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.     

Polygonal impact craters in Argyre region,Mars: Implications for geology and cratering mechanics

Abstract— Impact craters are not always circular; sometimes their rims are composed of several straight segments. Such polygonal impact craters (PICs) are controlled by pre‐existing target structures, mainly faults or other similar planes of weakness. In the Argyre region, Mars, PICs comprise ? 17% of the total impact crater population (>7 km in diameter), and PICs are relatively more common in older geologic units. Their formation is mainly controlled by radial fractures induced by the Argyre and Ladon impact basins, and to a lesser extent by the basin‐concentric fractures. Also basin‐induced conjugate shear fractures may play a role. Unlike the PICs, ridges and graben in the Argyre region are mostly controlled by Tharsis‐induced tectonism, with the ridges being concentric and graben radial to Tharsis. Therefore, the PICs primarily reflect an old impact basin‐centered tectonic pattern, whereas Tharsis‐centered tectonism responsible for the graben and the ridges has only minor influence on the PIC rim orientations. According to current models of PIC formation, complex PICs should form through a different mechanism than simple PICs, leading to different orientations of straight rim segments. However, when simple and complex PICs from same areas are studied, no statistically significant difference can be observed. Hence, in addition to enhanced excavation parallel to the strike of fractures (simple craters) and slumping along the fracture planes (complex craters), we propose a third mechanism involving thrusting along the fracture planes. This model is applicable to both simple and small complex craters in targets with some dominating orientations of structural weakness.     

Classification of the rift zones of venus: Rift valleys and graben belts

The spatial distribution of rift zones of Venus, their topographic configuration, morphometric parameters, and the type of volcanism associating with rifts were analyzed. This allowed the main characteristic features of rifts to be revealed and two different types of rift-forming structures, serving for classification of rift zones as rift valleys and graben belts, to be isolated. These structural types (facies) of rift zones are differently expressed in the relief: rift valleys are individual deep (several kilometers) W-shaped canyons, while graben belts are clusters of multiple V-shaped and rather shallow (hundreds of meters) depressions. Graben belts are longer and wider, as compared to rift valleys. Rift valleys are spatially associated with dome-shaped volcanic rises and large volcanos (concentrated volcanic sources), while graben belts do not exhibit such associations. Volcanic activity in the graben belts are presented by spacious lava fields with no apparent sources of volcanism. Graben belts and rift valleys were formed during the Atlian Period of geologic history of Venus, and they characterized the tectonic style of the planet at the late stages of its geologic evolution. Formation of this or that structural facies of the rift zones of Venus were probably governed by the thickness of the lithosphere, its rheological properties, and the development degree of the mantle diapirs associating with rift zones.     

Geologic mapping of the Amirani-Gish Bar region of Io: Implications for the global geologic mapping of Io

We produced the first geologic map of the Amirani-Gish Bar region of Io, the last of four regional maps generated from Galileo mission data. The Amirani-Gish Bar region has five primary types of geologic materials: plains, mountains, patera floors, flows, and diffuse deposits. The flows and patera floors are thought to be compositionally similar, but are subdivided based on interpretations regarding their emplacement environments and mechanisms. Our mapping shows that volcanic activity in the Amirani-Gish Bar region is dominated by the Amirani Eruptive Center (AEC), now recognized to be part of an extensive, combined Amirani-Maui flow field. A mappable flow connects Amirani and Maui, suggesting that Maui is fed from Amirani, such that the post-Voyager designation “Maui Eruptive Center” should be revised. Amirani contains at least four hot spots detected by Galileo, and is the source of widespread bright (sulfur?) flows and active dark (silicate?) flows being emplaced in the Promethean style (slowly emplaced, compound flow fields). The floor of Gish Bar Patera has been partially resurfaced by dark lava flows, although other parts of its floor are bright and appeared unchanged during the Galileo mission. This suggests that the floor did not undergo complete resurfacing as a lava lake as proposed for other ionian paterae. There are several other hot spots in the region that are the sources of both active dark flows (confined within paterae), and SO₂- and S₂-rich diffuse deposits. Mapped diffuse deposits around fractures on mountains and in the plains appear to serve as the source for gas venting without the release of magma, an association previously unrecognized in this region. The six mountains mapped in this region exhibit various states of degradation. In addition to gaining insight into this region of Io, all four maps are studied to assess the best methodology to use to produce a new global geologic map of Io based on the newly released, combined Galileo-Voyager global mosaics. To convey the complexity of ionian surface geology, we find that a new global geologic map of Io should include a map sheet displaying the global abundances and types of surface features as well as a complementary GIS database as a means to catalog the record of surface changes observed since the Voyager flybys and during the Galileo mission.     

