Topography of Martian central volcanoes

New topographic maps of six large central volcanoes on Mars are presented and discussed. These features are Olympus Mons, Elysium Mons, Albor Tholus, Ceraunius Tholus, Uranius Tholus, and Uranius Patera. Olympus Mons has the general form of a terrestrial basaltic shield constructed almost entirely from lava flows; but with 20 to 23 km of relief it is far larger. Flank slopes average about 4°. A nominal density calculated from the shield volume and the local free-air gravity anomaly is so high that anomalously dense lithosphere probably underlies the shield. Uranius Patera is a similar feature of much lower present relief, about 2 km, but its lower flanks have been buried by later lava flood deposits. Elysium Mons has about 13 km of local relief and average slopes of 4.4°, not significantly steeper than those of Olympus Mons. Its upper flank slopes are significantly steeper than those of Olympus Mons. We suggest Elysium Mons is a shield volcano modified and steepened by a terminal phase of mixed volcanic activity. Alternatively, the volcano may be a composite cone. Albor Tholus is a partially buried 3-km-tall shield-like construct. Ceranius and Uranius Tholus are steeper cone-like features with relief of about 6 and 2 km, respectively. Slopes are within the normal range for terrestrial basaltic shields, however, and topographic and morphologic data indicate burial of lower flanks by plains forming lavas. These cones may be lava shield constructs modified by a terminal stage of explosive activity which created striking radial patterns of flank channels. Differences among these six volcanoes in flank slopes and surface morphology may be primarily consequences of different terminal phases of volcanic activity, which added little to the volume of any construct, and burial of shallow lower flanks by later geologic events. Additional topographic data for Olympus Mons, Arsia Mons, and Hadriaca Patera are described. The digital techniques used to extract topographiv data from Viking Orbiter stereo images are also described.     

The record of impact cratering on the great volcanic shields of the Tharsis region of Mars

Mariner 9 images of the four great volcanic shields of the Tharsis region of Mars show many circular craters ranging in diameter from 100mm to 20 km. Previous attempts to date the volcanoes from their apparent impact crater densities yielded a range of results. The principal difficulty is sorting volcanic from impact craters for diameters $\text{?1}\text{km}$. Many of the observed craters are aligned in prominent linear and concentric patterns suggestive of volcanic origin. In this paper an attempt is made to date areas of shield surface, covered with high resolution images using only scattered small (?1 km) craters of probable impact origin. Craters of apparent volcanic origin are systematically excluded from the dating counts.The common measure of age, deduced for all surfaces studied, is a calculated “crater age” F′ defined as the number of craters equal to or larger than 1 km in diameter per 10⁶km². The conclusions reached from comparing surface ages and their geological settings are: (1) Lava flow terrain surfaces with ages, F′, from 180 to 490 are seen on the four great volcanoes. Summit surfaces of similar ages, F′ = 360 to 420, occur on the rims of calderas of Arsia Mons, Pavonis Mons, and Olympus Mons. The summit of Ascraeus Mons is possibly younger; F′ is calculated to be 180 for the single area which could be dated. (2) One considerably younger surface, F′ < 110, is seen on the floor of Arsia Mon's summit caldera. (3) Nearly crater free lava flow terrain surfaces seen on Olympus Mons are estimated to be less than half the age of a summit surface. The summit caldera floor is similarly young. (4) The pattern of surface ages on the volcanoes suggests that their eruption patterns are similar to those of Hawaiian basaltic shields. The youngest surfaces seem concentrated on the mid-to-lower flanks and within the summit calderas. (5) The presently imaged sample of shield surface, though incomplete, clearly shows a broad range of ages on three volcanoes—Olympus, Arsia, and Pavonis Mons.Estimated absolute ages of impact dated surfaces are obtained from two previously published estimates of the history of flux of impacting bodies on Mars. The estimated ranges of age for the observed crater populations are 0.5 to 1.2b.y. and 0.07 to 0.2b.y. Areas which are almost certainly younger, less than 0.5 or 0.07b.y., are also seen. The spans of surface age derived for the great shields are minimum estimates of their active lifetimes, apparently very long compared to those of terrestrial volcanoes.     

