Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Scattered disc wikipedia , lookup
Earth's rotation wikipedia , lookup
Sample-return mission wikipedia , lookup
Geomagnetic storm wikipedia , lookup
Heliosphere wikipedia , lookup
Planet Nine wikipedia , lookup
Space: 1889 wikipedia , lookup
Dwarf planet wikipedia , lookup
Planets beyond Neptune wikipedia , lookup
History of Solar System formation and evolution hypotheses wikipedia , lookup
Planets in astrology wikipedia , lookup
PSCI 1414 GENERAL ASTRONOMY LECTURE 8: THE SOLAR SYSTEM ALEXANDER C. SPAHN THE SOLAR SYSTEM Our ancestors long ago recognized the motions of the planets through the sky, but it has been only a few hundred years since we learned that Earth is also a planet that orbits the Sun. Even then, we knew little about the other planets until the advent of large telescopes. More recently, the dawn of space exploration has brought us far greater understanding of other worlds. We’ve lived in this solar system all along, but only now are we getting to know it. In this chapter, we’ll explore our solar system like newcomers to the neighborhood. We’ll begin by discussing what we hope to learn by studying the solar system, and in the process take a brief tour of major features of the Sun and planets. We’ll also explore the major patterns we observe in the solar system. Finally, we’ll discuss the use of spacecraft to explore the solar system, examining how we are coming to learn so much more about our neighbors. THE SOLAR SYSTEM Sometimes we study the planets individually—for example, when we map the geography of Mars or the atmospheric structure of Jupiter. Other times we compare the worlds to one another, seeking to understand their similarities and differences. This latter approach is called comparative planetology. Note that astronomers use the term planetology broadly to include moons, asteroids, and comets as well as planets. THE SOLAR SYSTEM A planet falls naturally into one of two categories according to the size of its orbit. The orbits of the four inner planets (Mercury, Venus, Earth, and Mars) are crowded in close to the Sun. In contrast, the orbits of the four outer planets (Jupiter, Saturn, Uranus, and Neptune) are widely spaced at great distances from the Sun. THE SOLAR SYSTEM All planetary orbits are nearly circular and lie nearly in the same plane. All planets orbit the Sun in the same direction: counterclockwise as viewed from high above Earth’s North Pole. Most planets rotate in the same direction in which they orbit, with fairly small axis tilts. The Sun also rotates in this direction. Most of the solar system’s large moons exhibit similar properties in their orbits around their planets, such as orbiting in their planet’s equatorial plane in the same direction as the planet rotates. THE SOLAR SYSTEM THE SOLAR SYSTEM CALCULATION CHECK 7-1 Which of the planets has an orbital path that is most nearly a perfect circle in shape? The shape of a planet’s orbit is given by the value of its eccentricity. The closer this value is to zero, the closer the orbit’s shape is to that of a perfect circle. According to the table, the orbit of Venus has the eccentricity closest to zero (0.007), making it the most circlelike of all planetary orbits. CONCEPT CHECK 7-3 A planet’s average density can be estimated by measuring its size and how much the planet’s gravity deflects a nearby spacecraft’s path. If the density of rocks recovered from a planet’s surface is lower than the planet’s average density, what can one infer about the density of the planet’s core? If the rocks on the surface have a density lower than the planet’s average density, then the planet’s core has a density greater than the planet’s average. This is not surprising, since gravity causes material of greater density to sink towards the center of a planet. CALCULATION CHECK 7-2 If Earth’s diameter is 12,756 km and Saturn’s diameter is 120,536 km, how many Earths could fit across the diameter of Saturn? If we divide Saturn’s 120,536-km diameter by Earth’s 12,756-km diameter—we find that 120,536 km ÷ 12,756 km = 9.449, so about 9½ Earth’s would fit across Saturn’s diameter. THE SOLAR SYSTEM The terrestrial planets (terrestrial means “Earth-like”) are the four planets of the inner solar system: Mercury, Venus, Earth, and Mars. These planets are relatively small and dense, with rocky surfaces and an abundance of metals in their cores. They have few moons, if any, and no rings. THE SOLAR SYSTEM The jovian planets (jovian means “Jupiter-like”) are the four large planets