Since the Space Age began in the late 1950s, more has been learned about the planets than in all preceding history. By the end of this century we will have made at least a reasonable beginning to the detailed exploration of our Solar System as part of the space programs of several countries. Studying different planets, each with its own characteristics, should show us how different planets have evolved to their present state through the same or different sets of dynamic processes, for the planets are the laboratories needed for observing processes beyond the range, in both time and extent, of our terrestrial environment. To understand our own planet better, we need a perspective that can only be acquired by comparative study of the other Terrestrial planets. Having considered the Earth and Moon in the previous chapter, in this chapter we shall relate the progress to date in studies of the other Terrestrial planets. It is not an accident that we have evolved on Earth rather than on Venus or Mars, and we should be aware of that fact when we tamper with our environment.
Let us begin with a brief overview of the Terrestrial planets. However, since we have just considered the Earth and Moon in the preceding chapter, we will not include them in the overview, but consider the remaining three--Mercury, Venus, and Mars.
Although one of the brighter objects in the heavens, most people have never seen Mercury. It is difficult to study from Earth because Mercury is so close to the Sun. Its maximum angular separation (greatest elongation) is only 28o on either side of the Sun. At this favorable position for viewing its phase corresponds to a quarter moon; the full phase occurs at superior conjunction, when Mercury is almost in line with the Sun (Figure 8.1). Swift orbital motion keeps the planet visible low above the horizon for only a few days each year, immediately after sundown or before sunup. From the northern hemisphere, the best time to see Mercury when it is a morning "star" is in the spring, and the best time when it is an evening "star" is in the fall.
[Figure 8.1]
Of the Terrestrial planets Mercury is next to the least massive after the Moon, being about 6 percent of Earth's mass, and next to the smallest in size, being less than twice the size of the Moon. Next to the Earth, however, it has the highest mean density, about 5.4 g/cm3, a point we shall come back to later. The general physical characteristics of the Terrestrial planets, excluding the Moon, are given in Table 8.1.
[Table 8.1]
Mercury's rotation period is two-thirds of its orbital period; thus the planet completes three rotations during two orbital revolutions. This combination of rotation and revolution periods, like that between the Earth and the Moon, is not accidental; it was apparently set up by the strong tidal force of the Sun, since different parts of the body of Mercury are at slightly different distances from the Sun. Such a tidal force is responsible for having slowed the planet's rotation, trapping it so that the ratio of its rotation to revolution period is 3:2. As a result, from one sunrise to the next one on Mercury takes 176 days; meanwhile the planet completes two orbits of the Sun.
Venus, the second closest planet to the Sun, is the third after the Sun and Moon in brightness in our night sky. Like Mercury, Venus goes through all the lunar phases, as Galileo first observed in 1609 (Figure 8.1). Since it has a larger orbit than Mercury, Venus swings farther out from the Sun as viewed from Earth, about 47o, or twice as far as Mercury. Venus remains visible as an evening "star" in the western sky or as a morning "star" in the eastern sky for weeks at a time.
Venus is closest among the Terrestrial planets in size to the Earth with its diameter, mass, and density being only slightly less than those of Earth. Venus possesses a mean density of 5.3 g/cm3, suggesting that its internal structure is similar to that of Earth and Mercury.
The most striking feature about Venus is a cloud cover that totally hides the surface in visible and infrared radiation. The clouds themselves are almost totally featureless in visible wavelengths, so the planet has a bland appearance with a pale yellow color. In the ultraviolet portion of the spectrum, the clouds do possess features that we will discuss later.
Venus's rotation was a mystery that eluded solution by optical or spectroscopic observations because of its cloud cover and the planet's slow rate of rotation. But Doppler shifts noted in radar observations solved the problem. The planet rotates in a retrograde direction, with its axis of rotation inclined only 2o from the perpendicular to its orbital plane. (Retrograde here means a direction of rotation reversed from that of revolution about the Sun.) The period of rotation, as determined from radar measurements, is 243 days. Because its revolution period is about 225 days, just slightly shorter than its rotation period, the Venusian "day" is 117 Earth days long, with 58.5 days of sunlight and 58.5 days of darkness. Thus the Sun rises on the western horizon and sets approximately twice during the Venusian orbit with respect to Earth.
[Box - Radar Mapping of Planetary Surfaces, Figure 8.2]
Mars has such an eccentric orbit that the closest approach at opposition between Earth and Mars comes every 15 to 17 years, when Mars is near perihelion in its orbit. At the last favorable opposition on September 28, 1988, Mars came close enough to Earth that it was about 24 arc seconds wide or fully 95 percent as large as it can ever be. At a time such as that even the most casual observers of the heavens are struck by the planet's brilliant ruddy color, far outshining the brightest stars.
Mars is a little more than half Earth's size, has about 11 percent of Earth's mass, and therefore has a mean density 75 percent of that of Earth. The Martian "day" lasts 24 hours and 37.4 minutes. Its axis of rotation tilts 25o from the perpendicular to its orbital plane, giving the red planet seasons like those of Earth, but they last twice as long because the orbital period is nearly 2 years.
