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## CUNY Brooklyn College Ages of Planetary Surfaces Paper

### Description

General instructions for assignments

• All responses must be typed.
• All calculations must be shown in full.
• All graphs must be electronically produced.
• All references must be cited using Chicago Manual of Style conventions.
• Sharing of phrasing or formatting with other students is prohibited.
• This work is the intellectual property of the instructor and Washington State University. All reproduction or retransmission in whole or in part is strictly prohibited.

Ref: LRO Quickmap

https://quickmap.lroc.asu.edu/ (Links to an external site.)

The Lunar Reconnaissance Orbiter (LRO) “quickmap” application is a mapping overlay tool, allowing the visual representation of data compiled from lunar research missions, including the LRO itself. We will use this data to estimate the ages of the Moon’s surface at two locations, one in the lunar lowlands and one in the lunar highlands.

To estimate surface age, we will compute the density of impact cratering, i.e., number of impact craters per million square km.

Navigate to the LRO quickmap website.

Select the three horizontal bars (the “hamburger” at upper left) to reveal the menus. Expand the “overlays” menu and select “anthropogenic features”; when successful the adjacent box will change color and several points will be marked on the lunar image. You can change the center of the image using click-and-drag, and you can change the zoom using a mouse wheel, or two-finger manipulation of a touch pad, or clicking on the +/- buttons (top right).

Lunar lowlands

Roam around till you find the “A11” marker for the Apollo 11 landing site. If you have trouble finding it, then at lower left select the compass icon and enter the coordinates 0.5,23.5 (i.e., selenographic latitude 0.5 degrees, longitude 23.5 degrees). Center the image on this location. Adjust the resolution to 500.00 m/px (meters per pixel). Estimate the total land area of this image: use the scale marker in the lower right corner. For example, on my monitor** I measured 50 km = 19 mm. My image was 224-by-151 mm, or 589.5-by-397.4 km. The total land area was thus 234,238 square km.

** Note that every monitor is different in pixel size and overall dimension.

Now we need to count the number of craters in this image. We could do this by eye, but fortunately there is a tool that helps. Expand the “geological features” menu and select “craters 5 to 20 km”, and “craters > 20 km”. Blue circles (difficult to see) and orange circles locate these craters in the image. Now count the total number of craters in the image.

Finally we need to compute the number of craters per million square km. This number is called the cratering density and is the standard unit used by planetary scientists. For example, if I count 20 impact craters then the cratering density is

$cratering\text{}density=\frac{20\text{}craters}{234,238\text{}sq\text{}km}×\frac{1,000,000\text{}sq\text{}km}{1\text{}million\text{}sq\text{}km}$

$cratering\text{}density=85\text{}craters\text{}per\text{}million\text{}sq\text{}km$

Lunar highlands

Perform the same exercise (above) for a lunar highland area. In this case we choose an area centered on the landing site of Apollo 16 “A16”. It is near latitude -9 and longitude 15.5 degrees.

If you are having trouble manipulating the lunar image, you can use the following two direct links to scaled and labeled images:

Crater density and age

The Apollo astronauts between 1969 – 1972 collected Moon rocks. The ages of these rocks were determined in a lab. The cratering density was measured. The results can be summarized in the following table:

 Age of the surface (billions of years) Cratering density (Cumulative # of craters) Rate of formation (# of new craters since the previous time) 3.0 90 — 3.2 110 110 – 90 = 20 3.4 151 151 – 110 = 41 3.6 245 245 – 151 = 94 3.8 468 4.0 1014 4.2 2356 4.4 5669

Questions

To complete this assignment, answer the following questions.

1. For the lunar lowlands image, report your crater count (number of craters), land area ( sq km), and cratering density (# of craters per million sq km). Report the same data for the lunar highlands image.
2. Which area has more cratering, lowlands or highlands? Which area has the older surface, lowlands or highlands?
3. “Age” means “the time since the surface rock became solid”. Explain why older surfaces have a higher number of accumulated impact craters.
4. Using the age table, approximately how old is the surface near the Apollo 11 landing site? near the Apollo 16 landing site?
5. Fill in the missing numbers in the third column of the table. What is meant by the column’s title, “rate of formation”?
6. The last row of the table (4.4 billion years) is farther in the past than the first row (3.0 billion years). Look at the third column. Write a sentence describing how the rate of formation of new craters has changed over time (e.g., was the impact rate higher in the more distant past? lower? same?)

Grading: All questions are weighted equally [5 points each]. Total points = [30].

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