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1.
Topographic information, surface structures and construction of the Martian Tharsis bulge are used to estimate the previous stresses across the low-lying peripheral margins of the crustal blocks in terms of simple compensation models. Hot mantle activity, crustal roots, isostasy, and late-stage extensive lithosphere thickening together with volcanic building have been in combined response to the high-elevated Tharsis bulge. The initial phases of the Tharsis building have been dominated by the mantle plume doming, followed by extrusional dome raising. The volcanism has been most important bulge building factor only after thickening of the crust. During the initial mantle-generated doming and igneous activity the thin-lithosphere block tectonics has been very important. There has been a compressional peripheral zone around the bulge giving rise to dorsa formation while the high bulge crests have been in tensional state. The situation may be favorable for comparative studies with other planets. We may have something to learn from this block tectonics on the one-plate planet Mars even in respect to the Earth's plate tectonic paradigm.On leave from Dept. of Astronomy, University of Oulu, Finland.  相似文献   

2.
This review provides explanations of how geodesy, rotation and gravity can be addressed using radioscience data of an orbiter around a planet or of the lander on its surface.The planet Mars is the center of the discussion.The information one can get from orbitography and radioscience in general concerns the global static gravitational field, the time variation of the gravitational field induced by mass exchange between the atmosphere and the ice caps, the time variation of the gravitational field induced by the tides, the secular changes in the spacecraft's orbit induced by the little moons of Mars named Phobos and Deimos, the gravity induced by particular targets, the Martian ephemerides, and Mars' rotation and orientation.The paper addresses as well the determination of the geophysical parameters of Mars and, in particular, the state of Mars' core and its size, which is important for understanding the planet's evolution.Indeed, the state and dimension of the core determined from the moment of inertia and nutation depend in turn on the percentage of light elements in the core as well as on the core temperature, which is related to heat transport in the mantle.For example, the radius of the core has implications for possible mantle convection scenarios and, in particular, for the presence of a perovskite phase transition at the bottom of the mantle.This is also important for our understanding of the large volcanic province Tharsis on the surface of Mars.  相似文献   

3.
A theoretical thermal evolution model of Mars is constructed, utilizing as constraints the available geophysical and geological data, including those provided by the Viking missions. The calculation includes conduction and subsolidus mantle convection. Calculated models indicate that Martian evolution can be roughly characterized by four different stages. (1) Core formation and crust differentiation: this stage starts from the planet formation to about 1 by thereafter. During this period, Martian core is separated and the initial crust is differentiated. (2) Heating, expansion, and mantle differentiation: this stage begins after the core separation and extends to about 3 by. First, mantle temperatures rise and reach partial melting. Between 2 and 3 by, extensive melting, differentiation, and outgassing occur. Planetary radius increases and extensional features observed at the surface are most likely generated at this stage. (3) Mature phase: after 3 by, the planet reaches maturity. Between 3 and 4 by slow and sustained evolution continues. Lithosphere thickens and partial melt zone deepens. (4) Cooling period: this stage represents the last phase of Martian history. The planet is cooling slowly. The partial melting zone shrinks and volcanic activity tapers off. At present, Martian lithosphere is about 200 km thick and the mantle is convecting slowly. The models suggest that the core is molten, and the calculated surface heat flux is 35 erg cm?2 sec?1.  相似文献   

4.
Jafar Arkani-Hamed 《Icarus》1975,26(3):313-320
The undulations of the Martian gravitational potential indicate lateral density variations in the Mars interior. A gravitating and solid Martian model deforms under the influence of these variations, producing stress differences of about 125 bars at a depth of about 200 km. Introduction of a partially molten core of 1300 km radius does not affect the stress distribution in the mantle significantly, whereas the assumption of a partially molten asthenosphere umderlying a solid lithosphere of about 300 km increases the stress differences appreciably. A strong linear correlation of the gravitational potential and the surface topography indicates that the extensive volcanism at the Tharsis region is a recent phenomenon. The high stresses associated with this region imply that there has been no extensive molten region within the upper 300 km since the volcanism.  相似文献   