Visual observers have made countless detailed maps of the numerous and varied surface features of Mars. Two observers are especially notable: the Italian astronomer Giovanni Schiaparelli (1835-1910) and the American astronomer Percival Lowell (1855-1916), who mapped and named many Martian features. Through a telescope the red planet appears to have Earthlike characteristics, such as white polar caps and large dark areas that vary with Martian seasons (Figure 8.3).
[Figure 8.3]
Recognition that Mars has polar caps dates back to the early 1800s. When Mars is closest to the Sun, the south pole is inclined toward it. As a result, the large southern polar cap recedes during the Martian summer, leaving behind a small residual cap or on some occasions none at all that can be seen from Earth. On the other side of the orbit, the north pole is inclined toward the Sun when the planet is farthest from the Sun. The residual polar cap never quite disappears during the Martian summer in the northern hemisphere.
Mars has also for some time been known to have an atmosphere in which vast yellow dust storms occur. Once or twice a Martian year one of these dust storms grows to global proportions, enveloping almost the entire planet in a dense shroud of dust. White clouds in the Martian atmosphere have also been observed from Earth.
Although various observed characteristics had hinted that Mars
might have strong similarities to our planet, spacecrafts have
revealed a surface topography that is more like the Moon than
the Earth. This quickly put to rest the many long-running romantic
notions that seasonal changes were due to vegetation and the inference
that Mars might thus be the home of intelligent life.
8.2. Terrain-Shaping Processes
Over the last few decades two events have forced a revolution in our thinking about planets, particularly the surfaces of the Terrestrial planets: (1) recognition of the Earth's thermal-tectonic activity and (2) results from planetary space exploration by the United States and the Soviet Union.
Almost everywhere across the surfaces of the Moon, Mercury, and Mars we see evidence of impact cratering and volcanic activity, with the relative percentage of the two varying from one body to the next. In addition, from radar studies of Venus it appears that evidence is there of both processes. For Earth, however, scientists have a very different picture: It is a planet presently dominated by thermal-tectonic activity.
During the first billion or so years of the Solar System a period of heavy bombardment, called the intense impact-cratering period, produced most of the impact craters seen on the Terrestrial planets. A by-product of the intense impact cratering was flooding with lava of immense impact basins to form the maria. This intense cratering period explains essentially why the surfaces of the Moon and Mercury look as they do. However, for the larger Terrestrial planets--Earth, Venus, and Mars--the second major terrain-shaping mechanism has remade almost all the surface in the case of Earth and part of the surfaces in the cases of Venus and Mars. This mechanism is the thermal-tectonic activity produced by convection in the mantle.
The surfaces of Venus, Earth, and Mars over billions of years were fractured, deformed, and in the case of Earth worn down by erosion from water, wind, and life. Volcanic material can reach a planet's surface to develop plains or volcanos only if the crust can be broken, allowing lava to flow onto the surface. Over the 4.6-billion year existence of the Terrestrial planets, the Earth has shown by far the greatest volcanic activity, with presumably Venus next (we really do not know at this point), followed by Mars, Mercury, and the Moon.
If heavily cratered terrain is the oldest type of surface, then its preservation implies a relatively stable history for a crust. Why should the smaller Terrestrial planets have old and, consequently, relatively stable crusts, whereas the larger Terrestrial planets have younger, in some cases much younger, portions to their surfaces because of thermal-tectonic activity? Just after the formation of the Terrestrial planets, these planets must have been completely molten spheres of rocky materials. Within a few hundred million years after formation, their surfaces solidified forming a thin crust that enclosed a molten interior. Large bodies bombarding their surfaces, however, could still puncture or crack the crust allowing molten material from the interior to flow onto the surface in great seas of lava.
The important point is that the Terrestrial planets would not have cooled at the same rate. This is because the thermal energy content of a planet depends on its volume, which is proportional to the cube of its radius. Whereas the rate that a planet cools will depend on its surface area, through which thermal energy is lost to space, and the surface area is proportional to the square of the planet's radius. Thus the ratio of a planet's thermal energy content to its rate of cooling is proportional to its radius, or radius cubed divided by radius squared, meaning that smaller planets radiate away energy more rapidly than do large planets. Smaller planets, consequently, cool faster than do larger planets.
Therefore, the Moon, being the smallest of the five bodies, should have cooled the most rapidly forming a thick protective crust which stopped the surface from undergoing significant alterations at least 3 billion years ago. The next to form a protective crust and cease radical surface changes should have been Mercury, since it is only slightly larger than the Moon, followed by Mars. Venus may still be undergoing some major changes in its surface terrain, the extensive cloud cover prevents us from being sure. However, the Earth's surface is still very much "alive," unlike the "dead" surface of the Moon, and should continue to experience dramatic change for the next 2 billion or so years.
8.3. Surfaces of the Terrestrial Planets
Observations from the Earth had hinted that Mercury might look like the Moon. But it was the three Mariner 10 flybys between March of 1974 and March of 1975 that showed that the planet is indeed heavily cratered like the Moon (Figures 8.4 and 8.5). Although the overall surface of Mercury is remarkably similar to that of the Moon, there are significant differences--differences that suggest a somewhat different surface evolution from that of other Terrestrial planets.
