The optical effects of small iron particles that darken but do not redden: Evidence of intense space weathering on Mercury

Submicroscopic iron particles larger than about 50 nm, infused throughout mineral grains or glasses, are abundant in planetary materials altered by their environment such as shocked meteorites and lunar agglutinate glasses. Such particles darken their host material but do not redden their spectra but to date there has been no theoretical treatment of their optical effects. Using Mie theory, we modify the Hapke (2001) radiative transfer model of the effects of space weathering to include these effects. Comparison with laboratory measurements shows that the new treatment reproduces the relationship between submicroscopic iron size, abundance and reflectance. We apply this new model to near-IR spectra of Mercury recently obtained by the MESSENGER spacecraft and find that submicroscopic iron is much more abundant on Mercury than in lunar soils, with typical total submicroscopic iron abundances near 3.5 wt.% compared to about 0.5 wt.% for lunar soils We also find that the ratio of iron particles that darken but do not redden to the abundance of very small iron particles that impart the red slope to space weathered material is much larger than lunar (6 vs. 2). Both the total submicroscopic iron abundance and ratio of particle size fractions are consistent with the higher production of melt and vapor in micrometeorite impact on Mercury relative to the Moon (Cintala, 1992) that enables more accumulation of space weathering products before sequestration by regolith overturn. The radiative transfer model cannot directly constrain the abundance of opaque minerals on Mercury because of ambiguities between the darkening effects of opaques and submicroscopic iron particles larger than 50 nm, but assuming the opaques are the ultimate source of the submicroscopic iron, our results place a lower limit of 4-20 wt.% on opaque abundance on Mercury depending on the composition of the opaque phase and whether titanium metal also contributes to the space weathering effect.     

Properties of the hermean regolith: iii. disk-resolved vis-NIR reflectance spectra and implications for the abundance of iron

Disk-resolved reflectance spectra of the surface of Mercury (longitudes 240-300°), obtained in the visual (vis) and near-infrared (NIR) spectral region, are presented and analyzed. The observations were made at the 2.6-m Nordic Optical Telescope with the ALFOSC low-resolution spectrograph on 20 and 22 June 1999 in the wavelength range 520-970 nm with a footprint size of 700 km on the mid-disk of Mercury. A method which enables more accurate correction for telluric line absorptions and atmospheric extinction than that applied on previously published vis-NIR spectra of Mercury is introduced. The resulting reflectance spectra are remarkably linear, lack significant absorption features, and have optical slopes comparable to remotely sensed lunar pure anorthosites. The relation between spectral slope and photometric geometry found by Warell (2002, Icarus 156, 313-317) is confirmed and is explained as caused by strongly backscattering particles with embedded submicroscopic metallic iron in a mature regolith. With the theoretical maturation model of Hapke (2001, J. Geophys. Res. 106 (E5), 10039-10073) an abundance of 0.05-0.3 wt% submicroscopic metallic iron in the regolith for silicate grain sizes in the range 10-80 μm is determined, implying a ferrous iron content in mafic minerals intrinsically lower than that of the lunar highlands. A binary crustal composition model with anorthite linearly mixed with pyroxene provides better spectral fits than a pure anorthitic composition. Comparison with mature lunar pure anorthosite spectra yields a confident upper limit to the FeO content of 3 wt% under the assumption that the surfaces are similarly matured, but this figure probably represents a considerable overestimate. The average mercurian regolith does not seem to be substantially more weathered than the most mature lunar highland soils in terms of abundance of submicroscopic metallic iron, indicating that a steady-state maturation level has been reached. However, the strong relation between optical spectral slope and photometric geometry may imply that the majority of regolith particles are more fine-grained than their lunar counterparts and that the regolith is admixed with complex agglutinate weathering products which are more abundant and more transparent than those of the lunar highlands. This is consistent with more energetic impacts and a higher rate of impact melt production in an iron-poor regolith. An observed relation between the spectral slope and latitude provides evidence that the Ostwald ripening process may be operating at equatorial latitudes on Mercury.     

