B. Ray Hawke

Mapping the FeO and TiO2 content of the lunar surface with multispectral imagery

The derivation of quantitative elemental concentrations from multispectral imaging of the Moon has long been a goal of lunar remote sensing. Concentration maps at the spatial resolutions available from the recent Clementine mission would provide a revolutionary new tool for understanding the origin and evolution of the lunar crust. Lucey et al. [1995] presented a method for extracting the concentration of Fe from multispectral imaging of the Moon. This paper examines and quantifies important aspects ofthat technique left unexamined by Lucey et al. which had the potential to severely limit its utility. These aspects include the effects of maturity, grain size, mineralogy, shading due to topography, and the presence of glass. We also present a new algorithm for derivation of TiO2 from multispectral imaging of both mare and highland units. We find that both techniques are only weakly sensitive to maturity and that they have about 1 wt % accuracy based on examination of the spectral properties and compositions of resolved lunar sampling stations presented by Blewett et al. [1997]. We also discuss these findings in the context of two contrasting views of the effect of composition on lunar spectral properties presented by Pieters and coworkers and Hapke and coworkers. We find the view of Hapke and coworkers to be more consistent with our observations. Using a global mosaic of Clementine multispectral data and these element derivation algorithms, we find that the global modal abundance of FeO is 4.5 wt %±1 wt % and the global modal abundance of TiO2 is 0.45 wt %±1 wt. %.

Imaging of lunar surface maturity

The physical evolution of the lunar surface with exposure to the space environment is termed maturation, and maturity is the degree to which a particular lunar soil possesses quantitative characteristics consistent with that exposure. Several quantitative measures or indices of maturity have been proposed and employed, including the abundance of solar wind gas, abundance of various types of agglutinates, various measures of grain size, and Is/FeO. Among the changes attendant with space exposure are striking changes in the optical characteristics of soils. Mature lunar soils are dark red and exhibit reduced spectral contrast relative to immature soils. This paper presents an optical maturity index that quantifies the spectral effects of maturation. We show that this optical maturity index correlates with other maturity indices about as well as the accepted maturity indices correlate among themselves and is only weakly coupled to composition. The modest correlations among maturity indices suggest important controls on individual maturity indices other than age, uncorrelated variations in the rates of accumulation of individual indicators, or variations in depths over which indicators are emplaced. In addition to mixing, these effects conspire to reduce the equivalence of maturity and duration of surface exposure. Optical maturity illustrates some of these effects, showing that ejecta of large and small craters mature at different rates and that the interiors and ejecta of large craters exhibit systematically different optical maturities. The same or analogous effects are likely to influence other maturity indices.

Clementine images of the lunar sample‐return stations: Refinement of FeO and TiO2 mapping techniques

Clementine UVVIS images of the lunar sample-return sites have been processed and used to produce refined calibrations for the iron and titanium determination algorithms of Lucey et al. [1995, 1996]. The high spatial resolution of the Clementine data permits individual sampling stations to be resolved at the Apollo 15, 16, and 17 landing sites. We find an excellent, linear correlation between the spectral Fe and Ti parameters and the average FeO and TiO 2 contents of soils sampled at each site or station. This correlation demonstrates that these techniques can confidently be applied to other areas of the Moon. The Luna 24 site does not fit the Ti relation found for other sites, suggesting that either its sample is nonrepresentative or the reported landing coordinates are incorrect.

