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1.
A long-popular model for producing Ganymede's bright terrain involves flooding of low-lying graben with liquid water, slush, or warm, soft ice. The model suffers from major problems, however, including the absence of obvious near-surface heat sources, the negative buoyancy of liquid water, and the lack of a mechanism for confining the flows to graben floors. We present new models for cryovolcanic resurfacing to overcome these difficulties. Tidal heating within an ancient Laplace-like orbital resonance (Showman and Malhotra 1997, Icarus 127, 93; Showman et al., 1997, Icarus 129, 367) provides a plausible heat source and could allow partial melting to occur as shallow as 5-10 km depth. Our favored mechanism for delivering this water to the surface invokes the fact that topography—such as a global set of graben—causes subsurface pressure gradients that can pump water or slush upward onto the floors of topographic lows (graben) despite the negative buoyancy of the liquid. These eruptions can occur only within the topographic lows; furthermore, as the low areas become full, the pressure gradients disappear and the resurfacing ceases. This provides an explanation for the observed straight dark-bright terrain boundaries: water cannot overflow the graben, so resurfacing rarely embays craters or other rough topography. Pure liquid water can be pumped to the surface from only 5-10 km depth, but macroscopic bodies of slush ascending within fractures can reach the surface from much greater depths due to the smaller negative buoyancy of slush. A challenge for these models is the short predicted gravitational relaxation timescale of topographic features at high heat flows; the resurfacing must occur before the graben topography disappears. We also evaluate alternate resurfacing mechanisms, such as pumping of liquid water to the surface by thermal expansion stresses and buoyant rise of water through a silicate-contaminated crust that is denser than liquid water, and conclude that they are unlikely to explain Ganymede's bright terrain. 相似文献
2.
We characterize the lithospheric structure of Isidis Planitia on Mars by analyzing Mars Global Surveyor and Mars Odyssey gravity and topography data using a flexural model of a thin elastic shell including bending and membrane stresses. Isidis Planitia is a circular, relatively flat plain formed near the end of the Early Noachian, at the edge of the highlands-lowlands boundary and the site of a large free-air gravity anomaly, features consistent with modification and filling of an impact basin. Our results suggest that the bulk density of the fill material inside Isidis must be more than 2600 kg m−3 and higher densities are probable. A comparison of the faulting observed at Nili Fossae to the predicted zone of extensional strain northwest of Isidis constrains the thickness of the elastic lithosphere to be 100-180 km thick beneath the basin at the time of loading. We also find that loads outside of the basin play a significant role in the interpretation of the tectonics; simplified models tend to overestimate the lithospheric thickness. We place relatively narrow bounds on the thermal gradient (3.4-6.5 K km−1) and heat flux (13.6-26 mW m−2) at Isidis at the time of loading. 相似文献
3.
The martian polar caps feature large chasmata and smaller trough systems which have no counterpart in terrestrial ice sheets. Chasma Boreale cuts about 500 km into the western part of the north-polar cap, is up to 100 km wide and up to 2 km deep. One possible formation mechanism is by a temporary heat source under the ice due to tectono-thermal or volcanic activity, which melts the ice from below. It is demonstrated by model simulations that this process is feasible, a moderately increased heat flux of 0.5-1 W m−2, sustained over at least tens of thousands of years, producing a topographic depression which resembles the real chasma. Associated meltwater discharge rates are small (), but can exceed 10 km3 a−1 if a stronger heat flux of 10 W m−2 is assumed. Local ice-flow velocities during the process of chasma formation can exceed 1 m a−1 at the head and scarps of the chasma. However, if the thermal anomaly shuts down, glacial flow quickly decreases, so that the chasma can stay open for an indefinite amount of time without an ongoing, sustaining process under the climate conditions of the most recent millions of years. 相似文献
4.