of the outer solar system: Jupiter, Saturn, Uranus, and Neptune. The jovian planets are much larger in size and lower in average density than the terrestrial planets, and they have rings and many moons. They lack solid surfaces and are made mostly of hydrogen, helium, and hydrogen compounds. Because these substances are gases under earthly conditions, the jovian planets are sometimes called “gas giants.” THE SOLAR SYSTEM All the planets except Mercury and Venus have moons (also called satellites). At least 170 satellites are known: Earth has 1 (the Moon), Mars has 2, Jupiter has at least 66, Saturn at least 62, Uranus at least 27, and Neptune at least 13. Like the terrestrial planets, all of the satellites of the planets have solid surfaces. THE SOLAR SYSTEM Seven of the Jovian satellites are roughly as big as the planet Mercury! Earth’s Moon and Jupiter’s satellites Io and Europa have relatively high average densities, indicating that these moons are made primarily of rocky materials. By contrast, the average densities of Ganymede, Callisto, Titan, and Triton are all relatively low. The interiors of these four moons also contain substantial amounts of water ice, which is less dense than rock. THE SOLAR SYSTEM the average densities of the planets and satellites give us a crude measure of their chemical compositions—that is, what substances they are made of. The most accurate way to determine chemical composition is by directly analyzing samples taken from a planet’s atmosphere and soil. Unfortunately, this isn’t possible for most worlds. Astronomers instead analyze sunlight reflected from the distant planets and their satellites. To do that, astronomers bring to bear one of their most powerful tools, spectroscopy, the systematic study of spectra and spectral lines. THE SOLAR SYSTEM spectral line: In a light spectrum, an absorption or emission feature that is at a particular wavelength. dark spectral line arises when light at a specific wavelength is at least partially absorbed so that the spectrum appears darker; it is also called an absorption line. A bright spectral line arises because at least some additional light is being emitted at a specific wavelength; it is also called an emission line. THE SOLAR SYSTEM There is a direct connection between these two types of spectra. These connections are summarized in three statements about spectra that are known as Kirchhoff’s laws. A hot opaque body, such as a perfect blackbody, or a hot, dense gas produces a continuous spectrum—a complete rainbow of colors without any spectral lines. Note: a blackbody is an idealized type of dense object that does not reflect any light at all; instead, it absorbs all radiation falling on it. THE SOLAR SYSTEM A hot, transparent gas produces an emission line spectrum—a series of bright spectral lines against a dark background. THE SOLAR SYSTEM A cool, transparent gas in front of a source of a continuous spectrum produces an absorption line spectrum—a series of dark spectral lines among the colors of the continuous spectrum. THE SOLAR SYSTEM If a planet has an atmosphere, then sunlight reflected from that planet must have passed through its atmosphere before coming back out. During this passage, some of the wavelengths of sunlight will have been absorbed. Hence, the spectrum of this reflected sunlight will have dark absorption lines. Astronomers look at the particular wavelengths absorbed and the amount of light absorbed at those wavelengths. Both of these depend on the kinds of chemicals present in the planet’s atmosphere and the abundance of those chemicals. THE SOLAR SYSTEM For example, astronomers have used spectroscopy to analyze the atmosphere of Saturn’s largest satellite, Titan. THE SOLAR SYSTEM Spectroscopy can also provide useful information about the solid surfaces of planets and satellites without atmospheres. When light shines on a solid surface, some wavelengths are absorbed while others are reflected. By comparing such a spectrum with the spectra of samples of different substances on Earth, astronomers can infer the chemical composition of the surface of a planet or satellite. THE SOLAR SYSTEM Spectroscopic observations from Earth and spacecraft show that the outer layers of the Jovian planets are composed primarily of the lightest gases, hydrogen and helium. In contrast, chemical analysis of soil samples from Venus, Earth, and Mars demonstrate that the terrestrial planets are made mostly of heavier elements, such as iron, oxygen, silicon, magnesium, nickel, and sulfur. THE SOLAR SYSTEM Temperature