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1.
We have reinvestigated the coupled thermal and crustal evolution of Mars taking new laboratory data concerning the flow behavior of iron-rich olivine into account. The low mantle viscosities associated with the relatively higher iron content of the martian mantle as well as the observed high concentrations of heat producing elements in a crust with a reduced thermal conductivity were found to promote phases of crustal recycling in many models. As crustal recycling is incompatible with an early separation of geochemical reservoirs, models were required to show no episodes of crustal recycling. Furthermore, admissible models were required to reproduce the martian crust formation history, to allow for the formation of partial melt under present day mantle conditions and to reproduce the measured concentrations of potassium and thorium on the martian surface. Taking dehydration stiffening of the mantle viscosity by the extraction of water from the mantle into account, we found that admissible models have low initial upper mantle temperatures around 1650 K, preferably a primordial crustal thickness of 30 km, and an initially wet mantle rheology. The crust formation process on Mars would then be driven by the extraction of a primordial crust after core formation, cooling the mantle to temperatures close to the peridotite solidus. According to this scenario, the second stage of global crust formation took place over a more extended period of time, waning at around 3500 Myr b.p., and was driven by heat produced by the decay of radioactive elements. Present-day volcanism would then be driven by mantle plumes originating at the core-mantle boundary under regions of locally thickened, thermally insulating crust. Water extraction from the mantle was found to be relatively efficient and close to 40% of the total inventory was lost from the mantle in most models. Assuming an initial mantle water content of 100 ppm and that 10% of the extracted water is supplied to the surface, this amount is equivalent to a 14 m thick global surface layer, suggesting that volcanic outgassing of H2O could have significantly influenced the early martian climate and increased the planet’s habitability. 相似文献
2.
Moderately and slightly siderophile element constraints on the depth and extent of melting in early Mars 总被引：1，自引：0，他引：1
The thermal history of Mars during accretion and differentiation is important for understanding some fundamental aspects of its evolution such as crust formation, mantle geochemistry, chronology, volatile loss and interior degassing, and atmospheric development. In light of data from new Martian meteorites and exploration rovers, we have made a new estimate of Martian mantle siderophile element depletions. New high pressure and temperature metal–silicate experimental partitioning data and expressions are also available. Using these new constraints, we consider the conditions under which the Martian mantle may have equilibrated with metallic liquid. The resulting conditions that best satisfy six siderophile elements—Ni, Co, W, Mo, P, and Ga—and are consistent with the solidus and liquidus of the Martian mantle phase diagram are a pressure of 14 ± 3 GPa and temperature of 2100 ± 200 K. The Martian mantle depletions of Cr and V are also consistent with metal–silicate equilibration in this pressure and temperature range if deep mantle silicate phases are also taken into account. The results are not consistent with either metal–silicate equilibrium at the surface or at the current‐day Martian core–mantle boundary. Recent measurements and modeling have concluded that deep (～17 GPa or 1350 km) mantle melting is required to explain isotopic data for Martian meteorites and the nature of differentiation into core, mantle, and crust. This is in general agreement with our estimates of the conditions of Martian core formation based on siderophile elements that result in an intermediate depth magma ocean scenario for metal–silicate equilibrium. 相似文献
3.
During late-stage planet formation, giant impacts produce localized mantle melt regions within which impactor iron droplets settle to the bottom near a permeability horizon. After accumulation, iron heated by the impact migrates downward to the core through colder, mostly solid mantle. The degree of thermal equilibration and partitioning of viscous heating between impactor iron and silicates depends on the mechanism of iron transport to the core. Simple estimates suggest that, following a giant impact, the temperature difference between iron delivered to the core and the mantle outside the impact heated region can be ∼103 K. Hot impactor iron mergers with the core where it may be efficiently mixed or remain stratified due to thermal buoyancy. In either case, collisional energy carried to the core by impactor iron helps establish conditions favorable for early core cooling and dynamo generation. In this study, we consider the end-member scenario in which impactor iron forms a layer at the top of the core. Energy transfer from the impactor iron layer to the mantle is sufficient to power a dynamo for up to ∼30 Myr even in the limit of a very viscous mantle and heat flux limited by conduction. Using two-dimensional finite element calculations of mantle convection, we show that large-scale mantle flow driven by the buoyancy of the impact thermal anomaly focuses plumes in the impact region and increases both dynamo strength and duration. Melting within the mantle thermal boundary layer likely leads to formation of a single superplume in the location of the impact anomaly driven upwelling. We suggest that formation of magnetized southern highland crust may be related to spreading and differentiation of an impact melt region during the impact-induced dynamo episode. 相似文献
4.