Lunar pure anorthosite as a spectral analog for Mercury

Abstract— Plans are underway for spacecraft missions to the planet Mercury beginning in the latter part of this decade (NASA's MESSENGER (MErcury, Surface, Space ENvironment, GEochemistry, Ranging) and ESA's BepiColombo). Mercury is an airless body whose surface is apparently very low in ferrous iron. Much of the mercurian surface material is expected to be optically mature, a state produced by the “space weathering” process from direct exposure to the space environment. If appropriate analog terrains can be identified on the Moon, then study of their reflectance spectra and composition will improve our understanding of space weathering of low‐Fe surfaces and aid in the interpretation of data returned from Mercury by the spacecraft. We have conducted a search for areas of the lunar surface that are optically mature and have very low ferrous iron content using Clementine ultraviolet‐visible (UV‐vis) image products. Several regions with these properties have been identified on the farside. These areas, representing mature pure anorthosites (>90% plagioclase feldspar), are of interest because only relatively immature pure anorthosites have previously been studied with Earth‐based spectrometry. A comparison of Mercury with the lunar analogs reveals similarities in spectral characteristics, and there are hints that the mercurian surface may be even lower in FeO content than the lunar pure anorthosites. We also investigate the potential for use of spectral features other than the commonly studied “1 μm” mafic mineral absorption band as tools for compositional assessment when spacecraft spectral measurements of Mercury become available. Most low‐Fe minerals plausibly present on Mercury lack absorption bands, but plagioclase possesses an iron impurity absorption at 1.25 μm. Detection of this diagnostic band may be possible in fresh crater deposits.     

An analysis of the Mariner 10 color ratio map of mercury

The results of a geological analysis of the Mariner 10 orange/UV color ratio man of Mercury (B. Hapke, C. Christman, B. Rava, and J. Mosher, Proc. Lunar Planet Sci. Conf. 11th 1980, pp. 817–822) are given. Certain errors that occured in reproducing the published version of the 1980 map are pointed out. The relationships between color and terrain are distinctly nonlunar. There is no correlation between color boundaries and the smooth plains on Mercury, in contrast to the strong correlation between color and maria-highlands contacts on the Moon. There are no large exposures of low-albedo, blue material that could be considered to be Mercurian analogs of high-FeTi lunar maria basalts on any part of Mercury imaged by Mariner 10. Three lines of evidence imply that the crust is low in Fe²⁺ and Ti⁴⁺: rays and ejecta blankets are bluer than most areas on Mercury; the Fe²⁺ band in Mercury's reflectance spectrum is very weak or nonexistent and the albedo contrasts are smaller than those on the Moon. There is no evidence in the spectral or albedo data that a lunar type of second wave of melting ever occured on Mercury; rather, the observations are most consistent with the hypothesis that the smooth plains are extrusive landforms derived from local material, possibly mobilized by the Caloris event. In several places correlations between color and topography can be explained if older, redder, higher-Fe materials underlie younger, bluer, lower-Fe surfaces. There is some evidence of late Fe-rich pyroclastic-like activity.     

A comparison of the ultraviolet to near-infrared spectral properties of Mercury and the Moon as observed by MESSENGER

Measurements of the disk-integrated reflectance spectrum of Mercury and the Moon have been obtained by the MESSENGER spacecraft. A comparison of spectra from the two bodies, spanning the wavelength range 220-1450 nm, shows that the absolute reflectance of Mercury is lower than that of the nearside waxing Moon at the same phase angle with a spectral slope that is less steep at visible and near-infrared wavelengths. We interpret these results and the lack of an absorption feature at a wavelength near 1000 nm as evidence for a Mercury surface composition that is low in ferrous iron within silicates but is higher in the globally averaged abundance of spectrally neutral opaque minerals than the Moon. Similar conclusions have been reached by recent investigations based on observations from both MESSENGER and Mariner 10. There is weak evidence for a phase-reddening effect in Mercury that is slightly larger in magnitude than for the lunar nearside. An apparent absorption in the middle-ultraviolet wavelength range of the Mercury spectrum detected from the first MESSENGER flyby of Mercury is found to persist in subsequent observations from the second flyby. The current model of space weathering on the Moon, which also presumably applies to Mercury, does not provide an explanation for the presence of this ultraviolet absorption.     

