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A Brief Tour Through the Solar System
From: Cambridge University Press
| By:
W.G. Ernst |
EDITOR'S INTRODUCTION |
Thanks to the space missions of recent decades, we are becoming increasingly familiar with earth's nearest celestial neighbours. Professor W.G. Ernst of Stanford University is the guide to a geological journey around the other planets and planetesimals that orbit the Sun. |
 he ancients studied the heavens and discovered that the positions of the stars are fixed relative to one another. Of course, the universe is expanding; however, because all objects are at virtually infinite distance and are moving directly away from each other, their relative positions with respect to an observer remain constant. |
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| Earth and Moon as viewed by Mariner 10. | |
Naturally, the early astronomers had no way of knowing about the red shift. They did puzzle over a small class of luminous objects they termed "wandering stars" because these bodies moved differentially through the firmament. We now know that these objects are other planets in our solar system. Why is Venus called the evening (or morning) star? It is because, having an orbit closer to the Sun than does our planet, observers on Earth can view Venus for only a few hours after sunset and before dawn. The innermost planet, Mercury, is so near to the Sun that it is detectable only in very favorable orbital positions just after sunset or immediately before sunrise. |
Unlike the self-luminous stars, planets primarily reflect light rather than generate it. This is a consequence of the fact that they are not massive enough to generate the high internal pressure and temperature conditions requisite to initiate hydrogen fusion. Gassy giants such as Jupiter emit slightly more radiative energy than is received from the Sun; however, this is due to the energy released during ongoing self-compression, not to nuclear fusion. |
Jupiter is only 0.1 percent the mass of the Sun, which is probably a good thing for us. Were it substantially more massive, it might have begun hydrogen burning, and then our solar system would have been characterized as a binary star system. Such a configuration would have provided a somewhat different environment in the vicinity of the Earth; however, inasmuch as, at its closest approach to us, Jupiter is approximately four times as far away from the Earth as is the Sun, we would not have been incinerated (or more correctly, primordial life would not have been destroyed). Also, it must be noted that binary star systems are common in the universe. |
An interesting but thus far unexplained relationship is that the distance to a planet from the Sun is approximately twice as far as that of the next inner one. This observation is known as Bode's rule. The sole exception is the gap between Mars and Jupiter; however, this region is the site of the asteroid belt. Such planetesimals, condensed rocky and iron-rich masses ranging from fine dust to 10 kilometers or more in diameter, may represent a fragmented planet. |
More likely is the hypothesis that planetary accretion in this zone of the ecliptic plane was inhibited due to gravitational disruption by the neighboring massive planet, Jupiter. Most meteorites currently circling about in our solar system are probably derived from the asteroid belt, flung into eccentric orbits by the gravitational pull of Jupiter. |
Each planet in our solar system is made up of various proportions of matter similar to iron and stony meteorites, as well as of condensed gas species (predominantly hydrogen, methane, carbon dioxide, water, and ammonia). Where abundant, the metal is concentrated in the planetary core, the stony material constitutes the surrounding mantle and crust, and the volatile constituents are present as surficial condensed ice layers, liquid oceans, and a planet-enveloping atmosphere. This is a reflection of the fact that during accretionary heating, such bodies underwent a gravitative differentiation process, whereby the densest substances were concentrated toward the center of the planet, with concentric outer layers consisting of progressively less dense matter. |
In the inner planets, at least, this chemical stratification took place during early-stage partial melting occasioned by radioactivity- and meteorite impact-generated self-heating. Reflecting the higher densities of metals and metal alloys, and the lower fusion temperatures of iron and iron-nickel alloys compared with magnesium silicates and oxides, the molten iron would have migrated down the gravity gradient to form a dense planetary core, surmounted by the silicate "slag" of the mantle. |
