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.     

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 Ascraeus Mons fan-shaped deposit: Volcano-ice interactions and the climatic implications of cold-based tropical mountain glaciation

Amazonian-aged fan-shaped deposits extending to the northwest of each of the Tharsis Montes in the Tharsis region on Mars have been interpreted to have originated from mass-wasting, volcanic, tectonic and/or glacial processes. We use new data from MRO, MGS, and Odyssey to characterize these deposits. Building on recent evidence for cold-based glacial activity at Pavonis Mons and Arsia Mons, we interpret the smaller Ascraeus fan-shaped deposit to be of glacial origin. Our geomorphological assessment reveals a number of characteristics indicative of glacial growth and retreat, including: (1) a ridged facies, interpreted to be composed of drop moraines emplaced during episodic glacial advance and retreat, (2) a knobby facies, interpreted to represent vertical downwasting of the ice sheet, and (3) complex ridges showing a cusp-like structure. We also see evidence of volcano-ice interactions in the form of: (1) an arcuate inward-facing scarp, interpreted to have formed by the chilling of lava flows against the glacial margin, (2) a plateau feature, interpreted to represent a subglacial eruption, and (3) knobby facies superimposed on flat-topped flows with leveed channels, interpreted to be subglacial inflated lava flows that subsequently drained and are covered by glacial till. We discuss the formation mechanisms of these morphologies during cold-based glacial activity and concurrent volcanism. On the basis of a Mid- to Late-Amazonian age (250-380 Ma) established from crater size-frequency distribution data, we explore the climatic implications of recent glaciation at low latitudes on Mars. GCM results show that increased insolation to the poles at high obliquities (>45°) forces sublimation of polar ice, which is transported to lower latitudes and deposited on the flanks of the Tharsis Montes. We assess how local orographic effects, the mass balance of the glacier, and the position of equilibrium line altitudes, all played a role in producing the observed geomorphologies. In doing so, we outline a glacial history for the evolution of the Ascraeus Mons fan-shaped deposit and compare its initiation, growth and demise with those of Arsia Mons and Pavonis Mons.     

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 determination of the rheological properties and effusion rate of an Olympus Mons lava

A new technique for the interpretation of lava flow morphology was applied to a lava flow on Olympus Mons. The yield stress of the flowing lava was determined subject to uncertainties in the estimates of the slope of Olympus Mons. The lava is most probably more silicic than the basaltic lavas of the Hawaiian shield volcanoes and its effusion rate appears to have been greater than those of typical Hawaiian flows.     

Distribution and time-variation of spire streaks at Pavonis Mons on Mars

We documented the distribution and the time-variation of the specific dark wind streaks at Pavonis Mons. We focused on the streaks we named “Spire Streaks”, which are overlapping spindle shaped dark streaks at the upper boundary of the coalesced dark streaks on Tharsis volcanoes. We investigated both visible and infrared images obtained by Viking orbiter camera, Mars Orbiter Camera (MOC), THEMIS, CTX and HiRISE of the spire streaks at Pavonis Mons. We also used topographic data obtained by Mars Orbiter Laser Altimeter (MOLA) to see the relationship between the topography and the distribution of the spire streaks. The spire streaks at Pavonis Mons provide us high-resolution information about the direction of the nighttime slope wind, and could be indirect clues for the time-variation of the nighttime environment. We conclude that the spire streaks are erosional features. However, some features of the spire streaks reported in this paper are outside the scope of previous modeling for erosional process, and we need a new category of model for the formation.     

Radar altimetry of South Tharsis,Mars

Martian altitudes were measured by radar during the oppositions of 1971 and 1963 using the 64-m antenna at Goldstone (California). The resultant topographic profiles substantiate a zonal classification of the volcanic flows blanketing the south flanks of Arsia Mons, and they confirm the existence of a secondary, parasitic shield attached from the SSW to the main Arsia shield. The secondary shield is about 400 km in diameter at its base and at least 4 km high at its center. South of Valles Marineris, the Tharsis plateau is bounded by the approximate longitudes of 80° in the east and 140° in the west. In the Sinai Planum, closely adjacent to Coprates Chasma, another rise has been detected, bounded by longitudes of 55° in the east and 80° in the west. A volcanic shield of diameter 80 km, capped with a 22 km caldera has been identified near the crest of the rise. Topographic highs of about 1 km are associated with heavily faulted tracts such as Claritas Fossae. The distribution and orientation of the lunar-mare-like ridges in Sinai Planum appear to be independent of the regional gradients. Segments of the chaotic terrain at the eastern terminus of Valles Marineris are located as much as 6 km below the level of the surrounding plains.     

