Elysium Rise Research Articles

Subsurface structure of the martian Elysium Rise revealed by gravitational field separation

The Elysium Rise in the Martian northern hemisphere is the second largest volcanic province on Mars. To understand the volcanic activity in the Elysium Rise, the gravitational characteristics of this area were studied using the Bouguer gravity data derived from Goddard Mars Model-3 (GMM-3). The Bouguer gravity anomaly in the Elysium Rise was separated into regional fields and residual fields and the subsurface density profiles were obtained by a three-dimensional density inversion. The different anomaly features of the large volcanoes in the Elysium Rise were exhibited by the separation and density inversion results. Elysium Mons is characterized by a positive residual gravity anomaly surrounded by an annular negative anomaly, which possibly results from the volcano loading on a lithosphere plate. Albor Tholus has a positive gravity anomaly but with no obvious annular negative anomalies, which may result from the relatively small mass deficit generated by the eruption of Albor Tholus, unlike the Elysium Mons. In addition, the residual field shows prominent positive gravity anomalies with 150 km wide, centred at 157.2°E, 12.1°N, which could possibly be a hidden impact crater with an untraceable surface morphology or topography. Our study reveals the subsurface mass source distributions and structures of the Elysium volcanoes, providing a new method to understand the tectonic framework of the Martian subsurface through the study of gravity.

Model Estimates of Non-Hydrostatic Stresses in the Martian Crust and Mantle: 1—Two-Level Model

Regions of maximum shear and tension–compression stresses in the Martian interior have been revealed using two types of models: the elastic model and the model with an elastic lithosphere of varied thickness (150–500 km) positioned on a weak layer that has partially lost its elastic properties. The weakening is simulated by a ten-fold lower value of the shear modulus down to the core boundary. The numerical simulation applies Green’s functions (load number method) with the step of 1 × 1 grade along latitude and longitude down to a depth of 1000 km. The boundary condition is the expansion of the latest data on Martian topography and the gravitational field (model MRO120D) in spherical harmonics up to the degree and order of 90 in relation to the reference surface that is assumed an equilibrium spheroid. The considered two-level compensation model assumes nonequilibrium relief and density anomalies at the crust–mantle boundary to be the sources of the anomalous gravitational field. Calculations are performed for two test models of Martian internal structure with the crust mean thicknesses of 50 to 100 km and mean density of 2900 kg/m3. Considerable tangential and simultaneously compressive stresses occur under the Tharsis region. The main regions of high shear and simultaneously extentional stresses are located in the Hellas region crust and in the lithosphere of the following regions: Argyre Planitia, Mare Acidalium, Arcadia Planitia and Valles Marineris. The zone of high maximum shear and extentional stresses has been found at the base of the lithosphere under the Olympus volcano and that under the Elysium rise.

Evidence of widespread degraded Amazonian-aged ice-rich deposits in the transition between Elysium Rise and Utopia Planitia, Mars: Guidelines for the recognition of degraded ice-rich materials

Widespread deposits surrounding mesas, in craters and in valley systems are observed in the transition zone between the Elysium Rise and the Utopia Planitia Basin. They are characterized by their relatively high albedo, the presence of ring-mold crater (RMC) morphologies and their pitted surfaces, with textures ranging from lineations and fish-scale-patterns to widely distributed knobs. These deposits are interpreted to be modified ice-rich material in the form of degraded deposits of concentric crater fill (CCF), lineated valley fill (LVF) and lobate debris aprons (LDA). The degraded CCF deposits are observed from 31.2–40°N, 138–150°E over an elevation range of almost 9 km. This wide-ranging distribution demonstrates that degraded ice-rich deposits exist at every altitude and latitude in the study area, indicating that icy mantle materials were initially deposited over extensive areas and were stable over a long time period, allowing the deposits to coexist and interact with different processes under very different conditions. The degraded LDA deposits represent the largest unit of modified ice-rich material, with an area of ∼15,700 km 2, and are populated with a range of ring-mold crater morphologies that is interpreted to be related to a degradational sequence between previously described RMC and newly observed RMCs that appear to be more degraded. A distinctive frequency difference in the distribution of normal and degraded RMCs permits an evaluation of different degradation stages of the LDA deposits; we show how an RMC distribution can be used as a key tool for evaluation of altered LDA, LVF and CCF deposits. Taken together, these observations suggest that ice-rich material has played a major role in shaping the present-day landscape in the transition zone between the Elysium Rise and the Utopia Planitia Basin, and they provide a link for understanding Amazonian-aged degradation processes of ice-rich deposits in an area with no significant topographic relief.