Remote sensing of lunar pyroclastic mantling deposits

Abstract Mantling deposits on the Moon are considered to be pyroclastic units emplaced on the lunar surface as a result of explosive fire fountaining. These pyroclastic units are characterized as having low albedos, having smooth fine-textured surfaces, and consisting in part of homogeneous, Febearing volcanic glass and partially crystallized spheres. Mantling units exhibit low returns on depolarized 3.8-cm radar maps, indicating an absence of surface scatterers in the 1- to 50-cm-size range. A number of reflectance spectra from several regional pyroclastic deposits are presented for the first time; these data support a previous interpretation that mantling units have a unique spectral signature which is indicative of the presence of a significant Fe-bearing volcanic glass component. The Rima Bode region is discussed as an example of an area in which several types of remote sensing data (including 3.8-cm radar, spectral reflectance, and multispectral vidicon data) were used to reconstruct the geologic events surrounding the emplacement of a regional pyroclastic mantling deposit. The recognition of numerous varieties of volcanic glass samples, especially relatively high-albedo (e.g., green, yellow) glasses, suggests the existence of additional, unrecognized mantling deposits with albedos higher than those studied to date. On the basis of the remote sensing data summarized and presented, five new areas have been identified which may represent higher-albedo regional pyroclastic deposits.

Compositional analyses of lunar pyroclastic deposits

Abstract The 5-band Clementine UVVIS data at ∼100 m/pixel were used to examine the compositions of 75 large and small lunar pyroclastic deposits (LPDs), and these were compared to representative lunar maria and highlands deposits. Results show that the albedo, spectral color, and inferred composition of most LPDs are similar to those of low-titanium, mature lunar maria. These LPDs may have consisted largely of fragmented basalt, with substantial components of iron-bearing mafic minerals (pyroxenes, olivine) and smaller amounts (if any) of volcanic glass. Several smaller LPDs also show substantial highland components. Three classes of very large deposits can be distinguished from most LPDs and from each other on the basis of crystallinity and possible titanium content of their pyroclastic components. One class has spectral properties that are dominated by high-titanium, crystallized “black beads” (e.g., Taurus–Littrow), a second consists of a mixture of high-titanium glasses and beads with a higher glass/bead ratio (Sulpicius Gallus) than that of Taurus–Littrow, and a third has a significant component of quenched iron-bearing volcanic glasses (Aristarchus) with possible moderate titanium contents. Although areally extensive, these three classes of very large pyroclastic deposits compose only 20 of the 75 deposits studied (∼27%), and eruption of such materials was thus likely to have been less frequent on the Moon.

FeO and TiO2 concentrations in the South Pole‐Aitken basin: Implications for mantle composition and basin formation

We use newly developed spectroscopic methods for determining FeO and TiO2 abundances to study the huge (2500 km diameter) South Pole-Aitken basin. The floor of this basin shows iron and titanium values which are weakly correlated and largely range from 7 to 14 wt % FeO and from 0.5 to 1.5 wt % TiO2. This composition is consistent with an approximate 1:1 mixture of lower crustal material like that found elsewhere on the Moon and mantle rock containing 10–16 wt % FeO and <0.1 wt % TiO2. Although mantle rock appears to compose about 50% of the floor deposits, models of the formation of large impact basins predict that virtually all the floor materials ought to be derived from the mantle.

Criteria for the detection of lunar cryptomaria

Cryptomaria are mare basalt deposits hidden or obscured by superposed higher albedo material or variations in albedo. They represent a record of the earliest mare volcanism, and may be a significant volumetric contribution to the volcanic and magmatic history of the Moon. In order to assess their global distribution and significance, criteria for the identification of cryptomaria are developed and techniques for locating them are described. These criteria and techniques include the presence of dark halo craters, identification by spectral mixing analysis, identification by geochemical evidence, association with light plains units, location within basin topography, proximity to known mare, relation to mascons indicated by gravity anomalies, and identification of the source of an obscuring agent, such as crater ejecta. On the basis of these criteria and techniques, several types of cryptomare are recognized, depending on the nature of ejecta and mare materials. Cryptomaria may be formed when maria are obscured by coverings of proximal or distal basin ejecta, or by crater ejecta dusting, or when ejecta covers over basalts which lack a distinctive 1µm absorption band. Using these concepts we outline three case studies: 1) the Schiller-Schickard region adjacent to the Orientale basin, classified as a basin-ejecta cryptomare and grading from distal to proximal, with possible crater-ejecta covering occurring in the southwestern portion of the region, 2) the Balmer basin, classified as a crater-ejecta-dusting cryptomare, and 3) the Australe basin, in which two types of cryptomare were identified: a) crater-ejecta-dusting on old mare patches and b) possible distal-basin-ejecta covering even older mare material. These case studies provide criteria for the further global identification and classification of cryptomaria and stress the need for utilization of multiple criteria and data sets.

