摘要:

There exist only a few reported measurements of quasi‐stationary (near dc) electric potentials over very large spatial scales (hundreds of kilometers or more) on the Earth's surface. Such measurements have typically been made using unpowered submarine telecommunications cables. The measurements pose unique experimental challenges and require careful procedures to avoid data contamination by electrode contact potentials and local ground currents. In addition, there are possible interpretational problems from pervasive, poorly understood, low‐frequency electric fields induced by ocean water motion through the Earth's stationary magnetic field. Nevertheless, estimates of the magnitude of the electric field computed from large‐scale potential difference measurements, made principally to date in the Pacific Ocean, can be used to place a limit on the size of the toroidal magnetic field at the core‐mantle boundary under certain conditions on the Earth's electrical conductivity profile. Thus, large‐scale electric potential measurements can serve as an adjunct probe of the Earth's dynamo process in addition to measurements of the poloidal magnetic field and its secular changes made at and above the surface of the Earth. A review of all of these data suggests that the toroidal and poloidal magnetic fields at the top of the core are comparable in

ISSN:0148-0227    

DOI:10.1029/93JE02548

年代:1993

数据来源: WILEY

摘要:

Infrared thermal emission spectroscopy is evaluated for its possible application to compositional mapping of the Moon's surface from lunar orbit. Principles of the mid‐IR (∼4–25 μm) technique, previous lunar ground‐based observations, and laboratory studies of Moon samples are reviewed and summarized. A lunar thermal emission spectrometer experiment is described, patterned after a similar instrument on the Mars Observer spacecraft. Thermal emission spectrometry from a polar‐orbiting lunar spacecraft could provide a valuable mapping tool to aid in exploration for lunar resources and help provide understanding of the origin of the Moon and history of lunar surface

ISSN:0148-0227    

DOI:10.1029/93JE02604

年代:1993

数据来源: WILEY

摘要:

The formation of a large volcano loads the underlying lithospheric plate and can lead to lithospheric flexure and faulting. In turn, lithospheric deformation affects the stress field beneath and within the volcanic edifice and can influence magma transport. Modeling the interaction of these processes is crucial to an understanding of the history of eruption characteristics and tectonic deformation of large volcanoes. We develop models of time‐dependent stress and deformation for the Tharsis volcanoes on Mars. By means of a finite element code, we calculate stresses and displacements due to a volcano‐shaped load emplaced on an elastic plate overlying a viscoelastic mantle. Models variously incorporate growth of the volcanic load with time and a detachment between volcano and lithosphere. The models illustrate the manner in which time‐dependent stresses induced by lithospheric plate flexure beneath the volcanic load may affect eruption histories, and the derived stress fields can be related to tectonic features on and surrounding Martian volcanoes. As a result of flexure there are three regions where stresses become sufficiently large to cause failure by faulting, according to the Mohr‐Coulomb criterion: at the surface of the plate just outward of the volcano, near the base of the elastic lithosphere beneath the center of the volcano, and on the upper flanks of the volcano early in its growth history. Normal faulting is the dominant mode of failure predicted for the first region, consistent with circumferential graben observed around the Tharsis Montes and with the scarp at the base of Olympus Mons, interpreted as a large‐offset, listric normal fault. Normal faulting, mostly radially oriented, is predicted for the second region. Failure in the third region is predicted to consist of thrust faulting, circumferentially oriented on the upper and middle flanks and radially oriented on the lower flanks. In models simulating a growing volcano, this portion of the edifice is subsequently covered by later units which exhibit lower stresses and are not predicted to fail; this volume of early failure remains the most highly stressed area in the edifice. Concentric terraces, interpreted by some workers as thrust faults, on the upper flanks of Olympus Mons may correspond to the predicted circumferential thrust features, if the most recent increments of volcano growth were relatively large, or in the presence of local material property or stress field variations. For volcanoes detached from the plate, predicted failure in the edifice takes the form of radial normal faulting near the volcano base. The addition of a local extensional stress arising from the regional topographic slope yields a pattern of predicted faulting which closely matches that observed on the Tharsis Montes, including the development of radial rifts on the lower volcano flanks to the northeast and southwest and the asymmetric formation of circumferential flank graben. This stress state is also consistent with an interpretation of the aureole deposits of Olympus Mons as the result of gravity sliding along a basal detachment. Our models also suggest an explanation for the lack of strike‐slip features, predicted by previously published flexural models, around the Tharsis volcanoes. For a given load increment, the first mode of near‐surface failure for most of the area immediately outward of the load is circumferential normal faulting and graben formation. As the volcano grows and the flexural response to the increasing load proceeds, the predicted failure mode in a portion of this annular region surrounding the volcano changes to strike‐slip faulting. Because normal faulting has been predicted to have taken place earlier, however, it is likely that release of later stresses will occur by reactivation and growth of these normal faults and graben rather than by the formation of new str

