ISSN:0148-0227    

DOI:10.1029/JB095iB09p14087

年代:1990

数据来源: WILEY

摘要:

This dedicated issue of theJournal of Geophysical Researchcontains papers dealing with all aspects of Martian science. The impetus for the issue was the Fourth International Conference on Mars, held in Tucson on January 10–13, 1990. The third conference had been held 7 years earlier (seeJournal of Geophysical Research,87(B12), 1982); in the intervening time period, significant new analysis of the existing data has taken place, and further robotic exploration of Mars has been both planned and carried out. The papers contained in this issue reflect the mature nature of our understanding of the Martian environment, and they contain new results that significantly affect our view of the plane

ISSN:0148-0227    

DOI:10.1029/JB095iB09p14089

年代:1990

数据来源: WILEY

摘要:

A model is presented for the thermal evolution of the Martian mantle and core and for the evolution of the Martian magnetic field. In the model, Mars is initially hot and completely differentiated into a core and mantle, consistent with evidence from SNC meteorites of early core formation in Mars. The subsequent evolution of Mars consists of a simple cooling, with interior temperature, surface heat flux, and core heat flux declining monotonically with time. Lithosphere thickness and mantle viscosity increase steadily through time. Heat transport across the mantle is accomplished by subsolidus convection which is parameterized by a Rayleigh number‐Nusselt number relation. The core contains a light alloying constituent, assumed to be sulfur. Initial sulfur concentrationxSis a principal parameter controlling core and magnetic field evolution. Model parameters such as core radius are functions ofxSand are chosen consistent with the overall density and moment of inertia of Mars. The Martian mean moment of inertiaIis allowed to vary between 0.365Mpr2pand 0.345Mpr2p(Mpis the mass of Mars andrpis the radius of Mars), in accordance with recent inferences of the planet's moment of inertia from measurements of its gravitational oblateness. Large, S‐rich, low‐density cores or small, S‐poor, high‐density cores are consistent with the density and moment of inertia of Mars. For a givenxS, mantle density and core‐mantle boundary pressure increase with increasingI, while core radius and pressure at the center of the core decrease with increasingI. The decrease with time in mantle temperature and surface heat flow and the increase with time in lithosphere thickness are essentially independent ofxS. The occurrence of inner core solidification depends mainly onxSand mantle viscosity ν. For models in which inner core freezing takes place, present inner core radius increases with decreasingxSfor a given ν and also increases with decreasing ν at fixedxS. For mantle viscosity about 1016m2s−1, inner core solidification requires less than about 16 wt % S in the core. With increasing ν, the maximumxSfor which inner core freezing occurs decreases; for ν = 5 × 1016m2s−1inner core freezeout requiresxSless than about 10 wt %. Relatively small, S‐poor cores would be largely solid at present, while relatively large, S‐rich cores would be largely liquid at present. When inner core solidification occurs, the inner core grows rapidly at first and then more gradually as the core cools. The fraction of the core that is solid at present is almost independent of moment of inertia. Current estimates of the Martian dipole moment may be consistent with a small magnetic field driven by weak thermal convection in a completely fluid core. If Mars does not have a present magnetic field andxSis less than about 16 wt %, the explanation may lie in the nearly complete solidification of the core or in the nonoperation of the dynamo; however, ifxSis ≥ 16 wt %, then nonexistence of a magnetic field may be explained by the absence of an inner core or by the nonoperation of the dynamo in a thermally or

ISSN:0148-0227    

DOI:10.1029/JB095iB09p14095

年代:1990

数据来源: WILEY

摘要:

Numerical calculations of fully three‐dimensional convection in constant viscosity, compressible spherical shells are interpreted in terms of possible convective motions in the mantles of Venus and Mars. The shells are heated both internally and from below to account for radiogenic heating, secular cooling, and heat flow from the core. The lower boundary of each of the shells is isothermal and shear stress free, as appropriate to the interface between a mantle and a liquid outer core. The upper boundary of each of the shells is rigid and isothermal, as appropriate to the base of a thick immobile lithosphere. Calculations with shear stress‐free upper boundaries are also carried out to assess the role of the rigid surface condition. The ratio of the inner radius of each shell to its outer radius is in accordance with possible core sizes in both Venus and Mars. A calculation is also carried out for a Mars model with a small core to simulate mantle convection during early core formation. Different relative proportions of internal and bottom heating are investigated, ranging from nearly complete heating from within to almost all heating from below. The Rayleigh numbers of all the cases are approximately 100 times the critical Rayleigh numbers for the onset of convection. Cylindrical plumes are the prominent form of upwelling in the models independent of the surface boundary condition so long as sufficient heat derives from the core. Thus major volcanic centers on Mars, such as Tharsis and Elysium, and the coronae and some equatorial highlands on Venus may be the surface expressions of cylindrical mantle plumes. The form of the downwelling sheets is significantly affected by the rigid boundary in that the sheets are more irregular in their horizontal structure than when the top boundary is shear stress free. In the mainly heated‐from‐within models, the downwelling sheets are also shorter and less temporally durable when the top boundary is rigid than when it is stress free. Thus the free motion of plates on the Earth facilitates extensive durable convective currents that drive the plates, while the stiffening of the lithospheres on Mars and Venus promotes a style of convection that is not particularly effective in breaking the lithosphere into plates. In the rigid top cases, the upper boundary layer surrounding the plumes appears to be interspersed with downwelling currents emanating radially from the plumes' axes; these currents may establish a stress field at the base of hotspot swells that could lead to radial fractures such as those on Tharsis. Models with rigid upper boundaries have higher interior temperatures than do similar models with shear‐stress‐free upper boundaries. On this basis, Venus not only has a higher surface temperature than Earth, but it would have a hotter mantle as well. Upwelling plumes are more numerous when the outer boundary is rigid (as compared with shear stress free). Flows dominated by a few stong plumes occur when the proportion of basal heating is large. Thus if the Martian crustal dichotomy were caused by a convective system dominated by spherical harmonic degree ℓ = 1, then the convection may have been driven strongly from below by a heating pulse accompanying core formation or from the flow of heat from an early hot core. If a convective mechanism is responsible for the crustal dichotomy, then the dichotomy is likely a very ancient feature. However, the small core models we consider consistently produce a convective pattern with a dominant ℓ = 2 signature which does not correlate with the Martian crustal dichotomy; a yet smaller core may be necessary to yield the ℓ = 1 pattern. The small core convective pattern does correlate with the geoid and topography signatures of Tharsis, which have strong ℓ = 2 components, and the model produces dynamic uplift comparable to the total topography of the Tharsis rise. Models with larger cores (i.e., with the probable inner to outer radius ratio of Mars' present mantle) generate 4 km of uncompensated topography, similar to estimates of the uncompensated Tharsis topography. Thus the Tharsis rise could have achieved its full height early in the evolution of Mars by mantle plume dynamic uplift. At present, the uncompensated portion of the Tharsis rise topography can be accounted for by dynamic uplift, obviating the need for elastic support. The present compensated portion of the Tharsis topography could be attributed to volcanic or magmatic crustal thickening or depletion of the underlying mantle. Large core models appropriate to the present Mars produce too many plumes to account for just two major volcanic centers (i.e., Tharsis and Elysium). Mantle plume activity could be focussed beneath regions like Tharsis if fracturing or thinning of the lithosphere in these regions has facilitated magma and heat transport across the lithosphere. A similar mechanism could be responsible for clustering of coronae on Venus. There are no deep‐seated, active, linear upwellings in the Venus models that could be associated with linear spreading centers in Aphrodite. If linear spreading is actually occurring in Aphrodite, the phenomenon is probably a shallow passive one, similar to mid

ISSN:0148-0227    

DOI:10.1029/JB095iB09p14105

年代:1990

数据来源: WILEY

摘要:

The mean density and mean moment of inertia are remotely accessible geodetic parameters which provide two integral constraints on the radial density profile of Mars. The mean density is presently well known, but until the axial precession rate is better determined, the moment of inertia will remain uncertain. Values anywhere in the interval from 0.345 MR2to 0.365 MR2can be obtained from the observed gravitational field, depending on how that field is partitioned into hydrostatic and nonhydrostatic components. Even if the moment were accurately known, a considerable degree of nonuniqueness would still remain concerning the density and composition. This nonuniqueness is illustrated by construction of a family of orthogonal polynomials in the normalized radius which make no contribution to either the mean density or the mean inertial moment. If the mean moment is near to the lower end of its plausible range, the nonuniqueness in density and composition is considerably diminished. In order to avoid unrealistically high densities in the deep interior, the Martian mantle must have a density appreciably lower than the terrestrial mantle and must consequently be significantly depleted in iron. On the other hand, if the mean moment is near the upper end of its plausible range, a much broader range of density profiles can be accommodated, including (but not restricted to) iron enriched mantles.