Remote sensing and geologic studies of the Schiller-Schickard region of the Moon

Near-infrared reflectance spectra, multispectral images, and photogeologic data for the Schiller-Schickard (SS) region were obtained and analyzed in order to determine the composition and origin of a variety of geologic units. These include light plains deposits, Orientale-related deposits, mare units, and dark-haloed impact craters (DHCs). Spectral data indicate that the pre-Orientale highland surface was dominated by noritic anorthosite. Near-IR spectra show that DHCs in the region have excavated ancient (>3.8 Ga) mare basalts from beneath highland-bearing material emplaced by the Orientale impact. Ancient mare basalts were widespread in the SS region prior to the Orientale event, and their distribution appears to have been controlled by the presence of several old impact basins, including the Schiller-Zucchius basin and a basin previously unrecognized. Both near-IR spectra and multispectral images indicate that light plains and other Orientale-related units in the SS region contain major amounts of local, pre-Orientale mare basalt. The amounts of local material in these deposits approach, but seldom exceed, the maximum values predicted by the local mixing hypothesis of Oberbeck and co-workers.

Regolith composition and structure in the lunar maria: Results of long‐wavelength radar studies

Radar measurements at 70-cm and 7.5-m wavelengths provide insight into the structure and composition of the upper 5–100 m of the lunar regolith and crust. We combine high-resolution (3–5 km) 70-cm radar data for the nearside with earlier calibrated full-disk observations at the same wavelength to provide a reasonable estimate of the lunar backscatter coefficient. These data are tested against models for echoes from a buried substrate and Mie scattering from surface and buried rocks. These mechanisms are expected to dominate the 70-cm radar echo, with their relative importance determined by the rock population, regolith depth, substrate roughness, and the loss tangent of the soil. Results indicate that the 70-cm radar echo for the maria comes largely from Mie scattering by rocks buried within the fine soil. Radar scattering from a buried substrate is not likely to greatly affect the observed return. We also compared the 70-cm and 7.5-m radar images to infrared eclipse temperature maps, crater-population age estimates for the maria, and to TiO2 and FeO abundances inferred from Earth-based telescopic and Clementine multispectral observations. These data imply that (1) the TiO2 (ilmenite) content of the regolith controls variations in 70-cm depolarized echo strength among mare units, with higher titanium abundance leading to lower echoes; (2) changes in the average 70-cm return for a given TiO2 abundance between maria of different ages do occur, but uncertainties in the current radar data do not allow us to uniquely distinguish between variations in rock population with age and calibration effects; (3) the 7.5-m radar echoes are controlled by the age of the mare basalt flows, with older deposits having a greater degree of fracturing and higher backscatter. Future mapping at 12.6-cm and 70-cm wavelengths will help to resolve some of the issues raised here.

Remote sensing studies of the terrain northwest of Humorum Basin

We have used near-infrared reflectance spectra and Earth-based radar data to investigate the composition and origin of the various geologic units northwest of Humorum basin as well as the stratigraphy of the Humorum pre-impact target site. The results of our spectral analysis indicate that at least a portion of the inner, mare-bounding ring is composed of pure anorthosite. Other highlands units in the region are dominated by noritic anorthosite. The anorthosites on the inner ring may have been derived from a layer of anorthosite that exists at depth beneath a more pyroxene-rich unit. Both Gassendi G and F craters expose mare material from beneath a highlands-rich surface unit that was emplaced as a result of the Letronne, Gassendi, and other impact events. This ancient basalt unit was emplaced after the formation of Humorum basin but prior to the Orientale impact.