Debris-covered piedmont glaciers along the northwest flank of the Olympus Mons scarp: Evidence for low-latitude ice accumulation during the Late Amazonian of Mars

We use Viking and new MGS and Odyssey data to characterize the lobate deposits superimposed on aureole deposits along the west and northwest flanks of Olympus Mons, Mars. These features have previously been interpreted variously as landslide, pyroclastic, lava flow or glacial features on the basis of Viking images. The advent of multiple high-resolution image and topography data sets from recent spacecraft missions allow us to revisit these features and assess their origins. On the basis of these new data, we interpret these features as glacial deposits and the remnants of cold-based debris-covered glaciers that underwent multiple episodes of advance and retreat, occasionally interacting with extrusive volcanism from higher on the slopes of Olympus Mons. We subdivide the deposits into fifteen distinctive lobes. Typical lobes begin at a theater-like alcove in the escarpment at the base of Olympus Mons, interpreted to be former ice-accumulation zones, and extend outward as a tongue-shaped or fan-shaped deposit. The surface of a typical lobe contains (moving outward from the basal escarpment): a chaotic facies of disorganized hillocks, interpreted as sublimation till in the accumulation zone; arcuate-ridged facies characterized by regular, subparallel ridges and interpreted as the ridges of surface debris formed by the flow of underlying ice; and marginal ridges interpreted as local terminal moraines. Several lobes also contain a hummocky facies toward their margins that is interpreted as a distinctive type of sublimation till shaped by structural dislocations and preferential loss of ice. Blocky units are found extending from the escarpment onto several lobes; these units are interpreted as evidence of lava-ice interaction and imply that ice was present at a time of eruptive volcanic activity higher on the slopes of Olympus Mons. Other than minor channel-like features in association with lava-ice interactions, we find no evidence for the flow of liquid water in association with these lobate features that might suggest: (1) near-surface groundwater as a source for ice in the alcoves in the lobe source region at the base of the scarp, or (2) basal melting and drainage emanating from the lobes that might indicate wet-based glacial conditions. Instead, the array of features is consistent with cold-based glacial processes. The glacial interpretations outlined here are consistent with recent geological evidence for low-latitude ice-rich features at similar positions on the Tharsis Montes as well as with orbital dynamic and climate models indicating extensive snow and ice accumulation associated with episodes of increased obliquity during the Late Amazonian period of the history of Mars.     

Phreato-magmatic dike-cryosphere interactions as the origin of small ridges north of Olympus Mons, Mars

Using images from the Mars Orbiter Camera, we have identified several linear ridges located 10-60 km north of the volcano Olympus Mons, Mars, at the edge of the Olympus Mons aureole materials. These ridges appear to be made of unconsolidated material by virtue of the many dust avalanche scars seen on their upper slopes. Based upon their morphology (several ridges have crater-like central depressions) and superposition relationships, the ridges appear to have formed very recently and post-date the formation of the youngest lava flows spilling over the northern escarpment of Olympus Mons. Several possible origins for the ridges, including an eolian, periglacial, or depositional origin have been considered, but we favor a ridge origin by a series of small explosive eruptions initiated by the intrusion of a dike into a volatile-rich substrate. To explore this process, we develop a numerical model for dike intrusion into a volatile-rich substrate that yields plausible dike widths between 2.4-3.5 m. The total volume of a single ridge system is ∼65×10⁶ m³, and we calculate that it may have taken only a few minutes to form. Viable solutions only exist when the thicknesses of the ice-rich layer is less than ∼1000-2000 m. This strongly suggests that the ice-rich region is limited in its vertical extent to a value of this order.     

