Data returned from the Mars Orbiter Laser Altimeter allows construction of a high precision digit... more Data returned from the Mars Orbiter Laser Altimeter allows construction of a high precision digital elevation model. Quantitative investigations into the geomorphic properties of drainage features, similar to ones carried out on Earth, are now possible.
ABSTRACT Water vapor diffusion is measured through porous media at Mars-like conditions and diffu... more ABSTRACT Water vapor diffusion is measured through porous media at Mars-like conditions and diffusion coefficients are obtained. A series of homogeneous porous glass disks exhibiting a range of pore sizes (10 to 220 microns) and thicknesses (0.25 to 8 cm) are used in initial experiments to validate the experimental setup and develop a system model. Correction factors to diffusion coefficients for gas-gas diffusion for this experimental geometry are obtained and applied to further samples. The values of porosity and grain/pore size studied cover the range of reasonable physical properties for near-surface Martian regolith. The diffusion coefficients for porous disks and unconsolidated Mars analog materials studied all fall within the range 5 to 15 cm2s-1, consistent with theoretical models. Calculated values of tortuosity factor also fall in the expected range of 2 to 5. Implications for ground-ice stability and equilibrium with the Martian climate are presented in the context of previous theoretical investigations.
ABSTRACT It has long been suggested that water ice can exist in extremely cold regions near the l... more ABSTRACT It has long been suggested that water ice can exist in extremely cold regions near the lunar poles, where sublimation loss is negligible. The geographic distribution of H-bearing regolith shows only a partial or ambiguous correlation with permanently shadowed areas, thus suggesting that another mechanism may contribute to locally enhancing water concentrations. We show that under suitable conditions, water molecules can be pumped down into the regolith by day-night temperature cycles, leading to an enrichment of H2O in excess of the surface concentration. Ideal conditions for pumping are estimated and found to occur where the mean surface temperature is below 105 K and the peak surface temperature is above 120 K. These conditions complement those of the classical cold traps that are roughly defined by peak temperatures lower than 120 K. On the present-day Moon, an estimated 0.8% of the global surface area experiences such temperature variations. Typically, pumping occurs on pole-facing slopes in small areas, but within a few degrees of each pole the equator-facing slopes are preferred. Although pumping of water molecules is expected over cumulatively large areas, the absolute yield of this pump is low; at best, a few percent of the H2O delivered to the surface could have accumulated in the near-surface layer in this way. The amount of ice increases with vapor diffusivity and is thus higher in the regolith with large pore spaces.
Physical review. E, Statistical, nonlinear, and soft matter physics, 2002
Probability density functions and conditional averages of velocity gradients derived from upper o... more Probability density functions and conditional averages of velocity gradients derived from upper ocean observations are compared with results from forced simulations of the two-dimensional Navier-Stokes equations. Ocean data are derived from TOPEX satellite altimeter measurements. The simulations use rapid forcing on large scales, characteristic of surface winds. The probability distributions of transverse velocity derivatives from the ocean observations agree with the forced simulations, although they differ from unforced simulations reported elsewhere. The distribution and cross correlation of velocity derivatives provide clear evidence that large coherent eddies play only a minor role in generating the observed statistics.
Physical review. E, Statistical, nonlinear, and soft matter physics, 2001
We investigate the consequences of fluid flowing on a continuous surface upon the geometric and s... more We investigate the consequences of fluid flowing on a continuous surface upon the geometric and statistical distribution of the flow. We find that the ability of a surface to collect water by its mere geometrical shape is proportional to the curvature of the contour line divided by the local slope. Consequently, rivers tend to lie in locations of high curvature and flat slopes. Gaussian surfaces are introduced as a model of random topography. For Gaussian surfaces the relation between convergence and slope is obtained analytically. The convergence of flow lines correlates positively with drainage area, so that lower slopes are associated with larger basins. As a consequence, we explain the observed relation between the local slope of a landscape and the area of the drainage basin geometrically. To some extent, the slope-area relation comes about not because of fluvial erosion of the landscape, but because of the way rivers choose their path. Our results are supported by numerically ...
Physical review. E, Statistical physics, plasmas, fluids, and related interdisciplinary topics, 2000
The power spectrum is measured in direct numerical simulations of the two-dimensional Navier-Stok... more The power spectrum is measured in direct numerical simulations of the two-dimensional Navier-Stokes equation and other two-dimensional flows with white-in-time forcing at large scales. For the Navier-Stokes equation the energy spectrum in the inertial range approaches k(-3) with increasing Reynolds number, with possible logarithmic corrections. A family of two-dimensional flows, including the surface quasigeostrophic equation, allows us to vary the locality of the "enstrophy" transfer, where enstrophy is the mean square of the convected quantity. Dimensional analysis based on the enstrophy dissipation correctly predicts the energy spectrum, whenever the enstrophy transfer can be assumed to be spectrally local. Otherwise, the enstrophy spectrum is steeper than would be expected on the basis of local transfer. In this case the data suggest a k(-1) passive scalar spectrum.
