Infrared spectral observations of asteroid 4 Vesta

ICARUS 26, 420--427 (1975)

Infrared Spectral Observations of Asteroid 4 Vesta H A R O L D P. LARSON XND U W E F I N K Lunar and Planetary Laboratory, University of Arizona, Tucson, Arizona 85721 R e c e i v e d April 3, 1975; revised April 23, 1975 A n ir s p e c t r u m of asteroid Vesta, the first of a n y asteroid, has been recorded at a spectral resolution o f 4 4 c m -1 w i t h a F o u r i e r spectrometer. A n electronic absorption b a n d is observed t h a t is assigned to an iron-rich p y r o x e n e (pigeonite) spectroscopically similar to t h a t found in certain eucrites. O t h e r i m p o r t a n t rock-forming minerals such as olivine and plagioclase feldspar are n o t observed. T h e r e is no evidence for compositional v a r i a t i o n with rotational phase angle. This spectroscopic picture of Vesta suggests considerable e v o l u t i o n including t h e m e l t i n g and differentiation o f silicates.

INTRODUCTION

The value of spectroscopy for studying planetary and stellar atmospheres is well established b u t only recently have spectroscopic methods become productive for astronomical studies of surface materials. The identification of ices on the surfaces of various objects in the solar system has been a particularly successful application of Fourier techniques at low resolution (Larson and Fink, 1972; Pilcher et al., 1972; Fink et al., 1973). For asteroid research there is particular significance in combining current experimental capabilities with recently acquired insights into remote mineralogical sensing (Adams, 1974). The asteroids are still best understood through statistical classification but there is increasing awareness of their individuality and, in the case of Vesta itself, its uniqueness. Asteroid surfaces m a y still bear record of processes active early in the history of the solar system. Part of that record evidently has been preserved and delivered to us in the form of meteorites but it is generally believed that the selection provided us b y nature is incomplete and does not represent the asteroid belt as a whole. Thus, spectroscopy of individual asteroids combined with analyses of terrestrial and meteoritic rock samples can provide new information leading to a more Copyright © 1975by AcademicPres~%Inc. All rights o f reproduction in any from reserved. Printed in Great Britain

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OBSERVATIONS

Figure 1 contains the spectral results recorded in May 1974 at the Catalina 1.6m telescope with the Fourier spectrometer described b y Larson and Fink (1975a). A spectral resolution of 44 cm -1 was achieved at a signal-to-rms noise ratio of about 50 for 13.75hr of actual integration time. We estimate that the K magnitude (2.2Fm) of Vesta was about 4.8 at the time of our observations. A lunar comparison spectrum is included in Fig. 1 for elimination of solar and telluric absorption features b y the ratio technique. Details of Vesta's spectrum agree well with those of the Moon for telluric features common to both such as the CO 2 bands at 2Fro and the sharp inflection at 1.47/zm in the 1.4/~m water vapor band. We used the ratio technique combined with numerical smoothing to help identify absorptions characterizing Vesta's surface composition. To emphasize the very broad electronic absorptions characteristic of rock-forming minerals we first strongly smoothed the data to eliminate noise and weak, narrow absorptions from consideration. We used the moving polynomial method of Savitsky and Golay (1964) since it symmetrically smears features and thus 42O