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.     

Variable features on Mars. IV. Pavonis Mons

A remarkable set of albedo changes has been uncovered by Mariner 9 photography of the upper slopes of the shield volcano Pavonis Mons, near its summit caldera. The most likely explanation of the event is aeolian transport of fine-grained particles. Since the atmospheric pressure in this locality is ～ 1.5 mb, minimum wind velocities above the surface boundary layer of about 110 m/s are necessary, corresponding to 0.51 of the speed of sound. Slope winds in this velocity range are expected near the upper flanks of major Martian volcanic constructs.     

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.     

Volcanic and structural history of the rocks exposed at Pickering Crater (Daedalia Planum, Mars)

In the western hemisphere of Mars Amazonian volcanism from Arsia Mons produced the smooth surfaces of Daedalia Planum and masks older rocks. Close to the southern termination of Daedalia Planum basement rocks are exposed in which are preserved craters that escaped or were only partially filled by this most recent volcanism. Pickering Crater is an approximately 130 km diameter crater. The youngest lavas flowed into this crater from Daedalia Planum by way of a NW rim breach, covering its western part. East of a well-defined flow front an older lava sequence with a distinctive platy surface and derived from a more proximal unestablished source to the northeast is exposed. Several units are identified within this sequence on the basis of surface texture, which is more subdued in progressively older rocks. Only local mapping of the flow front boundaries of these units is possible because of incomplete coverage by high resolution imagery. During emplacement of the older lavas a NE-SW striking en echelon graben system and parallel smaller troughs and dikes formed under inferred regional NW-SE extension. A much earlier strike-slip regime pre-dating the lavas exposed in the crater floor is postulated, based on the highly fretted nature of the rim of Pickering Crater and an elongated smaller crater to its northeast, approximately 40 km long in the NE-SW direction. The rims of these craters contrast with that of a smoother rimmed impact crater in the southeast that was excavated subsequent to strike-slip deformation but prior to the emplacement of platy surfaced lavas.     

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.     

Gravity studies of the Tharsis area on Mars

The Tharsis rise on Mars with a diameter of about 8000 km and an elevation up to 10 km shows extensive volcanism and an extensional fracture system. Other authors explained this structure by (I) an uplift due to mantle processes and by (II) volcanic construction. Gravity models of four profiles are in accordance with a total Airy isostatic compensation of the whole rise with mean crustal thicknesses of 50 km and 100 km. But two regions exhibit significant mass deficits: (i) the area between Olympus Mons and the three large Tharsis volcanoes and (ii) central Tharsis. This can be explained by (1) a heated upper mantle, (2) a chemically modified upper mantle, (3) a crustal thickening, or (4) a combination of these three processes. Crustal thickening is mainly a constructional process, but the mass deficit should contribute to a certain degree of uplift causing the extensional area of Labyrinthus Noctis. Gravity modelling results in a different isostatic state of the three Tharsis volcanoes. Pavonis Mons is not compensated, Ascraeus Mons is highly or totally compensated, and Arsia Mons is medium or not compensated. The large, flat volcanic structure Alba Patera has been explained by a hot spot with an evolution of a mantle diapir.The results have shown that the Tharsis rise is a very complex structure. The central and eastern part of the rise is characterized by extensional features and a mass deficit (Extensional Province). The western part is dominated by many volcanic features and a central elongated mass deficit (Volcanic Province). The northern part consists of Alba Patera. It seems unlikely that the whole rise has been generated by one stationary large axisymmetric plume or hot spot. There could have been one or more active hot spots with an evolution in space and time.Contribution Nr. 421, Institut für Geophysik der Universität Kiel, Germany.     

