The dependence of reflectance spectra of Mercury on surface terrain

ICARUS 59, 6(I--68 {19841

The Dependence of Reflectance Spectra of Mercury on Surface Terrain FAITH VILAS Lunar and Planetao' Laboratot3', Univer,sity o f Arizona, Tucson, Arizona 8,5721

MARTHA A. L E A K E Department o f Physics. Astronomy and Geology, Valdosta State College, Valdosta, Georgia 32698 AND

W E N D E L L W. M E N D E L L NASA Johnson Space Center/SN3, Houston. Tt'.~.a,~ 77058 Received N o v e m b e r 28, 1983: revised April 2. 1984 Reflectance spectra of Mercury, covering the spectral range of ~-t).3-I.1 /zm obtained during 1963-1976, were examined for any correlations with surface terrain. Mercury's 6.1385°/day rotational rate, the phases of the planet around maximum elongations, and bidirectional reflectance spectroscopy theory were used to identify the surface area associated with each spectrum. Data from 1974-1975, re-reduced with improved standard star flux ratios, show a weak absorption band in the near infrared not see in earlier analyses. Older spectra suggest that the western longitudes of the unimaged side of Mercury are similar to the rest of the planet. Spectra of the intercrater plains in the 0-90 ° quadrant suggest a possible absorption band. Spectra of areas dominated by Caloris Basin with the encompassing smooth plains may show Fe 2+ abundances in the soil comparable to lunar highlands soil. No striking differences between spectra of intercrater plains and spectra of smooth plains are found. The absorption features seen in spectra of Mercury are generally weaker than features seen in lunar spectra.

McCord, 1976)--have never been satisfactorily explained. In this study, the authors collected all known published and unpublished reflectance spectra of Mercury. Some data taken in 1974 and 1975 were re-reduced using improved standard star calibrations. Utilizing Mercury's 6.1385°/day rotational rate and the phases of the planet around maximum elongation, the spatial coverage of the planet's surface can be constrained for the given observations. Longitudinal intervals corresponding to the illuminated portions of the planet were calculated for the dates of observations. Hapke's bidirectional reflectance theory for a planetary surface (Hapke, 1981) was applied to define the sur-

INTRODUCTION

During the past 20 years, astronomers have taken telescopic reflectance spectra of Mercury in an attempt to understand the surface mineralogy of the planet. These data have always been difficult to obtain due to Mercury's close proximity to the Sun: the planet can only be observed around greatest elongation, an angular separation never greater than 27.7 ° from the Sun. Discrepancies between spectra--the most notable being an apparent absorption feature near 0.9 /zm attributed to an Fe 2 orthopyroxene band seen in some spectra (McCord and Adams, 1972; McCord and Clark, 1979) but not in others (Vilas and 60 0019-1035/84 $3.11(I Copyright t, 1984by Academic Press, htc. All rights o1 reproduction in any lbrm reserved.

MERCURY REFLECTANCE SPECTRA

61

face area of the planet which contributed the greatest amount of reflected sunlight to the spectra taken on individual dates. Mariner I0 images, radar profiles, and albedo data were then used to identify the reflectance spectra with geological units on the surface of Mercury.

vatory (Irvine et al., 1968). The observations used 9 or l0 narrowband interference filters covering a wavelength range of 314710,635 A. Relative intensities at the different wavelengths were derived from magnitudes using the conversion:

REFLECTANCE SPECTRA

and scaling the intensities to 1.000 for the 5012-,~ filter. The data are grouped around the maximum elongations on 13 June 1963 (23°W), 24 August 1963 (27°E), 24 May 1964 (25°W), and 6 May 1965 (27°W). In Figs. l a - d are plotted the unweighted means and standard deviations of reflectance values taken on dates around these elongations from the data in Table III of Irvine et al. (1968). Irvine e t al. does not list errors for the daily Mercury data, and general errors for all observations made during monthly intervals (Table IX) are not included in the errors for Fig. 1. Figure le is a previously unpublished reflectance spectrum (T. B. McCord, private communication) consisting of the un-

