Google Mars: Wind Processes

Transcription

1 Google Mars: Wind Processes This assignment will require the use of the latest version of Google Earth (version 5.0 or later), which you can download for free from You must have the latest version because you will need both the Earth and Mars capabilities of the program. Introduction Just as water can transport sediment as a bed load and a suspended load, so can wind. The main difference due to the vastly different densities and viscosities of air compared to water is the particle size. Whereas sand-sized and larger clasts (up to boulders) can be moved by enough stream power, wind rarely can move clasts larger than small pebbles and coarse sand. This material is then moved via surface creep (pebbles), saltation (sand), and suspension (silt and clay; dust ). These aeolian (wind-related) processes are illustrated in Figure 1. Figure 1. Processes of aeolian sediment transport. When the velocity of wind decreases, air loses its sediment-carrying ability in the same way that a river loses its capacity/competence as the water velocity decreases. When this happens, the sediment formerly being transported (i.e. eroded) must be redeposited. An interesting feedback process then occurs. Recall that surface friction is one way to slow down a moving fluid, be it water or air. Surface friction can arise from rough irregularities on a surface. For example, vegetation or topography can create such irregularities, places where air is forced to slow down and deposit its load. As the wind deposits more sediment, the pile of sediment that results creates more surface roughness. This forces the wind to slow down further, depositing even more sand, and creating a 1

2 GEOL 4315/5315/6315: Planetary Science larger and larger pile. As the obstruction slows the wind further, eventually the pile of sand grows to become a sand dune. Geologists have categorized sand dunes based upon their shape and how they are formed. The size and shape of a dune depends on the ability of winds to pick up and carry sand and the direction the winds blow. Some common dune morphologies include (Figure 2): Barchan: These crescent-shaped dunes are formed by winds blowing in a constant direction over relatively sand-poor areas. The tips of these dunes point downwind. Over time, transport and redeposition of the sand by the wind cause these dune to migrate downwind. Parabolic: These are also crescent-shaped dunes, but they have tips that point upwind. The sand in these dunes is anchored by vegetation and the crescent shape is formed by sand being removed from the inside out, a process known as deflation. They form on beaches where sand is abundant, vegetation is present, and where winds are moderate and tend to blow from a single direction. Barchanoid Ridge and Transverse: These dunes form in areas where winds blow in a constant direction with an abundant supply of sand. These make long crests and troughs at right angles to the wind and look like sea waves. Longitudinal: These dunes are form parallel to the average wind direction in places where there are two dominant winds and an ample sand supply. Other: Under special circumstances, other morphologies such as star, dome, and reversing can occur. Figure 2. Common aeolian dune morphologies. 2

3 In cross-section (Figure 3), a dune has a characteristically asymmetric shape. The upwind, or windward, side will be less steep than the downwind, or leeward, side. These two surfaces are also called the stoss slope and the slipface, respectively. The steepness of these surfaces is determined by the angle of repose. Recall, from our discussion of mass wasting, that the angle of repose for dry, non-cohesive sediment (like sand) is approximately 35. At this angle, the force of gravity pulling sand downslope is balanced by the friction between sand grains. As sand is blown in the wind direction, sand grains saltate up the windward side, eroding the sand dune and maintaining the shallow stoss slope in the process. At the crest (top edge) of the dune, the sand grains reach the steep slipface where the angle of repose is continually being exceeded and recovered by mass wasting. Over time, collapse of the dune on the leeward slope allows the dune to migrate. Previous positions of the slipface are preserved as cross-stratification within the dune. These are often seen in ancient aeolian sandstones as cross-bedding. Figure 3. Cross-section of a dune showing the asymmetry and migration resulting from aeolian transport and deposition. 3