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accurately simulates performing the ex- of integration time. The K magnitude of periment at lower resolution. The ratio Vesta was estimated as 5.9 for these spectrum of Vesta in Fig. 1 was produced observations. Comparison of these figures b y division of these strongly smoothed with those for the 1974 data reveals more spectra. The gaps in this curve indicated than an order of magnitude increase in inb y dotted lines correspond to regions of strumental sensitivity that resulted from strong telluric water vapor absorptions. A the use of InSb rather than PbS detectors. broad absorption centered near 2/~m is the The improved spectrum of Vesta in Fig. 2 most conspicuous spectral feature. The shows that all the weak, narrow absorpratio spectrum also shows the spectral re- tions below 2.7/~m are either solar or telflectivity to be falling toward 1/~m in agree- luric in origin. Thus, our preliminary asment with the spectrophotometric data of signment of three of the four weak features McCord et al. (1970). seen in the spectrum of Vesta in Fig. 1 to The relatively low signal-to-rms noise hydrated silicates and carbonaceous maratio in the spectrum of Vesta in Fig. 1 led terial appears not to be confirmed (Larson to considerable uncertainty in the inter- and Fink, 1975b). The spectral bandpass of pretation of certain weak features such as our latest observations did not include the the apparent aSsorptions on Vesta at 3.4, strongest of the four suspected weak, nar2.4, 2.2, and 1.7/~m. To aid our analysis we row absorptions on Vesta, that at 3.4/~m, reobserved Vesta at the earliest opportu- which we tentatively had assigned to CH nity during its 1975 apparition with very in carbonaceous material. Since meteorites much improved instrumental sensitivity. containing organic matter do show a weak Figure 2 contains the spectrum of Vesta band at this wavelength due to CH (Gaffey recorded in May 1975 at the Steward and Salisbury, 1975; Salisbury, 1975), imObservatory 2.3m telescope. I t displays proved observations of Vesta in this speca spectral resolution of 21 em -1 at a signal- tral region m a y still confirm our original to-rms noise ratio of about 130 for 47rain interpretation.

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Through an exhaustive comparison with meteorite spectra, Gaffey (1974) confirmed The first class of materials we compared Vesta's general agreement with spectra of with Vesta's spectrum were ices whose certain basaltic achondrites, in particular, near-infrared spectra are rich in both broad eucrites of his spectral type 1. Infrared and narrow features that cannot be con- p h o t o m e t r y of Vesta b y Johnson et al. fused with the more complex absorptions (1975) further supported the association of of minerals. None of the ices studied in our Vesta's surface composition with eucritelaboratory (H20, CO2, H2S, CH4, N H 3, and type material. NH4SH ) provided a match with any of This study not only confirms the above Vesta's spectral features, which thus interpretations through our detection of establishes that the surface of Vesta lacks the 2Fm pyroxene band but, more imporany of these substances. tant, it permits a more precise identification The broad absorption near 2Fm in the of the chemical composition of Vesta's ratio spectrum of Vesta in Fig. 1 can be pyroxene component that leads to some unequivocably assigned to Fe 2+ in a important conclusions. With a Fourier pyroxene mineral. Laboratory spectro- spectrometer the determination of the scopic studies of rocks (Hunt and Salisbury, band center of the pyroxene absorption is 1970; Adams, 1974) have established that not subject to the same uncertainties that among rock-forming minerals only the have influenced the calibration of the specpyroxenes display electronic bands at such trophotometric data at 0.9Fro. Also, no long wavelengths. A companion band near other mineral displays electronic transi0.9#m has already been observed on Vesta tions near 2Fro which eliminates the probb y McCord et al. (1970), who first noted that lem of mixtures of minerals that can conVesta's spectrum below 1.0Fm agreed well fuse interpretation of the 0.9Fm pyroxene with that of basaltic achondrites such as band. Our pyroxene identification rests Neuvo Laredo. They attributed the pyro- upon recent laboratory studies b y Adams xene feature seen on Vesta to a magnesian (1974). In a pyroxene the 0.9Fro feature pigeonite (CaSiO 3 < 15%, FeSiO 3 < 60%, may actually be found between 0.9 and and MgSiO 3 > 25%). Chapman (1971) re- 1.05Fm while the 2.0Fm feature varies in vised upward the iron or calcium compo- position between 1.8 and 2.3Fro. Through nent of the pyroxene through an improved spectral studies o f mineral samples of calibration procedure that shifted the known composition Adams found that the 0.9/~m feature to longer wavelengths. observed band positions in pyroxenes are SPECTRUM A~IALYSIS

