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G. G. Schaber

Publications and source records attributed to G. G. Schaber.

At least 19 recordsLinked to original sources
USGS2012

Cleopatra Patera on Venus: Venera 15/16 evidence for a volcanic origin

The non-concentric nature, anomalous depth, and terraced morphology of the nested craters that compose Cleopatra Patera are more closely analogous to volcanic craters (calderas) than multi-ring impact structures. Associated deposits northeast and downslope of the Patera first recognized on Venera 15/16 radar images are interpreted as volcanic plains related to, and perhaps cogenetic with, Cleopatra. The plains lavas probably originated as effusions from a radial rift zone or ring fissures. Rim deposits surrounding Cleopatra are asymmetric along the structural fabric of the region, also indicating fissure-type eruptions. Finally, a volcanic origin not only is easily reconciled with the tectonic setting of the Patera, it is almost required by the correlation between the Patera and regional structural trend.

Geophysical Research Letters↗
USGS2012

The geology and distribution of impact craters on Venus: What are they telling us?

Magellan has revealed an ensemble of impact craters on Venus that is unique in many important ways. We have compiled a data base describing the 842 craters on 89% of Venus' surface mapped through orbit 2578. (The craters range in diameter from 1.5 to 280 km.) We have studied the distribution, size‐density, morphology, geology, and associated surface properties of these craters both in the aggregate and, for some craters, in greater detail. We find that (1) the spatial distribution of craters is highly uniform; (2) the size‐density distribution of large craters (diameters ≥35 km) is similar to the young crater populations on other terrestrial planets but at a much lower density that indicates an average age of only about 0.5 Ga (based on the estimated population of Venus‐crossing asteroids); (3) unlike the case on other planets, the density of small craters (diameters ≤35 km) declines rapidly with decreasing diameters because of atmospheric filtering; (4) the spectrum of crater modification differs greatly from that on other planets: 62% of all craters are pristine, only 4% are embayed by lavas, and the remainder are affected by tectonism, but none are severely and progressively depleted (as extrapolated from the size‐density distribution of larger craters); (5) large craters have a progression of morphologies generally similar to those on other planets, but small craters are typically irregular or multiple rather than bowl shaped; (6) diffuse radar‐bright or ‐dark features surround some craters, and 367 similar diffuse “splotches” with no central crater are observed; and (7) other crater features unique to Venus include radar‐bright or ‐dark parabolic arcs opening westward and extensive outflows originating in crater ejecta. The first three of these observations are entirely unexpected. We interpret them as indicating that the planet's cratering record was erased by a global resurfacing event or events, the latest ending about 0.5 Ga, after which volcanic activity declined (but did not cease entirely). Since the last resurfacing event, a maximum of 10% of the planet has been resurfaced and only about 4% of the craters have been obliterated. Convective thermal evolution models support this interpretation (Arkani‐Hamed and Toksoz, 1984). Observations 3–7 confirm quantitatively the expectation that the dense atmosphere of Venus has strongly affected the production of craters. Large impactors have been relatively unaffected, intermediate‐sized ones have been fragmented and have produced overlapping or multiple craters, a narrow size range has produced shock‐induced “splotches” but no craters, and the smallest bodies have had no observable effect on the surface. The number of craters eliminated by the “atmospheric filter” is enormous, about 98% of the craters between 2 and 35 km in diameter that Magellan might have observed on a hypothetical airless Venus. Unique crater‐related features such as parabolas and outflow deposits demonstrate the roles of Venus' high atmospheric density and temperature in modifying the crater formation process. Finally, heavily fractured craters and lava‐embayed craters are found to have higher than average densities along the major fracture belts and rifted uplands connecting Aphrodite Terra and Atla, Beta, Themis, and Phoebe regiones. These craters thus provide physical evidence for recent volcanic and tectonic activity at a low level.