ISSN:0148-0227    

DOI:10.1029/JB095iB09p14131

年代:1990

数据来源: WILEY

摘要:

The primary objective of this study is to explore the range of uncertainty in the obliquity history of Mars associated with the present uncertainty in the axial precession rate. The obliquity, or angular separation between the spin axis and the orbit normal, is the most important parameter for determining the seasonal and latitudinal pattern of insolation. Thus significant variations in obliquity are a likely source of major climatic variations on Mars. The present obliquity is well known, and the torques acting to reorient the spin axis of Mars can be readily calculated for time spans of order 107years into the past (or future). The primary limitation to reconstructing the obliquity history is uncertainty in the mean moment of inertia of Mars, which governs its response to the applied torques. The range of axial precession rates corresponding to recent theoretical estimates of the moment of inertia is 8.29–8.77 arc sec/yr, but even the most recent observational limits are still much broader: 8–12 arc sec/yr. Nominal estimates of the axial precession rate suggest that resonant amplification of a number of small terms in the orbital inclination series will lead to significant variations in the obliquity of Mars, a behavior much different from the Earth. The major variations are on a 105year time scale, with significant amplitude modulation on a 106year time scale. Because of this resonant amplification, estimates of the obliquity history depend very sensitively on assumed values for the axial precession rate. Three different analytic techniques are applied to the obliquity problem. Both linear perturbation analysis and direct numerical integration of the governing differential equations can be used to obtain an obliquity time series, once a model value is selected for the moment of inertia. The linear solution, because of its simplicity, provides useful insight into the amplification of near‐resonant forcing, but fails in the case of exact resonance. The secular orbital theory of Laskar, which is complete to fifth order in eccentricity and inclination, has been used to integrate numerically a suite of 501 different obliquity histories, each spanning the 20‐m.y. interval centered on the present. Each of the computed histories corresponds to a different axial precession rate. Another approach, which utilizes the adiabatic invariance of action variables associated with the spin evolution, gives additional insight into the obliquity behavior expected during passage through resonance but does not yield an actual obliquity time series. On approach to resonance, the amplitude of obliquity oscillations increases dramatically, and on passage through resonance, the mean value about which the oscillations occur changes abruptly. Clearly, passage through a resonance involves either a change in the resonant frequency, which is determined by the internal mass distribution of Mars, or a change in the effective forcing frequency, due either to actual changes in the orbital configurations of the perturbing planets or merely to interference effects among closely spaced forcing frequencies. It has previously been hypothesized that the formation of Tharsis and/or core differentiation may have driven the spin axis precession rate through resonance with one of the orbital inclination eigenfrequencies early in the history of Mars, giving rise to very large obliquity oscillations. A major result of the present analysis is the observation that, within the plausible range of present axial precession rates, a very wide range of obliquity histories are possible, including some which involve resonance passages within the relatively recent past. As a result, obliquities as high as 51.4°, or as low as 0.2°, may have occurred within the la

ISSN:0148-0227    

DOI:10.1029/JB095iB09p14137

年代:1990

数据来源: WILEY

摘要:

The Mars Observer (MO) Mission, in a near‐polar orbit at 360–410 km altitude for nearly a 2‐year observing period, will greatly improve our understanding of the geophysics of Mars including its gravity field. To assess the expected improvement of the gravity field, we have conducted an error analysis based upon the mission plan for the Mars Observer radio tracking data from the Deep Space Network. Our results indicate that it should be possible to obtain a high‐resolution model (spherical harmonics complete to degree and order 50 corresponding to a 200‐km horizontal resolution) for the gravitational field of the planet. This model, in combination with topography from MO altimetry, should provide for an improved determination of the broad scale density structure and stress state of the Martian crust and upper mantle. The mathematical model for the error analysis is based on the representation of doppler tracking data as a function of the Martian gravity field in spherical harmonics, solar radiation pressure, atmospheric drag, angular momentum desaturation residual acceleration (AMDRA) effects, tracking station biases, and the MO orbit parameters. Two approaches are employed. In the first case, the error covariance matrix of the gravity model is estimated including the effects from all the nongravitational parameters (noise‐only case). In the second case, the gravity recovery error is computed as above but includes unmodelled systematic effects from atmospheric drag, AMDRA, and solar radiation pressure (biased case). The error spectrum of gravity shows an order of magnitude of improvement over current knowledge based on doppler data precision from a single station of 0.3 mm s−1noise for 1‐min integration intervals during three 60‐day periods. This first approach of noise only yielded an estimated total accuracy (omission plus commission) for a 200 km block size of 5.4 m for geoid undulations and 31 mGal for gravity anomalies. For the second case, corresponding to the unmodelled systematic effects, an additional error of 5% in the above statistics was obtained. Although the degradation in the accuracy of the mean gravity anomalies and mean geoid undulations is not very pronounced, significant degradation in the recovery of the harmonic coefficients was observed due to the unmodelled systematic nongravitational effects (mainly atmospheric drag). A worst result occurred for an individual 60‐day period, corresponding to maximum atmospheric drag, which gave significant degradation for the low‐degree terms out through degree 20. However, the overall accuracy for the combined solution of the three 60‐day periods for the second case of systematic effects gave only a small difference from the noise‐only solution. The results suggest that the spacecraft orbit could possibly be raised in altitude without significant loss of gravitational signal, because the atmospheric drag is the dominant error source. A change in altitude could also alleviate the large effects seen in the spectrum of the satellite resonant orders. A conservative error estimate for the gravity recovery, corresponding to the complete mapping period, was made based upon a simplified approach of scaling systematic and random effects so as to correspond to each orbit orientation period. These contributions of each orbit orientation were combined throughout the entire mapping mission to obtain the overall accuracy esti