plays a major role in determining whether the materials of which planets are made exist as solids, liquids, or gases. Hydrogen (H2) and helium (He) are gaseous except at extremely low temperatures and extraordinarily high pressures. By contrast, rock-forming substances such as iron and silicon are solids except at temperatures well above 1000 K. As you might expect, a planet’s surface temperature is related to its distance from the Sun. The four inner planets are quite warm. For example, midday temperatures on Mercury may climb to 700 K (801°F), and during midsummer on Mars, it is sometimes as warm as 290 K (63°F). The outer planets, which receive much less solar radiation, are cooler. Typical temperatures range from about 125 K (−234°F) in Jupiter’s upper atmosphere to about 55 K (−360°F) at the tops of Neptune’s clouds. THE SOLAR SYSTEM In addition to the eight planets, many smaller objects orbit the Sun. These objects fall into three broad categories: asteroids, transNeptunian objects, and comets. Asteroids (minor planets) are rocky bodies that orbit the Sun much like planets, but they are much smaller. Most known asteroids are found within the asteroid belt between the orbits of Mars and Jupiter (2.0 – 3.5 AU). The asteroid shown in this image, 433 Eros, is only 33 km (21 mi) long and 13 km (8 mi) wide—about the same size as the island of Manhattan. Because Eros is so small, its gravity is too weak to have pulled it into a spherical shape THE SOLAR SYSTEM The outer solar system hosts what are known as trans-Neptunian objects. As the name suggests, these are small bodies whose orbits lie beyond the orbit of Neptune. Since 1992 astronomers have discovered more than 900 other trans-Neptunian objects, including Pluto and Eris. Just as most asteroids lie in the asteroid belt, most trans-Neptunian objects orbit within a band called the Kuiper belt that extends from 30 AU to 50 AU. THE SOLAR SYSTEM Objects in the Kuiper belt can collide if their orbits cross each other. When this happens, a fragment a few kilometers across can be knocked off one of the colliding objects and be diverted into an elongated orbit that brings it close to the Sun. Such small objects, each a combination of rock and ice, are called comets. When a comet comes close enough to the Sun, the Sun’s radiation vaporizes some of the comet’s ices, producing long tails of gas and dust particles. Some comets appear to originate from locations far beyond the Kuiper belt. The source of these is thought to be a swarm of comets that forms a spherical “halo” around the solar system called the Oort cloud. This hypothesized “halo” extends to 50,000 AU from the Sun. THE SOLAR SYSTEM We can gather important clues about the interiors of terrestrial planets and satellites by studying the extent to which their surfaces are covered with craters. The planets orbit the Sun in roughly circular orbits. But many asteroids and comets are in more elongated orbits. Such an elongated orbit can put these small objects on a collision course with a planet or satellite. If the object collides with the solid surface of a terrestrial planet or a satellite, the result is an impact crater – a circular depression on a planet or satellite caused by the impact of a meteoroid. THE SOLAR SYSTEM The Moon’s surface has craters of all sizes. The large crater near the middle of this image is about 80 km (50 mi) in diameter, equal to the length of San Francisco Bay. This Reservoir in Quebec is the relic of a crater formed by an impact more than 200 million years ago. The crater was eroded over the ages, leaving a ring lake 100 km (60 mi) across. Lowell Crater of Mars is 201 km (125 mi) across. There are craters on top of craters. Note the lightcolored frost formed by condensation of carbon dioxide from the Martian atmosphere. THE SOLAR SYSTEM These smaller craters are thought to have been caused by impacts of relatively small objects called meteoroids, which range in size from a few hundred meters across to the size of a pebble or smaller. Meteoroids are the result of collisions between asteroids, whose orbits sometimes cross. The chunks of rock that result from these collisions go into independent orbits around the Sun, which can lead them to collide with the Moon or another world. THE SOLAR SYSTEM Why are craters so much rarer on Earth than on the Moon if both have been subjected to the same amount of bombardment? The answer is that Earth is a geologically active planet. Through plate tectonics—the motion of rocky plates over Earth’s surface—the continents