5.
Large impacts not only create giant basins on terrestrial planets but also heat their interior by shock waves. We investigate the impacts that have created the largest basins existing on the planets: Utopia on Mars, Caloris on Mercury, Aitken on Moon, all formed at ∼4 Ga. We determine the impact-induced temperature increases in the interior of a planet using the “foundering” shock heating model of Watters et al. (Watters, W.A., Zuber, M.T., Hager, B.H. [2009]. J. Geophys. Res. 114, E02001. doi:10.1029/2007JE002964). The post-impact thermal evolution of the planet is investigated using 2D axi-symmetric convection in a spherical shell of temperature-dependent viscosity and thermal conductivity, and pressure-dependent thermal expansion. The impact heating creates a superheated giant plume in the upper mantle which ascends rapidly and develops a strong convection in the mantle of the sub-impact hemisphere. The upwelling of the plume rapidly sweeps up the impact-heated base of the mantle away from the core-mantle boundary and replaces it with the colder surrounding material, thus reducing the effects of the impact-heated base of the mantle on the heat flux out of core. However, direct shock heating of the core stratifies the core, suppresses the pre-existing thermal convection, and cripples a pre-existing thermally-driven core dynamo. It takes about 17, 4, and 5 Myr for the stratified cores of Mars, Mercury, and Moon to exhaust impact heat and resume global convection, possibly regenerating core dynamos. 相似文献
6.
Mars is the fourth planet out from the sun. It is a terrestrial planet with a density suggesting a composition roughly similar
to that of the Earth. Its orbital period is 687 days, its orbital eccentricity is 0.093 and its rotational period is about
24 hours. Mars has two small moons of asteroidal shapes and sizes (about 11 and 6 km mean radius), the bigger of which, Phobos,
orbits with decreasing semimajor orbit axis. The decrease of the orbit is caused by the dissipation of tidal energy in the
Martian mantle. The other satellite, Deimos, orbits close to the synchronous position where the rotation period of a planet
equals the orbital period of its satellite and has hardly evolved with time. Mars has a tenous atmosphere composed mostly
of CO with strong winds and with large scale aeolian transport of surface material during dust storms and in sublimation-condensation
cycles between the polar caps. The planet has a small magnetic field, probably not generated by dynamo action in the core
but possibly due to remnant magnetization of crustal rock acquired earlier from a stronger magnetic field generated by a now
dead core dynamo. A dynamo powered by thermal power alone would have ceased a few billions of years ago as the core cooled
to an extent that it became stably stratified. Mars' topography and its gravity field are dominated by the Tharsis bulge,
a huge dome of volcanic origin. Tharsis was the major center of volcanic activity, a second center is Elysium about 100° in
longitude away. The Tharsis bulge is a major contributor to the non-hydrostaticity of the planet's figure. The moment of inertia
factor together with the mass and the radius presently is the most useful constraint for geophysical models of the Martian
interior. It has recently been determined by Doppler range measurements to the Mars Pathfinder Lander to be (Folkner et al. 1997). In addition, models of the interior structure use the chemistry of the SNC meteorites which are widely
believed to have originated on Mars. According to the models, Mars is a differentiated planet with a 100 to 200 km thick basaltic
crust, a metallic core with a radius of approximately half the planetary radius, and a silicate mantle. Mantle dynamics is
essential in forming the elements of the surface tectonics. Models of mantle convection find that the pressure-induced phase
transformations of -olivine to -spinel, -spinel to -spinel, and -spinel to perovskite play major roles in the evolution of mantle flow fields and mantle temperature. It is not very likely
that the -spinel to perovskite transition is present in Mars today, but a few 100 km thick layer of perovskite may have been present
in the lower mantle immediately above the core-mantle boundary early in the Martian history when mantle temperatures were
hotter than today. The phase transitions act to reduce the number of upwellings to a few major plumes which is consistent
with the bipolar distribution of volcanic centers of Mars. The phase transitions also cause a partial layering of the lower
mantle which keeps the lower mantle and the core from extensive cooling over the past aeons. A relatively hot, fluid core
is the most widely accepted explanation for the present lack of a self-generated magnetic field. Growth of an inner core which
requires sub-liquidus temperatures in the core would have provided an efficient mechanism to power a dynamo up to the present
day.
