An analysis of data on the evolution of the Earth's crust shows that it breaks from time to time /4/, which leads to accumulation of sedimentary strata in fault zones; this stage gives way to orogenesis, followed by aging (erosion and leveling) of the mountains, and finally their conversion to plain, platform regions. Consequently, the relief of the sial crust periodically grows complex, and then simple. At certain times, the "fault" may expose the basalt substratum, which is then covered with sediments and no longer shows on the surface. As a result, the frequency curves of earlier epochs apparently alter nated between one and two maxima. Moreover, in distinction from the recent curve, the oceanic level played a highly inferior role. This is proved by the fact that sediments were then deposited in shallow seas, while abyssal formations are known in a few, generally questionable instances. How does this historical outlook help in characterizing the surfaces of other planets? Let us start with Mars. Its surface, as we know, is relatively plain, with the exception of the dark belts of the maria. Are these analogs of the oceanic bottom or of the great terrestrial plains? In spite of the large distance to this "near" planet, we apparently can give a fairly definite answer to this question. The point is that rose-colored rocks prevail on Mars, occupying some 60-70% of its surface. Their rose color is generally regarded as a desert varnish. However, the Martian conditions are far from desert: the planet's average temperature is far below the freezing point. It is therefore logical to attribute the color of the Martian surface to acidic or related rocks, i. e. , most of the crust of Mars can be regarded as an analog of the terrestrial granite crust. In places, however, e. g. , in maria which are obviously lower than the continents (which is the reason for the accumu lation of moisture in them), the main surface component is apparently the crustal "basalt" of Mars. An analysis of Martian topography thus shows that the main peak of the frequency curve will be accompanied by a second peak, which, though an analog of the oceanic (basalt) bed of the terrestrial oceans, is much less developed than the second terrestrial maximum (its magnitude is similar to what has apparently been the case for the Earth in past epochs).
What is the position of Mars relative to the other planets? The Moon, the smallest of well-known planets, has a diameter of 3.5 thousand km and a differentiated basic-ultrabasic crust. Mars is 6.7 thousand km in diameter and is characterized by essentially different conditions than the Moon. Finally, the Earth, with its diameter of 12.7 thousand km, has a highly differentiated three-layer crust.
Applying the extension hypothesis /2/, we may find the reason for the differences. The crustal differences of the three planets are consistent with differences in their respective sizes, which in turn are functions of the inner development of the planets. Briefly this can be summarized as follows.
The granite crust has not developed yet on the Moon because of the small size of this planet. It apparently forms on planets reaching a size of the order of 6 thousand km, whose material acquires new physical properties. This should naturally lead to a transformation of the hypsograph which, from a curve characterizing a peridotite-basalt planetary surface (e. g. , the Moon), will change to a curve typical of planets with "granitized" crusts (Mars, Earth).
Depending on the particular stage of evolution of the granite part of the crust at which a given planet is observed, the latter can have a poorly differentiated (as now on Mars) or a highly differentiated (as on Earth) surface. During the evolution from the poorly differentiated surface characteristic of Mars to the highly differentiated (actually three-layered) structure of the Earth, the frequency curve characterizing the surface topography should undergo several radical changes. Periodically appearing and disappearing, the intermediate "basalt" level (which is only poorly developed on Mars) should have a progressively greater area on the curve.
We shall now say a few words about Venus, whose surface is not accessible to direct observation. The size of this planet, which measures 12.4 thousand km together with the atmosphere, indicates, in accordance with the previous discussion, that it is well advanced on its evolutionary path. The vigorous generation of the atmosphere on its surface also points to intense processes of relief formation. The composition of the Venusian atmosphere—enormous amounts of nitrogen, oxygen, and water vapor — can only be a consequence of considerable internal activity. The large quantities of nitrogen liberated from the interior are transformed to C74 and O86 and enormous amounts of water vapor as a result of certain known nuclear reactions, under the influence of cosmic factors /2/.
The rapid growth of Venus should have broken its granitic crust (there is no question but that granite is the main constituent of the Venusian shell) resulting in the formation of deep, apparently as yet rudimentary, oceanic basins. The frequency curve of Venusian heights and depths should there fore have two maxima. To sum up, certain theoretical premises /2/ enable us to develop a fairly reliable characterization of the relief of some planets of the solar system.