As a continuing process, the deeper portions of the terrestrial planets have become progressively devolatilized due to the thermally driven escape of gaseous species trapped by the accretionary process. This thermal annealing, for instance, accounts for the more fluid envelopes surrounding the Earth, as well as the carbon dioxide-rich atmosphere of Venus. In cases where a liquid iron-nickel core is present, fluid motion within it is thought to be responsible for the generation of a planetary magnetic field. The thermally driven chemical-density differentiation process described above, while generally applicable to the entire solar system, is exactly what scientists hypothesize to have transpired on Earth. |
Mercury
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| Mercury seen by Mariner 10. | |
So close to the Sun that visual observation from Earth is difficult, Mercury is only 5.5 percent of the mass of our planet. Like Venus, Mercury has no orbiting natural satellites. It is slightly larger than the Moon, and exhibits a similarly pockmarked surface, due chiefly to asteroid impacts. As on the Moon, dark lava flows floor the largest basins. |
Mercury has no atmosphere, and is characterized by a relatively low albedo (the percentage of incident sunlight bounced back; hence, the reflectivity of a substance). Its aggregate density is 5.4 grams per cubic centimeter; consequently, we surmise that it must have a large metallic core. (Masses of the individual planets can be deduced from their orbital paths around the Sun. Their specific sizes are determined by trigonometric measurements, conducted from Earth, and now, from space. Thus, the overall density of each planet is known, being simply the ratio of its mass to its volume.) Mercury possesses a weak dipolar magnetic field aligned with the planetary rotation axis. |
Venus
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| Hubble Space Telescope image of Venus. | |
The second planet is Earth's virtual twin. Its dimensions and density, 5.3 grams per cubic centimeter, are comparable to terrestrial values, and it possesses 81.5 percent of the mass of the Earth. However, Venus has an atmosphere that, unlike ours, is very dense, ninety times that of the Earth, and predominantly consists of carbon dioxide. |
On Earth, water is ubiquitous as globe-encircling oceans. Carbon dioxide is not abundant in the terrestrial atmosphere; instead, it is stabilized by precipitation of calcium carbonate plus or minus calcium-magnesium carbonate from seawater, and largely held in the solid portion of the planet as the sedimentary rocks limestone and dolomite. Because Venus is closer to the Sun, its H2O is completely volatilized; hence, CO2 cannot be dissolved, transported, and precipitated to become sequestered in marine carbonate strata, but instead is concentrated in the dense atmosphere. This thick atmosphere provides the planet with a high reflectivity (high albedo). |
In contrast to the Earth, Venus has a very high surface temperature, on the order of 500°C. This phenomenon is due partly to its proximity to the Sun and primarily to its dense atmosphere, which traps incoming solar radiation (the greenhouse effect). Radar imagery reveals large, circular structures (impact craters) as well as curious pancake-shaped volcanic domes and intricately fractured terrains. |
Venus lacks the bimodal topography of ocean basins and landmasses that characterizes Earth, but it does have two prominent, continent-like areas. Overall, however, the topography is typically subdued. Judging from its aggregate density, Venus must have a metallic core. It lacks a magnetic field, possibly because of its slow rate of rotation (once every 243 Earth days). Its spin is termed retrograde, because it rotates more slowly than its revolution (orbit) about the Sun. |
Moon
Within our solar system, humans are blessed with a rather large satellite, relative to the size of the central planet. Even so, the Moon contains only the equivalent of 1 percent of the mass of the Earth, it lacks an atmosphere, and consequently it possesses a very low albedo; volatile species could readily escape, and did because of the very weak gravity field. Its density, 3.3 grams per cubic centimeter, is relatively low, and suggests that the Moon has only a very small core--a conclusion supported by the absence of a dipolar magnetic field. |
Craters formed by meteorite impacts are of considerable but variable abundance on the Moon. It is rather similar in surface features to Mercury, but is less dense. The dark lunar maria are underlain by lavas similar to those that issue from the Hawaiian volcanoes, and some of these flow fields are more sparsely pockmarked than are others. The relatively light-colored lunar highlands consist of pulverized, fragmented rocks that evidently have been repeatedly smashed by incoming asteroids early in lunar history. Radiometric data obtained for specimens retrieved during the Apollo Project have corroborated this hypothesis conclusively, in as much as some lunar highland fragmental rocks are as old as 4.4 billion years, whereas the maria lavas were erupted only 3.2 to 3.8 billion years ago. |