ISSN:0148-0227    

DOI:10.1029/JB095iB09p14155

年代:1990

数据来源: WILEY

摘要:

The presence of komatiitic igneous rocks on Mars, based on geochemical evidence from SNC meteorites and Viking X ray fluorescence analyses of the regolith, suggests that massive and disseminated iron sulfide mineralization occurs near the Martian surface. Analogies are drawn between possible ultramafic Fe‐Ni sulfides on Mars and terrestrial pyrrhotite‐pentlandite ore deposits associated with Archean komatiites formed during early crustal development on Earth. Partial melting of the mantle as a result of high radiogenic heat production then, extrusion of turbulent high‐temperature ultramafic lavas, segregation of immiscible FeS melts during cooling, gravitational settling and fractional crystallization of sulfide minerals in magma chambers or lava flows produced massive and disseminated sulfide mineralization associated with terrestrial komatiites. Comparable processes probably occurred on Mars where, on account of the inferred higher Fe/(Fe + Mg) ratio of the X ray mantle (estimated to contain ∼4.5 wt % S), iron‐rich basaltic magmas were produced by partial melting at depths and temperatures exceeding 165 km and 1400°C, respectively. Adiabatic diapiric emplacement of these iron‐rich, very low viscosity basaltic melts transported significant concentrations of dissolved sulfur as S2−and HS−from the mantle. Ensuing sulfide mineralization may have been either thinly disseminated within ultramafic lavas erupting over large areas of Mars or concentrated locally at the base of structural depressions. Cumulate ore deposits several meters thick may occur at the base of intrusions or in near‐surface magma chambers. The evidence for insignificant plate tectonic activity on Mars and minimal interactions of Martian mantle with crust, hydrosphere and atmosphere has restricted the evolution of sulfide ore deposits there. Thus terrestrial porphyry copper and molybdenum deposits, granite‐hosted mineralization, and related continental crust‐derived ores, including PbS‐ZnS deposits in sedimentary rocks, are unlikely to have formed on Mars. Ultramafic Fe‐Ni sulfides and perhaps iron‐rich sediments (gossans and abiotic banded iron formations) derived from chemical weathering of the basaltic crust, as well as cumulate chromites, are likely to be the only

ISSN:0148-0227    

DOI:10.1029/JB095iB09p14169

年代:1990

数据来源: WILEY

摘要:

Multiring impact basins have profoundly influenced the geologic evolution of Mars. We compile and summarize the evidence for Martian impact basins and suggest eight new examples. Multiring basins on Mars define three morphologic subclasses with increasing basin size. Basins having diameters 300

ISSN:0148-0227    

DOI:10.1029/JB095iB09p14175

年代:1990

数据来源: WILEY

摘要:

The shapes and densities of crater size‐frequency distribution curves are used to constrain two major events early in Martian history: termination of high obliteration rates and viability of the multiple impact origin of the crustal dichotomy. Distribution curves of fresh craters superposed on uplands, intercrater plains, and ridged plains display shapes and densities indicative of formation prior to the end of heavy bombardment. This observation correlates with other geologic evidence, suggesting a major change in the erosional regime following the last major basin size impact (i.e., Argrye). In addition, the multisloped nature of the curves supports the idea that the downturn in the crater size‐frequency distribution curves reflects the size‐frequency distribution of the impactors rather than being the result of erosion. The crustal dichotomy formed prior to the heavy bombardment intermediate epoch based on distribution curves of knobby terrain; if the dichotomy resulted from a single gigantic impact, this observation places constraints on when this event happened. An alternate theory for dichotomy formation, the multiple‐impact basin idea, is questioned: since distribution curves of large basins as well as heavy bombardment era units are not represented by a −3 differential power law function, this study finds fewer basins missing on Mars compare to the Moon and Mercury than previously reported. The area covered by these missing basins is less than that covered the northe

ISSN:0148-0227    

DOI:10.1029/JB095iB09p14191

年代:1990

数据来源: WILEY