5.
The early history of Mars included two large-scale events of great significance: (1) the lowering and resurfacing of one-third of the crust, followed closely by (2) evolution of the Tharsis bulge. Tharsis development apparently involved two stages: (1) an initial rapid topographic rise accompanied by the development of a vast radial fault system, and (2) an extremely long-lived volcanic stage apparently continuing to the geologic present. A deformational model is proposed whereby a first-order mantle convection cell caused early subcrustal erosion and foundering of the low third of the planet. Underplating and deep intrusion by the eroded materials beneath Tharsis caused isostatic doming. Minor radial gravity motions of surficial layers off the dome produced the radial fault system. The hot underplate eventually affected the surface to cause the very long-lived volcanic second stage. Deep crustal anisotropy associated with the locally NE-trending boundary between the highland two-thirds and the lowland one-third caused the NE elongation of many features of Tharsis.  相似文献   

6.
The concept of block tectonics provides a framework for understanding many aspects of Tharsis and adjoining structures. This Tharsis block tectonics on Mars is manifested partly by mantle-related doming and partly by response to loading by subsequent volcanic construction. Although the origin of the volcanism from beneath Tharsis is a subject of controversy explanations have to include inhomogenities in Martian internal structure, energy distribution, magma accumulation and motion below the lithosphere. Thermal convection can be seen as a necessary consequence for transient initial phase of Martian cooling. This produced part of the elevated topography with tensional stresses and graben systems radial to the main bulge. The linear grabens, radial to the Tharsis center, can be interpreted to indicate rift zones that define the crustal block boundaries. The load-induced stresses may then have contributed on further graben and ridge formation over an extended period of time.On leave from Dept. of Astronomy University of Oulu, Oulu, Finland.  相似文献   

7.
Did tidal deformation power the core dynamo of Mars?   总被引：1，自引：0，他引：1  
Jafar Arkani-Hamed 《Icarus》2009,201(1):31-218
We first show that 7 out of the 20 giant impact basins of Mars recently reported by Frey [Frey, H., 2008. Geophys. Res. Lett. 35. L13203] trace a great circle on Mars. The other five basins trace another great circle and still the other three basins trace yet another great circle. The latter great circle is in good agreement with the pre-Tharsis equator of Mars that is estimated from modeling crustal magnetic anomalies [Arkani-Hamed, J., 2001. Geophys. Res. Lett. 28, 3409-3412] and diagonalizing the moment of inertia of Mars after removing the loading effects of Tharsis bulge [Sprenke, K.F., Baker, L.L., Williams, A.F., 2005. Icarus 174, 486-489]. It is shown in this paper that the three great circles were likely the equatorial plane of Mars at certain times and Mars experienced appreciable polar wander. The great circles also indicate that the asteroids that created the basins were satellites of Mars whose orbits decayed in time through spin-orbit coupling with tidally deforming Mars, and eventually impacted on the planet creating the giant basins at around 4 Ga. The orbital dynamics of four largest asteroids show that they could have orbited Mars for several hundred million years if they were retrograde satellites. Continual elliptical straining of otherwise circular fluid streamlines of the liquid core of Mars by tidal deformation could have exerted a strong strain that was large enough to overcome dissipation and excite the elliptical instability inside the core. We investigate the physical properties of the martian core that are required to allow the tidal deformation to power the core dynamo, i.e., the growth time of the elliptical instability to become shorter than the dissipation time. The tidal energy dissipation rate inside Mars caused by even only one of the 4 largest asteroids is found to be over two orders of magnitude greater than the magnetic energy dissipation rate in the core, indicating that if only one of the 4 largest asteroids were orbiting in retrograde sense, it would have likely powered the core dynamo of Mars for several hundred million years.  相似文献   

