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Chapter 3&4

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Astronomy 2021A/B

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CHAPTER 3 THE UNIVERSAL CONTEXT OF LIFE 3.1 The Universe and Life What major lessons does modern astronomy teach us about our place in the universe? Newton delivered the final, shattering blow to theAristotelian conception that Earth must by necessity be unique. The heavens could no longer be considered a separate realm made from different material (the ether or quintessence) and operating under different laws from Earth. Three ideas are especially important in framing the universal context for everything else we will study: -The universe is vase and old. Its vastness implies an enormous number of worlds on which life might possibly have arisen, and its old age means there has been plenty of time for life to being and evolve. -The elements of life are widespread. Observation shows that the basic chemical elements that make up Earth and life are present throughout the universe. -The same physical laws that operate on Earth operate throughout the universe. Every experiment and observation made to date has given additional support to Newton's conclusion that the laws of nature are the same everywhere. Together, these ideas reinforce the primary lesson of the Copernican revolution: we are not the center of the universe. 3.3 The Nature of the Worlds How do other worlds in our solar system compare to Earth? Today, we know that no other world in our solar system is much like Earth, at least on the surface. No other world has surface oceans of liquid water, an atmosphere rich in oxygen, or a climate so hospitable to life. The four inner planets (Mercury, Venus, Earth, and Mars) are much smaller than the four outer planets (Jupiter, Saturn, Uranus, and Neptune). These size differences reflect basic differences in planetary character. The four inner planets are made almost entirely of metal and rock, which makes their average densities several times that of water. They have solid surfaces and their atmospheres (if any) are quite thin com- pared to the planets themselves. Because Earth is a member of this group, we refer generally to these rocky worlds as terrestrial planets (terrestrial means “Earth-like”). Note that the terrestrial planets have few moons; Earth is the only one with a large moon, while Mercury and Venus have no moons and Mars has two very small moons. Jupiter, Saturn, Uranus, and Neptune are quite different in character and composition from the terrestrial planets. Because Jupiter is the largest member of this group, we refer generally to these worlds as jovian planets (jovian means “Jupiter-like”). Rather than metal and rock, the jovian planets are made largely of hydrogen, helium, and hydrogen compounds such as water (H2O), methane (CH4), and ammonia (NH3); this composition gives them much lower average densities than the terrestrial planets. The jovian planets of our solar system have at least 165 moons among them, with Jupiter alone having more than 60 known moons.All the jovian planets also have rings made up of countless small particles orbiting them, though only Saturn’s rings are easily visible from Earth. Because hydrogen compounds are generally gases under earthly conditions (except for water, which can be either solid, liquid, or gas on Earth), the jovian planets are often called “gas giants.” However, the pressure throughout most of their interiors is so high that these “gases” are not actually in the gas phase; instead, they may be compressed into liquid or into other high- density phases, which makes them behave quite differently than they do on Earth. Moreover, while the jovian planets contain metal and rock deep in their cores, the high pressure means that even these cores are unlikely to resemble the solid surfaces of the terrestrial worlds. There would be no place to “land” on the jovian planets; if you plunged into one of them, you would continue your descent until you were crushed by the growing pressure. Pluto, Eris, and other objects large enough for their own gravity to have made them round are now considered dwarf planets. The rest of the small bodies have traditionally been categorized in two groups: asteroids made mostly of metal and rock and comets made mostly of rock and ice. Most asteroids orbit in the region called the asteroid belt, which lies between the orbits of Mars and Jupiter. We now know that comets come from the distant reaches of the solar system, and they grow tails only when they come close enough to the Sun for the Sun’s heat to convert some of their ice into gas. comets come from two vast “reservoirs”: the Kuiper belt (Kuiper rhymes with “piper”), which occupies the region of the solar system beyond Neptune and in which we find both Pluto and Eris, as well as many similar but smaller objects, and a much more distant, spherically shaped region called the Oort cloud (Oort rhymes with “court”). Pluto and Eris are members of the Kuiper belt Asteroids and comets can also have their own small moons. Pluto, for example, is orbited by three moons: Charon, which is about half the diameter of Pluto itself, Most moons are very small and would be considered asteroids or comets if they orbited the Sun independently. But a few moons are planet- like in size. Jupiter’s moon Ganymede and Saturn’s moon Titan are larger than the planet Mercury. These and other relatively large moons are planet-like in almost every way except for their orbits. Some even have active geol- ogy or atmospheres. For example, Io is the most volcanically active world in our