Crater Degradation in the Martian Highlands
The idea behinde this project is entirely from the fine paper "Crater degradation in the Martian highlands" by Drs. Forsberg-Taylor, Howard and Craddock published in the Journal of Geophysical Research, Vol 109. Number E5, May 25, 2004. Nearly all of what I write below is adapted from it. The diagrams are also from this paper. If any of the following is in error, it is no doubt my mistake.
The question we are trying to answer is "What has caused the craters in the Martian Highlands to decay? Was it wind or water?".
Relevant Geological Processes
Over time, impact craters, like all geological features, undergo decay. There are a variety of specific geological processes that cause decay and by carefully studying a crater's elevation profile we can deduce which processes are actually involved. Also, by gathering statistics on a number of craters in the same region, we can double check our conclusions.
The paper by Forsberg-Taylor et. al. really just deals with two ways a crater can degrade. First there is wind-blown dust that can gradually fill up a crater. This is a pretty obvious way to degrade a fresh new crater. Planetary scientists call the geological process eolian modification.
The other major way craters on Mars are thought to decay is by "natural" collapse of the sides of the crater that might encouraged by weathering or flowing water. Right after a crater has been created the slope of the sides of the crater can be very steep. Based on the properties of the rock that make up the wall of the crater, there is a maximum slope or angle that is stable over long periods of time. This angle is typically 25 to 40 degrees and is called the angle of repose. Walls with a slope grater than this are inherently unstable and will eventually yield to gravity. The technical term for this kind of yielding is "mass wasting" or "mass movement". The mass wasting might be fast and dramatic (a landslide) or very slow (called creep), but it is inevitable.
This mass wasting can be encouraged by other geological processes. If there is some fluvial process, that is some flowing water, then it is easy to see how rock and dirt can be washed off the crater walls to fill in the crater floor. Over time, the crater becomes more shallow. Besides flowing water, maybe there is some process (encouraged by something in the water, or variation in temperature, etc.) that tends to break down the rock which encourages the collapse of the walls.
In Forsberg-Talyor et. al, for convenience, they bundle all the processes together and just use the term "fluvial erosion and deposition".
The Shape of A Crater
By relying on previous work, lab experiments and mathematical simulations, Forsberg-Taylor et. al discusses how the current shape of craters provides insights to how they are degrading.
We need to start at the beginning, which is the shape of a new impact crater as seen in cross section.
Over time, as a crater degrades its cross section will change. If the degradation is from wind borne material (aka eolian infiling), the debris blown sticks to the existing walls of the craters. This implies that the general shape of the crater tends to remain roughly the same. The figure below shows the eolian infiling degradation that a 50km impact crater would experience over time:
Each of the cross section lines show the profile of the crater at a different time. As you can see as it fills up, the shape remains similar.
This is very different from what happens when a crater undergoes "fluvial erosion". When the sides of a crater are washed into to crater basin the slope of the sides tend to get stepper while the slope of the floor tends to flatten. Again, we turn to Forsberg-Taylor et. al. for a graph generated by computer simulation:
This graph is also for a 50 km crater. If a crater is only being degraded by either eolian or fluvial processes and is old enough to have suffered significant degradation, you can take a cross section and gain insight into what has been happening over the last several million or billion years.
Also, notice that over time eolian infilling will eventually pretty much erase a crater. More and more debris will accumulate in and around it making the original crater less deep and harder to detect. Over time, the crater slowly disappears.
But that isn't the case with fluvial erosion! By moving material from the side of the crater to its floor you're mostly changing the crater's shape. This approach can't erase a crater. Craters suffering from fluvial erosion will remain visible until some other process erases them.
Finally, notice how the diameter of the crater changes over time. Crater diameters are generally measured from rim to rim. As the diagram shows, eolian infilling doesn't change the diameter much at all. However, with fluvial erosion, it is the rim wall that are being washed to the crater floor. This means the rim wall retreats, making the rim to rim measurement increase over time.
Looking At Many Craters
In Forsberg-Taylor et. al. there is a clever insight regarding many craters. If you look at the current depth of many craters relative to their original depth, you can make a frequency distribution graph. Remember that if eolian infilling is at work, this continuous process will create craters at every possible depth. So, the graph will illustrate this continuous process.
But, if fluvial degradation is at work, the graph must have a different shape. Relative depths where the crater is nearly completely filled in can't exist because there isn't a source of new material to fill the hole. Here's a frequency distribution graph for the Sinus Sabaeus region of relative crater depth:
This graph has separate lines for craters that appear fresh and those with various levels of degradation. The solid line at the top shows the totals. This is the sort of distribution that is predicted when fluvial degradation is at work.
Science Summary
By looking at the elevation cross section of a crater, you might be able to tell what geological process is responsible for its degradation.
By looking at the relative depth many craters, you might be able to tell what geological process is responsible for their degradation.
Cross Sections with GeoVirgil
Below is a copy of Figure 8 from Forsberg-Taylor et. al. According to its original caption, it shows "degraded craters with steep interior rims and possible rock outcrops with only modest gully development". It is a portion of a mosaic image from the Mars Orbital Camera in "Wide Angle" or low resolution mode.
Below is a screen shot of GeoVirgil showing a slightly larger region:
Although the newer MOC mosaic is at a higher resolution, the older Viking data displayed by GeoVirgil looks pretty much the same. The elevation cross sections look pretty steep with a somewhat flat bottom suggesting fluvial erosion. (The vertical black stripes are a function of NASA's data processing.)
Notice that the cross section on the left looks a little choppy. I was running GeoVirgil with the ~33MB elevation data base. It contains a 16x16 grid of elevation values for each 1 degree by 1 degree region of the surface of Mars. As you zoom in and draw cross sections of smaller features there are fewer elevation values available and the cross section gets choppy. So, it is better to work with larger features.
For some more fun, lets look at a 3D view of the above region generated by GeoVirgil. Here, elevation has been exaggerated by a factor of 5. The view is looking to the north.
As you might of expected, it shows a large, fairly flat area with some large and shallow craters. And now, another 3D view this time with the imagery colored coded based on elevation. Again, vertical elevation exaggerated by a factor of 5:
Before you settle down and start to analyze cross sections, it might be useful to grab your joystick and fly over the region to familiarize yourself with the area.
Project Ideas
Forsberg-Taylor et. al. looks at the Sinus Sabaeus Quadrangle of mars, a region on Mars from 0 to 30 degrees south latitude and 0 to 45 degrees east longitude. What about other parts of the Martian Highlands? Why not pick another similarly sized region adjacent to Sinus Sabaeus and try to create the above Figure 11! What if you try a region much further north, not in the highlands? Is there any region of Mars where crater degradation is dominated by eolian infill?