ASTRONOMY 1040 |
Class Notes #4 ALL NOTES ARE SUBJECT TO CHANGE |
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This set of notes covers chapter 6, which is required to read, but also includes some vital information from chapters 7 & 8, which you are not required to read. You should, however, read and study the notes for chapters 7 & 8, and you should at least skim them in the textbook. Chapter 6ScaleOne of the most vital concepts in astronomy -- and one that has many misconceptions attached to it -- is that of scale. In order to properly comprehend the enormity of the Universe. For this chapter, a solid concept of the scale of the Solar System is absolutely required.You need to know the rough sizes of planets (preferably in km) and how they compare, relative to each other, and the Sun. You don't have to know the exact figures, but you *do* need to know the approximate sizes of the Sun and planets, and roughly how big one is relative to the other. For example, the Sun is roughly 1.4 million km across, whereas the earth is nearly 13,000 across (or "in diameter," which is saying the same thing). That makes the Sun about 110 times the diameter of the Earth (109 times is closer, but you can round off). It's grade school math, by the way. If can help to think of things in terms of analogies. The book uses these frequently. For example, if the Sun is the size of a large beachball (say 24 inches), how big would Earth be? What about Jupiter? Saturn? Mars? (You figure it out, plus variations). Or if the Earth were the size of a softball (about 3.5 inches), how big would the Moon be? Jupiter? Jupiter? Saturn? Mars? (You figure it out, plus variations). What about other comparisons? What about the comparison of the Earth to Mars with common household objects, like oranges and grapes for example? (You need to know the relative sizes) Similarly, you need to know the rough distances between planets and how they compare, relative to each other. For the distances between planets and the Sun, it is convenient to think of distances in terms of the Earth's distance to the Sun, which we call an Astronomical Unit. For example, the Earth is one Astronomical Unit (AU) from the Sun, and Mars is 1.52 AU from the Sun. Look up or figure out some others. You can also think of things in terms of light-time. Consider that the Sun is about 8.3 light-minutes from Earth. What is the light distance between the Sun and Venus? What is the distance between the Sun and the dwarf planet Pluto? What about other planets? How do planets compare in terms of size, physical characteristics and distance from the Sun? Is there a theme here? Is there a pattern? What are the major differences between say, the Earth and Jupiter? Does this apply to other planets? By the way, you don't have to memorize all this. In fact at this point it might be a waste of time, or it might take away from your time studying other things. What you need to know some basic facts about the Earth for example, and then the comparative values for other objects. So if you know that the Earth is about 13,000 km across and the one is roughly a quarter of the size of Earth, simple division tells you that the Moon is about 3250 km across (actually, it is 3475 km, but we just need rough values). PlanetologyPlanetology is the study of planets, whereas Comparative Planetology is the study of the differences and similarities between planets.Until very recently, this study has been limited entirely to just one set of planets (plus their moons and other minor objects), namely those of our own solar system. But within the past decade or so, astronomers have begun collecting limited information about planets orbiting some nearby stars, as well as solar systems in the process of formation. One particularly important fact about the planets of our Solar System is that they are arranged into two major groups (plus natural satellites ("moons") and a few oddballs): Terrestrial PlanetsThe Latin name for Earth is Terra, from which we get the word terrestrial, meaning "of or like the Earth." This includes the first four planets away from the Sun: Mercury, Venus, Earth and Mars.These planets, sometimes also known as the "Inner Planets", share the characteristics of being relatively close, as well as relatively small, hard and rocky. The physical nature of the Terrestrial Planets, i.e. being composed largely of rocky and metallic elements, relates to being formed so close to the Sun. In essence, the heat of the Sun "boiled away" most of the lighter gases so prominent in the Jovian Planets. Jovian PlanetsAn old Latin name for Jupiter is Jove. (Actually "Jupiter" appears to derive from "Jovis Pater," or "Father Jove" indicating the king or father of the gods). Thus the planets known as Jovian Planets take Jupiter as their model. They are Jupiter, Saturn, Uranus and Neptune.These worlds are, compared to the Terrestrial Planets, all far from the Sun, large, and mostly composed of gases. They are often called the "Gas Giants." Avoid referring to them as the "Outer Planets" because that would also include Pluto, which is utterly unlike the Jovian Planets. The physical nature of the Jovians is due largely to having formed far from the Sun where it is cold. The Jovians, being quite massive, also have strong gravities. The combination of cold temperatures and strong gravities helps these Gas Giants hold onto even light gases such as hydrogen and helium, which are largely lost in the hot inner Solar System. Moons and Oddballs!MoonsAsteroids Comets Meteoroids Pluto and the other "dwarf planets" (Ceres and Eris), plus TNOs Formation of the Solar SystemAny reasonable theory of the formation of the solar system must explain the existence of the Sun, planets, moons, asteroids and so on -- as well as their arrangement and physical natures.The Cosmic Perspective outlines four major aspects of the Solar System that must be explained by any reasonable theory:
The Nebular TheoryThe basic idea today starts with a large nebula (cloud) of gas and dust. In the case of our Solar System, this must have been debris from a large exploded star because it contained elements (found now in the Sun and planets) that could only have been produced in a large star.The large cloud would slowly start to collapse under gravity (condensation), but it probably got a kick start from a shockwave from a nearby supernova explosion. In any event, it began to collapse. As it shrank, and the central regions grew more dense, it began to flatten out due to "centrifugal force." The central region by then was massive enough to continue collapsing despite the centrifugal force, forming the "protosun." Rather than continuing to fall in to the protosun, material in the "disk" region began to orbit it instead. Smaller areas of mass concentration in the disk began to form as particles began to collide with one another and stick together (accretion). By then entire disk region had become a "protoplanetary disk." As the protosun contracted, it heated according to the laws of thermodynamics. So did the protoplanets, but due to their smaller sizes, the heating was much smaller as well. The inner part of the protosun was the most dense, as well as hottest. The protosun gave off energy at various wavelengths, but mostly infrared. Eventually it probably also began to glow visibly, but was nowhere near as bright as it is today. In the protoplanets, the heavier elements sank to the centers to form cores (such as Earth's largely iron core). It was cool enough yet in the protosolarsystem that the planets all likely had large gaseous atmospheres similar to what the Jovian Planets have today. Eventually, the continuing contraction of the protosun produced temperatures high enough for the onset of nuclear fusion -- at this moment the Sun became a full-fledged star and the Solar System was born. The new Sun emitted large numbers of charged particles in a gusty solar wind much more powerful than today. This solar wind blew away the light gas atmospheres of the Inner Worlds, leaving the denser cores that have become the Terrestrial Planets of today. But the Jovian Planets were too cold and far away to be so greatly affected. Particles in an atmosphere must travel at a certain speed (escape speed) in order to be lost to space. But the lower temperatures at the distances of the Jovian Planets means slower speeds. Combined with the stronger gravitational pulls of the Jovians (requiring higher escape speeds), the atmospheric particle simply did not have enough energy to fly off into space. As a result, the Jovian Planets retained their thick atmospheres. Jupiter and the other Jovians were so far from the Sun -- past the so-called "Frost Line" -- that hydrogen compounds (such as water) remained as ices. Comets formed from icy materials far from the Sun. Between Mars and Jupiter, several smaller planetoids likely formed and perhaps crashed into each other, breaking up and forming the asteroids and meteoroids. Evidence for the Nebular TheoryNeedless to say, the theory does not violate any known laws of physics and in fact was constructed from the laws of physics. That is, it is a plausible theory. And the one that best fits the observational data.There is abundant evidence that planets experienced a period of massive bombardment from space billions of years ago, at a time consistent with the Nebular Theory. The bombardment likely was from impacting asteroids and other debris left over from the formation of the Solar System. An impact between a large object and the Earth may well have produced our Moon (this is the "Giant Impact" theory of lunar formation. Other collisions or near collisions could explain the unusual orbits of Mercury and Pluto, and the unexpected orbital inclinations of Venus and Uranus. Portions of this theory, such as the protoplanetary disk and the protosun, appear to be supported by observational evidence. Although we cannot observe these stages for our Sun or Solar System, they have been observed elsewhere in the Milky Way. Today we even have observational evidence for more than 100 planets orbiting other stars (largely through detecting the way they affect the stars they orbit). The Titus-Bode LawFirst discovered by German Johann Titus in 1766, this is an rough guide to the spacings of planets away from the Sun.Another German, Johan Bode, elaborated on it and it is today called either Bode's Law or the Titus Bode Law, although the term "law" is perhaps a bit too strong a word. Start with this sequence of numbers: 0 3 6 12 24 48 96 192 384 Then add 4 to each and divide each by 10. This gives the approximate distances of the major planets to the Sun: .4 .7 1.0 1.6 2.8 5.2 10.0 19.6 39 in terms of astronomical units Note that nothing was seen at 2.8 AU. The "celestial police" eventually discovered the asteroids there. Also note that it breaks down after Uranus. Neptune is at 29 AU, not 39, but Pluto is at 39.