The astronomer is generally limited to observing Mercury during astronomical twilight with the planet close to the horizon. The planet is viewed through a large and rapidly changing airmass, causing severe differential refraction of the light from the planet's surface. Scattered sunlight is also present. Careful monitoring of a standard star through the same airmass interval in which Mercury is observed is necessary to correctly calibrate the extinction for the observations. The times to observe Mercury during the year are limited, and spectrophotometric observations may be restricted by the photon counting capabilities of available instrumentation. Hence, very few reflectance spectra of Mercury exist. Data selected for this study covered approximately the 0.3- to 1.1-/zm spectral region. This provided a sufficient spectral range for mineralogically diagnostic information to be seen, and allowed comparison among spectra taken on different dates. Photometry covering more limited intervals within this range (e.g., UBV photometry; observations made from 3147-5012 ,~ by Irvine e t al. (1968)) or at longer wavelengths (e.g. McCord and Clark's (1979) data between 1.1 and 2.5/zm) were not considered. Given the improvements in instrumentation over the last 20 years, the more recent data should be considered to contain more reliable compositional information. All dates discussed in this paper are Universal time. Spectral reflectance values presented in this paper in graphic form are available from the authors on request. The earliest reflectance spectra included here were taken on thirteen dates during the years 1963-1965 at the Boyden Obser-

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FIG. 1. Mercury reflectance spectra produced by averaging spectra calculated from observations made around maximum elongations (see text): (a) June 1963, (b) August 1963, (c) May 1964, (d) May 1965, (e) June 1969, (f) December 1969, (g) March 1972, (h) September 1974, (i) October 1974, (j) March 1975. The lack of error bars for June 1963 only signifies that one night of observations is graphed here.

62

VILAS, LEAKE, AND MENDELL

weighted mean and standard deviation of four observations made on the nights of 17 and 18 June 1969 near the maximum elongation of 23 June 1969 (23°W). The observations used 22 narrowband interference filters covering a wavelength range of 3190-10,530 A. We have applied a calibration technique utilizing known reflectance values of areas on the Moon, which was used in the reduction of subsequent spectral reflectance data of Mercury (McCord and Adams, 1972). The data are scaled to 1.000 for the 5640-A filter. The observatory where these observations were made is unknown. The data published by McCord and Adams (1972) are presented in unaltered form in Figs. If and g. The same filters and calibration techniques were used for these data and the June 1969 data. Figure If is the unweighted mean and error of four observations taken on 26 D e c e m b e r 1969 near the maximum elongation of 27 D e c e m b e r 1969 (20°E) at the Cerro Toiolo Inter-American Observatory. Figure lg is the unweighted mean and error of six observations taken on 13 March 1972 near the elongation of 14 March 1972 (18°E) at the Kitt Peak National Observatory. Reflectance data of Mercury were taken on eight dates in 1974 and on four dates in 1975 at the Cerro Tololo Inter-American Observatory around the maximum elongations of 1 October 1974 (26°E) and 6 March 1975 (27°W). These were first published as composite spectra for each observing run (Vilas and McCord, 1976). These observations used 24 narrowband interference filters covering a wavelength range of 335(I10,640 A. As part of this study, these data were reexamined. The data reduction procedure for the Mercury/Sun reflectance spectra is described in Vilas and McCord (1976). One or both of the standard stars 109 Virginis and c~ Lyrae were observed during each night in 1974. The standard stars e Aquarii and c~ Aquarii were observed during each night of 1975. Improved standard star calibrations (P. Owensby and