4 White Sands National Monument, New Mexico We will examine some aeolian features that are close to home. Many of you may have visited them before (if you haven t, I highly recommend the trip)! Read the instructions carefully and answer all the questions on separate pages (not here) Start Google Earth on your computer. Be sure you are in Earth mode and not in Sky or Mars mode (use the menu that looks like the planet Saturn). Familiarize yourself with the controls (use the mouse and menus) and how to navigate (particularly the Fly To tab under Search ). See the Help menu or the Google Earth website ( for more information. 2. Navigate to the following coordinates (which we ll call Site A ): , (Type the coordinates exactly as shown into the space under the Fly To tab under Search and hit Enter. Google Earth should take you right there!) a. What kind of dunes are these? Make a sketch of these dunes. Be sure to annotate your sketch with features of interest and the wind direction you infer. b. How large are these dunes, and how far apart are they spaced? (Use the Ruler under the Tools menu.) c. Where is the source of the sediment? (You may need to zoom out and move around the image to find out.) d. Do you know what kind of sand this is i.e., what is it made of? (You will have to do a little research if you don t know the answer!) 3. Now navigate to these coordinates (which we ll call Site B ): , a. What kind of dunes are these? Make a sketch of these dunes. Be sure to annotate your sketch with features of interest and the wind direction you infer. b. How large are these dunes, and how far apart are they spaced? (Use the Ruler under the Tools menu.) c. Where are these dunes with respect to the sediment source and the dunes at Site A? (You may need to zoom out and move around the image to find out.) d. What is different between these dunes and the dunes at Site A? (You may need to zoom out and move around the image to find out.) 4. Now navigate to these coordinates (which we ll call Site C ) and zoom in: ,

5 a. The dunes here are similar to those at Site B. What features (other than the dunes!) do you see here? Make a sketch of what you see. Be sure to annotate your sketch with features of interest. b. What do you think the (non-dune) features you see here represent? (Figure 3 may help here.) c. Based on what you see here, can you deduce what the wind pattern was in the past? (Figure 3 may help here.) 5. Now navigate to these coordinates (which we ll call Site D ): , a. What kind of dunes are these? Make a sketch of these dunes. Be sure to annotate your sketch with features of interest and the wind direction you infer. b. How large are these dunes, and how far apart are they spaced? (Use the Ruler under the Tools menu.) c. Where are these dunes with respect to the sediment source and the dunes at Sites A, B, and C? (You may need to zoom out and move around the image to find out.) d. What is different between these dunes and those at Sites A, B, and C? (You may need to zoom out and move around the image to find out.) 6. Devise a hypothesis to explain the patterns you observe among wind direction, sediment supply, dune type, and the amount of vegetation/water. Nili Patera, Mars We will now explore some aeolian features that are much further away! The atmosphere of Mars is thin, about 1% that of Earth. Even so, the winds can blow up to 100 km/hr and, for example, can create dust storms that periodically engulf the entire planet. Note the items in bold which are questions for you to answer on separate pages (not here) 1. In order to use Google Earth to view Mars data you will need to put the program in Mars mode (use the menu that looks like the planet Saturn). See the Help menu or the Google Earth website ( for more information. 2. Under the Fly To tab (under Search ) type the following place name and hit Enter (type it exactly as shown): Nili Patera 3. The place you are looking at is in the Syrtis Major Planum region of Mars. This region is a vast, basaltic shield volcano that spans about 1000 km across. Nili Patera is the younger of two volcanic calderas at the center. Can you see the western edge of the caldera and the concentric ring fractures? Draw a labeled sketch. 5

6 4. Near the center of the screen, you should see a rectangular strip of imagery that is a different color (gray-blue) than the surroundings (reddish). (If you don t, there may be something wrong with Google Earth, so ask Dr. Hurtado!) Zoom into and explore the rectangular area. 5. If you click on the square in the middle of the rectangular area, you ll see an information box about the image (PSP_005684_1890). What kind of image is this and where did it come from? Note that images of this type have a level of detail comparable to the White Sands images you worked with earlier. Close the information box when you are done reading about the image. 6. Zoom out a little bit so you can see the whole rectangular area. Compare the northern (top) part of image PSP_005684_1890 to the southern (bottom) part. What difference do you see i.e., what features are present in the south that are not present in the north? 7. Zoom in a moderate amount and explore the southern part of image PSP_005684_1890. What are the dark features in the southern part of the image? Describe them and draw a labeled sketch. 8. How large are the dark features? How far apart are they spaced? Compare this to what you see at White Sands. 9. What color are the features? What are the features made of? Compare this to what you see at White Sands. 10. Can you give a specific name to the dark features? What can you infer about the process that made them? Compare this to what you see at White Sands. 11. Can you find any Mars examples of what you saw at Site C at White Sands? Write down the latitude/longitude coordinates of the location (shown at the bottom of the screen) if you do! 12. Zoom in (almost) as far as you can on one of the dark features. Describe/sketch what you see. What are the smallest things/patterns you can discern? What are they and how were they formed? 13. Carefully explore the northern part of image PSP_005684_1890 again. Can you find any features at all that might be like White Sands? Describe, draw, and interpret them if you do. Don t forget latitude/longitude! 14. Do you think any of the features in image PSP_005684_1890 are active? Give some evidence to support your hypothesis. 6