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USGS1963

Astrogeologic studies, annual progress report, August 25, 1961 - August 24, 1962

This report, which covers the period August 25, 1961 to August 24, 1962, is the third of a series describing the program of research conducted by the U.S. Geological Survey on behalf of the National Aeronautics and Space Administration. The major long-range objectives of the program are to determine and map the stratigraphy and structure of the Moon's crust, to work out from these the sequence of events that led to the present condition of the Moon's surface, and to determine the processes by which these events took place. This report is presented in four parts with a separate summary. Part A: Lunar and planetary investigations (with map supplement). Geologic mapping of the Moon involves discrimination of the different materials exposed on the lunar surface and their assignment to geologic units, the determination of the photometric characteristics of these materials, delineation of the boundaries of the units and determination of their stratigraphic sequence. Part B: Crater investigations. These include field studies of terrestrial craters, hypervelocity impact experiments with rocks and other materials, studies of the behavior of rocks under high shock loads, and experimental and field studies of shock metamorphism in rocks. Part C: Cosmochemistry and petrography. Studies are being made of the chemical, petrographic, and physical properties of materials of extraterrestrial origin and of certain materials associated with terrestrial impact structures. The studies reported in Part C are concerned with tektites, terrestrial impactites, metallic spherules formed by the Meteor Crater impact event, and particles of possible cosmic origin. Part D: Studies for space flight program. These studies are undertaken to aid in the design of space flight experiments and the planning of space missions. The report that makes up Part D is an evaluation of the lunar flux of secondary particles derived from primary impact events on the lunar surface.

Open-File Report↗

USGS1978

Lunar landscape morphometry

This report outlines some general methods for analyzing the geometry of planetary topography and illustrates them with results obtained for the Moon. Its contents apply to selection of landing sites for spacecraft, planning traverses for vehicles, and to other aspects of planetary exploration that involve numerical expression of topographic form primarily for engineering purposes. The report is divided into three separate sections. Part I treats descriptive and predictive techniques for terrain slope. Part II describes the analysis of fine-scale surface roughness for the NASA Lunar Roving Vehicle, which was first deployed on Apollo mission 15 in the Hadley Rille area of the Moon. Part III applies the method of numerical taxonomy to multivariate characterization of lunar terrain and geologic map units. Although the findings described here are for the Moon only, the approaches and techniques could be used to solve terrain-related problems on any of the terrestrial planets. The author hopes that this study will stimulate further research in both terrestrial and extraterrestrial landscape morphometry.

Open-File Report↗

USGS2026

Lunar-VISE landing site selection and characterization at Mons Gruithuisen Gamma

The Lunar Vulkan Imaging and Spectroscopy Explorer (Lunar-VISE) was selected for a Commercial Lunar Payload Services (CLPS) delivery to the Gruithuisen domes region of the Moon as part of NASA’s Payloads and Research Investigation on the Surface of the Moon (PRISM) program. The Gruithuisen domes are chemically and morphologically distinct from their surroundings with a thorium-rich, silicic composition. The Lunar-VISE instrument payload is designed to investigate the compositional and thermophysical properties of dome materials in order to understand how late-stage silicic volcanism occurred on the Moon. Selection of a landing site required balancing science and exploration goals with the safety requirements for landing and rover trafficability. Science required access to boulders, potential exposures of bedrock, and if possible, rover access to the dome edge to enable observations of the surrounding maria. Safety considerations included landing hazards, maintenance of line-of-sight communications between the lander and rover, and any early morning or late afternoon shadows that would limit the mission duration. After consideration of several candidate landing sites, a 100-meter diameter landing ellipse centered on 36.45715° N, 319.20398° E, was selected near the edge of a topographic step and blocky ejecta crater (recently named Mareta) near the summit of Mons Gruithuisen Gamma. This location enables access to a field of boulders excavated by a relatively fresh impact providing a diversity of boulders for investigations, as well as views to the surrounding mare and Mons Gruithuisen Delta dome off of the dome edge via only a short rover traverse outside the landing ellipse (traverse < 100 m) while meeting safety requirements in accord with the CLPS risk posture.