8.
Abstract— The age, structure, composition, and petrogenesis of the martian lithosphere have been constrained by spacecraft imagery and remote sensing. How well do martian meteorites conform to expectations derived from this geologic context? Both data sets indicate a thick, extensive igneous crust formed very early in the planet's history. The composition of the ancient crust is predominantly basaltic, possibly andesitic in part, with sediments derived from volcanic rocks. Later plume eruptions produced igneous centers like Tharsis, the composition of which cannot be determined because of spectral obscuration by dust. Martian meteorites (except Allan Hills 84001) are inferred to have come from volcanic flows in Tharsis or Elysium, and thus are not petrologically representative of most of the martian surface. Remote‐sensing measurements cannot verify the fractional crystallization and assimilation that have been documented in meteorites, but subsurface magmatic processes are consistent with orbital imagery indicating thick crust and large, complex magma chambers beneath Tharsis volcanoes. Meteorite ejection ages are difficult to reconcile with plausible impact histories for Mars, and oversampling of young terrains suggests either that only coherent igneous rocks can survive the ejection process or that older surfaces cannot transmit the required shock waves. The mean density and moment of inertia calculated from spacecraft data are roughly consistent with the proportions and compositions of mantle and core estimated from martian meteorites. Thermal models predicting the absence of crustal recycling, and the chronology of the planetary magnetic field agree with conclusions from radiogenic isotopes and paleomagnetism in martian meteorites. However, lack of vigorous mantle convection, as inferred from meteorite geochemistry, seems inconsistent with their derivation from the Tharsis or Elysium plumes. Geological and meteoritic data provide conflicting information on the planet's volatile inventory and degassing history, but are apparently being reconciled in favor of a periodically wet Mars. Spacecraft measurements suggesting that rocks have been chemically weathered and have interacted with recycled saline groundwater are confirmed by weathering products and stable isotope fractionations in martian meteorites.  相似文献   

9.
We present the results of extensive numerical modeling of the Martian interior. Yoder et al. in 2003 reported a mean moment of inertia of Mars that was somewhat smaller than the previously used value and the Love number k2 obtained from observations of solar tides on Mars. These values of k2 and the mean moment of inertia impose a strong new constraint on the model of the planet. The models of the Martian interior are elastic, while k2 contains both elastic and inelastic components. We thoroughly examined the problem of partitioning the Love number k2 into elastic and inelastic components. The information necessary to construct models of the planet (observation data, choice of a chemical model, and the cosmogonic aspect of the problem) are discussed in the introduction. The model of the planet comprises four submodels—a model of the outer porous layer, a model of the consolidated crust, a model of the silicate mantle, and a core model. We estimated the possible content of hydrogen in the core of Mars. The following parameters were varied while constructing the models: the ferric number of the mantle (Fe#) and the sulfur and hydrogen content in the core. We used experimental data concerning the pressure and temperature dependence of elastic properties of minerals and the information about the behavior of Fe(γ-Fe ), FeS, FeH, and their mixtures at high P and T. The model density, pressure, temperature, and compressional and shear velocities are given as functions of the planetary radius. The trial model M13 has the following parameters: Fe#=0.20; 14 wt % of sulfur in the core; 50 mol % of hydrogen in the core; the core mass is 20.9 wt %; the core radius is 1699 km; the pressure at the mantle-core boundary is 20.4 GPa; the crust thickness is 50 km; Fe is 25.6 wt %; the Fe/Si weight ratio is 1.58, and there is no perovskite layer. The model gives a radius of the Martian core within 1600–1820 km while ≥30 mol % of hydrogen is incorporated into the core. When the inelasticity of the Martian interior is taken into account, the Love number k2 increases by several thousandths; therefore, the model radius of the planetary core increases as well. The prognostic value of the Chandler period of Mars is 199.5 days, including one day due to inelasticity. Finally, we calculated parameters of the equilibrium figure of Mars for the M13 model: J 2 0 = 1.82 × 10?3, J 4 0 = ?7.79 × 10?6, e c-m D = 1/242.3 (the dynamical flattening of the core-mantle boundary).  相似文献   

