Problem footprints in Magellan altimetry data

The intensity, time-delay, and frequency content of radar echoes from the Magellan altimetry system are reduced to several parameters that are of great use in addressing many geological issues of the surface of Venus. These parameters include planetary radius, power reflection coefficient (reflectivity, both uncorrected and corrected for diffuse scattering), rms slope, and scattering functions (the behavior of backscatter as a function of incidence angle). Because the surface of Venus often reflects radio energy in unpredictable ways, models of radar scattering and their associated algorithms occasionally fail to accurately solve for the above surface parameters. Methods for identifying possible 'problem' altimetry data footprints, and techniques for resolving some key ambiguities are presented

Lithological and textural controls on radar and diurnal thermal signatures of weathered volcanic deposits, Lunar Crater region, Nevada

Radar backscatter intensity as measured by calibrated synthetic aperture radar (SAR) systems is primarily controlled by three factors: local incidence angle, wavelength-scale roughness, and dielectric permittivity of surface materials. Radar observations may be of limited use for geological investigations of surface composition, unless the relationships between lithology and the above characteristics can be adequately understood. In arid terrains, such as the Southwest U.S., weathering signatures (e.g. soil development, fracturing, debris grain size and shape, and hill slope characteristics) are controlled to some extent by lithologic characteristics of the parent bedrock. These textural features of outcrops and their associated debris will affect radar backscatter to varying degrees, and the multiple-wavelength capability of the JPL Airborne SAR (AIRSAR) system allows sampling of textures at three distinct scales. Diurnal temperature excursions of geologic surfaces are controlled primarily by the thermal inertia of surface materials, which is a measure of the resistance of a material to a change in temperature. Other influences include albedo, surface slopes affecting insolation, local meteorological conditions and surface emissivity at the relevant thermal wavelengths. To first order, thermal inertia variations on arid terrain surfaces result from grain size distribution and porosity differences, at scales ranging from micrometers to tens of meters. Diurnal thermal emission observations, such as those made by the JPL Thermal Infrared Multispectral Scanner (TIMS) airborne instrument, are thus influenced by geometric surface characteristics at scales comparable to those controlling radar backscatter. A preliminary report on a project involving a combination of field, laboratory and remote sensing observations of weathered felsic-to basaltic volcanic rock units exposed in the southern part of the Lunar Crater Volcanic Field, in the Pancake Range of central Nevada is presented. Focus is on the relationship of radar backscatter cross sections at multiple wavelengths, apparent diurnal temperature excursions identified in multi-temporal TIMS images, surface geometries related to weathering style, and parent bedrock lithology

Overview of Venus geology: Preliminary description of terrain units for Venus global geological mapping

Venus terrain units can be categorized on the basis of morphology, reflectivity, backscatter, roughness, and emissivity. Morphology can be inferred from Magellan left-looking nominal incidence angle image mosaics, right-looking coverage, and more limited left-looking stereo. The typical resolution is about 300 m down to about 120 m near periapsis in the cycle one nominal coverage. The scale of geologic mapping governs definition of mappable terrain units. Initial global mapping is being compiled at a scale of 1:50 million. At this scale, the smallest individual features that can be mapped are about 125 km. The categories of terrain types are plains, complex ridge terrain, features with morphology suggesting volcanic or volcano-tectonic origin, features interpreted to be tectonic in origin, crater units, and surficial units such as splotches and streaks. Brief descriptions of terrain units are provided

