Surface maps. Introductory text.

This section of the Atlas presents relief maps of the surfaces of the terrestrial planets - Mercury, Venus, the Earth and Mars, and the Moon and Phobos (one of the moons of Mars). The scale, the map compilation, the choice of a common projection and common way of relief representation were aimed at showing the general surface features and the characteristic distinctions of these celestial bodies.

Certain relief features from the initial phase of surface formation, are only presented on Mercury today (the same is true of the Moon). Craters are the predominant relief feature of Mercury. The majority of them are of impact origin. Their dimensions vary from many kilometers (Beethoven, Tolstoj Crateres and others) down to the limit of resolution of the most detailed images. The largest impact crater (basin) on Mercury - Caloris Planitia -is up to 1,300 km in diameter.
Two types of topography can be delineated on the surface of Mercury: highland regions characterized by a large number of craters, and mare type surfaces which are slightly cratered and called smooth plains.
Regions of highland relief are predominant in the southern hemisphere with many large and highly eroded craters what confirms the ancienty of these surfaces on Mercury. Highlands are evidently noticeable between the accumulations of large-scale craters. They are characterized by a large number of crateres with the diameter range of 5-15 km. These still existing regions are obviously the results of ancient volcanic activity on Mercury.

Smooth plains of Mercury are widely spread in the northern hemisphere (Sobkou, Suisei, Borealis Planitiae and others). They form small basins within the close to equator zones. Their diameter is not more than 400 km. Smooth plains are probably, the result of flood basaltic volcanic activity. Significantly few craters can be seen here. And the surface is complicated by dorsa and fossae. In this respect they are very similar to the Moon's inland mare.

The results of tectonic activity can also be marked out on Mercury. These are systems of fractures around the large craters and basins. Tectonic features typical of Mercury are clearly seen in the structural network of fractures only. These are gigantic rupes - scarps originated in the thrust-faults of zones of the planetary crust. Discovery, Hero, Fram Rupes and others are the largest of them. They have the heights of up to 2 km and the length of more than 1,000 km.
The information about the general planetary relief peculiarities of the surface of Venus on the whole was obtained at the end of the 70s - the beginning of the 80s only with the help of artificial satellites of the planet - the American spacecraft "Pioneer-Venus 1" and the Soviet interplanetary automatic stations - "Venera 15, 16". Altitudes are figured for Venus from a certain assumed datum. As such a level we can assume a sphere of the median radius equal to 6,051.6 km (it divides all the area of the surface in half), or a sphere of mean radius equal to 6,051.4 km, or, for example, a sphere of the radius equal to 6,051.0 km. The latter is used for mapping Venus. Mountainous regions, hilly planitiae and lowlands are marked out relative to this datum of Venus.

Hilly planitiae with marks from 0 up to +2 km occupy the largestpart of the planetary surface. Smoothed hilly or ridgy relief with the overfall of altitudes being not more than 0.5 km is typical of them. There are craters of an impact origin but their depth is significantly less than for the craters on the other celestial bodies.  Basaltic materials determine mainly the composition of the plains of Venus. The result of the analysis is indicative ; of the character and the composition of the >. rocks on the surface for the landing places of the automatic stations "Venera 9,10,13,14" i and "Vega 1".

Morphological features of tectonic and volcanic origin were extracted for the planitiae of Venus using high resolution radar 1 imagery obtained from the interplanetary automatic stations "Venera 15,16". These are chaotic deformations, first of all. They are considerable as for their area. These regions are conventionally called "parquet" since they are formed by a system of dorsam and vallcs with a cross diagonal and orthogonal structure of imagery (Fortuna, Laima Tesserae and others). The ring-shaped structures of tectonic and magmatic origin called о voids and coronac are not of considerable interest. These arc groups of transverses having length from 170 to 600 km which arc composed by residual mountains or mensae of ring-shaped concentric dorsam. They are often covered by later lava sheets (Feronia, Rananeida Coronae and others).

Lowlands with marks from 0 to -2 km occupy small areas on the surface of Venus. The largest part of them is concentrated in the northern hemisphere of the planet. Snegu-rochka Planitia with the diameter of more than 3,000 km and the deepest depression of Venus - Atalanta Planitia - are marked out in the northern polar zone. In the southern direction from the North Pole Guinevere, Sedna, Leda Planitiae have merged into a longitudinal zone . subjacent to the polar zone. Here, besides craters of impact origin, a lot of tholis, calderas and fissures are revealed within these planitiae. These relief features indicate their volcanic origin.