IRSPECTRALOBSERVATIONS

systematically related to Ca 2+ and Fe 2+ content. He has prepared empirical curves relating both the 0.9 and 2.0/~m band centers observed in a given pyroxene to its chemical composition. To locate the position of Vesta's 2ftm pyroxene absorption, the spectral reflectivity curve in Fig. 1 was given analytic representation through a least-squares-adjusted polynomial. From this curve, we located the absorption maximum at 2.06 ± 0.01/~m. Although the Moon is completely adequate for elimination of solar and telluric absorption features, it will impose its own color and weak silicate features upon any other object with which it is ratioed. We have considered this effect carefully in connection with previous studies of surface compositions of objects in the solar system. For general albedo characteristics a solartype stellar comparison such as y Boo is certainly preferred but if the Moon is the only convenient comparison object available, as was the case for our 1974 Vesta observations, then it can be used without serious compromise. We show in Fink et al. (1973, Fig. 1) a comparison of the ir spectra of ~ Boo and the Moon. The Moon's color is certainly reddened compared with the solar-type star. A ratio spectrum of these two objects, however, would show t h a t near 2/~m the slope is nearly flat and becomes increasingly steep only toward 1 ftm. This behavior is also seen in the Gemini 7 lunar albedo measurements by Condron et al. (1968). Division by the lunar spectrum would produce shifts to longer wavelengths in the positions of broad absorptions in the asteroid's spectrum near l'/~m but at 2/~m the effect is negligible. The Moon's own silicate absorptions are extremely weak. Spatially resolved telescopic observations of the Moon's surface have revealed the 0.95/~m pyroxene band and its variations with lunar topography but no absorptions beyond 1 ~m were observed (McCord and Johnson, 1970). The Gemini 7 lunar albedo curves also showed a slight inflection near 1/~m but no evidence of the 2~m silicate feature. Our own experience with ratio spectra of the Moon with solar-type stars indicates t h a t the Moon's 2ftm silicate feature is not detectable. Our lunar data are

OF V ES TA

423

always recorded with the Moon drifting over the spectrometer's input aperture to average the real color differences of the lunar surface. These considerations mean t h a t our determination of the center of Vesta's 2/~m pyroxene band does not contain any systematic shifts resulting from our use of the Moon as a comparison object. Other effects t h a t are more difficult to quantize such as the assumption of band symmetry and Vesta's own ir color can affect the position and interpretation of Vesta's 2/~m band but we have no reason to suspect anything more than a minor influence. The interpretation to follow would not be affected if our estimated uncertainty of ±0.01/~m were increased by a factor of 2 or even more to include some presently unrecognized shift. Using Adam's calibration of band center versus FeSiO 3 (Fs) component (Adams, 1974, Fig. 3, top), our spectroscopic measurement of 2.06/~m implies a pyroxene on Vesta with 75% Fs. From a second curve (Adams, 1974, Fig. 5, top), 15°/'o CaSiO 3 (Wo) is indicated. The remainder is attributed to the MgSiO 3(En) component. Thus the pyroxene on Vesta deduced from the infrared data has the composition Wol 5Enl 0Fs75, a ferropigeonite. Adams and McCord (1972) have shown t h a t when the centers of the 0.9 and 2.0/~m bands of iron-bearing pyroxenes are plotted against each other, a useful curve is produced. This "pyroxene line" is another way of expressing the systematic variation of band position with Ca 2+ and Fe 2+ content. Bands shifted for any reason from their true position in a pure pyroxene will plot off the pyroxene line. Possibilities include the presence of other silicate phases with strong absorptions, the presence of certain ions such as Fe 3+, Ti 3+, A13+, etc., or systematic errors in the experiment or spectral analysis. In Fig. 3 we have reproduced the pyroxene line from coordinates taken from Adams (1974). The location of Vesta on this line uses the 2.06fern band position of this study and the 0.95pm measurement of Chapman (1971). Given the scatter in the calibration itself and the uncertainties in the band positions, the location of Vesta on the pyroxene line is

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acceptable. We indicate on Fig. 3 the trends in mineralogical type as one proceeds along the pyroxene line. We also include from Gaffey (1974) the locations of the basaltic achondrite meteorite types. Among both mineral types (pigeonites) and meteorite types (eucrites) Vesta occupies a very iron-rich position. The composition of its pyroxene (W%sEn10Fs75) is quite close to those eucrites that match its spectrum so well, for example, Neuvo Laredo (WotlEnz4Fs65), Pasamonte (up to 70% ,Fs) and Petersburg (up to 70% Fs) (Duke and Silver, 1967). Pyroxene seldom occurs b y itself in rock formations. Other important rock-forming silicates that m a y be present include olivine and plagioclase feldspar, both of which have broad ir absorptions that can be sought in Vesta's spectrum. Olivine displays a broad Fe z+ electronic band at about 1.05~m that shifts slightly with iron content and disappears completely for pure fosterite. Within limits imposed b y the signal-to-noise ratio and interfering water vapor absorptions affecting this region of Vesta's spectrum, we see no evidence for olivine. Very strong olivine absorption on Vesta would already have been detected in the spectrophotometric observations below