The dependence of reflectance spectra of Mercury on surface terrain

Reflectance spectra of Mercury, covering the spectral range of ～0.3–1.1 μm 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 Fe²⁺ 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.     

Mercurian volcanism questioned

The Mariner 10 television team has argued that extensive plains on Mercury were formed by volcanism and compared them with the demonstrably lunar maria. I believe, however, that in stratigraphic relations, surface morphology, and albedo contrast, the Mercurian plains more closely resemble the lunar light plains. These lunar plains were interpreted as volcanic on the basis of data comparable to that available to the Mariner 10 investigators but have been shown by the Apollo missions to be of impact origin. The plains on Mercury might also be formed of impact materials, perhaps of impact melt or other basin ejecta that behaved more like a fluid when emplaced that did lunar basin ejecta.     

Mercury crater statistics from MESSENGER flybys: Implications for stratigraphy and resurfacing history

The primary crater population on Mercury has been modified by volcanism and secondary craters. Two phases of volcanism are recognized. One volcanic episode that produced widespread intercrater plains occurred during the period of the Late Heavy Bombardment and markedly altered the surface in many areas. The second episode is typified by the smooth plains interior and exterior to the Caloris basin, both of which have a different crater size-frequency distribution than the intercrater plains, consistent with a cratering record dominated by a younger population of impactors. These two phases may have overlapped as parts of a continuous period of volcanism during which the volcanic flux tended to decrease with time. The youngest age of smooth plains volcanism cannot yet be determined, but at least small expanses of plains are substantially younger than the plains associated with the Caloris basin. The spatial and temporal variations of volcanic resurfacing events can be used to reconstruct Mercury's geologic history from images and compositional and topographic data to be acquired during the orbital phase of the MESSENGER mission.     

On the origin of the lunar smooth-plains

Before the Apollo 16 mission, the material of the Cayley Formation (a lunar smooth plains) was theorized to be of volcanic origin. Because Apollo 16 did not verify such interpretations, various theories have been published that consider the material to be ejecta of distant multiringed basins. Results presented in this paper indicate that the material cannot be solely basin ejecta. If smoothplains are a result of formation of these basins or other distant large craters, then the plains materials are mainly ejecta of secondary craters of these basins or craters with only minor contributions of primary-crater or basin ejecta. This hypothesis is based on synthesis of knowledge of the mechanics of ejection of material from impact craters, photogeologic evidence, remote measurements of surface chemistry, and petrology of lunar samples. Observations, simulations, and calculations presented in this paper show that ejecta thrown beyond the continuous deposits of large lunar craters produce secondary-impact craters that excavate and deposit masses of local material equal to multiples of that of the primary crater ejecta deposited at the same place. Therefore, the main influence of a large cratering event on terrain at great distances from such a crater is one of deposition of more material by secondary craters, rather than deposition of ejecta from the large crater. Examples of numerous secondary craters observed in and around the Cayley Formation and other smooth plains are presented. Evidence is given for significant lateral transport of highland debris by ejection from secondary craters and by landslides triggered by secondary impact. Primary-crater ejecta can be a significant fraction of a deposit emplaced by an impact crater only if the primary crater is nearby. Other proposed mechanisms for emplacement of smooth-plains formations are discussed, and implications regarding the origin of material in the continuous aprons surrounding large lunar craters is considered. It is emphasized that the importance of secondary-impact cratering in the highlands has in general been underestimated and that this process must have been important in the evolution of the lunar surface.     