From recent close encounters with Comets Wild-2 and Tempel 1 we learned that their surfaces are very rugged and no simple uniform layers model can be applied to them. Rather, a glaciological approach should be applied for describing their surface features and behavior. Such intrinsically rugged surface is formed in our large scale experiments, where an agglomerate of ∼200 μm gas-laden amorphous ice particles is accumulated to form a 20 cm diameter and few cm high ice sample. The density, tensile strength and thermal inertia of our ice sample were found to be very close to those found by Deep Impact for Comet Tempel 1: density 250-300 kg m−3 vs DI 350-400 kg m−3; tensile strength 2-4 kPa vs DI 1-10 kPa; thermal inertia 80 W K−1 m−2 s1/2 vs <100 W K−1 m−2 s1/2 and <50 W K−1 m−2 s1/2. From the close agreement between the thermal inertias measured in our ice sample, which had no dust coverage and that of Comet Tempel 1, we deduce that the low thermal inertia is an intrinsic property of the fluffy structure of the ice as a result of its low density, with an addition by the broken terrain and not due to the formation of a dust layer. Upon warming up of the ice, water vapor migrates both outward into the coma and inward. Reaching cooler layers, the water vapor condenses, forming a denser ice crust, as we show experimentally. We also demonstrate the inward and outward flow of water vapor in the outer ice layers through the exchange between layers of D2O ice and H2O ice, to form HDO. 相似文献
5.
Insight into the state of the early martian lithosphere is gained by modeling the topography above surface breaking thrust faults in the southern Thaumasia region. Crater counts of key surface units associated with the faulting indicate a scarp emplacement in the late Noachian-early Hesperian periods between 4.0 and 3.7 Gyr. The seismogenic layer thickness at the time of faulting is constrained to 27-35 km and 21-28 km for the two scarps investigated, implying paleo geothermal gradients of 12-18 and 15-23 K km−1, corresponding to heat flows of 24-36 and 30-46 mW m−2. The heat flow values obtained in this study are considerably lower than those derived from rift flank uplift at the close-by Coracis Fossae for a similar time period, indicating that surface heat flow is a strong function of regional setting. If viewed as representative for magmatically active and inactive regions, the thermal gradients at rifts and scarps span the range of admissible global mean values. This implies , with the true value probably being closer to the lower bound. 相似文献
6.
We have quantitatively assessed the resurfacing sources and styles in eighteen mapped venusian quadrangles, about 30% of the venusian surface. Each quadrangle was split into 0.5° by 0.5° boxes, which were then identified as corona materials, large volcano materials (>100 km diameter), intermediate volcano materials (10-100 km), small edifice materials (<10 km), flow materials from rifts or fractures, plains without an identifiable source, impact crater materials and highly deformed materials, or data gaps. We find that coronae resurface approximately 21%, small edifices 22% and large volcanoes about 6% of the surfaces analyzed. Plains with no identifiable source account for an average of 35% of the surface assessed. Small edifices resurface on a scale of 10-100 s of km2; large edifices resurface areas of 104-105 km2. Coronae have greatly varying amounts of associated volcanism, with some coronae producing negligible flow deposits and others producing deposits of 104-106 km2. The areas identified as plains with no visible source occur on small scales (102 km2) to large scales (> 105 km2). Our results indicate that the majority of plains resurfacing by volcanism can be tied to an identifiable source, that fields of small edifices contribute more to resurfacing than we had anticipated, and that resurfacing styles do not appear to have evolved over the time period represented by the surface geology in the mapped quadrangles. All of the units that we quantified occur throughout the histories of the regions mapped. We favor plains resurfacing to have occurred over at least 100 myr, which implies terrestrially reasonable resurfacing rates. 相似文献
7.
We suggest that the regions of smooth terrain which were observed on Comet 9P/Tempel 1 by the Deep Impact spacecraft were formed by blowing ice grains in an outburst of gas from the comet interior. When gas is released from 10 to 20 m deep layers which were heated to 135 K, it is released quiescently onto the surface by individual conduits. If large amounts of gas are released, the drainage system cannot release them fast enough and wider interconnected channels are formed, leading to sudden outburst of gas. Instability triggering a sudden shift of flow is well known in subglacial drainage of water. The ballistic trajectory of the ice particles reach a distance of 3 km in the atmosphereless comet, whose gravity is 0.034 cm s−1, if ejected at an angle of 45° at a speed of 95 cm s−1. This speed is close to the speeds measured in laboratory experiments: 167, 140×sini and 167 cm s−1, for particles of 0.3, 1000 and 14-650 μm, respectively. Blowing of ice grains can overcome the 1650 m long horizontal section of smooth terrain i1 (Fig. 1), whereas simple flow of material downhill would stop close to the foot of the hill. The ice particles at the end of their trajectory have a horizontal velocity component and this low velocity ballistic sedimentation would lead to formation of lineaments on the smooth terrain, like in solid-particulate volcanic eruptions. 相似文献
8.
