CHAPTER 1: ORIGIN OF THE PLANETS & THE SOLAR SYSTEM TODAY
The Solar System
1. Figure 1.3: The Solar System consists of the Sun, nine planets, 61 moons and a multitude of asteroids, comets and meteoroids.
2. The orbits of the planets are elliptical around the sun
3. The planets generally revolve in the same direction around the sun and within the plane of the ecliptic except for Pluto, which is tilted at 17o to the ecliptic.
4. Most of the moons revolve around the planets in the same direction as the planets revolve around the sun.
5. Meteoroids, asteroids and comets also follow orbits around the sun.
6. The rotations of planets, moons and other bodies are inherited from the rotation of the ancient gas cloud from which they formed.
The Terrestrial (Rocky) Planets
1. Closest to the sun and consist of Mercury, Venus, Earth and Mars.
2. Are generally small, rocky bodies sharing many similarities and also differences.
3. Densities greater than 3 gm/cm3.
4. Composed mainly of silicates in addition to Fe and Ni.
5. Volcanism is largely basaltic, a black rock relatively rich in Mg, Si, O and Ca.
6. Differences observed among the rocky planets reflect factors such as size and distance from the Sun rather than composition. This tells us that the these rocky planets formed more or less from similar material early in the history of the solar system.
The Jovian (Gaseous) Planets
1. Occur beyond the orbit of Mars and consist of Jupiter, Saturn, Uranus, Neptune and Pluto.
2. Generally larger then the terrestrial planets.
3. Densities less than 3 gm/cm3.
4. Each (except for Pluto) consist of a solid core (probably rocky) surrounded by a thick atmosphere composed of methane, ammonia, hydrogen, helium and other gases. Pluto lacks a thick atmosphere and instead consists of a solid core with a thick outer layer of ice.
5. Most Jovian Planets have multiple moons.
6. Most Jovian Planets have impressive ring systems composed of dust- to boulder-sized particles of mostly ice.
Theories on the Origin of the Solar System
1. During the late 1700s, Buffon envisioned the planets as having formed from the sun. He proposed that the gravitational attraction of passing comets had pulled hot, gaseous material from the sun. This material later cooled and condensed to form the planets. Thus according to Buffon, the sun was much older than the planets.
2. Another early theory called for condensation (solidification) of the planets from a hot, gaseous cloud called the solar nebula rather than having the planets derived from the sun itself.
3. In the early 20th Century, scientists generally discounted the idea of condensation of planets directly from a hot gas cloud and instead favored the hypothesis that the planets and other solar system bodies aggregated from cold clouds of dust and gases. According to this model, the planets initially formed as cold spheres resulting from slow accumulating of dust and gas.
4. The cold origin of planets was initially put forward in the planetesimal hypothesis, developed early in the twentieth century by Chamberlin (a geologist) and Moulton (an astronomer). According to this model, the gravitational pull from a passing star supposedly extracted solar gaseous materials from the sun. These gaseous materials then surrounded the sun and began condensing small, solid bodies about the size of asteroids (tens to hundreds of km in diameter) called planetesimals. These planetesimals eventually aggregated to form the early planets, which then continued to grow by attracting still more particles.
5. Figure 1.6: Once the cold, growing planets became sufficiently large, gravitational attraction took over. The planets eventually achieved a mass that caused gravitational collapse, resulting in internal heating and softening, followed by separation of the different materials into discrete layers. Denser material sunk into the planet interiors whereas lighter material migrated or floated to the surface.
1. Figure 1.2: Another idea, called the nebular hypothesis, was put forward in the mid-twentieth century by astronomers von Weizacher and Kuiper. The nebular hypothesis has the sun and planets forming at the same time. According to this model, our solar system began as a gigantic, disc-shaped interstellar cloud of gases and dust about 5 or 6 billion years ago .
2. It was envisioned that slow rotation of this interstellar cloud gradually caused much of it's mass to become concentrated near the center of the disc. This centralized mass underwent further compression through gravitational attraction until eventually reaching temperatures of several million degrees causing thermonuclear reactions to begin. This centralized, thermonuclear mass became the early sun.