Tectonic histories between Alba Patera and Syria Planum, Mars

Syria Planum and Alba Patera are two of the most prominent features of magmatic-driven activity identified for the Tharsis region and perhaps for all of Mars. In this study, we have performed a Geographic Information System-based comparative investigation of their tectonic histories using published geologic map information and Mars Orbiter Laser Altimetry (MOLA) data. Our primary objective is to assess their evolutional histories by focusing on their extent of deformation in space and time through stratigraphic, paleotectonic, topographic, and geomorphologic analyses. Though there are similarities among the two prominent features, there are several distinct differences, including timing deformational extent, and tectonic intensity of formation. Whereas Alba Patera displays a major pulse of activity during the Late Hesperian/Early Amazonian, Syria Planum is a long-lived center that displays a more uniform distribution of simple graben densities ranging from the Noachian to the Amazonian, many of which occur at greater distances away from the primary center of activity. The histories of the two features presented here are representative of the complex, long-lived evolutional history of Tharsis.     

MANGALA VALLES, MARS: ASSESSMENT OF EARLY STAGES OF FLOODING AND DOWNSTREAM FLOOD EVOLUTION

The Mangala Valles system is an ∼ ∼900 km fluvially carved channel system located southwest of the Tharsis rise and is unique among the martian outflow channels in that it heads at a linear fracture within the crust as opposed to a collapsed region of chaos as is the case with the circum-Chryse channels. Mangala Valles is confined within a broad, north–south trending depression, and begins as a single valley measuring up to 350 km wide that extends northward from a Memnonia Fossae graben, across the southern highlands toward the northern lowlands. Approximately 600 km downstream, this single valley branches into multiple channels, which ultimately lose their expression at the dichotomy boundary. Previous investigations of Mangala Vallis suggested that many of the units mapped interior to the valley were depositional, related to flooding, and that a minimum of two distinct periods of flooding separated by tens to hundreds of millions of years were required to explain the observed geology. We use infrared and visible images from the THermal EMission Imaging System (THEMIS), and topographic data from the Mars Orbiting Laser Altimeter (MOLA), to investigate the nature of the units mapped within Mangala Vallis. We find that the geomorphology of the units, as well as their topographic and geographic distribution, are consistent with most of them originating from a single assemblage of volcanic flow deposits, once continuous with volcanic flows to the south of the Memnonia Fossae source graben. These flows resurfaced the broad, north–south trending depression into which Mangala Vallis formed prior to any fluvial activity. Later flooding scoured and eroded this volcanic assemblage north of the Mangala source graben, resulting in the present distribution of the units within Mangala Vallis. Additionally, our observations suggest that a single period of catastrophic flooding, rather than multiple periods separated by tens to hundreds of millions of years, is consistent with and can plausibly explain the interior geology of Mangala Vallis. Further, we present a new scenario for the source and delivery of water to the Mangala source graben that models flow of groundwater through a sub-cryosphere aquifer and up a fracture that cracks the cryosphere and taps this aquifer. The results of our model indicate that the source graben, locally enlarged to a trough near the head region of Mangala, would have required less than several days to fill up prior to any spill-over of water to the north. Through estimates of the volume of material missing from Mangala (13,000–20,000 km³), and calculation of mean discharge rates through the channel system (∼ ∼5 × 10⁶ m³ s⁻¹), we estimate that the total duration of fluvial activity through the Mangala Valles was 1–3 months.     