10.
Jafar Arkani-Hamed 《Icarus》2009,204(2):489-498
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.  相似文献   

11.
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.  相似文献   

12.
Most (～90%) of the estimated original volume of outgassed water on Mars cannot be satisfactorily accounted for by exospheric escape or storage in the atmosphere, as frost, or in the permanent north polar ice cap. The balance may be stored as ground ice in the Martian cryosphere, a zone of permanently frozen ground that is protected from the atmosphere by a debris cover. Ground ice can exist throughout the entire cryosphere, but it need not fill it. If the ground ice does fill the cryosphere, then excess water can exist in a confined aquifer. The theoretical distribution of ground ice can be tested by identification of forms on the Martian surface that may be related to the presence of subsurface ice. The observed features that are most likely to reflect ground ice are thermokarst-like pits and debris flows. Landforms with ambivalent origins include polygonally patterned ground, lobate ejecta blankets, craters with central pits, and curvilinear features. The most persuasive morphologic evidence for ground ice is thermokarst pits and debris flows; the thermokarst pits are primarily located in the volcanic regions of Tharsis and Elysium. The association of ice-related features with these volcanic areas suggests that these forms are not directly latitude dependent. Activation by orbital variations could produce periodic, multiple episodes of melting that are dependent upon latitude. The presence of ice-related features in both hemispheres and the equatorial region of Mars indicates that ground ice may be—or have been—present over the entire planet, as predicted by the cryosphere model.  相似文献   

13.
Maps of the vector components of the Mars crustal magnetic field are constructed at the mapping altitude (360 to 410 km) using a selected set of data obtained with the Mars Global Surveyor magnetometer during 2780 orbits of the planet in 1999. Forward modeling calculations are then applied to six relatively strong and isolated, dominantly dipolar, magnetic anomalies for the primary purpose of estimating bulk directions of magnetization. Assuming that the magnetizing field was a (dipolar) core dynamo field centered in the planet, paleomagnetic pole positions are calculated for the six primary source bodies together with that for a seventh anomaly analyzed earlier. In agreement with several previous studies, it is found that six of the seven pole positions are clustered in what is now the northern lowlands in a region centered northwest of Olympus Mons (mean pole position: 34°±10° N, 202°±58° E). Assuming that the dynamo dipole moment vector was approximately parallel to the rotation axis, the modeling results therefore suggest a major reorientation of Mars relative to its rotation axis after magnetization was acquired. Such a reorientation may have been stimulated by internal mass redistributions associated with the formation of the northern lowlands and Tharsis, for example. A comparison of the mean paleo (magnetic) equator to the global distribution of crustal fields shows that magnetic anomalies tend to occur at low paleolatitudes. The same appears to be true for the Noachian-aged valley networks, which exhibit a broad spatial correlation with the magnetic anomalies. A possible interpretation is that the formation of magnetic anomalies and the valley networks was favored in the tropics where melting of water ice and snow was a stronger source of both surface valley erosion and groundwater recharge during the earliest history of the planet. This would be consistent with models in which hydrothermal alteration of crustal rocks played a role in producing the unusually strong martian magnetic anomalies.  相似文献   

14.
Of the terrestrial planets, Earth and probably Mercury possess substantial intrinsic magnetic fields generated by core dynamos, while Venus and Mars apparently lack such fields. Thermal histories are calculated for these planets and are found to admit several possible present states, including those which suggest simple explanations for the observations; whule the cores of Earth and Mercury are continuing to freeze, the cores of Venus and Mars may still be completely liquid. The models assume whole mantle convection, which is parameterized by a simple Nusselt-Rayleigh number relation and dictates the rate at which heat escapes from the core. It is found that completely fluid cores, devoid of intrinsic heat sources, are not likely to sustain thermal convection for the age of the solar system but cool to a subadiabatic, conductive state that can not maintain a dynamo. Planets which nucleate an inner core continue to sustain a dynamo because of the gravitational energy release and chemically driven convection that accompany inner core growth. The absence of a significant inner core can arise in Venus because of its slightly higher temperature and lower central pressure relative to Earth, while a Martian core avoids the onset of freezing if the abundance of sulfur in the core is ?15% by mass. All of the models presented assume that (I) core dynamos are driven by thermal and/or chemical convection; (ii) radiogenic heat production is confined to the mantle; (iii) mantle and core cool from initially hot states which are at the solidus and superliquidus, respectively; and (iv) any inner core excludes the light alloying material (sulfur or oxygen) which then mixes uniformly upward through the outer core. The models include realistic pressure and composition-dependent freezing curves for the core, and material parameters are chosen so that the correct present-day values of heat outflow, upper mantle temperature and viscosity, and inner core radius are obtained for the earth. It is found that Venus and Mars may have once had dynamos maintained by thermal convection alone. Earth may have had a completely fluid core and a dynamo maintained by thermal convection for the first 2 to 3 by, but an inner core nucleates and the dynamo energetics are subsequently dominated by gravitational energy release. Complete freezing of the Mercurian core is prohibited if it contains even a small amount of sulfur, and a dynamo can be maintained by chemical convection in a thin, fluid shell.  相似文献   