Interannual variability of water ice clouds over major martian volcanoes observed by MOC

Using Mars Global Surveyor Mars Orbiter Camera daily global maps, cloud areas have been measured daily for water ice clouds associated with the topography of the major volcanoes Olympus Mons, Ascraeus Mons, Pavonis Mons, Arsia Mons, Elysium Mons, and Alba Patera. This study expands on that of Benson et al. [Benson, J.L., Bonev, B.P., James, P.B., Shan, K.J., Cantor, B.A., Caplinger, M.A., 2003. Icarus 165, 34-52] by continuing their cloud area measurements of the Tharsis volcanoes, Olympus Mons and Alba Patera for an additional martian year (August 2001-May 2003) and by also including Elysium Mons measurements from March 1999 through May 2003. The seasonal trends in cloud activity established by Benson et al. [Benson, J.L., Bonev, B.P., James, P.B., Shan, K.J., Cantor, B.A., Caplinger, M.A., 2003. Icarus 165, 34-52] for the five volcanoes studied earlier are corroborated here with an additional year of coverage. For volcanoes other than Arsia Mons, interannual variations that could be associated with the large 2001 planet encircling dust storm are minimal. At Arsia Mons, where cloud activity was continuous in the first two years, clouds disappeared totally for ∼85° of LS (LS=188°-275°) due to the dust storm. Elysium Mons cloud activity is similar to that of Olympus Mons, however the peak in cloud area is near LS=130° rather than near LS=100°.     

The role of the wind-transported dust in slope streaks activity: Evidence from the HRSC data

Slope streaks are gravity-driven albedo features observed on martian slopes since the Viking missions. The debated mechanism of formation could involve alternatively dry granular flow or wet mass wasting. A systematic mapping of slope streaks from the High Resolution Stereo Camera is presented in this paper. Two regions known for their slope streaks activity have been studied, the first one is located close to Cerberus lava flow, and the second one is inside the Olympus Mons Aureole. The statistics of slope streaks shapes measured from orthorectified images confirm previous results from Mars Orbiter Camera surveys. Preferential orientations of slope streaks are reported. Slope streaks occur preferentially on west facing slopes at latitudes lower than 30° N for Olympus and on south-west facing slopes for Cerberus. Wind directions derived from a General Circulation Model during the dusty season correlate with these orientations. Furthermore, west facing slopes at Olympus have a thicker dust cover. These observations indicate that slope streaks are dust avalanches controlled by the preferential accumulation of dust in the downstream side of the wind flow. The paucity of slope streaks at high latitudes and their preferential orientation on south-facing slopes have been presented as an evidence for a potential role of H₂O phase transition in triggering or flow. The potential role of H₂O cannot be ruled out from our observations but the dust avalanche model together with the atmospheric circulation could potentially explain all observations. The role of H₂O might be limited to a stabilizing effect of dust deposits on northward facing slopes at intermediate latitudes (30° N-33° N) and on all slopes further north.     

Ice-lubricated gravity spreading of the Olympus Mons aureole deposits

Gravity sliding and spreading at low strain rates can account for the general morphology and structure of the aureoles and basal scarp of Olympus Mons. Detachment sliding could have occurred around the volcano if either pore-fluid pressures were exceptionally high (greater than 90%) or the rocks had very low resistance to shear (about 1 × 10⁵ Pa or 1 bar). Because of the vast areal extent and probable shallow depth of the detachment zone, development of ubiquitous, high pore-fluid pressures beneath aureole-forming material was unlikely. However, a zone of sufficiently weak material consisting of about 10% interstitial or interbedded ice could have been present. If so, a simple rheologic model for the aureole deposits can be applied that consists of a thin ductile layer overlain by a thicker brittle layer. According to this model, extensional deformation would have occurred near the shield and compressional deformation in its distal parts. Proximal grabens and distal corrugations on aureole surfaces support this model. A submarine slide at Kitimat Arm, British Columbia, is a valid qualitative analogy for the observed features and inferred emplacement style of the aureole deposits. Ground-ice processes have been considered the cause of many geologic features on Mars; a 3% average concentration of ground ice in the regolith is predicted by theoretical models for the ice budget and cryosphere. Ice may have been deposited in higher concentrations below the aureole-forming material; the source of the ice could have been juvenile water circulated hydrothermally by Olympus Mons volcanism. The basal scarp of Olympus Mons apparently demarcates the transition between the upper, stable part of the shield and its lower part that decoupled and formed the aureole deposits. This transition may reflect a change in the bulk shear strength of the shield, caused either by a radial dependence in the abundance of ice or fluid in the shield materials or by the concentration of intrusive dikes within the volcano. Other Martian volcanoes exhibit virtually no evidence of similar large-scale gravity spreading and basal scarps. Perhaps such evidence, if it existed, has been buried by lava flows, or perhaps the smaller size of other volcanoes did not permit the development of these features.     