Electromagnetic waves propagating transverse to the magnetic field, containing inhomogenous and loss cone plasma, may become unstable due to the excitation of resonant proton, resonant electron and drift cyclotron instabilities. Resonant proton instability gets excited in inhomogenous plasma, irrespective of the presence of temperature anisotropy, loss cone or temperature gradient. However, the growth rate of this instability is much smaller than the other two instabilities. The maximum growth rates of resonant electron instability are enhanced with the increase of loss cone index, gradients in transverse temperature and magnetic field, and with the decrease of temperature anisotropy and gradients in density and parallel temperature. The drift cyclotron instability exists in a bounded range of wave numbers and its growth rate increases with the increase of electron temperature, density and magnetic field gradient, and with the decrease of proton temperature and temperature anisotropy. In the region of ring current for beyond plasmapause the resonant proton and resonant electron instabilities have the characterstic frequencies around 0.1Ωp and growth rates ~10?6Ωp and 10?3Ωp, respectively. In the ring current region the drift cyclotron instability is not excited whereas in the plasma sheet region the frequency and growth rate of this instability are around Ωp and 10?2Ωp, respectively. These instabilities can accelerate the ring current particles along the magnetic field lines and dump them into the auroral region. 相似文献
9.
The MHD instabilities of a temperature-anisotropic coronal plasma are considered. We show that aperiodic mirror instabilities
of slow MHD waves can develop under solar coronal conditions for weak magnetic fields (B < 1 G) and periodic ion-acoustic instabilities can develop for strong magnetic fields (B > 10 G). We have found the instability growth rates and estimated the temporal and spatial scales of development and decay
of the periodic instability. We show that the instabilities under consideration can play a prominent role in the energy balance
of the corona and may be considered as a large-scale energy source of the wave coronal heating mechanism. 相似文献
10.
Javier Ruiz 《Icarus》2005,177(2):438-446
The heat flow from Europa has profound implications for ice shell thickness and structure, as well as for the existence of an internal ocean, which is strongly suggested by magnetic data. The brittle-ductile transition depth and the effective elastic thickness of the lithosphere are here used to perform heat flow estimations for Europa. Results give preferred heat flow values (for a typical geological strain rate of 10−15 s−1) of 70-110 mW m−2 for a brittle-ductile transition 2 km deep (the usually accepted upper limit for the brittle-ductile transition depth in the ice shell of Europa), 24-35 mW m−2 for an effective elastic thickness of 2.9 km supporting a plateau near the Cilix impact crater, and >130 mW m−2 for effective elastic thicknesses of ?0.4 km proposed for the lithosphere loaded by ridges and domes. These values are clearly higher than those produced by radiogenic heating, thus implying an important role for tidal heating. The ?19-25 km thick ice shell proposed from the analysis of size and depth of impact structures suggests a heat flow of ?30-45 mW m−2 reaching the ice shell base, which in turn would imply an important contribution to the heat flow from tidal heating within the ice shell. Tidally heated convection in the ice shell could be capable to supply ∼100 mW m−2 for superplastic flow, and, at the Cilix crater region, ∼35-50 mW m−2 for dislocation creep, which suggests local variations in the dominant flow mechanism for convection. The very high heat flows maybe related to ridges and domes could be originated by preferential heating at special settings. 相似文献
11.
