R. Stephen Saunders

COASTAL GEOMORPHOLOGY OF THE MARTIAN NORTHERN PLAINS

The paper considers the question of the formation of the outflow channels and valley networks discovered on the Martian northern plains during the Mariner 9 mission. Parker and Saunders (1987) and Parker et al. (1987, 1989) data are used to describe key features common both in the lower reaches of the outflow channels and within and along the margins of the entire northern plains. It is suggested, that of the geological processes capable of producing similar morphologies on earth, lacustrine or marine deposition and subsequent periglacial modification offer the simplest and most consistent explanation for the suit of features found on Mars.

Venus volcanism: Classification of volcanic features and structures, associations, and global distribution from Magellan data

A preliminary analysis of a global survey of Magellan data covering over 90% of the surface and designed to document the characteristics, location, and dimensions of all major volcanic features on Venus has revealed over 1660 landforms and deposits. These include over 550 shield fields (concentrations of small volcanoes <20 km in diameter), 274 intermediate volcanoes between 20 and 100 km diameter with a variety of morphologies, 156 large volcanoes in excess of 100 km diameter, 86 calderalike structures independent of those associated with shield volcanoes and typically 60–80 km in diameter, 175 coronae (annulus of concentric ridges or fractures), 259 arachnoids (inner concentric and outer radial network pattern of fractures and ridges), 50 novae (focused radial fractures forming stellate patterns), and 53 lava flood-type flow fields and 50 sinuous lava channels (all of which are in excess of 102–103 km in length). The vast majority of landforms are consistent with basaltic compositions; possible exceptions include steep-sided domes and festoons, which may represent more evolved compositions, and sinuous rules, which may represent more fluid, possibly ultramafic magma. The range of morphologies indicates that a spectrum of intrusive and extrusive processes have operated on Venus. Little evidence was found for extensive pyroclastic deposits or landforms, consistent with the inhibition of volatile exsolution and consequent disruption by the high surface atmospheric pressure. The large size of many volcanic features is evidence for the presence of very large magma reservoirs. The scale of resurfacing implied by individual features and deposits is typically much less than 125,000 km2. The areal distribution, abundance, and size distribution relationships of shield fields, arachnoids, novae, large volcanoes, and coronae strongly suggest that they are the surface manifestation of mantle plumes or hot spots and that the different morphologies represent variations in plume size and stage and thermal structure of the lithosphere. Maps of the global distribution of volcanic features show that they are broadly distributed globally, in contrast to the plate boundary concentrations typical of Earth. However, they are not randomly distributed on the surface of Venus. An observed deficiency of many volcanic features in several lowland areas of Venus may be due to an altitude-dependent influence of atmospheric pressure on volatile exsolution and the production of neutral buoyancy zones sufficient to form magma reservoirs; this would favor lava floods and sinuous channels at low elevations and edifices and reservoir-related features at higher elevations. A major concentration of volcanic features is observed in the Beta/Atla/Themis region, an area covering about 20% of the planet and centered on the equator. This region is unique in that it is the site of local concentrations of volcanic features with concentrations 2–4 times the global average, an interlocking network of rift and deformation zones, several broad rises several thousand kilometers in diameter with associated positive gravity anomalies and tectonic junctions, and evidence for volcanically embayed impact craters. Although the region as a whole does not appear to be anomalously older or younger than the rest of Venus, there is evidence that the most recent volcanic activity on the planet occurs here, and the presence of this series of concentrations suggests that the mantle in this region is anomalous. Analysis of the impact crater population shows that it cannot be distinguished from a completely spatially random population (Phillips et al., this issue), and several end-member models for this distribution are possible: (1) single production age or “spasmodic or catastrophic volcanism” model: craters have accumulated subsequent to a global volcanic resurfacing event about one-half billion years ago (Schaber et al., this issue); (2) vertical equilibrium or “leaky planet” model: craters are removed by slow accumulation of lava over the whole planet leading to a range of volcanic degradation states for craters; (3) regional resurfacing or “collage” or “cookie-cutter” model: craters are removed largely instantaneously by superposition of features and deposits; the horizontal scale of resurfacing does not exceed the horizontal scale of randomness of the crater population. Our data on the scale and location of resurfacing are consistent with the regional resurfacing model and with the catastrophic resurfacing model. The nature and abundance of impact craters definitely degraded by volcanism also favor these two models, although uncertainty exists as to whether all such craters have been detected. Although a process toward the regional resurfacing end-member model presently seems most plausible, distinction between the three models requires an understanding of the mode and timing of emplacement of the volcanic plains that make up the majority of the surface and which are not clearly related to the edifices and features mapped in this study. In addition, the resurfacing mechanisms involved in the catastrophic resurfacing models are not yet explicitly enough formulated to test with the existing data. An equilibrium resurfacing model implies a volcanic flux of 0.5 km3/yr, a value similar to the present rate of intraplate volcanism on Earth (0.3–0.5 km3/yr). This value is broadly comparable to that implied by the edifices and deposits on Venus mapped in this study. Geologically recent volcanism on Venus is dominated by features interpreted to be related to mantle plumes.

