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At least 163 records · Page 9Linked to original sources
USGS1987

Thermal evolution of a differentiated Ganymede and implications for surface features

Thermal evolution models are presented for Ganymede, assuming a mostly differentiated initial state of a water ocean overlying a rock layer. The only heat sources are assumed to be primordial heat (provided by accretion) and the long-lived radiogenic heat sources in the rock component. As Ganymede cools, the ocean thins, and two ice layers develop, one above composed of ice I, and the other below composed of high-pressure polymorphs of ice. Subsolidus convection proceeds separately in each ice layer, its transport of heat calculated using a simple parameterized convection scheme and the most recent data on ice rheology. The model requires that the average entropy of the deep ice layer exceeds that of the ice I layer. If the residual ocean separating these layers becomes thin enough, then a Rayleigh-Taylor-like (“diapiric”) instability may ensue, driven by the greater entropy of the deeper ice and merging the two ice mantles into a single convective layer. This instability is not predicted by linear analysis but occurs for plausible finite amplitude perturbations associated with large Rayleigh number convection. The resulting warm ice diapirs may lead to a dramatic “heat pulse” at the surface and to fracturing of the lithosphere, and may be directly or indirectly responsible for resurfacing and grooved terrain formation on Ganymede. The timing of this event depends rather sensitively on poorly known rheological parameters, but could be consistent with chronologies deduced from estimated cratering rates. Irrespective of the occurrence or importance of the heat pulse, we find that lithospheric fracturing requires rapid stress loading (on a time scale ⪅10 4 years). Such a time scale can be realized by warm ice diapirism, but not directly by gradual global expansion. In the absence of any quantitative and self-consistent model for the resurfacing of Ganymede by liquid water, we favor resurfacing by warm ice flows, which we demonstrate to be physically possible, a plausible consequence of our models, compatible with existing observations, and a hypothesis testable by Galileo. We discuss core formation as an alternative driver for resurfacing, and conclude that it is less attractive. We also consider anew the puzzle of why Callisto differs so greatly from Ganymede, offering several possible explanations. The models presented do not provide a compelling explanation for all aspects of Ganymedean geological evolution, since we have identified several potential problems, most notably the apparently extended period of grooved terrain formation (several hundred million years), which is difficult to reconcile with the heat pulse phenomenon.

Icarus↗
USGS1987

Large-scale volcano-ground ice interactions on Mars

The process of volcano-ground ice interaction on Mars is investigated by thermodynamic calculations and observations of Viking Orbiter images. We develop a numerical model of volcano-ground ice interaction that includes heat transport by conduction, radiation from the surface, heat transfer to the atmosphere, and H 2 O phase changes in an ice-rich permafrost. We consider eruption of lava flows over permafrost, and intrusion of sills into permafrost. For eruption of lava over permafrost, most of the heat in the flow is lost by radiation and atmospheric effects. The amount of H 2 O liquid and vapor produced is small, and its removal would not be sufficient to cause collapse that would lower the surface of the lava flow below the surrounding terrain. For intrusion of a sill, most of the heat in the sill eventually goes into H 2 O phase changes, producing much larger amounts of water that could have profound geomorphic and geochemical effects. Approximate meltwater discharge rates are calculated for both extrusive and intrusive interactions. We examine two large regions of large-scale volcano-ground ice interactions. Near Aeolis Mensae, intrusion of a complex of dikes and sills into ice-rich ground has produced substantial melting, with mobilization and flow of material. This interaction probably also produced large quantities of palagonite tuff and breccia. Morphologic evidence for progressive fluidization implies that meltwater was stored beneath the surface for some time, and that most of the release of water and volcanic mudflow took place late in the interaction. Northeast of Hellas, several large channels emanate from the area near the volcano Hadriaca Patera. If genetically related to the volcanic activity, large collapse features at the sources of some channels must have originated due to heat from large buried magma bodies. A channel emerging directly from the base of Hadriaca Patera may have originated from release of heat from thick extruded material. Other small channels in the region results from heat released from surface lava flows. Inferred channel discharges may be compared to discharge rates calculated for lava-ground ice interactions. Such comparisons show that meltwater probably accumulated beneath the surface and then was released rapidly, with a discharge rate limited by soil permeability. Volcano-ground ice interaction has been a widespread and important geologic process on Mars, and may be the primary source of palagonites making up the ubiquitous Martian dust.

