEM RESEARCH Vol. 45 No. 1 2011

energy of which reaches the energy of the body destruction, no global heating to high temperatures occurs (Keil et al., 1997). In other words, on asteroids, at considerable pressures and temperatures in the epi center of the impact explosion, the surface material is only partly melted and evaporated independently of the composition of the material of the colliding bodies. Even under such extreme conditions on the asteroid, formation of breccias (partly melted conglomerates of particles that are mostly different in both physical state and chemical and mineralogical composition), rather than continuous melts, is most probable (Dodd, 1981; Keil, 2000; Bischoff et al., 2006). From this, it follows that the impact metamorphism of the surface material of asteroids is rather heterogeneous and local. Due to the occasional character of the impact pro cesses, there are specific peculiarities in the evolution of each of the asteroids and the individual formations on its surface. Depending on the value and the direc tion of the velocity of the falling body, the bottom of the impact crater can be filled with both the material of the impactor and the material of the asteroid itself (Pierazzo and Melosh, 2000). This means that the fall of smaller bodies on the asteroids may result in the transfer or delivery of the material of the other type (since it was formed on the other parent body). Even if the impact crater or the material ejecta from it was mainly formed from the material of the asteroid, the substantial impact loadings and high temperatures may lead to the changes in the structure and the com position of the rocks and minerals (crushing, mixing, heating, partial melting, and removal of volatiles) (see, for example, Korzhinskii, 1957). As we know from numerous research of meteorites--fragments of aster oids--the surface material of the latter was subjected to the intense and multiple impact reprocessing con taining a number of short term, mainly reducing, pro cesses, as they were accompanied by high pressures and temperatures on one hand and led to the forma tion of oxygen depleted nonequilibrium melts or con densates on the other hand (Dodd, 1981; Scott et al., 1992; Ryan and Melosh, 1998; Keil, 2000, Wasserman and Melosh, 2001). Moreover, it was recently found that the chemical reaction, which is characteristic of high temperature and pressure mantle processes, took place at the con densation of the silicate material vapors accompany ing the impacts of solid bodies (Mao and Bell, 1977). The impulse laser modeling of the impact melting, evaporation, and condensation of the iron containing samples of augite and peridotite in the helium atmo sphere showed that the processes are accompanied by the chemical disproportionation of bivalent iron when Fe2+ is transformed to Fe0 and Fe3+ (Yakovlev et al., 2009). The composition of the successive layers of the condensate obtained in this experiment was deter mined; and its analysis suggested an unambiguous conclusion: when the concentration of Fe2+ decreases, not only the content of Fe0 but also the content of Fe3+


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grows, and the latter increases several times faster. According to the interpretation proposed by the authors, the high density of the gas in the volume unit of the vapor, produced during impact, as if it "locks" the liberated oxygen in the "system": for a certain duration, it is left in this volume, which increases the probability of its reaction with FeO and iron oxidation to the trivalent state. The same method was applied to examine the impact condensate on the fine fraction particles of the lunar regolith samples returned by the Luna 16 spacecraft. The comparative study of its com position confirmed the effect of the disproportion ation reaction of bivalent iron under the natural con ditions on the atmosphereless celestial bodies: in the successive layers of the impact condensate, the ratios of the valent forms of iron averaged Fe0 : Fe2+ : Fe3+ = 1.2 : 1.9 : 0.7 (Yakovlev et al., 2009; Gerasimov et al., 2002). This result is important in interpreting the observed spectral characteristics of solid atmosphere less celestial bodies. Coming back to the asteroids we have considered, let us discuss the possible influence of the impact pro cesses on their material. C type asteroids (to which Hygiea belongs) are believed to be the parent bodies of meteorites--carbonaceous chondrites, whereas S type asteroids (to which Philomela belongs), the parent bodies of ordinary chondrites, because their reflec tance spectra are similar (Gaffey et al., 1989). Labora tory studies of carbonaceous chondrites of the most primitive groups (CI, CM, CO, and CV) showed that their material was not subjected to heating higher than 100­200 K as a rule, and it is enriched with the layered hydrosilicates containing bound water (see Dodd, 1981; Rubin, 1997; Huss et al., 2006, for example). Due to the impact events inducing local heating and melting, we can suppose that the material of carbon aceous chondrite type dehydrates and their composi tion becomes similar to that of ordinary chondrites, which results in the corresponding transformation of the shape of reflectance spectra. Exactly such a behav ior of the spectral characteristics of carbonaceous chondrites was demonstrated by crushed samples heated to different temperatures in the laboratory (Hiroi et al., 1993). It is necessary to note that, though the considered asteroid 135 Hertha belongs to the M type (Tholen, 1989), it can be partly covered with hydrosilicates. This follows from the presence of the diagnostic absorption band of H2O/OH in its reflec tance spectra at 3.0 µm (Rivkin et al., 2000), the weak absorption band of Fe3+ at 0.44­0.45 µm (according to our data, see Figs. 1 and 2), and from the relatively low IRAS albedo, 0.14 (Tedesco et al., 2004). As we have already noted, the shape of Hertha's reflectance spec tra changed from slightly convex (typical of M­S type asteroids) to flat or concave (typical of C­F type asteroids) (Fig. 2). The similar changes in the reflec tance spectra were found in the other hydrated aster oid of the M type, 21 Lutetia, during its rotation (Busarev, 2008). As the studies of Lutetia fulfilled with