slowly change their positions over eons, new material flows onto the surface from the interior, and old surface material is pushed back into the interior. These processes, coupled with erosion from wind and water, cause craters on Earth to be erased over time. THE SOLAR SYSTEM The Moon, by contrast, is geologically inactive. There are no volcanoes and no motion of continents (and, indeed, no continents). Furthermore, the Moon has neither oceans nor an atmosphere, so there is no erosion as we know it on Earth. With none of the processes that tend to erase craters on Earth, the Moon’s surface remains pockmarked with the scars of billions of years of impacts. THE SOLAR SYSTEM In order for a planet to be geologically active, its interior must be at least partially molten. Even if the surface does not undergo plate tectonics—as in the case of Jupiter’s moon Io— a semi-molten interior is required to generate volcanoes with molten lava. Therefore, we would expect geologically inactive (and hence heavily cratered) worlds like the Moon to have less molten material in their interiors than does Earth. THE SOLAR SYSTEM The smaller the terrestrial world, the less internal heat it is likely to have retained, and, thus, the less geologic activity it will display on its surface. The less geologically active the world, the older and hence more heavily cratered its surface. This rule means that we can use the amount of cratering visible on a planet or satellite to estimate the age of its surface and how geologically active it is. THE SOLAR SYSTEM Another, more direct tool for probing the interior of any planet or satellite is an ordinary compass, which senses the magnetic field outside the planet or satellite. The needle of a compass on Earth points north because it aligns with Earth’s magnetic field. Magnetic fields arise whenever electrically charged particles are in motion. The consensus among geologists is that the Earth’s magnetic field is caused by the motion of the liquid portions of its interior. THE SOLAR SYSTEM THE SOLAR SYSTEM A planet or satellite with a global magnetic field has liquid material in its interior that conducts electricity and is in motion, generating the magnetic field. This process for producing a magnetic field is called a dynamo. A planet’s rotation helps to sustain the magnetic field. Thus, by studying the magnetic field of a planet or satellite, we can learn about that world’s interior. Many spacecraft carry devices called magnetometers to measure magnetic fields. THE SOLAR SYSTEM Mars has a very interesting magnetic field that sheds light on its early history. Mars has no planet-wide magnetic field like Earth, but portions of its rocky surface are magnetized. This information actually tells us a lot about the Martian past. During the planet’s first billion years or so, the interior was still hot and molten. Electric currents in the flowing molten material could then produce a planet-wide magnetic field. As surface material cooled and solidified, the crust became magnetized by the planet-wide field, and remained magnetized even after the internally generated planet-wide field shut down. THE SOLAR SYSTEM The Jovian planets are composed primarily of hydrogen and helium, not substances like iron that conduct electricity. How, then, can they have a dynamo that generates strong magnetic fields? Deep inside Jupiter and Saturn, the pressure is so great and hydrogen atoms are squeezed so close together that electrons can hop from one atom to another. This hopping motion creates an electric current, just as the ordered movement of electrons in the copper wires of a flashlight constitutes an electric current. In other words, the highly compressed hydrogen deep inside Jupiter behaves like an electrically conducting metal; thus, it is called liquid metallic hydrogen. THE SOLAR SYSTEM Uranus and Neptune also have magnetic fields, but they cannot be produced in the same way: Because these planets are relatively small, the internal pressure is not great enough to turn liquid hydrogen into a metal. Instead, it is thought that both Uranus and Neptune have large amounts of liquid water in their interiors and that this water has molecules of ammonia and other substances dissolved in it. Under the pressures found in this interior water, the dissolved molecules lose one or more electrons and become electrically charged. FOR NEXT TIME… • Reading: Finish chapter 7 (7.6 – 7.8) and read the first few sections of chapter 8 • Homework: Homework 6 (Chapter 7 questions 4, 7, 8, 10, 11, 33. Due Wednesday)