High mountain plana on Venus (they are analogous to the mainlands on the Earth) are characterized by the marks of +2 - +11 km. The largest of them - Ishtar and Aphrodite Terrae and Beta Regio - are marked out by a very complex and rugged topography, large-scale linear and circle-shaped structures, stretched mountain systems. The greatest among these regions - Aphrodite Terra - is approximately equal to Africa as for its area. Large-scale linear nonuniformity looks like crushing zones or transform faults on the Earth.

The structure of the second by size highland of Venus - Ishtar Terra - is of interest as well. Its area is approximately equal to the square of Australia. The high mountain Lakshmi Planum is located in its south-western part. The Lakshmi Planum is circled, by the Akna and Freyja Montes in the north and north-west, and it borders upon the Maxwell Montes in the east. Here a large-scale Cleopatra Cratcris is well notable. Beta Regio is an example of a large-scale volcanic structure. This regio is quite possible to be a gigantic shield volcano.

The morphology of the surface of Venus on the whole meets the notion about primary spreading of the regional tectonic and volcanic processes. The activity of these processes is likely to be partially preserved in our epoch. At the same time, a conception on the evidence of structures (spreading zones) caused by processes of global tectonics, is developed: That would allow to consider Venus to be more like the Earth as for their geology.

The basic geological conceptions are naturally bounded up with the Earth since it has been studied the best of the terrestrial planets. Today most of the scientists hold a point of view that the solid cover of the Earth (lithosphere) is a totality of mobile blocks or plates with the dimensions from 2 thousand to 6 thousand km. Their dynamics is the surface result of convective flows in the mantle.
The global system of the mid-oceanic dorsa with the axial rift zones stretched for 64 thousand km is clearly marked out on the bed of the Global Ocean. Permanent accumulation of a new oceanic crust takes place on the axis of the rift zone because of the mantle substance to outgo through fractures. Lithospheric plates move apart from the axis of the rift sliding on the softened underlying layer (astenosphere). I.e. the process of spreading is caused by convective processes in the mantle.
The largest fractures in the bed of the oceans and sometimes in the continents (so called transform faults) are the result of interaction of solid edges when lithospheric plates move along the astenosphere. The seismic zones on the borders of plates are caused by the same reason. A part of the plates includes the present mainlands - North America, European and Asian mainlands and others. The other part is completely oceanic - the Pacific, the Carribean, the Nazca plates and others.

The solid oceanic lithosphere deepens into the mantle in the deep-sea throughs located on the periphery of the oceanic basins. The hypocenters of earthquakes are indicative of it since their plane goes sloping under the mainland down the depth of about 700 km.
So the lithospheric plates draw together, and one of the plates underthrusts the other within these zones - the zones of subduction. Lighter melts and separate relief features which cannot sink into the mantle, are accumulated building up mainlands. Volcanism is widely developed along the borders of the plates where they are drawn together and absorbed. The well-known "firing ring" of the Pacific is an example of this process. Island arcs are regarded to be zones where the mainland plates undcrthrust the oceanic ones. Catenae of island arcs bordering upon the north-western and western parts of the Pacific - the Aleutian, the Kuril, the Japan Islands, the Indonesian archipelago, - are typical examples of this structure.

The real dimensions of the mainlands are larger than their part above water, and they include mainland shoal-shelf. That is a peculiarity of the mainlands of the Earth's crust. The length of the mainland shelf zone makes often up to hundreds of kilometers in the direction of the ocean where it divides the mainland part from the ocean bed by a sharp rupes.
The borders of the lithospheric plates are evident to be the main structural features of the Earth's crust along with the lithospheric zones of accumulation and absorption. The most contrast modern relief features are connected with them. The same is true of seis-micity and for the largest part of magmatic phenomena. So, the movement and interaction of the lithospheric plates defined in the geological past, and define today the process of the Earth's surface development.

It has been noted above, that the features of spreading are marked in a number of regions on Venus. Alongside, it should be emphasized that as distinct from the Earth, structures similar to the oceanic troughs and associated with the zones of subduction are not revealed on surface of Venus. This doesn't allow us to answer the question if the processes of horizontal dislocations of Venus had or have a global character and to what extent they are similar to the processes of tectonics of the lithospheric plates on our planet.