Earth
The third planet has an overall density of 5.5 grams per cubic centimeter, and a strong dipolar magnetic field, due to an iron-rich core. The Earth is distinct from all the other inner, rocky planets in possessing distinct continents and ocean basins. Perhaps most important, H2O is present as a significant constituent of the atmosphere, as the dominant component in the liquid, globe-encircling oceans, and in the solid polar ice caps. |
The Earth is quite appropriately called the blue planet. Its atmosphere is largely composed of nitrogen (78 percent) and oxygen (21 percent), with only minor amounts of carbon dioxide, and vanishingly small concentrations of noble gases and more reduced volatile species (methane, ammonia and hydrogen). Would you anticipate that the Earth would have a high or low albedo? |
Earth certainly must have sustained a similar number of meteorite impacts as the Moon; however, the surface of our planet preserves only a few of the geologically more recent impact sites; most meteorite craters presumably have been reworked and destroyed through erosion or mountain building. Volcanoes are of several distinct types and are confined to roughly linear chains. Most mountainous regions also form distinct belts, especially those wrinkled zones exhibiting evidence of continental contraction. Of course, what also differentiates the Earth from the rest of the solar system is biologic activity--the myriad and marvellously intricate forms of life. |
Mars
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| Mars seen by Hubble Space Telescope. | |
Our outer solar system neighbor, the red planet, has two tiny satellites. Mars is often likened to Earth, but in fact it is much colder (as might be surmised by its brilliant white polar ice caps), is quite a bit smaller, and is less dense. Having only 10.7 percent of the mass of the Earth, its aggregate density is only 3.9 grams per cubic centimeter, indicating the presence of a relatively small metallic core; Mars lacks a dipolar magnetic field. |
The tenuous atmosphere is only approximately 1 percent as thick as that of Earth (Mars possesses a relatively low albedo), and consists chiefly of carbon dioxide. A good portion of this volatile component is present as frozen carbon dioxide ("dry ice"), along with H2O ice, covering the martian polar regions. Like the Earth, its spin axis is tilted slightly relative to the plane of the ecliptic, so Mars experiences seasons in which the ice caps reciprocally wax and wane. It also rotates at approximately the same rate as the Earth. The surface of Mars, which contains modest amounts of iron, is strongly oxidized, accounting for the distinctly red hue of the planet. |
The surface of Mars exhibits impact features and volcanic edifices. The southern hemisphere is heavily cratered; one such feature, Hellas, is 2000 kilometers in diameter, the largest known crater in the solar system. Huge, broad, basaltic shield volcanoes (so called because their shape is reminiscent of a gently curved shield), similar to Mauna Loa, Mauna Kea, and the lunar maria, are numerous, especially in the northern hemisphere. The largest, Olympus Mons, is 600 kilometers across at the base and rises 25 kilometers into the thin martian atmosphere. |
The surface of Mars also displays canyons that have dendritic (tree-like) patterns. One such sinuous feature is Valles Marineris, which is 4000 kilometers long, 100 to 250 kilometers wide, and 7 kilometers deep. At some stage of the history of the red planet, this declivity appears to have resulted from erosion by running water. A less plausible alternative hypothesis of its origin is that this depression is a down-dropped block, bounded on either side by faults. Episodic dust storms and wind-blown sand have been encountered above and on the martian surface by the Mariner 9 spacecraft and the Viking lander. Even though Mars possesses a very tenuous atmosphere, it does have energetic weather patterns; sustained wind velocities of up to 270 kilometers per hour have been measured. |
Asteroids
Judging from meteorites falling on and recovered from the surface of the Earth, planetesimals residing in this belt consist of two distinct types, stony and iron. Stony meteorites consist predominantly of magnesium and ferrous meteorites consist predominantly of magnesium (and ferrous iron) silicates, whereas metallic meteorites are made up of an alloy of native iron (90 percent) and nickel (10 percent). A few meteorites contain both silicate and metal, and an even rarer group of stony meteorites contains carbonaceous matter. The immense gravitational attraction of the fifth planet probably continually disrupts the self-accumulation of a planet-sized mass in this orbit, thus maintaining the asteroids as a swarm of small, condensed bodies orbiting about the Sun in a broad band. |