Rheologies and ages of lava flows on Elysium Mons,Mars

We present results of our study of the rheologies and ages of lava flows in the Elysium Mons region of Mars. Previous studies have shown that the geometric dimensions of lava flows reflect rheological properties such as yield strength, effusion rate and viscosity. In this study the rheological properties of lava flows in the Elysium Mons region were determined and compared to the rheologies of the Ascraeus Mons lava flows. We also derived new crater size-frequency distribution measurements (CSFDs) for the Elysium lava flows to identify possible changes in the rheological properties with time. In addition, possible changes in the rheological properties with the distance from the caldera of Elysium Mons were analyzed.In total, 35 lava flows on and around Elysium Mons were mapped, and divided into three groups, lava flows on the flanks of Elysium Mons, in the plains between the three volcanoes Elysium Mons, Hecates and Albor Tholus and lava flows south of Albor Tholus. The rheological properties of 32 of these flows could be determined. Based on our morphometric measurements of each individual lava flow, estimates for the yield strengths, effusion rates, viscosities, and eruption duration of the studied lava flows were made. The yield strengths of the investigated lava flows range from ～3.8 × 10² Pa to ～1.5 × 10⁴ Pa, with an average of ～3.0 × 10³ Pa. These yield strengths are in good agreement with estimates for terrestrial basaltic lava flows. The effusion rates are on average ～747 m³ s^?1, ranging from ～99 to 4450 m³ s^?1. The viscosities are on average ～4.1 × 10⁶ Pa s, with a range of 1.2 × 10⁵ Pa s to 3.1 × 10⁷ Pa s. The eruption durations of the flows were calculated to be between 6 and 183 days, with an average of ～51 days. The determined rheological properties are generally very similar to those of other volcanic regions on Mars, such as on Ascraeus Mons in the Tharsis region. Calculated yield strengths and viscosities point to a basaltic/andesitic composition of the lava flows, similar to basaltic or andesitic a’a lava flows on Earth.Absolute model ages of all 35 lava flows on Elysium Mons were derived from crater size-frequency distribution measurements (CSFD). The derived model ages show a wide variation from about 632 Ma to 3460 Ma. Crater size-frequency distribution measurements of the Elysium Mons caldera show an age of ～1640 Ma, which is consistent with the resurfacing age of Werner (2009). Significant changes of the rheologies with time could not be observed. Similarly, we did not observe systematic changes in ages with increasing distances of lava flows from the Elysium Mons caldera.     

Photogrammetric measurements of Olympus Mons on Mars

Mariner 9 took many pictures of the giant Olympus Mons during its year in orbit around Mars. Control points have been identified on the top of Olympus Mons, on the volcanic shield, and on the surrounding plains, and their locations have been measured on the television pictures. These measurements were used to compute the aerographic coordinates and the planetary radii of the points. The radii at some of the points were derived from radar elevation measurements and from radio occultation measurements. The mountain rises about 21 km above its base.     

The elevation of Olympus Mons from limb photography

Using a novel photogrammetric technique, the relative elevations of a set of control points on the Martian volcano Olympus Mons (Nix Olympica) have been measured from Mariner 9 limb pictures that include the volcano in the foreground. The summit of Olympus Mons is found to be 22 ± 1 km above the mean level of the surrounding plain by correlating the elevation results with previously reported planet radii at nearby occultation points. Vertical elevation differences as great as 5 km were measured between close points on the basal scarp.     

