Name Date Partners Comparative Planetology by Mary Lou West after Paul Johnson and Ron Canterna Purpose : to become familiar with the major features of the planets of the solar system, especially the Earth, Moon and Mars. This is also an exercise in comparing different estimates of the same quantity and analyzing the assumptions supporting each one. Introduction : After the planets of the solar system formed together about 4.6 billion years ago they were modified by several different processes such as impact cratering, plate tectonics, volcanism, and erosion by ice, wind, and water. These processes did not act uniformly on all the bodies, and we would like to find out why. Equipment: Laminated planet images, ruler, Earth images, Mars images, colored pencils, maps of NJ, newspapers Procedure : 1. Categorizing Images Examine the laminated images of planetary features. Place them into multiple categories such as clouds, rivers, mountains, whole planets, etc. List your categories and their member images. Tell which solar system body you think is pictured on each image: Image Planet Image Planet 1 10 2 11 3 12 4 13 5 14 6 15 7 16 8 17 9 18
2 2. Cratering Density Since direct dating of rocks (by their radioactive decay) is possible for only a small sample of material from other worlds we will calibrate and use the indirect method of crater density to get a rough idea of the ages of areas on the moon, Mars, and the Earth. A. Moon Data Table 1: the Moon Mission Area N craters +/- N 1/2 craters/area Age, Gyr Apollo 14 100 4.0 Fra Mauro Apollo 15 100 3.3 Hadley Rille Apollo 16 Descartes 100 3.7 Apollo 17 Taurus- 100 3.9 Littrow Fresh surface 100 Q: The largest number of craters/area is in the location. Plot a graph of crater density vs. age, and put error bars on the crater density points by +/- N 1/2. Draw a smooth curve through the points' error bars. Q: How has the cratering rate (slope of the curve) changed with time? Q: Why might this happen? (Is the supply of meteors steady?) Q: Draw a straight line through the first two points of your curve. See where it crosses the time axis. When was the major cratering bombardment finished? Your curve of craters/km2 vs. age will be your model of the "aging" of planetary surfaces. You can approximate the recent part of the curve by a straight line with a much lower slope. Using a ruler, draw a straight line to approximate the curve from 2 Ga to 0 years ago. On the curve at age= 2 Ga the number of craters/ is N =. This calibrates the straight line so that the age of any location with fewer craters/unit area than N is given by the proportion Age = 2 * 10^9 years * (the location's craters/ ) N
3 B. The Earth Local: On the Earth find a region with impact craters, and calculate the crater density for it. Enter this in Data Table 2. Data Table 2: Earth Location Area, N +/- N 1/2 craters Craters/area Approximate age Q: Is this an over-estimate or an under-estimate of the true crater density? (Hint: Does the area you used go halfway to the next crater?) As an approximation we can use your moon's cratering curve to infer the ages of this region. Enter this calculate value in Data Table 2. Global: So far humans have found 178 craters on the Earth's dry land. Since the Earth's radius is roughly 6000 km and the surface area of a sphere is 4πR 2, the Earth's total area is roughly km2. About 30% is dry land, or about km2. This means that the rough average for the Earth is about = 178 craters/ dry area = craters/. Q: Is this more or less than the cratering density on various places on the moon? C. Mars On Mars find a region with impact craters and also linear dimensions listed, and calculate the crater density for it. Fill in Data Table 3. Again use your Moon's cratering curve to estimate the age of this surface. Data Table 3: Mars Location Area, N +/- N 1/2 craters Craters/area Approximate age On Mars the largest crater density found in the class is craters/. D. Comparison of Worlds One way to compare these three worlds is to look at their most heavily cratered regions. The largest number of craters/km2 is on the Moon, on Earth (the over-all average), and on Mars. Q: Now list the three worlds in order of oldest (many craters) to youngest (few craters) with respect to their surfaces: Q: Give two reasons for this.
4 3. Geological Maps Use the various books and maps available to locate on Earth and on Mars all examples of the following landforms. Use colored pencils to sketch them in on your maps. blue Ice red Volcanic mountains green Rift valleys yellow Pushed up or folded mountains (may be rims of huge craters) brown Heavily cratered terrain. 4. Scaling a lunar crater to the Earth The lunar crater Eratosthenes is 60 km wide. On your map of New Jersey find the scale and figure out how big this would be. Tear out a rough newspaper circle to match the crater Eratosthenes to this scale. Place your paper crater on the NJ map as if the crater had formed in Trenton. Would Montclair be destroyed? If the crater was in New York, would Montclair be destroyed? If the crater was in Philadelphia, would Montclair be destroyed? 5. Discussion Q: Discuss briefly why the Earth, Moon, and Mars are so different in surface age and landforms. Give at least three reasons.
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