3. Figure 1.2: The embryonic sun was surrounded by an envelope of gas and dust called the solar nebula. Turbulence within the nebula first caused condensation of planetesimals. The cold planetesimals, in addition to dust and gases, then collided rapidly and aggregated to form 9 or 10 protoplanets of similar composition. Moons may have either formed in a similar manner around their host planets or were possibly later captured from elsewhere by the planet's gravitational attraction. The remaining planetesimals developed very elliptical orbits and were eventually tossed out of the inner solar system by Jupiter's gravity to become comets.
4. Figure 1.6: The cold homogenous accretion model states that as protoplanets grew, their progressively stronger gravitational fields swept up even more material from the dust cloud until they eventually became large, compositionally homogenous planetary bodies of even greater size than present, but of much lower density. The growing gravitational fields eventually caused the large protoplanets to contract and become more dense. This contraction led to differentiation where most of the heavier elements migrated to the center of the protoplanets while lighter elements moved towards their surfaces. Much of the light gases of H and He were lost to space.
5. The four inner, terrestrial planets were relatively small and had weaker gravitational fields relative to the outer, gaseous planets. The inner terrestrial planets therefore lost far more of their lighter elements than did their larger counterparts.
6. The sun's radiation, or solar wind, further modified the compositions of the planets by blowing away any remaining nebular gases to the outer part of the solar system.
7. Recently, the hot heterogeneous accretion model has been examined. According to this model, the internal zonation of the planets developed during, not after, accretionary accumulation. Accretion is thought to have begun with the solar nebula at a time when the gases were still very hot (> 1000 C). (A) As the nebula began to cool, primitive Fe and Ni accumulated first to form the metallic core of the planet. (B) Silicates accreted later around the earlier formed core as temperatures continued to drop. (C) Finally, the mantle differentiated to form the crust.
Dawn of Earth's History
Figure 1.5: Summary of the proposed stages for the early evolution of the Earth.
Chemical and Thermal Evolution
1. The early earth at around 4.5 billion years ago had as much as five times the amount of radioactive heat production as now.
Following Accretion, Heating of the Early Earth was Caused by:
(a) Initial heating due to gravitational contraction which may have raised the temperature of the earth's center by 1000o C.
(b) Radioactive heat production that raised the temperature by an additional 2,000 C.
(c) Intense meteorite bombardment prior to 4 billion years ago.
2. Figure 1.6b: Early heating of the earth may have been so intense as to completely melt the planet for a short time.
3. A distinct core and mantle was created by about 4.5 billion years ago when the earth's surface was covered by an immense ocean of molten magma.
4. Even today, continued activity from volcanoes and hot springs indicate that heat is still being released from the interior. It is estimated that, due to the thermal insulation provided by the earth's crust and mantle, the interior is today only midway in its cooling history even though much of the radioactive elements have long since decayed.
Origin of the Crust
1. Figure 1.6b: Evidence suggests that the earth's crust differentiated (separated) from the underlying mantle on the basis of chemical composition. During mantle differentiation, relatively light elements such as Si, O, Al, K, Na, Ca, C, N, H and He rose to the surface to form the crust, seawater and atmosphere (Figure 1.8).
2. Isotopic dating indicates that continental crust did not become stable until about 3.9 - 4.1 billion years ago, almost half a billion years after formation of the core and mantle.
Origin and Evolution of the Atmosphere and Seawater
Several hypotheses exist to explain the origin of the earth's atmosphere. They all operate under the assumption that:
(a) Figure 1.8: Considerable hydrogen and helium escaped into space during early differentiation of the earth (Figure 1.6b). Most of the remaining hydrogen was locked up in water.
(b) The early atmosphere had practically no molecular O2. Abundant oxygen came much later as a result of slow accumulation over geologic time.
(c) The earth's early atmosphere may have been much like that of Jupiter today and contained gases similar to those found today in meteorites. These early gases consisted primarily of methane, ammonia and water vapor.