Sapas Mons,Venus: evolution of a large shield volcano

Magellan radar image data of Sapas Mons, a 600 km diameter volcano located on the flanks of the Arla Rise, permit the distinction of widespread volcanic units on the basis of radar properties, morphology, and spatial and inferred temporal relations, each representing a stage or phase in the evolution of the volcano. Six flow units were identified and are arranged asymmetrically about the volcano. Although there is some evidence for overlapping of units, the stratigraphy clearly indicates a younging upwards sequence. The estimated volume of this 2.4 km high volcano is 3.1 × 10⁴ km³, which is comparable to the largest Hawaiian shield (Mauna Loa, 4.25 × 10⁴ km³), but it is significantly less than an estimated volume for the entire Hawaiian-Emperor chain (1.08 × 10⁶ km³) and less than the lower diameter (100 × 150 km) island of Hawaii (11.3 × 10⁴ km³). Although it is difficult to clearly identify a single lava flow, estimates of apparent single flow volumes range from 4 km³ (for an average unit 5 flow of 3.4 km width, 10 m thickness, and 121 km length) to almost 59 km³ (for a 17.8 km wide, l0 m thick, 330 km long unit 1 flow). Estimates of total volumes for the units show that four of the six flow units have volumes that are within a factor of 1.2 of each other, one unit is approximately three times more voluminous, and the latest unit has a very small volume. Flows within a given unit are very distinct relative to flows in other units with respect to average lengths, aspect ratio, radar brightness, and planimetric outline. There is a weak distinction in rms slope between units and emissivity is correlated with altitude, not unit boundaries. A pair of 25 km diameter scalloped-margin domes occur at the summit and are the source of the last stage of eruptions on Sapas; steep fronts and high aspect ratios suggest that associated flows may have had a high viscosity. Graben form a circumferential structure 75–100 km in diameter surrounding the summit domes and are interpreted to be indicative of subsidence over a central magma reservoir. Radial fractures with associated small edifices cut the lower flanks of the edifice but are not observed within the summit ring of graben; these are interpreted to be the expression of near-surface dykes and may have been emplaced during a period of enhanced activity that correlates with the most voluminous flow unit. Unlike at Hawaii, however, these dykes and small edifices do not seem to be the source of significant flank eruptions. Although some effusive activity may have accompanied their emplacement, the majority of lava flows at Sapas appear to be radial to a single, near-summit point located between the two summit domes.Calculated effusion rates range from 1.5 × 10³ m³/s to 3.1 × 10⁵ m³/s; these values suggest that rates were high compared with the Earth and decreased with time. These rates, and the volumes calculated, give eruption durations for the various units that range from 18 days to over 20 years. If eruption is caused by the influx of magma from depth and rupture of an overpressurized chamber, this suggests a variable flux over the history of the volcano. The late-stage eruptions which formed the summit domes are interpreted to be the result of fractional crystallization and/or volatile build-up in the chamber, following a period of decreased supply from depth.Local topography and gravity, as well as regional geology support the presence of a mantle plume at Sapas. The similar properties of large volumes of magma over the total history of the volcano, as well as the prolonged period of magma supply and gradual waning, are consistent with a plume origin. These inferences and the observations allow us to characterise the history of the volcano as follows: arrival of the mantle plume caused uplift of topography and surrounding plains formation: continued supply of smaller volumes of material permitted construction of the edifice; development of a magma reservoir (predicted by theory to form at shallow depths) modified eruption characteristics by permitting storage and homogenization of magma; unbuffered conditions prevailed for the majority of eruptions, producing flows of similar volumes but decreasing flow lengths; a period early on of enhanced supply led to buffered chamber conditions, resulting in the eruption of the voluminous flow unit and the emplacement of many lateral dykes; evacuations from the chamber and cooling towards the last stages caused distributed summit collapse and formation of the ring graben; and finally the gradual waning of supply allowed evolution of the magma which produced the late-stage, possibly viscous flows and dome construction. Preliminary observation of Sapas and two other volcanoes at different elevations suggests that altitude-dependent chamber development and growth may influence the complexity of lava flows and determine the existence of collapse calderas. Many features at Sapas are representative of large volcanoes on Venus and thus Sapas Mons is a good example of a typical plume-associated edifice. Sapas differs in many ways from Kilauea, a terrestrial type shield volcano, but these differences can be understood in the context of the Venus environment.     