Magellan stereo images and Venusian geology

Areas of Venus imaged by Magellan radar with multiple viewing conditions provide unique data that will contribute to the solution of venusian geologic problems and provide a basis for quantitative comparison of venusian landforms with those on other planetary bodies. Three sets of images with different viewing conditions have been acquired: (1) left-looking with variable incidence angles (cycle 1 profile), (2) right-looking with nearly constant incidence angles (cycle 2 profile), and (3) left-looking with variable incidence angles that are almost always smaller than those in (1) (cycle 3 profiles). The unique data provided by paired images of the same scene with different incidence angles arises from image displacements caused by the relief of individual landforms at scales comparable to the ground-range and azimuth resolutions of the images. There are two aspects of the data: (1) Stereopsis achieved by simultaneous viewing of paired left-looking images of the same scene permits three-dimensional perception and interpretation of the morphologies of landforms at resolutions much finer than the altimetry footprints. (2) Measurements of differences of image displacements (parallax) on paired images with known imaging geometries provide quantitative estimates of the relief and shapes of landforms. The potential scientific contributions of the data can be grouped into two interrelated classes: (A) geologic mapping, analysis, and interpretation and (B) topical studies that involve topographic measurements. Stereopsis, without quantitative measurements, enhances geologic mapping, analysis, and interpretation of the rock units of Venus to a degree that cannot be overestimated. In geologic mapping, assemblages of landforms, assessments of backscatter and variations in backscatter, and fine-scale topography are used to define and characterize geologic map units that represent laterally continuous deposits or rock units. Stereopsis adds the important dimension of local relief for characterization of geologic units at a scale that is not possible with Magellan altimetry or products derived from the altimetry. Relative ages of the geologic units are determined using the well-known principles of superposition and intersection. Here, the perception of relief is invaluable because superposition relations among the geological units are more readily and clearly established. The recognition of folds, faults, and fault systems, regardless of their orientations, is facilitated with stereopsis so that sequences of deformation of the geologic units can be determined and structural analyses vastly improved. Shapes of landforms are readily perceived so that they can be properly interpreted. The end result of the mapping, analyses, and interpretations is a geologic history of Venus that includes the sequences of formation and deformation of various geologic units. Measurements of relief at the finest scale possible are necessary for numerous topical studies. Standard altimetry will provide the necessary information on the relief of most large landforms, but it tends to underestimate the relief of small landforms and distorts their shapes. Although special processing of the altimeter echoes improves the estimates of the relief and shapes of some landforms, there are uncertainties in the interpretations of the echoes. Examples of topical studies requiring measurements of relief are given

The unique radar scattering properties of silicic lava flows and domes

Silicic (silica-rich) lava flows, such as rhyolite, rhyodacite, and dacite, possess unique physical properties primarily because of the relatively high viscosity of the molten lava. Silicic flows tend to be thicker than basaltic flows, and the resulting large-scale morphology is typically a steep-sided dome or flow lobe, with aspect ratios (height/length) sometimes approaching unity. The upper surfaces of silicic domes and flows are normally emplaced as relatively cool, brittle slabs that fracture as they are extruded from the central vent areas, and are then rafted away toward the flow margin as a brittle carapace above a more ductile interior layer. This mode of emplacement results in a surface with unique roughness characteristics, which can be well-characterized by multiparameter synthetic aperture radar (SAR) observations. In this paper, we examine the scattering properties of several silicic domes in the Inyo volcanic chain in the Eastern Sierra of California, using AIRSAR and TOPSAR data. Field measurements of intermediate-scale (cm to tens of m) surface topography and block size are used to assess the mechanisms of the scattering process, and to quantify the unique roughness characteristics of the flow surfaces

SHARAD radar sounding of the Vastitas Borealis Formation in Amazonis Planitia

Amazonis Planitia has undergone alternating episodes of sedimentary and volcanic infilling, forming an interleaved sequence with an upper surface that is very smooth at the kilometer scale. Earlier work interprets the near-surface materials as either young, rough lava flows or ice-rich sediment layers, overlying a basement comprising the Vastitas Borealis Formation and earlier Hesperian plains. Sounding radar profiles across Amazonis Planitia from the Shallow Radar (SHARAD) instrument on the Mars Reconnaissance Orbiter reveal a subsurface dielectric interface that increases in depth toward the north along most orbital tracks. The maximum depth of detection is 100–170 m, depending upon the real dielectric permittivity of the materials, but the interface may persist at greater depth to the north if the reflected energy is attenuated below the SHARAD noise floor. The dielectric horizon likely marks the boundary between sedimentary material of the Vastitas Borealis Formation and underlying Hesperian volcanic plains. The SHARAD-detected interface follows the surface topography across at least one of the large wrinkle ridges in north central Amazonis Planitia. This conformality suggests that Vastitas Borealis sediments, at least in this region, were emplaced prior to compressional tectonic deformation. The change in radar echo strength with time delay is consistent with a loss tangent of 0.005–0.012 for the column of material between the surface and the reflector. These values are consistent with dry, moderate-density sediments or the lower end of the range of values measured for basalts. While a component of distributed ice in a higher-loss matrix cannot be ruled out, we do not find evidence for a dielectric horizon within the Vastitas Borealis Formation that might suggest an abrupt change from an upper dry layer to an ice-rich lower deposit