The Outgassing Hypothesis
1. Figure 1.8: In 1951, a geologist by the name of W.W. Rubey favored the theory that most gases in the earths early atmosphere were derived from the interior of the planet through igneous transfer via volcanoes and hotsprings. This process is known as outgassing.
2. The trace amount of He and Ar found in our present atmosphere represent daughter products of U and K decay, respectively.
3. By assuming formation of the atmosphere through continuous outgassing from volcanoes and hot springs, we can reasonably account for all the N, He, Ar and water vapor occuring in the atmosphere today. Oxygen has a separate origin as a product of photosyntheses over the course of 2-3 billion years.
Photochemical Dissociation Hypothesis
1. The Photochemical Dissociation Hypotheses assumes an early earth atmosphere much like that found on the planet Jupiter today which is dominated by methane, ammonia and water vapor.
2. According to this model, the early atmosphere of the earth was devoid of an ozone layer which today acts to filter out incoming ultraviolet radiation. Without an ozone layer in the early atmosphere, ultraviolet light was able to reach the earths surface and cause several reactions to take place within the primitive atmosphere.
Reactions of Ultraviolet Light with Primitive Earth Atmosphere:
(a) Dissociation of water vapor into hydrogen and oxygen with most hydrogen escaping into space: 2H2O + uv light = 2H2 + O2
(b) Newly formed molecular oxygen reacted with methane to form carbon dioxide and more water: CH4 + 2O2 = CO2 +2H2O
(c) Oxygen also reacted with ammonia to form nitrogen and water: 4NH3 + 3O2 = 2N2 + 6H2O
(d) After all the CH4 and NH3 were converted to CO2 and N2, then excess O2 could accumulate as more water vapor dissociated. Over time, our present atmosphere of N2, CO2 and O2 may have formed.
Oxygen from Photosynthesis
1. The early earth may have additionally contained a great deal of CO2 in the primitive atmosphere.
2. The appearance of photosynthetic cyanobacteria about 3.5 billion years ago instigated the process of photosyntheses in which these early life forms extracted CO2 from the atmosphere and released O2 as a by-product. Over the course of hundreds of millions of years, O2 slowly began to accumulate in the atmosphere.
Origin of Seawater
1. The rate of seawater accumulation is directly tied to atmospheric production of water vapor following chemical differentiation of the earth. In other words, the outgassing hypothesis can also account for the accumulation of water on the earths surface.
2. The question remains, however, whether the atmosphere and oceans accumulated slowly at a more or less uniform rate or did they accumulate rapidly during the early stages of earth history?
3. Some suggest that intense early bombardment of the earth by icy comets may have contributed to the planet's supply of water and gasses, implying that the atmosphere and seawater formed early and rapidly.
4. On the other hand, if seawater accumulated slowly in a manner similar to the O2 buildup by photosynthesis, then the earth's water supply may have been pretty well established by around 2.5 billion years ago.
The Moon and Planets
1. The moon is a small, dense rocky object pock-marked by impact craters and numerous basalt flows.
2. Seismic measurements from seismometers placed on the moon by astronauts have determined that the moon is layered. The crust of the moon, where measured, is around 65 km thick. The moon is covered by a thin veneer of regolith (mixture of gray pulverized rock fragments and small dust particles) overlying a 2 km thick layer of shattered and broken rock. Below the broken-rock zone is about 23 km of basalt, followed by 40 km of feldspar-rich rock. The mantle composition is unknown but possibly similar to the Earth's mantle. The lithosphere is possibly as much as 1000 km thick and any asthenosphere would occur at deeper levels.
3. The Moon's surface includes light-colored mountainous areas called highlands, which are heavily cratered and primarily composed of plagioclase-rich rocks called anorthosite that formed early in the history of the moon (4.5 billion years ago).
4. The smooth, dark-colored lowland impact craters are called maria (singular mare) which are nearly circular and filled with basaltic lava flows.
