CraterCounting_Lab
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Cincinnati State Technical and Community College *
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MISC
Subject
Astronomy
Date
Feb 20, 2024
Type
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9
Uploaded by UltraFlower12994
Credit: Nicole Kelly, UW Astronomy Clearinghouse. Modified by J. Huber.
PSC 105 Name: _____________________________ Crater Counting on Mars Introduction Impact craters are the dominant landform on most surfaces of the solid worlds in our Solar System. These impact craters have formed over the course of the 4.6 billion years of our Solar System’s history. The number of craters on a surface increases with the length of time a surface has been exposed to space. The number of impact craters in some defined area on a surface of a world is referred to as the crater density
of that surface. By comparing the crater density on one part of a world to the crater density on another part of the world, the relative ages
of the two surfaces can be determined. Ideally, planetary scientists would like to find the absolute age
(e.g. a number, in years) in order to infer more about the history of that surface or world. If you want to find the absolute age of the surface you are studying, you need a sample from that surface. As you have learned, the Apollo mission brought back lots of rock samples from six unique sites on the Moon. By measuring the ages of the rocks (via radiometric dating) from these six sites and counting the craters at these sites, we can determine how the crater density is related to the absolute age at each of these sites. In this class, we make the assumption that the cratering rate measured by Apollo on the Moon is typical of the cratering rate of all
terrestrial bodies in the inner Solar System (N.B. This is a very important
assumption!). We can now extend our measurements of the crater density on the Moon to estimate the ages of various regions on the surface of Mars. Goals In this lab, you will be investigating four regions on the surface of Mars that span the history of the planet. You will calculate the crater density of each of the four surfaces and will then compare this number to the calibrated graph for the Moon. This will allow you to determine the absolute age of each of the four surfaces. Procedure There are many different ways to quantitatively represent the crater density of a surface. One of the most commonly used is to choose an area and count the total number of craters larger than a specific diameter (D). In this simple form, the crater density can be reduced to a single number by choosing a specific diameter and specific amount of area. This allows us to easily compare the crater densities between the two surfaces by simply comparing two numbers. In this class, we use a value of 10 km for the crater diameter (D) and one million km
2
for the area. The resulting number is represented as N(10) and is read as “the total number of craters that have a diameter
of 10 km over one million square km.”
The data we will use in this exercise come from the Mars Global Surveyor (MGS) spacecraft. The MGS was a global mapping mission that examined the entire planet from September 1997 to January 2007. The images
we will use come from the atlas of the surface compiled by MGS and published in 2002: http://www.msss.com/mars_images/moc/moc_atlas/ Examine the above image carefully. Using your knowledge of crater density and age, make a prediction of the relative surface age
(eg. young or old) for each of the four regions: Region 1: ____________________ Region 2: ____________________ Region 3: ____________________ Region 4: ____________________ In the images on the last four pages of this exercise, each of the regions labeled above are divided into six smaller panels that each have a surface area of 87,838 km
2
. The total area of the six small regions is 527,028 km
2
. At the upper right and lower left corner of the images is a black dot that represents the actual size of 10 km craters on the image. Use this as a guide to determine the size of the craters on the image. 2 1 3 4
Collect the Data:
Follow these steps for each of the four images: •
Count the total number of carters larger than 10 km in each of the panels and record that number in the column labeled “N(10) in Image” in the corresponding data table. Work from the upper left panel to the lower right panel. •
Since each panel has an area of 87,838 km
2
, you need to multiply the number of craters you counted by 11.4 to find the number of craters in 1 million km
2
(1,000,000 / 87,838 = 11.4), Record this number in the next column. •
Now that you know the value of N(10), use the lunar surface calibrated graph below to determine the age of each panel in the image. Record this age in the last column. If you cannot determine the age, record a zero for the age. •
Add up the total number of craters you counted in the six panels and record this number in the first column of the last row of the data table. •
Multiply the total number of craters by 1.9 to get the value of N(10) for the area covered by all six panels (1,000,000 / 527,028 = 1.9). Record this number in the second column of the last row. •
Use the calibrated lunar graph below to determine the age of the whole image and record this number in the last cell. 1
10
100
1000
4.1
4
3.5
3
2.5
2
1.5
1
N(10
)
Age (Byr)
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Region 1 Region 2 Panel N(10) in Image N(10) in 10
6
km
2
Age in Byr 1 2 3 4 5 6 Total Region 3 Region 4 Q
uestions 1.
Did your absolute ages confirm your predictions of the relative ages for the four regions? Explain. Panel N(10) in Image N(10) in 10
6
km
2
Age in Byr 1 2 3 4 5 6 Total Panel N(10) in Image N(10) in 10
6
km
2
Age in Byr 1 2 3 4 5 6 Total Panel N(10) in Image N(10) in 10
6
km
2
Age in Byr 1 2 3 4 5 6 Total
2.
To obtain the absolute ages for the four Martian regions, you compared the N(10) values you calculated to the relation established from the N(10) values of the regions visited by Apollo astronauts on the Moon. How do we know the ages of surfaces on the moon? 3.
Calculate the age of Region 4 if you removed one
10 km crater from the image. Show all work. 4.
What is the difference from the original age you determined for Region 4? 5.
Based on your measurements and calculations above (and especially on the answer to the previous question), explain why it is difficult to accurately determine the age of a very young
surface (age < 2 Byr) using this method. 6.
If the Earth was cratered at the same rate as the Moon and Mars, how many craters larger than 10 km should North America (area = 25x10
6
km
2
) have if it is about 1 Byrs old? 7.
Currently the state of Kentucky has zero 10 km impact craters. What happened to them?
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