15.
A scenario for the evolution of the Martian atmosphere consistent with various data of the Viking 1 and 2 and the Mariner 9 has been presented: Mars was formed from Renazzo-type meteorites polluted by the products of supernova explosion. A dense ancient Martian atmosphere has been swept away by the solar wind and the present tenuous atmosphere was supplied recently by the volcanic gas from the Tharsis region, after the occurrence of the magnetic field.  相似文献   

16.
Abstract— Radiometric age dating of the shergottite meteorites and cratering studies of lava flows in Tharsis and Elysium both demonstrate that volcanic activity has occurred on Mars in the geologically recent past. This implies that adiabatic decompression melting and upwelling convective flow in the mantle remains important on Mars at present. I present a series of numerical simulations of mantle convection and magma generation on Mars. These models test the effects of the total radioactive heating budget and of the partitioning of radioactivity between crust and mantle on the production of magma. In these models, melting is restricted to the heads of hot mantle plumes that rise from the core‐mantle boundary, consistent with the spatially localized distribution of recent volcanism on Mars. For magma production to occur on present‐day Mars, the minimum average radioactive heating rate in the martian mantle is 1.6 times 10?12 W/kg, which corresponds to 39% of the Wanke and Dreibus (1994) radioactivity abundance. If the mantle heating rate is lower than this, the mean mantle temperature is low, and the mantle plumes experience large amounts of cooling as they rise from the base of the mantle to the surface and are, thus, unable to melt. Models with mantle radioactive heating rates of 1.8 to 2.1 times 10 ?12 W/kg can satisfy both the present‐day volcanic resurfacing rate on Mars and the typical melt fraction observed in the shergottites. This corresponds to 43–50% of the Wanke and Dreibus radioactivity remaining in the mantle, which is geochemically reasonable for a 50 km thick crust formed by about 10% partial melting. Plausible changes to either the assumed solidus temperature or to the assumed core‐mantle boundary temperature would require a larger amount of mantle radioactivity to permit present‐day magmatism. These heating rates are slightly higher than inferred for the nakhlite source region and significantly higher than inferred from depleted shergottites such as QUE 94201. The geophysical estimate of mantle radioactivity inferred here is a global average value, while values inferred from the martian meteorites are for particular points in the martian mantle. Evidently, the martian mantle has several isotopically distinct compositions, possibly including a radioactively enriched source that has not yet been sampled by the martian meteorites. The minimum mantle heating rate corresponds to a minimum thermal Rayleigh number of 2 times 106, implying that mantle convection remains moderately vigorous on present‐day Mars. The basic convective pattern on Mars appears to have been stable for most of martian history, which has prevented the mantle flow from destroying the isotopic heterogeneity.  相似文献   

