Astro Today 9th Ed. Ch. 8 Moon and Mercury Discussion 3,5,6,7,15 3. Employ the concept of escape speed to explain why the Moon and Mercury have no significant atmospheres. Both the Moon and Mercury have low masses and, therefore, weak surface gravities and low escape velocities. To make matters worse, both objects experience high daytime temperatures because they are in the inner solar system and rotating slowly. At these temperatures, most if not all atmospheric molecules will have velocities in excess of these low escape velocities. Neither object, then, has been able to hold an atmosphere. 5. What does it mean to say that Mercury has a 3:2 spin orbit resonance? Why didn't Mercury settle into a synchronous orbit around the Sun, as the Moon did around Earth? Mercury's orbit is not synchronous with its rotation in a 1 to 1 ratio, like the Moon, but in a 3 to 2 ratio. It rotates 3 times (relative to the stars) for every two orbits around the Sun. This was the result of Mercury's highly eccentric orbit and allows the same side of Mercury to face the Sun every other orbit at perihelion. It is at perihelion that the tidal forces are the strongest on Mercury and this forced the denser side of Mercury to face the Sun every other orbit. If Mercury maintained it's perihelion speed for its entire orbit, it would have a 1:1 spin-orbit resonance. So it's as if Mercury DOES have a 1:1 resonance assuming a circular orbit with a radius equal to the perihelion distance from the Sun. 6. What is a scarp? How are scarps thought to have formed? Why do scientists think that the scarps on Mercury formed after most meteoritic bombardment ended? A scarp is basically a cliff that extends a greater distance than the vertical drop of the cliff. On Mercury, the scarps drop off a few km and extend as far as 400 km. They probably formed during a cooling stage of the planet when the interior temperature dropped and it actually shrunk in size causing the crust to wrinkle. This stage appears to have happened after most of the heavy bombardment because the scarps cross the craters more than the craters overlap and erase the scarps. 7. What is the primary source of erosion on the Moon? Why is the average rate of lunar erosion so much less than on Earth? There is no water or wind erosion and no change due to plate tectonics because none of these exist on the Moon. The primary source of erosion on the Moon is cratering, especially by micrometeors. While meteors do strike the airless Moon very frequently, Earth has many more sources of erosion. 15. How is the varying thickness of the lunar crust related to the presence or absence of maria on the Moon? When the lunar material differentiated, Earth's tidal forces played an important role in the Moon's internal development. Denser mantle material settled closer to the Earth; lighter crustal material settled farther away. Therefore, the mantle is somewhat “off-center,” and the crust on the near side of the Moon is thinner than the crust on the far side. Because of this difference in crust thickness, lava was able to erupt more easily onto the near side when large meteoroids struck, and maria formed more easily. On the far side, the crust was harder to penetrate and maria were generally unable to form. ---------------------------------------------------------- AT 9th edition Ch. 9 Discussion 3,8,11,14 3. What is our current best explanation of Venus's slow, retrograde spin? Our best explanation is that Venus was struck by a large object at just the right angle and speed to slow and even reverse its spin. 8. What component of Venus' atmosphere causes the planet to be so hot? Explain why there is so much of this gas in the atmosphere of Venus, compared with its presence in Earth's atmosphere. What happend to all the water that Venus must have had when the planet formed? The main component contributing to the greenhouse effect on Venus is carbon dioxide. However, the cloud droplets of sulfuric acid and the sulfur dioxide also play a role in trapping heat. There is more CO2 in Venus' atmosphere than Earth's because much of Earth's CO2 is trapped in oceans. Venus has lost its water due to the runaway greenhouse effect. As the CO2 level increased, the atmospheric temperature increased forcing surface water into the vapor form. That water vapor diffused up to the upper atmosphere where it was dissociated into H and O2 which were light enough to escape the planet altogether. 11. How are the impact craters of Venus different from those found on other bodies in the solar system? There are very few small impact craters on Venus, because meteoroids smaller than about 1 km are destroyed as they travel through the thick atmosphere. The smaller impact craters that do exist appear to have been created by larger bodies that shattered prior to impact. There are also few very large craters, but such large meteoroids were more common in the very early days of the solar system, and ancient large craters may have been filled in by more recent lava flows on the surface. 14. Given that Venus, like Earth, has a partially molten iron-rich core, why doesn't Venus also have a magnetic field? The dynamo effect for creating a magnetic field requires a liquid, electrically conductive interior and rapid rotation. Venus lacks the rapid rotation since it takes 243 Earth days to rotate once relative to the stars. ---------------------------------------------------------- AT 9th Ed. Ch. 10 Mars Disc. 5,6,8,9 5. Describe the major large-scale surface features of Mars. Mars has the Tharsis bulge, a high altitude region mostly in the Northern hemisphere which contains the largest shield volcanoes in the solar system. On the Eastern side of the Tharsis bulge is the huge, canyon-like fault Valles Marineris. The Northern part of the planet is systematically lower in altitude and younger than the southern half, as indicated by a lower crater density and more signs of flooding by water. The southern hemisphere contains its own, low-altitude region: the circular Hellas Basin. 6. Why were Martian volcanoes able to grow so large? Mar's surface has a lower surface gravity than Earth and Venus which allows mountains to reach a greater height before the crust can no longer support them. Another factor is the lack of plate tectonics on Mars, which allows hot spots under the crust to feed the same volcano indefinitely. 8. What is the evidence that water once flowed on Mars? Is there liquid water on Mars today? Many examples of runoff channels and outflow channels exist on Mars. Runoff channels are like old river channels, while outflow channels are caused by more catastrophic, large-scale flooding (like the breaking of a natural dam). There are also what appear to be lake shores that receded over time. Martian rovers have found "blueberries" and white silica deposits which were probably formed in liquid water environments. There are several observations pointing towards possible liquid water on Mars today, but they all have alternative explanations. For example, the "brine flows" or RSL (Recurrent Slope Lineae) look like water stains that change with the season, but they are probably just granular flows (because they only occur on slopes greater than ~25 degrees). There are also gullies and alluvial fans that are probably younger than 2 million years, which is young compared to the age of any large scale feature on Mars. 9. Is there water on Mars today, in any form? Yes. It exists in small quantities in the atmosphere and can sometimes even form fog. It exists as ice on the polar ice caps. Large quantities are also hidden under the surface as a "permafrost" in latitudes over 50 degrees away from the equator. Recently, brine lakes have been detected by ground-penetrating radar 1.5 km under the surface near the Martian South Pole. These are thought to be liquid water with dissolved salts which prevent the water from freezing.