Coefficient of normal restitution of viscous particles and cooling rate of granular gases

We investigate the cooling rate of a gas of inelastically interacting particles. When we assume velocity dependent coefficients of restitution the material cools down slower than with constant restitution. This behavior might have large influence to clustering and structure formation processes.Comment: 3 figures, Phys. Rev. E (in press

Dynamical Organization around Turbulent Bursts

The detailed dynamics around intermittency bursts is investigated in turbulent shell models. We observe that the amplitude of the high wave number velocity modes vanishes before each burst, meaning that the fixed point in zero and not the Kolmogorov fixed point determines the intermittency. The phases of the field organize during the burst, and after a burst the field oscillates back to the laminar level. We explain this behavior from the variations in the values of the dissipation and the advection around the zero fixed point.Comment: 4 pages, REVTex, 3 figures in one ps-fil

Developed turbulence: From full simulations to full mode reductions

Developed Navier-Stokes turbulence is simulated with varying wavevector mode reductions. The flatness and the skewness of the velocity derivative depend on the degree of mode reduction. They show a crossover towards the value of the full numerical simulation when the viscous subrange starts to be resolved. The intermittency corrections of the scaling exponents of the pth order velocity structure functions seem to depend mainly on the proper resolution of the inertial subrange. Universal scaling properties (i.e., independent of the degree of mode reduction) are found for the relative scaling exponents rho which were recently defined by Benzi et al.Comment: 4 pages, 5 eps-figures, replaces version from August 5th, 199

Extensive water ice within Ceres’ aqueously altered regolith: Evidence from nuclear spectroscopy

The surface elemental composition of dwarf planet Ceres constrains its regolith ice content, aqueous alteration processes, and interior evolution. Using nuclear spectroscopy data acquired by NASA’s Dawn mission, we determined the concentrations of H, Fe, and K on Ceres. The data show that surface materials were processed by the action of water within the interior. The non-icy portion of Ceres’ C-bearing regolith contains similar amounts of H to aqueously altered carbonaceous chondrites, but less Fe. This allows for the possibility that Ceres experienced modest ice-rock fractionation, resulting in differences between surface and bulk composition. At mid-to-high latitudes, the regolith contains high concentrations of H, consistent with broad expanses of water ice, confirming theoretical predictions that ice can survive for billions of years just beneath the surface

Extensive water ice within Ceres’ aqueously altered regolith: Evidence from nuclear spectroscopy

The surface elemental composition of dwarf planet Ceres constrains its regolith ice content, aqueous alteration processes, and interior evolution. Using nuclear spectroscopy data acquired by NASA’s Dawn mission, we determined the concentrations of H, Fe, and K on Ceres. The data show that surface materials were processed by the action of water within the interior. The non-icy portion of Ceres’ C-bearing regolith contains similar amounts of H to aqueously altered carbonaceous chondrites, but less Fe. This allows for the possibility that Ceres experienced modest ice-rock fractionation, resulting in differences between surface and bulk composition. At mid-to-high latitudes, the regolith contains high concentrations of H, consistent with broad expanses of water ice, confirming theoretical predictions that ice can survive for billions of years just beneath the surface

mTORC1 is essential for early steps during Schwann cell differentiation of amniotic fluid stem cells and regulates lipogenic gene expression.

Schwann cell development is hallmarked by the induction of a lipogenic profile. Here we used amniotic fluid stem (AFS) cells and focused on the mechanisms occurring during early steps of differentiation along the Schwann cell lineage. Therefore, we initiated Schwann cell differentiation in AFS cells and monitored as well as modulated the activity of the mechanistic target of rapamycin (mTOR) pathway, the major regulator of anabolic processes. Our results show that mTOR complex 1 (mTORC1) activity is essential for glial marker expression and expression of Sterol Regulatory Element-Binding Protein (SREBP) target genes. Moreover, SREBP target gene activation by statin treatment promoted lipogenic gene expression, induced mTORC1 activation and stimulated Schwann cell differentiation. To investigate mTORC1 downstream signaling we expressed a mutant S6K1, which subsequently induced the expression of the Schwann cell marker S100b, but did not affect lipogenic gene expression. This suggests that S6K1 dependent and independent pathways downstream of mTORC1 drive AFS cells to early Schwann cell differentiation and lipogenic gene expression. In conclusion our results propose that future strategies for peripheral nervous system regeneration will depend on ways to efficiently induce the mTORC1 pathway

Paleo-Evolution of Martian Subsurface Ice and Its Role in the Polar Physical and Isotopic Layering