Journal of Geophysical Research E: Planets↗
USGS2000

Orbital radar studies of paleodrainages in the central Namib Desert

Orbital radar images of the central Namib Desert show clearly the extent of relict fluvial deposits associated with former courses of the Tsondab and Kuiseb rivers. South of the Kuiseb River, radar data show the existence of a drainage network developed in calcrete-cemented late Tertiary fluvial deposits. The sand-filled paleovalleys are imaged as radar-dark tones in contrast to the radar-bright interfluves where the calcreted gravels occur. The drainage network developed as a result of local runoff from indurated gravels and channeled surface and subsurface flow to the sites of the many interdune lacustrine deposits found in the area. (C) Elsevier Science Inc., 2000.Orbital radar images of the central Namib Desert show clearly the extent of relict fluvial deposits associated with former courses of the Tsondab and Kuiseb rivers. South of the Kuiseb River, radar data show the existence of a drainage network developed in calcrete-cemented late Tertiary fluvial deposits. The sand-filled paleovalleys are imaged as radar-dark tones in contrast to the radar-bright interfluves where the calcreted gravels occur. The drainage network developed as a result of local runoff from indurated gravels and channeled surface and subsurface flow to the sites of the many interdune lacustrine deposits found in the area.

Remote Sensing of Environment↗
USGS1999

SAR studies in the Yuma Desert, Arizona: Sand penetration, geology, and the detection of military ordnance debris

Synthetic Aperture Radar (SAR) images acquired over part of the Yuma Desert in southwestern Arizona demonstrate the ability of C-band (5.7-cm wavelength), L-band (24.5 cm), and P-band (68 cm) AIRSAR signals to backscatter from increasingly greater depths reaching several meters in blow sand and sandy alluvium. AIRSAR images obtained within the Barry M. Goldwater Bombing and Gunnery Range near Yuma, Arizona, show a total reversal of C- and P-band backscatter contrast (image tone) for three distinct geologic units. This phenomenon results from an increasingly greater depth of radar imaging with increasing radar wavelength. In the case of sandy- and small pebble-alluvium surfaces mantled by up to several meters of blow sand, backscatter increases directly with SAR wavelength as a result of volume scattering from a calcic soil horizon at shallow depth and by volume scattering from the root mounds of healthy desert vegetation that locally stabilize blow sand. AIRSAR images obtained within the military range are also shown to be useful for detecting metallic military ordnance debris that is located either at the surface or covered by tens of centimeters to several meters of blow sand. The degree of detectability of this ordnance increases with SAR wavelength and is clearly maximized on P-band images that are processed in the cross-polarized mode (HV). This effect is attributed to maximum signal penetration at P-band and the enhanced PHV image contrast between the radar-bright ordnance debris and the radar-dark sandy desert. This article focuses on the interpretation of high resolution AIRSAR images but also Compares these airborne SAR images with those acquired from spacecraft sensors such as ERS-SAR and Space Radar Laboratory (SIR-C/X-SAR).Synthetic Aperture Radar (SAR) images acquired over part of the Yuma Desert in southwestern Arizona demonstrate the ability of C-band (5.7-cm wavelength), L-band (24.5 cm), and P-band (68 cm) AIRSAR signals to backscatter from increasingly greater depths reaching several meters in blow sand and sandy alluvium. AIRSAR images obtained within the Barry M. Goldwater Bombing and Gunnery Range near Yuma, Arizona, show a total reversal of C- and P-band backscatter contrast (image tone) for three distinct geologic units. This phenomenon results from an increasingly greater depth of radar imaging with increasing radar wavelength. In the case of sandy- and small pebble-alluvium surfaces mantled by up to several meters of blow sand, backscatter increases directly with SAR wavelength as a result of volume scattering from a calcic soil horizon at shallow depth and by volume scattering from the root mounds of healthy desert vegetation that locally stabilize blow sand. AIRSAR images obtained within the military range are also shown to be useful for detecting metallic military ordnance debris that is located either at the surface or covered by tens of centimeters to several meters of blow sand. The degree of detectability of this ordnance increases with SAR wavelength and is clearly maximized on P-band images that are processed in the cross-polarized mode (HV). This effect is attributed to maximum signal penetration at P-band and the enhanced PHV image contrast between the radar-bright ordnance debris and the radar-dark sandy desert. This article focuses on the interpretation of high resolution AIRSAR images but also compares these airborne SAR images with those acquired from spacecraft sensors such as ERS-SAR and Space Radar Laboratory (SIR-C/X-SAR).

