ersc-assignment-2
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1P94
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Astronomy
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Apr 3, 2024
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ERSC Assignment 2
Planetary Science (Brock University)
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ERSC Assignment 2
Planetary Science (Brock University)
Scan to open on Studocu
Studocu is not sponsored or endorsed by any college or university
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ERSC 1P94 –
Assignment 2, Crater Counting (Kunowsky & Soffen) In this lab exercise you will determine the age of a planetary surface using the crater counting method. Crater counting uses the frequency of occurrence of given size craters (by diameter) to estimate the age of a surface since it was last reworked. You will assess the quantity of craters in 4 size classes of: 8 to 16 km, greater than 16 up to 32 km, greater than 32 up to 64 km, and greater than 64 up to 128 km. To do this you will need images of a planetary surface and some method of measuring/counting the craters, both of which are in this document. The number of craters in a size class (in a normalized area) are then plotted on a chart which is used to read off the likely age. The Images, chart, and questions are in this file. If you do the assignment completely in Word, which we recommend
, then you save this file as a PDF
to upload under Tests & Quizzes when you enter your answers. •
Save this worksheet with all circles, graphs and answers as a PDF before submitting it.
If you submit a ‘pages’ file or ‘Word’ file
, it will receive 0 marks (i.e., a loss of 18 marks). To save as PDF go to File
, then Save as
, and change the file type
to PDF. •
Add your surname at the beginning of your pdf’s file name
. Link to the crater counting YouTube tutorial (a must see!):
Crater Counting Tutorial on YouTube •
Your answers depend on the quality of your counts. Even with the generous ranges we accept, poor counting will lose marks. This is how scientific study works; theory alone is not sufficient, application and execution matter just as much. •
Below are two cratered-surface images. They have scale bars
and a set of circles
to use for sizing/counting. Some craters have already been sized for you! Include them in your counts
. •
Craters that are less than 8 km across are marked with red dots
, do not count those craters or features smaller than them
, they are too small for our size classes. Instructions
: 1)
Watch the YouTube tutorial, read the Examples &
FAQs
at the end of this document, and read the Background & Methods
pdf. Review Module 3’s Falling Space Objects
. 2) Now, on to our Martian images! Click on the image, you will notice the entire image is selected (little handles appear on the edges). 3)
A click on a circle selects the circle and then you can move it (
with your mouse, or the arrow keys
). Copy the circle and paste it to make a new one (CTRL + C then CTRL + V, or just CTRL + D)
. You’re going to want to be zoomed in. If you stretch a circle resize it with the scale bar. 4)
Move a circle over a crater of the proper size range. In the example above, the crater is more than 32km but less than 64 km in diameter; it is therefore in the 32-64 km category (red circle). The picture has a 32km green circle to show the crater is too large for that group. 5)
While a circle is selected, (it still has the little handles for rescaling and moving on it), press CRTL +D (hold control key and press D) to make a second circle of the same size near the first one. Move the second circle to another crater and generate the next circle with CTRL+D again. Move across the image systematically until all craters of that size are filled. Probably best to count whilst using the provided circles (make tic marks on a sheet). You have CTRL + Z to undo a mistake. Notes: •
Please be careful not to count ejecta blankets (see Examples & FAQs
at the end of this document). •
Post your questions with a specific title
in the Forum. •
Relax and count purposefully. Recommended music for crater counting is Gustav Holst’s
The Planets suite
(esp. Jupiter, Bringer of Jollity
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Assignment 2, Crater Counting (Kunowsky & Soffen) Image name: Kunowsky crater
, north of the dichotomy. Image Size: 1491 km x 940 km = 1,401,540 km
2
(
5pts –
for outlined craters
) Downloaded by Timi Odetoyinbo (timiodetoyinbo@gmail.com)
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ERSC 1P94 –
Assignment 2, Crater Counting (Kunowsky & Soffen) Image name: Soffen Crater
, south of the dichotomy. Image Size: 1491 km x 940 km = 1,401,540 km
2
(5pts for outlined craters)
Be certain to include the craters measured for you in your total counts. Downloaded by Timi Odetoyinbo (timiodetoyinbo@gmail.com)
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Assignment 2, Crater Counting (Kunowsky & Soffen) Question 1: Upload the PDF of this completed document with images (including circles placed), counts, and graph (below) as a PDF
. 5 points for each of 2 counted images, 8 marks for the graph = 18 marks. If you upload something other than a PDF you will receive 0 marks. Both images are 1491 km x 940 km = 1,401,540 km
2
Hint –
since the area you measure is greater than 1,000,000 km
2
, the number of craters per 1,000,000 km
2
is less than the actual number of craters you counted Martian Crater Density Data Table (2 points each, total 8 points) Northern Hemisphere (
Kunowsky
) Southern Hemisphere (