International audienceMars harbors ice deposits in several forms, on the surface and in the subsurface, which exchange with each other on various timescales. We seek to study the pore ice evolution over millennial time scales and how it contributes to and affects the Polar cap's evolution. We calculate the evolution of SubSurface Ice (SSI) pore filling by coupling two models, the Mars LMD Global Climate Model, which calculates the atmospheric and surface evolution on an annual timescale, and the dynamical version of the Mars Subsurface Ice Model, which calculates the evolution of the SSI on a millennial timescale. The SSI latitudinal boundary fluctuates over more than 25° in one obliquity cycle, overall extending equatorward of latitude ±35° at high obliquity, and receding to about ±60° at low obliquity. In locations where the SSI is stable continuously over orbital cycles, the simulations predict layering caused by a sublimation front at the SSI top boundary. Between 5 and 2.5 Myr ago, the subsurface lost at least ∼95 m of polar equivalent layer ice. The SSI flux routinely reaches ∼1 mm/Mars year. In addition to the direct contribution to the growth of the North Polar Layered Deposits (NPLD), the SSI causes variations in the NPLD accumulation rate due to the changes in the SSI distribution that affect the seasonal energy budget. These variations are comparable to the change in rate due to variations in orbital elements. When running paleo-climate simulations, particularly to reconstruct the NPLD profile, changes in the SSI distribution should be considered

Stratigraphic and Isotopic Evolution of the Martian Polar Caps from Paleo‐Climate Models.

International audienceExposed scarps images and ice-penetrating radar measurements in the North Polar Layered Deposits (NPLD) of Mars show alternating layers that provide an archive of past climate oscillations, that are thought to be linked to orbital variations, akin to Milankovitch cycles on Earth. We use the Laboratoire de Météorologie Dynamique (LMD) Martian Global Climate Model (GCM) to study paleoclimate states to enable a better interpretation of the NPLD physical and chemical stratigraphy. When a tropical ice reservoir is present, water vapor transport from the tropics to the poles at low obliquity is modulated by the intensity of summer. At times of low and relatively constant obliquity, the flux still varies due to other orbital elements, promoting polar layer formation. Ice migrates from the tropics towards the poles in two stages. First, when surface ice is present in the tropics, and second, when the equatorial deposit is exhausted, from ice that was previously deposited in mid-high latitudes. The polar accumulation rate is significantly higher when tropical ice is available, forming thicker layers per orbital cycle. However, the majority of the NPLD is sourced from ice that temporary resided in the mid-high latitudes and the layers become thinner as the source location moves poleward. The migration stages imprint different D/H ratios in different sections in the PLDs. The NPLD is isotopically depleted compared to the SPLD in all simulations. Thus we predict the D/H ratio of the atmosphere in contact with NPLD upper layers is biased relative to the average global ice reservoirs

Water Group Exospheres and Surface Interactions on the Moon, Mercury, and Ceres

Water ice, abundant in the outer solar system, is volatile in the inner solar system. On the largest airless bodies of the inner solar system (Mercury, the Moon, Ceres), water can be an exospheric species but also occurs in its condensed form. Mercury hosts water ice deposits in permanently shadowed regions near its poles that act as cold traps. Water ice is also present on the Moon, where these polar deposits are of great interest in the context of future lunar exploration. The lunar surface releases either OH or H2O during meteoroid showers, and both of these species are generated by reaction of implanted solar wind protons with metal oxides in the regolith. A consequence of the ongoing interaction between the solar wind and the surface is a surficial hydroxyl population that has been observed on the Moon. Dwarf planet Ceres has enough gravity to have a gravitationally-bound water exosphere, and also has permanently shadowed regions near its poles, with bright ice deposits found in the most long-lived of its cold traps. Tantalizing evidence for cold trapped water ice and exospheres of molecular water has emerged, but even basic questions remain open. The relative and absolute magnitudes of sources of water on Mercury and the Moon remain largely unknown. Exospheres can transport water to cold traps, but the efficiency of this process remains uncertain. Here, the status of observations, theory, and laboratory measurements is reviewed

Extensive water ice within Ceres’ aqueously altered regolith: Evidence from nuclear spectroscopy

International audienceWater ice beneath the surface of Ceres The dwarf planet Ceres in the inner solar system is thought to have a crust made of a mixture of rock and ice. Prettyman et al. used neutron and gamma-ray spectroscopy from the Dawn spacecraft to peer below Ceres' surface and map the subsurface composition. They found evidence for water ice across the dwarf planet, with water making up a larger fraction of the material near the poles than around the equator. Together with their measurements of other elements, these results aid our understanding of Ceres' composition and constrain models of its formation. Science , this issue p. 5