T. B. McCord, private communication) permit the recalculation of these spectra. Uncertainties were discovered in the 109 Vir observations during the 1974 observing run, therefore the only reflectance data for Mercury considered were those in which c~ L y r was used as the standard star. This limited the number of observation dates to five nights. An additional night of observations was eliminated after examining records of sky conditions during this observing run. The data were then grouped by phase angle. Improved c~ Lyr/Sun spectral flux ratios were then used to reduce eight observations of Mercury on 29 September 1974 (Fig. lh), and thirty-seven observations on 5-7 October 1974 from which a weighted average and error were calculated (Fig. It). Fifty-six observations of Mercury on 8-11 March 1975 were reduced using improved ¢~ Aqr/Sun spectral flux ratios. (The new" calibration produced no change in the ~: A q r / a Aqr ratio.) Figure Ij is a weighted average and error for these four nights. These data were scaled to 1.000 for the 5670-J~ filter. McCord and Clark (1979) show a comparison between a portion of a reflectance spectrum of Mercury and Apollo 16 soil, presented here in unaltered form in Fig. 2. The spectrum is a composite of fifty-one observations made on the nights of 21-23 April 1976, near the maximum elongation of 28 April 1976 (21°E), using a circular-variable-filter spectrometer with 120 bandpasses covering the spectral range of 0.652.50 /xm. The data shown are included in the spectral region considered in this study. Additional data were taken by Tepper and Hapke (1977) on 6 February 1977 near the maximum elongation of 29 January 1977 125°W). B. Hapke (private communication) requested that these data not be considered since probable atmospheric H20 contamination would make the results questionable. S U R F A C E C O V E R A G E OF T H E P L A N E T

The rotational period of Mercury is expected to equal two thirds of its orbital pe-

MERCURY REFLECTANCE SPECTRA

63

v•l.04 >_ 0.96

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FIG. 2. A portion of April 1976 Mercury reflectance spectrum (data points) compared to a laboratory spectrum of Apollo 16 soil (continuous line) showing the 0.89-/xm Fe z+ orthopyroxene absorption feature. The sloped continuum (reddening) seen in Fig. 1 spectra has been removed. See McCord and Clark (1979).

riod, or 58.6462 days. By measuring the changes in the shadows of features on the planet's surface in Mariner 10 images, Klaasen (1975) calculated a rotational period of 58.661 +- 0.017 days, a value in agreement with the expected period within the calculated error. Therefore, we used the adopted convention of two thirds of the orbital period in this study. Table I contains the phase angle, central

APR 76

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meridian, terminator, and bright limb for each date when observation(s) of Mercury were made. These values were calculated from the tabulation of Smith and Bruce (1971). The physical ephemerides are for Oh or 12h on the date of observation, depending upon what we estimated were close to the probable times of the observations. Positive phase angles represent eastern elongations (as viewed in the Earth's sky, the I

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FIG. 3. Longitude intervals for dates of observation for Mercury reflectance spectra. The broad side is the bright limb tapering to a point for the terminator (indicating the greater contribution of light to the spectrum from the bright limb area). The hatched lines mark the 10-190 ° interval photographed by Mariner 10.

64

VILAS. LEAKE, AND MENDELL

western side of Mercury is illuminated, however, given the hemisphere presented toward the Earth, the eastern longitudes are illuminated); negative phase angles represent western elongations. Figure 3 is a graph of longitude interval vs date of observation for the reflectance spectra of Mercury. Smith and Bruce ( 197 I) demonstrate that 54 sidereal periods of Mercury are equal to 13.00600 tropical years of the Earth, such that every 13 years the Mercurian physical ephemeris repeats itself. Smith and Reese (1968) report that this quasi-commensurability allows only certain areas of Mercury's surface to be visible from different locations on the

"FABLE 1 PHYSICAL EPHEMERIDES FOR SPECTRAl REFLECTANCE OBSERVATIONS OF MERCURY

Observation date (UT)

Phase angle (o)

16 Jun 1963 14 A u g 1963 15 A u g 1963 18 A u g 1963 19 Aug 1963 22 A u g 1963 25 A u g 1963 26 A u g 1963 19 May 1964 20 May 1964 22 May 1964 15 May 1965 18 May 1965 17 Jun 1969 1 8 J u n 1969 26 Dec 1969 13 Mar 1972 29 Sep 1974 5 Oct 1974 6 Oct 1974 7 Oct 1974 8 Mar 1975 9 Mar 1975 10 Mar 1975 11 Mar 1975 21 A p t 1976 22 A p t 1976 23 A p t 1976

