5 February 2009
Welcome to part 2 of our attempt at tackling The Beast. If you missed Part 1, check it out here. We are working our way, slowly but surely, through the monstrous 2001 Mars Orbital Camera paper by Malin and Edgett. This paper summarizes the results from MOC, which revolutionized the scientific community’s view of Mars. This week we’re going to be looking at the section discussing surface patterns and properties. This section especially focuses on the new discoveries that MOC’s high resolution permitted.
Malin and Edgett report that much of the surface of Mars is covered by “mantling units” that probably are made up of dust that settles out of the atmosphere. In many places, the mantles are thick enough to have developed their own textures. For example, near the equator they are smooth, but at higher latitudes the mantles get “roughened”, possibly due to volatiles such as ice escaping from the soil.
Interestingly, the mantled terrains are not all the same brightness. I tend to hear the word “dust” and picture the light-toned red stuff that blocks the sunlight on the rover solar panels, but there are mantled deposits that are dark too! Could there be dark dust that we don’t know about yet?
The mantles aren’t all the same thickness either: on the Tharsis rise (a notoriously dusty area) there are some places where the lava flows are hidden beneath tens of meters of dust, and other places where the flows are still visible, and the wind has carved troughs and grooves in the remaining dust mantle.
In other places, the mantled units are thick enough that they make the surface look “out of focus”, although fresh craters prove that there was nothing wrong with the camera.
An especially interesting observation that Malin and Edgett made is that most of the time, surfaces that appeared rough in low-resolution Viking images look smooth in MOC, while smooth-looking Viking images are almost always rough at high-resolution!This is quite different from the moon, where the mare are smooth at all scales.
The paper goes on to consider the brightness or “albedo” of different regions on Mars. They note that although many brightness variations are due to dust cover, others have to do with the intrinsic properties of the bedrock. In other words, some rocks are just lighter-toned than others! (Notice that I didn’t say lighter-colored! Brightness is not color. My adviser has beaten that into me!) This isn’t a particularly surprising result, but it shows that not every piece of bedrock on Mars is the same. Other places that appear to be lighter or darker are actually just showing evidence of their relative roughness or smoothness. Rough surfaces have more things that can cast shadows, so they look darker.
The authors also note that many surfaces on Mars appear to be covered with small-scale ridges and grooves. The ridges don’t have the same shape as yardangs (rocks carved by the wind), and tend to be closely spaced and a few meters high.It’s not certain how these ridges form but Malin and Edgett suggest that they may be related to erosion along parallel cracks (a.k.a. joints), or fossilized sand ripples, or the sublimation of ice. Their favored hypothesis is that they are due to erosion uncovering fossilized dunes.