solar system, Europa must occasionally have water or ice flowing across its surface, and Titan has an atmosphere thicker than Earth’s. Moos vary in composition in a way that correlates with their locations in the solar system. Our Moon is made mostly of rock, much like the rocky composition of the terrestrial planets. In contrast, the moons of the jovian planets generally contain large amounts of ice, including water ice (H2O). When you combine this icy composition with the fact that some of the large jovian moons have internal heat and geological activity, you can see why scientists suspect that some of these moons may hide oceans beneath their icy surfaces. Why do worlds come in different types? The nebular theory is the idea that our solar system was born from the gravitational collapse of an interstellar cloud, or nebula, of gas and dust. The particular cloud that gave birth to our own solar system is usually called the solar nebula. The solar nebula presum- ably began as a large, diffuse cloud, roughly spherical in shape. We do not know precisely what caused this cloud to start collapsing under its own gravity, but observations of similar clouds that exist today show that they can indeed collapse and give birth to stars. Once the collapse began, the laws of physics ensured that the solar nebula would heat up, spin faster, and flatten into a disk as it shrank in size. The heating occurred because gas particles tend to move faster as they fall inward under gravity, much like the way a falling brick speeds up as it approaches the ground. The particles in the solar nebula collided with each other as they fell inward, transforming their energy of motion into heat. The cloud became hottest near the center, where the Sun formed. This heating was a consequence of the law of conservation of energy; this law states that energy can be neither created nor destroyed, but only transformed from one form to another. The cloud began with a great deal of gravitational potential energy— energy that it had because its particles were far from the cloud center—and it is this energy that ultimately was transformed into heat. The cloud began with a great deal of gravitational potential energy— energy that it had because its particles were far from the cloud center—and it is this energy that ultimately was transformed into heat. In the center of the disk, gravity drew together enough material to form the Sun. In the surrounding disk, material had to begin clumping in some other way and to grow in size until gravity could start pulling it together into planets around a seed. These seeds form like snowflakes in the clouds from cold temperature. Some atoms or molecules in a gas may bond and solidify. Hydrogen and helium gas made up 98% of the solar nebula’s mass and did not condense, so the vast majority of the nebula remained gaseous. However, other materials could condense wherever the temperature allowed. In the inner solar system, where temperatures were high, only rock and metal could condense; hydrogen com- pounds remained gaseous. Farther out, where temperatures were much lower, hydrogen compounds could condense to make solid bits of ice. The first particles to condense were microscopic in size and orbited the Sun with the same orderly, circular paths as the gas from which they condensed. Individual particles therefore moved at nearly the same speed as neighboring particles, so “collisions” were more like gentle touches. Under these circumstances, particles could stick together through electrostatic forces—the same static electricity that makes hair stick to a comb. Small particles thereby began to combine into larger ones and eventually formed planets. The general process by which particles stick together and grow larger is called accretion. We refer to particles that grew to the size of mountains or larger as planetesimals, which means “pieces of planets.” We can use the processes of condensation and accretion to explain why the solar system ended up with two types of planets. In the inner solar system, where only metal and rock could condense into solid particles, the planetesimals ended up being made of metal and rock. Further growth became more difficult once the planetesimals reached these relatively large sizes. Gravitational encounters between planetesimals tended to alter their orbits, particularly those of the smaller planetesimals. With different orbits crossing each other, collisions be- tween planetesimals occurred at higher speeds and hence became more destructive. Such collisions produced fragmentation more often than accretion. Only the largest planetesimals avoided being shattered and grew into full-fledged, terrestrial planets. The planet formation process probably began similarly in the outer solar system, except the lower temperatures meant that ices condensed along with metal and rock. Because ices were more abundant than rock and metal, icy planetesimals grew to larger sizes in the outer solar system than the rocky planetesimals of the inner solar system. With large masses, their gravity became strong enough not only to capture but also to hold onto some of the hydrogen and helium gas that made up the vast majority of the surrounding solar nebula. As the growing planets accumulated gas, their gravity grew stronger still, allowing them to capture even more gas. Ultimately, the jovian planets grew so much that they bore little resemblance to the icy seeds from which they started. The same processes of heating, spinning, and flattening that made the disk of the solar nebula should have also affected the gas drawn by gravity to the young jovian planets. Each jovian