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Inner Solar System - Terrestrial Planets 1 •Exterior (gas & droplets) •Surface (solids & liquids) •Interior (solid) 2) Exterior: Atmosphere Huge Effect 3) Surface Little Effect 1) Interior Terrestrial Planets - Inside and Out These typically do not change much after formation: Conserved Quantity • Mass & Size Mass • Distance from Sun Orbital energy & angular momentum • Composition Atomic elements (except decay) • Rotation Angular momentum Only affected by exchanges with space: • Impacts (asteriods or comets) • Atmospheric loss Interior: Formation Properties 2 On the previous slide, the properties of a planet that are listed on the left are set at the time the planet is formed and do not change much afterward. The reason the things don’t change much is that they are related to fundamental laws of nature that say that certain things, like energy, are conserved. Thus, for example, the rotation rate of a planet (listed in the left-hand column on the slide) does not change much after the planet is formed, because the angular momentum (listed at the right on the slide) is conserved. At the bottom on the previous slide, it is noted that impacts by asteroids or comets can make changes in a planet after it is formed. Also, a planet can gradually lose some of its atmosphere to space. • Mantle: Molten: melted rock • Lithosphere: Stiff: solid “frozen” rock Thickness depends on t Interior: Internal Structure 3 We have been able to figure out what the interior of the earth is like by studying the behavior of seismic waves that are generated by earthquakes or nuclear weapons tests. The waves refract as they propagate through the earth as light does in going from air into water. From the arrival times at different locations on the surface, we can get an idea of the earth’s interior composition. This same technique is used to explore for oil. • Heating: Impacts - Early on ( more than 3.8 billion years ago) Differentiation - Mostly shortly after formation Radioactivity- Ongoing, decreasing, relatively small by now Negligible heating of interior by Sun. Differentiation is the process by which heavier elements, like iron, sink toward the center of the planet’s interior, while lighter elements rise. • Cooling: Conduction (molecular vibrations) Convection (movement of rock in the mantle, see next slide) Eruption (volcanoes) All are ongoing at least until mantle freezes. Interior: Heating and Cooling 4 Convection cools the mantle material in much the same way that stirring your coffee will make it tend to cool off faster. When you stir your coffee, you cause hotter coffee from the bottom of your cup to come up to the surface, where it can give up some of its heat easily to the air. Without the stirring, the coffee at the surface would cool off, but the coffee below it would stay warm, since it could not contact the cool air directly. Convection in the earth’s mantle is the motion of hotter mantle material up from near the earth’s core toward the surface, where, like the coffee in contact with the air, it can give up its heat more rapidly to the rocks of the earth’s crust. After losing some of its heat, the cooled mantle material descends again toward the earth’s core, where it picks up heat from the core that it can carry up to the crust on its next convection cycle. 5 Without convection, the earth’s interior could cool only through conduction. Conduction is the process whereby heat flows from a hotter material to a cooler one, with which it is in contact, without either material moving. Heat is lost through the walls of your house in winter by conduction. If coffee in your cup does not move, it loses heat through the walls of your cup by conduction (which is why the cup feels hot). If the mantle material inside a planet like the earth does not move, then it loses heat through the surface of the planet by conduction. Heat loss by conduction is generally much slower than heat loss by convection. 6 2 1 2D example: 3D example: 1 4 Surface 8 1 Interior 4 4 S/I 2 2 6 Surface 24 1 Interior 8 6 S/I 3 • Rate of cooling increases with surface area • Total heat increases with volume • Close packed elements hide each other’s surfaces, but have the same volume. Interior: Surface Area vs. Volume In the 2-D example on the previous slide, a single square has a surface perimeter of length 4 meters which encloses an interior area of 1 square meter. For this square, we have 4 meters of surface to each square meter of interior area. If we pack 4 such squares together, we get a bigger square. This larger square has a surface perimeter of length 8 meters which encloses an interior area of 4 square meters. For this larger square, we have 2 meters of surface to each square meter of interior area. Our square got bigger, but the ratio of its surface perimeter to its enclosed area got smaller. This happened because many of the surfaces of the component squares were brought into contact, so that they are no longer surfaces. 