Remote Sensing of Environment↗
USGS1999

The importance of SAR wavelength in penetrating blow sand in Northern Arizona

Multifrequency and polarimetric Airborne Synthetic Aperture Radar (AIRSAR) images of a sand streak in northern Arizona demonstrate the ability of C- (6-cm wavelength), L- (24 cm), and P-band (68 cm) radar signals to backscatter, respectively, from increasingly greater depths reaching 2 m or more in active blow sand. The studied sand streak, located on a salient of Ward Terrace at lat 35??48.4'N and long 111??15.0'W, is sourced in the normally dry Little Colorado River channel and migrates northeastward toward the Moenkopi Plateau Scarp. The sand streak is 'radar dark' compared to the 'radar-bright' surface of Ward Terrace on all C-band images. It is, however, penetrated completely at P-band and is invisible on these images. The sand streak is recognizable on the LHH image but is poorly defined on the LHV image. High resolution AIRSAR images constitute the primary data set analyzed; in addition, a C-band radar image acquired by the European Remote Sensing (ERS) satellite is also presented. The spaceborne image data confirms the results of the aircraft data at C-band.Multifrequency and polarimetric Airborne Synthetic Aperture Radar (AIRSAR) images of a sand streak in northern Arizona demonstrate the ability of C- (6-cm wavelength), L- (24 cm), and P-band (68 cm) radar signals to backscatter, respectively, from increasingly greater depths reaching 2 m or more in active blow sand. The studied sand streak, located on a salient of Ward Terrace at lat 35??48.4???N and long 111??15.0???W, is sourced in the normally dry Little Colorado River channel and migrates northeastward toward the Moenkopi Plateau Scarp. The sand streak is `radar dark' compared to the `radar-bright' surface of Ward Terrace on all C-band images. It is, however, penetrated completely at P-band and is invisible on these images. The sand streak is recognizable on the LHH image but is poorly defined on the LHV image. High resolution AIRSAR images constitute the primary data set analyzed; in addition, a C-band radar image acquired by the European Remote Sensing (ERS) satellite is also presented. The spaceborne image data confirms the results of the aircraft data at C-band.

Remote Sensing of Environment↗
USGS1993

Surficial geology of the Safsaf region, south-central Egypt, derived from remote-sensing and field data

We used a decorrelation-stretched image of Landsat Thematic Mapper (TM) Bands 1, 4, and 7 and field data to map and describe the main surficial units in the hyperarid Safsaf region in south-central Egypt. We show that the near-infrared bands on Landsat TM, which are sensitive to very subtle changes in mineralogy common to arid regions, significantly improve the geologist's capability to discriminate geologic units in desert regions. These data also provide the spatial and spectral information necessary to determine the migration patterns and provenance of eolian materials. The Safsaf area was the focus of our post flight field studies using Shuttle Imaging Radar (SIR) data following the discovery of buried paleochannels in North Africa. Most of the channels discernible on SIR images are not expressed in TM data, but traces of a few channels are present in both the SIR and the TM data within the Wadi Safsaf area. Here we present a detailed digital examination of the SIR and the TM-band reflectance and reflectance-ratio data at three locations of the more obvious surface expressions of the buried channels. Our results indicate that the TM expressions of the channels are not purely topographic but are more compositional in nature. Two possibilities may account for the TM expressions of the buried channels: 1) concentrations of windblown, iron-rich materials that accumulated along subtle curvilinear topograpohic traps, or 2) curvilinear exposures of an iron-rich underlying unit of the flat sand sheet.