Physical review. E, Statistical physics, plasmas, fluids, and related interdisciplinary topics, 2000
We study statistical properties of two-dimensional turbulent flows. Three systems are considered:... more We study statistical properties of two-dimensional turbulent flows. Three systems are considered: the Navier-Stokes equation, surface quasigeostrophic flow, and a model equation for thermal convection in the Earth's mantle. Direct numerical simulations are used to determine one-point fluctuation properties. Comparative study shows universality of probability density functions (PDFs) across different types of flow. For instance, the PDFs for derivatives of the advected quantity are the same for the three flows, once normalized by the average size of fluctuations. The single-point statistics is surprisingly robust with respect to the nature of the nonlinearity.
Comet-asteroid transition object 107P/Wilson-Harrington was observed near its October 22, 2009 pe... more Comet-asteroid transition object 107P/Wilson-Harrington was observed near its October 22, 2009 perihelion passage to search for activity. No activity was detected. Consequently, we place limits on possible dust production of 0.013 kg/s at 1.23 AU. Furthermore, the data was not sufficient to constrain a rotation period; however, it is clear that the rotation period is > 4hr. Our data is
Permanently stable ground ice is found beneath a permanently frost free surface on Mars, and simi... more Permanently stable ground ice is found beneath a permanently frost free surface on Mars, and similar conditions exist in the Antarctic Dry Valleys. This phenomenon is due to a balance of the vapor pressure of the ice with the atmospheric humidity in the presence of large amplitude temperature oscillations. An exactly solvable model example shows that the fraction of time the atmosphere needs to be saturated to stabilize the ice decreases with temperature amplitude. It is estimated that for conditions that prevail on Mars today, the mean temperature needs to be about 5K lower than the frost point temperature for ground ice to be stable. A decomposition method to evaluate the contribution of short term weather events to ground ice stability is developed; when applied to a study site in the Dry Valleys, it reveals that the coldest periods contribute most to stabilization.
Data returned from the Mars Orbiter Laser Altimeter allows construction of a high precision digit... more Data returned from the Mars Orbiter Laser Altimeter allows construction of a high precision digital elevation model. Quantitative investigations into the geomorphic properties of drainage features, similar to ones carried out on Earth, are now possible.
ABSTRACT Water vapor diffusion is measured through porous media at Mars-like conditions and diffu... more ABSTRACT Water vapor diffusion is measured through porous media at Mars-like conditions and diffusion coefficients are obtained. A series of homogeneous porous glass disks exhibiting a range of pore sizes (10 to 220 microns) and thicknesses (0.25 to 8 cm) are used in initial experiments to validate the experimental setup and develop a system model. Correction factors to diffusion coefficients for gas-gas diffusion for this experimental geometry are obtained and applied to further samples. The values of porosity and grain/pore size studied cover the range of reasonable physical properties for near-surface Martian regolith. The diffusion coefficients for porous disks and unconsolidated Mars analog materials studied all fall within the range 5 to 15 cm2s-1, consistent with theoretical models. Calculated values of tortuosity factor also fall in the expected range of 2 to 5. Implications for ground-ice stability and equilibrium with the Martian climate are presented in the context of previous theoretical investigations.
ABSTRACT It has long been suggested that water ice can exist in extremely cold regions near the l... more ABSTRACT It has long been suggested that water ice can exist in extremely cold regions near the lunar poles, where sublimation loss is negligible. The geographic distribution of H-bearing regolith shows only a partial or ambiguous correlation with permanently shadowed areas, thus suggesting that another mechanism may contribute to locally enhancing water concentrations. We show that under suitable conditions, water molecules can be pumped down into the regolith by day-night temperature cycles, leading to an enrichment of H2O in excess of the surface concentration. Ideal conditions for pumping are estimated and found to occur where the mean surface temperature is below 105 K and the peak surface temperature is above 120 K. These conditions complement those of the classical cold traps that are roughly defined by peak temperatures lower than 120 K. On the present-day Moon, an estimated 0.8% of the global surface area experiences such temperature variations. Typically, pumping occurs on pole-facing slopes in small areas, but within a few degrees of each pole the equator-facing slopes are preferred. Although pumping of water molecules is expected over cumulatively large areas, the absolute yield of this pump is low; at best, a few percent of the H2O delivered to the surface could have accumulated in the near-surface layer in this way. The amount of ice increases with vapor diffusivity and is thus higher in the regolith with large pore spaces.