Planetary Science Journal↗

USGS2023

Mapping planetary bodies

As the United States and its space agency, the National Aeronautics and Space Administration (NASA), looks to send humans back to the Moon, many other countries and their space agencies are also sending orbiters, rovers, and sample return missions across the Solar System. We are living in an extraordinary age of planetary exploration, where every mission builds on the decades of advancements in satellite design and onboard instrumentation. Once we acknowledge this, we can turn to understanding and analyzing the wealth of collected data. Fortunately, the principles and methods used for terrestrial mapping can also be used for extraterrestrial bodies. Herein, we introduce the concepts and some challenges to mapping planetary bodies like the Moon, Mercury, Mars, and the numerous moons we have visited in our outer Solar System. These spacecrafts including orbiter and fly-by missions are often loaded with novel instrument types from intricate pushbroom cameras and multi- and hyper-spectral cameras to radar and laser altimeter instruments.

Book chapter↗

USGS1998

The seventy-fourth Christmas bird count. 315. Southern Dorchester County, Md

Because limited information is available regarding preferences for nocturnal habitat during winter, we studied use of nocturnal habitats by American woodcock (Scolopax minor) wintering in the Georgia Piedmont (1994-95). During the evening crepuscular period, woodcock on the wintering grounds move from forested to field habitats, presumably to feed, conduct courtship displays, roost, and avoid predators. We conducted crepuscular flight surveys and tracked radio-marked woodcock to compare the use of fields of different sizes (<5.5 ha, 5.540.0 ha, >40.0 ha) and types (seed tree-clearcuts, fallow-old fields, hayfields, pastures). Fields > 5.5 ha were used more frequently than fields <5.5 ha (P < 0.001). Seed tree-clearcuts and fallow-old fields were more frequently used than pastures (P = 0.003). Woodcock also most frequently used fields with greater foliage volume at 0.82.0 m in height and a high percentage of bare soil (P < 0.001). Nocturnal use of fields or forests by radio-marked woodcock did not differ among age or sex classes. However, females moved an average of 230 + 32.1 m between diurnal and nocturnal locations while males moved 525 + 53.1 m ( P = 0.085). Movements differed among moon phases (P < 0.003), ranging from 579 + 79.6 m during the new moon to 213 + 50.5 m during the full moon. To manage habitat on the wintering grounds, seed tree-clearcuts and fallow-old fields should be created or maintained near preferred diurnal habitats.

Journal of Wildlife Management↗

USGS1998

The seventy-fourth Christmas bird count. 315. Southern Dorchester County, Md

Journal of Wildlife Management↗

USGS1972

Preliminary examination of lunar samples: Part A: a petrographic and chemical description of samples from the lunar highlands

More than four-fifths of the surface of the Moon consists of a profoundly cratered irregular surface designated terra or highlands by analogy with the terrestrial continents. These terra regions have much higher albedos than the physiographically lower and much smoother mare regions. The difference in albedo can now be ascribed to a fundamental difference in the chemical and mineralogical character of these two regions. Lunar samples from landing sites in the mare regions and high-resolution photographs taken from lunar orbit have shown that the lunar maria are underlain by extensive lava flows. Isotopic dating of samples from four mare regions (refs. 7-1 to 7-4) indicate that mare volcanism covered a time span of 600 million years, beginning approximately 3.7 billion years ago. The intensely cratered character of terra regions is due to both the greater antiquity of these parts of the Moon and the higher flux of incoming objects that hit the Moon during very early lunar history (ref. 7-5). In contrast with the mare region, the origin of the underlying material of the terra is not easily inferred from physiographic criteria. The surface manifestations of early plutonic or extrusive igneous activity — if indeed they ever existed — were erased from the terra regions by the intense early bombardment of the lunar surface. Some portions of the highlands may be exceptions to this generalization; in particular, large craters such as Ptolemaeus, Hipparchus, Albategnius, and Alphonsus. The regions bounded by these craters are much smoother than the typical densely cratered highlands. These regions are generally assumed to be physiographic lows that have, in some way, been filled by younger material. The nature of the material and the mechanism by which it was introduced into the basins are not well understood. On the basis of rather detailed studies of the physiographic and albedo characteristics of the basin material, it has been suggested (ref. 7-6) that the filling of the highland basins was a result of volcanic processes similar to those which filled the large mare basins. Some highland basins also contain hilly, hummocky regions that bear no relation to large crater rims or crater ejecta. These regions have been interpreted as extrusive igneous features formed by viscous, silicic, igneous liquids (ref. 7-6).

