CRITERIA FOR THE DETECTION OF LUNAR CRYPTOMARIA

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1 CRITERIA FOR THE DETECTION OF LUNAR CRYPTOMARIA IRENE ANTONENKO 1, JAMES W. HEAD 1, JOHN F. MUSTARD t and B. RAY HAWKE 2 a Department of Geological Sciences, Brown University, Providence, Rhode Island 02912; 2planetary Geosciences, Hawaii Institute of Geophysics and Planetology, University of Hawaii, Honolulu, Hawaii (Received 28 December 1994) Abstract. 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 1 /zm 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 southwestem 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 distalbasin-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. 1. Introduction Prior to the Apollo 16 mission, the Cayley Formation (intermediate to high albedo "light" plains) were thought to be of volcanic origin (Figure la) because of their smooth nature, their filling of highland craters, their stratigraphic position and their age based on crater counts, which placed them between the latest basin-scale impact events, such as Imbrium, and the eafliest mare (Wilhelms, 1970, 1976). Thus, it was thought that a significant phase of highland (high-albedo) volcanism characterized the eafly Imbrian history of the Moon. The Apollo 16 mission was targeted to light plains units in the Descartes region of the central highlands to explore these units and their possible volcanic source regions (Hinners, 1972). Discovery of pervasive impact generated breccias at the Apollo 16 site by Astronauts John Young and Charles Duke (Young et al., 1972; Muehlberger et al., 1972; LSPET, 1972) caused this model to fall into disfavour. Subsequent to the Apollo 16 mission, interpretation of the light plains at the site centered on two major ideas: 1) the emplacement of Earth, Moon and Planets 69: , 1995 KluwerAcademicPublishers. Printed in the Netherlands.

2 142 IRENE ANTONENKO ET AL. Models of Light Plains Origin a) b) Basin Ejecta Pre-Apolio 16: Highlands (High-Albedo) Volcanism Post-Apollo 16: Ballistic Erosion and Sedimentation (Impact Breccias) Fig. 1. Differences between (a) volcanic and (b) ballistic erosion and sedimentation theories for the origin of lunar light plains. the plains as primary basin ejecta, either from Imbrium (Eggleton and Schaber, 1972) or Orientale (Chao et al., 1973, 1975; Chao, 1974; Moore et al., 1974), or 2) through the mixing of ejecta from basin impacts with local material (Oberbeck et al., 1974; Oberbeck, 1975; Head, 1972). The wide range of crater ages shown by light plains, however, has led some investigators to continue to support a volcanic origin for many light plains deposits (e.g., Neukum, 1977). Continued reassessment of the impact cratering process led to the conclusion that light plains can be produced by ballistic erosion and sedimentation processes (Oberbeck et al., 1974; Oberbeck, 1975). Ejecta associated with large cratering events impacts and mixes with the local target material in relatively predictable ways, obscuring pre-impact topography and resulting in the formation of deposits of relatively smooth light plains in low-lying regions (Figure lb). Thus, these light plains do not represent an ejecta "blanket" laid down over the surface, but rather an ejecta deposit which consists of a dynamic mixture of primary ejecta and local material, where the proportion of primary ejecta material to local target material decreases exponentially with radial distance from the primary crater (Oberbeck et al., 1974; Oberbeck, 1975). One implication of this ballistic method of light plains formation is that these ejecta deposits cover and obscure underlying stratigraphic units. For example, when high albedo ejecta is mixed with pre-existing impact breccias, light plains are produced, but when the same ejecta is mixed with a pre-existing low albedo mare deposit, the resulting light plains can obscure the mare unit, creating a cryptomare (Head and Wilson, 1992), or hidden märe. Such cryptomaria have been identified by the analysis of dark-halo craters (Schultz and Spudis, 1979, 1983; Hawke and Bell, 1981; Bell and Hawke, 1984), which penetrate through the light ejecta deposit to the dark mare material below and emplace the low albedo material in a halo around the crater (Figure 2). Multispectral imaging has also proved useful in identifying cryptomaria since the light plains emplacement process incorporates local material in the ejecta deposit, which can be recognized by mixing analysis of spectral endmembers (Mustard et al., 1992).

3 .. CRITERIA FOR THE DETECTION OF LUNAR CRYPTOMARIA 143 Fig. 2. Diagrammatic representation of the formation of a cryptomare by the emplacement of a high albedo basin ejecta deposit on top of a pre-existing low albedo mare deposit. These hidden maria may be sampled by dark halo impact craters, which penetrate the regional ejecta to excavate mare material, emplacing it in a halo around the crater. Craters that are too small to penetrate to the mare substrate will not form dark halos. If volcanism continues to occur after the emplacement of the regional ejecta deposit, and the formation of the cryptomare, post-basin mare patches will be formed. The existence of cryptomaria indicates that our understanding of the areal extent and volume of early lunar volcanism is incomplete. If cryptomaria are common occurrences on the Moon, then our estimates of volcanic volumes and fluxes may be low, thus affecting our understanding of the thermal history and evolution of the Moon, and the key early stages of secondary crust formation (Taylor, 1989) and mare basalt petrogenesis (Neal and Taylor, 1992; Longhi, 1992; Head and Wilson, 1992). It is, therefore, important to be able to identify cryptomaria and the extent of their occurrence. To this end, this paper will endeavor to develop criteria for the recognition and documentation of cryptomaria. We then draw on these techniques and criteria, together with previous analyses, to develop three case studies in different regions of the Moon; the Schiller-Schickard region in the southwestem nearside, the Balmer basin region in the southeastem nearside, and the Australe region on the southeastem limb and farside. 2. Cryptomare Identification 2.1. DARK HALO CRATERS Cryptomare can be identified by the presence of dark-halo impact craters (Schultz and Spudis, 1979, 1983; Hawke and Bell, 1981; Bell and Hawke, 1984). Dark-halo craters are identified by a relatively symmetrical halo of dark material surrounding an impact crater. Care taust be taken not to confuse these with endogenic dark halo craters, which are formed by pyroclastic activity around a volcanic vent (Schultz and Spudis, 1979). Such volcanic dark halo craters are offen aligned with fissures

4 144 IRENE ANTONENKO ET AL. and linear rilles and are elongated (Head and Wilson, 1979), while impact-derived dark halo craters are generally circular. Other distinguishing characteristics of dark-halo impact craters area depth/diameter ratio of about 1:5 for fresh craters smaller than 10 km, the presence of braided ejecta facies and secondary craters, and an uplifted rim. Endogenic dark halo craters tend to have smaller depth/diameter ratios, untextured ejecta deposits, no secondary craters, very low likelihood of any ray-like patterns, and commonly lack a sharply raised rim (Head and Wilson, 1979). The identification of an impact origin for a dark halo crater, however, may not always be sufficient. Some dark halos surrounding craters of definite impact origin have been shown to be deposits of impact melts (Howard and Wilshire, 1975; Hawke and Head, 1977). Such deposits can be recognized by their preferential concentration in topographic lows (Schultz and Spudis, 1979), the presence of cracks (Hawke and Head, 1977) and fluid flow features, and a spectral signature indicative of highland compositions (Bell and Hawke, 1984). Furthermore, impact melt deposits are generally confined to narrow zones near the rim crest and are surrounded by normal albedo ejecta deposits (Bell and Hawke, 1984). Schultz and Spudis (1979) note that albedo contrasts between impact melts and ejecta deposits decrease rapidly with time, thus misidentification of impact melts as basaltic dark halos should only be of concem for very young craters. It has also been suggested (e.g., Taylor, 1981) that dark ejecta deposits may be produced if small quantities of a dark projectile, such as a very dark carbonaceous chondrite, are incorporated into the ejecta material. Bell and Hawke (1984) conducted spectral studies of several dark halo craters and observed that none of their craters showed the spectral signature of contamination by carbonaceous projectile material. They conclude that the formation of dark halos by projectile contamination is an insignificant process on the lunar surface. However, in light of such complications, impact derived dark halo craters should be studied using spectral analysis, whenever possible, to confirm that the dark halo material is indeed basaltic. Dark halo impact craters form by excavating buried low albedo mare material and emplacing it on top of an overlying, higher albedo, regional ejecta deposit (Figure 2). Craters must therefore be large enough to penetrate the overlying ejecta thickness and excavate sufficient mare material to form a dark halo (Antonenko and Head, 1994). Furthermore, craters must be sufficiently old to permit the soil making up the upper part of the ejecta deposit to mature (Schultz and Spudis, 1979; Hawke and Bell, 1981), since an immature dark halo crater may appear bright until the combined effects of space weathering cause the soll of the halo to darken (Pieters et al., 1993). Over a hundred such dark halo craters, greater than 1 km in diameter, have been identified by Schultz and Spudis (1979,1983) in a global lunar survey. When dark halo craters are present in large quantities in an area, their size distribution and location can offer clues to the three-dimensional geometry of a cryptomare deposit (Antonenko and Head, 1994). The placement of the dark halo