7 In the 3-D example on the previous slide, a single cube has a surface area of 6 square meters which encloses an interior volume of 1 cubic meter. For this cube, we have 6 square meters of surface to each cubic meter of interior area. If we pack 8 such cubes together, we get a bigger cube. This larger cube has a surface area of 24 square meters which encloses an interior area of 8 cubic meters. For this larger cube, we have 3 square meters of surface to each cubic meter of interior area. Our cube got bigger, but the ratio of its surface area to its enclosed volume got smaller. This happened because many of the surfaces of the component cubes were brought into contact, so that they are no longer surfaces. • Works for any given shape: Surface area increases with square of size Interior volume increases with cube of size Sphere: R ~ R*R ~ R*R*R Area : 4 R 2 4 3 R 3 Volume R Surface Area 3 Volume : A sphere of twice the radius has 8 times the volume, but it has only 4 times the surface area. Since this larger sphere must cool its volume through its surface, it will cool only half as effectively. Interiors: Square-Cube Law 8 Interior: Smaller Planets Cool Faster • Rate of cooling increases with surface area • Total heat increases with volume • Time to cool: ~ R*R ~ R*R*R Total Heat / Rate of Cooling ~ R • Relative Thickness of lithosphere increases with decreasing size. • Time to cool: Total Heat / Rate of Cooling ~ R • Relative Thickness of lithosphere increases with decreasing size. 9 Of these, only Earth has a liquid outer core. We know this because of the magnetic field that it generates. The magnetic field causes your compass to point toward the north magnetic pole. On Venus or Mars, you would not find your compass to be nearly so useful, because the magnetic field is very much weaker. The recent NASA Messenger satellite, now orbiting Mercury, has led us to update the intelligence of the previous slide. Mercury does have a molten outer core, and it does have a magnetic field 10 The magnetic field of the earth resembles that of a bar magnet. This is the latest information on the interior structure of Mercury. 11 Interior: Effects on Larger Planets (Earth & Venus) Effects on Surface: • Thin lithosphere • Convection in Molten mantle • Motion of mantle against lithosphere Faults Volcanoes Cracks & valleys Mountain ranges Effect on Atmosphere: • Outgassing Builds and replenishes atmosphere Adds greenhouse gases 12 13 The scale of the convective upwellings, which we call convection cells, on the earth can be seen in this map. Only a handful of very large cells cover the globe, as we see in the movie. Venus has convective cooling, but no Earthlike plate tectonics. • No surface water. • Very high surface temperatures. • Whole surface layer is relatively buoyant, not just “continents” in limited regions of the surface. Lots of recent volcanism. • Surface changes relatively rapidly. 14 Interior: Effects on Smaller Planets (Moon & Mercury) Effects on Surface: • Thick lithosphere • No Convection near surface • Dead geology Surface dominated by cratering Effect on Atmosphere: • Outgassing stops Bulk of atmosphere escapes into space No greenhouse gases Recent new knowledge about Mercury from the NASA Messenger Spacecraft Why so large a metal core? • Composition of the material from which the planet formed at this small distance from the sun. • Not result of very large impact. • Built of much the same material as the other terrestrial planets, but with the qualification that there was almost no water in the environment. Recent geologic activity? • Well, it sure looks dead. 15 Recent new knowledge about Mercury from the NASA Messenger Spacecraft What is the surface made of? • Analyze X-rays and gamma-rays emitted from the surface. • Caused by either radioactive decay or by interaction with energetic cosmic rays. • Regions of the surface that were sampled in this way are shown at the right. Fig. 1. Regions of Mercury (footprints) sampled by XRS during analyzed flares, numbered according to Table 1. Outline colors reflect derived Mg/Si ratios: white, Mg/Si ≈ 0.6; yellow, Mg/Si ≈ 0.5; blue, Mg/Si ≈ 0.4. Arrow indicates spatially resolved measurement of a portion of northern plains material (16). Mercury observed by the NASA Messenger Spacecraft Schematic Illustration of the Operation of MESSENGER's GammaRay Spectrometer (GRS) Galactic cosmic rays interact