« ZurückWeiter »
information is contained in the complete reflectivity curve than in UBV measurements, but they can still be related to each other. It would, of course, be desirable to extend the range of reflectivity measurements, particularly into the infrared where there are a variety of highly diagnostic solid absorption bands, but nearly all asteroids are so faint that they are difficult to observe with available systems beyond 1.1 p.m.
EARLY SPECTRAL REFLECTIVITY MEASUREMENTS OF THE
The first spectral reflectivity study of an asteroid by McCord and his associates (1970) turned out to be particularly exciting. Measurements of Vesta made at Cerro Tololo in December 1969 and with a different filter set at Mt. Wilson in October 1968 showed a very deep absorption band centered near 0.915 pm. The band is the most prominent absorption band yet found on any solid solar system body. McCord, Adams, and Johnson have interpreted the composition indicated by the spectral reflectivity curve of Vesta to be that of certain basaltic achondrite meteorites (Mg-rich orthopyroxene or pigeonite). This identification, of course, refers to the composition of the Vesta surface minerals that, because of their abundance and albedo, contribute the bulk of Vesta's reflected light. A subsequent study of Vesta (Johnson and Kunin, 1971) has shown that the primary characteristics of the spectral reflectivity curve do not change as Vesta rotates. The asteroid was observed continuously for a few hours with the Mt. Wilson 152 cm reflector and no changes were detected except for statistically marginal evidence for the dark side being somewhat more reflective (relatively) than the light side in the violet. This change is in the same direction as a correlation of UBV color with lightcurve reported by Gehrels (1967). Two runs showing approximately opposite sides of the asteroid are presented in figure 3. It is particularly noteworthy that the 0.9 pm absorption band remains unchanged in position on opposite sides of the asteroid because band position is a sensitive indicator of mineralogical composition. Evidently the gross surface composition of Vesta is quite homogeneous on a large scale. We observed the three other bright asteroids (Ceres, Pallas, and Juno) in June 1970, using twilight time on the 508 cm reflector. Good signals were obtained during the short intervals available to us for observing, but standardization was difficult because of lack of time. Certain fluctuations for individual filters in the reduced data for two of the asteroids can be ascribed to the poor calibration of the particular standard star against which they were observed. These have been smoothed out, but the smoothings do not change the major characteristics of the spectral reflectivity curves. The spectral reflectivities of the three asteroids are plotted with Vesta as a reference in figure 4. Pallas is much brighter than the other asteroids in the violet, confirming Bobrovnikoff’s early conclusion and UBV data. Juno shows a reflectivity
Figure 3.-Spectral reflectivity of 4 Vesta. Individual runs on approximately opposite sides of the asteroid are plotted on the mean curve of 26 runs. Mt. Wilson 152 cm reflector, February 16, 1970.
Figure 4.—Spectral reflectivity curves for Ceres, Pallas, and Juno (with Vesta for comparison); Mt. Palomar 508 cm reflector, June 24 to 29, 1970. Square indicates scaling to unity at 0.56 um. Alternative methods of reducing Ceres data against to-Ceti shown for A > 0.7 um. Some smoothing applied to Pallas and Juno to correct for deviations due to the standard star n-Piscium.
peak near 0.7 pm and has a much redder slope in the visible than the other three major asteroids. None of the first three asteroids shows a noticeable absorption band to compare with that of Vesta, although Juno does diminish in reflectivity near 1.0 p.m. Ceres is quite bright in the blue but falls off sharply in the ultraviolet, which confirms its unusual UBV color shown in figure 1. All four major asteroids are different in color, but we do not feel confident of making a unique identification on the basis of these preliminary data. The flat, even bluish, trend of the reflectivity curve for Pallas is suggestive of ices, but the low albedos that have been inferred for Pallas (Matson;2 Veverka, 1970) are inconsistent with ices. John Adams has told us that metallic meteorites show similar flat spectral reflectivities, but no definitive identification is possible until a wide variety of meteorites (such as carbonaceous chondrites) have been studied and their reflectivities cataloged.