[Figure 8.4]
[Figure 8.5]
The surface of Mercury is pockmarked with craters ranging from hundreds of meters up to hundreds of kilometers across. Some of the bright craters have extensive ray systems like those on the Moon. Compared to the Moon, Mercury and Mars are deficient in craters in the range of a few tens of kilometers. There are some conspicuous differences between Mercury and the Moon, however. Craters on the Moon's highlands are densely packed, with rims of young craters overlying old craters, and the mare regions possess sharp boundaries. On Mercury, by contrast, craters are often interspersed with relatively smooth plains, giving the terrain a speckled appearance.
Like the Moon, both Mercury and Mars have dark maria, as shown in Figure 8.6, while those for Mars and the Moon are almost indistinguishable, maria on Mercury exhibit small but significant differences. Twenty or so of Mercury's maria are several hundred kilometers across, while Caloris, the largest one is more than 1000 km wide. The interior surface of Caloris Basin resembles Orientale Basin on the Moon (Figure 7.19). Scarps or cliffs a few kilometers high and often hundreds of kilometers long (Figure 8.6) cut across maria and craters alike.
[Figure 8.6]
The basins of the maria and many other features on Mercury, as on the Moon, were created in the first half billion or so years of its existence during the intense impact-cratering period. Because Mercury and the Moon cooled at a faster rate than did Venus, Earth, and Mars, the results of this intense cratering period can still be seen on the surfaces of Mercury and the Moon, and they have remained virtually unchanged for at least the last 3 billion years.
Venus's surface is hidden by total cloud cover. Radar studies suggest that the planet's surface possesses geologic features suggestive of impact cratering, volcanism, and primitive tectonic activity. Pioneer Venus radar studies from orbit confirmed Earth-based observations, and a contour map produced from them is shown in Figure 8.7. While 65 percent of the Earth's surface is ocean basin lying on average about 5 km below sea level, fully 60 percent of Venus's surface lies within 0.5 km of the mean radius of the planet, and only 5 percent is more than 2 km above the mean radius. Despite this limited spread in elevation, the maximum distance between the highest and lowest points is about 13 km, which is comparable to that on Earth.
[Figure 8.7]
There are two continentlike features rising well above the average surface level. One highland area, Ishtar Terra (Figure 8.7), is unlike anything seen on the Moon, Mercury, or Mars. It is larger than continental United States and stands several kilometers above the mean planet radius. In elevation it is similar to the Tibetan Plateau on Earth but about twice its size. Maxwell Montes on the eastern end of Ishtar Terra contains the highest point on Venus's surface--about 11 km above the mean radius or 2 km higher than Mount Everest is above sea level. The Venusian mountains containing Maxwell Montes rise out of the Ishtar Terra plateau in the same way that the Himalayas stand on the Asian plate. An artist's rendering of this continentlike feature is shown in Figure 8.8. The other continentlike region, Aphrodite Terra, is also comparable in size with continental United States.
[Figure 8.8]
Also disclosed by radar observations of Venus's hidden surface are some almost circular features between a few tens of and a thousand kilometers in diameter which may be impact craters and basins. From Soviet spacecraft Venera 15 and 16, new radar images that provide better resolution of the surface, however, suggest that some of these circular features are volcanic caldera flanked by large lava flows. Other large features include a 1000 km long trough a few hundred kilometers wide and a few kilometers deep which is similar in scale to the Martian Valles Marineris (partly shown in Figure 8.11). Maxwell Montes is a large, low, circular dome some hundreds of kilometers across with a central depression 100 km or so in diameter, similar in many respects to a volcanic peak. If truly a volcano, it is about 25 percent larger than Olympus Mons on Mars (Figure 8.12). There are two other regions of suspected volcanic activity: Beta Regio and the eastern end of Aphrodite Terra (Figure 8.7).
Although there has been no direct observation of a volcanic eruption, several lines of indirect evidence suggest that major volcanic activity has occurred within the last few tens of millions of years, and according to some studies maybe as recently as the last 50 years. This evidence includes the above-mentioned radar images of possible volcanic craters and lava flows, major changes in the amount of volcanic gases in the clouds, and radio noise characteristic of lightning around volcanic peaks. Not one of these arguments by itself is entirely convincing, but taken together they present very persuasive evidence for active volcanos on Venus. Since there is no clear evidence for lithospheric plate formation and subsequent motion, Venusian volcanos are probably isolated hot spots in the crust like Mauna Loa in Hawaii rather than like the strings of steep volcanic cones that characterize Earth's subduction zones where one plate is being forced under another.
The two Soviet landers, Venera 9 and 10, sent back the first photographs of the Venusian surface in 1975, while Venera 13 and 14 provided four more, with at least one in color. Sunlight filtering through the cloudy atmosphere supplies enough light to make the surface look like a dark, overcast day on Earth. The atmospheric color is decidedly orangish, since blue photons of incoming sunlight are absorbed and scattered by clouds. The Venera 13 view in Figure 8.9 shows a rock-strewn plain with a dark, fine-grained material interspersed between rock outcroppings. Although having a number of noticeable differences, the view roughly resembles Martian terrain (Figure 8.13). Venera 14 landed on a terrain different from that for Venera 13. Its view was of a plain of broken rock layers that extend to the horizon. Samples were collected by both spacecraft and analyzed. Their composition suggests that the material is basaltic rock, an igneous rock extruded from the interior and generally silicon-poor and metal-rich. Such basaltic rock is common on the Earth and the Moon.
