ISSN:0148-0227    

DOI:10.1029/93JE01899

年代:1993

数据来源: WILEY

摘要:

The first full‐disk radar images of Mercury were obtained on August 8 and 23, 1991. These images were constructed using the Very Large Array (VLA) in Socorro, New Mexico, to receive and map radar flux at 3.5 cm (Xband) which was continuously transmitted from the 70‐m Jet Propulsion Laboratory/Deep Space Network antenna at Goldstone, California. Approximately 77% of the surface was imaged, at resolutions as good as 150 km. About half of the hemisphere photographed by Mariner 10 was imaged, as well as most of the hemisphere which has not previously been photographed. At the time of the observations, the north pole was visible, and the feature with the highest same sense (SS) circular reflectivity in the images is near the nominal polar position. The peak SS reflectivity of this feature was 7.9%, and the circular polarization ratio throughout much of the feature is>1. Our best estimate of the size of the feature is that its diameter is ≲350 km. We interpret the feature to be indicative of the presence of ices because of the signal strength and polarization characteristics. The ices must be very clean and were thus probably deposited in a relatively short time period. The most likely place to find these ices is in any permanently shadowed areas in the polar regions, which would be very cold. In comparison to absolute reflectivities of other icy bodies and regions in the solar system, the reflectivity of the north polar feature is slightly depressed. This is probably because the ices do not fill an observational resolution cell (i.e., there is incomplete areal coverage), or are covered by a layer of dust or soil, which absorbs some of the incoming and outgoing radar energy, or both. A covering layer would also protect any ices from erosion by energetic sources. Other prominent features on the unphotographed hemisphere correspond to positions where atmospheric sodium enhancements have been measured from Earth. This may indicate that these locations are large basins similar to the Caloris basin, where an atmospheric potassium enhancement has been measured, which may be the result of increased degassing in the disrupted surface and near surface. Direct comparison of the radar echoes from our features with those from the Caloris basin are hard to make, as Caloris was not imaged by us in a favorable geometry. Features on the photographed hemisphere are smaller and mostly associated with craters and crater complexes, most notably the Kuiper crater and its env

ISSN:0148-0227    

DOI:10.1029/93JE01581

年代:1993

数据来源: WILEY

摘要:

Maxwell Montes, standing up to 7 km above the adjacent highland plateaus, constitute the highest mountain belt on Venus. Because the thickness of the crust is likely to be limited by the gabbro‐garnet granulite‐eclogite phase transitions, this relief is difficult to reconcile with the assumption of thermodynamic equilibrium and a standard Airy isostatic model. We explore the hypothesis that the crust‐mantle boundary is not in phase equilibrium, but rather is rate limited by the temperature‐dependent volume diffusion of the slowest ionic species. Under the simplifying assumption that the mountains formed by uniform horizontal shortening of the crust and lithospheric mantle at a constant rate, we solve the one‐dimensional thermal evolution problem. The time‐dependent density structure and surface elevation are calculated by assuming a temperature‐dependent reaction rate and local Airy isostatic compensation. For a rate of horizontal strain of 10−15s−1or greater, the rise in temperature at the base of the crust during mountain formation is modest to negligible, the deepening lower crust is metastable, and surface elevation increases as the crust is thickened. For strain rates less than 10−16s−1, in contrast, crustal temperature increases with time because of internal heat production, and the lower crust is more readily transformed to the dense eclogite assemblage. For such models a maximum elevation is reached during crustal shortening. While this maximum relief is 7 km or more for some models, a smaller density contrast between crust and mantle than assumed here (500 kg m−3) and incorporation of horizontal heat transport would lessen this value. We therefore favor formation of the mountain belt at a strain rate at least of order 10−15s−1. By this reasoning, Maxwell Montes must be comparat

ISSN:0148-0227    

DOI:10.1029/93JE01626

年代:1993

数据来源: WILEY

摘要:

New methods for estimating erosion at Meteor Crater, Arizona, indicate that continuous ejecta deposits beyond 1/4–1/2 crater radii from the rim (0.25R‐0.5R) have been lowered less than 1 m on the average. This conclusion is based on the results of two approaches: coarsening of unweathered ejecta into surface lag deposits and calculation of the sediment budget within a drainage basin on the ejecta. Preserved ejecta morphologies beneath thin alluvium revealed by ground‐penetrating radar provide qualitative support for the derived estimates. Although slightly greater erosion (2–3 m) of less resistant ejecta locally has occurred, such deposits were limited in extent, particularly beyond 0.25R‐0.5R from the present rim. Subtle but preserved primary ejecta features (e.g., distal ejecta lobes and blocks) further support our estimate of minimal erosion of ejecta since the crater formed ∼50,000 years ago. Unconsolidated deposits formed during other sudden extreme events (e.g., landslides) exhibit similarly low erosion over the same time frame; the common factor is the presence of large fragments or large fragments in a matrix of finer debris. At Meteor Crater, fluvial and eolian processes remove surrounding fines leaving behind a surface lag of coarse‐grained ejecta fragments that armor surfaces and slow vert

ISSN:0148-0227    

DOI:10.1029/93JE01580

年代:1993

数据来源: WILEY

摘要:

Wrinkle ridges of Arcadia Planitia were examined to determine their morphology, spatial distribution, and the amount of crustal shortening and strain they accommodate. Ridges trend generally northward, but their orientation and distribution are strongly controlled by the relief of the underlying knobby material. Ridges begin or end at inselbergs of older terrain and are associated with buried craters. Arcadia Planitia ridges have an average width of 3425 m and accommodate an average folding shortening of 3 m and a faulting shortening of 55 m; mean total shortening is 57 m. Three east‐west transects were constructed at 20° 25°, and 28°N to estimate regional shortening and strain. Average total shortening across the transects is about 900 m, corresponding to a regional compressive strain of 0.06%. The total shortening and compression across Arcadia Planitia are less than in Lunae Planum. Faults associated with the Arcadia ridges are inferred to have a westward dip compared with an eastward dip for Lunae Planum ridges. The generally levels of compression and symmetric orientation of the ridges suggest a regionally organized stress system. Differences in the shortening and strain between Lunae Planum and Arcadia Planitia may reflect the effects of local lithology or azimuthal variations in the stress f

ISSN:0148-0227    

DOI:10.1029/93JE01324

年代:1993

数据来源: WILEY

摘要:

A simple two‐reservoir degassing model for the Earth decribes rather well the current degassing rate of4He which is equal to (3±1)×106cm−2s−1according to data for the helium polar wind and the measured3He/4He ratio. This value of the helium degassing and loss rate provides an important constraint in modeling of noble gases, and some recent models do not fit this constraint. Scaling this value to the known amounts of40Ar in the atmospheres of Mars and the Earth and to U/K ratios in their surface rocks, this results in a crude estimate of the helium degassing rate on Mars, which is equal to 2.2×105cm−2s−1. Nonthermal escape of He is calculated using the daytime mean atmospheric models for low, mean, and high solar activity with He mixing ratio ƒHe= 1 ppm. Three processes contribute to He escape: (1) electron impact ionization and photoionization above the ionopause followed by solar wind sweeping away of the ions formed (1.3×105cm−2s−1), (2) collisions with hot oxygen atoms formed mainly by recombination of O2+(2.8×104cm−2s−1), (3) charge exchange of He+and CO2, N2, and CO between the exobase and ionopause (6×103cm−2s−1). The derived mixing ratio for He is ƒHe= 1.4 ppm for the adopted degassing rate. The He 584 Å airglow intensity is equal to 37 R and 67 R at low and high solar activity, respectively. The intensities are 105 R and 240 R for ƒHe= 7 ppm, and 10 R and 16 R for ƒHe= 0.3 ppm. These brightnesses should be detectable using the Extreme Ultraviolet Explorer satellite. The measurement of helium on Mars is important for the determination of the planetary abundance of U and Th, element differentiation in the primordial nebula (by comparison with the Earth and Venus) and in the interior of Mars (by comparison with the U and Th fraction in the surface rocks on Mars measured by the Mars 5 and Phobos orbiters). This measurement makes possible an estimate of the radiogenic heat flux which is important for the t

ISSN:0148-0227    

DOI:10.1029/93JE00534

年代:1993

数据来源: WILEY

摘要:

The atmospheres of rocky asteroids are unlikely to have ever been anything more than tenuous exospheres. However, it is possible that the densities of radiogenic heavy noble gases might have once been high enough to have implanted observable quantities in the regoliths that became meteorites. For this to have happened, a significant fraction of these species must have been photoionized and accelerated by the electromagnetic fields associated with the solar wind, rather than escaping thermally. Then, some fraction of the photoions would be accelerated into the asteroid's surface. Analytical and numerical results presented here suggest that acceleration of photoions by the solar wind motional field is a significant loss process for Xe on asteroids about 200 km in radius or larger, if the Xe is thermalized by its interactions with the surface. For Ar, photoion acceleration can only become important for asteroids nearly 500 km in radius. Thus photoion acceleration, previously invoked for lunar samples, could be responsible for excess fission‐produced Xe found associated with solar wind Xe in howardite meteorites. The lack of such Xe in other types of meteorites may reflect either smaller parent bodies or later times of regolith exposure. Similarly, the failure to observe solar‐wind‐associated radiogenic40Ar in meteorites is consistent with the much smaller likelihood that Ar will be photoio

ISSN:0148-0227    

DOI:10.1029/93JE01458

年代:1993

数据来源: WILEY

摘要:

The thermal history of cometary nuclei during residence in the Oort cloud is studied with the use of the very low thermal conductivity of amorphous ice recently obtained byKouchi et al. [1992a]. The heat sources included are (1) radioactive nuclides40K,232Th,235U, and238U with their chondritic abundances, and (2) latent heat released in transition from amorphous ice to crystalline ice. We model the cometary nucleus as a porous aggregate of grains with each individual grain being composed of a refractory core and an icy mantle. It is assumed that the ice is initially amorphous. The bulk thermal conductivity of a cometary nucleus is assumed to be expressed by the product of the thermal conductivity of individual grains and a reduction factor resulting from the porous structure of the nucleus. Numerical results of the thermal history are presented for various conditions including one case which includes heating by26Al decay. It is shown that the thermal histories are clearly classified into two distinct types depending mainly on the nucleus thermal conductivity κ. (1) Comets with small κ experience a runaway increase in the internal temperature to higher than 120 K during residence in the Oort cloud, in which case most of the ice in the nucleus crystallizes. (2) Comets with a sufficiently large κ on the other hand, do not exhibit a runaway heating and the temperature is limited to<100 K so that the initial amorphous ice is almost completely preserved. A criterion of nuclear ice crystallization is presented in an analytic expression derived from the analysis of the physical processes of the crystallization. A brief discussion is given on the implications of the results for the sources of volatile molecules observed in the co

ISSN:0148-0227    

DOI:10.1029/93JE01325

年代:1993

数据来源: WILEY

摘要:

Astronomical observations indicate that formation and destruction of dust mantles on cometary nuclei may be the cause for erratic and systematic variations of cometary activity, i.e. emission of dust. A laboratory experiment (KOSI‐9) has been performed to study the evolution of a dust mantle on top of a sublimating ice‐dust mixture in vacuum. A sample consisting of water ice with a 10% (by weight) admixture of olivine grains has been insolated in three periods at variable intensities from 200 to 1900 W/m2. Both increasing surface temperature of the sample and decreasing gas and particle emissions indicated the formation of a dust mantle during the first period. During the second insolation period after the gas flux had reached a critical value of a few 1021water molecules m−2s−1, avalanches of mantle material occurred on the inclined sample surface, broke up the mantle locally, and opened up a fresh icy surface. Enhanced ice and dust particle emission resumed for some time from these spots. A large number of the emitted dust particles were of a fluffy aggregate structure, i.e., they had large cross section to mass ratios compared to compact particles. During the third period the critical gas flux was not reached and no enhanced dust and ice emission was observed. A dry dust mantle of a few millimeters thickness developed during the course of the experiment. Consequences of these findings for cometary scenarios are di

ISSN:0148-0227    

DOI:10.1029/93JE01134

年代:1993

数据来源: WILEY

摘要:

Fall data indicate that a significant, elongate cluster of co‐orbital H chondrite falls in May between 1855 and 1895 (H Cluster 1) records encounters with two or three closely spaced and probably related meteoroid stream components, each of which was met near its perihelion. Although meteorites included in the Cluster vary widely in petrographic type (3–6), shock facies (a‐d), and21Ne exposure age (<5 to 50 Ma), they have a distinct labile trace element signature that confirms a common thermal history and, thus, a common source region within an H chondrite parent body. Hence, meteorites selected by one criterion (fall parameters) as distinguishable from all other H chondrites, are distinguished from them by another completely different criterion (contents of labile trace elem

ISSN:0148-0227    

DOI:10.1029/93JE01331

年代:1993

数据来源: WILEY