The Aristarchus-Harbinger region of the moon: Surface geology and history from recent remote-sensing observations

The region including the Aristarchus Plateau and Montes Harbinger is probably the most diverse, geologically, of any area of comparble size on the Moon. This part of the northwest quadrant of the lunar near side includes unique dark mantling material; both the densest concentration and the largest of the sinuous rilles; apparent volcanic vents, sinks, and domes; mare materials of various ages and colors; one of the freshest large craters (Aristarchus) with ejecta having unique colors and albedos; and three other large craters in different states of flooding and degradation (krieger, Herodotus, and Prinz). The three best-authenticated lunar transient phenomena were also observed here.This study is based principally on photographic and remote sensing observations made from Earth and Apollo orbiting space craft. Results include (1) delineation of geologic map units and their stratigraphic relationships; (2) discussion of the complex interrelationships between materials of volcanic and impact origin, including the effects of excavation, redistribution and mixing of previously deposited materials by younger impact craters; (3) deduction of physical and chemical properties of certain of the geologic units, based on both the remote-sensing information and on extrapolation of Apollo data to this area; and (4) development of a detailed geologic history of the region, outlining the probable sequence of events that resulted in its present appearance.A primary concern of the investigation has been anomalous red dark mantle on the Plateau. Based on an integration of Earth- and lunar orbit-based data, this layer seems to consist of fine-grained, block-free material containing a relatively large fraction of orange glass. It is probably of pyroclastic origin, laid down at some time during the Imbrian period of mare flooding.     

History of tectonic deformation in the interior plains of the Caloris basin,mercury

Analysis of images from the Messenger MDIS narrow angle camera imply that at least part of the radial graben of the Pantheon Fossae structure, and probably the structure as a whole, predate the deformation that led to circumferential ridges on the Caloris interior plains. This follows from structural analysis and comparison with similar geological relationships on Venus and the Moon, where graben are known to both postdate and predate ridges. Observations suggest that the Pantheon Fossae radial graben (extension) formed first, pre-dating observed circumferential graben (also extension), with ridges (compression) formed in between. This scenario puts constraints on the models for the deformation of the Caloris basin and its vicinity. Our observations and analysis are consistent with Pantheon Fossae having formed in a similar manner to Venusian astra/novae, where radial dikes that propagate away from a magmatic center led to graben formation. Our results also have implications for the length of time between the emplacement of the basin volcanic fill and the onset of the compressional stresss regime that led to ridge-formation. If the Pantheon Fossae structure formed before the emplacement of ridges, as we suggest, this means that compressional stresses took some time to develop sufficiently to deform the volcanic plains. Since the Caloris interior plains had to have been already in place when Pantheon Fossae formed, and since these plains represented a significant load to the underlying lithosphere, it is striking that compression took some time to develop. These observations may provide new information about the rigidity of the basin-filling material and will help constrain models for the mechanisms and timing of events within and around the Caloris basin.     

Episodes of floods in Mangala Valles,Mars, from the analysis of HRSC,MOC and THEMIS images

The Mangala Valles is a 900-km long outflow channel system in the highlands adjacent to the south-eastern flank of the Tharsis bulge. This work was intended to answer the following two questions unresolved in previous studies: (1) Was there only one source of water (Mangala Fossa at the valley head which is one of the Medusae Fossae troughs or graben) or were other sources also involved in the valley-carving water supply, and (2) Was there only one episode of flooding (maybe with phases) or were there several episodes significantly separated in time. The geologic analysis of HRSC image 0286 and mapping supported by analysis of MOC and THEMIS images show that Mangala Valles was carved by water released from several sources. The major source was Mangala Fossa, which probably formed in response to magmatic dike intrusion. The graben cracked the cryosphere and permitted the release of groundwater held under hydrostatic pressure. This major source was augmented by a few smaller-scale sources at localities in (1) two mapped heads of magmatic dikes, (2) heads of two clusters of sinuous channels, and (3) probably several large knob terrain locals. The analysis of results of crater counts at more than 60 localities showed that the first episode of formation of Mangala Valles occurred ～3.5 Ga ago and was followed by three more episodes, one occurred ～1 Ga ago, another one ～0.5 Ga ago, and the last one ～0.2 Ga ago. East of the mapped area there are extended and thick lava flows whose source may be the eastern continuation of the Mangala source graben. Crater counts in 10 localities on these lava flows correlate with those taken on the Mangala valley elements supporting the idea that the valley head graben was caused by dike intrusions. Our observations suggest that the waning stage of the latest flooding episode (～0.2 Ga ago) led to the formation at the valley head of meander-like features sharing some characteristics with meanders of terrestrial rivers. If this analogy is correct this could suggest a short episode of global warming in Late Amazonian time.     