[Figure 8.9]
The possibility of impact craters and basins on Venus suggests a surface billions of years old, while the possibility of volcanos, rift valleys, and plateaus point to youthful parts of the surface that may be only millions of years old. Unlike Mercury and the Moon, Venus appears to be a relatively active planet that may resemble the early Earth.
[Box - Space Missions to Venus and Mars]
The fine, delicate streaks called "canals" and sketched by observers on early Martian maps are quite clearly illusory. The Mariner and Viking pictures revealed these canals to be dark-floored craters or irregular dark patches aligned by chance and linked unconsciously by early observers into lines that looked like canals.
Like the Moon and Mercury, Mars has a different topographic pattern in each hemisphere, but it is more diverse and complex than either the Moon or Mercury. Mars's northern hemisphere is generally lower than the mean radius of the planet by a couple of kilometers, possesses few craters, and has been altered by intense volcanic activity. The extensive lava flooding occurred at various times after the cessation of the intense impact-cratering period. The southern hemisphere, however, has a densely cratered surface, averaging a couple of kilometers greater than the mean radius. Its crust has not changed appreciably throughout the planet's life.
There are about 16 smooth, circular basins containing lava-flood plains on Mars. One that has long been observed from Earth is Hellas, an almost craterless basin about 1800 km wide or about one and a half times the size of the largest lunar sea Mare Imbrium. The small number of impact craters on the maria suggests that they formed after the intense impact-cratering period ceased some 4 billion years ago. Estimates of the age of the ancient lava plains are about 3 billion or so years.
The abundance of craters in some regions of the southern hemisphere is comparable with that in the highlands of the Moon. The similarities between the cratered Martian southern hemisphere and the lunar highlands (Figure 8.4) prompts speculation that the two are about the same age. Thus almost half the surface of Mars is ancient terrain, with many of its landforms having remained essentially unchanged over the last 4 billion years.
From spacecraft photographs one sees that Mars has a wide variety of channels in the oldest terrain. They appear to have formed between 3 and 4 billion years ago, shortly after the end of the intense cratering period. Figure 8.10 is an oblique-angle photograph of a channel that has the appearance (left center) of some drainage systems on Earth. Thus it is speculated that water flowed on the Martian surface and is now held as subsurface ice. Since both polar caps contain water ice, it is not unreasonable to hypothesize that a time existed in the past when a thicker and warmer atmosphere permitted liquid water to exist on the surface.
[Figure 8.10]
An important departure from the character of the major portion of the Martian terrain is the Tharsis ridge shown in Figure 8.11. This area is different because of three large volcanos running diagonally along the crest of the ridge and a spectacular, isolated volcanic structure, Olympus Mons (Figure 8.12), which is similar to, but much larger than, Mauna Loa and Mauna Kea in Hawaii as seen from the bottom of the Pacific Ocean. In addition to these large shield volcanos rising some 20 km above the surrounding plains, there are flattish saucer-shaped volcanos, some of which lack a significant number of impact craters on their slopes, suggesting that they are relatively young. Tharsis ridge is 10 km or so above the average Martian radius; the volcanos extend above it. Like the suspected volcanos on Venus, Martian volcanos are more akin to Earth's "hot-spot" volcanos in the middle of a plate, such as Mauna Loa in Hawaii, rather than to lines of volcanos setting astride plate boundaries where one plate is forced underneath another.
[Figure 8.11]
[Figure 8.12]
In the equatorial region lies the spectacular canyon Valles Marineris, which cuts across the middle of a plateau. It is nearly 4000 km long, up to 100 km wide in some places, and at least 4 km deep (Figure 8.11). At its western end lies a complex pattern of intersecting fault valleys. Valles Marineris runs radially away from Tharsis ridge and probably results from the faulting that accompanied the evolution of Tharsis since its formation. However, thermal-tectonic activity on Mars is much weaker than that on Earth.
From orbit, the dominant features of the region around the Viking 1 lander are craters. From the ground, there are only a few obvious craters in the immediate vicinity of the lander. If Mars were like the Moon, then there should be visible several small craters that are tens of meters in diameter. Their absence indicates that the Martian atmosphere is dense enough to burn up small meteoroids or material ejected from large impacts before they reach the surface and that surface erosion is vigorous enough to obliterate small craters. Thus there is not the profusion of small craters seen on the Moon.
The area photographed by the Viking 1 lander in the Chryse region is a gently rolling landscape, yellowish brown in color, strewn with rocks and dotted with drifts of fine-grained material. Within 30 m or so of the lander lie several outcrops of bedrock, which in many ways resemble the semidesert regions of the American Southwest but without vegetation (Figure 8.13). Chemical analysis by the lander found elements such as silicon, oxygen, iron, magnesium, and aluminum, or those common in Earth's crust, but organic analysis failed to detect any organic molecules (those containing carbon, one of the essential ingredients for life). On Earth, soil in even the most dry, sterile-looking valleys contains many organic compounds.