Chaos formation by sublimation of volatile-rich substrate: Evidence from Galaxias Chaos, Mars

Galaxias Chaos deviates significantly from other chaotic regions due to the lack of associated outflow channels, lack of big elevation differences between the chaos and the surrounding terrain and due to gradual trough formation. A sequence of troughs in different stages is observed, and examples of closed troughs within blocks suggest that the trough formation is governed by a local stress field rather than a regional stress field. Moreover, geomorphic evidence suggests that Galaxias Chaos is capped by Elysium lavas, which superpose an unstable subsurface layer that causes chaotic tilting of blocks and trough formation. Based on regional mapping we suggest a formation model, where Vastitas Borealis Formation embedded between Elysium lavas is the unstable subsurface material, because gradual volatile loss causes shrinkage and differential substrate movement. This process undermines the lava cap, depressions form and gradually troughs develop producing a jigsaw puzzle of blocks due to trough coalescence. Observations of chaos west of Elysium Rise indicate that this process might have been widespread along the contact between Vastitas Borealis Formation and Elysium lavas. However, the chaotic regions have probably been superposed by Elysium/Utopia flows to the NW of Elysium Rise, and partly submerged with younger lavas to the west.

Formation, erosion and exposure of Early Amazonian dikes, dike swarms and possible subglacial eruptions in the Elysium Rise/Utopia Basin Region, Mars

Hundreds of narrow, linear ridge segments are found in the transition zone between the Elysium Rise and the Utopia basin, occurring as both single and multiple ridges. The ridges are distinctive because of their very linear, steep-sided nature, their often sharp ridge crest (which sometimes is fractured), their association with stubby flows, their continuity over long distances and their cross-cutting of different terrain. The linear ridges are interpreted to be single dikes and dike swarms, either emplaced as normal dikes or as dikes emplaced subglacially feeding an explosive or effusive eruption. Five dike swarms are identified, having lengths ranging from 10–45 km and being between 1–7 km wide, while single ridges are up to 20 km long and 100–500 m wide. In the areas of dike swarms, crustal dilatation is estimated to vary from 15–60%. Dikes emplaced en echelon suggest that variations in the local stress field caused rotation during dike emplacement and dikes crosscutting flow units imply that dike emplacement can account for some of the observed linear fractures in the area. The ridges both modify and constrain Early Amazonian flows and flood plain deposits suggesting intense dike emplacement in the Early Amazonian. The association with different stages of inverted craters, as well as some features of ice-related origin (possible ice-cauldron and tindar-like features), indicate that the dikes may have been exposed due to eolian erosion and loss of volatile rich units subsequent to their emplacement.