Assessment of planetary geologic mapping techniques for Mars using terrestrial analogs: The SP Mountain area of the San Francisco Volcanic Field,Arizona

We photogeologically mapped the SP Mountain region of the San Francisco Volcanic Field in northern Arizona, USA to evaluate and improve the fidelity of approaches used in geologic mapping of Mars. This test site, which was previously mapped in the field, is chiefly composed of Late Cenozoic cinder cones, lava flows, and alluvium perched on Permian limestone of the Kaibab Formation. Faulting and folding has deformed the older rocks and some of the volcanic materials, and fluvial erosion has carved drainage systems and deposited alluvium. These geologic materials and their formational and modificational histories are similar to those for regions of the Martian surface. We independently prepared four geologic maps using topographic and image data at resolutions that mimic those that are commonly used to map the geology of Mars (where consideration was included for the fact that Martian features such as lava flows are commonly much larger than their terrestrial counterparts). We primarily based our map units and stratigraphic relations on geomorphology, color contrasts, and cross-cutting relationships. Afterward, we compared our results with previously published field-based mapping results, including detailed analyses of the stratigraphy and of the spatial overlap and proximity of the field-based vs. remote-based (photogeologic) map units, contacts, and structures. Results of these analyses provide insights into how to optimize the photogeologic mapping of Mars (and, by extension, other remotely observed planetary surfaces). We recommend the following: (1) photogeologic mapping as an excellent approach to recovering the general geology of a region, along with examination of local, high-resolution datasets to gain insights into the complexity of the geology at outcrop scales; (2) delineating volcanic vents and lava-flow sequences conservatively and understanding that flow abutment and flow overlap are difficult to distinguish in remote data sets; (3) taking care to understand that surficial materials (such as alluvium and volcanic ash deposits) are likely to be under-mapped yet are important because they obscure underlying units and contacts; (4) where possible, mapping multiple contact and structure types based on their varying certainty and exposure that reflect the perceived accuracy of the linework; (5) reviewing the regional context and searching for evidence of geologic activity that may have affected the map area yet for which evidence within the map area may be absent; and (6) for multi-authored maps, collectively analyzing the mapping relations, approaches, and methods throughout the duration of the mapping project with the objective of achieving a solid, harmonious product.     

Tectonic pattern of the crater Grimaldi region of the Moon

The tectonics of the Grimaldi area are described and analyzed in detail from high-resolution Lunar Orbiter photographs.Rille grabens are long and narrow fault zone structures of lunar terra. The polygonal rille graben pattern indicates the importance of lunar internal activity with an adjoining thin lithosphere in the areal tectonics at the time of rille grabening. The graben subsidence developed during tensional bending of this thin terra lithosphere. The en échelon graben offsets indicate the existence of strikeslip movements along the main fault under tensional lithosphere conditions.In some places mare ridge ranges continue in the direction of the rille graben indicating the connection of these structures to each other as part of the lunar tectonic evolution. The very thin mare lithosphere was affected more easily and over a longer period of time by lunar internal forces. The effect of older structural units is thus less conspicuous within mare areas. Proposed Riedel-shear-like structures indicate a slight shortening and compression of the mare basin lithosphere during movements along lava-covered zones of weakness.     

