Objective: Identification of the main topographic features on Mars and determination their relative ages.

Transcription

1 Objective: Identification of the main topographic features on Mars and determination their relative ages. In this lab we use the terms older and newer (or younger ) to describe the relative ages of individual geographical features on the surface of Mars. A feature which formed later in the history of the planet will be referred to as younger or newer than those features which formed earlier. You will use the interactive maps of Mars compiled from recent images collected by the Mars Global Surveyor and Mars Odyssey spacecraft at Google Mars ( Turn on the computer at your station and navigate to this site. You may also find it helpful to use the images at mars.html You will use two methods (described below) to determine the sequence of formation for adjacent geographic structures on the surface of Mars. You will work through a series of exercises, answering questions about each feature to help make these determinations. Your score for this assignment will be based upon the accuracy of an ordered list of features from youngest to oldest, and the annotations you make on the sketch map, on the last page of this handout: Method 1 - Crater Density: Rocks of all sizes have been falling on Mars since the planet formed. The impacts of these rocks produced craters. If the craters are not weathered away, nor covered up by lava or dust, then the number of craters per unit area is so large that they generally overlap. Large portions of Mars are covered with such overlapping craters. Figure 1. Lightly cratered region B has been modified by some event that obscured or obliterated the the craters evident in region A. We say that surface of region B is newer than region surface of region A However, if the craters are covered by subsequent lava flows (as on some portions of the Moon and Mars) or eroded away by the action of wind and water (as on Earth) or covered with dust (as with very small craters on the Moon and Mars), then the number of visible craters per unit area will be reduced. Comparing the number of craters per unit area in different regions allows us to determine which region was more recently modified by some crater-obscuring event. Method 2 - Superposition: Superpose means to place above, or over top of. Whatever is on top of, or partially obliterates, another feature may be taken to be younger (more recent) than the 1/8

2 Figure 2. The later impact formed a crater superposed upon the crater formed by an earlier impact. underlying features. In the images above, the the crater labeled A was formed after the crater labeled B Figure 3. Use the buttons in the upper right hand corner of the map to change the view between elevation, infrared, and visible wavelength images of mars. EXERCISES The colors of the elevation map correspond to distances above a mean altitude (there is no liquid water on Mars from which to establish a sea level ). This elevation is designated 0 km and corresponds to a yellowgreen color on the map. On the last page of this hand-out you will find a sketch map representing the entire surface of Mars between 70 N latitude and 70 S latitude. You will use this map to indicate the location of some of the major features we will examine: four major volcanoes, a very large canyon two very large craters even larger roughly circular smooth regions In the following exercises, you will identify these, and some other major features, on the surface of Mars and determine their relative ages. You will then list all of the features you have identified from most recent to oldest. Your grade for this lab will be based on a correctly labeled sketch map and a correctly ordered list. 2/8

3 Procedure 1. Identify these features on the sketch map located on the last page of this write-up: a. Label each of the four volcanoes with a V, b. Label very large canyon with a B c. Label each of the two very large craters marked with VLC d. Label the VLC which has a double rim with DR (compare the VISIBLE images of the two very large craters) e. Label each of the two VERY large roughly "circular smooth" regions with CS (These regions are many times lager than the objects you labeled "VLC") Roughly half of Mars consists of plains (fairly smooth regions with a few features such as volcanoes or the large canyon superposed), and roughly half consists of regions where the craters generally overlap. From radar surveys of the surface, and photographic images obtained at oblique angles, we know that the heavily cratered region is at higher average elevation than the smoother plains. Hence, we call the cratered region the highlands. The thick dotted line meandering across the entire width of the sketch map marks the boundary between these two regions. 2. Based upon crater density, it is apparent that the region south of the thick dotted line is the surface (circle your answer). a. NEWER b. OLDER We can now begin make a more precise estimate of the relative ages of the two regions. 3. Find at least two "very large" craters that do not lie in the highlands. Indicate their locations on the sketch map by drawing small circles. 4. The image at right is of one of the VLC on your map. Find this crater and zoom in to a region about as wide as the image. Count the number of craters that are at least as large as the circled craters and which lie entirely in the smooth plains. Record that value in the space below. Nplain = Figure 4 -- A region just past the edge of the Highlands. The circled craters will be counted as "large craters" 3/8