Compare the bold numbers in the table above to the actual distances of the planets frome the Sun, expressed in Astronomical Units. The Titus-Bode Law is an interesting mathematical curiosity, for which there is no known physical reason. Since no one has been able to define a precise mechanism by which it might work, it is not widely accepted today, and is seen rather as an interesting coincidence. However, some astronomers believe that there may be some underlying reason, likely related to the interplay of gravitational fields of the planets and Sun, that could give the Titus-Bode Law more credibility. But such a reason hasn't yet been found. ======================== I am not requiring that you read chapters 7 & 8, but you should at least skim it. Here are some of the main points that may be touched on in the next test: Chapter 7The Age of the Earth (and the Solar System)In the 1600s, Irish Archbishop Usher (among others) used Biblical "begats" (working backwards from the present using the lifespans of various characters in the Bible to establish a timeline), and apparently some astronomical considerations, determined that the first day of Creation was Sunday, 23 October 4004 BC (by Hebrew tradition, the day begins at sundown the previous day, so this would be the evening of 22 October). Some different dates and even precise times have been attributed to Usher, but this seems to be the date he actually decided on. Other church chronologists came to similar findings.)In the 1800s, geologists considering the deposition of rock layers and other evidence realized that the Earth must be millions to hundreds of millions of years old. Examination of the Grand Canyon brought that figure up to at least a couple of billion years. In the 20th Century, physicists used radiometric dating of rocks and meteorites to deduce an age of about 4.5 billion years. Radioactive elements have "half lifes," meaning that after a certain period of time, half of the original radioactive material will be left, and the other half will be turned into one or more daughter products. Radioactive materials occur normally in Nature, and slowly change from one element to another by emitted subatomic particles at a certain rate. By knowing the halflife of the element (or isotope, really) from lab studies, and then looking at the ratio of the original element to the known daughter products in a rock, the age of the sample can be obtained. For instance, Uranium 238 changes into Lead 206 (through a series of intermediate stages), with a halflife of about 4.5 billion years. Thus in a sample with equal amounts of Uranium 238 and Lead 206, the age would be 4.5 billion years (other considerations being equal). The oldest rocks on Earth have been measured at about 4 to 4.2 billion years, whereas some meteorites have been measured at about 4.6 billion years. Internal Structure of Terrestrial PlanetsConsidering the mass and densities of the Terrestrial Planets, and applying the laws of Physics, we use out study of the Earth as a basic model for the other planets. The Internal structure of the Earth is deduced by considering its density, physics and seismic waves.The Earth's overall density is 5.52 times that of water, making it the densest planet in the Solar System. Since rocks near the surface of the Earth are much less than that, we can conclude that the interior of the Earth must be much higher than the surface. The models developed indicate that the Earth has a two-part core composed mostly of iron (density of 7+). The laws of thermodynamics require that the interior of Earth must be at a much higher temperature than the surface, and that the interior iron core be partly or completely molten. Seismic studies using the propagation of earthquake waves through the planet bear this out. There are two kinds of earthquake waves: P or compressional waves, and S or shear waves. The P waves are back and forth waves like sound, whereas the S waves are sideways waves. Typically, the S waves do the most damage in earthquakes. S waves also have another interesting characteristic, they do not travel through liquids. Seismologists have found that while P waves propagate completely through the Earth. S waves are stopped at a certain distance (about 2900 km or 1800 miles), indicating a large liquid mass. The conclusion is that it is molten iron. Due to the propagation of P waves and predictions of physics based on pressure and temperature, the inner core of the Earth (about 2400 km or 1500 miles across) is solid iron. The internal structure of the other Terrestrial Planets are based on this model of the Earth, allowing for changes in density and other parameters. Surfaces of the Terrestrial PlanetsSeveral forces shape the surfaces of the planets:
In the early 20th Century Alfred Wegener noticed that the continents seemed to fit together somewhat like jigsaw puzzle pieces. Since it looked like today's continents had broken apart from one larger continent and were drifting apart, this became known as Continental Drift -- but there was no real evidence. In the 1950s, oceanographers discovered an interesting pattern of magnetism in the rocks on the bottom of the Atlantic. The patterns formed "mirror" images on the East and West sides of what is know called the Mid-Atlantic Ridge. The idea is that new ocean floor is being formed as lave comes out of the ridge, pushing the old ocean floor (and the continents) away from the ridge. In 1963 Canadian geophysicist Tuzo Wilson formalized this idea into what we call today, "Plate Tectonics."
