James R. Salvail

The water regime of asteroid (1) Ceres

Abstract An analytical model is presented which describes the water regime of asteroid Ceres. The model predicts surface and subsurface temperatures, water fluxes, and ice depths as well as integrated water supply rates and residual atmospheric/surface water as a function of time, lattitude, and assumed regolith properties. We find that ice could have survived for 4.5 byr at depths of only 10–100 m near the equator and less than 1.0 to 10 m at latitudes greater than 40°. The current global water supply rate is expected to be between 30 and 300 g sec−1, which corresponds to a near surface number density of ∼1 × 104molecules cm−3. At least one current interpretation of the near-infrared reflectance of Ceres requires coverage by a very thin layer of ice down to 48° lat. Our model suggests that ice with the overall albedo of Ceres (0.09) could exist in transient steady state with the above supply rate only at latitudes greater than 80°. Otherwise the water loss rate from Ceres would exceed the supply rate by orders of magnitude. Stirring of the regolith substantially alters the distribution of ice only at latitudes greater than 70°. At lower latitudes ice depletion is faster than impact rehomogenization at all relevant depths and time scales. Possible contributors to the spectral feature at 3.1 μm other than free surface ice, such as interlayer ice within the optical surface, are considered.

The loss and depth of CO2 ice in comet nuclei

Abstract An analytical model has been developed to simulate the chemical differentiation of a homogeneous, initially unmantled cometary nucleus composed of water ice, putative unclathrated CO 2 ice, and silicate dust in specified proportions. Selective sublimation of any free CO 2 ice present in a new comet should produce a surface layer of water ice and dust overlying the undifferentiated core. This surface layer modifies the temperature of buried CO 2 ice and restricts the outflow of gaseous CO 2 . On each orbit, water sublimation closer to perihelion temporarily reduces the thickness of the water ice and dust layer and liberates dust. Most of the dust is blown off the nucleus, but a small amount of residual dust remains on the surface (cf. H. L. F. Houpis, W. H. Ip, and D. A. Mendis, 1986, Astrophys. J. , in press). Our model includes the effects of nucleus rotation, arbitrary orientation of the rotation axis, latitude, heat conduction into the interior of the nucleus, restriction of CO 2 gas outflow by the water ice and dust layer, and the use of thermal conductivities for both amorphous and crystalline water ice as appropriate, featuresthat were not included in the Houpis et al. model. The model also accounts for the erosion of the water ice surface, which Houpis et al. appear to have accounted for and which is an important effect. Specifically, we investigate the effects of varying the permeability of the surface water ice layer, the mass fraction of CO 2 , the orbit and the latitude, using the orbital parameters of Comets Halley and Tempel 2. It is found that CO 2 gas production should exceed H 2 O gas production beyond ∼3 AU, and at 1 AU CO 2 gas production should be between 20 to 25% of H 2 O gas production. The depth of CO 2 ice and the variation in the depth of CO 2 ice throughout an orbit are affected significantly by the perihelion of the orbit. The effects due to water ice permeability are significant but much less than expected on the basis of flow area. Latitude and CO 2 concentration produce relatively small effects. Under all conditions considered here, CO 2 ice should always be found within ∼1 m from the surface of comet nuclei if it is present as a free species to begin with. This result is probably generally valid for unmantled portions of most comets and qualitatively simulates the behavior of an abundant, highly volatile component in an H 2 O/silicate matrix. Comparison of these and similar results with observations could yield information regarding the permeability and chemical composition of cometary material and suggest sampling strategies to minimize fractionation effects. The method is applicable to other nonwater ices.

The effect of volume phase changes, mass transport, sunlight penetration, and densification on the thermal regime of icy regoliths

Abstract A thermal model is presented which quantitatively accounts for the effects of sublimation condensation, and convection throughout a volume of a porous ice crust subjected to solar insolation. The effect of penetration of insolation into ice that is translucent to visible radiation but opaque to infrared radiation is also included. The governing energy differential equation, which also satisfies conservation of mass and accounts for the possibility of free molecular or continuum flow, is solved for various conditions defined by a reasonable range of thermal conductivities, sunlight absorption coefficients, and pore sizes. Quasi-steady-state temperatures, H 2 O mass fluxes, and rates of change of mass density for the ice are computed as functions of depth and time of day. We find that, when the effects of latent heat and mass transport are included in the model, the increase in the near surface temperature (the boundary condition for crustal heat flow) brought about by the “solid-state greenhouse” is greatly diminished. If the lowest thermal conductivities reported for the surface of Europa (∼100 erg/cm sec K) are assumed to apply to the upper crust as well, the melting point could be approached at very shallow depth. However, this is precluded except as a transient and shallow phenomenon which could not affect deep crustal temperatures because densification would raise the thermal conductivity to ≥1000 erg cm −1 sec −1 K in the underlying material in a geologically negligible period of time. If thermal conductivities ≥1000 erg/cm sec K are assumed, then the greenhouse effect raises near-surface temperatures ≤35 K, but the densification is slow enough that deep crustal temperatures would be augmented, allowing for melting at a depth of 7–19 km, depending on assumptions concerning tidal dissipation. Thus, when the effects of latent heat, mass transport, and densification are all taken into account, the existence of a significant solid-state greenhouse effect can be shown to be compatible both with morphological evidence for significant crustal strength and with evidence for decoupling of the icy shell from the lithosphere.