Surficial properties in Melas Chasma, Mars, from Mars Odyssey THEMIS data

We examine the nature of the surface layer in a small area of the Melas Chasma region as determined from high-resolution thermal and visible Mars Odyssey Thermal Emission Imaging System (THEMIS) data as well as how our conclusions compare to past analyses. At THEMIS resolution, the thermal structure is dominated by local control and all significant thermal variations can be linked to morphology. Thus, THEMIS provides us with detailed images that contain thermophysical information as well, allowing us to create a surficial geologic map intended to reflect the surface structure of the region. Eight units have been defined: (i) blanketed plateaus with thermally distinct craters and fractures, (ii) blanketed canyon walls with rocky edges, (iii) indurated and/or rocky canyon wall slide material partially covered by aeolian material, (iv) an anomalous wall region with fluvial-like depressions partially filled with particulate material, (v) indurated and/or rocky ridged and non-ridged canyon floor landslide material mingled with aeolian material, (vi) sand sheets, (vii) indurated and/or rocky rounded blocks intermingled with small areas of aeolian material, and (viii) transverse dunes. The THEMIS thermal data support conclusions from previous studies but also reveal much more structure than was seen in the past. We have found that all significant thermal variations in this region can be linked to morphology but all morphological variations cannot be linked to significant thermal variations. THEMIS visible images provide an intermediate resolution that bridges the gap between MOC and Viking and allow for a more meaningful interpretation of the geologic context of a region. Surfaces indicate that landslides were an important geologic process long ago, shaping the canyon walls and floor, while aeolian processes have subsequently altered the surface layer in many locations and may still be active.     

Elysium planitia,mars: Regional geology,volcanology, and evidence for volcano-ground ice interactions

Geological mapping of Elysium Planitia has led to the recognition of five major surface units, in addition to the three volcanic constructs Elysium Mons, Hecates Tholus, and Albor Tholus. These units are interpreted to be both volcanic and sedimentary or erosional in origin. The volcano Elysium Mons is seen to have dominated constructional activity within the whole region, erupting lava flows which extend up to 600km from the summit. A major vent system, covering an area in excess of 75 000 km², is identified within the Elysium Fossae area. Forty-one sinuous channels are visible within Elysium Planitia; these channels are thought to be analogous to lunar sinuous rilles and their formation in this region of Mars is attributed to unusually high regional topographic slopes (up to ~ 1.7). Numerous circumferential graben are centered upon Elysium Mons. These graben, located at radial distances of 175, 205–225, and 330km from the summit, evidently post-dated the emplacement of the Elysium Mons lava flows but pre-dated the eruption of extensive flood lavas to the west of the volcano. A great diversity of channel types is observed within Elysium Fossae. The occurrences of streamlined islands and multiple floor-levels within some channels suggests a fluvial origin. Conversely, the sinuosity and enlarged source craters of other channels suggests a volcanic origin. Impact crater morphology, the occurrence of chaotic terrain, probable pyroclastic deposits upon Hecates Tholus and fluvial channels all suggest extensive volcano-ground ice interactions within this area.NASA Summer Intern.     

Radiating graben-fissure systems in the Ulfrun Regio area, Venus

Radiating graben-fissure systems are common on Venus. Most are thought to be underlain by mafic dykes, fed by centrally-located magmatic centres. From previous work it has been shown that these magmatic plumbing systems can extend out up to 2000 km or more and that interaction between neighbouring systems can provide insight into the relative chronology of their magmatic centres. Systematic mapping of graben-fissure systems has potential as a tool for regional magmatic chronology and correlation on Venus.This methodology is applied to the Ulfrun Regio area (200-240°E, 0-25°N) where we mapped 47,000 graben and fissures. From these, 66 radiating systems comprised of 13,000 individual graben and fissures, and having radii of up to 2000 km have been identified, and are interpreted to be underlain by dyke swarms focussed on magmatic centres. Cross-cutting relationships among these systems and with the Hecate Chasma rift zone have been examined to provide a relative chronology for the magmatic centres. Two trends emerged: (a) an apparent younging from the southwest to northeast of the study area and (b) a cluster of older ages in the southwest, linked to the Atla Regio mantle plume.     

