EARTH AS AN EVOLVING PLANETARY SYSTEM Part 9

362 Comparative Planetary Evolution Figure 10.4 Comparison of channel cross-sections for cata-clysmic flood channels on Mars (upper two) with N. Kasei straits (Gibraltar and Bosporus) and river chan-nels on the Earth. Modified from Baker (2001). Ayres m 500 Gibraltar 0 0 5 10 km Bosporus Missoula Amazon Mississippi Altai before the terminal large impact event about 3.9 Ga. Continued fracturing and volcanism on Mars extended to at least 1000 Ma and perhaps 100 Ma. Venus Comparison with the Earth. Unlike the other terrestrial planets, Venus is similar to the Earth both in size and mean density (5.24 and 5.52 g/cm3, respectively) (Table 10.1). After correcting for pressure differences, the uncompressed density of Venus is within 1% of that of the Earth, indicating that both planets are similar in composition, with Venus having a somewhat smaller core/mantle ratio. Although both planets have similar amounts of N2 and CO2, most of the Earth’s CO2 is not in the atmosphere but in carbon-ates. Venus also differs from the Earth by the near absence of water and the high density and temperature of its atmosphere. As described later, Venus may at one time have lost massive amounts of water by the loss of hydrogen from the upper atmosphere. Unlike the Earth, Venus lacks a satellite, has a slow retrograde rotation (244 Earth days for one rota-tion), and does not have a measurable magnetic field. Because Venus orbits the Sun in only 225 days, the day on Venus (244 Earth days) is longer than the year. The absence of a magnetic field in Venus may be caused by the absence of a solid inner core because, as described in Chapter 5, crystallization of an inner core may be required for a dynamo to operate in the outer core of a planet. Of the total Venusian surface, 84% is flat rolling plains, some of which are more than 1 km above the average plain elevation. Only 8% of Members of the Solar System 363 the surface is true highlands; the remainder (16%) lies below the average radius, forming broad, shallow basins. This is unlike the topographic distribution on the Earth, which is bimodal because of plate tectonics (Fig. 10.5). The unimodal distribution of elevation on Venus does not support the existence of plate tectonics on Venus today. The spectacular Magellan imagery indicates that unlike the Earth, deformation on Venus is distributed over thousands of kilometers rather than occurring in narrow orogenic belts (Solomon et al., 1992). There are numerous examples of compressional tectonic features on Venus, such as Maxwell Montes deformational belt in the western part of Ishtar Terra (Fig. 10.6). Ishtar Terra is a highland about 3 km above the mean plan-etary radius surrounded by compressional features suggestive of tectonic convergence resulting in crustal thickening. Maxwell Montes stands 11 km above the surrounding plains and shows a wrinkle-like pattern suggestive of compressional deformation (Kreep and Hansen, 1994). The deformation in this belt appears to have occurred passively in response to horizontal stresses from below. Coronae are large circular features (60–2600 km in diameter with most 100–300 km) with a great diversity of morphologies (Stefan et al., 2001). Almost all coronae occur between 80° N and 80° S latitude and show a high concentration in equatorial areas. Venus is the only planet known to have coronae. An approximately inverse correlation between crater and corona density suggests that the volcano–tectonic process that forms coronae may be the same process that destroys craters (Stefanick and Jurdy, 1996). The most widely accepted models for the origin of coronae are those involving mantle plumes. A rising plume creates a region of uplift accompanied by radial deformation and dyke emplacement (Copp et al., 1998). Volcanism may also accompany this stage. As the plume head spreads at the base of the lithosphere, it elevates the surface, producing annuli in some coronae. This is generally followed by collapse as the plume head cools. 65 Venus 60 55 50 45 40 35 30 25 Earth 20 15 10 5 Figure 10.5 Comparison of relief on Venus and the Earth. Surface height is plotted in 1-km intervals as a function of surface area. Height is measured from the sphere of average planetary radius for Venus and from the sea level for the Earth. Modified from Pettengill et al. (1980). 0 −6 −4 −2 0 2 4 6 Altitude (km) 364 Comparative Planetary Evolution Figure 10.6 Magellan image of Maxwell Montes, the highest mountain range on Venus, which stands 11 km above the average diameter of the planet. The complex pattern of intersecting ridges and valleys reflects intense folding and shearing of the crust. Courtesy of U. S. Geological Survey. Another unique and peculiar feature of the Venusian surface is the closely packed sets of grooves and ridges known as tesserae, which appear to result from compression. A combination of structural, mechanical, topographic, and geologic evidence suggests that tesserae record interaction of deep mantle plumes with an ancient, globally thin litho-sphere, resulting in regions of thickened crust (Hansen et al., 1999). Perhaps the most important data from the Magellan mission are those related to impact craters (Kaula, 1995). Unlike the Moon, Mars, and Mercury, Venus does not preserve a record of heavy bombardment from the early history of the solar system (Price and Suppe, 1994). Crater size–age distribution shows an average age of the Venusian surface of only 600 to 400 Ma, indicating extensive resurfacing of the planetary surface at this time. Most of this resurfacing is with low-viscosity lavas, presumably mostly basalts as inferred from the Venera geochemical data. Crater distribution also indicates a rapid decline in the resurfacing rate within the last tens of millions of years. However, results suggest that some large volcanoes (72 Ma), some basalt flows (128 Ma), some rifts (130 Ma), and Members of the Solar System 365 many coronae (120 Ma), are much younger than the average age the resurfaced plains and probably