-98.3 69.5 71.0 75.5 77.0 81.9 87.0 88.9 --116.(I 113.7 109.2 83.0 77.1 -121.5 118.8 70.7 89.1 73.8 86.4 88.9 91.5 78.2 -76.6 - 75.11 -73.4 77.4 81.3 85.3

Central Bright Terminator meridian limb I ~) (,,) (o) 238.4 136.(I 140.8 155.4 160.3 175.4 190.7 195.9 179.6 185.2 196.1 241.1 255.3 19.6 25.2 246.2 88.6 197.0 228,2 233,6 239.0 353.6 358.6 3.6 8.6 257.8 262.5 267.2

328.4 46.2 50.8 65.5 70.3 85.4 10(1.7 1(15,9 269.6 275.2 286.1 331.1 345.3 109.6 115.2 156.3 358.5 107.1 138.2 143.6 149.0 83.6 88.6 93.6 98.6 167.9 172.4 177.2

246.8 156.7 160.0 171).[) 173.4 183.5 193.7 197.0 205.6 208.9 215.4 234.1 242.4 51.1 54.(I 265.6 89.4 213.3 231.8 234.7 237.5 341.8 345.2 348.6 352.0 270.5 271.1 271.9

Earth. During a 13-year interval, Northern hemisphere observations will emphasize Mercurian central meridian longitudes of 90 and 270 °, while longitudes of 0 and 180° are poorly viewed. However, an observer in the Southern hemisphere finds that during the same time interval, all longitudes except those near 90 and 270 ° will be viewed equally well as the central meridian. The central meridians for observations from thc Northern hemisphere (e.g., 13 March 1972: 21-23 April 1976) do favor 90 and 270 °. As most of the observations discussed here were made at Southern hemisphere observatories, relatively complete spectral coverage of Mercury's surface has occurred over the 13-year interval in which these spectra were acquired. To vary future coverage of the planet's surface, observations should continue in the Southern hemisphere, although the longitudinal intervals presented to the Earth will vary slowly through the 13-year interval. To further define the portions of Mercury's illuminated disk which contributed the most light to the detector for a given night (or nights), we utilized Hapke's (1981) bidirectional reflectance formula for a planetary surface of close packed particles of random shape. Table II contains r, the bidirectional reflectance calculated along the luminance equator for western elongations with phase angles g -- -70, -90, and -110 °, for equal increments of sin e. The angle of emission, e, is measured in degrees along the luminance equator from the western half of the disk ( - 9 0 °) through the central meridian (0°) to the eastern half of the disk (+90°). The angle of incidence, i, corresponding to the location along the luminance equator for a given phase is also listed. The bidirectional reflectance values are multiplied by 1000 simply to provide more convenient numbers for examination. The illuminated portions of Mercury for the western elongations with phase angle g = - 7 0 , - 9 0 , and - 110° are shown in Fig. 4. Theoretical brightness contours demonstrate how the intensity of reflected light

MERCURY

REFLECTANCE

65

SPECTRA

T A B L E II MERCURIAN BIDIRECTIONAL REFLECTANCE ALONG THE LUMINANCE EQUATOR FOR DIFFERENT PHASE ANGLES

e

Sin e

(°)

-90.0 -71.8 -64.2 -58.2 -53.3 -48.6 -44.4 -40.5 -36.9 -33.4 -30.0 -26.7 -23.6 -20.5 - 17.5 - 14.5 - 11.5 -8.6 -5.7 -2.9 0.0 2.9 5.7 8.6 11.5 14.5 17.5

g = -70 °

- 1.00 -0.95 -0.90 -0.85 -0.80 -0.75 -0.70 -0.65 -0.60 -0.55 0.50 -0.45 -0.40 -0.35 -0.30 -0.25 -0.20 -0.15 -0.10 -0.05 0.00 0.05 0.10 0.15 0.20 0.25 0.30

g = -110 °

i (°)

r x 1000

i (o)

r × 1000

i (°)

r × 1000

20.0 1.8 -5.8 -11.8 -16.9 -21.4 -25.6 -29.5 -33.1 -36.6 -40.0 -43.3 -46.4 -49.5 -52.5 -55.5 -58.5 -61.4 -64.3 -67.1 -70.0 -72.9 -75.7 -78.6 -81.5 -84.5 -87.5