Safsaf region↗
USGS1989

Uranium-series dated authigenic carbonates and Acheulian sites in southern Egypt

Field investigations in southern Egypt have yielded Acheulian artifacts in situ in authigenic carbonate deposits (CaCO 3 -cemented alluvium) along the edges of now-aggraded paleovalleys (Wadi Arid and Wadi Safsaf). Uranium-series dating of 25 carbonate samples from various localities as far apart as 70 kilometers indicates that widespread carbonate deposition occurred about 45, 141 and 212 ka (thousand years ago). Most of the carbonate appears to have been precipitated from groundwater, which suggests that these three episodes of deposition may be related to late Pleistocene humid climates that facilitated human settlement in this now hyperarid region. Carbonate cements from sediments containing Acheulian artifacts provide a minimum age of 212 ka for early occupation of the paleovalleys.

southern Egypt↗
USGS1989

New evidence for global tectonic zones on Venus

Venera 15 and 16 spacecraft images show clear evidence of major crustal disruptions on Venus which have been interpreted to indicate crustal divergence. Complementary to the divergent zones are mountain belts that border the continent-like high terrains. The requisite transcurrent motions appear to be manifested as diffuse shear zones. The rift zones form an interconnected transpolar system which ties in with previously recognized equatorial disruption zones, suggesting a global tectonic network. Several independent lines of evidence suggest that the tectonism may be geologically young. -Authors

Geophysical Research Letters↗
USGS1982

Subsurface valleys and geoarcheology of the eastern Sahara revealed by shuttle radar

The shuttle imaging radar (SIR-A) carried on the space shuttle Columbia in November 1981 penetrated the extremely dry Selima Sand Sheet, dunes, and drift sand of the eastern Sahara, revealing previously unknown buried valleys, geologic structures, and possible Stone Age occupation sites. Radar responses from bedrock and gravel surfaces beneath windblown sand several centimeters to possibly meters thick delineate sand- and alluvium-filled valleys, some nearly as wide as the Nile Valley and perhaps as old as middle Tertiary. The now-vanished major river systems that carved these large valleys probably accomplished most of the erosional stripping of this extraordinarily flat, hyperarid region. Underfit and incised dry wadis, many superimposed on the large valleys, represent erosion by intermittent running water, probably during Quaternary pluvials. Stone Age artifacts associated with soils in the alluvium suggest that areas near the wadis may have been sites of early human occupation. The presence of old drainage networks beneath the sand sheet provides a geologic explanation for the locations of many playas and present-day oases which have been centers of episodic human habitation. Radar penetration of dry sand and soils varies with the wavelength of the incident signals (24 centimeters for the SIR-A system), incidence angle, and the electrical properties of the materials, which are largely determined by moisture content. The calculated depth of radar penetration of dry sand and granules, based on laboratory measurements of the electrical properties of samples from the Selima Sand Sheet, is at least 5 meters. Recent (September 1982) field studies in Egypt verified SIR-A signal penetration depths of at least 1 meter in the Selima Sand Sheet and in drift sand and 2 or more meters in sand dunes.

eastern Sahara↗
USGS1981

Stratigraphy of the Caloris basin, Mercury

Caloris basin, Mercury’s youngest large impact basin, is filled by volcanic plains that are spectrally distinct from surrounding material. Post-plains impact craters of a variety of sizes populate the basin interior, and the spectra of the material they have excavated enable the thickness of the volcanic fill to be estimated and reveal the nature of the subsurface. The thickness of the interior volcanic plains is consistently at least 2.5 km, reaching 3.5 km in places, with thinner fill toward the edge of the basin. No systematic variations in fill thickness are observed with long-wavelength topography or azimuth. The lack of correlation between plains thickness and variations in elevation at large horizontal scales within the basin indicates that plains emplacement must have predated most, if not all, of the changes in long-wavelength topography that affected the basin. There are no embayed or unambiguously buried (ghost) craters with diameters greater than 10 km in the Caloris interior plains. The absence of such ghost craters indicates that one or more of the following scenarios must hold: the plains are sufficiently thick to have buried all evidence of craters that formed between the Caloris impact event and the emplacement of the plains; the plains were emplaced soon after basin formation; or the complex tectonic deformation of the basin interior has disguised wrinkle-ridge rings localized by buried craters. That low-reflectance material (LRM) was exposed by every impact that penetrated through the surface volcanic plains provides a means to explore near-surface stratigraphy. If all occurrences of LRM are derived from a single layer, the subsurface LRM deposit is at least 7.5–8.5 km thick and its top likely once made up the Caloris basin floor. The Caloris-forming impact would have generated a layer of impact melt 3–15 km thick; such a layer could account for the entire thickness of LRM. This material would have been derived from a combination of lower crust and upper mantle.

Icarus↗