5 CRITERIA FOR THE DETECTION OF LUNAR CRYPTOMARIA 145 craters can be used to delineate the boundary of the cryptomare and to estimate the areal extent. Size of the dark halo craters can be used to approximate the thickness of the cryptomare deposit and the overlying ejecta deposits. The smallest dark-halo craters can be assumed to detect the top of the cryptomare, while the largest detect the bottom. The difference in excavation depths of these extremes gives an estimate of cryptomare thickness (Antonenko and Head, 1994) SPECTRAL MIXL'qG ANALYSIS Spectral mixing analysis takes advantage of the fact that a significant portion of local material can be mixed into the distal ejecta deposit of a large crater or basin (e.g., Oberbeck, 1975). Therefore, basin or crater ejecta emplaced on top of a mare deposit will incorporate a portion of the local mare basalts, which may be detected on the basis of their spectral signature. Using spectrally distinct endmembers (e.g., mare material, highland material and fresh crater material, Mustard et al., 1992), the analysis procedure can also determine in which proportions these endmembers contribute to the spectral signature of a specific area. In this manner, the amount of local mare material mixed into the ejecta deposit can be determined (Mustard et al., 1992). A significant mare component can indicate the presence and areal extent of a cryptomare. This method is limited by the fact that in areas where the ejecta deposit is thick, i.e. proximal to a primary crater or basin, the local underlying mare basalt component incorporated into the ejecta deposit may not be significant and so would not be identified by the analysis. For example, Inghirami W is a dark halo crater that has been shown, by the spectral studies of Bell and Hawke (1984), to clearly excavate basaltic material from beneath light plains deposits. However, it lies approximately 100 km outside (closer to the Orientale basin) of the region which contains a significant mare component, as defined by Mustard et al. (1992) and Head et al. (1993) (see Figure 7). It is possible that the Orientale ejecta at Inghirami W is too thick to incorporate a significantly detectable amount of the underlying mare material. The identification of a mare component in highlands by spectral mixing analysis is not in itself sufficient evidence to postulate the presence of a regional cryptomare deposit because ancient mare deposits, emplaced considerably before the end of terminal bombardment, may have been thoroughly obliterated as a discrete volcanic layer and reworked and mixed in with highland material. For such cases, little morphologic evidence of these basalts would remain, but they would still exert an influence on the spectral character of the area in which they were emplaced (Hawke and Spudis, 1980). No such specific mare remnants have yet been identified, but the availability of high resolution spectrometer and multispectral imaging data (e.g., Clementine) may reveal such deposits in the future. Thus, additional evidence, such as regional extent and the presence of dark halo craters, should be used for positive cryptomare identification.

6 146 IRENE ANTONENKO ET AL GEOCHEMICAL EVIDENCE Remotely sensed geochemical data can also be used to identify the presence of a local mare component in an ejecta deposit. For example, X-ray fluorescence experiments provide Mg/A1 ratio data, which is indicative of surface compositions (Adler et al., 1972). Mg/A1 concentration values for highlands range from 0.16 in the farside highlands to 0.49 in the Apollo 16 area (Andre and Adler, 1980), while Mg/A1 concentration values for maria range from 0.48 in Mare Smythii (Bielefeld, 1977) to as high as 2.2 in parts of the nearside maria (Andre and Adler, 1980) with a peak in the histogram at 0.64 (Maxwell and Andre, 1981). Thus, high Mg/A1 values are indicative of mare compositions, and the identification of high Mg/A1 values in a candidate area can suggest the presence of a cryptomare (Maxwell and Andre, 1981). Similarly, 7-ray data can provide Th, Fe and Ti concentrations. Typical Th values for highlands range from 0.24 ppm on the western limb to 3.5 ppm in the Albategnius region, while typical Th values for maria range from 0.82 ppm in Mare Undarum to 6.1 ppm in Mare Cognitum (Metzger et al., 1977). Data points which contained a mix of highland and mare in the collection area are not included in these range estimates. Fe values for highlands fange from 2.3% Fe on the rar western limb to 10.8% Fe on the eastern nearside, while mare values range from 6.3% Fe in Mare Smythii to 14.4% Fe in Mare Crisium (Davis, 1980). Ti values for highlands range from 0.0% Ti on the south-eastern limb to 2.4% Ti on the northeastem limb, while mare values range from 0.4% Ti in Mare Crisium to 5.3% in Mare Tranquillitatis (Davis, 1980). For all of Th, Fe and Ti, possible highland and mare values overlap, with the mare values having particularly large ranges. Thus it is not sufficient to cite high values to indicate the presence of a cryptomare. Instead, Th, Fe and Ti values of a candidate cryptomare area should be shown to be high with respect to the surrounding highlands or comparable to nearby mare, for positive identification. Several authors have used just such techniques to suggest the presence of buried mare deposits. Schultz and Spudis (1979) used high Th data to show that basalts had been excavated by dark-halo craters from beneath light plains in the Balmer area. They also used Mg/Si intensity ratios from north-east of Mare Smythii to show that significant buried mare material had been mixed into the overlying plains material. Clark and Hawke (1987, 1991) used X-ray and other orbital data to investigate further the Balmer basin cryptomaria and the ancient mare basalts east of Smythii. Similarly, Hawke et al. (1979, 1985) and Hawke and Spudis (1980) used geochemical data to suggest the presence of buried basalts in the Mare Marginis region, in the Van de Graaff region, in the terrain north of Taruntius crater, north-west of Milne basin, and several other regions. Clearly, geochemical data can be a powerful tool in the identification of cryptomare. It should be noted that geochemical anomalies may also represent features other than cryptomare, such as thoroughly admixed ancient maria or excavated basalt

7 CRITERIA FOR THE DETECTION OF LUNAR CRYPTOMARIA 147 dikes or other intrusives (Head and Wilson, 1992). Thus, other evidence, particularly the presence of dark halo craters, should be used in confirming cryptomare identification LIGHT PLAINS Known cryptomaria are often associated with light plains deposits (Head and Wilson, 1992; Hawke and Bell, 1981; Bell and Hawke, 1984; Hawke and Spudis, 1980; Mustard et al., 1992; Head et al., 1993). Bell and Hawke (1984) noted that some light plains may have a common origin as basin-related deposits emplaced on flat basaltic plains. This view is supported by the existence of light plains deposits on the lunar nearside, which exhibit distinctive spectral properties that may be a result of mixing between basin ejecta and pre-existing mare units, which occurred during the emplacement process (Charette et al., 1974; Bell and Hawke, 1984). Furthermore, the existence of flat expanses of terrain should have facilitated the formation of light plains deposits by debris surges associated with large impact structures (Hawke and Bell, 1981; Bell and Hawke, 1984), thus also supporting a connection between light plains and buried mare deposits. The distribution of lunar light plains deposits is shown in Figure 3. Many light plains surround the Orientale and Imbrium basins, and are concentrated at the transition zones between the continuous ejecta and secondary crater deposits associated with these basins (Wilhelms, 1987). Similar relations should also occur between light plains and other lunar basins. For example, the light plains illustrated in Figure 3 may also be related to the Humorum, Nubium, Nectaris and Tranquillitatis basins on the nearside and the Mendeleev, Moscoviense and Milne craters on the farside. Figure 3 thus provides a guide to the locations of potential cryptomaria. It is also possible that older light plains units may have been significantly degraded such that they are no longer recognized as light plains units. Thus, older, cryptomare-forming light plains units may exist which are not represented in Figure 3. The absence of identified light plains units in a cryptomare candidate region should, therefore, not be viewed as contrary evidence to the presence of cryptomaria BASIN LOCATION It has long been known that most exposed mare volcanic deposits occur within areas of low-lying, impact-basin-related topography (e.g., Head, 1976; Wilhelms, 1987), an observation accounted for at least in part by the propensity of fluid lava to flow downhill and pond in lows. Head and Wilson (1992) have proposed that this occurrence is also related to the development of neutral buoyancy zones at the base of the crust, and that magma driving pressures are generally insufficient to extrude lava onto the lunar surface unless the crust has been thinned by basin or crater formation processes. Thus it is logical to look for cryptomaria in regions