SPECTRAL REFLECTIVITIES OF 11 ASTEROIDS
We used the Mt. Wilson 152 cm reflector in October 1970 to measure the spectral reflectivities of 11 asteroids. An example of the data for one of these asteroids (192 Nausikaa) is shown in figure 5. The error bars are standard deviations of the means of nine runs. A fairly prominent absorption band is
Figure 5.-Spectral reflectivity for 192 Nausikaa, mean of 9 runs; Mt. Wilson 152 cm reflector, October 10, 1970. £2-Ceti was used as the standard star. Error bars are standard deviations of the mean. Preliminary reduction.
apparent, though less deep than that of Vesta. We wish to postpone attempting a conclusive mineralogical identification until further observations of 192 have been reduced. Chapman (1971) will discuss the final reduction for these 11 asteroids and others observed after October 1970. The spectral reflectivity curves for the 11 asteroids, including 192, are shown in figure 6. An approximate indication of the standard deviations of the points in the middle portions of the reflectivity curves is indicated in the figure. The smooth curves were drawn through the error bars. Most of the indicated features are probably real, but some of the smaller bumps and dips should await confirmation and improvements in our standardization. The reflectivity curves have been plotted in three groups in figure 6. The Vesta curve is also shown for comparison with each group. The top curves are those with the bluest trend and the bottom group contains the reddest, but we do not intend to suggest three distinct groupings from what may be a more or less continuous spectrum of color trends.
Figure 6.-Spectral reflectivity curves for asteroids 1, 2, 3, 11, 13, 16, 17, 29, 40, 79, and 192 (with Vesta for comparison). Mt. Wilson 152 cm reflector, October 9 to 12, 1970. Observations reduced against to-Ceti and smooth curves scaled approximately to unity at 0.56 um; typical error bars are indicated.
Although the reflectivities shown here have not been corrected for any reddening with phase and the observations cover a range of phases among the various asteroids shown, there is no correlation between the phase angle at time of observation and the apparent color trend. The differences between most of these asteroid spectral curves far exceed effects due to phase angle, particle size, and similar variables. These differences are almost certainly due to compositional variations among the asteroids. The wide range of compositions implied is most significant. An imperfect correlation between color trend and semimajor axis is evident for these 12 asteroids, but of course the statistics are poor.
The members of the reddish group are very dark in the ultraviolet and show prominent inflection points near 0.4 and 0.7 p.m. (The upturns in the far ultraviolet for 40 Harmonia and 79 Eurynome may not be real.) A possible cause for the broad relative absorption near 0.5 pm for the reddish asteroids is a band due to Ti”. Asteroids 1 Ceres, 2 Pallas, and 13 Egeria show a bluish trend, except that Ceres and Egeria have sharp turndowns toward the ultraviolet. Except for 3 Juno, the intermediate asteroids (11 Parthenope, 16 Psyche, and 29 Amphitrite) lack the rise near 0.7 pm characteristic of the redder asteroids. All the intermediate asteroids are moderately reflective in the ultraviolet, except 16, which is very reflective by comparison. Some of the asteroids other than 4 and 192 show hints of the 0.9 pm absorption band, but we must await reduction of additional observations of these objects to be sure. It is certainly a fair generalization that absorption bands as prominent as that of Vesta are unusual.
Two of the 12 asteroids studied are members of the same Hirayama family (Brouwer's 25th family). These two asteroids (17 Thetis and 79 Eurynome) have reflectivity curves that are identical to each other to within observational errors. This observation is consistent with the hypothesis that the family is composed of fragments from a single asteroid. We are attempting to observe other pairs of family members to see if this is a general rule.
SUMMARY AND CONCLUSIONS
The results of the MIT program that have been presented here are preliminary. Altogether we have obtained fairly comprehensive spectral reflectivity observations for 23 asteroids, and some data on 9 others. Of these, 12 have been partially reduced and described in this paper; the remainder will be reduced very soon. Through cooperation with Dennis Matson, we have obtained data on 12 asteroids that were included in his thermal infrared program.” These preliminary results are most promising because they demonstrate that the asteroids have a wide variety of surface compositions and that many of the spectral reflectivities do contain diagnostic bands and inflections that may lead to precise mineralogical identifications. Even when explicit