Transitional morphology in West Deuteronilus Mensae, Mars: Implications for modification of the lowland/upland boundary

Abstract West Deuteronilus Mensae, which lies along the lowland/upland boundary of Mars at 45° latitude, 345° longitude, exhibits both gradational ( L. A. Rossbacher, 1985 ,in Models in Geomorphology (Woldenberg, Ed), pp. 343–372, Allen & Unwin, Boston) and fretted terrain ( R.P. Sharp, 1973 , J. Geophys. Res. 78, 4073–4083) boundary types. To the west, the boundary is gradational and to the east it is defined by fretted terrain. The two types of boundaries do not merge with one another within the west Deuteronilus Mensae region, however. The gradational boundary materials appear to overlap the fretted terrain by about 500 km. Contacts between units associated with the gradational boundary and the fretted terrain boundary can both be recognized in the region of overlap. The fretted terrain can be identified as much as 300 km north of the gradational boundary in this region. Lowland units associated with the gradational boundary embay canyons of the fretted terrain in a topographically conformal fashion. Changes in the fretted terrain across the gradational boundary (from uplands to lowlands) include a reduction of canyon wall slopes and depths, such that the fretted terrain north of the gradational boundary appears mantled but not obscured. There are at least two major classes of processes which might explain the lateral overlap: (1) erosion of stratified upland terrain and (2) deposition of plains materials onto the sloping upland margin and fretted terrain. Erosion of stratified upland terrain does not adequately explain the plainward decrease in crater densities across the gradational boundary nor is it consistent with evidence that lowland plains material appears to onlap the sloping upland margin in several places. Of the possible plains emplacement mechanisms, eolian deposition would not produce the sharp, apparently topographically conformal gradational unit contacts. Volcanics plains emplacement would not preserve the complex geometry of the underlying fretted terrain. Sediment deposition in either a liquid or ice-covered sea could produce the draped appearance of the fretted terrain. Outflow channels along the lowland/upland boundary, particularly those of the circum-Chryse and west Elysium regions, may have flooded the northern lowlands to depths of tens to hundreds of meters. The gradational unit contacts may represent the shorelines of such a sea. The scale of characteristic morphologies along these contacts may require an unfrozen condition of sufficient duration to allow lacustrine-style wave erosion and redistribution of material along the contacts. Our limited understanding of the Martian paleoclimate and H 2 O inventories allows the possibility of clement periods in the past, and other geological evidence (e.g., small valley networks and outflow channels) strongly suggests an extensive role of liquid water.