Geologic mapping of the Zal region of Io

We produced a regional geologic map of the Zal region of Io's antijovian hemisphere using Galileo mission data. We discuss the geologic features, summarize the map units and structures that are present, discuss the nature of volcanic activity, and present an analysis of the volcanic, tectonic, and gradational processes that affect the region. The Zal region consists of five primary types of geologic materials: plains, mountains, paterae floors, flows, and diffuse deposits. The flows and patera floors are similar, but are subdivided based on uncertainties regarding emplacement environments and mechanisms. The Zal region includes two hotspots detected by Galileo: one along the western scarp of the Zal Patera volcano and one at the Rustam Patera volcano (name submitted to IAU). A third hotspot at the nearby At'am Patera volcano (name submitted to IAU) is the source of diffuse and pyroclastic materials that blanket north Zal Mons. The western bounding scarp of Zal Patera is the location of a fissure vent that is the source of multiple silicate lava flows. The floor of Zal Patera has been partially resurfaced by dark lava flows, although portions of the patera floor appear bright and unchanged during the Galileo mission. This suggests that the floor did not undergo complete resurfacing as a flooding lava lake but does contain a compound flow field. Mountain materials exhibit stages of degradation; lineated material degrades into mottled material. We have explored the possibility that north and south Zal Mons were originally one structure. We propose that strike-slip faulting and subsequent rifting separated the mountain units, opened a fissure which serves as a vent for lava flow, and created a depression which, by further extension during the rifting event, became Zal Patera. With comparison to other regional maps of Io, this work provides insight into the general geologic evolution of Io.     

Geomorphologic mapping of the Menrva region of Titan using Cassini RADAR data

We made a detailed geomorphologic map of the Menrva region of Titan, using Cassini RADAR data as our map base. Using similar techniques and approaches that were applied to mapping Magellan radar images of Venus, and earlier, more generalized Titan maps, we were able to define and characterize 10 radar morphologic units, along with inferred dunes and fluvial channels, from the RADAR data. Structural features, such as scarps, ridges, and lineaments were also identified. Using principles of superposition, cross-cutting, and embayment relations we created a sequence of map units for this region. We interpret Menrva to be a 440 km wide degraded impact basin, in agreement with earlier studies by Elachi et al. (Elachi, C. et al. [2006]. Nature 441, 709-713) and Wood et al. (Wood, C.A., Lorenz, R., Kirk, R., Lopes, R., Mitchell, K., Stofan, E., and the Cassini RADAR Team [2010]. Icarus 206, 334-344), and identify it as the oldest feature in the map region. Exogenic processes including hydrocarbon fluid channelization forming the Elivagar Flumina channel network and dune fields resulting from aeolian activity are the current geologic processes dominating our map area, and these processes have contributed to the erosion of the crater’s ejecta field. There is evidence of multiple episodes of channel formation, erosion and burial by aeolian deposits, as observed elsewhere on Titan by e.g., Barnes et al. (Barnes, J.W. et al. [2005]. Icarus 195, 400-414). Channel outflow regions have morphologies suggestive of streams formed by flash floods, and dune fields are small and restricted rather than forming large dune seas, consistent with a desert-like environment for this region with low supply of hydrocarbon particles, also consistent with other studies by e.g., Lorenz et al. (Lorenz, R.D. et al. [2008a]. Planet. Space Sci. 56, 1132-1144). There is no evidence of cryovolcanism or non-impact-related tectonic activity in the Menrva region, although this region is too small to infer anything about the roles of these processes elsewhere on Titan. This work suggests detailed geomorphologic mapping can confidently be applied to Cassini RADAR data, and we suggest that more extensive mapping should be done using RADAR, ISS, and VIMS data geographically distributed across Titan to assess its usefulness for a future combined RADAR-ISS-VIMS-based global geologic map.     

On the resurfacing of Ganymede by liquid-water volcanism

A long-popular model for producing Ganymede's bright terrain involves flooding of low-lying graben with liquid water, slush, or warm, soft ice. The model suffers from major problems, however, including the absence of obvious near-surface heat sources, the negative buoyancy of liquid water, and the lack of a mechanism for confining the flows to graben floors. We present new models for cryovolcanic resurfacing to overcome these difficulties. Tidal heating within an ancient Laplace-like orbital resonance (Showman and Malhotra 1997, Icarus 127, 93; Showman et al., 1997, Icarus 129, 367) provides a plausible heat source and could allow partial melting to occur as shallow as 5-10 km depth. Our favored mechanism for delivering this water to the surface invokes the fact that topography—such as a global set of graben—causes subsurface pressure gradients that can pump water or slush upward onto the floors of topographic lows (graben) despite the negative buoyancy of the liquid. These eruptions can occur only within the topographic lows; furthermore, as the low areas become full, the pressure gradients disappear and the resurfacing ceases. This provides an explanation for the observed straight dark-bright terrain boundaries: water cannot overflow the graben, so resurfacing rarely embays craters or other rough topography. Pure liquid water can be pumped to the surface from only 5-10 km depth, but macroscopic bodies of slush ascending within fractures can reach the surface from much greater depths due to the smaller negative buoyancy of slush. A challenge for these models is the short predicted gravitational relaxation timescale of topographic features at high heat flows; the resurfacing must occur before the graben topography disappears. We also evaluate alternate resurfacing mechanisms, such as pumping of liquid water to the surface by thermal expansion stresses and buoyant rise of water through a silicate-contaminated crust that is denser than liquid water, and conclude that they are unlikely to explain Ganymede's bright terrain.     