[Figure 8.13]
The multilayered polar regions are still another type of Martian topography. A region near the south pole appears in Figure 8.14. The layered deposits probably hold appreciable quantities of frozen water mixed with dust beneath a carbon dioxide coating. Periodic changes in the Martian climate may be responsible for the deposition of the successive layers of material.
[Figure 8.14]
As we discussed in Chapter 7, evidence from the intense impact-cratering period has been erased from the Earth's surface. Even the oldest rocks, which are 3.5 to 3.8 billion years old, are of no help in identifying this early intense cratering period because they are mostly isolated outcroppings, many covered with ice. (However, later impact craters still exist, as discussed in Chapter 7.) Earth also has basaltic plains, like maria on the Moon, Mercury, and Mars--not old ones, as on these Terrestrial planets, but very young ones. The major ones on Earth are formed by the addition of new material to the lithospheric plates at the midocean ridges.
It is unlikely that Earth ever went through an appearance like that of the Moon, Mercury, and Mars, in which maria were formed from lava's flooding a huge impact basin. In all probability, the thermal-tectonic activity of Earth's surface has always been too great to have allowed maria to last any appreciable length of time. Thus if we could watch a time-lapse movie of the evolution of the surface of all the Terrestrial planets, it is unlikely that they would all start out the same and begin to depart from each other later. Rather, the surfaces of the Moon and Mercury, which have thick lithospheres--thicker than those of Mars, Venus, and Earth--have not been fractured and then deformed by convection. This is so because the Moon and Mercury cooled very quickly, extinguishing any tectonic activity if it ever existed. And, the Earth has probably always shown vigorous tectonic activity, with Venus's less than that of Earth and Mars even less. Thus the Moon and Mercury have the oldest surfaces, Mars's surface is old but with some youngish features, Venus one guesses to have a mixture of old and young, and Earth's surface is comparatively very young.
8.4. What Influences Planetary Atmospheres?
We are all aware of the characteristics of Earth's atmosphere and its consequent importance in maintaining life; Mercury and the Moon, in contrast, have almost no atmosphere, while Venus and Mars have carbon dioxide atmospheres and the Jovian planets have extensive hydrogen-helium atmospheres. How did this diversity in planetary atmospheres arise?
There are several factors that determine why a planet's atmosphere is the way it is:
A planet's distance from the Sun and its mass are important in determining which molecular constituents it can retain in its atmosphere. From our earlier discussion on random thermal motion, we know that the higher the temperature or smaller the mass of a molecule (or both), the greater will be the average thermal velocity and the more likely that gas particles can escape an atmosphere. Using a planet's temperature, mass, and radius and the masses and thermal velocities of its atmospheric constituents, astronomers conclude that if a molecular constituent's thermal velocity is near one-third the escape velocity, about half that chemical species will escaped from the atmosphere within weeks. For a planet to retain various molecular components indefinitely, the mean thermal velocity must be less than a tenth of the escape velocity.
The massive Jovian planets, with their large escape velocities (several tens of kilometers per second), have held their primeval atmospheres of hydrogen and helium, whereas the less massive Terrestrial planets, with smaller escape velocities (several kilometers per second), have lost, if they ever had, these light gases. Venus, Earth, and Mars, however, have managed to retain atmospheric water molecules as well as such heavier gases as carbon dioxide and nitrogen. Mercury and the Moon lack any appreciable atmosphere because these two bodies have small masses; moreover, Mercury being quite close to the Sun is consequently very hot.
Chemical behavior is an important factor affecting a planet's atmosphere, which for Terrestrial planets is very different from that of Jovian planets. Because the Terrestrials formed from rocky materials, their early atmospheres should have been largely composed of such gases as carbon dioxide, nitrogen, and some water. Venus and Mars still have that kind of atmosphere. However, planets such as Jupiter and Saturn are more nearly like the Sun in chemical composition than like the Terrestrials and thus show chemical behavior different from that of Terrestrial planets.
The incoming rays from the Sun, which are primarily in the visible part of the spectrum, penetrate a planet's atmosphere of clear gases and are absorbed by the surface layers, warming them. In turn, the surface reradiates energy in the infrared region of the spectrum back out into the coldness of interplanetary space. (The reradiated energy is in the infrared region because the warming of the surface by sunlight maintains it at a temperature of only several hundred kelvins.) Passage of the reradiated infrared photons outward through the atmosphere into space, however, is hindered by any infrared absorbers such as carbon dioxide and water vapor. They absorb infrared photons and in turn reradiate much of the energy back to the surface. This process is called the greenhouse effect, after the similarity of the process to that of the glass in a greenhouse, which prevents heat radiation produced inside a greenhouse from escaping (Figure 8.15).
[Figure 8.15]
Depending on the chemical composition and chemical activity of a planet's atmosphere, the greenhouse effect will be more or less effective in trapping incoming solar radiant energy. This causes a warming of the low atmosphere and surface, which can profoundly affect the atmospheric chemistry. Venus is a good example of the long-term consequences of the greenhouse effect. Estimates are that the mean surface temperature of Mars, Earth, and Venus are about 5, 35, and 500 K warmer, respectively, than they would be without the greenhouse effect.
