ISSN:0148-0227    

DOI:10.1029/94JE03386

年代:1995

数据来源: WILEY

ISSN:0148-0227    

DOI:10.1029/JE100iE01p01513

年代:1995

数据来源: WILEY

摘要:

The possibility of measuring the inclination of Mercury's spin axis and the amplitude of its 88‐day forced libration in longitude by a lander‐orbiter measurement system is investigated. The experiment would involve ranging to a small lander placed at a high latitude on Mercury from a satellite in a near‐circular polar orbit, which is accurately tracked from the Earth. These measurements can be used to determine the polar moment of inertia of the anticipated fluid outer core plus solid inner core, and thus to obtain information on the outer radius of the fluid core. Covariance analyses are carried out to assess the performance of a simulated data set consisting of 40 separate arcs with orbiter to lander ranging of about 1‐m accuracy and high‐accuracy Earth to orbiter ranging and Doppler tracking data. The effects of the uncertainties in a number of unestimated parameters are also considered. The force model includes the gravity field up to degree and order 14, solar tidal distortion of Mercury, and a fairly complex formulation for solar and Mercury radiation pressure on the spacecraft body and on the antenna. The total uncertainties for the estimated libration amplitude and the spin axis inclination are shown to be 0.26 and 0.03 arc seconds, respectively, for the model employed. The resulting uncertainty for the fractional moment of inertia of the core is 0.0045. Moreover, the Love numberk2can be determined with an accuracy of 0.01. Thus information on the elastic properties of Mercury's mantle also can be

ISSN:0148-0227    

DOI:10.1029/94JE02833

年代:1995

数据来源: WILEY

摘要:

A rift zone over 6000 km in total length runs along the border of Lada Terra, a highland in the southern hemisphere of Venus, and Lavinia Planitia, a basin that has been interpreted as a site of early‐stage mantle downwelling. Along the length of the rift are a number of volcanic centers of widely varying morphology and volcanic output. These include coronae, radially fractured domes, and large flow fields similar in scale to terrestrial flood basalts. We develop a model for the origin of extension related to passive rifting in response to stresses created by the adjacent downwelling. Volcanism and extension at other rifts on Venus, such as Devana Chasma, have been attributed to deep‐seated mantle plume activity. In contrast, we interpret the origin of extension and volcanism along the Lada rift to be linked to upwelling and decompression melting of mantle material due to rifting and, possibly, to counterflow associated with downwelling. Extension occurred generally prior to the formation of volcanic centers and the eruption of large‐scale flow fields, although most of the volcanic centers have been fractured by continued extension along the rift. Current debate over the formation of terrestrial flood basalts centers on the necessity of preexisting extension and stretched and thinned lithosphere to produce enhanced decompression melting within a large plume head or mantle thermal anomaly. Our studies of large‐scale flow fields associated with the Lada rift and coronae on Venus indicate that extension is a prerequisite for the formation of the majority of large‐scale flow units

ISSN:0148-0227    

DOI:10.1029/94JE02334

年代:1995

数据来源: WILEY

摘要:

The high vapor pressure of volatile metal halides and chalcogenides (e.g., of Cu, Zn, Sn, Pb, As, Sb, Bi) at typical Venus surface temperatures, coupled with the altitude‐dependent temperature gradient of ∼8.5 K km−1, is calculated to transport volatile metal vapors to the highlands of Venus, where condensation and accumulation will occur. The predicted geochemistry of volatile metals on Venus is supported by observations of Cu, Zn, Sn, Pb, As, Sb, and Bi minerals around terrestrial volcanic vents, spectroscopic observations of CuCl in volcanic gases at Kilauea and Nyiragongo, and large enrichments of these and other volatile elements in terrestrial volcanic aerosols. A one‐dimensional finite difference vapor transport model shows the diffusive migration of a thickness of 0.01 to>10 μm/yr of moderately to highly volatile phases (e.g., metal halides and chalcogenides) from the hot lowlands (740 K) to the cold highlands (660 K) on Venus. The diffusive transport of volatile phases on Venus may explain the observed low emissivity of the Venusian highlands, hazes at 6‐km altitude observed by two Pioneer Venus entry probes, and the Pioneer Venus entry probe anomalies

ISSN:0148-0227    

DOI:10.1029/94JE02708

年代:1995

数据来源: WILEY

摘要:

High spectral resolution imagery produced by imaging spectrometers enables the discrimination of geologic materials whose surface expression is subpixel in scale. Moreover, the use of such data makes it possible to distinguish materials which are characterized only by subtle differences in the spectral continuum. We define the “continuum” as the reflectance or radiance spanning the space between spectral features. The capability to distinguish subpixel targets will prove invaluable in studies of the geology of the Earth and planets from airborne and spaceborne imaging spectrometers. However, subpixel targets can only be uniquely identified in a truly optimal sense through the application of data reduction techniques that model the spectral contribution of both target and background materials. Two such techniques are utilized herein. They are a spectral mixture analysis approach and a low probability detection routine based on orthogonal subspace projection. These techniques were applied to the problem of detecting two different volcanic tuff units, one basaltic and one rhyolitic, in two different scenes of data measured by the airborne visible/infrared imaging spectrometer (AVIRIS). These tuff units have limited exposures from an overhead perspective and have spectral signatures which differ from those of background materials only in terms of subtle slope changes in the reflectance continuum. Of the two approaches, it was found that the low probability detection algorithm was more effective in highlighting those pixels that contained the target tuff units while suppressing the response of undesired background materi

ISSN:0148-0227    

DOI:10.1029/94JE02637

年代:1995

数据来源: WILEY

摘要:

Recent calculations of the Martian obliquity suggest that it varies chaotically on timescales longer than about 107years and varies between about 0 and 60°. We examine the seasonal water behavior at obliquities between 40 and 60°. Up to several tens of centimeters of water may sublime from the polar caps each year, and possibly move to the equator, where it is more stable. CO2frost and CO2‐H2O clathrate hydrate are stable in the polar deposits below a few tens of meters depth, so that the polar cap could contain a significant CO2reservoir. If CO2is present, it could be left over from the early history of Mars; also, it could be released into the atmosphere during periods of high obliquity, causing occasional periods of more‐clement cl

ISSN:0148-0227    

DOI:10.1029/94JE02801

年代:1995

数据来源: WILEY

摘要:

Thermal drag, a variant of the Yarkovsky effect, may act on small asteroids with sizes from a few meters to a few tens of meters. Yarkovsky thermal drag comes from an asteroid's absorbing sunlight in the visible and reradiating it in the infrared. Since the infrared photons have momentum, by action‐reaction, they kick the asteroid when they leave its surface. The reradiation, which is asymmetric in latitude over the asteroid, gives a net force along the asteroid's pole. Due to the asteroid's thermal inertia, averaging this force over one orbital period produces a net drag if the spin axis has a component in the orbital plane. A regolith‐free basaltic asteroid 60 m in radius can shrink its semimajor axis by 2 AU (the distance from the asteroid belt to the Earth) over the age of the solar system. Regolith‐free iron asteroids evolve at about half the rate of basaltic ones. These calculations ignore planetary perturbations, collisions, erosion, etc. The rate of evolution varies inversely with the asteroid's radius for the size range considered here, so that smaller objects evolve faster than larger ones. The rate‐radius relation fails for objects smaller than a few meters because the thermal skin depth becomes comparable to the size of the asteroid. Basaltic asteroids covered by regoliths more than a few centimeters deep evolve much more slowly than regolith‐free ones. Thermal drag tends to circularize orbits. It can increase or decrease orbital inclinations. An object whose spin axis points in random directions over its lifetime displays little change in orbital inclination. Thermal drag appears to have little to do with the delivery of chondrites from the asteroid belt; the thermal drag timescale (108years for meter‐sized objects) is long compared with their cosmic ray exposure ages, and aphelia in the asteroid belt are not expected for mature thermal drag orbits. However, Yarkovsky thermal drag may act on the recently discovered near‐Earth asteroids, which have radii of 10–30 m. Asteroid 1992 DA, for instance, might have its orbit shrunk by 0.1 AU in 3×107years, removing it from an Earth‐crossing orbit. The near‐Earth asteroids also tend to have small to moderate orbital eccentricities, as expected for highly evolved thermal drag objects. However, the time needed to bring them in from the asteroid belt (about 109years) is long compared with the collisional and dynamical lifetimes (both about 108years) for Earth‐crossing objects, arguing against their empla

ISSN:0148-0227    

DOI:10.1029/94JE02411

年代:1995

数据来源: WILEY