Effects of the passage of Comet C/2013 A1 (Siding Spring) observed by the Shallow Radar (SHARAD) on Mars reconnaissance orbiter

The close passage of Comet C/2013 A1 (Siding Spring) to Mars provided a unique opportunity to observe the interaction of cometary materials with the Martian ionosphere and atmosphere using the sounding radar SHARAD (SHAllow RADar) aboard Mars Reconnaissance Orbiter. In two nightside observations, acquired in the 10 h following the closest approach, the SHARAD data reveal a significant increase of the total electron content (TEC). The observed TEC values are typical for daylight hours just after dawn or before sunset but are unprecedented this deep into the night. Results support two predictions indicating that cometary pickup O+ ions, or ions generated from the ablation of cometary dust, are responsible for the creation of an additional ion layer

Radar sounder evidence of thick, porous sediments in Meridiani Planum and implications for ice‐filled deposits on Mars

Meridiani Planum is one of the most intensely studied regions on Mars, yet little is known about the physical properties of the deposits below those examined by the Opportunity rover. We report the detection of subsurface echoes within the Meridiani Planum deposits from data obtained by the Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) instrument. The delay time between the surface and subsurface returns is indicative of materials with a real dielectric constant of 3.6 ± 0.6. The real dielectric constant is strongly modulated by bulk density. Newly derived compaction relationships for Mars indicate that the relatively low dielectric constant of the Meridiani Planum deposits is consistent with a thick layer of ice‐free, porous, basaltic sand. The unique physiographic and hydrologic setting of Meridiani Planum may have provided an ideal sediment trap for eolian sands. The relatively low gravity and the cold, dry climate that has dominated Mars for billions of years may have allowed thick eolian sand deposits to remain porous and only weakly indurated. Minimally compacted sedimentary deposits may offer a possible explanation for other nonpolar region units with low apparent bulk dielectric constants

Subsurface structure of Planum Boreum from Mars Reconnaissance Orbiter Shallow Radar soundings

We map the subsurface structure of Planum Boreum using sounding data from the Shallow Radar (SHARAD) instrument onboard the Mars Reconnaissance Orbiter. Radar coverage throughout the 1,000,000- km2 area reveals widespread reflections from basal and internal interfaces of the north polar layered deposits (NPLD). A dome-shaped zone of diffuse reflectivity up to 12 ls (1-km thick) underlies twothirds of the NPLD, predominantly in the main lobe but also extending into the Gemina Lingula lobe across Chasma Boreale. We equate this zone with a basal unit identified in image data as Amazonian sand-rich layered deposits [Byrne, S., Murray, B.C., 2002. J. Geophys. Res. 107, 5044, 12 pp. doi:10.1029/2001JE001615; Fishbaugh, K.E., Head, J.W., 2005. Icarus 174, 444–474; Tanaka, K.L., Rodriguez, J.A.P., Skinner, J.A., Bourke, M.C., Fortezzo, C.M., Herkenhoff, K.E., Kolb, E.J., Okubo, C.H., 2008. Icarus 196, 318–358]. Elsewhere, the NPLD base is remarkably flat-lying and co-planar with the exposed surface of the surrounding Vastitas Borealis materials. Within the NPLD, we delineate and map four units based on the radar-layer packets of Phillips et al. [Phillips, R.J., and 26 colleagues, 2008. Science 320, 1182– 1185] that extend throughout the deposits and a fifth unit confined to eastern Gemina Lingula. We estimate the volume of each internal unit and of the entire NPLD stack (821,000 km3), exclusive of the basal unit. Correlation of these units to models of insolation cycles and polar deposition [Laskar, J., Levrard, B., Mustard, J.F., 2002. Nature 419, 375–377; Levrard, B., Forget, F., Montmessin, F., Laskar, J., 2007. J. Geophys. Res. 112, E06012, 18 pp. doi:10.1029/2006JE002772] is consistent with the 4.2-Ma age of the oldest preserved NPLD obtained by Levrard et al. [Levrard, B., Forget, F., Montmessin, F., Laskar, J., 2007. J. Geophys. Res. 112, E06012, 18 pp. doi:10.1029/2006JE002772]. We suggest a dominant layering mechanism of dust–content variation during accumulation rather than one of lag production during periods of sublimation