A methodology for constraining lava flow rheologies with MOLA

Topography as measured by the Mars Orbiter Laser Altimeter (MOLA), when supplemented with imaging data, can be used to infer physical emplacement processes in lava flows on Mars with a level of detail analogous to what can be done with unobserved lava flow eruptions on Earth. MOLA, Viking Orbiter and Mars Orbiter Camera (MOC) data are used to develop new inferences regarding the rheology of a typical lava flow near Elysium Mons on Mars. We present a technique that uses MOLA Precision Experiment Data Records (PEDRs) to directly determine the longitudinal thickness profile of lava flows. This technique is preferable to using gridded topography derived from MOLA, particularly for features such as lava flows, with thickness variations at the same scale as their surroundings. Thickness profiles and underlying slope estimates can then be compared with results from rheologic models. The longitudinal thickness profile of the Elysium example discussed here exhibits a concave-up flow surface that is consistent with an exponential viscosity increase. The viscosity shows a relative increase of about 50 times over the length of flow examined when the density of the lava increases as a result of lava degassing.     

Cover

Pangboche crater (10. 4 km in diameter) lies close to the summit of Olympus Mons volcano, Mars, at an elevation of ~ 20. 9 km above the datum. These increasingly high resolution views show the crater just south of the caldera of the volcano (top left), an oblique view of the entire crater (top right), the terraces on the inner western wall (middle right) and the sequence of uplifted lava fl ows in the wall (bottom). Mouginis‐Mark discusses on pages 51–62 how the geomorphology of the crater may have been influenced by the lack of volatiles in the target and the thin atmosphere at this elevation at the time of formation, and contrasts Pangboche crater with more typical fresh craters found at lower elevations. Image courtesy of NASA/JPL.     

Morphology and geological structure of the western part of the Olympus Mons volcano on Mars from the analysis of the Mars Express HRSC imagery

The images of the western part of Olympus Mons and adjacent plains acquired by the HRSC camera onboard the Mars Express spacecraft were studied. The morphology, topography, and color of the surface were investigated. The surface age was determined by the frequencies of impact craters. The examination of the HRSC images combined with an analysis of the MOC imagery and MOLA altitude profiles have shown that the Olympus Mons edifice, at least in its western part, is composed of not only lavas but also of sedimentary and volcanic-sedimentary rocks consisting of dust, volcanic ash, and, probably, H₂O ice that precipitated from the atmosphere. These data also indicate that glaciations, traces of which are known on the western foot of Olympus Mons (Lucchitta, 1981; Milkovich and Head, 2003), probably also covered the gentle upper slopes of the mountain. It is probable that the ice is still there, protected from sublimation by a dust blanket. Confirming (or rejecting) its presence is a challenge for the scheduled radar sounding with the MARSIS instrument mounted on the Mars Express spacecraft as well.__________Translated from Astronomicheskii Vestnik, Vol. 39, No. 2, 2005, pp. 99–116.Original Russian Text Copyright © 2005 by Basilevsky, Neukum, Ivanov, Werner, S. van Gesselt, Head, Denk, Jaumann, Hoffmann, Hauber, McCord, the HRSC Co-Investigator Team.     

Inflated flows on Daedalia Planum (Mars)? Clues from a comparative analysis with the Payen volcanic complex (Argentina)