Spectra of extremely reduced assemblages: Implications for Mercury

Abstract— We investigate the possibility that Mercury's crust is very reduced with FeO concentrations of less than ?0.1 wt%. We believe that such a surface could have a composition of enstatite, plagioclase, diopside, and sulfide, similar to the mineral assemblages found in aubritic meteorites. To test this hypothesis, we investigated the spectra of aubrites and their constituent minerals as analogs for the surface of Mercury. We found that some sulfides have distinctive absorption features in their spectra shortwards of ?0.6 μm that may be apparent in the spectrum of such an object. Determination of the surface composition of Mercury using orbital x‐ray spectroscopy should easily distinguish between a lunar highlands and enstatite basalt composition since these materials have significant differences in concentrations of Al, Mg, S, and Fe. The strongest argument against Mercury having an enstatite basalt composition is its extreme spectral redness. Significant reddening of the surface of an object (such as Mercury) is believed to require reduction of FeO to nanophase iron, thus requiring a few percent FeO in the material prior to alteration.     

Crater size-shape profiles for the moon and mercury: Terrain effects and interplanetary comparisons

New crater size-shape data were compiled for 221 fresh lunar craters and 152 youthful mercurian craters. Terraces and central peaks develop initially in fresh craters on the Moon in the 0–10 km diameter interval. Above a diameter of 65 km all craters are terraced and have central peaks. Swirl floor texture is most common in craters in the size range 20–30 km, but it occurs less frequently as terraces become a dominant feature of crater interiors. For the Moon there is a correlation between crater shape and geomorphic terrain type. For example, craters on the maria are more complex in terms of central peak and terrace detail at any given crater diameter than are craters in the highlands. These crater data suggest that there are significant differences in substrate and/or target properties between maria and highlands. Size-shape profiles for Mercury show that central peak and terrace onset is in the 10–20 km diameter interval; all craters are terraced at 65 km, and all have central peaks at 45 km. The crater data for Mercury show no clear cut terrain correlation. Comparison of lunar and mercurian data indicates that both central peaks and terraces are more abundant in craters in the diameter range 5–75 km on Mercury. Differences in crater shape between Mercury and the Moon may be due to differences in planetary gravitational acceleration (gMercury=2.3gMoon). Also differences between Mercury and the Moon in target and substrate and in modal impact velocity may contribute to affect crater shape.     

Martian cratering revisited: Implications for early geologic evolution

Improved crater statistics from varied Martian terrains are compared to lunar crater populations. The distribution functions for the average Martian cratered terrain and the average lunar highlands over the diameter range 8–2000 km are quite similar. The Martian population is less dense by approximately 0.70 from 8 to 256 km diameter and diverges to proportionally lower densities at greater diameters. Crater densities on Martian “pure” terra give a lower limit to the Mars/Moon integrated crater flux of 0.75 since the last stabilization of the respective planetary crusts. The crater population >8 km diameter postdating the Martian northern plains is statistically indistinguishable from that population postdating the lunar maria. Monte Carlo simulations were performed to constrain plausible mechanisms of crater obliteration. The models demonstrate that if the crater density difference between the lunar and Martian terra has been due to resurfacing processes, random intercrater plains formation cannot be the sole process. If plains preferentially form in and obliterate larger craters, then the observed Martian distribution retains its “shape” as the crater density decreases. This result is consistent with the morphology of Martian intercrater plains.     