26.87 21.90 20.26 19.09 18.14 17.31 16.56 15.86 15.20 14.56 13.94 13.33 12.72 12.11 11.49 10.87 10.22 9.55 8.85 8.12 7.35 6.53 5.65 4.69 3.65 2.50 1.21

0.0 - 18.2 -25.8 -31.8 -36.9 -41.4 -45.6 -49.5 -53.1 -56.6 -60.0 -63.3 -66.4 -69.5 -72.5 -75.5 -78.5 -81.4 -84.3 -87. I -90.0

26.79 21.47 19.45 17.93 16.65 15.51 14.45 13.45 12.49 11.55 10.61 9.68 8.75 7.79 6.81 5.81 4.76 3.67 2.51 1.30 0.00

-20.0 -38.2 -45.8 -51.8 -56.9 -61.4 -65.6 -69.5 -73.1 -76.6 -80.0 -83.3 -86.4 -89.5

26.72 20.25 17.56 15.47 13.66 12.01 10.46 8.97 7.52 6.08 4.64 3.20 1.73 0.24

varies across a planetary surface. The reflectance distribution has been smoothed to a resolution equal to 1/40 of the apparent disk diameter. Although the figure depicts N

g = -90 °

N

only western elongations, the distribution for eastern elongations is a mirror image. The brightest portion of the constant albedo planet lies on the luminance equator at the bright limb. At smaller phase angles, as more of the planet is illuminated, the brightness distribution across the disk becomes more uniform. CORRELATION

: -70 o

S

S

g = -90 °

g : -llO o

FIG. 4. The illuminated portions o f M e r c u r y for three different phase angles at w e s t e r n elongation. Contours s h o w the loci o f points o f equal bidirectional reflectance (brightness) a c r o s s the surface as s e e n by an o b s e r v e r on Earth.

The above discussion presents a framework for examining the reflectance spectra with respect to surface terrain of Mercury. Trask and Guest (1975) first defined the two Mercurian geologic units discussed here, the intercrater plains and the smooth plains. The intermediate results of this

66

VILAS, LEAKE, AND MENDELL

study (Vilas et al., 1982) have been completely superceded by the conclusions drawn following the more rigorous approach taken here. Early Spectra

Even though the spectra taken during 1963-1965 lack sufficient spectral resolution to show detailed mineralogical information, some general conclusions can be drawn. All of the spectra examined in this study show the same slope for wavelengths shorter than 0.6-0.7 gin, except for obviously anomalous points (e.g., Fig. Ig). At longer wavelengths, the 1963-1965 data exhibit the same increase in slope (reddening) with increase in phase angle seen for the Moon. (Irvine et al. (1968) noted the same change for all of their spectra of Mercury when grouped only by phase.) Three of the spectra (Figs. la, c and d) cover the portion of Mercury not photographed by Mariner 10. Figure Ib covers an area of Mercury known from Mariner" 10 to be predominantly intercrater plains. The similarity between these spectra of the unimaged side and spectra of known terrain suggests that there is no radical difference in composition on the unimaged side, and provides supporting evidence for the idea suggested by other remotely sensed data that the terrain is probably dominated by intercrater plains from the longitude of 24{)360 ° (0 °) neat" the Mercurian equator. AIbedo data (Murray et al., 1972) imply that the intercrater plains in the 0-90 ° quadranl extend from 360 (0 °) to 330 ° . Radar data (Zohar and Goldstein, 1974) indicate the presence of smooth plains between 222 and 235 °, grading into intercrater plains from 240 to 252 '~. lntercrater Plains

