Leszek Czechowski

Solid state convection in the icy satellites: numerical results

Abstract Solid state convection in the mantle of icy satellites of giant planets is investigated using numerical models. We consider differentiated and non-differentiated satellites with both free (that is no horizontal stresses) and rigid (that is zero velocity) boundary conditions at the body surface. Internal heating originates from long lived radioactive elements uniformly distributed within rocky component of satellites (with initial chondritic concentration). Two modes of internal heating by radioactive sources are considered: uniform distribution of radioactive elements within undifferentiated bodies and heating of the icy mantle by heat flux from the rocky core. The calculations were performed for the range of Rayleigh number from 10 000 to 300 000. It corresponds to the medium size icy satellites in radii from the range 200 – 800 km. We found that convection velocity of the order of 1 mm yr −1 is typical for considered bodies. This is at least one order of magnitude less than the velocity in the Earths mantle but even such slow convection could be an important factor changing global tectonic activity, asymmetry of hemispheres, gravitational field and shape of the satellites.

Comparison of Early Evolutions of Mimas and Enceladus

Thermal history of Mimas and Enceladus is investigated from the beginning of accretion to 400 Myr. The numerical model of convection combined with the parameterized theory is used. The following heat sources are included: short lived and long lived radioactive isotopes, accretion, serpentinization, and phase changes. The heat transfer processes are: conduction, solid state convection, and liquid state convection. We find that temperature of Mimas’ interior was significantly lower than that of Enceladus. If Mimas accreted 1.8 Myr after CAI then the internal melting and differentiation did not occur at all. Comparison of thermal models of Mimas and Enceladus indicates that conditions favorable for the start of tidal heating lasted for a short time (~107 yr) in Mimas and for ~108 yr in Enceladus. This could explain the Mimas—Enceladus paradox. In fact, in view of the chronology based on cometary impact rate, one cannot discard a possibility that also Mimas was for some time active and it has the interior differentiated on porous core and icy mantle.

Thermal history and large scale differentiation of the Saturn’s satellite Rhea

Thermal history of Rhea from the beginning of accretion is investigated. We developed a numerical model of convection combined with the parameterized theory. Large scale melting of the satellite’s matter and gravitational differentiation of silicates from ices are included. The results are confronted with observational data from Cassini spacecraft that indicate minor differentiation of the satellite’s interior. We suggest that partial differentiation of the satellite’s interior is accompanied (or followed) by the process of light fraction uprising to the surface. The calculation indicates that the partial differentiation of the matter of the satellite’s interior is possible only for narrow range of parameters. In particular, we found that the time from the formation of CAI (calciumaluminum rich inclusions in chondrites) to the end of accretion of Rhea is in the range of 3–4 My.

Solid state convection in the icy satellites: discussion of its possibility

Abstract Surface features of Enceladus, Tethys, Dione, Iapetus, Miranda, Ariel, and Titania indicate that these satellites, with radii from the range 252–879 km, are highly or at least moderately modified due to internal tectonic activity. Detailed studies of known surfaces show that Enceladus is probably still geologically active at present while Tethys, Dione, Miranda, Ariel, and Titania were active in the recent past. Convection is one of the processes responsible for the evolution of the bodies of Solar System, including the evolution of icy satellites. We focus on studying the possibility of convection within the medium sized icy satellites. Thermally driven convection of solid satellite material as potential cause of surface evolution is considered for two cases: non-differentiated icy-mineral satellites and differentiated satellites with icy mantle and rocky core. Discussion of the parameters of icy/rocky mixture indicates that the Rayleigh number is higher than critical value for onset of convection.

Remarks on the Iapetus’ bulge and ridge

Iapetus is a medium sized icy satellite of Saturn. It has two spectacular features: the equatorial ridge (ER) and the abnormally large flattening. The flattening is usually explained in terms of large non-hydrostatic fossil equatorial bulge (EB) supported by a thick lithosphere. Here we show, building on the principle of isostasy, that EB and ER could be a result of low density roots underlying the lithosphere below the equator. The low density matter formed the layer over the core of the satellite. Such situation was unstable. The instability led to origin of axially symmetric plumes that formed equatorial bulge and equatorial ridge. So, we explain both: EB and ER in the frame of one hypothesis.

On Geophysical Explanation of some Irregular Variations of the Mean Latitudes of Latitude Stations

Mean latitude variations computed by Orlovs or other filters have some irregular variations in addition to secular ones. These are of the order of ±0.05 to ±0.1, they can last several years and sometimes show regional similarities. In looking for an explanation of such latitude variations several physical mechanisms have been investigated. The most probable one is the mechanism of stress propagation in the lithosphere and asthenosphere. The consequent gravitational and deformational effects could explain both the magnitude and the time dependance of the irregular latitude variations.

Early Thermal History of Rhea: The Role of Serpentinization and Liquid State Convection

Early thermal history of Rhea is investigated. The role of the following parameters of the model is investigated: time of beginning of accretion, tini, duration of accretion, tac, viscosity of ice close to the melting point, η0, activation energy in the formula for viscosity, E, thermal conductivity of silicate component, ksil, ammonia content, XNH3, and energy of serpentinization, cserp. We found that tini and tac are crucial for evolution. All other parameters are also important, but no dramatic differences are found for realistic values. The process of differentiation is also investigated. It is found that liquid state convection could delay the differentiation for hundreds of My. The results are confronted with observational data from Cassini spacecraft. It is possible that differentiation is fully completed but the density of formed core is close to the mean density. If this interpretation is correct, then Rhea could have accreted any time before 3–4 My after formation of CAI.

The magmatic activity on asteroids

The tidal effects on a fractured asteroid are considered. The asteroid is assumed to consist of two parts. In gravitational field of another body the motion of one part of the asteroid in relation to second part may be initiated. The necessary conditions for this motion are determined and amount of heat that can be generated is calculated for some cases. It is suggested that metamorphic episodes found in some meteorites are the results of such heating.

Density distribution in medium-sized icy satellites of giant planets

We discuss processes that determine the distribution of density in the medium-sized icy satellites (MIS). Gravitational differentiation, porosity and phase transitions lead to a spherical distribution, while thermal convection, large impacts and tidal deformation can result in a non-spherical distribution. According to our previous research, convective patterns in MIS can consist of one or two convective cells for radiogenic and tidal heating. The shift of the center of mass ΔrCM and the ratio of moments of inertia IZ/IXY are calculated using a numerical model of convection. A new dimensionless number C is introduced to describe the deformation of the surface. We found that ΔrCM can reach ≈0.5% of the satellite radius for the one-cell pattern. With the two-cell pattern the moment of inertia, IZ, can be reduced by ≈0.4%. The impact cratering could be one cause of significant changes in ΔrCM and IZ /IXY but only for the smallest of the MIS. Tidal deformation could result in the enhancement of mass redistribution caused by other mechanisms.

The Origin of Hotspots and The D” Layer

Most of the volcanoes occur at plate boundaries. However there are also about 50 other volcanic centers, that are either remote from plate boundaries or that are anomalous in the volume or chemistry material. They are known as hotspots. The hotspots are believed to be a result of narrow hot ascending convective currents — so called “plumes”. The origin of plumes are usually linked to the processes in D″ layer, just above core-mantle boundary.