Mercury: Radar images of the equatorial and midlatitude zones

Radar imaging results for Mercury's non-polar regions are presented. The dual-polarization, delay-Doppler images were obtained from several years of observations with the upgraded Arecibo S-band (λ12.6-cm) radar telescope. The images are dominated by radar-bright features associated with fresh impact craters. As was found from earlier Goldstone-VLA and pre-upgrade Arecibo imaging, three of the most prominent crater features are located in the Mariner-unimaged hemisphere. These are: “A,” an 85-km-diameter crater (348° W, 34° S) whose radar ray system may be the most spectacular in the Solar System; “B,” a 95-km-diameter crater (343° W, 58° N) with a very bright halo but less distinct ray system; and “C,” an irregular feature with bright ejecta and rays distributed asymmetrically about a 125-km source crater (246° W, 11° N). Due south of “C” lies a “ghost” feature (242° W, 27° S) that resembles “A” but is much fainter. An even fainter such feature is associated with Bartok Crater. These may be two of the best mercurian examples of large ejecta/ray systems observed in an intermediate state of degradation. Virtually all of the bright rayed craters in the Mariner 10 images show radar rays and/or bright rim rings, with radar rays being less common than optical rays. Radar-bright craters are particularly common in the H-7 quadrangle. Some diffuse radar albedo variations are seen that have no obvious association with impact ejecta. In particular, some smooth plains regions such as the circum-Caloris plains in Tir, Budh, and Sobkou Planitiae and the interiors of Tolstoj and “Skinakas” basins show high depolarized brightness relative to their surroundings, which is the reverse of the mare/highlands contrast seen in lunar radar images. Caloris Basin, on the other hand, appears dark and featureless in the images.     

Constraints on Mercury’s surface composition from MESSENGER and ground-based spectroscopy

The composition and chemistry of Mercury’s regolith has been calculated from MESSENGER MASCS 0.3-1.3 μm spectra from the first flyby, using an implementation of Hapke’s radiative transfer-based photometric model for light scattering in semi-transparent porous media, and a linear spectral mixing algorithm. We combine this investigation with linear spectral fitting results from mid-infrared spectra and compare derived oxide abundances with mercurian formation models and lunar samples. Hapke modeling results indicate a regolith that is optically dominated by finely comminuted particles with average area weighted grain size near 20 μm. Mercury shows lunar-style space weathering, with maturation-produced microphase iron present at ∼0.065 wt.% abundance, with only small variations between mature and immature sites, the amount of which is unable to explain Mercury’s low brightness relative to the Moon. The average modal mineralogies for the flyby 1 spectra derived from Hapke modeling are 35-70% Na-rich plagioclase or orthoclase, up to 30% Mg-rich clinopyroxene, <5% Mg-rich orthopyroxene, minute olivine, ∼20-45% low-Fe, low-Ti agglutinitic glass, and <10% of one or more lunar-like opaque minerals. Mercurian average oxide abundances derived from Hapke models and mid-infrared linear fitting include 40-50 wt.% SiO₂, 10-35 wt.% Al₂O₃, 1-8 wt.% FeO, and <25 wt.% TiO₂; the inferred rock type is basalt. Lunar-like opaques or glasses with high Fe and/or Ti abundances cannot on their own, or in combination, explain Mercury’s low brightness. The linear mixing results indicate the presence of clinopyroxenes that contain up to 21 wt.% MnO and the presence of a Mn-rich hedenbergite. Mn in M1 crystalline lattice sites of hedenbergite suppresses the strong 1 and 2 μm crystal field absorption bands and may thus act as a strong darkening agent on Mercury. Also, one or more of thermally darkened silicates, Fe-poor opaques and matured glasses, or Mercury-unique Ostwald-ripened microphase iron nickel may lower the albedo. A major part of the total microphase iron present in Mercury’s regolith is likely derived from FeO that is not intrinsic to the crust but has been subsequently delivered by exogenic sources.     

Whole-disk spectrophotometric properties of Mercury: Synthesis of MESSENGER and ground-based observations

Disk-integrated and disk-resolved measurements of Mercury’s surface obtained by both the Mercury Dual Imaging System (MDIS) and the Mercury Atmospheric and Surface Composition Spectrometer (MASCS) onboard the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft were analyzed and compared with previous ground-based observations of Mercury at 11 wavelengths. The spectra show no definitive absorption features and display a red spectral slope (increasing reflectance with increasing wavelength) typical of space-weathered rocky surfaces. The MDIS spectra show evidence of phase reddening, which is not observed in the MASCS spectra. The MDIS spectra are commensurate with ground-based observations to within 10%, whereas the MASCS spectra display greater discrepancies with ground-based observations at near-infrared wavelengths. The derived photometric calibrations provide corrections within 10% for observations taken at phase angles less than ∼100°. The derived photometric properties are indicative of a more compact regolith than that of the lunar surface or of average S-type asteroids. The photometric roughness of the surface is also much smoother than the Moon’s. The calculated geometric albedo (reflectance at zero phase) is higher than lunar values. The lower reflectance of immature units on Mercury compared with immature units on the Moon, in conjunction with the higher geometric albedo, is indicative of more complicated grain structures within Mercury’s regolith.     