Inflation is an emplacement process of lava flows, where a thin visco-elastic layer, produced at an early stage, is later inflated by an underlying fluid core. The core remains hot and fluid for extended period of time due to the thermal-shield effect of the surface visco-elastic crust. Plentiful and widespread morphological fingerprints of inflation like tumuli and lava rises are found on the Payen volcanic complex (Argentina), where pahoehoe lava flows extend over the relatively flat surface of the Pampean foreland and reach at least 180 km in length.The morphology of the Argentinean Payen flows were compared with lava flows on Daedalia Planum (Mars), using Thermal Emission Imaging System (THEMIS), Mars Orbiter Laser Altimeter (MOLA), Mars Orbiter Camera (MOC), Mars Reconnaissance Orbiter (MRO)/High-Resolution Imaging Science Experiment (HiRISE). THEMIS images were used to map the main geological units of Daedalia Planum and determine their stratigraphic relationships. MOLA data were used to investigate the topographic surface over which the flows propagated and assess the thickness of lava flows. Finally, MOC and MRO/HIRISE images were used to identify inflations fingerprints and assess the cratering age of the Daedalia Planum’ s youngest flow unit which were found to predate the caldera formation on top of the Arsia Mons. The identification of similar inflation features between the Daedalia Planum and the Payen lava fields suggests that moderate and long lasting effusion rates coupled with very efficient spreading processes could have cyclically occurred in the Arsia Mons volcano during its eruptive history. Consequently the effusion rates and rheological proprieties of Daedalia lava flows, which do not take into account the inflation process, can be overestimated. These findings raise some doubts about the effusion rates and lava rheological properties calculated on Martian flows and recommends that these should be used with caution if applied on flows not checked with high-resolution images and potentially affected by inflation. Further HiRISE data acquisition will permit additional analysis of the flow surfaces and will allow more accurate estimates of effusion rates and rheological properties of the lava flows on Mars particularly if this data is acquired under a favourable illumination.     

The seasonal behavior of water ice clouds in the Tharsis and Valles Marineris regions of Mars: Mars Orbiter Camera Observations

The Mars Global Surveyor Mars Orbiter Camera was used to obtain global maps of the martian surface with equatorial resolution of 7.5 km/pixel in two wavelength ranges: blue (400-450 nm) and red (575-625 nm). The maps used were acquired between March 15, 1999 (L_s=110°) and July 31, 2001 (L_s=205°), corresponding to approximately one and a quarter martian years. Using the global maps, cloud area (in km²) has been measured daily for water ice clouds topographically corresponding to Olympus Mons, Ascraeus Mons, Pavonis Mons, Arsia Mons, Alba Patera, the western Valles Marineris canyon system, and for other small surface features in the region. Seasonal trends in cloud activity have been established for the three Tharsis volcanoes, Olympus Mons, and Alba Patera. Olympus, Ascraeus, and Pavonis Mons show cloud activity from about L_s=0°-220° with a peak in cloud area near L_s=100°. One of our most interesting observational results is that Alba Patera shows a double peaked feature in the cloud area with peaks at L_s=60° and 140° and a minimum near L_s=100°. Arsia Mons shows nearly continuous cloud activity. The altitudes of several of these clouds have been determined from the locations of the visual cloud tops, and optical depths were measured for a number of them using the DISORT code of Stamnes et al. (1988, Appl. Opt. 27, 2502-2509). Several aspects of the observations (e.g., cloud heights, effects of increased dust on cloud activity) are similar to simulations in Richardson et al. (2002, J. Geophys. Res. 107, 5064). A search for short period variations in the cloud areas revealed only indirect evidence for the diurnal cloud variability in the afternoon hours; unambiguous evidence for other periodicities was not found.     

Rheological constraints on martian landslides

We use a dynamic finite-difference model to simulate martian landslides in the Valles Marineris canyon system and Olympus Mons aureole using three different modal rheologies: frictional, Bingham, and power law. The frictional and Bingham modes are applied individually. Fluidized rheology is treated as a combination of frictional and power-law modes; general fluidization can include pore pressure contributions, whereas acoustic fluidization does not. We find that general fluidization most often produces slides that best match landslide geometry in the Valles Marineris. This implies that some amount of supporting liquid or gas was present in the material during failure. The profile of the Olympus Mons aureole is not well matched by any landslide model, suggesting an alternative genesis. In contrast, acoustic fluidization produces the best match for a lunar slide, a result anticipated for dry crust with no overlying atmosphere. The presence of pressurized fluid during Valles Marineris landsliding may be due to liquid water beneath a thin cryosphere (<1-2 km) or flash sublimation of CO₂.     

