Assignment 2-Worksheet-Craters Milankovic-Schaberle W2023 (2)
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Brock University *
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Course
1P94
Subject
Geography
Date
Dec 6, 2023
Type
Pages
7
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ERSC 1P94 – Assignment 2, Crater Coun9ng (Milankovic-Schaeberle) In this lab exercise you will determine the age of a planetary surface using the crater coun8ng method. Crater coun8ng uses the frequency of occurrence of given size craters (by diameter) to es8mate the age of a surface since it was last reworked. You will assess the quan8ty 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/coun8ng the craters, both of which are in this document. The number of craters in a size class (in a normalized area) are then ploLed on a chart which is used to read off the likely age. The images, chart, and ques8ons are in this file. If you do the assignment completely in Word, which we recommend
, then it should be straighOorward, you save it as a PDF and upload it under Tests & Quizzes when you enter your answers. •
Save this worksheet with all circles, graphs and answers as a PDF before submiLng 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 coun9ng YouTube tutorial (a must see!):
Crater Coun8ng Tutorial on YouTube •
Your answers depend on the quality of your counts. Even with the generous ranges we accept, poor coun8ng will lose marks. This is how scien8fic study works; theory alone is not sufficient, applica8on and execu8on maLer just as much. •
Below are two cratered-surface images. They have scale bars
and a set of circles
to use for sizing/coun8ng. 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. Instruc9ons
: 1)
First, read the
Background & Methods pdf, then watch the YouTube tutorial
, and read the Examples &
FAQs
in this document. Review Module 3’s Falling Space Objects
. 2) Now, on to our Mar8an images! Click on the image, you will no8ce the en8re image is selected (liLle 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 s8ll has the liLle 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 systema8cally un8l all craters of that size are filled. Probably best to count whilst using the provided circles (make 8c 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 ques8ons with a specific 9tle
in the Forum. •
Relax and count purposefully. Recommended music for crater coun8ng is Gustav Holst’s
The Planets suite
(esp. Jupiter, Bringer of Jollity) Examples & Frequently Asked Ques9ons:
ERSC 1P94 – Assignment 2, Crater Coun9ng (Milankovic-Schaeberle) 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)
Some8mes a crater has another crater in it, but centered features in a larger crater are usually central uplik (as discussed in Module 3, page 5 Falling Space Objects,
under Stage 3: ModificaMon
). 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 uplik.
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 oddi8es, 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 below) No, they are lumps (hills, which go up not down) or pits.
ERSC 1P94 – Assignment 2, Crater Coun9ng (Milankovic-Schaeberle) Image name: Milankovic crater
, above the dichotomy. Image Size: 1491 km x 940 km = 1,401,540 km
2
(
5pts – for outlined craters
) Be certain to include
the craters already marked for you in your counts. The crater centered
in the larger crater is very unusual so we have marked it to avoid confusion. Image name: Schaeberle Crater
, below the dichotomy. Image Size: 1491 km x 940 km = 1,401,540 km
2
(5pts for outlined craters)
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ERSC 1P94 – Assignment 2, Crater Coun9ng (Milankovic-Schaeberle) Be certain to include
the craters already marked for you in your counts.
ERSC 1P94 – Assignment 2, Crater Coun9ng (Milankovic-Schaeberle) Ques9on 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 Why do we grade numerical answers and
the counted images? Because quality maLers. 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 par8al marks in ranges, those ranges are set up generously
, but full marks require a good effort here. Mar9an Crater Density Data Table (2 points each, total 8 points)
Northern Hemisphere (
Milankovic
)
Southern Hemisphere (
Schaeberle
)
Crater Size Range (km)
Number of craters in image (your answers in Sakai
) (Ques9ons 2-5)
Number of craters per 1,000,000 km
2
(use on the graph
)
Number of craters in image (your answers in Sakai
) (Ques9ons 6-9)
Number of craters per 1,000,000 km
2
(use on the graph
)
8-16 (Yellow)
Q2: 16
11.420
Q6: 106
75.660
16-32 (green)
Q3: 22
15.703
Q7: 154
109.921
32-64 (red)
Q4: 3
2.141
Q8: 57
40.685
64-128 (blue)
Q5: 1
0.714
Q9: 13
9.279
ERSC 1P94 – Assignment 2, Crater Coun9ng (Milankovic-Schaeberle) The Graph:
Use blue for Milankovic
, red for Schaeberle Crater
. Move the dots to the values you calculated. Posi8on a line by selec8ng it and using the arrow keys (or by dragging the ends). (
8 pts
) The colour coded triangles at the boLom 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 lek 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).
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ERSC 1P94 – Assignment 2, Crater Coun9ng (Milankovic-Schaeberle) Report your ages from the graph by observing the intercept at the right axis (you will enter these values in Sakai): Ques9on 10:
(2 pts) Mar8an Northern (Milankovic) Hemisphere Surface Age = 3.725 billion years old Ques9on 11:
(2 pts) Mar8an Southern (Schaeberle) Hemisphere Surface Age = 4.275 billion years old Concluding Ques9ons 12) Based on your data, what is the age difference between the two Mar8an Hemispheres? (
2pts
) (Subtract younger from older) 4.275 - 3.725 = 0.55 Billion Years 13) Let’s use the 16 – 32 km crater class (green) as a good representa8ve. 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 = 86 % decrease 14) Given that cratering in that size range has decreased by 86 % (your answer in 13) over a span of 12 years, what does that tell you about how impact frequency changed in the early solar system. ( 3 pts, this is a mul8ple choice ques8on in Sakai) Save this file with all craters circled, table and graph completed, and answers to ques9ons. Then through File
go to Save As
and change the filetype to PDF. Upload the PDF in ques9on 1 on Sakai. The file must have your images with circles, counts, table, graph and ques9ons 1-14 (i.e., everything).