Received 10 May 1997 相似文献
7.
Knowledge of the interior structure of Mars is of fundamental importance to the understanding of its past and present state as well as its future evolution. The most prominent interior structure properties are the state of the core, solid or liquid, its radius, and its composition in terms of light elements, the thickness of the mantle, its composition, the presence of a lower mantle, and the density of the crust. In the absence of seismic sounding only geodesy data allow reliably constraining the deep interior of Mars. Those data are the mass, moment of inertia, and tides. They are related to Mars’ composition, to its internal mass distribution, and to its deformational response to principally the tidal forcing of the Sun. Here we use the most recent estimates of the moment of inertia and tidal Love number k2 in order to infer knowledge about the interior structure of the Mars.We have built precise models of the interior structure of Mars that are parameterized by the crust density and thickness, the volume fractions of upper mantle mineral phases, the bulk mantle iron concentration, and the size and the sulfur concentration of the core. From the bulk mantle iron concentration and from the volume fractions of the upper mantle mineral phases, the depth dependent mineralogy is deduced by using experimentally determined phase diagrams. The thermoelastic properties at each depth inside the mantle are calculated by using equations of state. Since it is difficult to determine the temperature inside the mantle of Mars we here use two end-member temperature profiles that have been deduced from studies dedicated to the thermal evolution of Mars. We calculate the pressure and temperature dependent thermoelastic properties of the core constituents by using equations state and recent data about reference thermoelastic properties of liquid iron, liquid iron-sulfur, and solid iron. To determine the size of a possible inner core we use recent data on the melting temperature of iron-sulfur.Within our model assumptions the geodesy data imply that Mars has no solid inner core and that the liquid core contains a large fraction of sulfur. The absence of a solid inner is in agreement with the absence of a global magnetic field. We estimate the radius of the core to be 1794 ± 65 km and its core sulfur concentration to be 16 ± 2 wt%. We also show that it is possible for Mars to have a thin layer of perovskite at the bottom of the mantle if it has a hot mantle temperature. Moreover a chondritic Fe/Si ratio is shown to be consistent with the geodesy data, although significantly different value are also possible. Our results demonstrate that geodesy data alone, even if a mantle temperature is assumed, can almost not constrain the mineralogy of the mantle and the crust. In order to obtain stronger constraints on the mantle mineralogy bulk properties, like a fixed Fe/Si ratio, have to be assumed. 相似文献
8.
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
9.
Depth-dependent interior structure models of Mercury are calculated for several plausible chemical compositions of the core and of the mantle. For those models, we compute the associated libration amplitude, obliquity, tidal deformation, and tidal changes in the external potential. In particular we study the relation between the interior structure parameters for five different mantle mineralogies and two different temperature profiles together with two extreme crust density values. We investigate the influence of the core light element concentration, temperature, and melting law on core state and inner and outer core size. We show that a sulfur concentration above 10 wt% is unlikely if the temperature at the core-mantle boundary is above 1850 K and the silicate shell at least 240 km thick. The interior models can only have an inner core if the sulfur weight fraction is below 5 wt% for core-mantle boundary temperature in the 1850-2200 K range. Within our modeling hypotheses, we show that with the expected precision on the moment of inertia the core size can be estimated to a precision of about 50 km and the core sulfur concentration with an error of about 2 wt%. This uncertainty can only be reduced when more information on the mantle mineralogy of Mercury becomes available. However, we show that the uncertainty on the core size estimation can be greatly reduced, to about 25 km, if tidal surface displacements and tidal variations in the external potential are considered. 相似文献
10.