Polar wander of Mars: Evidence from giant impact basins

We investigate the polar wander of Mars in the last ∼4.2 Ga. We identify two sets of basins from the 20 giant impact basins reported by Frey [Frey, H., 2008. Geophys. Res. Lett. 35, L13203] which trace great circles on Mars, and propose that the great circles were the prevailing equators of Mars at the impact times. Monte Carlo tests are conducted to demonstrate that the two sets of basins are most likely not created by random impacts. Also, fitting 63,771 planes to randomly selected sets of 5, 6, or 7 basins indicated that the identified two sets are unique. We propose three different positions for the rotation pole of Mars, besides the present one. Accordingly, Tharsis bulge was initially formed at ∼50 N and moved toward the equator while rotating counterclockwise due to the influence of the two newly forming volcanic constructs, Alba Patera and Elysium Rise. The formation of the giant impact basins, subsequent mass concentrations (mascons) in Argyre, Isidis, and Utopia basins, and surface masses of volcanic mountains such as Ascraeus, Pavonis, Arsia and Olympus, caused further polar wander which rotated Tharsis bulge clockwise to arrive at its present location. The extensive polar motion of Mars during 4.2–3.9 Ga implies a weak lithosphere on a global scale, deduced from a total of 72,000 polar wander models driven by Tharsis bulge, Alba Patera and Elysium Rise as the major mass perturbations. Different compensation states, 0–100%, are examined for each of the surface loads, and nine different thicknesses are considered for an elastic lithosphere. The lithosphere must have been very weak, with an elastic thickness of less than 5 km, if the polar wander was driven by these mass perturbations.

Thermally distinct craters near Hrad Vallis, Elysium Planitia, Mars

Evidence of volcano–ground ice interactions on Mars can provide important constraints on the timing and distribution of martian volcanic processes and climate characteristics. Northwest of the Elysium Rise is Hrad Vallis, a ∼ 370 m deep, 800 km long sinuous valley that begins in a source region at 34° N, 218° W. Flanking both sides of the source region is a lobate deposit that extends ∼ 50 km perpendicular from the source and is an average of ∼ 40 m thick. Previous studies have suggested the formation of the Hrad Vallis source region was the result of explosive magma–ice interaction and that the lobate deposit is a mudflow; here we use newly available MOLA, MOC, and THEMIS data to investigate the evidence supporting this hypothesis. Within the lobate deposit we have identified 12 craters with thermal infrared signatures and morphologies that are distinct from any other craters or depressions in the region. The thermally distinct craters are distinguished by their cool interiors surrounded by warm ejecta in the nighttime THEMIS IR data and warm interiors surrounded by cool ejecta in the daytime THEMIS IR data. The craters are typically 1100–1800 m in diameter (one crater is ∼ 2300 m across) and 30–40 m deep, but may be up to 70 m. The craters are typically circular and have central depressions (several with interior dune fill) surrounded by ∼1 to >6 concentric fracture sets. The distribution of the craters and their morphology suggests that they are likely the result of the interaction between a hot mudflow and ground ice.

Constraints on the Martian lithosphere from gravity and topography data

Localized spectral admittances of the large Martian volcanoes are modeled by assuming that surface and subsurface loads are elastically supported by the lithosphere. In order to model the case where the load density differs from that of the crust, a new method for calculating gravity anomalies and lithospheric deflections is developed. The modeled gravity anomalies depend upon the elastic thickness, crustal thickness, load density, and crustal density, and these parameters were exhaustively sampled in order to determine their effect on the misfit between the observed and modeled admittance function. We find that the densities of the Martian volcanoes are generally well constrained with values of 3200 ± 100 kg m−3, which is considerably greater than those reported previously. These higher densities are consistent with those of the Martian basaltic meteorites, which are believed to originate from the Tharsis and Elysium volcanic provinces. The crustal density is constrained only beneath the Elysium rise to be 3270 ± 150 kg m−3. If this value is representative of the northern lowlands, then Pratt compensation is likely responsible for the approximately 6‐km elevation difference between the northern and southern hemispheres. The elastic thicknesses of the major Martian volcanoes (when subsurface loads are ignored) are found to be the following: Elysium rise (56 ± 20 km), Olympus Mons (93 ± 40 km), Alba Patera (66 ± 20 km), and Ascraeus Mons (105 ± 40 km). We have also investigated the effects of subsurface loads, allowing the bottom load to be located either in the crust as dense intrusive material or in the mantle as less dense material. We found that all volcanoes except Pavonis are better modeled with the presence of less dense material in the upper mantle, which is indicative of either a mantle plume or a depleted mantle composition. An active plume beneath the major volcanoes is consistent with recent analyses of cratering statistics on Olympus Mons and the Elysium rise, which indicate that some lava flows are as young as 10–30 Myr, as well as with the crystallization ages of the Shergottites, of which some are as young as 180 Myr.