8 148 IRENE ANTONENKO ET AlL. NEARSIDE N FARSIDE N ~,~.,, ~~ ~, ~~,i ~:~ s Fig. 3. Location and distribution of known lunar light plains deposits (from Howard et al., 1974). Light plains tend to be concentrated around Orientale and Imbrium basins, at a distance of 1 basin radius or more (Wilhelms, 1987). Similar relationships between light plains and other basins may also exist. s of lowlying or ancient basin topography, where the presence of mare flooding is plausible. Lunar basins larger than 220 km in diameter are shown in Figure 4, along with the gross physical provinces associated with basin processes. Some of these basins may contain cryptomare deposits and thus merit further consideration. However, the presence of old degraded basins suggests that even older basins may exist which are not currently recognized (Howard et al., 1974). Such basins are particularly good cryptomare candidates, since it is likely that they would have been covered by the ejecta deposits of younger basins. This concept is well illustrated by the basins located within the Orientale ejecta deposit in Figure 4. One particular basin to the south, Mendel-Rydberg, is already known to contain cryptomare (Head et al., 1993). Generally old, highly degraded basins, and thus any cryptomaria they may contain, would be very difficult to recognize since the degraded and disrupted nature of the basin rim and rings might preclude identification by these usual basin characteristics. The solution may lie in laser altimetry data. Details ofknown basins, from laser altimetry (Roberson and Kaula, 1972; Wollenhaupt and Sjogren, 1972), provide insight into other basins and how they may be identified. For example, use of altimetry data helped to identify the South Pole-Aitken basin, and lead to the possible identification of cryptomare in South Pole-Aitken (Head et al., 1993). Clementine altimetry data (Nozette et al., 1994; Zuber et al., 1994) should also prove useful in identifying other degraded basins, and may also be used to identify relatively flat and smooth areas typical of candidate cryptomaria.

9 ...!..:. I CRITERIA FOR THE DETECTION OF LUNAR CRYPTOMARIA 149 NEAR SIDE N FAR SIDE N A~LL i..«.~ ") ~i ~... ~ MARE FMBRIAN NECTARIAN BASINS BASINS $ NN N~ CRATERED HEAVILY CRATERED Fig. 4. Location of lunar basins larger than 220 km in diameter and the gross physical provinces associated with these basins and their formation processes (from Howard et al., 1974). Old degraded basins may be cryptomare candidates. Ejecta from the youngest basins can act as an obscuring agent. These ideas are illüstrated by the Orientale ejecta (defined as part of the Imbrian Basins unit), which demonstrates the extent of surface area influenced by a single basin forming event, and shows how several basins may be obscured in this manner PROXIMITY TO KNOWN MARE The extrusion of mare basalts is likely to be related to long-lived source regions at depth in the lunar mantle, with lifetimes considerably longer than the time of formation of individual impact basins and their deposits (Head and Wilson, 1992). Therefore, it is not unreasonable to expect to see many pulses of magma extrusion in a single area and it is likely that some pulses will predate an ejecta emplacement event, while others will postdate this event. Such a situation would result in patches of mare occurring on or near a light plains unit, which itself obscures cryptomare deposits (Figure 2). Furthermore, extrusion of mare lavas on the lunar surface may also be related to a specific set of circumstances, such as the overpressurization of magma stalled at neutral buoyancy zones, and the presence of basin topography which allows the low magma driving pressures to be sufficient for lavas to reach the surface (e.g., Head and Wilson, 1992). This complex set of circumstances will tend to be satisfied at relatively few locations, the number of which will decrease with time as the cooling of the Moon causes magma to stall at greater and greater depths (Head and Wilson, 1992). The presence of a mare patch is, therefore, indicative that conditions for magma extrusion were favourable in this area, at some point in time. It is reasonable to suppose that conditions for magma extrusion were

10 150 IRENE ANTONENKO ET AL. also favourable at earlier times, thus the presence of mare patches may indicate the presence of older, obscured mare in the same area. On the basis of these two reasons, it is prudent to search for cryptomaria in the area of known mare deposits. The existence ofknown mare patches near a candidate cryptomare may add support to the interpretation that flooding has indeed occurred in the region in question. It is possible that mme extrusion may have completely ceased before an ejecta emplacement event, in which case all mare material in the area would be buried by the overlying ejecta deposit. In such cases, no overlying mare patches would be present in the area. Thus, the absence of mare patches in a cryptomare candidate region should not be viewed as contrary evidence GEOPHYSICAL EVIDENCE It has been shown that positive lunar gravity anomalies (mascons) are associated with circular mare-filled basins (Muller and Sjogren, 1968). Mascons are formed because the combination of an uplifted mantle plug and the mass of the subsequently emplaced mare basalt is sufficiently greater than the mass deficiency caused by the excavation of the basin, producing a pronounced positive anomaly in the lunar gravity field (Wise and Yates, 1970; Solomon and Head, 1980). The earliest marefilled impact basins (e.g., Tranquillitatis) formed in a period of time when the thermal gradient favored viscous relaxation processes and subdual of basin and moho topography, thus anomalies tended to be smoothed out or lost (Solomon et al., 1982). Later, thickening of the lithosphere with time moderated viscous relaxation processes allowing mascons to be retained, and an increase in mare volcanism in the period b.y. ago further added to the anomalous mass. The thicker lithosphere was capable of supporting the combined mass contributions, which have lasted to the present (Solomon and Head, 1980). Careful analysis of high-resolution gravity data may provide clues to mass concentrations that are due to early cryptomare deposits formed subsequent to the time of significant viscous relaxation but during the period of basin formation and early filling. Clearly, other sources can contribute to mass concentrations (e.g., moho topography and igneous intrusions), but together with other evidence, their identification may support the presence of cryptomaria. To date, mascons have been identified in Serenitatis, Humorum, Imbrium, Orientale, Grimaldi, Crisium, Nectaris and Smythii (Solomon and Head, 1980). Highresolution gravity data does not exist for the farside of the Moon, however, improvements to the gravity data set afforded by Clementine (Zuber et al., 1994) may allow for the identification of farside mascons. Since mascons are related to circular maria (e.g., Muller and Sjogren, 1968), identification of such a mascon in a candidate region such as the Balmer basin (Haines et al., 1978) may be used, together with other evidence, to support an argument for the existence of a cryptomare deposit (Maxwell and Andre, 1981).

11 CRITERIA FOR THE DETECTION OF LUNAR CRYPTOMARIA IDENTIFICATION OF SOURCES OF OBSCURING AGENTS Since cryptomaria require sufficient ejecta from non-local, high albedo sources to obscure their low albedo, it is useful to identify the source of an obscuring agent. A nearby basin may be able to produce the ejecta deposit required. For this reason, cryptomaria might be sought in the vicinity of large basins. The locations of large basins and the gross physical provinces associated with the basin-forming processes are shown in Figure 4. The material surrounding Orientale is dominated by Orientale ejecta and demonstrates the extent of surface area which may be influenced by ejecta from a single basin. It should be noted that several large craters may also be capable of providing ejecta in sufficient quantities to act as an obscuring agent (e.g., Hawke et al., 1985). Likewise, a combination of crater and basin ejecta may work together to obscure a region. For example, ejecta deposits from Orientale, Schrödinger, and Apollo all overlie the South Pole-Aitken basin, and in places even overlap (Head et al., 1993, Fig. 29b). Thus, a variety of basins and craters should be considered as the sources of obscuring agents for any cryptomare candidate CRITERIA GUIDEL1NES The list of criteria we have outlined above can be used to aid in the identification and documentation of cryptomaria. No single criterion alone is sufficient to either positively identify or rule out a cryptomare candidate. For example, dark halo craters, while powerful evidence for the presence of an underlying low-albedo deposit, should not be the sole evidence used to identify a cryptomare. To strengthen the identification of a cryptomare deposit, dark halo crater evidence should be supplemented by other supporting criteria such as location in a basin and identification of the source of obscuring agents. Further corroboration by such definitive evidence as spectral mixing or geochemical data would be conclusive. It should be noted that not all of the lines of evidence discussed carry equal weight in the identification of cryptomaria. There are two basic types of evidence: 1) Defining evidence, which clearly defines the presence of an obscured basaltic unit. 2) Supporting evidence, which presents arguments to support the likelihood that a basaltic unit may have been formed and obscured in the area in question. The cryptomare criteria are classified according to their type in Table I. Clearly, several lines of evidence should always be used, and thus a variety of data sets is required, employing a variety of data acquisition instruments, such as multispectral imaging systems, imaging spectrometers, 7-ray and X-ray detectors, and laser altimeters. Furthermore, if all lunar cryptomaria are to be identified, global coverage is required, something that is not available for many of the data sets currently at our disposal, particularly the 7-ray and X-ray data coverage, which is confined to small regions and narrow swaths (Amold et al., 1972; Adler