Ganymede has an intrinsic magnetic field which is generally considered to originate from a self-excited dynamo in the metallic core. Driving of the dynamo depends critically on the satellite's thermal state and internal structure. However, the inferred structure based on gravity data alone has a large uncertainty, and this makes the possibility of dynamo activity unclear; variations in core size and composition significantly change the heat capacity and alter the cooling history of the core. The main objectives of this study is to explore the structural conditions for a currently active dynamo in Ganymede using numerical simulations of the thermal history, and to evaluate under which conditions Ganymede can maintain the dynamo activity at present. We have investigated the satellite's thermal history using various core sizes and compositions satisfying the mean density and moment of inertia of Ganymede, and evaluate the temperature and heat flux at the core-mantle boundary (CMB). Based on the following two conditions, we evaluate the possibility of dynamo activity, thereby reducing the uncertainty of the previously inferred interior structure. The first condition is that the temperature at the CMB must exceed the melting point of a metallic core, and the second is that the heat flux through the CMB must exceed the adiabatic temperature gradient. The mantle temperature starts to increase because of the decay of long-lived radiogenic elements in the rocky mantle. After a few Gyr, radiogenic elements are exhausted and temperature starts to decrease. As the rocky mantle cools, the heat flux at the CMB steadily increases. If the temperature and heat flux at the CMB satisfy these conditions simultaneously, we consider the case as capable of driving a dynamo. Finally, we identify the Dynamo Regime, which is the specific range of internal structures capable of driving the dynamo, based on the results of simulations with various structures. If Ganymede's self-sustained magnetic field were maintained by thermal convection, the satellite's metallic core would be relatively large and, in comparison to other terrestrial-type planetary cores, strongly enriched in sulfur. The dynamo activity and the generation of the magnetic field of Ganymede should start from a much later stage, possibly close to the present. 相似文献
11.
A comparison of the internal structure of Earth-like planets is unavoidable to understand the formation and evolution of the solar system, and the differences between Earth’s, Mars’, and Venus’ atmospheres, surfaces and tectonic behaviors. Recent studies point at the role of core structure and dynamics in the evolution of the atmosphere, mantle and crust. On Earth, the crust thickness and the radius and physical state of the cores are known for almost one century, since the advent of seismological observations, but the lack of long-term surface-based geodetic, electromagnetic and seismological observations on the other planets, results in very large uncertainties on the crust thickness, on the temperature and composition of their mantle, and on the size and physical state of their cores. According to the currently available geodetic data, Mars’ dimensionless mean-moment-of-inertia ratio is equal to 0.3653±0.0008. When combined with geochemical observations and with the inputs of laboratory experiments on planetary materials at high pressure and high temperature, this result constrains a narrow range of density values for Mars’ mantle and favors a light [6200-6765 kg m−3] sulfur-rich core, but it still allows for a 1600-1750 km range for the core radius, i.e. an uncertainty at least ten times larger than the precision obtained in 1913 by Gutenberg for the Earth’s core. Mars’ mantle density distribution may be explained by a large range of temperatures and mineralogical compositions, either olivine- or pyroxene-rich. The unknown mean thickness of Mars’ crust makes necessary a number of working assumptions for the interpretation of gravimetric and magnetic data. The situation is worse for Venus, and the most conservative model of its deep interior is a transposition of the Earth’s structure scaled to Venus’ radius and mass. The temperature conditions at the surface of this planet hardly make possible long-term ground-based measurements, but this is indeed feasible at the surface of Mars. Precise measurements of Mars’ crust thickness, core radius and structure, and the proof of the existence or absence of an inner core, would put tight constraints on mantle dynamics and thermal evolution, and on possible scenarios leading to the extinction of Mars’ magnetic field about 4.0 Ga ago. Long-lasting surface-based geodetic, seismological and magnetic observations would provide this information, as well as the distributions as a function of depth of the density, elastic and anelastic parameters, and electrical conductivity. Current studies on the structure of Earth’s deep interior demonstrate that the latter data set, when constrained by laboratory experiments, may be inverted in terms of temperature, chemical, and mineralogical compositions. 相似文献
12.