Icarus↗
USGS1987

Generation of liquid water on Mars through the melting of a dusty snowpack

The possibility that snowmelt could have provided liquid water for valley network formation early in the history of Mars is investigated using an optical-thermal model developed for dusty snowpacks at temperate latitudes. The heating of the postulated snow is assumed to be driven primarily by the absorption of solar radiation during clear sky conditions. Radiative heating rates are predicted as a function of depth and shown to be sensitive to the dust concentration and the size of the ice grains while the thermal conductivity is controlled by temperature, atmospheric pressure, and bulk density. Rates of metamorphism indicate that fresh fine-grained snow on Mars would evolve into moderately coarse snow during a single summer season. Results from global climate models are used to constrain the mean-annual surface temperatures for snow and the atmospheric exchange terms in the surface energy balance. Mean-annual temperatures within Martian snowpacks fail to reach the melting point for all atmospheric pressures below 1000 mbar despite a predicted temperature enhancement beneath the surface of the snowpacks. When seasonal and diurnal variations in the incident solar flux are included in the model, melting occurs at midday during the summer for a wide range of snow types and atmospheric pressures if the dust levels in the snow exceed 100 ppmw (parts per million by weight). The optimum dust concentration appears to be about 1000 ppmw. With this dust load, melting can occur in the upper few centimeters of a dense coarse-grained snow at atmospheric pressures as low as 7 mbar. Snowpack thickness and the thermal conductivity of the underlying substrate determine whether the generated snow-melt can penetrate to the snowpack base, survive basal ice formation, and subsequently become available for runoff. Under favorable conditions, liquid water becomes available for runoff at atmospheric pressures as low as 30 to 100 mbar if the substrate is composed of regolith, as is expected in the ancient cratered terrain of Mars.

Icarus↗
USGS1987

Valles Marineris, Mars: Wet debris flows and ground ice

Detailed study of the Valles Marineris equatorial troughs suggests that the landslides in that area contained water and probably were gigantic wet debris flows: one landslide complex generated a channel that has several bends and extends for 250 km. Further support for water or ice in debris masses includes rounded flow lobes and transport of some slide masses in the direction of the local topographic slope. Differences in speed and emplacement efficiency between Martian and terrestrial landslides can be attributed to the entrainment of volatiles on Mars, but they can also be explained by other mechanisms. Support that the wall rock contained water comes from the following observations: (1) the water within the landslide debris must have been derived from wall rock; (2) debris appears to have been transported through tributary canyons; (3) locally, channels emerged from the canyons; (4) the wall rock apprarently disintegrated and flowed easily; and (5) fault zones within the troughs are unusually resistant to erosion. The study further suggests that, in the equatorial region of Mars, material below depths of 400-800 m was not desiccated during the time of landslide activity (within the last billion years of Martian history). Therefore the Martian ground-water or groundice reservoir, if not a relic from ancient times, must have been replenished.

Icarus↗
USGS1988

Search for volatiles on icy satellites: I. Europa

New reflectance spectra have been obtained for both the leading and trailing sides of Europa, using the Cooled Grating Array Spectrometer (CGAS) of the NASA Infrared Telescope Facility (IRTF). The spectra are of higher precision than any yet obtained. Spectra of Europa's trailing side (central meridian longitude ≈300°) obtained in 1985 show two weak absorptions near 2.2 and 2.3 μm. Both of these features as well as others are seen in spectra obtained by R. N. Clark, R. B. Singer, P. D. Owensby, and F.P. Fanale (1980a, Bull. Amer. Astron. Soc. 12, 713–714) at similar central meridian longitude. Data obtained with an improved detector array in 1986, however, do not show the absorptions seen in the 1980 and 1985 spectra. It is not clear why the newest data do not show the apparent absorptions seen in previous years, but the suggestion is that either the 1980 and 1985 data are spurious or that the material responsible for the weak absorptions is no longer detectable. Analysis of the 1980 and 1985 data did not reveal any obvious source of systematic error capable of introducing spurious features, but we are skeptical of any explanation that cites transient deposition, movement, and/or destruction of material on Europa's trailing side to account for the nondetection of the features in the 1986 data. If the weak absorptions seen in the 1980 and 1985 data are real, they can be interpreted as indicating the transient spectroscopic presence of a molecular component on Europa's trailing side different from the water ice that is known to be the dominant surface constituent. Further monitoring is required to determine if the apparent absorptions are real.