The Moon is one of the largest moons of the Solar system. Its size is 1.5 times smaller than the size of Mercury. Similar features are marked for the morphology of the Moon and Mercury surface structures. Because of its significant mass, the stages of the thermal history of the Moon seem to be similar to the typical processes of the terrestrial-type planets. These processes result in differentiation of the substance from its interiors to the cover and define laws of its surface structure which are analogous to the planetary ones.
Megarelief of the Moon originated because of the early ending of its thermal history. It is characterized by the evidence of the two main types of surfaces - mares and mainlands. The largest part of the lunar surface is represented by the mainland shield with a very high density of craters of an impact origin. The diameter of the craters varies from the largest multi-ringed basins like Mare
Orientale, to the microcraters being noticeable on the samples of the Moon's rock delivered to the Earth. There are extended mountain systems on the highlands of the Moon. They mainly frame large-scale depressions in the Moon's crust like, for example, Montes Cordillera cycling Mare Orientale or the Caucasus, the Apennines and the Alps around Mare Imbrium.
The oceanic depressions - the mares of the Moon - occupy only 17% of the Lunar surface. They are mainly located on its visible side. The mares are supposed to be large-scale basins of an impact origin filled up by lava. Such are Mare Imbrium (diameter is 1,200 km), Mare Serenitatis (800 km), Mare Crisium (500 km) and others.

The tectonic structure of the Moon's surface is characterized by a system of radial and ring-shaped fractures which are genetically connected with the large-scale craters and basins. Rectilinear and sinuous fossae, fissures and rupes like, for example, the well-known Recti Montes and Mare Nubium, arc relief features of tectonic origin.

Asymmetry clearly expressed for the location of the main structures in the crust, is typical of the surface of Mars as well as of the surfaces of the Earth, the Moon and Mercury. The main structures are parts of highlands and mare depressions with significantly different features of the morphology and hyp-sometry.

The mainlands of the crust of Mars are slightly elevated (3-4 km above) relative to the reference 0 surface (a sphere with the radius equal to 3,393.4 km) and occupy practically all the southern hemisphere. Craters of an impact origin are the predominant relief feature of the highlands. Several of them have gigantic sizes, such as, for example, Schiaparelli (diameter is 400 km), Huygens, Antoniadi Crateres and others.
The largest part of them is strongly eroded by exogenous processes. The two largest circle-shaped depressions - Argyre and Hellas Planitiae - are located in the highland hemisphere of Mars. They are most likely of a meteorite origin.
The northern hemisphere of Mars differs by a low hypsometric level and more plane surface on which significantly fewer craters are to be distinguished. The northern polar rcgio is represented by Vastitas Borealis several parts of which are 3 km below the mean datum. The vast Arcadia, Acidalia, Utopia Planitiae and the smaller ones - Chryse, Amazonis and other Planitiae - border upon Vastitas Borealis in the south. The surface of the planitiae is composed of basaltic lava. Their morphology is mainly characterized by small features - craters, dorsa, lava rupes, eolian and cryogenic structures.

The borderlands between the oceanic planitiae and the highland are marked by a global mpes with the height up to 2 km and by a system of fractures. Two gigantic tholi -Tharsis Tholus and Elysium Planum - are evident in this transitional zone. The largest of Mars and the Solar system volcanoes - the Olympus (height is 26 km), the Arsia (25 km), the Ascraeus (25 km) and the Pavonis (22 km) Montes - are marked within the volcanic Tharsis Tholus. The montes of lower heights - the Elysium Mons and the Hecates Tholus with the heights of up to 10 km - are evident within the volcanic Elysium Planum.
As distinct from the Earth (and maybe, from Venus), no obvious indications of horizontal dislocations of the lithospheric blocks are observed on the surface of Mars. So, significant overfalls of the global relief are possible to be mainly caused on the planet by vertical dislocations of the parts of the planet's crust. The similar mechanism of relief forming is explained by the well-known conception of fixism which was actively developed conformably to the Earth in the period of the 40s-60s.

Rift-shaped structures and systems of fractures in the areas of volcanic upwarpings are indicative of tectonic activity on Mars in the remote past. The largest rift-shaped structure of Mars is a system of Vallcs Marineris. It extends for more than 4,000 km along the equatorial zone and divides the eastern part of the Tharsis Tholus. The smallest as for the size system of Kasei Vallcs breaks up the Tharsis Tholus in its north-eastern part. There arc meandering valles of dry river beds and river systems on the surface of Mars. These structures may be possible indications of the fluvial erosion processes on the planet in the remote past.

The relief of Phobos surface became one of supplementary arguments for the conception on the prevalent impact origin of craters on the planetary surfaces. Phobos is a solid celestial body. The relief of its surface was exclusively developed under the influence of cosmogenic factors since processes typical for the terrestrial planets, never took place inside them. The surface of Phobos is rich in craters. The largest of them is Stickney. Its diameter is about 11 km which is about one third of the moon's diameter. It is possible to distinguish three groups of morphological features on the surface of Phobos besides the crateres of an impact origin. They are: rectilinear depressions caused, possibly, by the formation of the Stickney craters, catenae of craters having an oblong and irregular form, parallel linear structures of grooves. Their origin is connected with the tidal influence of Mars.
The sketch map gives the main idea of the relief of the surface of Deimos. It is given in section XII.