1 ~m since the short wavelength limit of this band extends to 0.7Fm. Plagioclase feldspar normally accompanies pyroxene in basaltic rocks. Pure samples are spectroscopically neutral in the region 1-2.5/~m b u t Fe 2+ present as an impurity produces an electronic band at about 1.25~m. The Fe 2+ absorption in plagioclase has not yet been systematically related to mineralogical composition. Thus, the presence or absence of the Fe 2+ plagioclase feature has limited interpretative value although Gaffey (1974) used this absorption effectively in his spectroscopic classification of meteorites. He found that the plagioclase component ofeucrites manifests itself spectroscopically as a weak inflection near 1.25~m while for howardites with less plagloclase the inflection disappears. Although there appears to be an inflection near 1.23Fm in Vesta's ratio spectrum in Fig. 1, the low signal-to-noise ratio in this region does not permit a convincing identification of this mineral. We expect further observations of Vesta in late 1975 will provide a more definite conclusion regarding the presence of this mineral. We conclude our spectral analysis with an attempt to correlate Vesta's spectral features with its rotational period. Varia-

IR SPECTRAY,, OBSERVATIONS OF VESTA

tions in the light curves of asteroids, including Vesta's, often show several maxima and minima which are usually attributed to a nonspherical shape. According to Taylor's fight-curve analysis (1973), Vesta is a spheroid with variation in reflected light caused b y one diameter's being 15% longer than the other two. Variations in refleeted light could likewise be produced b y compositional differences, especially for an object displaying the broad, deep absorptions evident on Vesta. This study provides a test to help distinguish between these two alternatives. We divided our data into two sets representing observations made during the minima and maxima of Vesta's light curve, respectively. We thank Mr. Ronald Taylor for providing us with the light curve of Vesta he recorded on June 9, 1974, at the Catalina 61" telescope. From his curve we determined the rotational phase applicable to our observations the previous month. For each of the two sets of data we performed the same spectral analysis previously described and summarized in Fig. 1. The pyroxene band still dominated the spectra with its center at 2.04/xm for data recorded during the light-curve minima and at 2.08/x for observations during the light-curve maxima. As m a y be seen in Fig. 3, this wavelength spread represents only a slight change in the composition of Vesta's pyroxene. Since no evidence could be found relating Vesta's surface composition to its light curve, the purely geometric interpretation of Taylor is, therefore, preferred. Discussion

The spectroscopic evidence just presented has permitted the positive identification of one mineral on Vesta's surface, a ferropigeonite, and it has also placed constraints on the presence of other important rock-forming minerals. Olivine can be present only as a minor constituent or it must be quite iron-deficient so as not to produce the characteristic ir spectrum of this mineral. Plagioclase feldspar likewise is restricted to being a minor constituent with little Fe z+ substituting as an impurity. This spectroscopic picture of Vesta's surface is

425

to be compared with current theories of the origin and evolution of our solar system. Of these theories, equilibrium condensation of mineral phases from a solar nebula appears most successful in explaining many of the detailed properties of meteorites and the bulk properties of the planets and their satellites (Grossman and Larimer, 1974). In this theory the primordial condensates calculated to form below 1500K are found to be similar to the minerals found in ordinary chondrites, principally pyroxene, olivine, feldspar, nickel-iron, and troilite. The ferromagnesian silicates olivine and pyroxene are expected to be increasingly iron-rich in the asteroid belt in accordance with a recognized trend of increased oxidation of iron with heliocentric distance in the inner solar system. The iron-rich pyroxene we identify on Vesta agrees with this predicted character of ferromagnesian silicates condensing beyond Mars b u t the absence on Vesta of an iron-rich olivine that should accompany the pyroxene is inconsistent with a primordial chondritic composition based on equilibrium condensation. Thermal activity could explain the lack of olivine, a mechanism already suggested b y the general agreement noted in this and previous studies between Vesta's spectrum and that of basaltic achondrite meteorites. There is general agreement among meteoriticists over the igneous origin of achondrites in their parent bodies through melting and differentiation of silicates in a gravitational field (Anders, 1964). Wood (1962) describes achondrites as "unequivocably igneous" and "virtually identical in mineralogy and texture to the class of terrestrial igneous rocks known as diabases," magmatic intrusions such as dikes and sills. When it is found in terrestrial formations, the ferropigeonite we identify on Vesta is characterized as '% product of rapid crystallization and its occurrence is restricted to lavas and other relatively quickly chilled rocks" (Deer et al., 1962). Combining this petrologic evidence from terrestrial and extraterrestrial samples with the pyroxene determination of this spectroscopic study, we conclude that significant thermal activity including the melting and differentiation of silicates must have occurred on Vesta at on~