planet came to be surrounded by its own disk of gas, spinning in the same direction as the planet rotated. Moons could accrete from icy planetesimals within these disks, and that probably explains the formation of most of the large moons of the jovian planets. The smaller moons likely were captured asteroids or comets. Nearly all of the captures would have happened early in the solar system’s history, when the jovian planets were still surrounded by disks of gas that could exert friction to slow down passing asteroids or comets. The general lack of moons among the terrestrial planets also makes sense: Captures were far less likely since the terrestrial planets were not surrounded by large disks of gas, and there was no place for large moons to accrete. You can probably see how the nebular theory accounts for the existence of so many asteroids and comets: They are simply “leftover” planetesimals from the era of planet formation. Asteroids are the leftover rocky planetesimals of the inner solar system, while comets are the leftover icy planetesimals of the outer solar system. Asteroids tend to reside in the asteroid belt because the influence of Jupiter’s gravity “herds” them in a way that makes them less likely to suffer collisions than asteroids in other regions of the solar system. Therefore, while most asteroids in other regions of the inner solar system long ago crashed into one of the planets, asteroids of the asteroid belt had a decent chance of surviving to the present day. The Kuiper belt comets probably reside in the same general region in which they formed. This region, which lies beyond the orbit of Neptune, was relatively low in density. So while none of the planetesimals grew large enough to become a fifth jovian planet, some grew to the size of Pluto and Eris. The Oort cloud comets are now thought to have originated in regions where they crossed the orbits of the jovian planets. When one of these comets passed near a jovian planet, it was likely to be flung out to a great distance by the planet’s gravity, in much the same way that scientists have taken advantage of Jupiter’s gravity to accelerate spacecraft to planets beyond. While it may sound strange for gravity to fling an object away, it’s a direct consequence of the law of conservation of energy: When two objects interact through their gravity, their combined energy must remain unchanged, which means that one will lose energy and the other will gain it. The majority of helium and hydrogen gas in the solar nebula never became part of any planets, and got cleared way from the energetic light from the sun and the solar wind. It is a stream of charged particles continually blown outward in all directions from the Sun. Once the gas cleared, the compositional fate of the planets was sealed. If the gas had remained longer, it might have continued to cool until hydrogen compounds condensed into ices even in the inner solar system. Should we expect habitable worlds to be common? Both theory and observation support the idea that most stars are born surrounded by spinning disks of gas and dust. Thus, based only on what we see in our solar system, we would expect to find many other planetary systems with terrestrial and jovian planets laid out in the same general way as they are in our solar system. We might therefore expect habitable terrestrial planets and habitable jovian moons to be common throughout the galaxy. Prior to the discoveries of extrasolar planets, astronomers generally assumed that this would indeed be the case, with most other planetary systems laid out much like ours. The reality appears to be more complex. Many of the other planetary systems so far discovered have planets with unexpected orbits, such as jovian planets orbiting close to their stars. These systems also happen to be easier to discover than planetary systems laid out like our own, so we don’t yet know whether planetary systems like ours are the rule or the exception. The bottom line is that unless there is something dramatically wrong with our ideas about how planets form, it seems almost inevitable that our galaxy contains many worlds that have liquid water and hence would seem to be suitable homes for life. 3.5 Changing Ideas about the Formation of the Solar System How did the nebular model win out over competing models? Around 1755, German phi- losopher Immanuel Kant proposed that our solar system formed from the gravitational collapse of an interstellar cloud of gas. About 40 years later, French mathematician Pierre-Simon Laplace put forth the same idea independently (he apparently was unaware of Kant’s proposal). Because an interstellar cloud is usually called a nebula, the idea of Kant and Laplace became known as the nebular hypothesis. Overall, we can group the many known properties of our own solar system into a list of four major features that a theory of its formation must explain: 1. Orderly motions of large bodies. The theory should explain the orga- nized patterns that we see in the orbits and rotations of the larger objects of our solar system. Recall, for example, that all the plan- ets orbit the Sun with nearly circular orbits, all going in the same direction and in nearly the same plane. The orbital direction— counterclockwise as viewed from far above Earth’s North Pole—is the same as the direction of the Sun’s rotation, the direction of most planet rotations, and the direction in which most large moons orbit their planets. 