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31.
Mars is the fourth planet out from the sun. It is a terrestrial planet with a density suggesting a composition roughly similar
to that of the Earth. Its orbital period is 687 days, its orbital eccentricity is 0.093 and its rotational period is about
24 hours. Mars has two small moons of asteroidal shapes and sizes (about 11 and 6 km mean radius), the bigger of which, Phobos,
orbits with decreasing semimajor orbit axis. The decrease of the orbit is caused by the dissipation of tidal energy in the
Martian mantle. The other satellite, Deimos, orbits close to the synchronous position where the rotation period of a planet
equals the orbital period of its satellite and has hardly evolved with time. Mars has a tenous atmosphere composed mostly
of CO with strong winds and with large scale aeolian transport of surface material during dust storms and in sublimation-condensation
cycles between the polar caps. The planet has a small magnetic field, probably not generated by dynamo action in the core
but possibly due to remnant magnetization of crustal rock acquired earlier from a stronger magnetic field generated by a now
dead core dynamo. A dynamo powered by thermal power alone would have ceased a few billions of years ago as the core cooled
to an extent that it became stably stratified. Mars' topography and its gravity field are dominated by the Tharsis bulge,
a huge dome of volcanic origin. Tharsis was the major center of volcanic activity, a second center is Elysium about 100° in
longitude away. The Tharsis bulge is a major contributor to the non-hydrostaticity of the planet's figure. The moment of inertia
factor together with the mass and the radius presently is the most useful constraint for geophysical models of the Martian
interior. It has recently been determined by Doppler range measurements to the Mars Pathfinder Lander to be (Folkner et al. 1997). In addition, models of the interior structure use the chemistry of the SNC meteorites which are widely
believed to have originated on Mars. According to the models, Mars is a differentiated planet with a 100 to 200 km thick basaltic
crust, a metallic core with a radius of approximately half the planetary radius, and a silicate mantle. Mantle dynamics is
essential in forming the elements of the surface tectonics. Models of mantle convection find that the pressure-induced phase
transformations of -olivine to -spinel, -spinel to -spinel, and -spinel to perovskite play major roles in the evolution of mantle flow fields and mantle temperature. It is not very likely
that the -spinel to perovskite transition is present in Mars today, but a few 100 km thick layer of perovskite may have been present
in the lower mantle immediately above the core-mantle boundary early in the Martian history when mantle temperatures were
hotter than today. The phase transitions act to reduce the number of upwellings to a few major plumes which is consistent
with the bipolar distribution of volcanic centers of Mars. The phase transitions also cause a partial layering of the lower
mantle which keeps the lower mantle and the core from extensive cooling over the past aeons. A relatively hot, fluid core
is the most widely accepted explanation for the present lack of a self-generated magnetic field. Growth of an inner core which
requires sub-liquidus temperatures in the core would have provided an efficient mechanism to power a dynamo up to the present
day.
Received 10 May 1997 相似文献
32.
An Approach for the Classification of Urban Building Structures Based on Discriminant Analysis Techniques 总被引：4，自引：0，他引：4
Recognition of urban structures is of interest in cartography and urban modelling. While a broad range of typologies of urban patterns have been published in the last century, relatively little research on the automated recognition of such structures exists. This work presents a sample‐based approach for the recognition of five types of urban structures: (1) inner city areas, (2) industrial and commercial areas, (3) urban areas, (4) suburban areas and (5) rural areas. The classification approach is based only on the characterisation of building geometries with morphological measures derived from perceptual principles of Gestalt psychology. Thereby, size, shape and density of buildings are evaluated. After defining the research questions we develop the classification methodology and evaluate the approach with respect to several aspects. The experiments focus on the impact of different classification algorithms, correlations and contributions of measures, parameterisation of buffer‐based indices, and mode filtering. In addition to that, we investigate the influence of scale and regional factors. The results show that the chosen approach is generally successful. It turns out that scale, algorithm parameterisation, and regional heterogeneity of building structures substantially influence the classification performance. 相似文献
33.
In the present study, the temperature- and pressure-dependent transport and thermal properties, i.e., viscosity, phonon thermal conductivity, thermal expansivity and heat capacities, as well as electronic and radiative thermal conductivities, have been derived for the mantles of super-Earths. These properties are necessary to understand the interior dynamics and the thermal evolution of those planets. We assume that the mantles consist of MgSiO3 perovskite (pv), but we discuss the effects of the post-perovskite transition, and we elaborate on an addition of periclase MgO and incorporated Fe. However, MgO is found to only significantly influence the phonon thermal conductivity – the viscosities, heat capacities and thermal expansivities of pv and MgO remain comparable. We use the Keane theory of solids, which takes into account the behavior of solid matter at the infinite pressure limit, adopt the Keane equations of state, and adjust for pv and MgO by comparison with experimental high-pressure and high-temperature data. We find the theory of the infinite pressure limit of Keane to be in excellent agreement with recent ab initio studies and experiments. To calculate the melting curve, we further use the Lindemann–Stacey scaling law and fit it to available experimental data. The best data fitting melting temperature for pv reaches 5700 K at 135 GPa and increases to 20,000 K at 1.1 TPa, corresponding to the core-mantle boundary of a 10 Earth mass super-Earth (10MEarth). We find the pv adiabatic temperature (with a potential temperature of 1700 K) to reach 2570 K at 135 GPa and 5000 K at 1.1 TPa. To calculate the pressure-and temperature-dependent viscosity, we use the semi-empirical homologous temperature scaling to relate enthalpy change, and hence viscosity, to the melting temperature. We find that the resulting activation volume of pv decreases from 2.8 cm3/mol at 25 GPa to 1.4 cm3/mol at 1.1 TPa-resulting in a viscosity increase by ～15 orders of magnitude through the adiabatic mantle of a 10MEarth planet. Furthermore, the thermal expansivity (of pv and MgO) decreases by a factor of eight, and the total thermal conductivity (phonon, radiative and electronic) of an Earth-like pv/MgO composite increases by a factor of seven through an adiabatic mantle of a 10MEarth super-Earth. At higher temperatures, i.e., for super-adiabatic temperature profiles, the electronic and radiative thermal conductivities strongly increase and dominate the conductive heat transport. All findings indicate an increase of heat transfer solely by conduction in the lower mantles of super-Earths. Thus our results disagree with Earth-biased full-mantle convection assumptions made by previous models for super-Earths, and additionally raise questions about the differentiation of massive rocky exoplanets and their ability to generate magnetic fields or sustain plate tectonics. 相似文献
34.