Lab 3

Crater counting and age estimates.

Now we will try to put our estimated ages on a firmer footing. The impact rate was not constant, it decreased rapidly about 3.9 billion years ago as a large population of impactors was removed from the solar system by impact or ejection. The impactors are also not the same size, and small ones greatly outnumber large ones. It's more useful to us to collect crater counts in size ranges and plot the results normalized to a standard counting area. We will experiment using the same illustrations as last week. Results can be plotted on this graph (print it) or via a spreadsheet or graphing program - your choice.

The assignment:

1. We will move on to Mars, but still try to find images with a range of crater densities. Try to find one each from an old cratered highlands area, a medium-age plains surface and a young smooth area. I suggest you use Map-A-Planet for this:

USGS Map-A-Planet (Mars)

But... choose areas with relatively uniform crater density. If a boundary between heavily cratered and lightly cratered terrain runs through your area, your counting will mix up two different surface ages. You need enough craters to make it worthwhile, so for the smoother area, choose a larger region if necessary. We will normalize to 1 million square km so the different counting areas are not a problem.

Here are some examples:

Sabaeus Sinus (18 degrees south)

Lunae Planum (12 degrees north)

Amazonis Planitia (52 degrees north)

2. For each one, give basic location information and feature names if appropriate, website you obtained it from, and approximate dimensions of the image, showing how you determined the size.

A warning is needed here. These are photomosaic maps, not original images as we were using for the Moon. Each area I chose is in a standard map projection: Simple Cylindrical. Check the Map-a-planet result page - labels around the map give details not seen in my images. My maps are square, 8 degrees wide. One degree on Mars is about 60 km, giving us a rough size... except that meridians converge towards the pole. The areas are really trapezoidal, not square. The areas scale with the cosine of the map's midpoint latitude.

Example: if your area is 5 degrees across (N-S and E-W) with its centre at 48 degrees North, its area is (5 x 60 km) x (5 x 60 km) x cos(48).

3. Count craters in the size ranges shown on the graph, adjusted for a million square kilometer counting area. Tabulate results. Plot on a paper copy of the graph or via a spreadsheet or graphing program. Summarize your results briefly - what ages do you find, and are they reasonable or not?.

Hand in after 2 weeks (just before the reading break) - to instructor's office, under the door if he's not there.