Jupiter
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| Hubble Space Telescope view of Jupiter. | |
Jupiter is the largest of the giant, gassy planets; its mass is more than twice that of all the other solar system planets, satellites, and asteroids combined. The overall density of this behemoth is 1.3 grams per cubic centimeter; it is an enormous ball of compressed gases and clouds of ice crystals, probably with a small rocky and/or metallic core. |
The dense atmosphere is composed principally of hydrogen (H2), helium (He), ammonia (NH3), methane (CH4), and their condensates. It is famous for turbulent circulation patterns, including the enigmatic red spot. Slight compositional differences are reflected in the color contrasts, which help to identify convection cells. The red spot is just such a feature and is one of numerous, immense Jovian hurricanes; this evolving cyclonic system has a long diameter of approximately 22,000 kilometers--twice the diameter of the Earth. |
Sequential photographs from space provide striking proof of the dynamic circulation of this stupendous weather system, divided into circumferential latitudinal, jet stream-like belts girdling the rotational axis of the planet. Jupiter, like most of the outer giants, spins rapidly, 360 degrees, in just under ten hours Earth time. Also, due to self-compression, Jupiter emits twice as much radiative energy as it receives from the Sun. |
Jupiter is surrounded by a flock of sixteen satellites, of which the outer dozen are relatively minor bodies. The inner four, termed the Galilean satellites (so called because Galileo discovered them with his primitive telescope), are as large as, or larger than, our Moon. Under favorable night viewing conditions, you can see them with a good pair of binoculars. Closest to Jupiter, Io has a density of 3.5 grams per cubic centimeter, a surface covered by dark lavas and numerous sulfur-spewing volcanoes. Io lacks meteorite impact scars due to periodic repaving by volcanic activity. |
Farther out, Europa is characterized by a density of 3.2 grams per cubic centimeter, and displays a network of fractures in what appears to be a frozen H2O ocean. Meteorite impact craters are rare on this relatively smooth-surfaced satellite. The next outboard satellite, Ganymede, has a density of 2.0 grams per cubic centimeter, and possesses more numerous craters; in addition, it consists of faulted, grooved, down-dropped blocks partly overwhelmed by ice and frost. |
Callisto, outermost of the large Jovian moons, with a density of 1.8 grams per cubic centimeter, exhibits a surface consisting primarily of pockmarked H2O ice. Clearly, the inner Galilean satellites have a higher proportion of stony materials in their planetary constitutions, whereas, farther from Jupiter, the satellites are progressively enriched in volatiles and condensed gases (ices). |
Saturn
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| Voyager 1 view of crescent Saturn and rings. | |
The sixth planet in the solar system is the second biggest, having 30 percent of the mass of Jupiter. Like its inner neighbor, but on a somewhat lesser scale, it emits small amounts of radiation. Saturn has an aggregate density of 0.7 grams per cubic centimeter, hence consisting almost exclusively of gaseous constituents and icy equivalents. Its atmosphere is fully as turbulent as that of Jupiter. |
Its most famous feature is a series of equatorial, disk-shaped, nested rings. The radius of the disk is 65,000 to 75,000 kilometers, and the overall width of the rings, or bands, is approximately 10,000 kilometers; somewhat surprisingly, the thickness of the disk is less than 1 kilometer. The rings consist of particulate matter, including dust, rocky debris, ices, and oil droplets. |
One of Saturn's seventeen moons, Titan, is larger than Mercury. Titan possesses a hazy atmosphere rich in nitrogen, ethane, and methane, some of which appears to be condensed into an ocean of liquid nitrogen and hydrocarbon; it also displays polar ice caps. |
Uranus
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| Moons and rings around Uranus, seen by Hubble Space Telescope. | |
Uranus has a density of 1.2 grams per cubic centimeter and a thin system of narrow rings. Like Jupiter and Saturn, it is surrounded by a swarm of satellites, fifteen in number. Because of the abundance of methane and other hydrocarbons, its atmosphere is obscured by a photochemical haze similar to terrestrial smog. The spin axis of Uranus nearly coincides with the zodiacal plane. The change of seasons as well as day-night variations must be enormous on Uranus compared to Earth and most of the other planets in the solar system. |