Venus tectonics: An overview of Magellan observations

The nearly global radar imaging and altimetry measurements of the surface of Venus obtained by the Magellan spacecraft have revealed that deformational features of a wide variety of styles and spatial scales are nearly ubiquitous on the planet. Many areas of Venus record a superposition of different episodes of deformation and volcanism. This deformation is manifested both in areally distributed strain of modest magnitude, such as families of graben and wrinkle ridges at a few to a few tens of kilometers spacing in many plains regions, as well as in zones of concentrated lithospheric extension and shortening. The common coherence of strain patterns over hundreds of kilometers implies that even many local features reflect a crustal response to mantle dynamic processes. Ridge belts and mountain belts, which have characteristic widths and spacings of hundreds of kilometers, represent successive degrees of lithospheric shortening and crustal thickening. The mountain belts of Venus, as on Earth, show widespread evidence for lateral extension both during and following active crustal compression. Venus displays two principal geometrical variations on lithospheric extension: the quasi-circular coronae (75–2600 km diameter) and broad rises with linear rift zones having dimensions of hundreds to thousands of kilometers. Both are sites of significant volcanic flux, but horizontal displacements may be limited to only a few tens of kilometers. Few large-offset strike slip faults have been observed, but limited local horizontal shear is accommodated across many zones of crustal stretching or shortening. Several large-scale tectonic features have extremely steep topographic slopes (in excess of 20°–30°) over a 10-km horizontal scale; because of the tendency for such slopes to relax by ductile flow in the middle to lower crust, such regions are likely to be tectonically active. In general, the preserved record of global tectonics of Venus does not resemble oceanic plate tectonics on Earth, wherein large, rigid plates are separated by narrow zones of deformation along plate boundaries. Rather tectonic strain on Venus typically involves deformation distributed across broad zones tens to a few hundred kilometers wide separated by comparatively undeformed blocks having dimensions of hundreds of kilometers. These characteristics are shared with actively deforming continental regions on Earth. The styles and scales of tectonic deformation on Venus may be consequences of three differences from the Earth: (1) The absence of a hydrological cycle and significant erosion dictates that multiple episodes of deformation are typically well-preserved. (2) A high surface temperature and thus a significantly shallower onset of ductile behavior in the middle to lower crust gives rise to a rich spectrum of smaller-scale deformational features. (3) A strong coupling of mantle convection to the upper mantle portion of the lithosphere, probably because Venus lacks a mantle low-viscosity zone, leads to crustal stress fields that are coherent over large distances. The lack of a global system of tectonic plates on Venus is likely a combined consequence of a generally lesser strength and more limited horizontal mobility of the lithosphere than on Earth.

Surface modification of Venus as inferred from Magellan observations of plains

In Sedna Planitia, clear stratigraphic relations can be discerned among volcanic flow units. Young flows exhibit SAR specific cross section values similar to fresh terrestrial basalt flows, whereas older flows exhibit backscatter signatures similar to degraded terrestrial basalt flows. Total degradation of ∼1 m depth over ∼0.6 b.y. is inferred for the Sedna area from radar signatures, impact crater abundances, and ejecta superposition relations with respect to volcanic flow units. Analyses of parabolic ejecta deposits associated with the crater Stuart imply that the material is typically centimeters in thickness. A relatively small fraction (∼10%) of Venusian impact craters exhibit prominent parabolic ejecta deposits. These craters are interpreted to be relatively young and parabolic deposits are interpreted to be dispersed by aeolian activity over at least tens of millions of years. The inferred dispersal rate (<10−3 μm/yr) is too low to produce the degradation of flows at Sedna Planitia, and it is concluded that the dominant flow modification process is in situ weathering. In addition, elevation dependent weathering is inferred in western Ovda Regio, where plains above 6054 km radius have enhanced reflection coefficients as compared to adjacent plains at lower elevations. The inferred rate of generation of high reflection coefficient materials is no more than ∼10−2μm/yr, based on the inability of aeolian activity to cover high-reflectivity surfaces with normal reflection coefficient materials and the ubiquitous nature of high-reflectivity surfaces at high elevations. Surface modification rates on Venus are orders of magnitude lower than on Earth. Venusian rates are also much lower than the inferred rate of aeolian dispersal of friable materials on Mars but are comparable to the estimated rate of weathering and erosion of Martian bedrock. Low surface modification rates imply that it will be possible to determine regional-scale age variations on Venus based on the degree of preservation of volcanic landforms and microwave signatures.

Aeolian features on Venus - Preliminary Magellan results

Magellan synthetic aperture radar data reveal numerous surface features that are attributed to aeolian, or wind processes. Wind streaks are the most common aeolian feature. They consist of radar backscatter patterns that are high, low, or mixed in relation to the surface on which they occur. A data base of more than 3400 wind streaks shows that low backscatter linear forms (long, narrow streaks) are the most common and that most streaks occur between 17°S to 30°S and 5°N to 53°N on smooth plains. Moreover, most streaks are associated with deposits from certain impact craters and some tectonically deformed terrains. We infer that both of these geological settings provide fine particulate material that can be entrained by the low-velocity winds on Venus. Turbulence and wind patterns generated by the topographic features with which many streaks are associated can account for differences in particle distributions and in the patterns of the wind streaks. Thus, some high backscatter streaks are considered to be zones that are swept free of sedimentary particles to expose rough bedrock; other high backscatter streaks may be lag deposits of dense materials from which low-density grains have been removed (dense materials such as ilmenite or pyrite have dielectric properties that would produce high backscatter patterns). Wind streaks generally occur on slopes < 2° and tend to be oriented toward the equator, consistent with the Hadley model of atmospheric circulation. In addition to wind streaks, other aeolian features on Venus include yardangs(?) and dune fields. The Aglaonice dune field, centered at 25°S, 340°E, covers ∼1290 km^2 and is located in an ejecta flow channel from the Aglaonice impact crater. The Meshkenet dune field, located at 67°N, 90°E, covers ∼17,120 km^2 in a valley between Ishtar Terra and Meshkenet Tessera. Wind streaks associated with both dune fields suggest that the dunes are of transverse forms in which the dune crests are perpendicular to the prevailing winds. Dunes on Venus signal the presence of sand-size (∼60 to 2,000 μm) grains. The possible yardangs are found at 9°N, 60.5°E, about 300 km southeast of the crater Mead. Although most aeolian features are concentrated in smooth plains near the equator, the occurrence of wind streaks is widespread, and some have been found at all latitudes and elevations. They demonstrate that aeolian processes operate widely on Venus. The intensity of wind erosion and deposits, however, varies with locality and is dependent on the wind regime and supply of particles.