426

L ~ R S O ~ AND F ~

time. Vesta's surface has been at least partially flooded with lava or, alternatively, intrusions beneath the surface have been exposed b y impact craters. This hypothesis becomes especially compelling with the recent analysis of the Ibitira meteorite b y Wilkening and Anders (1974). This meteorite is the only known vesicular basalt, that is, containing gas bubbles trapped in a rapidly cooling lava. These authors estimate that the flow from which Ibitira came was between 2.5 and 20m thick. This implies a parent body capable of supporting a magma chamber of significant proportions. Ibitira's existence demands igneous activity in its parent body on the same scale as is spectroscopically suggested on Vesta. Such thermal activity has been postulated for meteorite parent body candidates such as the asteroids to explain the gross differences in meteorite types including the melting of the irons, the silicate differentiation in the aehondrites and the metamorphism in the ordinary chondrites. Vesta's ir spectrum thus constitutes an independent, in situ, verification of the required thermal activity on an asteroid itself. A number of heat sources have been investigated to explain the thermal histories revealed in meteorites of intense heating followed by rapid cooling in asteroid-sized parent bodies. Two mechanisms, extinct radioactivity (Fish et al., 1960) and inductive heating (Sonett et al., 1970) have received considerable attention although neither scheme has yet received complete experimental confirmation. Unfortunately, the spectroscopic data of this study cannot identify the dominant mechanism. One observation that m a y be useful, however, involves the strength of Vesta's pyroxene feature. No other asteroid studied spectrophotometrically displays such a prominent pyroxene band (Chapman et al., 1973). We suggest that if a heating episode acting on the surface rather than on the core of the asteroids was effective in the asteroid belt then many more examples of Vesta's spectral behavior ought to be seen. Since this is nat the case, then such sources as chemical heating or enhanced luminosity of the Sun are less compatible with the observational data than are core-heating processes.

The uniqueness of Vesta's spectrum need not be extended to Vesta as an object, however. Other large asteroids probably shared Vesta's capability to melt and differentiate but either they were broken up through collisions or they somehow managed to contain the differentiated silicates from view. Vesta itself m a y have experienced a nearly catastrophic collision that so fractured its mantle that lava from a deep magma chamber poured over extensive portions of its surface. Perhaps the impact crater near Vesta's south pole proposed by Taylor (1973) to explain Vesta's light curve marks this event. The results of this first ir spectroscopic study of an asteroid emphasize the potential, and limitations, of remote mineralogical analysis. Additional spectroscopic observations of asteroids combined with laboratory comparison surveys will be required to help solve fundamental problems of solar system geochemistry, particularly the origin of organic matter. We have presented evidence that at least the largest asteroids have evolved independently in response to certain properties of the primordial solar system that are not yet completely understood. This realization elevates a certain few of the asteroids to objects as unique as the planets themselves and places their properties among the most sensitive tests of any theory of the origin and evolution of the solar system. ACKNOWLEDGMENTS Especially useful discussions were held with G. Chapman, M. Drake, G. Sill, and L. Wilkening. This research was supported by NASA Grants NGR 03-002-332 and NSG 7070. REFERENCES ~-~)AMS, J . B., AND MCCORD, T. B. (1972). Electronic spectra of pyroxenes and interpretation

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IR SPECTRAL OBSERVATIONS OF VESTA

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