Soffen
) Crater Size Range (km) Number of craters in image (your answers in Sakai
) (Questions 2-5)
Number of craters per 1,000,000 km
2
(use on the graph
)
Number of craters in image (your answers in Sakai
) (Questions 6-9)
Number of craters per 1,000,000 km
2
(use on the graph
)
8-16 (Yellow)
17 12.13 100 71.35 16-32 (green)
3 2.14 86 61.36 32-64 (red)
0 0 52 37.10 64-128 (blue)
1 0.71 12 8.56 Why do we grade numerical answers and
the counted images? Because quality matters. It is possible
to get to the correct values by randomly placing circles, so your counted images are evaluated against the work of a more experienced counter. Your values as entered in Sakai are evaluated against a scale of full and partial marks in ranges, those ranges are set up generously
, but full marks require a good effort here. Downloaded by Timi Odetoyinbo (timiodetoyinbo@gmail.com)
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Assignment 2, Crater Counting (Kunowsky & Soffen) Downloaded by Timi Odetoyinbo (timiodetoyinbo@gmail.com)
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Assignment 2, Crater Counting (Kunowsky & Soffen) The Graph:
Use blue for Kunnowsky Crater
, red for Soffen Crater
. Move the dots to the values you calculated. Position a line by selecting it and using the arrow keys (or by dragging the ends). (
8 pts
) The colour coded triangles at the bottom highlight where to plot your data above the horizontal axis as instructed in the video and method sheet. The values for the scaled counts are logarithmic (see the video) on the left side of the graph. Each horizontal line is spaced at the same value as the last label below it. E.g., the next line above “10” is 20, then 30 etc.
If you have value of zero (0), do not consider it in the placement of your line. The slope of your lines should match the slope of the isochrons (the diagonal lines across the graph as shown in the video). Downloaded by Timi Odetoyinbo (timiodetoyinbo@gmail.com)
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Assignment 2, Crater Counting (Kunowsky & Soffen) Report your ages from the graph by observing the intercept at the right axis (you will enter these values in Sakai): Question 10:
(2 pts) Martian Northern Hemisphere Surface Age = 3.5 billion years old Question 11:
(2 pts) Martian Southern Hemisphere Surface Age = 4.2 billion years old Concluding Questions 12) Based on your data, what is the age difference between the two Martian Hemispheres? (
2pts
) (Subtract younger from older) 4.2 - 3.5 = 0.7 Billion Years 13
) Let’s use the 16 –
32 km crater class (green) as a good representative. By what percentage has the cratering in that class decreased in the younger hemisphere, compared to the older hemisphere? (
2pts
) [(Older actual crater count - Younger actual crater count)/older actual crater count] * 100 = % decrease (3/86)*100=3.45% 14) Given that cratering in that size range has decreased by ___% (your answer in 13) over a span of ___ years (your answer in 12), what does that tell you about how impact frequency changed in the early solar system. ( 3 pts, this is a multiple choice question in Sakai) This indicates that the frequency of impacts is steadily reducing. The heavy period of solar system formation is most likely the explanation for so many impacts earlier on. Save this file with all craters circled, table and graph completed, and answers to questions. Then through File
go to Save As
and change the filetype to PDF. Upload the PDF in question 1 on Sakai. The file must have your images with circles, counts, table, graph and questions 1-14 (i.e., everything). Downloaded by Timi Odetoyinbo (timiodetoyinbo@gmail.com)
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Assignment 2, Crater Counting (Kunowsky & Soffen) Examples & Frequently Asked Questions: A. What is an ejecta blanket? The image below is of a crater that has an ejecta blanket. The ejecta blanket is material disrupted by the impact and distributed around the outside of the crater like a blanket. In the middle image, the orange outline shows the extremity of the ejecta blanket, this is what you do not want. The right-most image shows the actual crater rim circled in green which is what you want to do. You are concerned with crater rims, not the ejecta blankets. No, the colour of circles used in these examples are not relevant.
B. What are these things in the middle of craters and should I circle them too?
(image at right)
Sometimes a crater has another crater in it, but centered features in a larger crater are usually central uplift (as discussed in Module 3, page 5 Falling Space Objects,
under Stage 3: Modification
). This occurs in larger craters as material rebounds from impact and it is not a crater so it should not be circled, only the crater rims should be circled. The image at right shows a crater with central uplift.
C. Do I need to size & count a crater at the edge? You need to size & count any crater that has 50% or more of its rim showing in the image. D. Inside the circle or under the exterior rim of the circle? Take a look at the circles over the scale bars, that is why the scale bars are on the image. If you can see the crater peaking out from the exterior rim of the circle, go up a size. Ensure your circles are sized properly. E. What are these? (image at right)
Space oddities, there are only a couple, or none at all depending on this semester’s images. Don’t bother with them.
F. Is this a crater? (images at right) No, they are lumps (hills, which go up not down)
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