摘要:

The production of magnetic fields by hypervelocity meteoroid impact has been proposed to explain the presence of paleomagnetic fields in some lunar samples as well as on the lunar surface. Impact‐generated magnetic fields also may be significant for the paleomagnetic record on a variety of cratered surfaces in the solar system, such as the Moon, Mercury, Phobos, and asteroids. Previous experiments with the two‐stage hydrogen light gas gun at the NASA Ames Vertical Gun Range demonstrated that hypervelocity impacts can produce impact‐generated magnetic fields by the expansion of an impact‐derived ionized vapor cloud (impact‐generated plasma).Here we further characterize impact‐generated magnetic fields and plasma with hypervelocity impact experiments at the NASA Ames Vertical Gun. Spherical projectiles (0.32 cm Fe, Cu; 0.64 cm Al, nylon) with velocities from 5.2 to 6.0 km/s were impacted at impact angles of 15°, 30°, 60°, and 90° (measured from horizontal) into powdered dolomite, silica sand, and aluminum plate. Three sets of experiments using search coils as magnetic detectors characterized impact‐generated magnetic fields as functions of ambient field strength and orientation, projectile/target composition, and impact angle. Experiments using Langmuir probes indicated a charged particle density (between 109and 1011ions/el cm−3) and an electron temperature (∼4500 K) of the impact‐generated plasma, the inferred source of impact‐induced magnetic fields. These new results demonstrate that impact‐generated magnetic fields at the laboratory scale exhibit spatial and temporal complexity dependent on impact angle, velocity, and projectile/target composition thereby suggesting that crater‐related paleomagnetism associated with this mechanism should exhibit similar complexity with spatial wavelengths on the order of a

ISSN:0148-0227    

DOI:10.1029/91JE02012

年代:1991

数据来源: WILEY

摘要:

The Fe2+ions in pyroxenes occurring in basalts on surfaces of terrestrial planets are vulnerable to oxidation, particularly in magma in contact with the atmosphere; this produces Fe3+‐bearing assemblages that may mask spectral features of remnant Fe2+in pyroxenes in telescopic reflectance spectra of oxidized planetary surfaces. To assess such interferences, reflectance spectra (0.3–2.6 urn) were obtained for a variety of pyroxenes that had been heated in air at elevated temperatures. Mossbauer spectral measurements enabled the relative proportions of Fe3+and Fe2+ions and Fe3+‐bearing phases to be determined in the reaction products. Nanophase hematite having particle diameters smaller than 10 nm obscures the pyroxene Fe2+/M2 site crystal field (CF) bands of oxidized enstatite and pigeonite. Intensification of Fe2+→ Fe3+intervalence charge transfer transitions in calcic clinopyroxenes mask pyroxene Fe2+/M1 site CF bands, as well as the Fe2+/M2 site CF bands of augites and subcalcic hedenbergites. The ferric iron derived from oxidation of Fe2+ions may remain as structural Fe3+ions in the oxidized calcic clinopyroxenes or exist as clusters of nanophase hematite particles having diameters of a few uni

ISSN:0148-0227    

DOI:10.1029/91JE01893

年代:1991

数据来源: WILEY

摘要:

Although magnetic field measurements have not been made on the surface of Venus or, for that matter, below the Pioneer Venus Orbiter (PVO) periapsis altitude of ∼150 km, the theory of the large‐scale ionospheric magnetization that results from the solar wind interaction can be used in concert with the available data to infer the existence of nonzero fields down to the surface. Here it is argued that magnetometer measurements in the upper part of the “diffusion region” of the ionosphere can be extrapolated to lower altitudes because of the particularly simple physics that determines the altitude profile of the large‐scale magnetic field in this region. The magnetic field at the bottom of the ionosphere inferred by this method using the PVO data is up to tens of nanoteslas when large‐scale ionospheric fields are present, suggesting the likely presence of fields of similar strength below. Given the negligible intrinsic field of Venus inferred from the PVO measurements, these fields must result from the solar wind interaction. The implications for Mars, which may have an analogous solar wind interaction, is that magnetometers on the surface could regularly observe varying fields of similar magnitude

ISSN:0148-0227    

DOI:10.1029/91JE01865

年代:1991

数据来源: WILEY

摘要:

The tectonics of the Tellus Regio highland of Venus have been examined using Pioneer Venus altimetry and gravity data and Venera 15/16 radar images. A thin elastic shell model has incorporated the Pioneer Venus data to calculate both global, long‐wavelength and regional, shorter‐wavelength stresses for various assumed values of crust, lithosphere, and mantle thickness and modes of compensation. The effects of these parameters on the calculated stress magnitudes have been examined. The model also allows calculation of crustal thickness and mantle density anomalies. The resulting calculated stress fields have been compared to the surface morphology observed in the Venera 15/16 radar images and interpreted in terms of the stress history. The best fitting parameters are consistent with minor amounts of lithospheric flexure being necessary to produce the observed surface features. The highland region is underlain by a thick (15–20 km) crust which is responsible for most of the topographic support. A positive density anomaly exists below this crust. The denser mantle may represent the down welling portion of a flow field, in which the horizontal flow feeding the downwelling has entrained mobile crustal material, dynamically maintaining a thickened crustal root near the stagnation point of the flow below Tellus Regio. This hot crustal material may have intruded the overlying cooler crust, causing lithospheric inflation and surface volcanism. This scenario is in agreement with the available gravity and radar data and can explain the general surface morphology of Tellus

ISSN:0148-0227    

DOI:10.1029/91JE01892

年代:1991

数据来源: WILEY

摘要:

New telescopic CCD multispectral imaging of the lunar near side and 330–870 nm spectroscopy of selected regions are presented as aids in estimating compositional differences relevant to locating potential lunar resources such as ilmenite (FeTiO3) and solar wind‐implanted3He and H. Conversion of 400/560 nm CCD ratio images to weight percent TiO2using a version of the Charette et al. (1974) empirical relation allows construction of a new TiO2abundance map for the lunar maria accurate to ±2 wt % HO2. This map provides a consistent view of TiO2distribution among mature mare soils and can be used to estimate areas potentially rich in ilmenite. Western Mare Tranquillitatis exhibits the highest TiO2abundances (>8 wt %), followed by regions near Flamsteed P and in northern Oceanus Procellarum. A 950/560 nm CCD ratio mosaic of the full Moon provides estimates of relative surface maturity on local scales as defined by the degree of agglutinate production due to micrometeorite impacts. Since high3He concentrations are correlated with mature ilmenite‐rich soils, a combination of relative surface maturity maps and TiO2abundance maps may be useful for estimating3He (and possibly H) distribution on local scales. While dark mantle materials are also potential sources of ilmenite,3He, and H, their compositional differences from mare soils prevent accurate remote mapping of their TiO2abundance and relative maturity. Reflectance spectra (relative to a more rigorously defined MS‐2 standard region) of dark mantle materials and high‐TiO2mare areas exhibit previously undocumented near‐UV (<350 nm) changes in spectral slope apparently related to compositional variations. Determination of the primary lunar sites for resource utilization will be dictated by the future goals and priorities of the lunar reso

ISSN:0148-0227    

DOI:10.1029/91JE02045

年代:1991

数据来源: WILEY

摘要:

A morphometric analysis of north–south trending graben at 35°N in northern Tharsis is used to provide the first quantitative measurements of crustal extension in and strain for areas of the Martian crust that have undergone both extreme and mild deformation. These estimates also demonstrate the viability of the Viking data for making such quantitative assessment Graben in the region accommodate ∼8 km of post Early Amazonian east–west extension. This extension corresponds to a net regional strain of 0.45 – 0.61%; local strains vary from<1% to 5%. Extension is nonuniform and localized near 110° (south of Alba Patera and north of Ceraunius Fossae) because of reactivation of preexisting fractures in underlying basement and possibly due to superposition of stress fields from Tharsis and Alba Patera. A major mechanical discontinuity in the shallow crust, probably representing the base of or a strength contrast within the megaregolith, is observed. It occurs at a depth of 2 km on the profile flanks and is depressed to about 7.5 km near 110° due to loading of the crust by the volcano. A number of more shallow discontinuities occur within the construct and probably represent lithologi

ISSN:0148-0227    

DOI:10.1029/91JE02005

年代:1991

数据来源: WILEY

摘要:

A relation for the temperature evolution of multicomponent porous ices is derived from the energy equation of multicomponent systems. It is demonstrated that the predominant energy transfer mechanisms can be incorporated into an effective thermal conductivity (λeƒƒ), which is strongly temperature dependent, and that the heat conduction equation can be written in a Fourier type form. The vapor of volatile ices significantly contributes to λeƒƒby the transfer of latent heat of sublimation but otherwise hardly affects the energy balance. The general results are applied to porous water ice, porous H2O–CO2ice and porous H2O–CO ice. In all cases it is found that the major part of the deposited energy per unit volume is consumed for heating of the ice. Depending on the material parameters and temperature, energy is transferred mainly by solid state conduction or by the transport of latent heat due to sublimation‐condensatio

ISSN:0148-0227    

DOI:10.1029/91JE01894

年代:1991

数据来源: WILEY