摘要:

A recent narrow‐band‐filtered CCD image by Mendillo et al. (1990) has shown that a sodium corona, produced near Io, extends at least 400 Jupiter radii in the planet's equatorial plane. Isophotes indicate that the polar to equatorial extents are in ∼1 to 3 proportions. The image can be reproduced by a model which includes both a high‐ and an intermediate‐speed distribution, with source rates of 2.2 and 1.1 × 1026atoms s−1, respectively. The high−speed distribution was ejected from Io with a velocity tangential to the satellite orbit of 57 km s−1(∼74 km s−1relative to Jupiter) plus an isotropic Maxwellian velocity distribution of ∼25 km s−1. This distribution likely corresponds to a charge exchange source of plasma torus sodium ions which are neutralized in the near‐Io atmosphere and are ejected relative to Jupiter with a corotational velocity (74 km s−1) plus a thermal ion (25 km s−1) Maxwellian distribution. The intermediate speed distribution was ejected from Io with a tangential speed near 20 km s−1(37 km s−1relative to Jupiter) plus an isotropic Maxwellian velocity distribution of ∼12 km s−1. This distribution corresponds to the same nonthermal sodium atoms earlier identified near Io in the sodium directional features (Pilcher et al., 1984). Modeled Doppler profiles of the sodiumDlines show that moderate resolution spectroscopy would prove to be highly diagnostic of both the velocity distribution of the source near Io and the resulting periodic time variations expected. On the scale of a few astronomical units, the sodium corona displays a cometary comalike appearance because of the acceleration of sodium atoms induced by solar radiation pressure. Finally, at distances of roughly 1000 planetary radii the effects of the ∼400‐hour photoionization lifetime of sodium atoms should begin to b

ISSN:0148-0227    

DOI:10.1029/91JE02505

年代:1991

数据来源: WILEY

摘要:

The presence of sulfur dioxide (SO2) on Io, together with the fact that the surface layer of Io has extremely high porosity, suggests the possibility of diffusion of this volatile within the surface, as well as exchange between the surface and an atmosphere. We investigate the former possibility through the development of a surface layer thermal model and subsequent calculations of the thermally driven diffusion flux of SO2within the layer. The major factors affecting the diffusion process are the temperature and temperature gradient in the surface layer throughout the day (which are results of the thermal model), and the porosity and grain size in the surface layer. Our results indicate that the net transport of SO2in the near‐surface region is downward into the subsurface, causing near‐surface depletion of SO2‐Near‐surface depletion would result in a layer of reduced thermal inertia overlying the bulk of the surface, consistent with thermal eclipse observations of Io. For our nominal model with 10‐μm grains and a porosity of 85%, the peak net diurnal downward flux reaches nearly 8×10−3g

ISSN:0148-0227    

DOI:10.1029/91JE02208

年代:1991

数据来源: WILEY

摘要:

The impulsive plasma wave bursts detected by the Pioneer Venus Orbiter electric field detector in the nightside ionosphere of Venus have been attributed to atmospheric lightning. However, it has also been argued that the wave bursts are generated locally by plasma instabilities. The waves associated with local instabilities are most probably electrostatic in nature, while lightning‐generated waves should be whistler mode waves, at least at the lowest frequencies. It may be possible to identify the wave modes through analysis of the wave polarization. We show that for typical ionospheric parameters the whistler mode wave electric field should be polarized predominantly perpendicular to the ambient magnetic field. However, the signals are often impulsive in nature, with durations much less than the spacecraft spin period, and an individual wave event is likely to be aliased since the event may occur at arbitrary spin phase and arbitrary intensity. Statistical analysis of the wave polarization using data acquired in the third nightside periapsis season shows that the 100‐Hz data suffer from interference, which may be due to the interaction of the spacecraft with the ambient plasma. The interference is removed through visual inspection of the data, and we show that the 100‐Hz waves are polarized perpendicular to the ambient magnetic field provided we restrict the data to those intervals in which the magnetic field is sufficiently far from horizontal to allow vertical propagation within the whistler mode resonance cone. The 100‐Hz waves detected outside of the resonance cone are polarized parallel to the magnetic field, as are the waves at higher frequency. The waves consequently fall into two classes: whistler mode waves which are most likely due to atmospheric lightning, since the low phase speed of the whistler mode argues against an in situ instability, and a mode that is polarized parallel to the ambient field. This latter mode may be analogous to the anomalous parallel polarized wave fields detected in the terrestrial ionosphere above thunde