Icarus↗
USGS1988

Global color and albedo variations on Io

Three multispectral mosaics of Io have been produced from Voyager imaging data: a global mosaic from each of the Voyager 1 and Voyager 2 data sets and a high-resolution mosaic of the region surrounding the volcano Ra Patera. The mosaics are maps of normal albedo and color in accurate geometric map formats. Io's photometric behavior, mapped with a two-image technique, is spatially variable, especially in the bright white areas. The disk-integrated color and albedo of the satellite have been remarkably constant over recent decades, despite the volcanic activity and the many differences between Voyager 1 and 2 images (acquired just 4 months apart). This constancy is most likely due to the consistent occurrence of large Pele-type plumes with relatively dark, red deposits in the region from long 240 to 360°. A transient brightening southeast of Pele during the Voyager 1 encounter was probably due to real changes in surface and/or atmospheric materials, rather than to photometric behavior. The intrinsic spectral variability of Io, as seen in a series of two-dimensional histograms of the multispectral mosaics, consists of continuous variation among three major spectral end members. The data were mapped into five spectral units to compare them with laboratory measurements of candidate surface materials and to show the planimetric distributions. Unit 1 is best fit by the spectral reflectance of ordinary elemental sulfur, and it is closely associated with the Peletype plume deposits. Unit 2 is strongly confined to the polar caps above about latitude ±50°, but its composition is unknown. Unit 5 is probably SO 2 with relatively minor contamination; it is concentrated in the equatorial region and near the long-lived Prometheus-type plumes. Units 3 and 4 are gradational between units 1 and 5. In addition to SO 2 and elemental sulfur, other plausible components of the surface are polysulfur oxides, FeCl 2 , Na 2 S, and NaHS.

Icarus↗
USGS1988

Cratering history of Miranda: Implications for geologic processes

Miranda's surface is divisible into cratered terrain and coronae. The cratered terrain is the most heavily cratered of the terrains and presumably is the oldest. The frequency of craters in the cratered terrain is variable and related to position on the satellite. The coronae are also variably cratered. Elsinore and Arden Coronae have similar crater frequencies and may have formed simultaneously. They are of intermediate agompared to the cratered terrain and to Inverness Corona, which is the youngest major terrain. Graben formation appears to have occurred both before and after the formation of the coronae reflecting periods of global expansion. Miranda's surfaces are, in general, the least cratered and therefore inferred to be the youngest within the Uranian system.