The 1969-1976 data have sufficient spectral resolution to permit their examination for specific mineralogical reflectance features. Four of the spectra (Figs. le, g, h, and j) c o v e r terrain in the 0-90 ° quadrant,

which is predominantly intercrater plains. Absorption by teiluric H20 in the Earth's atmosphere has not been completely corrected in the June 1969 spectrum (Fig. le) near 0.7, 0.8, and 1.1/xm. There is evidence for a broad feature between 0.75 and 0.95 ixm. H o w e v e r , considering the errors for this early spectrum, this result is suspect. McCord and Adams (1972) first interpreted the dip in the March 1972 spectrum (Fig. lg) as a possible Fe z+ pyroxene feature at 0.95 txm. The presence of a dip at 0.82/xm may suggest that incomplete removal of atmospheric H20 is the origin of this proposed pyroxene feature. The March 1972 data contain no evidence for the broader feature suggested in the June 1969 spectrum. Within the errors, no mineralogical features can be seen in the revised September 1974 spectrum (Fig. lh). In the revised March 1975 spectrum (Fig. lj), the incomplete removal of the atmospheric H20 is seen at or near 0.7, 0.8, 0.93, and I. I p.m. There is again the suggestion of a broad, shallow absorption feature between 0.73 and 0.96/~m. The possible presence of a shallow absorption feature between 0.75 and 0.95/xm in spectra of the intercrater plains suggests that the surface soil contains the Fe 2~ ions present in pyroxenes. The quality of these spectra do not allow any absorption feature to be studied quantitatively. Caloris Basin and S m o o t h Plains

The smooth plains are considered to be younger terrain than the intercrater plains, volcanic in origin, emplaced after the impact which formed Caloris Basin. The Mariner 10 images of Mercury terminate near 190° longitude. We make the assumption that Caloris Basin and the encompassing smooth plains extend symmetrically about the basin center (32°N, 195°W). The basin rim would then have a lower latitude limit of 15°N and upper limit of 45°N. The smooth plains would extend to a latitude of 5°S and, near the basin center, to a longi-

MERCURY REFLECTANCE SPECTRA tude of 235 °. Radar data (Zohar and Goldstein, 1974) show smooth terrain from 222 to 235 ° at 9°N latitude, which suggests that the smooth plains extend further than simple symmetry would suggest near the southwest side of Caloris. Below this region, intercrater plains dominate the southern half of the 90-190 ° quadrant, and are assumed to extend to further western longitudes. McCord and Clark (1979) show that the most recent spectrum of Mercury (Fig. 2) has an absorption feature, centered near 0.89/xm, which closely resembles the Fe 2+ orthopyroxene feature in the spectrum for an Apollo 16 (lunar highlands) soil. For this spectrum, the sub-Earth point on Mercury is located at slightly less than 2°S latitude. A reasonable assumption can be made that >50% of the light contributing to this spectrum is from smooth plains, McCord and Adams (1972) interpreted the dip at 0.95 /zm in the December 1969 spectrum (Fig. If) as possibly due to pyroxene, but this spectrum is subject to the same limitations as the March 1972 spectrum. The revised October 1974 spectrum (Fig. li) shows a definite broad absorption feature between 0.8 and 1.0/xm. The spectrum is sufficiently free of atmospheric H20 contamination to merit serious consideration. The sub-Earth point of Mercury for this spectrum is at 4.6°N latitude. Although the bright limbs for the three observation dates fall in the intercrater plains region, a major light contribution (>50%) to the spectrum comes from the smooth plains and the Caloris Basin.