Dike emplacement on Venus and on Earth

Many long linear troughs are visible in synthetic aperture radar images of the surface of Venus. Their geometry closely resembles that of dike swarms on Earth. Such troughs will be formed if dikes fail to reach the surface, or if the magma in them is partially drained. Dikes that form closed ellipses in plan view can be produced by intrusions near a depression, and intersecting dikes can result if the stress field changes with time. Because of the absence of erosion and sedimentation, all these effects are better displayed on Venus than they are on Earth. The magnitude of the regional stress on Venus can be estimated from the dike patterns to be about 3 MPa, or similar to that of Earth. Dikes consisting of en echelon segments whose total length is 1500 km or more occur on both Earth and Venus. On Earth their widths are generally greater than 30 m and are sometimes as great as 250 m, and they are likely to be similar on Venus. The melt velocity during their emplacement is predicted to be a few meters per second, and the calculated rate of heat loss is sufficiently slow that they can remain molten as they propagate for distances of more than 1000 km. The same calculations apply to sills, though less is known about their geometry. The results therefore suggest that magma intrusion in thin sheets extending over 1000 km or more could produce regional crustal thickening and uplift on both Earth and Venus.

Properties of Filamentary Sublimation Residues from Dispersions of Clay in Ice

During vacuum sublimation experiments on simulated Martian polar deposits and cometary dirty ices, a fluffy filamentary sublimate residue material with unique physical properties was produced. The silica-to-silica bonds that we believe join the particles together are the result of conditions that may exist in some Martian polar deposits and on some cometary surfaces. Submicron particles of montmorillonite clay thinly dispersed (1 : 1000 clay/water) and not contacting one another in water ice can form very-low-density structures (density as low as 10 ~ gcm 3) during sublimation of the ice. The lightweight constructs, when viewed in scanning electron microscopy micrographs, are composed of long network chains of the clay particles. The material is sufficiently electrically conductive to drain away the scanning electron microscopy charge. It is also resistant (no change in electronic properties are apparent) to scanning electron microscopy electron-beam heating for hours in vacuo. Infrared spectra and X-ray diffraction patterns of the sublimate residues show little difference from spectra and patterns of the original minerals. Heating in an oven, in air, to 370°C produces little change in the structure of the sublimate residual material. The particle bonding forces are strong and produce a resilient, elastic lightweight material. The material is porous and will allow vapors to diffuse through it, and its thermal conductivity is very low. These properties produce a high-performance vacuum insulation. This material may have applications for insulating ice bodies (solid cryogens) in space. The incoming heat is partially carried away by the out-flowing water vapor. ~ 1986 Academic Press. Inc.

Magellan: Mission Summary

The Magellan radar mapping mission is in the process of producing a global, high-resolution image and altimetry data set of Venus. Despite initial communications problems, few data gaps have occurred. Analysis of Magellan data is in the initial stages. The radar system data are of high quality, and the planned performance is being achieved in terms of spatial resolution and geometric and radiometric accuracy. Image performance exceeds expectations, and the image quality and mosaickability are extremely good. Future plans for the mission include obtaining gravity data, filling gaps in the initial map, and conducting special studies with the radar.

An Overview of Venus Geology

The Magellan spacecraft is producing comprehensive image and altimetry data for the planet Venus. Initial geologic mapping of the planet reveals a surface dominated by volcanic plains and characterized by extensive volcanism and tectonic deformation. Geologic and geomorphologic units include plains terrains, tectonic terrains, and surficial material units. Understanding the origin of these units and the relation between them is an ongoing task of the Magellan team.