12 152 IRENE ANTONENKO ET AL. Evidence Type Comments TABLE I Classification of evidence for cryptomare identification Dark Halo Craters D Spectral Mixing D Geochemistry D Light Plains S Basin Location S Mare Patches S Geophysics S Obscuring Agents S More or less the only line of evidence which is necessary for the identification of cryptomare. Can also be used as probes to the 3-D geometry of the cryptomare deposit. Not required for identification since overlying ejecta may be too thick. Can be used to indicate areal extent of cryptomare. Not required for identification because of the high variability of both highland and mare geochemistry. May be able to indicate the character of the underlying mare. Often associated with cryptomare and suggests the mechanism of obscuration. Indicates good sites for cryptomare searches. Often associated with visible mare, suggesting preferred sites of lava extrusion. Indicates good sites for cryptomare searches. Indicates that lava extrusion conditions have been mer at some time in this location and suggests the potential for earlier episodes as well. Identifies the presence of factors offen associated with known mare. Indicates the potential for mare deposits to be obscured. D = defining evidence. S = supporting evidence. et al., 1972), and high resolution gravity data, which is still incomplete for the farside of the Moon. In addition, global high-resolution imaging spectrometer data would provide one of the most fundamental advances in our ability to identify and characterize cryptomare deposits. 3. Modes of Cryptomare Occurrences On the basis of these criteria and techniques several different types of cryptomaria or possible cryptomaria can be identified and we have organized these into two categories: 1) traditional cryptomaria, which include a variety of basin-ejectacovering and crater-ejecta-dusting types, and 2) non-traditional cryptomaria, which include ejecta covering oflimb basalts (Figure 5). Here we examine these categories in detail PROXIMAL BASIN EJECTA COVERING When a cryptomare is located close to a large impact basin, the ancient mare deposit will be covered by an ejecta deposit that is thick and continuous (Figure 5a). For example, Scott et al. (1974) calculated, on the basis of partially buried craters, that

13 CRITERIA FOR THE DETECTION OF LUNAR CRYPTOMARIA 153 Modes of Cryptomare Oeeurrenees: a) Traditionah ~ "--.~= ~,~.-~ DH C Basin Ejecta ( P r o x i ~ Cryptomare Signature Swamped by Basin Ejecta c) Crater Ejecta (High-Albedo) Covering -..', ~ (f ~~~(~///))J~ Cryptomare Signature Presurved in Mixture with Basin Ejecta Non-Traditionah ~ DHC Umb Basalts (No Strong l~tm Band) b) d) Fig. 5. Types of cryptomaria. Distinctions depend on the nature of ejecta and mare materials. the thickness of Orientale ejecta may exceed 4 km at the Cordillera Mountains, or about km from the basin center. In such cases, local mixing by secondary impacts will not be an important process. Hence, the local mare component will be small to non-existent relative to the primary ejecta material (Oberbeck et al., 1974; Oberbeck, 1975) and so the cryptomare signature may not be detectable by spectral and orbital geochemical analyses. Thus, the identification of proximal cryptomaria depends on the presence of dark halo craters. Such a situation may be occurring with Inghirami W, a dark halo crater located in an area which is not identified by spectral mixing analysis (see Figure 7). It is possible that the ejecta here is too thick to allow the mare component to be detected. Based on the depth/diameter equations of Pike (1974) and the maximum excavation depths of Stöffler et al. (1975), we estimate that for ejecta deposits 1-2 km thick, impact craters would have to be on the order of 10's to many 10's of km in diameter in order to excavate the mare material. If these estimates are correct, then large crater and basin-sized impacts would be required to penetrate ejecta deposits greater than 2 km thick. Such heavily buried cryptomaria may, thus, never be identified. When proximal ejecta deposits are less than 2 km thick, it is reasonable to expect that dark halo craters will form, allowing the cryptomare to be identified. Other cryptomare identification criteria may then be cited as supporting evidence DISTAL BASIN EJECTA COVERING When the cryptomare is somewhat removed from a large impact basin, the ejecta deposit will be thinner (Figure 5b). For example, the calculations of Scott et al. (1974) indicate that the thickness of Orientale ejecta may be as low as 100 m at a distance of 1400 km from the basin center. Secondly, at greater distances from the primary basin, a larger percentage of the local material will be incorporated

14 154 IRENE ANTONENKO ET AL. into the resulting deposit by secondary cratering processes (Oberbeck et al., 1974; Oberbeck, 1975). The combined effects of a thin ejecta blanket and a large percentage of incorporated local material allow the cryptomare signature to be preserved (Mustard et al., 1992; Head et al., 1993; Blewett et al., 1993). In these cases, cryptomaria can be identified by spectral mixing analysis, geochemical evidence, and the presence of dark halo craters. Other criteria may be cited as supporting evidence CRATER EJECTA COVERING Ejecta from nearby craters can also produce a high-albedo covering over a mare surface (Mustard et al., 1992; Schultz and Spudis, 1979); thus, proximity to large basins is not always required to produce light plains. For example, Hawke and Bell (1981), Bell and Hawke (1984), Head et al. (1993) and Greeley et al. (1993) note that, based on crater counts, some of the light plains within the Schiller- Zucchius region are too young to have been formed by the Orientale event, and so may represent post-orientale mare that have been covered by bright ejecta from nearby craters such as Zucchius. Thus, mare material may be covered by the thick, continuous deposits of a single, proximal, impact crater. Such a situation occurs at Copernicus crater. Here, continuous Copernican ejecta obscures underlying mare material (Pieters et al., 1985), the existence of which has been confirmed by the presence of a dark halo crater whose spectra clearly indicate that basaltic material has been excavated (Bell and Hawke, 1984). At distances further removed from an impact crater, mare material may still be obscured by the compound effects of discontinuous, distal ejecta deposits of several nearby impact craters. Such a situation may be occurring in the Balmer crater region, where Hawke and Spudis (1980) and Hawke et al. (1985) propose that several nearby craters have contributed to a thin cover of highland material which overlies older basaltic deposits. It is possible for such thin, high albedo, deposits to obscure low albedo mafic materials at even greater distances from craters. Using spectral analysis of Copernicus rays, Pieters et al. (1985) have shown that ejecta deposits as thin as m and containing only 20-25% high albedo material within the primary/local ejecta mixture are still capable of obscuring the low albedo of underlying mare deposits. Such a thin covering would allow the cryptomare signature to be detected through the ejecta deposit (Figure 5c). This type of cryptomare can, therefore, be identified by geochemical evidence, spectral mixing analysis, and the presence of dark halo craters. Again, all other criteria may be cited as supporting evidence NON-TRADITIONAL CRYPTOMARE: LIMB BASALTS Spectral data from the Galileo Earth-Moon I flyby revealed several regions on the western limb (Grimaldi, western Mare Nubium) which contain basalts with exceptionally weak 1 #m absorption bands, with respect to the nearside basalts (Greeley

