«Chapter 2 The Terrestrial Planets General View Unlike the gaseous giant planets, which are preserved essentially unmodiﬁed in structure and ...»
The Terrestrial Planets
Unlike the gaseous giant planets, which are preserved essentially unmodiﬁed in structure
and composition since their origin 4.55 billion years ago, the terrestrial (inner) planets
(Fig. 2.1) experienced dramatic changes in the course of their evolution. This is evident in their interior structure, geology, surface landforms, and atmospheres (Fig. 2.2a).
The evolution of the inner planets was mainly controlled by both endogenous and exogenous factors involving the original storage of radionuclides, impacts, and distance from the Sun. Endogenous factors were mostly driven by heavy bombardment by asteroidsize bodies in an early epoch dating back to 4.0 billion years ago, and internal heat due to long-lived radio isotope decay, their storage being strongly dependent on the size/ mass of the planet. Impacts scarred the surface and left behind numerous craters, while internal heating was responsible for tectonic processes and widespread volcanism.
Distance from the Sun determines the incident radiation input to the planet and, hence, its thermal balance. Heat release from the interior (internal ﬂux) is negligible, in contrast to the case of the outer giant planets where it exceeds the solar incident ﬂux. Exogenous factors signiﬁcantly contributed to the planetary evolution through migration of small bodies from the outer solar system regions and collisional processes.
Planetary geology is closely related to differentiation of the planetary interiors into shells (core, mantle, and crust), accompanied by widespread tectonics and volcanism, as is clearly manifested by different patterns of the surface landforms.
All terrestrial and gaseous-icy planets have differentiated interiors, as is supported by the measured quadrupole moments of their gravitational ﬁelds and the respectively deduced dimensionless moment of inertia, I = C/MR2 (Fig. 2.2b). Here C is the moment of inertia about polar axis, M and R are mass and radius of the planet, respectively. Note that for an ideal sphere of uniformly distributed density, I = 0.4;
the less uniform the mass distribution (massive heavy core and lighter mantle in the interior), the lower the I value. This is why for the only partially differentiated © Springer Science+Business Media New York 2015 21 M.Ya. Marov, The Fundamentals of Modern Astrophysics, DOI 10.1007/978-1-4614-8730-2_2 22 2 The Terrestrial Planets Fig. 2.1 Terrestrial planets (in order from the Sun) Mercury, Venus, Earth, Mars (not in scale) (Author’s mosaics of NASA images) Fig. 2.2 (а) Internal structure of the terrestrial planets and the Moon. The order in the arrangement of main regions (core, mantle, crust) is a consequence of the differentiation of their constituent matter into shells, their extension depending on the size (mass) of the planet, the abundances of major components, and the condensation temperature in the formation zone. Mass of the body predetermines the core state and the crust (lithosphere) thickness (Adapted from Wikipedia).
(b) Parameters of the interiors (temperature T, pressure P, and dimensionless moment of inertia I) of the terrestrial planets and the Moon as compared to those of Jupiter (Credit: the Author).
(c) Velocity waves propagation and density variations within Earth based on seismic observations.
The main regions of Earth and important boundaries are labeled. This model was developed in the early 1980s and is called PREM for Preliminary Earth Reference Model (Adapted from Wikipedia) General View 23 Moon with a small core I = 0.392, whereas the minimum I values (less than 0.3) are pertinent to the gaseous giants Jupiter and Saturn with heavy iron-silicate cores and light hydrogen-helium envelopes.
Impacted structures are most clearly seen on the atmosphereless bodies such as Mercury and the Moon. Heavily cratered terrains are also preserved on the Mars surface, though in the presence of its atmosphere, ancient craters were eroded by the processes of weathering. Unlike the main mechanism of global plate tectonics on Earth, the geologies of Mars and Venus are different. On Mars, there are great ancient shield volcanoes elevated up to 26 km above the mean surface level, which (despite the relatively small size of the planet) are among the highest in the solar system. Another remarkable geologic pattern is Valles Marineris—an enormous feature 100 km wide and 8 km deep that extends along the equator for more than 3,000 km. It is poorly associated, however, with the global tectonics. On Venus, geologic structures have been revealed only with radar techniques because its very thick atmosphere and clouds fully obscure the surface when it is observed in optical wavelengths. There are no obvious tectonic features on the planet. Instead, numerous volcanoes were mostly pertinent to the planet’s thermal evolution. Volcanic activity appears to have terminated quite recently, less than 100 million years ago, though some planetary geologists believe that some limited activity is preserved today.