Neptune
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| Voyager 2 view of Neptune. | |
The eighth planet, nearly identical to Uranus, is slightly more dense at 1.7 grams per centimeter, and it has only eight moons. Both Uranus and Neptune consist chiefly of methane and condensed ices. Neptune exhibits two dark oval areas, one of which--the great dark spot--is comparable in size to the Earth. Like Jupiter's red spot, the great dark spot is thought to represent an enormous, counterclockwise rotating, cyclonic weather system. Neptune also appears to be surrounded by thin, incomplete rings or arcs of particulate matter, probably dust and/or ices. |
Pluto
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| Hubble portrait of Pluto and its moon, Charon. | |
We have relatively little information about this tiny planet, which is smaller than our Moon; our spacecraft have not yet explored it. Pluto has a markedly elliptical orbit and may actually be a large asteroid gravitatively captured by Neptune. Similar to Uranus, Pluto's spin axis nearly coincides with the ecliptic plane. Pluto has a density of 1.1 grams per centimeter and consists chiefly of solidified gases, probably predominantly frozen nitrogen. Its sole moon, Charon, is similar in composition and density to Pluto and represents approximately one-eighth the mass of that planet. |
Earth's place in the solar system
Assuming that meteorite populations are (and were) relatively uniformly distributed throughout our portion of the solar system, the density of an impact crater that we observe on a rocky planet's surface provides a relative measure of the extent of surface reworking caused by the internal mechanisms of the planet. The more numerous the impact craters are, the slower the bodily circulation, volcanism, deformation, and consequent surficial reworking by mass flow within the planet must be. Intensely pockmarked planets are thus nearly dead, or at least are less active internally than those reworked, modified bodies that lack an abundance of such craters. |
Comparing the size of the Moon, Mercury, Mars, Venus, and the Earth with their abundance of impact features, it is apparent that most of our planet's surface has formed during the very recent geologic past, whereas internal engines of the Moon, Mercury, and Mars slowed or stalled and became almost totally inactive several billion years ago. |
Venus is least known because of its dense, obscuring carbon dioxide cloud cover; however, radar imagery has provided enough topographic control to tentatively conclude that surface activity, and therefore internal circulation, probably ceased there more than 500 million years ago. |
Although the Earth possesses chemical characteristics similar to its sunward, torrid sister Venus, as well as its outer, frigid brother, Mars, it is obviously very different in surface aspect compared with these neighboring planetary siblings. This difference is understandable based on relative sizes and internal heat budgets, as discussed above. However, it is also a consequence of the position of our planet relative to the energy reaching the planetary surface-the radiant solar flux. Solid, liquid, and gaseous H2O are stable at and near the Earth's surface. |
The proximity of Venus to the Sun is responsible for its higher surface temperatures and lack of liquid H2O oceans; accordingly, as noted in the descriptions of the individual planets, the carbon dioxide compliment of Venus occurs as a gas and resides in the dense Venusian atmosphere. Mars, in contrast, is so distant from the Sun that, on its surface, H2O frost is presently confined to seasonal polar caps (which actually consist mostly of "dry ice"--solid CO2); the carbon dioxide on Mars is thus locked up in icy solid layers. |
Photographs taken by spacecraft of the inner solar system clearly show that in comparison to the dense, hot, carbon dioxide-enshrouded Venus, and the arid, cold, nearly atmosphereless Mars, as well as deader-than-a-doornail Mercury and the Moon, the Earth's envelope is defined by a particularly hospitable and dynamic interplay among atmosphere, hydrosphere, and the variegated surface of the solid planet. The presence of liquid water has allowed for the storing of the terrestrial budget of carbon dioxide as carbonate strata (deposited from seawater) in the rocky crust, rather than in the atmosphere. In addition, the origin and sustenance of carbon-based life itself clearly require liquid H2O. We have much to be grateful for, and many reasons to cherish the environmental conditions that characterize planet Earth. |
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