15 CRITERIA FOR THE DETECTION OF LUNAR CRYPTOMARIA 155 et al., 1993; Pieters et al., 1993). Since a strong 1 #m absorption is one of the spectral characteristics used to identify mare material, spectral detection of such cryptomaria may, therefore, be complicated. The physical reason for the weak 1 #m band is unknown. However, orbital geochemical data of appropriate resolution should be able to distinguish such hidden limb basalts. For these cryptomaria, the presence of dark halo craters is the best argument for their identification (Figure 5d). Other criteria, such as the location of the candidate area within a basin, the identification of a mascon associated with the basin, and the presence of mare patches either within or near this basin would be helpful in supporting the identification of such a cryptomare. 4. Case Studies In order to illustrate how the criteria and techniques outlined above can be used to identify cryptomaria and determine their mode of occurrence, we have applied these techniques, combined with preexisting analyses, to three areas; the Schiller- Schickard region, the Balmer basin and the Australe basin. For each area, we review the available evidence and then identify the cryptomare type SCHILLER-SCHICKARD The Schiller-Schickard area is located approximately 1400 km south-east of the Orientale basin, between the craters Schiller and Schickard (Figure 6 and 7). The topography here is subdued, showing many craters with smooth floors, and craters that have been obscured, presumably by the emplacement of Orientale ejecta (Offield, 1971; Karlstrom, 1974) and other preexisting plains surfaces. In Lunar Orbiter photos (Figure 6), the area can be seen to contain numerous occurrences of smooth plains which have an intermediate albedo. Several authors (Schultz and Spudis, 1979; Hawke and Bell, 1981; Bell and Hawke, 1984; Mustard et al., 1992; Head et al., 1993) have identified this region as a cryptomare. Several lines of evidence for the identification of this region as a cryptomare are illustrated in Figure 7. The presence of dark halo craters of definite impact origin in the Schiller-Schickard region has been documented by Schultz and Spudis (1979, 1983). Based on the distribution of these dark halo craters, they infer the existence of a buried mafic unit, which underlies highlands-rich deposits emplaced by the Orientale basin. This buried mafic unit may represent several different structures, such as mare deposits, intrusive plutons or dikes, or simply another underlying ejecta deposit which contains a higher proportion of mafic materials (such as the lunar mantle) than the overlying deposit. The nature of this mafic unit, therefore, cannot be confirmed without further information. Hawke and Bell (1981) and Blewett et al. (1994) used earth-based telescopic spectra to demonstrate the mare basalt affinities of many of these dark halo craters. Mustard et al. (1992) and Head

16 156 IRENE ANTONENKO ET AL. Fig. 6. Portion of Lunar Orbiter IV Frame 167M showing the Schiller-Schickard area. The large crater in the center, with mare patches north and south, is Schickard crater. The elongated crater to the south is Schiller. Many smooth plains can be seen south of Schickard crater, and textured Orientale ejecta is visible in the north-west section of the image.

17 CRITERIA FOR THE DETECTION OF LUNAR CRYPTOMARIA % I I o o I! 15% T Marseniusy ~ß O 30 S. 45",, 60%- Baade Rydberg (~~ Oo t I~ k o -, o i ~ o ; "'5 ~,~?~o ~,,/ f"% o ~:~ o p ' ~, J - Phocy ides "~~ S chillei"~ß i ~ --- / Scheiner ~(:2~ CD -60os J t i ~ i 255 o Fig. 7. Map of the Schiller-Schickard area, illustrating several cryptomare criteria, which support the identification of a cryptomare at this location. Dark halo craters (from Schultz and Spudis, 1979) are indicated by open circles. The dashed curve delineates an area where Mustard et al. (1992), with the use of spectral mixing analysis, have identified a 25% or greater mare component in the local regolith. The location of nearby mare is illustrated by black patches, which indicate post-orientale volcanism. The dark halo crater located at approximately 2900 longitude and 45 latitude is Inghirami W. Its position well outside the area identified by Mustard et al. (1992) suggests that the ejecta deposit here is thicker. The fact that Inghirami W is closer to the Orientale basin supports this interpretation. et al. (1993) used spectral mixing analysis to confirm these strong mafic affinities and to demonstrate the presence of a significant mare component in the light plains. An area containing 25% or more of this component is delineated by the dashed curve in Figure 7. Their results are further confirmed by the spectral analysis of Blewett et al. (1994). These findings argue for a cryptomare interpretation for the mafic unit proposed by Schultz and Spudis (1979, 1983). The strength of the mafic signature within the dark halos argues against an underlying ejecta deposit with high quantities of incorporated mafic materials. In such cases, the dark halos should be diluted by large quantities of feldspathic material, originating from the underlying ejecta deposit, and so have weaker mafic signatures. Furthermore, the spectral studies of Blewett et al. (1994) indicate that the pre-orientale highland surface was dominated by noritic anorthosite, thus the presence of a layer of mafic-rich ejecta does not seem likely. The large component of mare material in the light plains argues against subsurface plutons or dikes, since emplacement of the ejecta deposit would be unlikely to sample to such depths in sufficient quantities to produce the signature observed. The size of the area of enhanced mafic signature, shown by the dashed curve in Figure 7, measures approximately km 2 (Mustard et al., 1992). Such an areal extent supports a mare deposit interpretation for the underlying mafic unit, indicating a cryptomare. The many nearby mare patches

18 158 IRENE ANTONENKO ET AL. (e.g., Hawke and Bell, 1981), such as the two post-orientale patches on the floor of Schickard crater (e.g., Greeley et al., 1993; Head et al., 1993), show that extrusive igneous activity has occurred subsequent to the emplacement of Orientale ejecta in this region. This observation also supports a cryptomare interpretation for the buried mafic unit. Other supporting evidence includes the abundance of light plains in this area, and the proximity to the Orientale basin (Hawke and Bell, 1981). This region has been mapped as predominantly light plains material (Scott et al., 1977; Wilhelms et al., 1979; Offield, 1971; Karlstrom, 1974). The area1 extent and distribution of these light plains is illustrated in Figure 3, centered at approximately 60 W and 45 S, where a concentration of particularly large light plains deposits can be found. Note the proximity of these units to the Orientale basin (Figure 3). On the basis of the gradational nature between these light plains and the outer facies of the Hevelius formation, the outermost unit of the Orientale ejecta sequence, Scott et al. (1977) have interpreted these light plains to be ejecta from the Orientale basin. Furthermore, the Schickard plains have been dated, using crater count ages, as being the same age as the Orientale event (Greeley et al., 1993) and the sculptured texture of the Orientale ejecta can be seen to reach as far as the Schickard crater. Thus, the Orientale basin seems to be a likely source for the obscuring agent required to form the cryptomare. From the evidence presented, it is clear that one or more cryptomare deposits exist in the Schiller-Schickard region. The ability of spectral mixing analysis to detect a regionally distributed mare component at this range from Orientale suggests that this is a distal traditional cryptomare. However, Figure 7 shows that some dark halo craters can be found to the west of the spectrally defined cryptomare region, arguing for the presence of proximal cryptomare. It therefore seems that the setting of the Schiller-Schickard cryptomare is best described as a distal basin ejecta covering which grades westward to a proximal orte. Such an interpretation is supported by the distance from Orientale, which ranges from km, or 3-50rientale radii. The interpretation of the Schiller-Schickard cryptomare is further complicated by recent studies of the Schiller plains, which occur to the west of Schiller crater within the defined cryptomare area of Figure 7. Greeley et al. (1993) presented crater statistics for these plains indicating that they are younger than the Orientale basin. Clearly, they could not have been emplaced by the Orientale event. Furthermore, Hawke and Bell (1981) and Blewett et al. (1994) note that these plains exhibit mare-type ridges, appear to embay adjacent terrain and flood some small craters, including Orientale secondaries, suggesting a volcanic origin. A volcanic interpretation is supported by the location of this area within the Schiller-Zucchius basin (Hawke and Bell, 1981; Head et al., 1993; Greeley et al., 1993; Blewett et al., 1993, 1994). Spectral studies (Blewett et al., 1994) confirm a basaltic composition for these plains and also indicate that ~ 15 % highland material is incorporated in the plains deposit. Hawke and Bell (1981), Head et al. (1993) and Blewett et al. (1993,