Icarus↗
USGS1988

The global distribution, abundance, and stability of SO2 on Io

Sulfur dioxide distribution and abundances, bolometric hemispheric albedos, and passive surface temperatures on Io are modeled and mapped globally from Voyager multispectral mosaics, Earth-based spectra, and photometric descriptions. Photometric models indicate global average values for regolith porosity of 75-95% and macroscopic roughness with a mean slope angle of ~30??. Abundances of SO2 suggested by observations at uv-visible wavelengths and at 4.08 ??m are partially reconciled by intimate-mixing models; 30-50% SO2 coverage of the integral disk is indicated. Three major spectral end members, with continuous mixing, are recognized from the Voyager multispectral mosaics; one of these end members is identified as SO2. Intimate-mixing models with the three spectal end members are used to produce abundance maps for the optical surface; ~30% of Io's total optical surface consists of SO2. The SO2 is concentrated in the bright equatorial band and is relatively deficient in the region of Pele-type volcanic reuptions (long 240??-360??) and the polar regions. Temperatures are computed to vary over a 40??K range, at the same illumination angle, according to variations in surface bolometric hemispheric albedo. The brightest (and locally coldest) areas correspond to areas rich in SO2 and are concentrated in an equatorial band (??30?? lat), but many small cold patches occur elsewhere. These cold patches have radiative equilibrium temperatures ???120??K at the subsolar point, resulting in SO2 saturation vapor pressures ???10-8 bar. Midlatitude areas and the region of Pele-type plume eruptions are generally warmer (due to lower albedos). These results for surface temperatures and SO2 abundances and distribution support the regional coldtrapping model for the surface and atmospheric SO2 presented by F.P. Fanale, W.B. Banerdt, L.S. Elson, T.V. Johnson, and R.W. Zurek (1982, In Satellites of Jupiter (D. Morrison, Ed.), pp. 756-781, Univ. of Arizona Press, Tucson), although the region of Pele-type volcanic eruptions may be better characterized by the regolith condtrapping/volcanic-venting model of D.L. Matson and D.B. Nash (1983, J. Geophys. Res. 88, 4771-4783). The bright equatorial band is especially effective at slowing the formation of polar caps of SO2, both by reducing the sublimation rate near the subsolar point and by coldtrapping the SO2 in the equatorial region, so that competing processes of sputtering and volcanic resurfacing may prevent the formation of polar SO2 caps.

Icarus↗
USGS1989

Recharge of the early atmosphere of Mars by impact-induced release of CO2

Channels on the Martian surface suggest that Mars had an early, relatively thick atmosphere. If the atmosphere was thick enough for water to be stable at the surface, CO 2 in the atmosphere would have been fixed as carbonates on a relatively short time scale, previously estimated to be 1 bar every 10 7 years. This loss must have been offset by some replenishment mechanism to account for the numerous valley networks in the oldest surviving terrains. Impacts could have released CO 2 into the atmosphere by burial, by shock-induced release during impact events, and by addition of carbon to Mars from the impacting bolides. Depending on the relationship between the transient cavity diameter and the diameter of the resulting crater, burial rates as a result of impact gardening at the end of heavy bombardment are estimated to range from 20 to 45 m/10 6 years, on the assumption that cratering rates in Mars were similar to those of the Nectarian Period on the Moon. At these rates 0.1-0.2 bar of CO 2 could have been released every 107 years as a result of burial to depths where dissociation temperatures of carbonates were reached. Modeling of large impacts suggests that an additional 0.01 to 0.02 bar of CO 2 could have been released every 10 7 years during the actual impacts. In the unlikely event that all the impacting material was composed of carbonaceous chondrites, a further 0.3 bar of CO 2 could have been added to the atmosphere every 10 7 years by oxidation of meteoritic carbon. Even when supplemented by the volcanically induced release of CO 2 , these release rates are barely sufficient to sustain an early atmosphere if water were continuously present at the surface. The results suggest that water may have been only intermittently present on the surface early in the planet's history.

Icarus↗
USGS1989

Viking landing sites, remote-sensing observations, and physical properties of Martian surface materials