Here, we calculate the mineralogy of the Martian lower crust and upper mantle as a function of pressure and temperature with depth using four bulk compositions (average crust, Gusev basalt, olivine‐phyric shergottite, and primitive average mantle). We then use this mineralogy to extract rock properties such as density and seismic velocities, describe their changes with varying conditions and geotherms, and make predictions for the crust–mantle boundary. Mineralogically, all compositions produce garnet, orthopyroxene, clinopyroxene in varying proportions at high pressures, with differences in minor minerals (spinel, ilmenite, rutile, and/or K‐feldspar). According to our calculations, the average crust and Gusev basalt compositions have the potential to yield higher densities than the average mantle composition, particularly for thicker crusts and/or colder geotherms. Therefore, recycling of the Martian crust into the mantle could occur through the process of crustal delamination, if not kinetically inhibited. However, our results show that, depending on crustal thickness, the crust may not be easily distinguishable from the mantle in seismic properties. 相似文献
13.
S. M. Molodensky 《Solar System Research》2005,39(1):54-72
In the first part of this study (Molodensky, 2004; hereinafter, paper I), a survey of the theory of tides and nutation of the Earth was given with the inclusion of the dynamical effects of the elastic mantle and liquid core in the framework of a very simple model of a homogeneous, incompressible core. Here, effects of the inhomogeneity, compressibility, and viscosity of the liquid core are considered, along with those of electromagnetic coupling of the liquid core with the mantle and solid inner core. Errors in the present-day measurements of the amplitudes of forced nutation (of the order of 20 arc s) are well below not only the dynamical effects of the Earths liquid core but also the effects of the inelasticity of the mantle, the dynamical effects of the solid inner core, and the possible effects of electromagnetic coupling between the liquid core, solid inner core, and mantle. This opens up new avenues for astrometric studies of the mechanical properties and electrical conductivity of the mantle and solid inner core at very low frequencies. The modern data on the amplitudes and phases of the Earths forced nutation cannot be interpreted entirely unambiguously, since the following factors remain unknown: (i) the role of the core-mantle electromagnetic coupling compared to the coupling due to core viscosity near the core-mantle boundary and (ii) the frequency dependence of the creep function of the mantle at low frequencies. In large measure, the effects of electromagnetic and viscous coupling can be separated if high-precision data on the tidal variations in the gravitational force at periods of about a day are invoked and allowances are made for the differences between the effects of viscous and electromagnetic coupling on the amplitudes and phases of forced nutation and on the tidal variations in the gravitational force. Here, ranges of possible values of the creep function are constructed for periods from one hour to one day; for these calculations, values consistent with the entire set of data on the forced nutation of the Earth are assumed for the effective dynamical flattening of the liquid core-mantle and liquid core-solid inner core boundaries (these values describe the ellipticity of the boundaries of the inner core, outer core, and mantle, as well as the electromagnetic coupling between the liquid core, mantle, and solid inner core).Translated from Astronomicheskii Vestnik, Vol. 39, No. 1, 2005, pp. 61–80.Original Russian Text Copyright © 2005 by Molodensky. 相似文献
14.
Estimates of the martian elastic lithosphere thickness Te imply that Te increased from around 20 km in the Noachian to about 70 km in the Amazonian period. A phase of rapid lithospheric growth is observed during the Hesperian and we propose that this elastic thickness history is a consequence of the martian crustal rheology and its thermal evolution. A wet crustal rheology is found to generate a mechanically incompetent layer in the lower crust during the early evolution and the rapid growth of Te during the Hesperian results from the disappearance of this layer due to planetary cooling. The incompetent layer and the related rapid lithospheric growth are absent for a dry basaltic crustal rheology, which is therefore incompatible with the observations. Furthermore, we find that the observed elastic thickness evolution is best compatible with a wet mantle rheology, although a dry mantle cannot be ruled out. It therefore seems likely that rheologically significant amounts of water were retained in the Martian crust and mantle after planetary accretion. 相似文献
15.