17.
The rheology of the Martian mantle and the planet's initial temperature is constrained with thermal evolution models that include crust growth and test the conditions for magnetic field generation in the core. As observations we use the present-day average crustal thickness of 50-120 km as estimated from the Mars Global Surveyor gravity and topography data, the evidence for the crust being produced mostly early, with a rate declining from the Noachian to the Hesperian, and the evidence for an early magnetic field that likely existed for less than a billion years. We use the fact that the rate of crust growth is a function of temperature, which must be above the solidus in the sub-lithosphere mantle, and the mantle convection speed because the latter determines the rate at which melt can be replenished. The convection speed is a strong function of viscosity which, in turn, is a strong function of temperature and also of the water content of the mantle. We use a viscosity parameterization with a reference viscosity evaluated at 1600 K the value of which can be characteristic of either a dry or a wet mantle. We further consider the Fe-FeS phase diagram for the core and compare the core liquidus estimated for a sulphur content of 14% as suggested by the SNC meteorite compositions with the core temperatures calculated for our cooling models. Two data sets of the Fe-FeS eutectic temperature have been used that differ by about 200 K [Böhler, R., 1996. Fe-FeS eutectic temperatures at 620 kbar. Phys. Earth Planet. Inter. 96, 181-186; Fei, Y., Bertka, C.M., Finger, L.W., 1997. High-pressure iron-sulphur compound, Fe3S2, and melting relations in the Fe-FeS system. Science 275, 1621-1623] at Martian core-mantle boundary pressure and in the eutectic composition by 5 wt%. The differences in eutectic temperature and composition translate into a difference of about 400 K in liquidus temperature for 14 wt% sulphur.We find it premature to rule out specific mantle rheologies on the basis of the presently available crustal thickness and crust growth evidence. Rather a trade-off exists between the initial mantle temperature and the reference viscosity. Both a wet mantle rheology with a reference viscosity less than 1020 Pas and a dry mantle rheology with a reference viscosity of 1021 Pas or more can be acceptable if initial mantle temperatures between roughly 1700 and 2000 K are allowed. To explain the magnetic field history, the differences in liquidus temperatures matter. For a liquidus temperature of about 1900 K at the Martian core-mantle boundary as calculated from the Böhler et al. eutectic, a dry mantle rheology can best explain the lack of a present-day dynamo. For a liquidus temperature of about 1500 K at the core-mantle boundary as calculated from the Fei et al. eutectic all models are consistent with the observed lack of dynamo action. The reason lies with the fact that at 14 wt% S the Martian core would be close to the eutectic composition if the Fei et al. data are correct. As inner core growth is unlikely for an almost eutectic core, the early field would have been generated by a thermally driven dynamo. Together with the measured strength of the Martian crustal magnetization this would prove the feasibility of a strong thermally driven dynamo.  相似文献   

18.
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.  相似文献   

19.
The tectonics of the Tharsis and adjoining areas is considered to be associated with the convection in the Martian mantle. Convection and mantle plume have been responsible for the primary uplift and volcanism of the Tharsis area. The radial compressional forces generated by the tendency for downslope movement of surface strata, vertical volcanic intrusions and traction of mantle spreading beneath Tharsis were transmitted through the lithosphere to form peripheral mare ridge zones. The locations of mare ridges were thus mainly controlled by the Tharsis-radial compression. The load-induced stresses then contributed on further ridge formation over an extended period of time by the isostatic readjustment which was reponsible for long-term stresses in the adjoining areas. Extrusions, changes in internal temperature and possible phase changes may also have caused changes in mantle volume giving rise to additional compressional forces and crustal deformations.On leave from Dept. of Astronomy, University of Oulu, Oulu, Finland  相似文献   

20.
Studies extending over three decades have concluded that the current orientation of the martian rotation pole is unstable. Specifically, the gravitational figure of the planet, after correction for a hydrostatic form, has been interpreted to indicate that the rotation pole should move easily between the present position and a site on the current equator, 90° from the location of the massive Tharsis volcanic province. We demonstrate, using general physical arguments supported by a fluid Love number analysis, that the so-called non-hydrostatic theory is an inaccurate framework for analyzing the rotational stability of planets, such as Mars, that are characterized by long-term elastic strength within the lithosphere. In this case, the appropriate correction to the gravitational figure is the equilibrium rotating form achieved when the elastic lithospheric shell (of some thickness LT) is accounted for. Moreover, the current rotation vector of Mars is shown to be stable when the correct non-equilibrium theory is adopted using values consistent with recent, independent estimates of LT. Finally, we compare observational constraints on the figure of Mars with non-equilibrium predictions based on a large suite of possible Tharsis-driven true polar wander (TPW) scenarios. We conclude, in contrast to recent comparisons of this type based on a non-hydrostatic theory, that the reorientation of the pole associated with the development of Tharsis was likely less than 15° and that the thickness of the elastic lithosphere at the time of Tharsis formation was at least ∼50 km. Larger Tharsis-driven TPW is possible if the present-day gravitational form of the planet at degree 2 has significant contributions from non-Tharsis loads; in this case, the most plausible source would be internal heterogeneities linked to convection.  相似文献   

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