Book chapter↗

USGS2013

Updating the planetary time scale: focus on Mars

Formal stratigraphic systems have been developed for the surface materials of the Moon, Mars, Mercury, and the Galilean satellite Ganymede. These systems are based on geologic mapping, which establishes relative ages of surfaces delineated by superposition, morphology, impact crater densities, and other relations and features. Referent units selected from the mapping determine time-stratigraphic bases and/or representative materials characteristic of events and periods for definition of chronologic units. Absolute ages of these units in some cases can be estimated using crater size-frequency data. For the Moon, the chronologic units and cratering record are calibrated by radiometric ages measured from samples collected from the lunar surface. Model ages for other cratered planetary surfaces are constructed primarily by estimating cratering rates relative to that of the Moon. Other cratered bodies with estimated surface ages include Venus and the Galilean satellites of Jupiter. New global geologic mapping and crater dating studies of Mars are resulting in more accurate and detailed reconstructions of its geologic history.

Ciencias Da Terra↗

USGS2015

Cryovolcanism in the outer solar system

Cryovolcanism is defined as the extrusion of liquids and vapors of materials that would be frozen solid at the planetary surface temperatures of the icy bodies of the outer solar system. Active cryovolcanism is now known to occur on Saturn's moon Enceladus and on Neptune's moon Triton and is suspected on Jupiter's moon Europa, while evidence for past cryovolcanic activity is widespread throughout the outer solar system. This chapter examines the mechanisms and manifestations of cryovolcanism, beginning with a review of the materials that make up these unusual ‘‘magmas’’ and the means by which they might erupt and concluding with a volcanologist's tour of the farthest reaches of the solar system.

Book chapter↗

USGS2005

187Os/188Os And Highly Siderophile Element Systematics Of Apollo 17 Aphanitic Melt Rocks

Generally chondritic relative abundances and high absolute abundances of the highly siderophile elements (HSE: Ru, Rh, Pd, Re, Os, Ir, Pt, Au) in Earth's upper mantle provide strong evidence that these elements were added to the Earth following the last major interaction between its metallic core and silicate fraction. So called "late accretion" may have added materials comprising as much as 0.8% of the total mass of the Earth and possibly a similar proportion of mass to the Moon. We have begun to study the chemical nature of late accreted materials to the Earth-Moon system by examining the HSE contained in lunar impact-melt rocks. The HSE contained in melt rocks were largely added to the Moon during the period of time from the origin of the lunar highlands crust (4.4-4.5 Ga) to the end of the late bombardment period (ca. 3.9 Ga). These materials provide the only direct chemical link to the late accretionary period. The chemical fingerprints of the HSE in late accreted materials may enable us to ascertain under what conditions and where in the solar system the late accreted materials formed. The 1870s/1880s ratios (reflecting long-term Re/Os), coupled with ratios of other HSE, can be diagnostic for identifying the nature of the impactor. A critical issue, however, will be deconvolving the exogenous from indigenous components. Herein we examine the Os isotopic and HSE systematics of Apollo 17 aphanitic melt rocks 73215 and 73255. The HSE in these rocks were likely added at ~3.9 Ga from the impactor that formed the Serenitatis basin.

Conference Paper↗

USGS1975

Origin of lunar light plains

Most Cayley-type Imbrian-age plains deposits and adjacent mantled slopes, including those at the Apollo 16 site, may be composed at least near the surface of ejecta from the Orientale basin, the youngest multiringed impact basin on the Moon. The distribution and apparent age of the plains deposits and preliminary data on the highly feldspathic breccias collected by the Apollo 16 crew indicate that these surficial materials are neither locally derived nor part of the Imbrium ejecta. Stratigraphic relations, crater size-frequency distributions, and dating by erosional morphology of superposed craters have established Cayley light-plains deposits as younger than the Imbrium basin and older than mare material. All such crater-dated Cayley-type plains on both the near and far sides of the Moon are contemporaneous within the limits of the technique. Furthermore, comparisons of the crater size-frequency distributions of the Hevelius Formation (Orientale ejecta) and the plains show that the Cayley and Hevelius Formations are indistinguishable in age. The surface and near-surface materials of the Apollo 16 plains, therefore, are contemporaneous with Orientale basin ejecta not with Imbrium ejecta, in which both crater densities and crater degradation are greater than in the light plains. Imbrium ejecta may, however, be present on the surface at the Apollo 16 site where excavated by craters from depth. Limited age data now available on the Apollo 16 samples are consistent with this interpretation. We conclude that rocks of Orientale provenance may be predominant on and near the surface at the Apollo 16 site. This hypothesis implies that the catastrophic Orientale impact struck a highland area underlain by highly feldspathic material and spread it over much of the Moon. Thus, much of the lunar highlands crust need not consist of anorthositic materials to any significant depth. These conclusions apply to the plains at the Apollo 16 site and most of the other Cayley-type plains, but we do not exclude the possibility that, moonwide, the plains may be polygenetic.