67

an Fe 2+ absorption feature possibly attributable to pyroxene in the soil. The two spectra which may largely represent the smooth plains near Caloris Basin are of greatest interest. A possible Fe 2+ feature which is observed in these spectra is very similar to a lunar highlands soil spectrum containing approximately 5.5% FeO (McCord and Clark, 1979). This possibility is supported by the comparison of the normal albedos at 5540 A of the lunar highlands (0.10-0.11) and the Mercurian smooth plains (0.12-0.13) (Strom, 1984). Evidence is slowly accumulating to indicate that Mercury and the Moon, while probably subjected to like surface weathering processes, are somewhat dissimilar. The Mercurian smooth plains--often compared to the lunar maria--show a much weaker absorption feature than seen in the lunar maria. This can be explained by either some opaque material partially masking the actual absorption feature, or a smaller amount of Fe 2+ ions present in the soil. If opaque material is present, the high normal albedos of the different terrains on Mercury (Strom, 1984) would not be expected. Perhaps a smaller amount of oxidized Fe on Mercury implies that the majority of the Fe present on Mercury's surface is now in metallic form, or the surface content of Fe is diminished as a result of the formation of the proposed Fe core. The quality of spectra of Mercury should be improved through the use of newer spectrographs coupled with two-dimensional detectors (e.g., charge-coupled devices). The rapidly changing background sky can be then mapped concurrently with the planet's spectrum. Space Telescope, while DISCUSSION still constrained to observe Mercury The existing reflectance spectra show no around maximum elongations, has the castriking differences corresponding to varia- pability of further improving spectra by tions in terrain on Mercury. Additional evi- eliminating the atmospheric problems endence supports the conclusion that the un- tirely. Future spectra of Mercury should be imaged surface is similar to the portion of considered with attention to location on the the planet photographed by Mariner 10 and planet's surface, effects of high temperaprobably composed of intercrater plains. ture, and effects of particle size in the regoSpectra of the intercrater plains suggest lith.

68

VILAS, LEAKE, AND MENDELL ACKNOWLEDGMENTS

The authors thank Dr. T. B. McCord and Ms. P. Owensby for providing Mercury and standard star data. The work was partially supported by NASA Grant NGL-03-002-002.

REFERENCES HAPKE. B. (1981). Bidirectional reflectance spectroscopy: 1. Theory. J. Geophys. Res. 86, 3039-3054. IRVINE, W. M., T. SIMON, D. H. MENZEL, C. PIKOOS, .aND A. T. YOUNG (1968). Multicolor photoelectric photometry of the brighter planets: Ill. Observations from Boyden Observatory. Astron. J. 73, 807828. KLAASEN, K. P. (1975). Mercury rotation period determined from Mariner 10 photography. J. Geophys. Res. 80, 2415-2416. McCORD, T. B., AND J. B. ADAMS (1972). Mercury: Surface composition from the reflection spectrum. Science 178, 745-747. McCoRD, T. B., AND R. N. CLARK (1979). The Mercury soil: Presence of Fe 2~. J. Geophys. Res. 84, 7664-7668.

MURRAY, J. B., A. DOLLFUS, AND B. SMITH (1972). Cartography of the surface markings of Mercury. Icarus 17, 576-584. SMITH, B. A., AND T. C. BRUCE (1971). EphemerisJbr Physical Observations o f Mercury. New Mexico State University, Las Cruces. SMITH, B. A., AND E. J. REESE (1968). Mercury's rotation period: Photographic confirmation. Science 162, 1275-1277. STROM, R. (1984). Mercury. In Geology o f the Terrestrial Planets (M. Carr, Ed.), NASA SP 469. in press. TEPPER, L., AND B. HAPKE (1977). Mercury: Detection of a 1000 NM ferrous band. Ball. Amer. Astron. Soc. 9, 532, TRASK, N. J., AND J. E. GUEST (1975). Preliminary geologic terrain map of Mercury. J. Geophys. Res. 80, 2462-2477. VILAS, F., M. A. LEAKE, AND W. W. MENDELI (1982). Possible correlation of mineralogical variations with surface geology on Mercury. Bull. Amer. Astron. Soc. 14, 756. VILAS, F., AND T. B. McCORD (1976). Mercury: Spectral reflectance measurements (0.33-1.06/zm) 1974/ 75. Icarus 28, 593-599. ZOHAR. S., AND R. M. GOLDSTEIN (1974). Surface features on Mercury. Astron. J. 79, 85-91.