8.5. Atmospheric Structure and Evolution
Mercury's atmosphere is very tenuous, approximately a million billion times less dense than that of Earth. It seems to be supplied and constantly replenished by the solar wind. Helium and a couple of percent of atomic hydrogen have been identified as its principal constituents. The abundance of other atomic or molecular species, if they are present, is insignificant. No evidence has been found for atmospheric modification of any landform.
Some 4.6 billion years ago, just after Mercury formed, gases such as carbon dioxide escaping from the planet's interior may have temporarily created an atmosphere of some extent. But soon after forming, it would have escaped into space and vanished. This is because the planet is not massive enough to hold much of an atmosphere at such a small distance from the Sun and probably was endowed with less gaseous material during its formation than other Terrestrial planets.
The atmospheric pressure on Venus's surface is about 90 times greater than that of Earth while the surface temperature is some 2.5 times greater. Analysis of the lower atmosphere suggests that it is about 96 percent carbon dioxide and 3.4 percent nitrogen, with the remainder water vapor and some other gases (Table 8.2). Because of the high surface temperature on Venus, carbon dioxide was apparently not depleted, as it was on Earth, by reacting with primitive rocks to form carbonates and limestones and by absorption by water. Above 150 km atomic oxygen is the most abundant species. And finally, a huge cloud of hydrogen surrounds the planet far above the atmosphere.
[Table 8.2]
More speculation than on almost any other aspect of Venus has been given to the mysterious clouds that perpetually obscure the surface. In 1973, it was suggested that the clouds were composed of sulfuric acid droplets. From Pioneer Venus results, it appears that the clouds are indeed composed of sulfuric acid droplets and other particles, possibly free sulfur, so thick that during the descent of the probes they appeared to be passing through a blizzard. Data suggest that several other sulfur compounds also exist in the atmosphere. Although once thought to be an unchanging aspect of Venus, the clouds are probably continuously produced and destroyed rather than being an unchanging feature of the atmosphere. A key link in the chemical cycle producing the clouds is volcanic activity on the surface. The extent of the sulfuric acid clouds is controlled by a complicated cycle of atmospheric and crustal chemical reactions.
The clouds begin around 46 km above the planet's surface and seem to be confined to a fairly distinct layer, rising up to about 70 km. A thin haze exists above and below the cloud layer; the lower one has a surprisingly abrupt cutoff some 32 km above the surface. From the bottom of the haze down to the surface the atmosphere appears to be surprisingly clear.
The atmospheric circulation is the same in both hemispheres. A vigorous equatorial east-west jet stream is quite evident in the upper atmosphere, moving around the planet in only 4 days, opposite to the direction of the planet's slow spin (Figure 8.16). The wind velocity decreases at lower altitudes until at the surface it slows to a gentle breeze. The lower atmosphere apparently circulates because of differences in solar heating between the equatorial and polar regions. Clouds rise near the equator, spiral toward the poles, and descend in what appears to be an almost continuous flow. But we do not know why such high winds reverse direction in the upper atmosphere.
[Figure 8.16]
As on Venus, carbon dioxide is the most abundant constituent in the thin Martian atmosphere, amounting to about 95 percent (Table 8.2). We know that the atmosphere also contains about 2.7 percent nitrogen, about 2 percent argon, lesser amounts of atomic and molecular oxygen, and traces of other constituents.
The warmest daytime temperature is around 30o C at the Martian equator, while the nighttime temperature drops to -130o C. Over the polar regions it is even colder. During summer the north polar ice cap gets up to only -70o C--although very cold, not cold enough for the residual cap to be made of carbon dioxide ice. Thus it appears that it is water ice, which is consistent with finding more water vapor in the atmosphere at high latitudes near the poles.
A small, daily, and seasonally variable amount of water vapor has been monitored in the atmosphere. Because of the low atmospheric temperature and pressure (less than 1 percent of the Earth's sea level atmospheric pressure) near the surface of Mars, however, the amount of water vapor is far too low for rain or for water to exist as a liquid on open, flat ground.
Early morning fog lying in craters and other low places is probably evidence of an exchange of water vapor between subsurface or surface ice and the atmosphere. The Martian atmosphere also possesses clouds, which are most probably condensations of water and carbon dioxide ice.
One of the most exciting events during the active life of the Viking landers was the photographing of frost on the surface of Mars at the Viking 2 site (Figure 8.17). The frost occurred during the northern winter, between May and November of 1977. The composition of the frost is not known; the atmospheric temperature was too warm for it to have been pure carbon dioxide ice, and the atmosphere was too dry for it to have been pure water ice. The best speculation is that it was a mixture of carbon dioxide and water.
[Figure 8.17]
The skies at the locations of Viking 1 and 2 were yellowish brown in color and seemed to remain that way over the course of the Martian year. This color was probably due to dust particles suspended in the atmosphere below about 50 km (Figure 8.18). Surface winds can stir the atmosphere sufficiently to hold dust particles. From Viking data we know that the prevailing winds are westerly, as on Earth, with velocities up to 70 km/h at the surface and over 360 km/h at altitudes above 10 km. In fact, the winds are strong enough to create major dust storms that can engulf almost the whole planet and last for months. Undoubtedly the winds come from unequal solar heating of the Martian surface, driving air from high to low pressure areas, as on Earth.