Journal of Research of the U.S. Geological Survey↗

USGS2002

Exposure of delta smelt to dissolved pesticides in 2000

Delta smelt abundance in San Francisco Estuary has been declining since 1983. The exposure of delta smelt to toxic pesticides during larval and juvenile life stages may be one possible factor of this decline (Bennett and Moyle 1996; Moyle and others 1996). Although pesticides have been detected in the Delta (MacCoy and others 1995; Kuivila and others 1999), minimal data on pesticide concentrations and the duration of occurrence in delta smelt habitat are documented. A three-year study (1998– 2000) was undertaken by the U.S. Geological Survey (USGS) to quantify the exposure of larval and juvenile delta smelt to dissolved pesticides. Moon and others (2000) reported on the exposure of delta smelt to dissolved pesticides in 1998 and 1999, and this article follows up on Moon’s work and reports the results from late spring and summer of 2000.

Interagency Ecological Program Newsletter↗

USGS2026

Experimental lure design reveals the best attractants for increasing detection of multiple mesocarnivores

Many mesocarnivores have low detection rates that hinder practitioners' abilities to implement effective monitoring strategies. Using olfactory attractants (i.e. lures) may increase detection rates, but variation in effects among species is not well understood. Thus, investigating factors influencing detection of mesocarnivores, can inform and improve monitoring efforts. We evaluated the effects of lures and environmental covariates on the detection of plains spotted skunks Spilogale interrupta , striped skunks Mephitis mephitis , northern raccoons Procyon lotor , gray foxes Urocyon cinereoargenteus , coyotes Canis latrans , bobcats Lynx rufus and Virginia opossums Didelphis virginiana . We conducted surveys during January–May 2023 in southeast Oklahoma using motion-triggered cameras at randomly selected sites. We surveyed sites using a 4-camera cluster and leave-one-out lure design, where 3 cameras were randomly assigned 1 of 4 lures (i.e. skunk-based lure, fatty acid tablets, sweet lure or sardines) and 1 camera was a control (i.e. no lure). We analyzed species-specific detection data within an occupancy framework to determine the influence of lure combinations and environmental covariates (i.e. temperature, precipitation and moon illumination) on site-level detection patterns for each species. When lures influenced detection, we assessed species-specific preference among lures (and the control) within sites by comparing camera-level independent detections with Kruskal–Wallis and Dunn's tests at sites where the species was detected. The effect of lures varied among species; lure combinations influenced site-specific daily detection of plains spotted skunks, striped skunks, gray foxes, raccoons and bobcats, but did not affect detection of coyotes or opossums. One environmental covariate influenced detection of striped skunks (i.e. moon illumination), bobcats (i.e. temperature), and gray foxes (i.e. precipitation). Within sites, lure preference varied among species. Spotted skunks, gray foxes and raccoons preferred sardines whereas striped skunks preferred the skunk-based lure. Identifying lure preferences and effects of environmental factors can help optimize sampling and improve mesocarnivore monitoring efforts.

Oklahoma↗

USGS2000

Lunar Transient Phenomena: What do the Clementine Images Reveal?

Lunar Transient Phenomena (LTP) have been reported for at least 450 years. The events range from bright flashes, to reddish or bluish glows, to obscurations. Gaseous spectra and photometric measurements of the events have been obtained. Several theories have been offered as explanations for LTP, including residual volcanic activity or outgassing, bombardment by energetic particles, and piezoelectric effects. As the first set of digital multispectral images of the entire Moon, the Clementine data offer a unique opportunity to couple inferences of compositional relationships with lunar geomorphology in the regions of LTP. We have selected 11 regions from which numerous reliable historical reports of LTP exist. Our analysis of the Clementine multispectral images shows that many events occur in regions of bright, spectrally reddish deposits, which may be characteristic of volcanic ejecta. The events may be associated with outgassing of volatiles collected in or beneath mare basalt flows. We find that LTP tend to occur near the edges of maria, in agreement with a suggestion originally made by Cameron (1972. Icarus 16 , 339–387), and in other regions of crustal weakness. We also find that some of the reported events tend to be in craters with rims of distinctly different (bluer) composition. This compositional difference may result from recent slumping of the rim, accompanied by the appearance of fresher underlying material. In some cases, slumping may be triggered by the release of pockets of volatiles; in turn the slumping events may cause additional pockets of trapped material to be released. There are four instances in which Clementine multispectral images were acquired both before and after an event that was reported by a terrestrial team of amateur astronomers mobilized to observe the Moon during the mapping phase of Clementine. None of these four sets of images shows clear changes that could be attributed to the reported LTP.