19 CRITERIA FOR THE DETECTION OF LUNAR CRYPTOMARIA 159 Fig. 8. Portion of Lunar Orbiter IV Frame 9M showing the location of the Balmer basin. A large area of smooth plains occupies the center of the image. The area shown hefe corresponds roughly to that of the map of Figure ) suggest that the Schiller plains basalts have been contaminated by highland debris transported to the area by the nearby crater Zucchius, and possibly other young craters. The south-westem portion of the Schiller-Schickard region may, therefore, represent a third type of cryptomare deposit; crater-ejecta covering BALMER BASIN The Balmer basin is a pre-nectarian multi-ringed impact basin (Maxwell and Andre, 1981) located just east of Mare Fecunditatis (Figure 8). The basin structure exhibits two rings, approximately 210 and 450 km in diameter. Subsequent crater

20 160 IRENE ANTONENKO ET AL. 5 S : : 0 : : :. ~':~::_::_:_=_-_=.=.-::~i=-_-.=:_=:.=.=.=f:l4:::~.:-_--:~:i-' ~~~z.:::::: ~ Mare Materials L!ngren üs: 0: ~?-:::!:--~-i -~!:i ':::::::-::~:+- 4,::.:-:iiiiiii~i-'-::")~i ) I ~ Imbrian Plains -: ~..::. ~~~-t~-~ -:~-:-"~:i~-~!. ~:~:.. :.. i Ö :: --'-:~---:: _:--:--::::.la:perõ: " (Ip) :'--:~_~~~- -~:--:--":i:.' :::~':i'~:"i: Irnbrian/Nectarian Plains (INp) - i,". ::. Soulhe n "m" of rage Crater Materials (pre-n to C) Terra Materials (pre-n to N) 23os 60 E 80 o E ~ Balmer basin rings Dark halo craters Young, rayed craters Fig. 9. Map of the Balmer basin, illustrating several cryptomare criteria. These include the location and extent of light plains units, the presence of dark halo craters (from Schultz and Spudis, 1979; Hawke et al., 1985), indicated by open circles, the nearby mare patch, location within the Balmer basin, and the presence of young, rayed craters, indicated by black diamonds, which may be capable of providing the obscuring agent. The area covered by X-ray data is in the northem portion of the map. Modified from Maxwell and Andre (1981). impacts have highly modified the basin, offen obscuring the ring structure (Maxwell and Andre, 1981). Within the basin can be found light plains units of Imbrian and Nectarian age (Figure 9), which are candidates for cryptomare because of their location within a basin interior and their association with later mare deposits. Attention was first drawn to the Balmer area because of the presence of orbital geochemical anomalies (Hawke and Spudis, 1980). From Figure 9 it can be seen that a limited part of this area was covered by the Apollo 15 X-ray fluorescence experiment. The data (Figure 10) give Mg/A1 va]ues, which can be used to detect the presence of mafic compositions. Concentration values of > 0.58 for the Imbrian plains unit (Ip) and >0.51 for the Imbrian/Nectarian plains unit (INp) are obtained for the Balmer basin area (Maxwell and Andre, 1981). Recall that typical values for nearside basalts are concentrated around 0.64, while values lower than 0.49 are more indicative of feldspathic lithologies. Thus, it is likely that the plains units (Ip and INp) in the Balmer basin are dominated by mare material (Maxwell and Andre, 1981). Similarly, Mg/Si and A1/Si profiles of the Ip unit show compositions comparable to nearby mare basalts, particularly those of Mare Nectaris (Maxwell and Andre, 1981). 7-ray data is also available for the same limited region of the Balmer basin. The plains units here appear to be characterized by Th values of 4.0 ppm (Haines et al., 1978; Maxwell and Andre, 1981). Recalling that typical highland values tend to be lower than 3.5 ppm (Metzger et al., 1977), we can infer that Th values of 4.0 ppm indicate the presence of a mare basalt component. Fe

21 CRITERIA FOR THE DETECTION OF LUNAR CRYPTOMARIA E 75~E Fig. 10. Map of geochemical data from Apollo 15 X-ray experiments, showing Mg/A1 intensity and concentration values for the northem Balmer plains. The highest values are associated with the Imbrian plains (Ip) and Imbrian/Nectarian (INp) units of Figure 9, where Mg/A1 ratios approach typical nearside mare values. The inner ring of the Balmer basin is delineated by the dotted line. Modified from Maxwell and Andre (1981). values were found to range from % Fe for this region (Davis, 1980). These values approach those of nearby Mare Smythii, which ranges from % Fe. Values for the eastern farside highlands tend to fange from % Fe (Davis, 1980). If this fange can be applied to the Balmer area, then it would seem that the value range of % Fe suggests the presence of a mare component. Ti values for the Balmer region were found to range from % Ti (Davis, 1980). These values are well below those found for the nearby Mare Smythii (2.6% Ti), but they do tend to be higher than the values for the south-eastem farside highlands, which range from % Ti. Assuming this range is applicable to the Balmer area, then Ti values of % could also be indicative of a mare component. Thus, four lines of geochemical evidence, Mg/A1 concentrations, Th values and Fe and Ti percentages, all suggest the presence of mare material in the Balmer plains. Several other cryptomare criteria are illustrated in Figure 9. A diffuse cluster of dark halo craters has been identified around the Balmer basin area (Schultz and Spudis, 1979). The presence of dark halo craters indicates the existence of a buried mafic unit. This interpretation supports the geochemical evidence and together they indicate that the buried mafic unit is most probably a mare deposit because of its size and its detectability by geochemical methods. In addition, the identification of basin rings clearly illustrates that this region is located within an ancient multi-ringed impact basin, suggesting that crustal thinning has occurred in this area, making mare flooding more likely (e.g., Head and Wilson, 1992), and providing a topographic low for basalt accumulation. A mare patch at the west end of the basin adds support

22 162 IRENE ANTONENKO ET AL. to the possibility of longer-term mare volcanism in this region. Also shown in Figure 9 are the locations of several young rayed craters; Langrenus, Petavius B, Palitzsch B, and Hecataeus K (Wilhelms and E1-Baz, 1977). Furthermore, this area is surrounded by several major, Imbrian-aged, impact structures including Petavius, Humboldt, Ansgarius, and La Perouse (Hawke and Spudis, 1980; Hawke et al., 1985). All of these craters may be capable of collectively providing the necessary ejecta cover (Hawke and Spudis, 1980; Maxwell and Andre, 1981; Hawke et al., 1985) needed to obscure the underlying mare. Particularly notable are rays from Langrenus, which extend across the central portion of Balmer basin, contributing to the high albedo of the area. It is quite possible for Langrenus rays to obscure the low albedo of mare material, since it has been shown that the rays of Copemicus, although thin and containing relatively small amounts of high albedo material, are capable of increasing the albedo of Mare Imbrium in the vicinity of the crater rays (Pieters et al., 1985). The other nearby rayed craters should add to the obscuring effect. One more line of evidence for the existence of a cryptomare in the Balmer basin is the presence of a positive gravity anomaly (mascon), which reaches peak values greater than 40 milligals (Frontispiece, 1977; noted by Haines et al., 1978). Such an anomaly, usually associated with mare-filled basins (Muller and Sjogren, 1968; Solomon and Head, 1980), supports the probability of basalt filling in the Balmer basin. While there is varied and abundant evidence that a cryptomare exists in the Balmer basin region, there do not appear to be any large basins in the vicinity which would be capable of producing a significant ejecta deposit. The plains units in this region are mapped as Imbrian in age (Ip) or early Imbrian to late Nectarian (INp) (Wilhelms and E1-Baz, 1977). However, all surrounding basins (Smythii, Crisium, Tranquillitatis, and Nectaris) are dated at mid-late Nectarian or older (Wilhelms, 1987), and so could not have formed these plains. Moore et al. (1974) infer that ejecta from the Imbrium event may have spanned the Moon, however, secondary craters from Imbrium typically extend only to 2300 km (Wilhelms, 1987), thus it is unlikely that Imbrium ejecta contributions would be significant at Balmer basin, 3000 km distant. Clearly, no nearby basin could have provided ejecta in sufficient quantities to obscure the low albedo mare. Secondly, the fact that the mare signatures can be detected geochemically suggests that the surface contains a high proportion of local mafic material with respect to exotic feldspathic material, so that the geochemical signature is left relatively intact (Maxwell and Andre, 1981). This line of reasoning does not support the presence of a thick ejecta covering, which would deliver large quantities of exotic material that would effectively swamp the geochemical signature of the mare. Thus, neither a proximal nor a distal basin ejecta covering is likely for this location. On the other hand, the presence of several young, rayed craters (Wilhelms and E1-Baz, 1977) and Imbrian aged impacts nearby (Hawke and Spudis, 1980; Hawke et al., 1985) which are capable of collectively producing a higher albedo