ISSN:0148-0227    

DOI:10.1029/91JE02506

年代:1991

数据来源: WILEY

摘要:

The origins of the Sudbury Structure and associated Igneous Complex have been controversial. Most models call for a major impact event followed by impact‐induced igneous activity, although totally igneous models are still being proposed. Much of the controversy is due, in our opinion, to a misunderstanding of the size of the original Sudbury Structure. By analogy with other terrestrial impact structures, the spatial distribution of shock features and Huronian cover rocks at the Sudbury Structure suggest that the transient cavity was ∼100 km in diameter, which places the original final structural rim diameter in the range of 150–200 km. Theoretical calculations and empirical relationships indicate that the formation of an impact structure of this size will result in ∼104km3of impact melt, more than sufficient to produce a melt body the size of the Igneous Complex (present volume 4–8 × 103km3). For the Igneous Complex to be an impact melt sheet it must have a composition similar to that of the target rocks. Evidence for this has been presented previously for Sr and Nd isotopic data, which suggest a crustal origin. Here, we also present new evidence from least squares mixing models that the average composition of the Igneous Complex corresponds to a mix of Archean granite‐greenstone terrain, with possibly a small component of Huronian cover rocks. This is a geologically reasonable mix, based on the interpreted target rock geology and the geometry of melt formation in an impact event of this size. The Igneous Complex is differentiated, which is not a characteristic of previously studied terrestrial impact melt sheets. This can be ascribed, however, to its great thickness and slower cooling. That large impact melt sheets can differentiate has important implications for how the lunar samples and the early geologic history of the lunar highlands are interpreted. If this working hypothesis is accepted, namely, that both the Sudbury Structure and the Igneous Complex are impact in origin, then previous hybrid impact‐igneous hypotheses can be discarded and the Sudbury Structure can be studied specifically for the constraints it provides to large‐scale cratering and the formation of basin‐sized (multiring?)

ISSN:0148-0227    

DOI:10.1029/91JE02513

年代:1991

数据来源: WILEY

摘要:

The effective particle size of unconsolidated materials on the Martian surface can be determined from thermal inertia, due to a pore size dependence of thermal conductivity at Martian atmospheric pressures. Because dunes consist of a narrow range of well‐sorted, unconsolidated particles, they provide for a test of the relationship between particle size and thermal inertia calculated from midinfrared emission data for the Martian surface. We use two independent approaches. First, thermal inertia data indicate that Martian dunes have an average particle size of about 500±100 μm, or medium to coarse sand. Second, we determine expected dune particle sizes from grain trajectory calculations and the particle size transition from suspension to saltation. On Earth, the transition occurs for a grain when the ratio of the terminal fall velocity to the wind friction speed (u*t) is near unity; for grains atu*t, this occurs at about 52 μm. Terrestrial dune sands have a mean of 250 μm and are composed entirely of grains>52 μm. The corresponding Martian transition grain size is about 210 μm, suggesting that Martian dunes should be significantly coarser than terrestrial dunes. Grain saltation path length as a function of particle size also shows that under Martian conditions, larger grains than on Earth will become suspended. Both approaches indicate that Martian dune sand should be coarser than terrestrial dune sand. Thus, while terrestrial dune grains are in the fine to medium sand range, the average Martian dune sediments are probably medium to coarse sands. These results closely match the grain sizes determined from thermal inertia models, providing the first direct test of the validity of these models for actual Martian surface ma

ISSN:0148-0227    

DOI:10.1029/91JE02412

年代:1991

数据来源: WILEY

摘要:

A portion of Valles Marineris was mapped in detail in order to clarify the dominant processes responsible for the formation of Coprates Chasma. New crater counts indicate that the caprock on western Ophir Planum plateau has a Late Hesperian crater age, whereas trough floor preserved in western Coprates Chasma has an Early to Late Hesperian crater age. Caprock on western Ophir Planum correlates in relative age with the Syria Planum Formation, and the caprock may overlie Lower Hesperian ridged plains material. The conspicuous absence of wrinkle ridges on Ophir Planum may reflect burial by this later material. Trough floor material in Coprates Chasma correlates in relative age with ridged plains material on the adjacent Lunae Planum and Coprates plateaus and represents a structurally coherent block displaced downward by normal faulting. The crater counts and detailed structural relationships demonstrate the commonly accepted view that Coprates Chasma occupies a graben. The floor of this graben is only thinly mantled by intratrough deposits and is relatively intact; the trough shows no evidence for a chaotically broken and jumbled floor, as required by genesis models of collapse and subsurface drainage. Formation of Coprates Chasma by keystone collapse of locally elevated topography is not supported by available topographic data, but a general association between the trough and volcanotectonic activity in the Tharsis region is considered to be a likely explanation for the trough forming stresses. Subtle asymmetries in topography and boundary fault development across Coprates Chasma may imply a gentle, down‐to‐the‐north half‐graben geometry for the trough. Normal faults that define the northern margin of western Coprates Chasma and those that deform the adjacent Ophir Planum plateau change trend systematically with position, implying a 30° rotation of the local horizontal principal stresses in this region during trough growth. Faulting on Ophir Planum appears related to the faulting that created the Coprates Chasma trough. The geology and structure of Coprates Chasma are comparable to those of other troughs such as Melas, Ius, and perhaps Candor chasmata, suggesting that these troughs may also have formed as grabens. The revised crater age for western Ophir Planum indicates that the initiation of trough faulting and subsidence in the Valles Marineris system is early Late Hesperian and may have been synchronous across the system relative to the prevailing crater

ISSN:0148-0227    

DOI:10.1029/91JE02556

年代:1991

数据来源: WILEY

摘要:

Samples of lunar rocks and soils that have been examined in the laboratory both spectroscopically and chemically were analyzed with the goal of identifying and quantifying correlations between reflectance spectra and compositional properties of lunar materials. The method used to quantify spectral variations within the set of spectral samples is principal component analysis. The correlations between the chemical analysis of lunar samples and their spectral quantities are determined by using multivariate analysis. The combination of both methods yields functional relationships between the principal axes of variation and the chemical constituents, in particular, FeO, TiO2, SiO2, Al2O3, MgO, CaO, and MnO, and with the content of amorphous materials (glass) in lunar samples. To first order, the nature of these relationships is found to be linear. The results define a spectral‐chemical model that is applicable to derive abundances of chemical constituents and amorphous components from individual spectra. The accuracy of the model is better than ±2.4 wt % and ±4% for chemical constituents and amorphous components, respectively, making the model also sensitive to remotely sensed spectra of the lunar surf

ISSN:0148-0227    

DOI:10.1029/91JE02396

年代:1991

数据来源: WILEY

摘要:

Pyroxene reflectance and transmittance spectra have been examined in a search for systematic relationships between spectral features and compositional variations and to assess the applicability of reflectance Spectroscopy to pyroxene geothermometry. Orthopyroxenes containing up to ∼11 % Wollastonite show a positive correlation between Fe2+content and wavelength positions of the major absorption bands. Aluminum‐rich orthopyroxenes display absorption bands at lower than expected wavelengths. Spectral‐compositional relationships are more complex for clinopyroxenes, showing both positive and negative correlations between band positions and major cation abundances. These relationships are further complicated by the presence of significant amounts of other transition series elements such as Ti and Cr and by the presence of exsolved phases and compositional zonations. Even small‐scale exsolutions (<1 μm wide) seem to be sufficient to significantly alter spectral properties. Contours of the wavelength positions of band minima projected onto the pyroxene tetralateral generally exhibit concave downward shapes. The orientations of the contours relative to pyroxene geotherms are such as to effectively preclude the use of Spectroscopy to significantly constrain the temperatures of formation of p

ISSN:0148-0227    

DOI:10.1029/91JE02512

年代:1991

数据来源: WILEY

ISSN:0148-0227    

DOI:10.1029/91JE02471

年代:1991

数据来源: WILEY

ISSN:0148-0227    

DOI:10.1029/91JE02856

年代:1991

数据来源: WILEY