represent ongoing volcanic and tectonic activity (Price et al., 1996). The differences between Venus and the Earth, with the lower bulk density of Venus, affect the nature and rates of surface processes (weathering, erosion, and deposition), tectonic processes, and volcanic processes. Because a planet’s thermal and tectonic history depends on its size and the area/mass ratio as described later, Venus and the Earth are expected to have similar histories. However, the surface features of Venus are quite different from those of the Earth, raising questions about how Venus transfers heat to the surface and whether plate tectonics has ever been active. The chief differences between the Earth and Venus appear to have two underlying causes: (1) small differences in plan-etary mass leading to different cooling, degassing, and tectonic histories, and (2) differ-ences in distance from the Sun, resulting in different atmospheric histories. Surface Composition. Much has been learned about the surface of Venus from scientific missions by the United States and Russia. The Russian Venera landings on the Venusian surface have provided a large amount of data on the structure and composition of the crust. Results suggest that most of the Venusian surface is composed of blocky bedrock surfaces and that less than one-fourth contains porous, soil-like material (McGill et al., 1983). The Venera Landers have also revealed the presence of abundant volcanic features, complex tectonic deformation, and unusual ovoid features of probable volcanic–tectonic origin. Reflectance studies of the Venusian surface suggest that iron oxides may be important components. Partial chemical analyses made by the Venera Landers indicate that basalt is the most important rock type. The high K2O recorded by Venera 8 and 13 is suggestive of alkali basalt, and the results from the other Venera landings clearly indicate tholeiitic basalt, perhaps with geochemical affinities to terrestrial ocean-ridge tholeiites (Fig. 10.2). A Venusian crust composed chiefly of basalt is consistent with the presence of thousands of small shield volcanoes that occur on the volcanic plains, typi-cally 1 to 10 km in diameter and with slopes of about 5 degrees. The size and distribu-tion of these volcanoes resembles terrestrial oceanic-island and seamount volcanoes. Venusian Core. Venus has no global magnetic field, although it likely has a molten outer core with or without an inner core (Stevenson, 2003). The absence of a dynamo in the outer core probably reflects the lack of convection caused either because an inner core is absent or because the outer core is not cooling. If the inside of Venus is hotter than the corresponding depth in the Earth, which seems likely, an inner core is not expected. Alternatively, or in addition, the Venusian core may not be cooling at present because it is still recovering from heat loss associated with a resurfacing event some 500 Ma. Cooling and Tectonics. To understand the tectonic and volcanic processes on Venus, it is first necessary to understand how heat is lost from the mantle. Four sources of information are important in this regard: the amount of 40Ar in the Venusian atmosphere, lithosphere thickness, topography, and gravity anomalies. The amount of 40Ar in planetary atmospheres can be used as a rough index of past tectonic and volcanic activity because 366 Comparative Planetary Evolution it is produced in planetary interiors by radioactive decay and requires tectonic–volcanic processes to escape. Venus has about one-third as much 40Ar in its atmosphere as does the Earth, which implies less tectonic and volcanic activity for comparable 40K contents. In contrast to the Earth, where at least 90% of the heat is lost by the production and subduction of oceanic lithosphere, there is no evidence for plate tectonics on Venus. The difficulty of initiating and sustaining subduction on Venus is probably because of a combination of high mantle viscosity; high fault strength; and thick, relatively buoyant basaltic crust. Thus, it would appear that Venus, like the Moon, Mercury, and Mars, must lose its heat through conduction from the lithosphere, perhaps transmitted upward chiefly by mantle plumes. The base of the thermal lithosphere in terrestrial ocean basins is about 150 km deep, where the average geotherm intersects the wet mantle solidus. On Venus, however, where the mantle is likely dry, an average geotherm does not intersect the dry solidus, indicating the absence of a distinct boundary between the lithosphere and the mantle (Fig. 10.7). The base of the elastic lithosphere in ocean basins is at the 500° C isotherm, or about 50 km deep. Because 500° C is near the average surface temperature of Venus, there is no elastic lithosphere on Venus. Another important difference between Venus and Earth is the strong positive correlation between gravity and topography on Venus, implying compensation depths in the Venusian mantle of 100 to 1000 km. This requires strong coupling of the mantle and lithosphere, and hence the absence of an asthenosphere, agreeing with the thermal arguments presented previously. This situation may have arisen from a lack of water in Venus. One of the important consequences of a stiff mantle is the inability to recycle lithosphere through the mantle, again showing that plate tectonics cannot occur on Venus. The deformed plateaus and the lack of features characteristic of brittle deformation, such as long faults, suggest the Venusian lithosphere behaves more like a viscous fluid than a brittle solid. The steep-sided high-elevation plateaus on Venus, however, attest to the strength of the Venusian lithosphere. Figure 10.7 150 Comparison of geotherms from an average terrestrial ocean basin and Venus. Conduction is assumed to be the only mode of litho- 100 spheric heat transfer on Venus. Also shown are wet (0.1% water) and dry mantle solidi. 50 Base of Elastic Lithosphere OCEAN BASIN Wet Mantle Solidus Dry Mantle Solidus VENUS 00 400 800 1200 1600 Temperature (°C) ... - tailieumienphi.vn