Shun-ichiro Karato 《Icarus》2011,212(1):14-229
The rheological properties of the mantle of super-Earths have important influences on their orbital and thermal evolution. Mineral physics observations are reviewed to obtain some insights into the rheological properties of deep mantles of these planets where pressure can be as high as ∼1 TPa. It is shown that, in contrast to a conventional view that the viscosity of a solid increases with pressure (at a fixed temperature), viscosity will decrease with pressure (and depth) when pressure exceeds ∼0.1 TPa. The causes for pressure-weakening include: (i) the transition in diffusion mechanisms from vacancy to interstitial mechanism (at ∼0.1 TPa), (ii) the phase transition in MgO from B1 to B2 structure (at ∼0.5 TPa), (iii) the dissociation of MgSiO3 into MgO and SiO2 (at ∼1 TPa), and (iv) the transition to the metallic state (at ∼1 TPa). Some (or all) of them individually or in combination reduce the effective viscosity of constituent materials in the deep interior of super-Earths. Taken together, super-Earths are likely to have low viscosity deep mantle by at least 2-3 orders of magnitude less than the maximum viscosity in the lower mantle of Earth. Because viscosity likely decreases with pressure above ∼0.1 TPa (in addition to higher temperatures for larger planets), deep mantle viscosity of super-Earths will decrease with increasing planetary mass. The inferred low viscosity of the deep mantle results in high tidal dissipation and resultant rapid orbital evolution, and affects thermal history and hence generation of the magnetic field and the style of mantle convection. 相似文献
16.
One of the great discoveries of NASA's Galileo mission was the presence of an intrinsically produced magnetic field at Ganymede. Generation of the relatively strong (750 nT) field likely requires dynamo action in Ganymede's metallic core, but how such a dynamo has been maintained into the present epoch remains uncertain. Using a one-dimensional, three layer thermal model of Ganymede, we find that magnetic field generation can only occur if the sulfur mass fraction in Ganymede's core is very low (?3%) or very high (?21%), and the silicate mantle can cool rapidly (i.e. it has a viscosity like wet olivine). However, these requirements are not necessarily compatible with cosmochemical and physical models of the satellite. We therefore investigate an alternative scenario for producing Ganymede's magnetic field in which passage through an eccentricity pumping Laplace-like resonance in Ganymede's past enables present day dynamo action in the metallic core. If sufficient tidal dissipation occurs in Ganymede's silicate mantle during resonance passage, silicate temperatures can undergo a runaway which prevents the core from cooling until the resonance passage ends. The rapid silicate and core cooling that follows resonance escape triggers dynamo action via thermal and/or compositional convection. To test the feasibility of this mechanism we couple our thermal model with an orbital evolution model to examine the effects of resonance passage on Ganymede's silicate mantle and metallic core. We find that, contrary to expectations, there are no physically plausible scenarios in which tidal heating in the silicates is sufficient to cause the thermal runaway necessary to prevent core cooling. These findings are robust to variations in the silicate rheology, tidal dissipation factor of Jupiter (QJ), structure of the ice shell, and the inclusion of partial melting in the silicate mantle. Resonance passage therefore appears unlikely to explain Ganymede's magnetic field and we must appeal to the special conditions described above to explain the presence of the field. 相似文献
17.
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
18.