2. Two types of planets. We must also explain why the planets divide clearly into two groups, with the small, rocky terrestrial planets close together and close to the Sun while the large, gas-rich jov- ian planets are farther apart and farther out. 3. Small bodies. The planets are far outnumbered by small bodies— asteroids, comets (in both the Kuiper belt and the Oort cloud), and Kuiper belt objects—so we must also explain how these ob jects formed and came to be in their current orbits. 4. Exceptions to the rules. The generally orderly solar system also has some notable “exceptions to the rules.” For example, Earth is unique among the inner planets in having a large moon, and Uranus has an odd, sideways tilt.Asuccessful theory of our solar system must make allowances for such exceptions even as it explains the general rules. By the early twentieth century, however, scientists had found a few aspects of our solar system that the nebular hypothesis did not seem to explain well, at least in its original form. In particular, Laplace had proposed a physical mechanism by which he claimed the planets were made. His mechanism basically envisioned the planets forming in successive rings of gas that formed as the cloud contracted and spun faster, but the details are not important here. In- stead, the important point is that his mechanism was testable, and as scientists began to put it to the test, they found that it did not work. That is, Laplace’s mechanism could not actually build planets as he had thought Before too long, a new version of Buffon’s old idea began to gain favor. In this new version, instead of a direct collision with the Sun, scientists imagined a near-collision between the Sun and another star.According to this close encounter hypothesis, the planets formed from blobs of gas that had been gravitationally pulled out of the Sun during the near-collision. By the mid-twentieth century, these calculations showed that the close en- counter hypothesis could not account for either the observed orbital motions of the planets or the neat division of the planets into two categries. Laplace’s mechanism was discarded and replaced by the idea of condensation and accretion, and scientists soon realized that this important modification could indeed allow the nebular model to explain the major features of our solar system. Perhaps even more important, new discoveries about our solar system—such as learn- ing of the existence of the Kuiper belt and Oort cloud and learning more about the differing compositions of planets and moons—fit quite well into the nebular model. By the latter decades of the twentieth century, so much evidence had accumulated in favor of the nebular hypothesis that it achieved the status of a scientific theory—the nebular theory of our solar system’s birth. With the nebular theory, its clear that other planetary systems were to be expected, making the possibility of life on other worlds seem far more reasonable. Why isn't the nebular model set in stone? The new discoveries that are forcing reconsideration of the nebular theory fall into two major categories. First, there are the extrasolar planets with their surprising orbits. Second, there are new observations of young star systems, made possible by increasingly powerful telescopes that allow us to see the phenomena that accompany stellar birth. The basic model of the nebular theory, summarized in Figure 3.22, makes the process of planetary formation look rather smooth and calm, except perhaps in the late stages of accretion when shattering collisions become possible. However, observations of young stars show that their births are actually quite violent. Scientists developed the nebular model so that it neatly explains why jovian planets in our solar system exist only far from the Sun while the terrestrial planets exist only close in. The discovery of extrasolar planets that are massive like jovian planets but located in their inner solar systems was therefore quite unexpected, and it immediately caused scientists to begin questioning the nebular model. CHAPTER 4:  4.1 GEOLOGY AND LIFE • How is geology crucial to our existence? Geology appears to be crucial to our existence in at least three ways: Volcanism released most of the gas that made the at­ mosphere and the water vapor that condensed to form the oceans; plate  tectonics is crucial to the climate regulation that has kept Earth habitable over the long term; and Earth’s magnetic field has probably helped preserve the atmosphere from being stripped by the solar  wind. Geology: the study of any world with a solid surface. Geology is important to life on Earth in many ways, but three aspects of Earth’s geology stand out as being especially important: Volcanism. volcanoes are important to our existence on a much deeper level: Volcanic activity releases gases trapped in Earth’s interior, and these gases were the original source of Earth’s atmosphere and oceans. In addition, volcanism releases heat and creates chemical environments that, we suspect, helped lead to the origin of life on our planet. Platetectonics. Earth's surface has been shaped by the movement and recycling of rock between the surface and the interior. This process, calledplate tectonics, is best known for gradually rearranging the continents, but its most profound relevance to life involves Earth’s climate:According to modern understanding, plate tectonics is largely responsible for the long-term climate stability that has allowed life to evolve and thrive for some 4 billion years. Earth’s magnetic field. Our planet has a global magnetic field generated deep in its interior. You may know that the magnetic field has at least a few biological effects—for example, some birds use the magnetic field to help guide their migrations—b
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