5. The moon probably formed 4.6 billion years ago. One theory states that the moon formed in its present orbit by accretion during condensation of the solar nebula. A second theory suggests that the moon was captured by the earth.
6. Figure 1.4: The most widely accepted theory, however, is that the moon originated as a portion of the earth that was ejected during impact with a Mars-sized object about 4.5 billion years ago. The ejected material condensed to form the moon.
7. Intense meterorite impacts that occurred around 3.9 - 4.0 b.y. ago formed most of the craters seen on the moon today. Since that time, the Moon has remained a dead planet void of any tectonics or volcanism.
1. It's high density of 5.4 g/cm3 may be due to a large, metallic core about 3600 km in diameter.
2. Heavily pockmarked by ancient impact craters, many filled with basaltic flows.
3. Lacks an atmosphere and shows no evidence of tectonic activity (no evidence of moving lithospheric plates).
4. Mercury has a magnetic field about 1/100 as strong as that of the Earth. Planetary magnetic fields are typically formed by fluid motions in the core caused by rotation of the planet. Mercury's slow rotation (once every 59 days vs 24 hrs for earth) and lack of tectonic plate movements, however, pose problems with this interpretation.
1. Venus is about the same size and mass as Earth.
2. Thick atmosphere of CO2 prevents direct visual observation of the planets surface and is largely responsible for surface temperatures of about 500o due to the greenhouse effect.
3. Several spacecraft have landed on the surface and radioed back information from radar measurements of the surface topography. Spacecraft Magellan recently orbited Venus and has sent radar images back to earth.
4. Radar images show a surface consisting of broken rock fragments primarily basaltic in composition.
5. Vast volcanic plains and thousands of volcanoes shaped like broad domes, similar to those that occur today in Hawaii, dominate the surface. Several steeper-sided volcanoes indicate eruption of more Si-rich lava.
6. The topography also shows mountain ranges and rift valleys.
1. Mars is only 1/10 the size of earth and rotates once every 24.6 hours.
2. Mars has a thin atmosphere only 1/100 as dense as the Earth's and consists largely of CO2.
3. Mars has polar ice caps consisting mostly of CO2 and small amounts of water ice. The ice caps grow and shrink with the seasons.
4. The composition of the Earth and Mars may be similar. Mars has a reddish-brown surface covered by loose stones and windblown sand. Two Viking spacecraft had landed on the martian surface during the 1970s and analyzed the composition of the soils. Chemical analysis by the Viking spacecraft indicated clays and possibly gypsum, a mineral commonly precipitated from evaporating water.
5. The Viking spacecraft also monitored for earthquakes, but no earthquakes were recorded. The scarcity of earthquakes suggest that any former plate movements on Mars had now ceased.
6. Recently, the spacecraft Pathfinder landed on Mars and sent out its microrover, Sojourner, to study rocks on the surface. The rover found sedimentary and volcanic rocks much like what we have on earth.
7. Mars probably has a core that is completely solid since no magnetic field is apparent.
8. The SNC meteorites are considered martian in origin.
9. Recently, evidence of fossil bacteria were discovered in one of the martian meteorites, suggesting that simple life forms existed in the early martian crust.
10. The southern hemisphere is densly cratered and resembles the surfaces of the Moon and Mercury.
11. Craters are sparse in the northern hemisphere and large areas are relatively smooth, suggesting a younger surface. Huge shield volcanoes like Olympus Mons suggest extensive volcanism in the past. The youngest flows on Olympus Mons are probably less than 100 million years old. Long-lived sources of magma must still be present in the martian interior. Martian lithosphere also must be thick and strong in order to support the weight of Olympus Mons.
12. The martian surface also exhibits a system of huge canyons and branching valleys similar to those cut by intermittent desert streams on Earth. These features suggest that ice presently frozen beneath the surface may have melted during past warming episodes, creating torrential floods that carved these valleys.
13. Rain, lakes and streams may have existed early in martian history during a time of planetary differentiation and extensive volcanism. Mars eventually aquired a frozen regolith. Occasional melting of the frozen ground may have occurred during periods of magmatic activity or sudden changes in climate.