Far infrared photometry of planets: Saturn and Venus: Volume 38, Number 3 (1979), in the article,by R. Courtin,P. Léna,M. De Muizon,D. Rouan,C. Nicollier,and J. Wijnbergen,pp. 411–419

The global distribution of existing lunar topography suffers from a lack of measurements of far-side radii because of the sparsity of data types in the nonequatorial regions. This paper presents determinations of far-side lunar radii based on the reduction of photogrammetric measurements derived from selected Apollo 16 trans-Earth phase photographs. The regions covered in this analysis lie west of Mare Moscoviense between longitudes 90 and 130°E and latitudes 10 and 60°N. The determinations are made using control points appearing on both NASA topographic orthophoto maps and the Apollo 16 photographs. The estimated lunar radii are referred to these control points and determined with a relative accuracy of 500 m. The new lunar radii are used to generate a topographic map covering the area investigated. The map shows that, with the given spatial density of surface festures measured, basin-sized features can be resolved. In particular, the far-side craters Fabry, Riemann, and Szilard comprise a topographically depressed region about 500 km in diameter centered at 120°E and 38.5°N. The floor of this basin is 2.4 to 3.4 km below the reference sphere of 1738.0 km and 4.8 to 5.8 km below the northern rim of the basin. A comparison of the depth of the unfilled basin with the depths of maria-filled front-side basins leads to the conclusion that basalt fill of the near-side maria may be 2 km deep. The topographic map shows good correlation with geologic provinces of young plains and cratered terra in the far-side highland region investigated. Lack of correlation between sampled values of the state-of-the-art 16th-order and 16th-degree harmonic gravity field model and corresponding topographical values leads to the conclusion that the far-side region investigated is isostatically compensated.     

Lava flows at Arsia Mons, Mars: Insights from a graben imaged by HiRISE

HiRISE has imaged a graben wall on the western flank of Arsia Mons volcano, Mars. This graben is ∼3×16 km in plan-view size and is oriented almost perpendicular to the general volcano slope. We have identified 1318 individual sub-horizontal layers, which we interpret to be lava flows, in the 885 m high, nearly vertical, eastern wall of this graben. The average and median outcrop widths of each layer are 149 and 85 m, respectively. No layers extend >1.72 km across the width of the section, arguing against these being either areally-extensive ash or paleo-glacial deposits, which has implications for the reoccurrence interval of glacial events and/or the long-term magma production rate of the volcano. Measurements (N=118) made at a 100-m spacing across the width of the section reveal that there are, on average, 17.3 layers at each location. This implies an average layer thickness of ∼51 m. Locally, however, as many as 7 layers can be counted within a 70 m-high part of the section, implying, if these layers are indeed lava flows, that Arsia Mons occasionally erupted flows that were only ∼10 m thick.     

Composite graben tectonics of Alba Patera on Mars

The Alba Patera main graben zone is radial to the Tharsis bulge, indicating the importance of the Tharsis bulge-related peripheral rift tectonics. The concentric grabens around the Alba Patera area are also partly caused by crustal bending due to the central load of the Alba Patera volcano. These two graben sets partly coincide forming composite structures. Both tectonic systems were still active after the last major volcanic lava extrusions took place. After this, the crater chain grabens, radial to the northernmost part of the Tharsis bulge were formed. These collapse craters were evidently caused by the late-tectonic forces due to the northern Tharsis and adjoining lava loads, resulting in flexural tension and activating previous faults.