with the surface of Mercury to a depth of tens of centimeters, producing high-energy (“fast”) neutrons. These neutrons further interact with surface material, resulting in the emission of gamma rays with energies characteristic of the emitting elements and low-energy (“slow”) neutrons. Naturally occurring radioactive elements such as potassium (K), thorium (Th), and uranium (U) also emit gamma rays. Detection of the gamma rays and neutrons by GRS allows determination of the chemical composition of the surface 16 Mercury observed by the NASA Messenger Spacecraft Measurements of Mercury's surface by MESSENGER's Gamma-Ray Spectrometer (GRS) reveal a higher abundance of the radioactive element potassium, a moderately volatile element that vaporizes at a relatively low temperature, than previously predicted. Together with MESSENGER's X-Ray Spectrometer (XRS), it also shows that Mercury has an average surface composition different from those of the Moon and other terrestrial planets. "Measurements of the ratio of potassium to thorium, another radioactive element, along with the abundance of sulfur detected by XRS, indicate that Mercury has a volatile inventory similar to Venus, Earth, and Mars, and much larger than that of the Moon," says APL Staff Scientist Patrick Peplowski, lead author of one of the Science papers. These new data rule out most existing models for Mercury's formation that had been developed to explain the unusually high density of the innermost planet, which has a much higher mass fraction of iron metal than Venus, Earth, or Mars, Peplowski pointed out. Overall, Mercury's surface composition is similar to that expected if the planet's bulk composition is broadly similar to that of highly reduced or metal-rich chondritic meteorites (material that is left over from the formation of the solar system). Recent new knowledge about Mercury from the NASA Messenger Spacecraft What is the surface made of? • Analyze X-rays and gamma-rays emitted from the surface. • Not as exotic as everyone had hoped. 17 Recent new knowledge about Mercury from the NASA Messenger Spacecraft Observations of volatile elements in Mercury’s crust rule out multiple scenarios for its formation • No super-large collisions. • Not made exclusively from material bathed for long times in very high heat of the nearsun environment. Recent new knowledge about Mercury Messenger Spacecraft These diagrams illustrate the present scientific consensus on how the Moon was formed. The idea is that the Moon does not contain enough very dense material in its core to have formed the way Mars, Venus or Earth did. The recent Messenger orbiter observations of Mercury clearly show that the material making up Mercury’s surface layers has a material composition that is inconsistent with its having formed in this fashion. 18 Recent new knowledge about Mercury Messenger Spacecraft These diagrams illustrate the present scientific consensus on how the Moon was formed. The idea is that the Moon does not contain enough very dense material in its core to have formed the way Mars, Venus or Earth did. The recent Messenger orbiter observations of Mercury clearly show that the material making up Mercury’s surface layers has a material composition that is inconsistent with its having formed in this fashion. Recent new knowledge about Mercury from the NASA Messenger Spacecraft Observations of volatile elements and also of Mercury’s magnetic field force us to accept continued heating in the interior over long times. • Most likely source of heating in the interior is radioactive decay. 19 Recent new knowledge about Mercury from the NASA Messenger Spacecraft Close-up views of Mercury’s surface prove that volcanic flows occurred after the end of the major bombardment (after 3.8 billion years ago). Recent new knowledge about Mercury from the NASA Messenger Spacecraft Close-up views of Mercury’s surface prove that volcanic flows occurred after the end of the major bombardment (after 3.8 billion years ago). 20 Recent new knowledge about Mercury from the NASA Messenger Spacecraft Here we have a more global view. Credit: Courtesy of Science/AAAS and Brown University A View Looking Down on the North Pole of Mercury (Center). Red circles show the locations of impact craters larger than 20 km in diameter. The area of contiguous northern high-latitude smooth plains mapped by MESSENGER from orbit (inside the black line) covers 4.7 million square kilometers, over 6% of the surface of Mercury. Note that the number of craters superposed on the plains is much less than in the surrounding areas, indicating the relative youth of the plains. Recent new knowledge about Mercury from the NASA Messenger Spacecraft 21