1. Jupiter is about twice the mass of the other planets combined.
2. Jupiter is unusual in that it gives off twice as much energy as it receives from the sun, suggesting that it is still undergoing gravitational contraction.
3. Jupiter has an atmosphere composed primarily of H2, He, NH3 and CH4 surrounding a rocky core.
4. Surface may be a giant ocean of liquid hydrogen.
5. Colored atmospheric bands produced by high-speed winds. Giant red spot (storm).
Moons of Jupiter
6. The moon closest to Jupiter is Io and is colored with shades of yellow and orange, suggesting that it is covered by sulfur and sulfurous compounds. Io is volcanically active. Volcanic products include basaltic lava as well as molten sulfur flows and sulfurous gases. Geyser-like volcanic plumes of SO2 have been observed by the Voyager spacecraft. The heat energy which drives Io's volcanism may be caused by tidal stresses exerted by Jupiter's gravitational pull.
7. Europa, Ganymede and Callisto may have small metallic cores surrounded by thick mantles of ice and silicate minerals. Above the mantle are crusts of nearly pure ice in excess of 100 km thick. Europa is criss-crossed with fractures, suggesting that tidal stresses from Jupiter are manifested on the icy surface.
8. Ganymede (largest of Jupiter's moons) and Callisto have icy surfaces pitted by craters. Ganymede's surface contains dark areas covered by dust and impact debris, indicating ancient ice continents.
1. Saturn is known for its immense ring system.
2. The ring system is 10,000 km wide and a little over 100 m thick.
3. The Voyager spacecraft discovered that the major rings actually consist of hundreds of tiny ringlets.
4. Each ring is composed of dust- to boulder-sized particles consisting mostly of ice, some possibly stained with iron oxide. Color differences indicate slight compositional differences between the rings.
5. Saturn has an overall chemical composition similar to Jupiter.
6. Titan is the most distinctive among Saturn's moons. Titan is surrounded by an opaque, orange-colored atmosphere composed mostly of nitrogen with lesser amounts of ethane, acetylene, ethylene, and HCN. Titan may consist of 45% ice and 55% rocky matter. Surface temperature is estimated at around -180o C, in which case Titan may consist of ice continents surrounded by an ocean of liquid ethane and methane.
1. Rotates on its side.
2. Uranus has rings much like those encircling Jupiter.
3. Uranus has several moons, some with canyons while others are smooth.
1. Neptune is a bluish planet.
2. Winds exceed 1000 km/hr.
3. Neptune has visible white clouds of frozen methane.
4. Neptune has eight moons, six of which orbit in a direction opposite to the other two.
5. The largest of Neptune's moons is Tritan, which has a surface covered with solid nitrogen and methane.
1. Pluto is one-fifth the size of earth and 40 times farther from the sun.
2. Pluto is too small to be visible to the unaided eye.
3. It takes 248 years for Pluto to orbit the sun.
4. Pluto follows an elongated orbit, causing it at times to travel inside Neptune's orbit.
5. Pluto may possibly be a satellite of Neptune rather than a planet as originally thought.
6. Pluto is described as a dirty ice ball of frozen gases and rocky material.
7. Pluto has one moon, Charon, which is 1,300 km in diameter.
1. Asteroids are possibly fragments of broken planets.
2. Asteroids can reach 1,000 km in diameter, but most are only about 1 km or less across.
3. An extensive belt of asteroids occurs between the orbits of Mars and Jupiter.
4. Some asteroids have collided with the earth in the past.
1. Comets can be described as dirty snowballs of frozen gases in addition to rocky and metallic materials.
2. Some comets may contain organic material.
3. Comets develop a tail of dust and ionized gases when approaching the Sun due to the solar wind.
4. Millions of comets may orbit the Sun beyond Pluto.
5. Comets are thought to be relicts of the early Solar Nebula that were swept to the far reaches of the Solar System by the solar wind after formation of the planets.