Thermal evolution is assumed to be partially responsible for the formation of an atmosphere of secondary origin on a terrestrial planet after the primary atmosphere (presumably retained in the process of the planet’s accumulation) was lost.
Atmospheres exist on all terrestrial planets, but Mercury has only an extremely rareﬁed gas envelope equivalent to the Earth’s exosphere. The main properties of the atmospheres of these planets are summarized in Table 2.1.
We see that the atmospheres of the neighboring planets, Venus and Mars, dramatically differ from that of Earth: the pressure at the Venusian surface reaches 92 atm and the temperature is 735 K, whereas at the surface of Mars the average pressure is only 0.006 atm and the average temperature is about 220 K. The composition of the atmospheres of both planets is mostly carbon dioxide with relatively small admixtures of nitrogen and argon and a negligible mixing ratio of water vapor and oxygen (on Mars). In contrast to Venus, which has no seasonal variations because of the very small obliquity of its equator to the ecliptic, Mars, whose obliquity is nearly similar to that of the Earth’s, exhibits pronounced seasonal variations resulting in temperature contrasts between summer and winter hemispheres exceeding 100 K. At the winter pole the temperature drops below the freezing point for CO2 and thus “dry ice” deposits cover the Martian polar caps, though their main composition is water ice. Seasonal evacuation and release of carbon dioxide in the polar regions is one of the important drivers of atmospheric planetary circulation on Mars involving both meridional and zonal wind patterns.
However, atmospheric circulation on Venus is mainly characterized by the mechanism of super-rotation, or “merry-go-round” circulation, such that a zonal wind velocity of less than 1 m/s at the surface increases up to nearly 100 m/s near the upper cloud level at about 60 km. Venus’s clouds consist of quite concentric sulfuric acid droplets, and this unusual composition complements the picture of the very exotic and hostile environment of our closest planet, which until the middle of the last century was thought to be the Earth’s twin.
In terms of natural environment, Mars is another extreme, though more favorable in its climate and therefore much more accessible for future human expansion throughout the solar system. Historically, this planet was regarded as a potential target for ﬁnding life beyond Earth, and it is still addressed as a possible site that could harbor life at the microbial level and where extant or extinct life could be found. Indeed, unlike Venus, where an assumed early ocean was lost soon after the runaway greenhouse effect responsible for its contemporary climate conditions developed, ancient Mars appears to have had plenty of water until a catastrophic drought occurred on the planet about 3.6 billion years ago for reasons that are not yet well understood. There is evidence that contemporary Mars preserved a substantial part of its water storage; its original bulk is estimated to be equivalent to nearly
0.5 km of an average ocean deep. It is thought to be stored deep under the surface as permafrost and water lenses. Recent space missions revealed many speciﬁc geologic structures which conﬁrm such a scenario, as well as the existence of subsurface water at about 1 m depth unevenly distributed over the globe, mostly in the polar regions. Anyway, the general understanding is that ancient Mars had a much more clement climate when water covered much of its surface, until its quite dense atmosphere was lost and water ice was buried beneath thick sand-dust sediments.
We shall start the inner planets’ description with our home planet Earth and its large satellite, the Moon; together they form the Earth-Moon system. Because Earth is our home, we address other terrestrial planets ﬁrst of all in terms of better understanding Earth as a solar system planet and learning what predetermined the unique path of its evolution. In other words, Earth and the Moon should be perceived in the Earth 25 context of the family of Earth-like planetary bodies which store collectively invaluable data about our past history and may help to predict the future trends. However, Earth and the Moon also serve as an important basis for comprehending the peculiarities of natural conditions which formed on other inner planets. The Moon itself can be regarded as a frame of reference to highlight the key processes of the solar system evolution.
