Impact Cratering Lab Exercises
Part II: Features and Motion of Crater Ejecta
Experimental Impacts: Following a BaIIistic Trajectory
Open the Impact Ejecta Stack file. This is a movie of another experimental hypervelocity impact in a vacuum chamber at the NASA Ames Research Center. Animate the stack and observe what happens. This experiment has an aluminum plate with holes that split the ejecta curtain into packets. Notice that each component of the ejecta curtain is on a looping path called a ballistic trajectory. Click the double arrowheads (>>) in the ImageJ applet window and select StartupMacros. This activates the pencil tool, paintbrush tool, and flood fill tool. For this activity, you will use the pencil tool to mark individual pieces of ejecta.
Before continuing, you MUST convert the stack of images from grayscale to RGB color. To convert the stacks from grayscale to RGB color, select IMAGE > TYPE > RGB COLOR. Next, double-click the color picker (looks like an eyedropper). This opens the Color Palette. Choose a color that will stand out against the darker animation background. Click on the color and verify that the top of the larger two boxes below the palette is the color you chose. Right-click on the pencil tool and change the Pencil Width (pixels) to 10.
In the movie, choose three packets of ejecta that emerge from frame 15 or later. Mark the chosen ejecta packets with the pencil tool on each frame. Use the right or left arrow keys, or the Stack Tools, to flip through the frames. When you have marked 10–15 successive frames, run the animation at a slower speed. Notice the ballistic trajectory of the ejecta components you selected.
Stop the movie. Select IMAGE > STACKS > Z project. (NOTE: If at this point you are prompted to increase ImageJ's memory close both the image window and the ImageJ applet window and start over.) Select “OK” when the ZProjection window appears. This creates a new image that is an average of all 35 images that make up the movie. You should be able to see the marks you made with the pencil tool. Notice again the ballistic trajectory of each packet of ejecta. The highlighted packets of ejecta may be difficult to see. Enhance the image to emphasize these highlighted packets. (HINT: Select only the area surrounding the highlighted packets.) Print this enhanced image and include it with your lab writeup.
Open the Full Moon file. This is an Earth-based telescopic image of part of the full Moon. The bright and dark areas show brightness variations across the lunar surface. The arrow points to a small bright haloed impact crater. The reason this small crater is so prominent is because it is fresh. Most of the brightness you see in this image is the ejecta surrounding the crater. This image has its scale preset (in kilometers).
1. Measure the diameter of the bright halo and record your result. (Hint: A more accurate measurement can be made by using the Zoom Tool.)
Open the LO Crater file. This image was taken by Lunar Orbiter III in 1965. It is an image of the crater shown as a bright spot in the Earth-based telescopic full Moon image. The scale is 1 pixel = 2.15 meters. Use the top right corner box on the image to view the whole image on your screen.
2. Compare these two images of this crater.
a. Based on what is seen in the Lunar Orbiter picture, what is responsible for the brightness in the Earth-based image?
b. What does the ejecta around this crater appear to be made up of (referring to texture not composition)?
3. Measure the diameter of the crater and record your result.
4. How many times larger is the halo you measured on the Earth-based photograph than the crater that formed it?
Open the LO Crater Detail file. This image shows part of the crater and its ejecta blanket in more detail. Its scale has been preset (in meters). In this image you will study some of the characteristics of the ejecta deposit and determine the velocity of a boulder ejected from the crater. The distance, d, traveled by a fragment of ejecta is called the ballistic range and is mathematically expressed by:
d = (V2 sin2θ)/g
where V is the ejection velocity, θ is the ejection angle from the horizontal (surface), and g is the planet's gravity field. The Sun angle for this image is 17.88° above the horizon.
5. Measure the length and height of the rock marked A. To determine the height, measure the length of the rock. Then measure the length of the shadow it casts and derive its height from the same calculation used to find the crater depth in Part I. Record your results. What size room (in ft2) would be needed to store this rock?
6. Is the position of rock A the exact position where it initially landed? Explain your answer. (Hint: From question 5 you should have found out this is a pretty large rock. Think about what a rock this size would do to the lunar surface when ejected from a crater. Can you see rock A's original landing spot?)
7. Calculate the ejection velocity of rock A from the ballistic equation by measuring its distance from point C in the crater. Assume an ejection angle of 45°. Lunar gravity is 1.62 meters per second squared. Be careful where you measure the distance from (as discussed in question 6). Can you drive this fast (legally) in a car (you will need to convert from meters per second to miles/hour)?
The impact cratering lab exercises you have just completed demonstrate how images of the surface of a planet, or moon, can be used to make detailed measurements. These measurements reveal information about the physical properties of the planet in the image. This is the point of "remote sensing" — we don't have to actually set foot on the surface of another planet to learn about it. It is cheaper and easier to take pictures of the surface, either from orbit or from Earth, and then analyze them in labs. However, in order to understand the surface processes of another planet, we need to perform laboratory experiments like those used in this lab. Such lab experiments are completely under our control. Laboratories try to simulate what we see in pictures of the surface of another planet. In this way, a theory or model of what we think has taken place on the surface can be tested.