Yu. V. Skorov

Gas flow and dust acceleration in a cometary Knudsen layer

An analytical model of the innermost gas-dust coma region is proposed. The kinetic Knudsen layer adjacent to the surface of the cometary nucleus, where the initially non-equilibrium velocity distri ...

Stability of water ice under a porous nonvolatile layer: implications to the south polar layered deposits of Mars

Abstract The analysis of the Viking IRTM data led to the conclusion that the surface layer of the Mars south polar layered deposits has very low thermal inertia. This is consistent with the layered deposits being mantled by a layer consisting of very fine dust. Analysis of the heat and mass transport through such a layer allows us to estimate the minimum thickness of a dust mantle required to guarantee water ice stability beneath it. We find that if the thermal inertia is as low as 100 J m −2 s −1/2 K −1 and the dust layer is composed of particles of about 10 μm , then a 5 mm thick layer is sufficient to keep the ground water ice stable on at least seasonal time scales.

Simulation of gas flow in a cometary Knudsen layer

A numerical model of the nonequilibrium innermost coma region in active comets is proposed. The presence of dust grains is neglected for a first approximation. We consider the Knudsen layer adjacen ...

A model of short-lived outbursts on the 67P from fractured terrains

Aims. We develop a physical model to explain the potent outbursts that occurred in the fractured terrain of comet 67P near perihelion, and predict its temporal characteristics. Methods. The feasibility of the proposed mechanism is studied using a numerical model accounting for the relevant microscopic/macroscopic processes. We rely on the thermophysical, compositional, and geo-morphological data from the published measurements of respective instruments on board Rosetta. Results. The key idea of this novel mechanism is built around observations of fractures/cracks in the region of interest. It is argued that as the stresses on the nucleus increased during the perihelion approach, a crack deepening event occurred reaching the deeper material containing super-volatile ices in equilibrium with the surrounding. This sudden opening lead to a violent sublimation of the super-volatile ices. The time scales and mass release of this process are modeled and reported. In our modeling we pay attention to the question of the existence of super-volatile ices in the deeper interior for a long time, and the thermal equilibrium in the interior. Conclusions. The deepening of pre-existing cracks (fracture) into the material containing highly volatile ices can explain the observed outburst features. The sudden disequilibration of the steady-state reservoir of highly volatile ices results in a violent release of gas and dust. The proposed mechanism also explains the rapid shut down of this activity in accordance with the observations. The proposed mechanism is independent of solar illumination history of a given region, or the pre-existance of large sealed nucleus cavities.

Evolution of ice surfaces within porous near-surface layers on cometary nuclei

Abstract The small-scale structure of a cometary surface layer determines to a large extent the way in which cometary activity develops. The strong temperature variations on a rotating cometary nucleus and the corresponding sublimation and condensation processes make it probable that the surface is rough, with many caverns, cracks, and pores on size scales from meters down to the sub-millimeter range. Present work describes first results of theoretical models able to describe some of the aspects of the “shape-forming” processes active on a cometary nucleus. Monte Carlo simulations and analytical methods are used to study the gas flow within a porous surface layer. Dust pores with sublimation from ice filled bottoms as well as ice covered walls are considered. It is found that the erosion of an ice-filled channel embedded in a matrix composed of non-volatile material is effectively limited by the gas recondensing at the bottom due to the back flux of molecules reflected from the side walls. Inhomogeneities of the sublimating bottom tend to be leveled out, resulting in a sublimation surface of slightly parabolic shape. Hence it is correct to use Clausing formula for gas flux out of porous media modelled by a bundle of tubes, even if the shape of the sublimating surface at the bottom of each tube is a priori not known.

Estimating the Strength of the Nucleus Material of Comet 67P Churyumov-Gerasimenko