On the relationship between the Apollo 16 ancient regolith breccias and feldspathic fragmental breccias,and the composition of the prebasin crust in the Central Highlands of the Moon

Abstract Two types of texturally and compositionally similar breccias that consist largely of fragmental debris from meteorite impacts occur at the Apollo 16 lunar site: Feldspathic fragmental breccias (FFBs) and ancient regolith breccias (ARBs). Both types of breccia are composed of a suite of mostly feldspathic components derived from the early crust of the Moon and mafic impact-melt breccias produced during the time of basin formation. The ARBs also contain components, such as agglutinates and glass spherules, indicating that the material of which they are composed occurred at the surface of the Moon as fine-grained regolith prior to lithification of the breccias. These components are absent from the FFBs, suggesting that the FFBs might be the protolith of the ARBs. However, several compositional differences exist between the two types of breccia, making any simple genetic relationship implausible. First, clasts of mafic impact-melt breccia occurring in the FFBs are of a different composition than those in the ARBs. Also the feldspathic “prebasin” components of the FFBs have a lower average Mg/Fe ratio than the corresponding components of the ARBs; the average composition of the plagioclase in the FFBs is more sodic than that of the ARBs; and there are differences in relative abundances of rare earth elements. The two breccia types also have different provenances: the FFBs occur primarily in ejecta from North Ray crater and presumably derive from the Descartes Formation, while the ARBs are restricted to the Cayley plains. Together these observations suggest that although some type of fragmental breccia may have been a precursor to the ARBs, the FFBs of North Ray crater are not a significant component of the ARBs and, by inference, the Cayley plains. The average compositions of the prebasin components of the two types of fragmental breccia are generally similar to the composition of the feldspathic lunar meteorites. With 30–31% Al₂O₃, however, they are slightly richer in plagioclase than the most feldspathic lunar meteorites (～29% Al₂O₃), implying that the crust of the early central nearside of the Moon contained a higher abundance of highly feldspathic anorthosite than typical lunar highlands, as inferred from the lunar meteorites. The ancient regolith breccias, as well as the current surface regolith of the Cayley plains, are more mafic than (1) prebasin regoliths in the Central Highlands and (2) regions of highlands presently distant from nearside basins because they contain a high abundance (～30%) of mafic impact-melt breccias produced during the time of basin formation that is absent from other regoliths.     

Mercury surface composition: Integrating petrologic modeling and remote sensing data to place constraints on FeO abundance

A significant opaque component in Mercury’s crust is inferred based on albedo and spectral observations. Previous workers have favored iron-titanium bearing oxide minerals as the spectrally neutral opaque. A consequence of this hypothesis is that Mercury’s surface would have a high FeO content. An array of remote sensing techniques have not provided definitive constraints on the FeO content of Mercury’s surface. However, spectral observations have not detected a diagnostic 1 μm absorption band and have thus limited the FeO in coexisting silicates to <2 wt.% FeO. In this paper, we assess equilibrium among oxide and silicate minerals to constrain the distribution of iron between opaque oxides and silicates under a variety of environmental conditions. Equilibrium modeling is favored here because the geologic process that produced Mercury’s low-albedo intermediate terrain must have occurred globally, which favors a common widespread igneous process. Based on our modeling, we find that iron-rich ilmenite cannot occur with silicates that do not display a 1 μm absorption feature unless plagioclase abundances are high. However, such high plagioclase abundances are precluded by Mercury’s low albedo. Incorporating equilibrium crystallization modeling with spectral and albedo constraints we find the iron abundance of Mercury’s intermediate terrain is ?10 wt.% FeO. This intermediate iron composition matches constraints provided by visible albedo and total neutron absorption observed by MESSENGER. In fact, the total neutron absorption of mixtures of oxide, plagioclase, olivine and pyroxene for the oxide abundances estimated for Mercury, favor Mg-rich members of the ilmenite-geikielite solid-solution series. This work offers compositional constraints for Fe, Ti, and Mg that will be testable by various MESSENGER instrument data sets after it begins its orbital mission.     