Shield volcano topography and the rheology of lava flows on Io

Stereo and photoclinometry derived topography of shield-like volcanoes on Io indicate little relief (<3 km) and very low slopes (0.2° to 0.6°). Several shield volcanoes appear to be associated with broad rises of 1 to 3 km, but only 5 shield volcanoes have been identified with steep flank slopes (between 4° and 10°). These steep slopes are restricted to within 20-30 km of the summit, but where discernable, most of the lava flows observed on these edifices occur on the outer flanks where slopes are less than a degree. Despite their abundance, ionian shield volcanoes are among the flattest in the Solar System. The steepest volcanoes on Io are most comparable to large venusian shield volcanoes. Using simplistic Bingham rheologies we estimate the viscosity and yield strengths of ionian lavas. Yield strengths are estimated at 10¹-10² Pa, lower than most basaltic lavas. Viscosity estimates range from 10³ to 10⁵ Pa s, although these are probably upper limits. Actual values may have been as low as 10⁰ Pa s. Viscosity is sensitive to flow velocity, which is poorly known on Io. The best constraint on flow velocity comes from observations of the 1997 Pillan eruption, which bracket the eruptive phase to 132 day maximum, and more probably less than 50 days. Low slopes, long run-out distances and our estimated rheologic properties are consistent with (but not proof of) a low silica, low viscosity, high temperature composition for ionian lavas, supporting arguments for low-silica lava compositions such as basalt or komatiite. We cannot eliminate sulfur on rheologic grounds, however.     

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.     

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.     

Searching for evidence of hydrothermal activity at Apollinaris Mons,Mars

A multidisciplinary approach involving various remote sensing instruments is used to investigate Apollinaris Mons, a prominent volcano on Mars, as well as the surrounding plains for signs of prolonged hydrologic and volcanic, and possibly hydrothermal activity. The main findings include (1) evidence from laser altimetry indicating the large thickness (1.5–2 km at some locations) of the fan deposits draping the southern flank contrary to previous estimates, coupled with possible layering which point to a significant emplacement phase at Apollinaris Mons, (2) corroboration of Robinson et al. (Robinson, M.S., Mouginis-Mark, P.J., Zimbelman, J.R., Wu, S.S.C., Ablin, K.K., Howington-Kraus, A.E. [1993]. Icarus 104, 301–323) hypothesis regarding the formation of incised valleys on the western flanks by density current erosion which would indicate magma–water interaction or, alternatively, volatile-rich magmas early in the volcano’s history, (3) mounds of diverse geometric shapes, many of which display summit depressions and occur among faults and fractures, possibly marking venting, (4) strong indicators on the flanks of the volcano for lahar events, and possibly, a caldera lake, (5) ubiquitous presence of impact craters displaying fluidized ejecta in both shield-forming (flank and caldera) materials and materials that surround the volcano that are indicative of water-rich target materials at the time of impact, (6) long-term complex association in time among shield-forming materials and Medusae Fossae Formation.The findings point to a site of extensive volcanic and hydrologic activity with possibly a period of magma–water interaction and hydrothermal activity. Finally, we propose that the mound structures around Apollinaris should be prime targets for further in situ exploration and search for possible exobiological signatures.     

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.     

Shear-induced folding in Arsia Mons aureole: Evidence for low-latitude martian glaciations

Folds up to 50 km across have been identified on Arsia Mons aureole. Tharsis Province, Mars. The structures, located on Mars for the first time, are close to Aganippe Fossa and other huge faults which have behaved as left-lateral shear zones and then as extensional features. A tectonic scheme is proposed to explain the folds as shear-induced structures. Folding reveals a layered sequence in the aureole, and that is taken as a definitive evidence for its deposition by ice.If at least some of the Tharsis volcanoes aureoles are basal moraines, their study is critical, as they could contain a record of Mars paleoclimatic fluctuations. Martian past frozen lakes or oceans have been proposed, and some sediments found on the northern plains could have been deposited on the bottom of those basins. If this is so, those formations should be layered sequences and could also bear the traces of tectonic stresses, detectable as folds on Viking imagery. Correlation of these two kinds of evidence seems a promising line to tackle the Martian paleoclimatic problem.