the spectral frequency method (Busarev et al., 2007) in the range of the Fe3+ absorption band at 0.44­0.45 µm showed, there are many local heterogeneities of hydrates and/or oxidized material on its surface, and their sizes mainly range from several kilometers to sev eral tens of kilometers (Prokof'eva et al., 2005). Thus, the main causes of the absorption bands of Fe3+ at 0.44­0.45 µm and Fe3+­Fe2+ at 0.75­0.80 µm in the reflectance spectra of the asteroids are as fol lows: their "own" hydrated and/or highly oxidized compounds contained in their material (in particular, on the asteroids of primitive types), the delivery of such compounds during the falls of primitive bodies of carbonaceous chondrite composition with a high content of hydrosilicates (especially on the asteroids of magmatic types) (Busarev, 1998, 2002), and the chem ical reaction of disproportionation of bivalent iron during formation of the silicate material condensate in the impact processes (Yakovlev et al., 2009). Differ ent combinations of the listed causes cannot be excluded. Probably, if the hydrated and/or highly oxi dized compounds are present on the surface of aster oids, the influence of the impact processes leads to the strengthening of the absorption bands of Fe3+ or Fe3+­ Fe2+ in their reflectance spectra. At the same time, we see from the mean ratios obtained for the dispropor tionation of Fe2+ in the impact processes (Yakovlev et al., 2009) that their predominantly reducing char acter remains. Because of this, it is natural to suppose that, if the asteroid's surface is composed of ordinary chondrite type material, where such minerals as oliv ines and pyroxenes (more resistant than hydrosilicates to high temperatures and pressures) predominate, serious impact events result in no sharp changes in the composition of the material and/or in the degree of its oxidation and, consequently, in the spectral character istics of the observed hemisphere of the asteroid. Prob ably, this is confirmed by the reflectance spectra of Philomela obtained in our study (Fig. 5). It is worth stressing that the asteroids considered were observed at small phase angles of light under a stable atmospheric transparency. The absence of sub stantial errors in the observational data is confirmed by the same shapes of the reflectance spectra of asteroids at close values of the rotation phase. Because the spec tral properties of each of the asteroids are determined by the mean chemical and mineralogical composition of the material of its observed hemisphere, they are influenced by existing local heterogeneities in the material composition. Probably, such heterogeneities in the surfaces of Hygiea and Hertha causing the spec tral differences during rotation are the result of serious impact events. On the basis of the results and the cited published data, we assert that local heterogeneities in the surface material composition connected with impact events are quite probable on primitive types of asteroids (C, G, B, and F) or in the hydrated asteroids of other types and less probable on high temperature
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types of asteroids (M, S, E, and V), as their material is more resistant to high temperatures. The results show that, to estimate the spectral type and the corresponding mineralogy of each of the aster oids, one should determine the ranges of the variations in its spectral characteristics through the time interval comparable with the rotation period and to take them into account if the differences are noticeable. ACKNOWLEDGMENTS The spectral observations of asteroids were sup ported by the Russian Foundation for Basic Research (project no. 08 02 00931). REFERENCES
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