[Figure 8.18]
Since the Sun contains a significant noble gas abundance, the planets, having formed from the same basic material, should possess a significant amount of these heavy gases in their atmospheres. These gases are particularly useful in evolutionary studies because (1) they are too heavy to readily escape into space, and (2) they are chemically nonreactive and, consequently, are very difficult to incorporate into surface rocks. The surprising scarcity of neon, argon, krypton, and xenon in the Terrestrial planets' atmospheres suggests that atmospheres on Venus, Earth, and Mars probably formed from gases escaping from their interiors during volcanic eruptions.
Volcanic outgassing from the interiors of Venus, Earth, and Mars early on should have consisted primarily of carbon dioxide, nitrogen, and water vapor in approximately the same proportions that we observe in Terrestrial volcanic gases today. On Earth, water vapor condensed to form oceans, but nitrogen remained in a gaseous state. Most of the carbon dioxide combined with silicate rocks in the crust to form carbonate rocks, such as limestone, a reaction that occurs most efficiently in the presence of liquid water. If carbon dioxide could be somehow released from crustal rocks along with the small amount dissolved in the oceans, the amount in the Earth's atmosphere would equal about one-half that of the dense atmosphere of Venus.
If one assumes that Venus formed with about the same relative amount of water as Earth did, then the challenging question is what happened to it, since it is not on the surface in pools nor in the atmosphere. Venus receives about twice as much radiant energy from the Sun as does Earth, so that its atmospheric temperature should have been higher from the beginning. Because of the high temperature, water most likely stayed in vapor form, which would also have been the case for carbon dioxide. Eventually, these two molecules could have produced a runaway greenhouse effect that amplified the evaporation of water. Once the water vapor was part of the atmosphere, incoming ultraviolet photons from the Sun could dissociate the water molecules, allowing free hydrogen to escape over the planet's lifetime and the heavier oxygen to combine with crustal rocks to form oxides. Some water is also consumed in making sulfuric acid droplets in the clouds, and a tiny amount of water vapor is still present in the atmosphere.
On Mars, the outgassing of water vapor, carbon dioxide, and nitrogen was probably less complete than it was on Earth, yet carbon dioxide forms the largest part of the atmosphere of Mars. Mars appears at present to be in a cold phase, and a large amount of water is apparently stored in the polar caps and under the planet's surface as permafrost. There is speculation that such water ice may be a remnant of a denser atmosphere that Mars had in the first billion years or so of its existence. Even though Mars has always received less energy from the Sun compared to Earth, if that early atmosphere was a denser carbon dioxide (say 100 to 200 times more than at present) and water vapor atmosphere, then it could have acted to trap infrared radiation through the greenhouse effect. And this could have made the atmosphere warm enough to contain substantial amounts of water vapor. This increased amount of carbon dioxide could easily have been provided by outgassing from the body of the planet. However, over time, the formation of carbonate rocks removed carbon dioxide from the atmosphere and lowered both the temperature and pressure. Under such conditions, the atmosphere could no longer retain much water, and water could not exist as a liquid on the surface. Thus the era of water ended for Mars some time ago, with death due to the cold rather than heat, as was the case with Venus.
Why did the escape of water and consequent heat death on Venus or freezing of water and consequent cold death on Mars not occur on Earth, since its early atmosphere was probably similar in composition to those of both Venus and Mars? It is probable that the advent of photosynthesizing life on Earth began to replace carbon dioxide with oxygen about 2 billion years ago and prevented a substantial greenhouse effect; thus water stayed primarily in pools on the surface. Large expanses of liquid water moderated Earth's climate and provided an environment conducive to the further development of life, given there was adequate protection from solar ultraviolet radiation. Most of Earth's free oxygen, so necessary to animal life, comes from photosynthesis, through which oxygen is constantly replenished by green plants, plankton, and some bacteria. When living organisms began to extract carbon dioxide from the atmosphere, they helped save the Earth from the heat death that Venus has apparently experienced.
It has been known for many years that Earth has a magnetic field that extends far beyond the body of the Earth to form a magnetic envelope, or magnetosphere. Earth's magnetic field is attributed to electric currents flowing in the outer core, so the Earth is somewhat like a giant electromagnet. If other Terrestrial planets also have electric currents flowing in their deep interiors, then they too will have magnetospheres. Or we can turn the argument around: Detection by spacecraft of a magnetic field tells us something about the interior of a planet, that is, whether or not it is sufficiently fluid for matter to flow in the deep interior to generate electric currents.
An unexpected discovery of the Mariner 10 mission was that Mercury has a shock front similar to the wave surrounding the bow of a ship plowing through water. Like the one for Earth in Figure 7.11, Mercury's bow shock is caused by the onrushing solar wind particles colliding with the planet's magnetic field. Although Mercury does have a field, it is only 1 percent as strong as that of Earth. Consequently, it is much too weak to hold radiation belts such as Earth's Van Allen belts. The magnetic axis of its field almost coincides with Mercury's axis of rotation.