Evolution of the water regime of Phobos

Abstract An improved model of the evolution of the water regime of Phobos is presented. The central feature of this model is a time-dependent solar insolation that is influenced both by the increasing solar power output over geologic time and by the obliquity and eccentricity cycles of Mars and Phobos which vary over time scales of 10 5 to 10 6 years. A one-dimensional model is used to calculate temperatures, water fluxes, and ice depths over geologic time. Results are obtained at various latitudes for two assumed cases of porosity and pore size and for three putative values of the mass fraction of water initially allocated to Phobos. Results are obtained for one model that assumes a homogeneous distribution of water ice and for a second model that assumes that water ice is driven toward the surface by the internal thermal gradient near the poles. Results for the (more likely) surface-concentrated model indicate that ice may be found from 270 to 740 m at the equator and from 20 to 60 m at 80° lat depending on porosity and pore size, and subject to limits implied by assumptions of the initial mass fraction of free ice. A two-dimensional model is used to compute temperatures, heat and vapor fluxes, and ice removal/deposition rates for a two-dimensional grid over one obliquity cycle assuming that ice is distributed uniformly throughout Phobos. The heat and vapor fluxes and the ice removal/deposition rates are integrated over time to obtain the water loss at the surface, the lateral heat and vapor transport toward the poles, and the change in ice concentration at interior points. The results show that a relatively large amount of vapor is produced within 1 km of the surface, mostly at lower latitudes. Some of the vapor is transported to the surface where it is lost, and the remainder is transported to greater depths, where it is condensed at much lower rates over a much larger volume. For the large pore size/porosity case, we estimate that the current H 2 O loss rate is ∼3 g/sec. The possible effects of the radiative contributions from Mars, the shadowing by Mars, and the use of an ellipsoidal shape for Phobos are discussed.

The state of SO2 on Io's surface

We have modified an earlier model of Ios frozen surface SO2 and its influence on Ios atmosphere [Fanale et al., 1982]. In our new models, we consider four additional factors: (1) the latent heat of SO2 frost, (2) the rotation rate of Io, (3) thermal conduction and Ios internal heat flow, and (4) the deposition of some solar energy below the surface. The net result is a significant lowering of estimated surface temperatures and hence equilibrium SO2 pressures over the frost deposits. The atmospheric pressures are predicted to be much lower than those in the 1982 model of Fanale et al., and, in some cases, are low enough to be below the current upper limits set by recent Hubble Space Telescope observations. In the absence of local volcanic sources, even if Io were completely covered with frozen SO2, the atmosphere would be everywhere ballistic except very close to the subsolar point, where it would be marginally kinetic. Local kinetic atmospheres result only from SO2-rich plumes. The results of these models suggest that the sometimes reported posteclipse brightening of Io could be a real and temporally variable phenomenon and not an observational aberration.

A model of cometary gas and dust production and nongravitational forces with application to P/Halley

Abstract A program that computes gas and dust production rates and idealized nongravitational force components has been developed and applied to the case of Comet Halley. We use a modified form of our earlier comet model (F.P. Fanale and J.R. Salvali[(1984) Icarus 60 , 476–511] to which coma effects and a section on nongravitational forces have been added. The possibility of grain cohesion is also included. These models are used together with observations from 1910 and semiempirically derived data to investigate the effects of obliquity and thermal conductivity of the near thermal conductivity of the nucleus on gas and dust production. The results indicate that the thermal conductivity of the nucleus is of the order of 10 5 ergs/cm-s-°K, which implies that the ice near the surface is in the crystalline form. A general method is presented for calculating the radii of cometary nuclei using theoretically derived and semiempirically derived nongravitational force components. This method is used to calculate possible radii for Comet Halley that depend on the model variation chosen. The method used and the results presented herein should have greater significance and value when the observational data from Halleys current perihelion passage become available.

Mars: Long Term Changes in the State and Distribution Of H2O

A model for H2O distribution and migration on Mars has been formulated which takes into account: 1) thermal variations at all depths in the regolith due to variations in obliquity, eccentricity and the solar constant; 2) variations in atmospheric partial pressure of water (PH2O) caused by corresponding changes in polar surface insolation; and 3) the finite kinetics of H2O migration in both the regolith and atmosphere. Results suggest that regolith H2O transport rates are more strongly influenced by polar-controlled atmospheric PH2O variations than variations in pore gas PH2O brought about by thermal variations at the buried ice interface. The configuration of the ice interface as a function of assumed soil parameters and time is derived. Withdrawal of ice proceeds to various depths at latitudes 50° and transfer of H2O to the polar cap. The transfer has a somewhat oscillatory character, but only 1g/cm2 is shifted into and out of the regolith during each obliquity cycle. The net irreversible and inexorable transfer of H2O to higher latitudes involves between 1 × 106 km and 1 × 107 km3 of H2O over the history of Mars for most reasonable sets of assumptions. This mass is comparable to the amount of material removed from deflated terrain at mid and low latitudes and to the mass of the polar cap. We conclude that this process combined with periodic thermal cycles played a major role in development of the fretted terrain, deflationary features in general, patterned ground, the north polar cap and the layered terrain.