Cerberus Fossae, Elysium, Mars: a source for lava and water

Cerberus Fossae, a long fracture system in the southeastern part of Elysium, has acted as a conduit for the release of both lava and water onto the surface. The southeastern portion of the fracture system localized volcanic vents having varying morphology. In addition, low shields occur elsewhere on the Cerberus plains. Three locations where the release of water has occurred have been identified along the northwest (Athabasca and Grjota' Vallis) and southeast (Rahway Vallis) portions of the fossae. Water was released both catastrophically and noncatastrophically from these locations. A fluvial system that extends more than 2500 km has formed beginning at the lower flank of the Elysium rise across the Cerberus plains and out through Marte Vallis into Amazonis Planitia. The timing of the events is Late Amazonian.

Elysium‐Utopia flows as mega‐lahars: A model of dike intrusion, cryosphere cracking, and water‐sediment release

High‐resolution altimetry from the Mars Orbiter Laser Altimeter (MOLA) provides new data to test the interpretation that unusual flow‐like deposits in Elysium‐Utopia were emplaced as lahars (mass flows fluidized by water that are induced by volcanism). Using several data products derived from MOLA altimetry, we confirm that two major types of terrain dominate the region: (1) lava flows and (2) deposits consistent with emplacement by debris flows along medial channels, supporting the lahar hypothesis of previous workers. Associated terrain types are interpreted to be the result of modification of lavas by water and/or ice associated with the debris outflows and by later climatically controlled deposition and removal of a volatile‐rich dust layer at high latitudes. Both lava flows and debris flows originate from northwest‐trending, radial fossae on the flanks of the Elysium rise. The fossae are interpreted here to have been formed by laterally propagating dikes from Elysium Mons that have intersected or approached the surface. Fossae that are sources for only lava flows lie at higher elevations (−3400 m and above) than those that are the sources of lahars and channels (−3100 m to −4300 m). This discrepancy in source location suggests that the availability of groundwater responsible for lahar formation is elevation‐dependent and reflects a hydrostatic‐equilibrium distribution of water in the subsurface. These observations at Elysium allow a geologic test of the prevailing hypothesis on the configuration of the subsurface Martian hydrosphere: that an interconnected groundwater system at hydrostatic equilibrium may exist below a confining cryosphere. If water at a certain elevation is under hydrostatic pressure and this cryosphere is disrupted, the potential arises for groundwater to flow to the surface. We propose the following flow‐emplacement model in which geologic activity in northwest Elysium is consistent with the preceding hydrological model. Laterally propagating dikes from Elysium approached or intersected the surface on the flanks of the rise, resulting in effusion of lava flows and possibly pyroclastics. Disruption of the cryosphere by the dikes allowed groundwater to escape to the surface and form lahars by interaction with erupted and existing material at elevations at and below the maximum level of subsurface saturated ground. This model provides a consistent explanation for the major structures and units (both lava flows and lahars) and their elevation and spatial relationships in the Elysium‐Utopia region. This type of mega‐lahar, with groundwater as its primary water source, is rare on Earth and occurs on Mars due to Martian climatic and hydrologic conditions and the presence of a global confining cryosphere. This study supports the idea that groundwater outflow resulting from disruption of a confining cryosphere may be an important phenomenon in Martian geology. In particular, general association of major outflow with major volcanic areas on Mars is highly consistent with a model of disruption of a groundwater‐confining cryosphere by dikes expected to be intruded in these areas. Further geologic analyses of features and processes related to groundwater outflow will help constrain the degree and causes of variability within the Martian hydrologic system.