Heterogeneous distribution of H2O in the Martian interior: Implications for the abundance of H2O in depleted and enriched mantle sources 
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下载免费PDF全文 Francis M. McCubbin Jeremy W. Boyce Poorna Srinivasan Alison R. Santos Stephen M. Elardo Justin Filiberto Andrew Steele Charles K. Shearer 《Meteoritics & planetary science》2016,51(11):2036-2060
We conducted a petrologic study of apatite within 12 Martian meteorites, including 11 shergottites and one basaltic regolith breccia. These data were combined with previously published data to gain a better understanding of the abundance and distribution of volatiles in the Martian interior. Apatites in individual Martian meteorites span a wide range of compositions, indicating they did not form by equilibrium crystallization. In fact, the intrasample variation in apatite is best described by either fractional crystallization or crustal contamination with a Cl‐rich crustal component. We determined that most Martian meteorites investigated here have been affected by crustal contamination and hence cannot be used to estimate volatile abundances of the Martian mantle. Using the subset of samples that did not exhibit crustal contamination, we determined that the enriched shergottite source has 36–73 ppm H2O and the depleted source has 14–23 ppm H2O. This result is consistent with other observed geochemical differences between enriched and depleted shergottites and supports the idea that there are at least two geochemically distinct reservoirs in the Martian mantle. We also estimated the H2O, Cl, and F content of the Martian crust using known crust‐mantle distributions for incompatible lithophile elements. We determined that the bulk Martian crust has ~1410 ppm H2O, 450 ppm Cl, and 106 ppm F, and Cl and H2O are preferentially distributed toward the Martian surface. The estimate of crustal H2O results in a global equivalent surface layer (GEL) of ~229 m, which can account for at least some of the surface features on Mars attributed to flowing water and may be sufficient to support the past presence of a shallow sea on Mars' surface. 相似文献
19.
Effect of chlorine on near‐liquidus crystallization of olivine‐phyric shergottite NWA 6234 at 1 GPa: Implication for volatile‐induced melting of the Martian mantle 下载免费PDF全文
下载免费PDF全文 Benjamin J. Farcy Juliane Gross Paul Carpenter Jacob Hicks Justin Filiberto 《Meteoritics & planetary science》2016,51(11):2011-2022
Martian magmas are thought to be rich in chlorine compared with their terrestrial counterparts. Here, we experimentally investigate the effect of chlorine on liquidus depression and near‐liquidus crystallization of olivine‐phyric shergottite NWA 6234 and compare these results with previous experimental results on the effect of chlorine on near‐liquidus crystallization of the surface basalts Humphrey and Fastball. Previous experimental results showed that the change in liquidus temperature is dependent on the bulk composition of the basalt. The effect of chlorine on liquidus depression is greater for lower SiO2 and higher Al2O3 magmas than for higher SiO2 and lower Al2O3 magmas. The bulk composition for this study has lower Al2O3 and higher FeO contents than previous work; therefore, we provide additional constraints on the effect of the bulk composition on the influence of chlorine on near‐liquidus crystallization. High pressure and temperature crystallization experiments were performed at 1 GPa on a synthetic basalt, of the bulk composition of NWA 6234, with 0–4 wt% Cl added to the sample as AgCl. The results are consistent with previous notions that with increasing wt% Cl in the melt, the crystallization temperature decreases. Importantly, our results have a liquidus depression ?T (°C) from added chlorine that is consistent with the difference in bulk composition and suggest a dependence on both the bulk Al2O3 and FeO content. Our results suggest that the addition of chlorine to the Martian mantle may lower magma genesis temperatures and potentially aid in the petrogenesis of Martian magmas. 相似文献
20.
Conventional evolutionary models for Mars adopt a dry mantle solidus. Taking into account the condensation conditions in the preplanetary nebula in the accretion zone of Mars, it can be concluded that large amounts of water or hydrated silicates have condensed in those regions. Therefore, water influences significantly the melting behaviour and the viscosity of the silicatic material. A model for the calculation of the thermal history of a planet is constructed. On this basis, and use of water — saturated solidus — it is possible to derive that the core is not liquid, as given in models employing a dry mantle solidus, but solid to a large extent, which prevents the operation of a large-scale dynamo and explains in that way the lack of a magnetic field. With these assumptions one can construct a possible evolutionary scheme that covers early crust differentiation, a hot thermal past and the missing magnetic field at present. 相似文献