Important problems that confront future scientific exploration of Mars include the physical properties of Martian surface materials and the geologic processes that formed the materials. The design of landing spacecraft, roving vehicles, and sampling devices and the selection of landing sites, vehicle traverses, and sample sites will be, in part, guided by the physical properties of the materials. Four materials occur in the sample fields of the Viking landers: (1) drift, (2) crusty to cloddy, (3) blocky, and (4) rock. The first three are soillike. Drift materials is weak, loose, and porous. We estimate that it has a dielectric constant near 2.4 and a thermal inertia near 1 × 10 −3 to 3 × 10 −3 ( cal cm −2 sec 1 2 K −1 ) "> 1 × 10 −3 to 3 × 10 −3 (cal cm −2 sec 12 K −1 ) because of its low bulk density, fine grain size, and small cohesion. Crusty to cloddy material is expected to have a dielectric constant near 2.8 and a thermal inertia near 4 × 10 −3 to 7 × 10 −3 because of its moderate bulk density and cementation of grains. Blocky material should have a dielectric constant near 3.3 and a thermal inertia near 7 × 10 −3 to 9 × 10 −3 because of its moderate bulk density and cementation. Common basaltic rocks have dielectric constants near 8 and thermal inertias near 30 × 10 −3 to 60 × 10 −3 . Comparisons of estimated dielectric constants and thermal inertias of the materials at the landing sites with those obtained remotely by Earth-based radars and Viking Orbiter thermal sensors suggest that the materials at the landing sites are good analogs for materials elsewhere on Mars. Correlation of remotely estimated dielectric constant and thermal inertias indicates two modal values for paired values of dielectric constants and thermal inertias near (A) 2 and 2 × 10 −3 and (B) 3 and 6 × 10 −3 , respectively. These two modes are comparable to the dielectric constants and thermal inertias for drift and crusty to cloddy material, respectively. Dielectric constants and thermal inertias for blocky material are larger but consistent with values in the northern plains. Our interpretations are compatible with an aeolian origin for drift and similar materials elsewhere on Mars. The postulate that moderate dielectric constants and thermal inertias larger than 3 or 4 × 10 −3 are produced by cementation of soillike materials is partly consistent with the data. The average dielectric constant and thermal inertia and their correlation with one another suggest that most of the surface of Mars should present few difficulties to future surface exploration, but some surfaces may present difficulties for spacecraft that are not suitably designed.

Icarus↗
USGS1990

The extraordinary radar echoes from Europa, Ganymede, and Callisto: A geological perspective

This outline of plausible geologic explanations for the icy Galilean satellites' radar properties takes into consideration electromagnetic scattering models for the echoes, available empirical and theoretical information about regolith formation, and ice physics. The strange radar signatures arise because (1) ice is electrically different from silicates and/or (2) icy regoliths contain bulk-density (and hence refractive-index) structures absent within silicate regoliths. Ice's relatively high radar-frequency transparency compared with that of silicates permits longer photon path lengths, deeper radar sounding, and a greater number of scattering events. Consequently, scattering mechanisms that cannot contribute significantly to lunar echoes can dominate icy-satellite echoes. Possible phenomena unique to icy regoliths include (1) smoothing out of discontinuities between solid ejecta fragments and more porous surroundings under the action of thermal annealing to form refraction-scattering (RS) “lenses” and (2) formation of density enhancements in the shape of crater floors that result in RS and/or total internal reflection (TIR). In either case, high-order multiple scattering is more likely to be responsible for the echoes than low-order scattering. Radar/radio observations can constrain the order of the scattering and the scale of the structures responsible for the echoes but might not determine whether TIR or RS dominates the scattering. Multiwavelength investigations of the degree of correlation between radar properties and geologic terrain type should prove most useful, because inter- and intrasatellite variations in radar properties probably correspond to variations in ice purity, regolith thickness, and regolith thermal history and age.

Icarus↗
USGS1990

Volcanism in Northwest Ishtar Terra, Venus

Soviet Venera 15/16 radar images and topographic data for an area in NW Ishtar Terra, Venus (74°N, 313°E) contain evidence for the existence of a complex volcanic center. Located at the intersection of Akna and Freyja Montes (both elevated ∼4 to 7 km above the planetary reference radius of 6051 km), this volcanic center has a complex caldera system, possibly more than one major eruptive vent, and associated lava flows at lower elevations. Evidence for a volcanic center at NW Ishtar Terra includes: a smooth-surfaced unit with moderate radar brightness; adjacent (possibly associated) bright flows extending northwestward more than 300 km; an irregularly shaped depression 200 by 250 km in size and ∼2.0 km deep; arcuate slopes that appear to mark the rims of several semicircular depressions within the larger depression; and 5.5-km-elevation ridge separating volcanic deposits of northern Lakshmi Planum from those of NW Ishtar Terra. The lack of a notable shield construct at NW Ishtar and the similarity between smooth terrain and undivided plains of Lakshmi Planum suggest that volcanic units at NW Ishtar Terra may represent an earlier (perhaps younger) stage of volcanism than that attributed to Colette and Sacajawea Paterae of Lakshmi Planum. A westward migration in volcanic activity from Sacajewea to Colette NW Ishtar is supported by relative youth of Colette as compared to Sacajawea, the relatively young stage of volcanism at NW Ishtar Terra, and the superposition of this volcano on the banded terrain of Akna and Freyja Montes.