Stress differences in the Martian lithosphere: Constraints on the thermal state of Mars

Depth‐dependent density perturbations are determined inside the elastic part of the Martian lithosphere (assumed to be 300 km thick) using a thick elastic shell model, such that the deformed model under the loads of surface topography and the density perturbations gives rise to the observed topography and gravity anomalies of Mars specifies by spherical harmonics of degree 3–50. Large positive density perturbations, as high as ∼150 kg/m3, exist in the crust that are associated with Hellas, Isidis, Argyre, and Utopia basins, reflecting the mantle uplift in response to the impact excavation. Also, large positive density perturbations exist in the crust beneath Olympus (∼350 kg/m3) and Tharsis monts (∼200 kg/m3), indicating that these shield volcanoes are not isostatically compensated. On the other hand, negative density perturbations are associated with Alba Patera, Elysium rise, and the highland surrounding Hellas basin. The density perturbations gradually diminish with depth, becoming less than 20 kg/m3 by 250–300 km depth. The resulting stress differences within the crust and upper mantle are dominated by Olympus (up to 100 MPa), whereas other shield volcanoes create lesser stress differences. We estimate the brittle‐ductile transition temperature in the lithosphere, adopting the creep law of dry olivine for the rheology of the Martian lithosphere. The brittle‐ductile transition temperature beneath Olympus is ∼800C throughout the lithosphere, indicating that the upper parts of Mars must have been cold to support this volcano for 1–2 Gyr.

A geologic evaluation of thermal properties for the elysium and aeolis quadrangles of Mars

Thermal inertias with spatial resolutions as small as 2 by 5 km were combined in a regional study of the Elysium and Aeolis quadrangles of Mars (30°N to 30°S, 180°W to 225°W). The range of thermal inertias obtained was 1–14 (× 10−3 cal cm−2 s−½ K−1 = 41.84 j m−2 s−½ K−1), with the lowest values corresponding to the Elysium Rise and the highest values correlating with dark patches of aeolian material in the southern highlands. The thermal properties obtained from the high spatial resolution measurements are essentially identical to the results obtained from data with much poorer spatial resolution, indicating a regional homogeneity of surface properties at scales greater than 5 km. Aeolian features, both dark patches and bright materials associated with topographic obstacles such as craters, have higher thermal inertias than do their surroundings. Other surface features do not display distinctive thermal properties, even when clearly resolved from their surroundings. Comparison of the thermal inertias with global data sets shows a pronounced inverse correlation with albedo (consistent with globally observed trends) and brightnesses at red, green, and violet wavelengths but no prominent correlation with elevation. Thermal inertias for individual geologic units within the two quadrangles appear to be more strongly controlled by the location of the terrain in either the northern plains or the southern highlands than by properties intrinsic to the unit.

Elysium Region, Mars: Tests of lithospheric loading models for the formation of tectonic features

The second largest volcanic province on Mars lies in the Elysium region. Like the larger Tharsis province, Elysium is marked by a topographic rise and a broad free air gravity anomaly and also exhibits a complex assortment of tectonic and volcanic features. We test the hypothesis that the tectonic features in the Elysium region are the product of stresses produced by loading of the Martian lithosphere. We consider loading at three different scales: local loading by individual volcanoes, regional loading of the lithosphere from above or below, and quasi‐global loading by Tharsis. A comparison of flexural stresses with lithospheric strength and with the inferred maximum depth of faulting confirms that concentric graben around Elysium Mons can be explained as resulting from local flexure of an elastic lithosphere about 50 km thick in response to the volcano load. Volcanic loading on a regional scale, however, leads to predicted stresses inconsistent with all observed tectonic features, suggesting that loading by widespread emplacement of thick plains deposits was not an important factor in the tectonic evolution of the Elysium region. A number of linear extensional features oriented generally NW‐SE may have been the result of flexural uplift of the lithosphere on the scale of the Elysium rise. The global stress field associated with the support of the Tharsis rise appears to have influenced the development of many of the tectonic features in the Elysium region, including Cerberus Rupes and the systems of ridges in eastern and western Elysium. The comparisons of stress models for Elysium with the preserved tectonic features support a succession of stress fields operating at different times in the region. While the order in which those stress fields operated cannot be determined from present geological observations, thermal and mechanical arguments favor the hypothesis that any flexural uplift of the lithosphere by a mantle thermal anomaly preceded or occurred contemporaneously with emplacement of the largest volcanic loads.