ISSN:0148-0227    

DOI:10.1029/93JE03093

年代:1993

数据来源: WILEY

摘要:

We have used automatic extraction of digital elevation models (DEMs) from stereo imagery of Mars to produce DEMs of parts of the Aeolis quadrangle of Mars. The method uses automated stereo matching based on an adaptive least squares correlation technique, which can be applied to Viking Orbiter imagery. A demonstration of the technique is presented, which focuses on the volcano Apollinaris Patera (186°W, 9°S), and the channel Ma'adim Vallis (183°W, 20°S). The topographic information derived is compared with previous determinations of topography, and a brief discussion of the relationship of the topography to the processes that have acted in the area is given. DEMs derived from stereomatching are an invaluable resource for terrain interpretation, and will provide a complementary technique to direct topographic measurements such as future missions to Mars will prov

ISSN:0148-0227    

DOI:10.1029/93JE03153

年代:1993

数据来源: WILEY

摘要:

We examine the conditions under which ions impacting the Hermean surface can act as a regional source of enhanced atmospheric column through ion implantation and subsequent release at the surface. At most latitudes, energetic (a few keV) Na or K ions which impact the nightside surface, are released quickly (well before noon) upon warming of the surface. We show that the relative sunrise/sunset difference produced by ion implantation is {[n]SR− [n]ss}/[n]Ave= ƒ, where ƒ is the fraction of the photo‐ions recycled, [n]SRis the average zenith column above the sunrise portion of the illuminated hemisphere, [n]ssis the average zenith column in the sunset portion, and [n]Aveis the average column over the sunlit disk. Thus, to produce a large sunrise/sunset difference via ion implantation and subsequent release requires efficient (close to total) recycling. We show that the most extensive set of available data reduced to Na column abundance does not show any sunrise enhancement. We argue that the K data do not permit an unambiguous interpretation in favor of sunrise/sunset differences. We further find that if an efficient surface loss process for the alkali is not operating after sunrise, the initial Na and K distribution will relax into the bulk of the solid. Preserved abundance gradients of Na and K in lunar glasses suggest that the Arrhenius coefficients for impact glasses are likely more modest than those for laboratory glasses derived from rock by nonimpact processes, but the effect of the more modest diffusion rates is only to delay the efficient loss of Na by a few Earth days. We argue that implantation can lead to observable regional increases in the observed Na or K column densities only if it occurs at very high latitudes, where diffusion is slow. It is typically lower energy ions which impact at high latitude and these are both more numerous than the high energy ions and possess smaller average penetration depths; thus there are additional reasons to favor a high‐latitude locus for any possible prompt return related increases in zenith column. We find that we cannot rule out sputtering as a source process, as the low‐energy ions are efficient sputterers. Finally, we argue that the observed Na/K ratio in the atmosphere may be the expression of their different asymptotic rates of loss from the interiors of the regolith grains. If so, this allows us to fix the importance of sputtering and photon stimulated desorption relative to impact vap

ISSN:0148-0227    

DOI:10.1029/93JE02617

年代:1993

数据来源: WILEY

ISSN:0148-0227    

DOI:10.1029/93JE01778

年代:1993

数据来源: WILEY

ISSN:0148-0227    

DOI:10.1029/93JE03419

年代:1993

数据来源: WILEY