On the spectral reflectance and maturation darkening of lunar soils

Optical absorption and diffuse reflectance spectra were obtained for simulated lunar glasses of four different compositions, both in their as-quenched (reduced) states and following mild subsolidus oxidation. The transmission spectra, when normalized by the FeO content of the glasses, differed from one another only in the relative intensity of an unresolved band in the UV. For fixed melting conditions the strength of this band in the as-quenched glasses increased with increasing FeO, or with increasing TiO₂ for a fixed FeO content. Electron spin resonance (ESR) experiments have demonstrated the absence of Fe³⁺ or Ti³⁺ and the presence of metallic iron in these materials; all other transition-group elements were excluded in preparation. The unresolved UV absorption edge in the as-quenched reduced glasses is therefore tentatively ascribed to Fe²⁺Ti⁴⁺ intervalency charge transfer transitions. A similar UV edge was also produced by oxidation, leading to the conclusion that the assignment of this band would be ambiguous in the absence of an independent determination of the valence states of Fe and Ti. The relationship between the transmission spectra of polished samples and the reflectance spectra of sieved powders of the same materials is shown to be well described by the Kubelka-Munk approximation. Using this insight, it is possible to understand the spectral characteristics both of oxidation darkening of synthetic glass powders and of maturation darkening of lunar soils in terms of (1) the growth of the aforementioned charge transfer band(s) and (2) the development of opaque surface phases. It is shown that mechanism (1) is of primary importance in lunar highland materials and that mechanism (2) dominates in mare materials. The present results, coupled with previous findings, suggest that lunar soil maturation darkening may result from vitrification only if accompanied by (a) enrichment in the elements Fe and Ti, (b) changes in valence states of these elements, (c) partial crystallization of opaque phases such as iron, ilmenite or magnetite, or (d) a combination of (a), (b), and (c).     

Properties of the Hermean Regolith: II. Disk-Resolved Multicolor Photometry and Color Variations of the “Unknown” Hemisphere

Multicolor photometric observations of the “unknown” hemisphere of Mercury have been performed with the Swedish Vacuum Solar Telescope on La Palma at maximal elongations from the Sun in 1997 and 1998. A set of six interference filters with central wavelengths from 450 to 940 nm were used. Multicolor photometry of Mercury was performed on disk-resolved images of the unknown hemisphere (longitudes 160°-340°) with a highest resolution of ∼200 km (J. Warell and S. Limaye 2001, Planet. Space Sci.49, 1531-1552).Disk-integrated spectrophotometry shows that (1) the spectrum of Mercury displays a linear slope from 650 to 940 nm, indicating that the average mercurian regolith is considerably more mature than relatively immature pure anorthosite regions on the Moon; (2) there is negative evidence for the presence of the putative 1-μm absorption feature near 940 nm due to the presence of ferrous iron (Fe²⁺) in pyroxenes; and (3) no effect of phase reddening of the integrated disk is observed between phase angles of 63° and 84°.For the first time, disk-resolved spectrophotometry of Mercury's surface has been obtained, from which it is inferred that (4) the scattering properties of Mercury's regolith are more homogeneous than for the Moon and that there is no clear relation between reflectance and chemical properties at spatial scales of ∼300 km on the unknown hemisphere and (5) there exists an inverse relation of spectral slope with emission angle which is larger for Mercury than for the Moon, indicating that the average mercurian regolith is more backscattering and that this effect increases with wavelength.Finally, from filter ratio images of Mercury's disk it is found that (6) no color variations larger that 2% with respect to the surroundings are detected at a spatial resolution of ∼300 km.