Before the advent of planetary exploration with spacecraft, astronomers suspected that Earth's sister planet, Venus, might have a magnetic field comparable with and produced in the same way as Earth's field. However, after data from the first few Mariner and Venera missions were in, it was evident that the Venusian magnetic field is much smaller than Earth's field. Venus has a well-developed bow shock formed by solar wind particles impinging on the planet's magnetic field; but that weak field, like that of Mercury, is too feeble to trap solar wind particles in radiation belts. The weakness of Venus's magnetic field is probably due to the planet's very slow rotation.
Mars's magnetic field intensity is much less than that of Earth. There is apparently a feeble bow shock formed between the onrushing solar wind and the magnetosphere, but no radiation belts appear to exist.
In summary, Earth is the only Terrestrial planet with a field intense enough to retain radiation belts. This suggests that Earth is the only Terrestrials possessing sufficient circulation in a fluid core to generate a strong magnetic field.
[Box - Magnetic Fields, Figure 8.19]
8.6. Internal Structure of the Terrestrial Planets
The final aspect of the Terrestrials that we want to consider is their interiors; those regions hidden from direct observation. We overcome this limitation by calculating mathematical models of the interior from a planet's observed physical properties and from theoretical arguments about the physical laws governing a planet's internal structure. Ideally the physical properties needed are a planet's mass, mean density, shape, rotation rate, gravitational and magnetic field strengths, surface temperature, and chemical composition. However, even with less than all these pieces of information, an interior model can be calculated, but it is accordingly more speculative. Naturally, interior models would be more accurate if we had seismic data and rock samples, as we have for the Earth and Moon. Planetary interior models, and as we shall see later interior models of stars, are two of the prime examples of the use of scientific models to guide our thinking in the pursuit of knowledge.
When constructing an interior model for a planet, astronomers make the following assumptions about the appropriate physical processes going on inside planets:
Having constructed a model, astronomers can use it to predict how temperature, pressure, and density vary from the planet's center to its surface. Planetary models essentially show that other Terrestrial planets, like Earth, possess such layered zones as core, mantle, and crust, whereas Jovian planets have a different type of structure. Examples of interior models for the Terrestrials are given in Figure 8.20 and for the Jovians in Figure 9.13.
[Figure 8.20]
The values of its mass and radius imply that Mercury must contain a large fraction of iron, the only heavy element sufficiently abundant to account for the planet's high mean density. The iron and nickel content may be as much as 65 percent of Mercury's mass. By analogy with terrestrial, lunar, and meteoritic chemical abundances, we presume that silicates and oxides of iron are also prevalent. Additional evidence for a large iron-rich core comes from Mercury's magnetic field, which is intrinsic to the planet and is most likely the result of an internal mechanism that continuously generates the field in much the same way as does Earth. Chemical differentiation appears to have occurred very early in the planet's history, probably during the first half billion years. Since then, the surface has been largely undisturbed by thermal-tectonic processes.
The model that has been derived for Mercury is one with a crust overlying a silicate mantle, which in turn surrounds a molten (or partially molten) iron-rich core. The core radius may be as much as 76 percent of the planetary radius, a percentage that is considerably greater than that for any other Terrestrial, including Earth. Such a core should be adequate to generate the magnetic field observed by Mariner 10.
Since Venus is the Terrestrial planet closest to Earth in size and mass, it is reasonable to expect that it will generally be the planet most like the Earth. Of critical importance in developing a model for its interior is Venus's chemical composition: As we have seen from estimates of what chemical elements were likely to have been present at the time of formation of the Terrestrial planets and from observations of the planets' influences on the motion of spacecrafts, astronomers have made estimates for the iron and nickel content in the Terrestrial planets; as percentages of total mass, the iron and nickel content is as high as 65 percent for Mercury, up to 38 percent for Venus, up to 33 percent for the Earth and Moon, and only up to 26 percent for Mars.
Such an iron content for Venus suggests that it ought to have a molten core (like that of Earth) that occupies about the same fraction of Venus's interior (Figure 8.20) as does Earth's (or significantly smaller than the core of Mercury). Overlying the molten iron-rich core is a silicate mantle, and there is a crust on top of the mantle. It is possible that Venus (also like Earth) has an inner core of solid iron-rich material. Speculation for and against an inner core depends on estimates of the iron content of the planet.
Besides the Earth and Moon, Mars is the only planet for which scientists have seismic data. Both Viking landers carried instruments to record quakes on Mars. Unfortunately, only the one on Viking 2 worked, and in November of 1976, it is believed by some astronomers to have detected a quake. Any seismic activity on Mars, although it should be somewhat more extensive than that of the Moon, should also be much less than that of Earth.
If real, which it may not be, the quake data suggest that the average thickness of Mars's crust is greater than that of Earth. Earth's crust is about 0.5 percent of its radius, Mars should be closer to 1 percent, whereas that of the Moon's crust is about 4 percent. This suggests for Mars a lithosphere (crust and outer portion of the mantle) a couple of hundred kilometers thick on a body, which is presumably chemically differentiated. If so, Mars should have a silicate mantle and an iron-rich core of about 1500 km radius (Figure 8.20). Thus, of the five Terrestrial planets, Mars has the smallest percentage of iron and the smallest core for its size.
This completes our survey of the Terrestrial planets. We now want to move in the next chapter to a survey of the Jovian planets, which are the principle occupants of the outer Solar System.