Consideration is given to the estimates for the strength of the consolidated material forming the bulk of the nucleus of comet 67P Churyumov–Gerasimenko and those for the strength of the surface material overlying the consolidated material at the sites of the first and last contact of the Philae lander with the nucleus. The strength of the consolidated material was estimated by analyzing the terrain characteristics of the steep cliffs, where the material is exposed on the surface. Based on these estimates, the tensile strength of the material is in the range from 1.5 to 100 Pa; the shear strength, from ∼13 to ⩾30 Pa; and the compressive strength, from 30 to 150 Pa, possibly up to 1.5 kPa. These are very low strength values. Given the dependence of the measurement results on the size of the measured object, they are similar to those of fresh dry snow at –10°C. The (compressive) strength of the surface material at the site of the first touchdown of Philae on the nucleus is estimated from the measurements of the dynamics of the surface impact by the spacecraft’s legs and the geometry of the impact pits as 1–3 kPa. For comparison with the measurement results for ice-containing materials in terrestrial laboratories, it needs to be taken into account that the rate of deformation by Philae’s legs is four orders of magnitude higher than that in typical terrestrial measurements, leading to a possible overestimation of the strength by roughly an order of magnitude. There was an attemp to put one of the MUPUS sensors into the surface material at the site of the last contact of Philae with the nucleus. Noticeable penetration of the tester probe was not achieved that led to estimation of the minimum compressive strength of the material to be ⩾4 MPa4 This fairly high strength appears to indicate the presence of highly porous ice with grains “frozen” at contacts.

Is near-surface ice the driver of dust activity on 67P/Churyumov-Gerasimenko

Context. Mainly for historical reasons, nearly all of the current thermophysical models of dust activity rely on the poorly justified assumption of cohesionless dust lifted by a gas drag force against the weak nucleus gravity. The interpretation of Rosetta data and our understanding of comet activity is particularly sensitive to this assumption. Aims. We investigate the role that cohesion forces among the dust grains play in the evolution of temperature and pressure at the ice-dust interface and the resulting dust activity (lifting). Methods. We used a 1D thermophysical numerical model that provides a realistic description of cohesion forces among dust aggregates. Several conditions of solar illumination on the nucleus are investigated for the H 2 O, CO, and CO 2 ices below the dust layer. We examine a wide range of dust grain sizes. Results. The simulations confirm an increase in temperature and pressure at the ice boundary between the two model layers with respect to exposed pure ice. Furthermore, we show that a non-monotonic behavior of temperature and pressure versus layer thickness is expected at the ice-dust interface for fine aggregates (of sizes ≤30 μ m), but not for the larger grains. The ratio of vapor pressure to the physically determined tensile strength for various agglomerate sizes and layer thicknesses provides further evidence that the gas drag is not sufficient to remove dust grains of sizes Conclusions. In the framework of the presented model, which can be considered common in terms of assumptions and physical parameters in the cometary community, the dust removal by a gas drag force is not a plausible physical mechanism. The sublimation of not only water ice, but also of super-volatile ice (i.e., CO) is unable to remove dust grains for illumination conditions corresponding to 1.3 AU. A way out of this impasse requires revision of the most common model assumption employed by the cometary community.

Dynamics of Dust Particles of Different Structure: Application to the Modeling of Dust Motion in the Vicinity of the Nucleus of Comet 67P/Churyumov–Gerasimenko

We consider the estimates of the main forces acting on dust particles near a cometary nucleus. On the basis of these estimates, the motion of dust particles of different structure and mass is analyzed. We consider the following forces: (1) the cometary nucleus gravity, (2) the solar radiation pressure, and (3) the drag on dust particles by a flow of gas produced in the sublimation of cometary ice. These forces are important for modeling the motion of dust particles relative to the cometary nucleus and may substantially influence the dust transfer over its surface. In the simulations, solid silicate spheres and homogeneous ballistic aggregates are used as model particles. Moreover, we propose a technique to build hierarchic aggregates—a new model of quasi-spherical porous particles. A hierarchic type of aggregates makes it possible to model rather large dust particles, up to a millimeter in size and larger, while no important requirements for computer resources are imposed. We have shown that the properties of such particles differ from those of classical porous ballistic aggregates, which are usually considered in the cometary physics problems, and considering the microscopic structure of particles is of crucial significance for the analysis of the observational data. With the described models, we study the dust dynamics near the nucleus of comet 67P/Churyumov–Gerasimenko at an early stage of the Rosetta probe observations when the comet was approximately at 3.2 AU from the Sun. The interrelations between the main forces acting on dust aggregates at difference distances from the nucleus have been obtained. The dependence of the velocity of dust aggregates on their mass has been found. The numerical modeling results and the data of spaceborne observations with the Grain Impact Analyzer and Dust Accumulator (GIADA) and the Cometary Secondary Ion Mass Analyzer (COSIMA) onboard the Rosetta probe are compared at a quantitative level.