Previously, radio Doppler data, generated with NASA's Galileo spacecraft during its second encounter with Jupiter's moon Ganymede, were used to infer the locations and magnitudes of mass anomalies on Ganymede using point-mass models. However, the point-mass solutions do not provide the vertical and horizontal extent of the anomalous mass concentrations. Here, we provide the results of a new study using spherical cap disks to model Ganymede's mass anomalies. The spherical cap disk models not only provide the locations and magnitudes of the mass anomalies, but also their vertical and horizontal dimensions. The new models show that three disks, a positive mass located at (53.0° N, 127.0° W) and two negative masses located at (22.0° N, 87.0° W) and (49.0° N, 219.0° W), can explain the data. The magnitudes of the mass anomalies are on the order of 1018 kg. The diameters of the anomalies are a few thousand kilometers. The positive anomaly is about 100 meters thick and both negative anomalies have a thickness of less than a kilometer. We use the additional information provided by the disk models to investigate the viability of mass anomalies at Ganymede's surface by comparing the diameters of the anomalies to the sizes of regiones and sulci and the anomalies' thicknesses to accumulated layers of rock and clean ice on the surface. We find that the dimensions of the mass anomalies could be explained by concentrations of rock in the regio and rock-free ice in the sulci. These results confirm that mass anomalies may reside on or near Ganymede's surface and that positive mass anomalies are contained within areas of dark terrain and negative mass anomalies within bright terrain. 相似文献
12.
Monte Carlo simulations are used to model the July 14, 2005 UVIS stellar occultation observations of the water vapor plumes on Enceladus. These simulations indicate that the observations can be best fit if the water molecules ejected along the Tiger Stripes in the South Polar region of Enceladus have a vertical surface velocity of 300-500 m/s at the surface. The high surface velocity suggests that the plumes on Enceladus originate from some depth beneath the surface. The total escape rate of water molecules is 4-6×1027 s−1, or 120-180 kg/s, consistent with previous works, and more than 100 times the estimated mass escape rate for ice particles. The average deposition rate in the South Polar region is on the order of 1011 cm−2 s−1, yielding a resurfacing rate as high as 3×10−4 cm/yr. The globally averaged deposition rate of water molecules is about one order of magnitude lower. 相似文献
13.
A mid-ocean-ridge spreading analog is used to constrain the opening rates and brittle-ductile transition depths for two axisymmetric ridged bands on Europa. Estimates of brittle-ductile transition depth based on the morphologies of Yelland and Ino Lineae are combined with a conductive cooling model based on a mid-ocean ridge analog to estimate the opening rates and active lifetimes of the bands. This model limits local strain rates to ∼10−15-10−12 s−1, opening rates to 0.2-40 mm yr−1, and active lifetimes of the bands to 0.1-30 Myr. If the observed structures in the outer portions of ridged bands are indeed normal faults, the estimated range for the tensile strength of ice on Europa is 0.4-2 MPa, consistent with nonsynchronous rotation as the dominant driving mechanism for band opening. 相似文献
14.
Ronen Jacovi 《Icarus》2008,196(1):302-304
Titan's haze, formed by photolysis of C2H2, C2H4 and HCN, was found experimentally to trap Ar, Kr and Xe with efficiencies of 3.5 × 10−4, 1.9 × 10−3 and 6.5 × 10−2 [noble gas atom]/[carbon atom] in the polymer, respectively. The rate of aerosol formation and settling down of 3 × 10−13 kg m−2 s−1, as inferred from our experiments on CH4 photolysis in the far UV [Podolak, M., Bar-Nun, A., 1979. Icarus 39, 272-276], is sufficient to reduce the mixing ratios of 36Ar and 40Ar to their low values of (2.8 ± 0.3) × 10−7 and (4.3 ± 0.1) × 10−3, respectively, and those of Kr and Xe to below the detection limit of 10−8. 相似文献
15.
