If the resolution of the images at the end of the attached activity or in the online version below is not sufficient for your class, you can download the images or use different images from the Mars Global Surveyor site: http://www.msss.com/mars_images/moc/moc_atlas/.
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 km2 (106 km2) 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 used come from the atlas of the surface compiled by MGS and published in 2002: http://www.msss.com/mars_images/moc/moc_atlas/ (Links to an external site.)
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Examine the above image carefully. Before moving on to the questions below, use 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: ________?________
All of the craters with a diameter of 10 km or larger have been counted in each of the four regions and recorded in the final table below in the "raw" column as well as the "calibrated" cumulative crater count over 1 million square kilometers. Using the figure below, determine the absolute surface age of each region. Please note: the y-axis is logarithmic.
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In your discussion post, address each of the following questions (2pts each):
Did the absolute ages you found for each region confirm your predictions of the relative ages for the four regions? Explain why or why not.
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. Explain why this comparison is possible.
Calculate the age of Region 4 if you removed one 10 km crater from the image. What is the difference from the original age you determined for Region 4?
Explain why it is difficult to accurately determine the age of a very young surface (age < 2 Byr) using this method, in light of your response to #3.
Currently the state of Washington has zero 10 km impact craters. What happened to them? (Hint: Impactors that leave 10 km craters make it through the Earth's atmosphere to the ground.)
Full credit responses to this activity include clear and thoughtful answers to each of the 5 questions AND at least one constructive comment or question on one of your fellow classmate's posts that differs from your own. If your answers are wildly different from others, seek to (politely) understand why. It will help you and your fellow classmates learn! Please feel free to ask questions and comment on a variety of posts, and/or discuss other discrepant answers on this forum as well, but for full credit you need only address one other student's responses.
Please note: you will not be able to see other posts until you have submitted your own.
Credits: This activity was adapted from "Astronomy 150: The Planets Course Pack", 2015 by Toby Smith