Impact Cratering Lab Exercises
Part I: Impact Cratering Mechanics & Crater Morphology
Impact Cratering Mechanics
The Vertical Impact Stack file is a movie made of a stack of 24 individual images, or frames. It is a movie of an experimental vertical impact at the NASA Ames Research Center. This type of experiment uses small projectiles fired into targets, mostly composed of quartz sand, to simulate the physical properties of solid rock at very high velocities. The experiments take place in a vacuum chamber to simulate atmosphereless conditions. A projectile is fired from an ultra-high-speed gas gun at a velocity of 6 kilometers per second. Although the impact velocity is high by most standards, it is relatively slow compared to celestial body impacts, which mostly take place at velocities between approximately 10 and 72 kilometers per second. Nevertheless, experimental impacts such as these give us good insights into the formation of large impact craters.
Open the Vertical Impact Stack file. The scale of these images is 1 pixel = 0.156 centimeters. Set the scale of these images; Analyze > Set Scale. Enter the measured distance as 1 pixel, and the known distance as 0.156 centimeters. Enter "centimeter" in the "Unit of Length" field.
From the IMAGE menu, select STACKS > START ANIMATION. [A shortcut to starting, and stopping, any movie is to press the \ (backslash) key. There are also controls for starting/stopping the animation, and stepping through the frames, in the right side of the ImageJ applet window. Click on the double arrows (>>) then select Stack Tools.] Watch the movie. Notice how the crater's depth and diameter grow, and how the ejecta curtain moves. Slow down the movie to get a better view of the crater formation and ejecta curtain movement. Use the number keys (1-9) to run the movie at different speeds. (The higher the number, the faster the movie plays.) Next, cycle through the movie one frame at a time using the left or right arrow keys. Notice that each frame is numbered consecutively, with 1 being the first frame.
1. Compare the stages of formation of this experimental crater with those shown for a simple crater in the previous background section.
a. On which frame of the experimental impact does the crater reach maximum depth? Measure the depth and record your answer.
b. On which frame does it reach maximum diameter? Measure the diameter and record your answer.
2. Calculate the depth/diameter ratio of this crater.
3. On the second or third frame, measure the angle of the ejecta curtain with respect to the surface using the Angle Tool, and record your result.
Close this stack of images.
Open the Oblique Impact Stack file. This is another movie of an experimental hypervelocity impact in a vacuum chamber at the NASA Ames Research Center. This time the impact occurs at an angle of 30° from the vertical in order to show the effect of an oblique impact on the distribution of ejecta. Animate the stack, then step through the frames one at a time. Impacts that occur at angles to the surface form ejecta deposits that are asymmetrical, with most of the ejecta deposited on the downrange side of the crater.
4. On the second frame, measure the angle that the ejecta curtain makes with the surface on both the left and right side of the impact, and record your results.
a. In what direction in the images was the projectile traveling?
b. How did you reach this conclusion?
5. Is the shape of this crater different from a crater formed by a vertical impact? Explain your answer.
Close this stack of images.
Impact Crater Morphology
Open the LO Crater file. This is an image of a small crater on the Moon taken in 1965 by Lunar Orbiter III. You will measure the diameter and depth of this crater and calculate the depth/diameter ratio for comparison with the depth/diameter ratios of the experimental crater and simple craters on the Moon. There are two basic types of impact crater, simple and complex. A simple crater is bowl-shaped with little interior structure. A complex crater has terraced inner walls, a central peak or peaks, and a flat floor. Simple craters on the Moon have a depth/diameter ratio from 0.14 to 0.2, i.e., the diameter is about 5 to 7 times greater than the depth. For complex craters on the Moon (larger than 20 kilometers in diameter), the depth/diameter ratio ranges from 0.1 to 0.05, i.e., the diameter is from 10 to 20 times larger than the depth. This is because slumping of the inner walls and formation of the central peak causes a shallower depth.
In this image 1 pixel = 2.15 meters. Set the scale.
6. Measure the diameter of the crater and record your result.
7. Measure the length of the shadow cast by the crater rim and determine the depth of the crater. The depth of the crater can be determined from the length of the shadow cast by the crater rim on the floor of the crater. The depth of the crater is the shadow length times the tangent of the Sun angle, or D = L tan β, where D is the depth, L is the shadow length, and β is the Sun angle. When this image was taken the Sun was 17.88° above the horizon. (If you have not yet been introduced to trigonometric functions, e.g., tangent, see your instructor.) Click here for more information on determining crater depth.
8. Determine the depth/diameter ratio of this crater.
a. Does it agree with the ratio for the experimental crater you measured earlier, and the ratio for simple lunar craters?
b. What does this tell you about this crater?
Open the Ammonius and Lambert files. Tile the images to make it easier to view both at the same time. Select Windows > Tile. These lunar craters where photographed by the Apollo astronauts in orbit around the Moon. The interior structure of each crater is very different. One is a complex crater and the other is a simple crater. On the Moon, the change from simple to complex craters is at diameters between 15 and 20 kilometers.
9. Describe the major morphological differences between the craters and state which one is simple and which is complex.
10. From the characteristics alone, which crater is the largest? Explain your answer.
Cascade the images. Select WINDOW > CASCADE. The scale for the Ammonius image is 1 pixel = 0.038 kilometers, and the scale of the Lambert image is 1 pixel = 0.0835 kilometers.
11. Measure the diameters of Ammonius and Lambert. Do your measurements agree with your answer to question 10 above?
Open the Meteor Crater file. This is a picture of Meteor Crater in northern Arizona. Its diameter is 1.2 kilometers, and its depth is 183 meters. The crater was formed approximately 49,000 years ago. It is the largest, most well-preserved crater on Earth. Relatively few (about 120) impact craters have been identified on Earth because they are rapidly erased by erosion and plate tectonics. Seventy percent of Earth’s surface is water, so many craters formed on the ocean floors and were quickly eroded. However, impact craters are well preserved on other planets and the Moon. This is the reason you have been studying impact craters on the Moon.
12. What type of impact crater is Meteor Crater? How did you reach your conclusion?
13. How does this crater differ from the same type of crater on the Moon? What are some possible reasons for these differences?