Mars has polar caps composed of water ice and frozen Carbon Dioxide or "dry ice." These wax and wane with the seasons. The planet also is subject to occasional planet-wide dust storms. The Martian Rovers spacecraft have found what appears to be almost indisputable evidence that many of the Martian formations were formed under the influence of large amounts of water. Some of this evidence includes abundant minerals known to form in water, and "cross-bedded" rock formations like those on Earth that form at the bottom of a body of water. As of late May, 2008, another Martian lander, Phoenix, has successfully landed on the Red Planet and joined the search for water and life. Atmospheres of the Terrestrial PlanetsMercury essentially has no atmosphere. There are transient particles from the Sun that may temporarily be caught in the Mercurial gravity, but they do not stay long. The original cloud of hydrogen and helium thought to have enshrouded all planets in the early days was quickly blown away by the solar wind. Today, the environment is simply too hot and Mercury's gravity too weak to maintain a permanent atmosphere.Venus probably also had a Hydrogen and Helium atmosphere that was blown away by the solar wind. But its strong volcanic activity pumped massive amounts of Carbon Dioxide into the atmosphere, giving Venus about 100 times the atmospheric pressure of Earth. The Carbon Dioxide is a potent Greenhouse gas, and has promoted a "runaway" Greenhouse Effect, raising the average worldwide temperatures to 900 - 950 degrees F. Clouds, composed in large measure by sulfuric acid droplets, completely enshroud the planet. With such high temperatures, high pressure and unbreathable atmosphere, Venus may be pretty as viewed from Earth, but it is a nasty place. Earth is unique among planets. Its hydrogen-helium atmosphere also was blown away in the early days. This was replaced, as with Venus, by Carbon Dioxide mostly from volcanoes (plus Nitrogen). The major difference between Earth and Venus, though, is that Earth had lots of water (perhaps brought here by comets). The water formed clouds and rain followed. The rain "scrubbed" much of the Carbon Dioxide out of the atmosphere, putting it in the oceans, where it eventually was incorporated into rocks on the ocean floor. Meanwhile, plant life emerging on the planet's surface used photosynthesis to "fix" more of the Carbon Dioxide, sending it down through its roots into the soil. At the same time, the process of photosynthesis produced the Oxygen we now breathe. So our current atmosphere has mostly the leftover Nitrogen (about 77-78 percent, including some from decaying plants), photosynthesized Oxygen, and a very small amount of other elements and compounds including Argon, Carbon Dioxide and water vapor. Earth's atmosphere today is a third generation atmosphere. Mars is similar in some ways to Venus. It lost its Hydrogen-Helium atmosphere, and replaced it with Carbon Dioxide from volcanoes. There is increasing evidence that there was a good amount of water on Mars long ago, and this may have helped to "scrub" some of the Carbon Dioxide out. But if there was plant life, apparently there wasn't enough to make much of an effect in removing CO2 or in adding Oxygen. Actually the atmosphere of Mars is thinner (about 1/150th of Earth) than you might expect. Exactly where all the Carbon Dioxide and water went is not yet known. Satellites of the Terrestrial Planets
The Moon's composition is also interesting. Based on a study of its elements and isotopes, mostly from samples brought back by astronauts, it appears like the Earth in some ways, but surprisingly different in others The best theory today for its origin is that not long after the Earth formed, a large object, possibly the size of Mars, collided with our planet. Earth material mixed into the debris with that of the other object, and the debris went into orbit about the Earth. The debris eventually coalesced to form our (hybrid) Moon.
Chapter 8The Jovian WorldsAs noted earlier, the solar system is divided into two types of planets, separated by a wide gap in which we now can observe tens of thousands of small planetoids called asteroids. The first four planets away from the Sun are the small, hard and rocky Terrestrial planets, which Earth as the largest. Past Mars, there is a huge gap of hundreds of millions of kilometers before you get to the realm of the other type of planet, the Jovian worlds.Jove is an old name for Jupiter, so the prototype of this classification is Jupiter. They are all large and far from the Sun, and separated from each other by enormous distances. The Jovian planets are also known as "Gas Giants" because they are so large, and the primary components of them is gas. Although they all have various gases, the predominant gases are hydrogen and helium. In some ways, the Gas Giant planets are more like the Sun than they are like the terrestrial planets. By they are not large enough to develop the internal pressures and temperatures needed to turn them into stars. The Jovian or Gas Giant planets are massive, which gives them strong gravities, capable of retaining large numbers of natural satellites or moons. These moons are mostly ice, with some rock, but exhibit interesting geology. Jupiter's moon Io is an exception, as there is no ice there, but instead the most active volcanic surface in the Solar System. Another of Jupiter's moons, Europa, shows evidence of a possible water ocean under its frozen crust. This lends itself to the idea that some primitive life may exist there. Saturn's large moon Titan has a thick atmosphere (unlike other moons), composed mostly of nitrogen, the main component in Earth's atmosphere. We know less about Uranus and Neptune, which are smaller than Jupiter and Saturn, but much larger than the Terrestrial planets. Of course, way beyond Neptune is the realm of Pluto, now demoted from planethood to the title of dwarf planet. Actually, it is just one of many objects out there, some larger than planets. These are sometimes known as Trans-Neptunian Objects, or TNOS; they also are sometimes referred to as Kuiper Belt Objects. The Kuiper Belt is one of two large reservoirs of cometary material that are part of the Solar System. Comets, of course, are ancient objects, probably mostly ice, that occasionally fall in close to the Sun, thaw and make impressive looking displays in our sky! END
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