Icarus↗

USGS2018

Report of the IAU Working Group on Cartographic Coordinates and Rotational Elements: 2015

This report continues the practice where the IAU Working Group on Cartographic Coordinates and Rotational Elements revises recommendations regarding those topics for the planets, satellites, minor planets, and comets approximately every three years. The Working Group has now become a “functional working group” of the IAU and its membership is open to anyone interested in participating. We describe the procedure for submitting questions about the recommendations given here or the application of these recommendations for creating a new or updated coordinate system for a given body. Regarding body orientation, the following bodies have been updated: Mercury, based on MESSENGER results; Mars, along with a refined longitude definition; Phobos; Deimos; (1) Ceres; (52) Europa; (243) Ida; (2867) Šteins; Neptune; (134340) Pluto and its satellite Charon; comets 9P/Tempel 1, 19P/Borrelly, 67P/Churyumov-Gerasimenko, and 103P/Hartley 2, noting that such information is valid only between specific epochs. The special challenges related to mapping 67P/Churyumov-Gerasimenko are also discussed. Approximate expressions for the Earth have been removed in order to avoid confusion, and the low precision series expression for the Moon’s orientation has been removed. The previously on-line only recommended orientation model for (4) Vesta is repeated with an explanation of how it was updated. Regarding body shape, text has been included to explain the expected uses of such information, and the relevance of the cited uncertainty information. The size of the Sun has been updated and notation added that the size and the ellipsoidal axes for the Earth and Jupiter have been recommended by an IAU Resolution. The distinction of a reference radius for a body (here, the Moon and Titan) is made between cartographic uses, and for orthoprojection and geophysical uses. The recommended radius for Mercury has been updated based on MESSENGER results. The recommended radius for Titan is returned to its previous value. Size information has been updated for 13 other Saturnian satellites and added for Aegaeon. The sizes of Pluto and Charon have been updated. Size information has been updated for (1) Ceres and given for (16) Psyche and (52) Europa. The size of (25143) Itokawa has been corrected. In addition, the discussion of terminology for the poles (hemispheres) of small bodies has been modified and a discussion on cardinal directions added. Although they continue to be used for planets and their satellites, it is assumed that the planetographic and planetocentric coordinate system definitions do not apply to small bodies. However, planetocentric and planetodetic latitudes and longitudes may be used on such bodies, following the right-hand rule. We repeat our previous recommendations that planning and efforts be made to make controlled cartographic products; newly recommend that common formulations should be used for orientation and size; continue to recommend that a community consensus be developed for the orientation models of Jupiter and Saturn; newly recommend that historical summaries of the coordinate systems for given bodies should be developed, and point out that for planets and satellites planetographic systems have generally been historically preferred over planetocentric systems, and that in cases when planetographic coordinates have been widely used in the past, there is no obvious advantage to switching to the use of planetocentric coordinates. The Working Group also requests community input on the question submitting process, posting of updates to the Working Group website, and on whether recommendations should be made regarding exoplanet coordinate systems.