Icarus↗
USGS1990

Young volcanic deposits in the Valles Marineris, Mars?

A study of the interior deposits of the central Valles Marineris has led to the discovery of a sequence of deposits that cover the chasma floors and range in thickness from that of thin dust to several kilometers. The emplacement of the deposits was the last major event in the history of the Valles Marineris, following deposition of older layered interior beds, warping, faulting, erosion, and landslide emplacement. The young deposits are of three major types: (1) dark patches typically occurring along faults; (2) light-colored deposits locally associated with craters; and (3) rugged, mottled deposits with, in places, light-colored lobate fronts. These young materials may be of volcanic origin, as suggested by the low albedo and spectrally gray colors of some, their association with faults and possible volcanic craters and calderas, and their embayment relations and lobate margins. No other mechanism explains all the observed features and relations as well as volcanism. If these deposits are volcanic, implications are that (1) volcanism was associated with rifting in the Valles Marineris, (2) the volcanism was explosive in places, and (3) the volcanism may be as young as the late Tharsis volcanism and, locally, may well be recent.

Icarus↗
USGS1991

Photometric functions for photoclinometry and other applications

Least-squared fits to the brightness profiles across a disk or “limb darkening” described by Hapke's photometric function are found for the simpler Minnaert and lunar-Lambert functions. The simpler functions are needed to reduce the number of unknown parameters in photoclinometry, especially to distinguish the brightness variations of the surface materials from that due to the resolved topography. The limb darkening varies with the Hapke parameters for macroscopic roughness ( θ ) "> (θ) , the single-scattering albedo ( w ), and the asymmetry factor of the particle phase function ( g ). Both of the simpler functions generally provide good matches to the limb darkening described by Hapke's function, but the lunar-Lambert function is superior when viewing angles are high and when ( θ ) "> (θ) is less than 30°. Although a nonunique solution for the Minnaert function at high phase angles has been described for smooth surfaces, the discrepancy decreases with increasing ( θ ) "> (θ) and virtually disappears when ( θ ) "> (θ) reaches 30° to 40°. The variation in limb darkening with w and g , pronounced for smooth surfaces, is reduced or eliminated when the Hapke parameters are in the range typical of most planetary surfaces; this result simplifies the problem of photoclinometry across terrains with variable surface materials. The Minnaert or lunar-Lambert fits to published Hapke models will give photoclinometric solutions that are very similar (>1° slope discrepancy) to the Hapke-function solutions for nearly all of the bodies and terrains thus far modeled by Hapke's function.

Icarus↗
USGS1991

Modeling the reflectance spectrum of Callisto 0.25 to 4.1μm

The reflectance spectrum of Callisto from 0.2 to 4.1 μm is modeled using a simultaneous intimate plus areal mixture solution of ice and dark material which satisfies absorption band depths and reflectance levels. The model uses the radiative transfer theory based on Hapke's (1981, J. Geophys. Res. 86, 3039–3054) work, optical constants of materials and includes effects of grain size and abundance of each material. The best-fitting models contain 20–45 wt% ice in the optical surface. The models indicate that the ice component of the surface is fairly large gained and that the ice cannot account for major spectral features beyond approximately 2.5 μm. In this spectral region other hydrated minerals must dominate. A variety of reasonably well-fitting models were found and the amount of ice determined for these best fits was mathematically removed from the original Callisto spectrum. All of the spectra determined for the non-material were quite similar to each other and have absorption features that resemble hydrated silicates bearing both oxidation states of iron. Certain features in the Callisto non-ice spectrum can be duplicated by mixtures of Fe- and Mg-end member serpentines. Discrepancies indicate that other phases, possibly opaque minerals, are also required to match the entire spectrum. The unusual Fe-serpentines are commonly found in the matrices of primitive cabodnaceous chondrites, suggesting that other matrix phases may also be likely candidates for the Callisto non-ice material.