23 CRITERIA FOR THE DETECTION OF LUNAR CRYPTOMARIA 163 overprint (Maxwell and Andre, 1981) suggests that the cryptomare formed by crater ejecta covering. Recall that crater rays are capable of raising the albedo of a mare surface (Pieters et al., 1985). Furthermore, these rays may contain as little as 20-25% feldspathic material (Pieters et al., 1985) and still efficiently raise the surface albedo. This supports the observation of Maxwell and Andre (1981) that the Balmer plains units contain a high proportion of local mafic to exotic feldspathic material AUSTRALE BASIN Australe is an ancient multi-ringed basin, approximately 900 km in diameter, located on the south-eastern limb (Figure 11). It is believed that the basin structure was largely destroyed by smaller impacts before the eruption of surface basalts (Whitford-Stark, 1979). Basalt volcanism occurred in four episodes spanning late Nectarian to early Eratosthenian ages (Whitford-Stark, 1979), producing a multitude of discrete mare patches that together make up Mare Australe (Figure 12). Earlier basalt units appear to exhibit higher albedos than the younger units and most nearside maria (Wilhelms and E1-Baz, 1977; Whitford-Stark, 1979). This increase in unit albedo with age causes some of the earliest units to resemble light plains (Whitford-Stark, 1979) suggesting the possibility of cryptomäre in this area. Figure 12 shows several lines of evidence for the identification of this area as a cryptomare. Several dark halo craters have been documented by Schultz and Spudis (1979), suggesting the presence of mare material outside of the known mare patches. The mare patches themselves illustrate that several episodes of mare volcanism have occurred in this area and thus suggest the possibility that older, more extensive mare filling may have occurred at an even earlier time. Theoretical calculations of proposed lava filling of Australe craters, based on crater depth/diameter observations, indicate that almost complete flooding of the degraded Australe basin should have occurred at some time (Whitford-Stark, 1979). However, today we see only discrete patches of mare. Thus it is possible that more extensive mare volcanism occurred at an earlier time, but was subsequently covered by ejecta from various craters and basins, leaving only the youngest mare patches visible. The location of this region within a topographic low, produced by the impact basin, further supports the possibility of more extensive mare flooding. Between the mare units can be found deposits of light plains material and terra mantling material. The terra mantling material is described as an undulatory, moderately smooth surface of Imbrian age (Wilhelms and E1-Baz, 1977) and we interpret this to indicate ejecta deposit material from such nearby Imbrian-age craters as Schrödinger, Humboldt, and Petavius, for example. The bulk of the intermare area is mapped as material of partly mantled terra, a smooth or moderately rough surface of Nectarian or pre-nectarian age (Wilhelms and E1-Baz, 1977). We interpret this unit to potentially be composed of ejecta material from Nectaris and other basins such as Smythii and Crisium. This interpretation is supported

24 164 IRENE ANTONENKO ET AL. Fig. ll. PortionofLunarOrbiterIVFrame 10MshowingthemanymarepatchesinAustralebasin. The area shown in this image is slightly smaller than the area represented in the map of Figure 12. The craters Hamilton (H) and Jenner (J) are labeled for reference. by the work of Whitford-Stark (1979), who calculated the total amount of ejecta potentially deposited on Australe basin since its formation, using theoretical contributions from nearby impact craters and basins, including Nectaris, Smythii, Crisium, Schrödinger, Humboldt and Petavius among others. The total ejecta thickness values so obtained range from 200 m to greater than 1 km, with ejecta from Nectaris dominating the fill. It is therefore probable that the Nectarian and pre- Nectarian units described above would consist of a significant portion of Nectarian and other ejecta. This ejecta may be acting as an obscuring agent, covering up older mare material.

25 CRITERIA FOR THE DETECTION OF LUNAR CRYPTOMARIA 165 ' 1 Eratoslhenian age basars ~ Upper Imbdan age basa]ts 8öoE Lower/mid lmbdan age bäsalts Humboldl, Schrödlng~r Nectadan/lower Imbrian acle basalts 0! 100 E ~ 11 ~ E O 3oOs S. 0 a 0 5o S. 0 6o s. 70 E I! Fig. 12. Geologic map of the basalt units within Mare Australe, showing the approximate ages and relations to major ejecta contributing events. Other cryptomare criteria are also shown by the dark halo craters (from Schultz and Spudis, 1979), indicated by open circles, and the basin topography, indicated by the dashed line which represents the rim of the Australe basin (Wilhelms and El-Baz, 1977). The dot-dashed line represents a possible older basin. Modified from Whitford-Stark (1979). Whitford-Stark (1979) notes that the older mare patches display higher albedos than the younger mare. Furthermore, the two oldest mare units (Figure 12) are superimposed by secondary craters from Humboldt. These secondaries illustrate that the ejecta from Humbolt, and probably other craters as well, was emplaced on top of the oldest mare units. As has been shown by the Copernicus example, crater ejecta is capable of increasing the low mare albedo, even when relatively thin and present in only small quantities (Pieters et al., 1985). Clearly, then the oldest mare units have been lightened by crater ejecta (Whitford-Stark, 1979; Schultz and Spudis, 1979) and so represent cryptomare formed by crater ejecta covering. However, the presence of dark halo craters, which occur between the mare patches (Schultz and Spudis, 1979) suggests the existence of another, older cryptomare covered by distal basin ejecta. Whitford-Stark (1979) calculated the levels of mare filling in Australe basin, based on crater depth/diameter observations, from

26 166 IRENE ANTONENKO ET AL. which he created an isopach map of proposed mare thickness. This map (Whitford- Stark, 1979, his Figure 8) shows that the floor of the entire Australe basin should be almost completely flooded to depths of 500 m or more of mare fill; however, today only patches of mare are visible. Furthermore, the calculations of the total amount of ejecta potentially deposited on Australe by a variety of Nectarian-aged basins and Imbrian-aged craters (Whitford-Stark, 1979) suggests that there was sufficient ejecta emplaced in the Australe basin to obscure such an extensive mare deposit. The domination of this calculated ejecta by Nectaris material and the Nectarian to pre-nectarian age of the inter-mare units supports the interpretation that inter-mare units may be obscuring older mare units. From this evidence, and the presence of dark halo craters which occur between the currently observable mare patches, we infer the existence of an older cryptomare, predominantly of distal basin ejecta type, which fills most of the Australe basin. Thus it is probable that the mare patches visible today represent only the youngest of the mare filling episodes in Australe basin. 5. Conclusions Many light plains units are formed by large impact cratering events which emplace deposits, consisting of ejecta and local material, on top of the pre-existing topography, producing smooth high albedo plains. When the pre-impact stratigraphy includes mare basalt units, cryptomaria are produced. In such cases, local basaltic material will be incorporated in the impact emplaced deposit. The mixing of local and ejecta material allows the mare component to be recognized by spectral mixing analysis and the cryptomare to be identified. Other identification criteria include the presence of dark halo craters and geochemical evidence. Supporting evidence may include location within ancient impact basins and location in regions with abundant light plains units, proximity of known mare, associated mascons, and identification of the source of obscuring agents. This set of criteria can thus be used as a guideline for future searches for as yet unknown cryptomaria. As many different criteria as possible should be utilized to strengthen identification of the cryptomare. Based on the nature of the ejecta and mare materials, several types of cryptomare were identified. For impact basins, cryptomaria may be formed by either proximal or distal basin deposits, depending on their distance from the ejecta forming basin. Craters may also provide sufficient obscuration to form cryptomaria by primary crater ejecta emplacement. Non-traditional cryptomaria could be formed by ejecta emplaced on basalts which exhibit weak 1 #m band absorptions. No such nontraditional cryptomare have yet been identified, though it is not clear if this is because they don't exist or have just not been recognized. It is possible that limb basalts represent a very late stage of mare volcanism and so are unlikely to have been obscured by ejecta deposits, since the rate of cratering would have steeply