Using an electron transport model, we calculate the electron density of the electron impact-produced nighttime ionosphere of Mars and its spatial structure. As input we use Mars Global Surveyor electron measurements, including an interval when accelerated electrons were observed. Our calculations show that regions of enhanced ionization are localized and occur near magnetic cusps. Horizontal gradients in the calculated ionospheric electron density on the night side of Mars can exceed 104 cm−3 over a distance of a few tens of km; the largest gradients produced by the model are over 600 cm−3 km−1. Such large gradients in the plasma density have several important consequences. These large pressure gradients will lead to localized plasma transport perpendicular to the ambient magnetic field which will generate horizontal currents and electric fields. We calculate the magnitude of these currents to be up to 10 nA/m2. Additionally, transport of ionospheric plasma by neutral winds, which vary in strength and direction as a function of local time and season, can generate large (up to 1000 nA/m2) and spatially structured horizontal currents where the ions are collisionally coupled to the neutral atmosphere while electrons are not. These currents may contribute to localized Joule heating. In addition, closure of the horizontal currents and electric fields may require the presence of vertical, field-aligned currents and fields which may play a role in high altitude acceleration processes. 相似文献
16.
The areas of volcanic units on Venus have been measured on the 1:5000000 geological maps published by NASA/USGS. These data were used to obtain a frequency-area distribution. The cumulative frequency-area distribution of 1544 specific occurrence of units cover six orders of magnitude from the largest unit (30 × 106 km2) to the smallest (20 km2). The probability distribution function has been calculated. The medium and large volcanic units correlate well with a power-law (fractal) relation for the dependence of frequency on area with a slope of −1.83. There are fewer small units than the expected values provided by the power-law relation. Our measurements cover 21.02% of the planetary surface, 3.59% of the study area was found to be tessera terrain and is excluded from this study of volcanism. The measurements were restricted to areas where geological maps have been published. The analysis was performed on two independent areas of the planet, with a complete coverage of published maps. In both areas the largest volcanic unit covers a significant portion of the surface (58.75% and 63.64%, respectively). For the total measured volcanic units (excluding tessera), these two largest units (that could correspond to the same unit or not) cover the 61.18% and they are stratigraphically superimposed on older volcanic units which cover 3.37% of the area. The remaining area (35.45%) is occupied by younger volcanic units stratigraphically superimposed on the large volcanic unit(s). These results are based on the independent mapping of a large number of geologists with different ideas about the geodynamical evolution of Venus and different criteria for geological mapping. Despite this fact, the presence of these very large units is incompatible with the equilibrium resurfacing models, because their generation at different ages would destroy the crater randomness. Our frequency-area distribution of the mapped volcanic units supports a catastrophic resurfacing due to the emplacement of the largest unit(s) followed by a decay of volcanism. Our data for the frequency-area distribution of volcanic units provide new support for catastrophic resurfacing models. It is difficult to make our observations compatible with equilibrium, steady-state resurfacing models. 相似文献
17.
Recent high-resolution Cassini images of the south polar terrain of Enceladus reveal regions of short-wavelength deformation, inferred to be compressional folds between the Baghdad and Damascus tiger stripes (Spencer, J.R., Barr, A.C., Esposito, L.W., Helfenstein, P., Ingersoll, A.P., Jaumann, R., McKay, C.P., Nimmo, F., Waite, J.H. [2009a]. Enceladus: An active cryovolcanic satellite. In: Saturn after Cassini-Huygens. Springer, New York, pp. 683-722). Here, we use Fourier analysis of the bright/dark variations to show that the folds have a dominant wavelength of 1.1 ± 0.4 km. We use the simple model of lava flow folding from Fink (Fink, J. [1980]. Geology 8, 250-254) to show that the folds could form in an ice shell with an upper high-viscosity boundary layer of thickness <400 m, with a driving stress of 40-80 kPa, and strain rate between 10−14 s−1 and 10−12 s−1. Such deformation rates imply resurfacing of the SPT in 0.05-5 Myr, consistent with its estimated surface age. Measurements of fold topography and more sophisticated numerical modeling can narrow down the conditions of fold formation and provide valuable constraints on the thermal structure of the ice shell on Enceladus. 相似文献
18.