4 5. Without changing the size of the window or the zoom level, move your field of view to a region in the southern highlands and record the number of similarly sized large craters. Nhighlands = 6. Compute the ratio of these two numbers and record it below Nhighlands /Nplain = 7. Suppose now that we know the absolute age for the plains of Mars to be 3 billion years. Suppose also that the rate of cratering (number of craters formed per billion years) is, and has been, constant for the entire age of Mars. Using the ratio you calculated above, the age of the highlands would be roughly billion years. Noting that the age of the solar system, and all of the planets in it, is approximately 4.5 billion years, explain what is wrong with this result: What do you conclude about the rate of cratering? (circle one) a. The rate of cratering has been constant for the last 4.5 billion years. b. The rate of cratering has increased over the last 4.5 billion years. c. The rate of cratering has decreased over the last 4.5 billion years. 8. From the relative ages of features on the Moon and other airless bodies, it can be deduced that most of the really large asteroids in the solar system collided with each other (making smaller rocks) and with planets (making very large craters) during the first billion years after the formation of the solar system. Therefore the "very large" craters should appear mostly on the oldest surfaces. What does the existence of asteroids today and of "very large" craters on the plains on Mars imply about the possibility of the formation of new large craters on Mars? a. There is no possibility of new large craters on Mars. b. New, large craters will continue form on Mars, only infrequently. c. New, large craters will form on Mars frequently Explain your reasoning: 9. The two very large, roughly circular and smooth regions indicated on the sketch map (The Argyre and Hellas planitias)the were likely caused by the impacts of very large bodies. However, the smooth plains at the bottoms of these basins are lava flows, which appear to have formed quite late, after most of the bombardment and 4/8

5 cratering on Mars was over. This leaves us with a dilemma. Did these giant impacts happen early, when most impacts occurred; excavating large basins with raised rims (see figure), which filled with lava only very much later? Or, did these impacts occur later in the history of the planet, after most other impacts, with lava filling the basin at the time of the impact? Inspect the elevation map near the two large basins and identify a rim for each, or at least a fairly circular ring-like structure around each. Look for evidence of impact craters interrupting the rims of these basins. Using what you have learned about superposition, choose between: a. impact early in the history of Mars - lava flow much later b. impact later in the history of Mars - lava flow soon after impact Explain your reasoning: Does your choice agree with the earlier conclusion that most "very large" craters were formed rather early in the evolution of Mars? 10.Look at the area south of the great canyon, between longitudes 45 W and 100 W. This area is not uniformly cratered (or wrinkled). A boundary seems to occur at about longitude. The older surface is to the EAST WEST of this boundary (circle your answer). 11.Zoom in on the large canyon; compare the crater density within the canyon to the terrain just to the north and south of the rims. From this examination, you may concluded that: a. the canyon formed before the surrounding plains, and the lava floods that made the plains simply did not reach the canyon. b. the canyon might have formed as a boundary between two plains of very different ages c. the canyon might have formed after the plains Explain how you arrived at your conclusion (in terms of crater density and the superposition of features) 5/8

6 12.Notice on that each of the four large volcanoes has a crater at its center (that is, on top). Considering the crater density in the areas surrounding the volcanoes, are the craters at the tops of the volcanoes likely to be due to impacts? Explain your reasoning 13.The southernmost volcano is Arsia Mons, it is surrounded by an expanse of smooth terrain called the Tharsis Plateau. Look at the long structures on the flanks of the volcano, extending radially away from the crater. These are almost certainly due to eruptions of the volcano. Do these structures extend out onto the surrounding plain without interruption? In particular, examine the gully to the south of the crater. Using what you have learned about the superposition of features, the appearance of these radial features suggests that: a. Arsia Mons is older than the surrounding terrain. b. Arsia Mons is younger than the surrounding terrain. Explain your reasoning 14.At about Latitude 23 South, Longitude 217 West -- to the SW of Arsia Mons -- at the edge of the smooth terrain surrounding the Volcano, there are three craters which just overlap (see the image on the next page). Zoom in on this feature. Look, in the infrared image, at the northern rim of the crater labeled C in the figure. Which of these craters is the YOUNGEST feature. Are any of these three craters newer (YOUNGER) than the smooth terrain to the North and East? Explain your reasoning 15.If the ridges running SW to NE on the right side of the volcano were made by the event that formed canyon, this would indicate that: a. Arsia Mons is older than the canyon. b. Arsia Mons is younger than the canyon. 6/8

7 Clark College Astronomy 101 Your grade for this lab will be based on a correctly labeled sketch map and a correctly ordered list. Name: Partners: Rank the ages of the regions you examined in this lab from 1 (youngest) to 5 (oldest). highlands volcano (Arsia Mons) canyon (Valles Marineris) south of canyon, longitudes south of canyon, longitudes Extra Credit (5 points). At about 14 N 178 E, there is a large (about 375 km) oblong depression called the Orcus Patera. What kind of feature is this, and where would you place it on the list above? Explain your answers: FOR QUESTION 14 7/8

8 8/8