Physical review. E, Statistical, nonlinear, and soft matter physics, 2002
Probability density functions and conditional averages of velocity gradients derived from upper o... more Probability density functions and conditional averages of velocity gradients derived from upper ocean observations are compared with results from forced simulations of the two-dimensional Navier-Stokes equations. Ocean data are derived from TOPEX satellite altimeter measurements. The simulations use rapid forcing on large scales, characteristic of surface winds. The probability distributions of transverse velocity derivatives from the ocean observations agree with the forced simulations, although they differ from unforced simulations reported elsewhere. The distribution and cross correlation of velocity derivatives provide clear evidence that large coherent eddies play only a minor role in generating the observed statistics.
Physical review. E, Statistical, nonlinear, and soft matter physics, 2001
We investigate the consequences of fluid flowing on a continuous surface upon the geometric and s... more We investigate the consequences of fluid flowing on a continuous surface upon the geometric and statistical distribution of the flow. We find that the ability of a surface to collect water by its mere geometrical shape is proportional to the curvature of the contour line divided by the local slope. Consequently, rivers tend to lie in locations of high curvature and flat slopes. Gaussian surfaces are introduced as a model of random topography. For Gaussian surfaces the relation between convergence and slope is obtained analytically. The convergence of flow lines correlates positively with drainage area, so that lower slopes are associated with larger basins. As a consequence, we explain the observed relation between the local slope of a landscape and the area of the drainage basin geometrically. To some extent, the slope-area relation comes about not because of fluvial erosion of the landscape, but because of the way rivers choose their path. Our results are supported by numerically ...
Physical review. E, Statistical physics, plasmas, fluids, and related interdisciplinary topics, 2000
The power spectrum is measured in direct numerical simulations of the two-dimensional Navier-Stok... more The power spectrum is measured in direct numerical simulations of the two-dimensional Navier-Stokes equation and other two-dimensional flows with white-in-time forcing at large scales. For the Navier-Stokes equation the energy spectrum in the inertial range approaches k(-3) with increasing Reynolds number, with possible logarithmic corrections. A family of two-dimensional flows, including the surface quasigeostrophic equation, allows us to vary the locality of the "enstrophy" transfer, where enstrophy is the mean square of the convected quantity. Dimensional analysis based on the enstrophy dissipation correctly predicts the energy spectrum, whenever the enstrophy transfer can be assumed to be spectrally local. Otherwise, the enstrophy spectrum is steeper than would be expected on the basis of local transfer. In this case the data suggest a k(-1) passive scalar spectrum.
Physical review. E, Statistical physics, plasmas, fluids, and related interdisciplinary topics, 2000
We study statistical properties of two-dimensional turbulent flows. Three systems are considered:... more We study statistical properties of two-dimensional turbulent flows. Three systems are considered: the Navier-Stokes equation, surface quasigeostrophic flow, and a model equation for thermal convection in the Earth's mantle. Direct numerical simulations are used to determine one-point fluctuation properties. Comparative study shows universality of probability density functions (PDFs) across different types of flow. For instance, the PDFs for derivatives of the advected quantity are the same for the three flows, once normalized by the average size of fluctuations. The single-point statistics is surprisingly robust with respect to the nature of the nonlinearity.
Comet-asteroid transition object 107P/Wilson-Harrington was observed near its October 22, 2009 pe... more Comet-asteroid transition object 107P/Wilson-Harrington was observed near its October 22, 2009 perihelion passage to search for activity. No activity was detected. Consequently, we place limits on possible dust production of 0.013 kg/s at 1.23 AU. Furthermore, the data was not sufficient to constrain a rotation period; however, it is clear that the rotation period is > 4hr. Our data is
Permanently stable ground ice is found beneath a permanently frost free surface on Mars, and simi... more Permanently stable ground ice is found beneath a permanently frost free surface on Mars, and similar conditions exist in the Antarctic Dry Valleys. This phenomenon is due to a balance of the vapor pressure of the ice with the atmospheric humidity in the presence of large amplitude temperature oscillations. An exactly solvable model example shows that the fraction of time the atmosphere needs to be saturated to stabilize the ice decreases with temperature amplitude. It is estimated that for conditions that prevail on Mars today, the mean temperature needs to be about 5K lower than the frost point temperature for ground ice to be stable. A decomposition method to evaluate the contribution of short term weather events to ground ice stability is developed; when applied to a study site in the Dry Valleys, it reveals that the coldest periods contribute most to stabilization.
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