We investigate the effects of strain localization on the formation of Ganymede’s grooved terrain by numerically modeling the extension of an ice lithosphere in which the yield strength of the ice decreases as the magnitude of the plastic strain increases. We do this to more realistically model fault strength, which is expected to vary with slip during initial fault development. We find that the inclusion of strain weakening leads to the formation of periodic structures with amplitudes of 200-500 m, consistent with the observed amplitudes of Ganymede’s large-scale grooves. The morphology of the deformation that results from extension depends both on the thermal gradient, which sets the lithospheric thickness, and on the rate at which the yield strength of the ice decreases with increasing plastic strain. Slow weakening with strain leads to low-amplitude, periodic structures, whereas moderate to rapid weakening with strain leads to large-amplitude, non-periodic structures. The combined influence of the thermal gradient and the weakening rate leads to the formation of complex surface deformation and may help explain the variety of surface morphologies observed within the grooved terrain. 相似文献
19.
In this paper we analyze near-infrared thermal emission spectra of the spatially resolved nucleus of Comet 9P/Tempel 1 obtained by the NASA spacecraft Deep Impact. Maps of spectral reddening, the product X′ between the beaming function and directional emissivity, as well as surface temperature are constructed. Thermophysical modeling is used to estimate the degree of small scale surface roughness and thermal inertia by detailed reproduction of the empirical temperature map. Mie and Hapke theories are used in combination with numerically calculated beaming functions to analyze the X′ map and place constraints on composition and grain size of the surface material. We show that it is absolutely mandatory to include small scale surface roughness in thermophysical modeling of this object, since the resulting self heating is vital for reproducing the measured temperatures. A small scale self heating parameter in the range 0.6?ξ?0.75 is common, but smoother areas where 0.2?ξ?0.3 are also found. Contrary to models neglecting small scale surface roughness, we find that the thermal inertia of Comet 9P/Tempel 1 generally is high (1000-3000 J m−2 K−1 s−1/2), although it may be substantially lower (40-380 J m−2 K−1 s−1/2) in specific areas. We obtain a disk-averaged reddening of 3.5% kÅ−1, with statistically significant local variations around that value on a ±1.0% kÅ−1 level. Vast regions appear covered by small (∼0.1 μm) highly absorbing grains such as carbon or iron-rich silicates. Other regions appear dominated by somewhat larger (∼0.5 μm) and/or less absorbing grains such as troilite or magnesium-rich silicates. Surface variations in reddening, roughness, thermal inertia, composition and/or grain size are moderately to strongly correlated to the locations of morphological units on the surface. The existence of morphological units with differing physical properties may be primordial, hence reflecting a diversity in the building block cometesimals, or resulting from evolutionary processes. 相似文献
20.
Thermal inertia derivation techniques generally assume that surface properties are uniform at horizontal scales below the footprint of the observing instrument and to depths of several decimeters. Consequently, surfaces with horizontal or vertical heterogeneity may yield apparent thermal inertia which varies with time of day and season. To investigate these temporal variations, we processed three Mars years of Mars Global Surveyor Thermal Emission Spectrometer observations and produced global nightside and dayside seasonal maps of apparent thermal inertia. These maps show broad regions with diurnal and seasonal differences up to 200 J m−2 K−1s−1/2 at mid-latitudes (60° S to 60° N) and 600 J m−2 K−1s−1/2 or greater in the polar regions. We compared the seasonal mapping results with modeled apparent thermal inertia and created new maps of surface heterogeneity at 5° resolution, delineating regions that have thermal characteristics consistent with horizontal mixtures or layers of two materials. The thermal behavior of most regions on Mars appears to be dominated by layering, with upper layers of higher thermal inertia (e.g., duricrusts or desert pavements over fines) prevailing in mid-latitudes and upper layers of lower thermal inertia (e.g., dust-covered rock, soils with an ice table at shallow depths) prevailing in polar regions. Less common are regions dominated by horizontal mixtures, such as those containing differing proportions of rocks, sand, dust, and duricrust or surfaces with divergent local slopes. Other regions show thermal behavior that is more complex and not well-represented by two-component surface models. These results have important implications for Mars surface geology, climate modeling, landing-site selection, and other endeavors that employ thermal inertia as a tool for characterizing surface properties. 相似文献