Origin of the Olympus Mons aureole and perimeter scarp

The aureole materials that form an annulus of corrugated terrain surrounding Olympus Mons are considered to be the product of mass movement. The scarp at the mountain's foot formed as a result of this massive removal of material from the volcano's outer flanks. This interpretation is supported by a comparison of the amount of material originally available before scarp formation, and the present volume of aureole materials. On the basis of distribution, surface textures and theoretical considerations it is considered that the aureole was produced by a series of megaslides, rather than by a flow mechanism. Production of the megaslides may have been assisted by a period of widespread melting of permafrost.Paper presented at the European Workshop on Planetary Sciences, organised by the Laboratorio di Astrofisica Spaziale di Frascati, and held between April 23–27, 1979, at the Accademia Nazionale del Lincei in Rome, Italy.     

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.     

Geologic features of Wudalianchi volcanic field,northeastern China: Implications for Martian volcanology

Wudalianchi volcanic field, located in northeast China, consists of 14 Quaternary volcanoes with each volcano as a steep-sided scoria cone surrounded by gently sloping lava flows. Each cone is topped with a bowl-shaped or funnel-shaped crater. The volcanic cones are constructed by the accumulation of tephra and other ejecta. In this paper, their geologic features have been investigated and compared with some Martian volcanic features at Ascraeus Mons volcanoes observed on images obtained from High-Resolution Imaging Science Experiments (HiRISE), Mars Orbiter Camera (MOC), Context Imager (CTX) and Thermal Emission Imaging System (THEMIS). The results show that both Wudalianchi and Ascraeus Mons volcanoes are basaltic, share similar eruptive and geomorphologic features and eruptive styles, and have experienced multiple eruptive phases, in spite of the significant differences in their dimension and size. Both also show a variety of eruptive styles, such as fissure and central venting, tube-fed and channel-fed lava flows, and probably pyroclastic deposits. Three volcanic events are recognized at Ascraeus Mons, including an early phase of shield construction, a middle eruptive phase forming a low lava shield, and the last stage with aprons mantling both NE and SW flanks. We suggest that magma generation at both Wudalianchi and Ascraeus Mons might have been facilitated by an upwelling mantle plume or upwelling of asthenospheric mantle, and a deep-seated fault zone might have controlled magma emplacement and subsequent eruptions in Ascraeus Mons as observed in the Wudalianchi field, where the volcanoes are constructed along the northeast-striking faults. Fumarolic cones produced by water/magma interaction at the Wudalianchi volcanic field may also serve as an analogue for the pseudocraters identified at Isidis and Cerberus Planitia on Mars, suggesting existence of frozen water in the ground on Mars during Martian volcanic eruptions.     

Cratering on Mars with almost no atmosphere or volatiles: Pangboche crater

Pangboche crater (17.2°N, 226.7°E; 10.4 km dia.) lies close to the summit of Olympus Mons volcano, Mars, at an elevation of ~20.9 km above the datum. Given a scale height of 11.1 km for the atmosphere, this relatively large fresh crater most likely formed at an atmospheric pressure <1 mbar in essentially volatile‐free young lava flows. Detailed analysis of Pangboche crater from High Resolution Imaging Science Experiment (HiRISE) and Context Camera (CTX) images reveals that volatile‐related features (e.g., fluidized ejecta layers and pitted floor material) are absent. In contrast, abundant impact melt occurs on the floor, inner walls, and rim of the crater, and there is an extensive field of secondary craters that extend up to approximately 45 km from the rim crest. All of these attributes argue that it was the absence of volatiles in the target rocks at the time of crater formation, rather than the thin atmosphere, which had a controlling influence on crater morphology. Digital elevation data derived from the CTX images reveal that Pangboche crater has a depth of about 954 m (depth/diameter = approximately 0.092) and that uplifted target rocks comprise about 58% of the relief of the 180 m‐high north rim. As the target material comprised a sequence of layered lava flows, Pangboche crater may well represent the best crater on Mars for direct comparison with craters formed on the Moon (permitting variations in gravitational effects to be investigated) or on Mercury (allowing the role of the atmosphere to be studied).