Icarus↗
USGS1992

Triton's surface-atmosphere energy balance

We explore the energetics of Triton's surface-atmosphere system using a model that includes the turbulent transfer of sensible heat as well as insolation, reradiation, and latent heat transport. The model relies on a 1° by 1° resolution hemispheric bolometric albedo map of Triton for determining the atmospheric temperature, the N 2 frost emissivity, and the temperatures of unfrosted portions of the surface consistent with a frost temperature of ≅38 K. For a physically plausible range of heat transfer coefficients, we find that the atmospheric temperature roughly 1 km above the surface is approximately 1 to 3 K hotter than the surface. Atmospheric temperatures of 48 K suggested by early analysis of radio occultation data cannot be obtained for plausible values of the heat transfer coefficients. Our calculations indicate that Triton's N 2 frosts must have an emissivity well below unity in order to have a temperature of ≅38 K, consistent with previous results. We also find that convection over small hot spots does not significantly cool them off, so they may be able to act as continous sources of buoyancy for convective plumes, but have not explored whether the convection is vigorous enough to entrain particulate matter thereby forming a dust devil. Our elevated atmospheric temperatures make geyser driven plumes with initial upward velocities ≤10 m s −1 stagnate in the lower atmosphere. These “wimpy” plumes provide a possible explanation for Triton's “wind streaks.”

Icarus↗
USGS1992

Earth and Mars: Water inventories as clues to accretional histories

The Earth has 2.7 km of water on its surface. Its mantle contains at least 150 ppm water, and probably significantly more depending on the amount of undepleted mantle and subducted crustal water that is present. Geologic evidence suggests that a few hundred meters of water are close to the Martian surface, but evidence from SNC meteorites indicates that the Martian mantle is very dry, containing no more than about 35 ppm water. Part of the difference in water content of the mantles of the two planets is attributed to plate tectonics. However, the Earth's mantle appears to contain at least several times the water content of the Martian mantle, even accounting for plate tectonics. We attribute the difference to two possible causes. The first possibility is that melting of the Earth's surface during accretion, as a result of the development of a steam atmosphere, allowed impact-devolatized water at the surface to dissolve into the Earth's interior. In contrast, because of Mars' smaller size and greater distance from the Sun, the Martian surface may not have melted, so that the devolatilized water could not dissolve into the surface. A second and preferred possibility is that Mars, like the Earth, acquired a late volatile rich veneer, but it did not get folded into the interior as with the Earth, but instead remained as a water-rich veneer. The perception of Mars as having a wet surface, but a dry interior, is consistent with what we know of the geologic history of Mars, which can be viewed as the progressive intrusion and overplating of a water-rich crust by dry, mantle-derived volcanic rocks.

Icarus↗
USGS2004

Are the circular, dark features on Comet Borrelly's surface albedo variations or pits?

The highest resolution images of Comet 19P/Borrelly show many dark features which, upon casual inspection, appear to be low albedo markings, but which may also be shadows or other photometric variations caused by a depression in the local topography. In order to distinguish between these two possible interpretations we conducted a photometric analysis of three of the most prominent of these features using six of the highest quality images from the September 22, 2001 Deep Space 1 (DS1) flyby. We find that: 1. The radiance in the darkest parts of each feature increases as phase angle decreases, similarly to the radiance behavior of the higher albedo surrounding terrain. The dark features could be either fully illuminated low albedo spots or, alternatively, they could be depressions. No part of any of the three regions was in full shadow. 2. One of the regions has a radiance profile consistent with a rimmed depression, the second, with a simple depression with no rim, and the third with a low albedo spot. 3. The regolith particles are backscattering and carbon black is one of the few candidate regolith materials that might explain this low albedo. We conclude that Borrelly's surface is geologically complex to the limit of resolution of the images with a combination complex topography, pits, troughs, peaks and ridges, and some very dark albedo markings, perhaps a factor of two to three darker than the average 3-4% albedo of the surrounding terrains. Our technique utilizing measured radiance profiles through the dark regions is able to discriminate between rimmed depressions, rimless depressions and simple albedo changes not associated with topography. ?? 2003 Elsevier Inc. All rights reserved.

Icarus↗