27 CRITERIA FOR THE DETECTION OF LUNAR CRYPTOMARIA 167 declined by then and may not have been sufficient to form a regional cryptomare. Conversely, non-traditional cryptomare may simply not be identified, either because they are difficult to detect, particularly by spectral mixing analysis, or because they have been mistaken for traditional cryptomare, which is possible if spectral analysis is not used in the identification procedure. We applied our list of cryptomare criteria to three case study areas to illustrate how they may be used to recognize cryptomaria and identify the type. The Schiller- Schickard area was identified as a distal, grading to a proximal, basin ejecta covering cryptomare, with possible crater ejecta covering occurring in the south-western portion of the region. Balmer basin was recognized to be a cryptomare formed by crater ejecta dusting. Australe basin is believed to exhibit two types of cryptomare; a crater ejecta covering on old mare patches and a possible distal basin covering on even older mare basalts. It should be stressed that cryptomare identification and characterization should utilize as many of the criteria we have outlined and as wide a variety of data sets as possible. The need for such a multiple approach is clearly illustrated. However, such efforts are also hampered by a lack of areally complete data sets, which underscores the need for global coverage. Such data coverage can only be obtained by further lunar missions, which should include imaging systems, high resolution spectrometry, and geochemical experiments (i.e., multi-spectral imaging systems, which provide high resolution images capable of identifying dark halo craters, mare patches and light plains deposits; imaging spectrometers, whose data could be used for the identification of key minerals and spectral mixing analysis; X-ray fluorescence instruments, which could be used for identifying Mg/A1 ratios characteristic of mare deposits; -y-ray detectors, to identify Th, Fe and Ti concentrations indicative of mare deposits; and laser altimeters, for the identification of basin topography). The recent Clementine mission (Nozette et al., 1994) provided essentially global multispectral image and altimetry coverage which will be extremenly useful in the detection and documentation of additional cryptomare deposits. Future missions should include high resolution imaging spectrometers and geochemical experiments. 6. Future Directions It has been generally assumed that the timing and extent of lunar volcanism reflects the intemal thermal history of the Moon (e.g., Nyquist and Shih, 1992) and models of early lunar evolution often refer to basalt volume fluxes as tests of model accuracy (e.g., Kirk and Stevenson, 1989). If cryptomaria can be shown to comprise a significant portion of the total lunar volcanic output, then our understanding of volcanic volumes and fluxes may be incomplete, and interpretations regarding the thermal history and evolution of the Moon may be affected (Head and Wilson, 1992; Neal and Taylor, 1992). Clearly, there is a need to acquire a complete

28 168 IRENE ANTONENKO ET AL. understanding of the total areal extent and volume of all lunar maria; known, hidden and obliterated. Since the volumes and fluxes of the known maria are relatively well understood (summarized in Head and Wilson, 1992), the next step is to conduct a global census of cryptomaria. To complete the picture, a study of obliterated or remnant maria would be required. However, it is not clear how volumes for such destroyed units could be obtained, or even if such units exist. The definition of a set of guidelines for the identification of cryptomaria is an important requirement for such a lunar cryptomare census, one which we feel has been addressed in this paper. Fumre work should focus on the identification and documentation of new cryptomaria, and the quantification of the volumes and composition of those previously identified. We propose that Figures 3 and 4 represent a good basis for beginning such a search. Since cryptomaria are often associated with light plains, it is logical to look for them within areas which exhibit a concentration of light plains units. Furthermore, the existence of smooth expanses ofmare basalt should facilitate the production of light plains by basin ejecta deposits (Hawke and Bell, 1981; Bell and Hawke, 1984). Many light plains areas are shown in Figure 3 and these may be considered as cryptomare candidates. It should be noted that the ballistic erosion and sedimentation emplacement mechanism for light plains may be capable of producing a smooth surface even when emplaced over relatively rough highland topography (Oberbeck, 1975), thus not all light plains will necessarily be cryptomaria. Also, light plains emplaced by older basins may have been significantly degraded so that they are no longer recognized as such, thus not all cryptomaria will necessarily be associated with any of the light plains illustrated in Figure 3. Cryptomaria are also found within areas of basin topography that have been obscured by ejecta from younger basins. Several such basins are illustrated in Figure 4 and these should be considered as cryptomare candidates. It should be noted that many older basins will be so degraded that they may not yet be recognized and so would not be represented in Figure 4. Clearly, Figures 3 and 4 are incomplete in many ways with regard to identifying all possible cryptomare locations; however, they do present a starting point for the search for cryptomaria. Another reference for cryptomare locations is represented by the more than 100 dark halo craters identified by Schultz and Spudis (1979, their Figure 6). Areas showing high densities of dark halo craters represent good cryptomare candidates. Not all dark halo craters will represent a cryptomare since they may be tapping an intrusive pluton or dike (Head and Wilson, 1994), or another ejecta unit with a high mafic content instead of a mare deposit. Such instances will probably be rare; however, they should be kept in mind. Dark halo craters, therefore, represent another good starting point for cryptomare location. The recently acquired Clementine data set (Nozette et al., 1994) should prove particularly useful for the identification of cryptomaria and documentation of cryptomare characteristics. The global coverage of high resolution imagery should facilitate the identification of dark halo craters, light plains deposits, mare patches

29 CRITERIA FOR THE DETECTION OF LUNAR CRYPTOMARIA 169 and basin structures. The multiple wavelength filters will allow for basic spectral mixing analysis for the identification of cryptomaria as well as suggestions of the presence of key minerals, and the confirmation of dark halo craters and mare patches. Spectral mixing analysis may also be used to delineate the areal extent of the cryptomare, and dark halo craters, once confirmed, may be used to estimate the cryptomare thickness. Improved topography data should allow for the identification of basin topography and possibly new areas of flat topography which may represent light plains units. The increased coverage for farside gravity data will aid in the identification of farside lunar mascons, if any exist. Future work should also tackle the problem of characterizing the cryptomare basalts, identifying the basalt type, composition, Ti content and perhaps even identifying family groups of basalts. Data from instruments such as imaging spectrometers, with much higher spatial and spectral resolution than presently available, are essential to this type of further investigation. Higher resolution X-ray and 7-ray data with global coverage would also be essential in characterization. Once the remote identification and characterization of cryptomaria are accomplished, in situ surface exploration and sample retum missions can be targeted to the most critical areas for detailed documentation of composition, setting, and age. This series of steps would provide increasingly important information to document the onset and earliest stages of mare basaltic volcanism, one of the most important phases in planetary history (BVSP, 1981; Taylor, 1989). Acknowledgments This work was supported by the funding of NASA Grant NAGW-7 13 to James W. Head from the Planetary Geology and Geophysics Program of the Solar System Exploration Division, which is gratefully acknowledged. We would also like to thank Peter Neivert for help with drafting and Dr. Jessica Sunshine for the use of her Lunar Orbiter photo frame boundary subdual technique. References Adler, I., Trombka, J., Gerard, J., Lowman, R, Schmadebeck, R., Blodget, H., Eller, E., Yin, L., Lamothe, R., Osswald, G., Gorenstien, R, Bjorkholm, R, Gursky, H., Harris, B., Golub, L., and Harden, E R. Jr.: 1972, X-ray Fluorescence Experiment', in Apollo 16 Preliminary Scien«e Report, NASA, Washington, DC, Andre, C. G. and Adler, I.: 1980, 'Frontispiece', in J. J. Papike and R. B. Merrill (eds.), Proc. Conjl Lunar Highlands Crust, Pergamon, New York, N.Y,, Plate 1 and 2. Antonenko, I. and Head, J. W.: 1994, 'Cryptomaria in the Schiller-Schickard, Mare Humorum and Western Oceanus Procellarum Areas: Studies Using Dark-Halo Craters', Lunar and Planet. Sci. Conf. 25, Arnold, J. R., Metzger, A. E., Petersom L. E., Reedy, R. C., and Trombka, J. I.: 1972, «Gamma Ray Spectrometer Experiment', in Apollo 16 Preliminary Science Repart, NASA, Washington, DC, Basaltic Volcanism Study Project: 1981, Basaltic Volcanism on the Terrestrial Planets, Pergamon, NY, 1286 p.

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