Celestial Mechanics and Dynamical Astronomy↗

USGS1972

Preliminary mariner 9 report on the geology of Mars

Mariner 9 pictures indicate that the surface of Mars has been shaped by impact, volcanic, tectonic, erosional and depositional activity. The moonlike cratered terrain, identified as the dominant surface unit from the Mariner 6 and 7 flyby data, has proven to be less typical of Mars than previously believed, although extensive in the mid- and high-latitude regions of the southern hemisphere. Martian craters are highly modified but their size-frequency distribution and morphology suggest that most were formed by impact. Circular basins encompassed by rugged terrain and filled with smooth plains material are recognized. These structures, like the craters, are more modified than corresponding features on the Moon and they exercise a less dominant influence on the regional geology. Smooth plains with few visible craters fill the large basins and the floors of larger craters; they also occupy large parts of the northern hemisphere where the plains lap against higher landforms. The middle northern latitudes of Mars from 90 to 150† longitude contain at least four large shield volcanoes each of which is about twice as massive as the largest on Earth. Steep-sided domes with summit craters and large, fresh-appearing volcanic craters with smooth rims are also present in this region. Multiple flow structures, ridges with lobate flanks, chain craters, and sinuous rilles occur in all regions, suggesting widespread volcanism. Evidence for tectonic activity postdating formation of the cratered terrain and some of the plains units is abundant in the equatorial area from 0 to 120° longitude.Some regions exhibit a complex semiradial array of graben that suggest doming and stretching of the surface. Others contain intensity faulted terrain with broader, deeper graben separated by a complex mosaic of flat-topped blocks. An east-west-trending canyon system about 100–200 km wide and about 2500 km long extends through the Coprates-Eos region. The canyons have gullied walls indicative of extensive headward erosion since their initial formation. Regionally depressed areas called chaotic terrain consist of intricately broken and jumbled blocks and appear to result from breaking up and slumping of older geologic units. Compressional features have not been identified in any of the pictures analyzed to data. Plumose light and dark surface markings can be explained by eolian transport. Mariner 9 has thus revealed that Mars is a complex planet with its own distinctive geologic history and that it is less primitive than the Moon.

Icarus↗

USGS1976

Disharmony of the spheres: Recent trends in planetary surface nomenclature

Inadvisable departures from tradition in naming newly mapped features on Mars, Mercury, and the Moon have been implemented and proposed since 1970. Functional need for place names also has become confused with cartographic convenience. Much of the resulting new nomenclature is neither unique, efficient, nor imaginative. The longstanding classical orientation in Solar System geography needs to be firmly reasserted. The Mädler scheme for designating smaller craters on the Moon should be retained and extended to the farside. Names of surface features on other bodies might best reflect the traditional connotations of planet and satellite names: for example, most crates on Mars would be named for mythical heroes and military personalities in ancient history, craters on Mercury might commemorate explorers or commercial luminaries, and features on Venus would bear the names of famous women.

Icarus↗

USGS1977

Orientale and Caloris

Applications of experimental explosion-crater data to Orientale and recent geologic mapping of the basin have produced a new stratigraphy and genetic model for Orientale that are also applicable to Caloris. The inner-basin scarp of Orientale is thought to be a bench separating the upper parts of the basin from its deep bowl-shaped interior. The elongated and complexly fractured domes of the basin floor formed by inward compression in the terminal stages of the cratering sequence. The Inner Montes Rook are considered a central peak ring. The Montes Rook and the nonlineated knobby and associated smoother materials that overlie the Cordillera scarp around much of its circumference are the uppermost parts of the overturned rim flap which formed early in the cratering event. The knobs and smaller massifs are probably coherent blocks quarried from deep within the moon. They were among the last materials to leave the basin and had little radial momentum unlike the lineated Hevelius which formed earlier by disaggregation of the rim flap, secondary cratering, and the ground surge. The Cordillera scarp, best seen on the east side of the basin but poorly developed and discontinuous on the west, is a primary feature formed early in the crater excavation process by basinward motions of the walls and the fractured zone beyond the rim of the expanding cavity. The Cordillera scarp is overlain by ejecta over most of its extent, and post-basin internal slumping, previously thought to be important, must be a subordinate process in development of the scarp. The basin fill in Caloris has no counterpart in Orientale but the materials between the most prominent scarp and the weakly developed outer scarp appear to be the degraded and possibly mantled equivalents of the massifs and knobs associated with the Montes Rook. The radially lineated terrain that generally lies beyond the outer scarp of Caloris is considered the subdued counterpart of the Hevelius Formation, which generally shows the same relation to the Cordillera scarp at Orientale. Thus, the prominent innermost scarp of the Caloris basin is the equivalent of the Montes Rook. Beyond this scarp is the overturned flap covered by large blocks and massifs derived from a deep horizon in Mercury where the bedrock is more coherent than the upper impact-brecciated layers. The radially lineated deposits, as in Orientale, are earlier-arriving basin ejecta and secondary-crater materials mixed with the pre-basin surface all of which were modified by the ground surge. This comparison between Orientale and Caloris suggests that one or more buried ring structures should be present inside Caloris and that Mercury is also layered internally as is the moon. The differences in spacing and development of the ring structures or circumferential scarps of Orientale and Caloris are probably gravitational effects. ?? 1977.

Physics of the Earth and Planetary Interiors↗