We then discuss in a bit more detail the nature of other terrestrial planets, emphasizing our neighbor planets Venus and Mars, which are located very close (on a space scale) to Earth but evolved along completely different paths. Therefore, we may address them as extreme models of potential unfavorable Earth evolutionary paths, provided that the acting natural feedback mechanisms are slowed down. An important implication is that mankind should bear this in mind and carefully control the growing anthropogenic inﬂuence on the environment to prevent the development of risky scenarios.
Main Properties. Earth is the third planet in order from the Sun and the largest body of the four terrestrial planets. It orbits the Sun at a distance equal to semimajor axis а = 149.6 million km (1 AU) with a very small eccentricity, е = 0.017, which means that the orbit is nearly circular, although the distance varies as the Earth moves from perihelion in January to aphelion in July. The Earth moves along its orbit with a velocity V = 29.8 km/s, and its period of revolution is Рorb = 365.256 days (1 year). The sidereal period of rotation around its axis (relative to the stars, or 1 sidereal day) is Prot = 23h 56m 4.99s. The inclination of the Earth’s equator to the plane of circumsolar orbit (ecliptic) equals i = 23°27′, which ensures signiﬁcant seasonal changes on the planet.
Earth is a rather small body by the cosmic scale. Its equatorial radius is Re = 6,378 km, and its polar radius is Rp = 6,356 km; hence, its oblateness is 0.0034.
The pear-shaped ﬁgure of the Earth is called the geoid. The mass is ME = 5.974 × 1024 kg, the mean density r = 5.515 g/cm3, and the mean acceleration due to gravity is g = 9.78 m/s2 (it is a bit more at the poles compared to the equator).
The Earth’s surface is represented by the continents and the oceans, the oceans occupying nearly two-thirds of the whole surface. 94 % of the Earth’s bulk composition is O, Mg, Si, and Fe. Together with Al and Ca these are rock-forming elements, whereas C and N are volatile-forming elements. Their abundance is 1 %. The elemental composition of continents and oceans is quite different. The age of the Earth (dated as the time of the solar system origin based on the dating of calciumaluminum inclusions (CAIs) in the Allende and Efremovka meteorites; see Chap. 4 for more details) is established to be 4,567.5 ± 0.5 million years.
Geology. Earth is a geologically evolved planet. The main geological mechanism is global plate tectonics, which means that its outer shell (lithosphere) is not homogeneous but split into 12 large plates that are laterally mobile (Fig. 2.3). The mechanism 26 2 The Terrestrial Planets Fig. 2.3 The ﬂoor of the Earth’s oceans. The relief bears traces of lithospheric plate tectonics involving middle-ocean spreading zone and subduction zones at the edges of ocens connected with the most powerful volcanic and erthquakes activity (Credit: B. Heizen. At the right—Image of Earth from the Moon orbit taken by the Soviet Zond 7) involves a “spreading zone,” where hot lava ascends from the upper mantle pushing the lithosphere plates apart and ﬁlling “cracks” (rifts) between them, and subduction zones, where plates covered with sediments slowly plunge deep under continents. The seaﬂoor spreading hypothesis was proposed by the American scientist Harry Hess in 1960. The spreading zones coincide with the mid-ocean ridges that run globally at the bottom of all oceans. Both spreading and subduction zones are associated with the sources of the most powerful volcanic activity and earthquakes.
There are about 800 active volcanoes on the Earth’s surface, and numerous earthquakes occur annually.
Global plate tectonics is responsible for the drift of continents, which continuously retreat from each other, as was ﬁrst suggested by the German scientist Alfred Wegener as early as 1912, although the idea was not accepted until Hess’s hypothesis of spreading zones was known. Reconstruction of the process back in time has caused us to conclude that 300–200 million years ago there was a supercontinent Pangaea which disintegrated into several pieces, giving rise to the now existing continents. In support of this model, one may compare the contours of the eastern part of South America with the western part of Africa and see that they exhibit quite similar conﬁgurations. The model was also conﬁrmed by in-depth studies of the bottom of the ocean and magnetic properties of the early emerged lava ﬂows.