Identification II

I'm back from my week at the American Geoplysical Union conference in San Francisco. On Monday, I again met with students. In the past we had identified pretty simple features like impact craters, wind streaks and small rivers. It was time to move beyond the simple and look at something that wasn't "obvious". The subject of this post can be found at latitude = 7, longitude = -77.


Here, I'm curious about the long linear feature cutting through the image from north to south. What is it? Or, more importantly, how do you figure out what it is? First thing, as usual, I got an elevation cross section along part of the linear feature. It shows that the area to the left of the linear feature is lower and the area to the right is higher. By moving the mouse around the feature and watching the elevation displayed at the bottom of the screen I can see the difference in elevation is over 600 meters.
One of the things that makes planetary geology a little easier is that determining features can be matter of multiple choice. In this case, what might cause the difference in elevation between the left side and the right? Offhand, I can think of four possibilities:
Material on the left has been eroded away.
Material on the right has been added.
The area on the left has undergone some kind of collapse.
The area on the right has undergone some kind of uplift.

Also, notice that on the left side of the feature there are fewer craters than on the right. The number of craters is typically proportional to a geologic feature's age. New features can cover the old features they form on top of resulting in an area with no craters. Over time, impact craters accumulate. The oldest areas are dominated by craters. For an example of this, check out the southern highlands of Mars.

Uplift and collapse are more rare geological processes so first let's consider the more likely ones first. How might material have been added on the right to create the linear feature? There are several ways. Wind can deposit dust. Retreating glaciers can leave material behind. Lava can turn into rock to build up an area.
I don't see how wind blown dust could create the sharp difference in elevation that we see here. I'd expect it to be more gradual. Sadly, I don't know much about glacier deposits so I can't rule them out. Lava could easily create the feature on the right, but that is the older side. If linear feature was created by some process it is more likely it was created by the most recent process.
Since the crater record suggests that the left side is the newer side it makes sense to figure out how that side became the lower side. There are several ways the old surface on the left side could have been removed to expose what we see now. Wind can blow away part of a region. Advancing glaciers can scour a region. Meteorites can create impact craters. Flowing liquids, such as water, can erode whatever it flows over. Again, I wouldn't expect wind (or as the scientists call it "aeolian") erosion to form such a stark difference in height. It doesn't make sense for either deposition or erosion. Again, glaciers must be skipped since I don't know much about them.
That leaves either cratering or a river or some other water based removal of the old surface. These are easy to tell apart. Craters are round (or rarely, oval) while rivers are long. Simply by panning north a couple times you can see the feature isn't round. Panning north and zooming in reveals this significant feature:


Teardrop shaped features form in rivers with the pointy end pointing downstream. By drawing some elevation cross sections you can verify that downstream is downhill. With two pieces of evidence suggesting which direction the mouth of the river lies. Let's turn south and search for the headwaters. Going south, you eventually find:


At the start of the feature, it looks like a regular river with two banks not very far apart. As it flowed north it got very, very wide. We've mostly been looking at the right bank. You could also explore the left bank. Further evidence for this being a river/water feature can be found by identifying a couple tributaries.

This feature isn't reminiscent of typical rivers on Earth. On Earth, rain water feeds streams that combine to create rivers. They constantly flow with a seasonally determined amount of water. If rivers on Mars look different, perhaps the source of water is different.