Identifying Wind Streaks

In my last post I described a student project to figure if there are global wind patterns on Mars. Now, I have a few things to say about searching for wind streaks.

When searching for wind streaks it is important to correctly identify what you see. There are some features that might confuse you. For example, here is an impact crater


On the west side of the crater is something that looks like a bright, mostly vertical feature. Most wind streaks are bright, so is this a wind streak? No. This is bright feature is the result of strong sunlight reflecting off the crater rim. The shape of this bright feature doesn't match the teardrop shape we saw on authentic wind streaks. From the shape of the above bright feature you can't make sense of what way the wind is blowing. The wind streak should trail off on the downwind (leeward) side of the feature, not go along the side of the feature. Another clue that is a bright feature is a function of the sun is that there is a shadow on the other side of the crater.

Not everything that has a teardrop shape is a wind streak.


The behavior (and the physics) of wind blown dust is very similar to silt and dirt traveling downstream with a river. In both cases you have material in suspension being carried along. Depending on turbulence material can be carried away or deposited. So, features formed by wind can look like features formed by rivers. When making an identification like this pay attention to the surrounding area and use it to help determine if you're in, for example, a riverbed.

Wind streaks tend to be pretty thing thin. So, they have don't really stick up above the surrounding terrain. However, many teardrop shaped features formed by flowing water have a significant elevation profile.


The dust and sand that forms a wind streak tends to have a very small thermal inertia value. However, there are two problems with using thermal inertial to identify wind streaks. First, the thermal inertial detector has, compared the the Viking camera, fairly poor resolution. Most wind streaks are smaller then a single thermal inertia pixel. So, the thermal inertia value combines all the wind streak and a bunch of surrounding terrain instead of having several values inside the wind streak like you'd need. Second, if a wind streak is thin enough the thermal inertia value of the underlying surface is also detected. While a very thin layer of surface dust might cool quickly, perhaps the rock underneath received enough sunlight to heat up and will take a long time to cool down. So, the received thermal inertia value is a combination of both the dust and the rock.

The dust and sand that forms a wind streak tends to have a very small thermal inertia value. However, there are two problems with using thermal inertial to identify wind streaks. First, the thermal inertial detector has, compared the the Viking camera, fairly poor resolution. Most wind streaks are smaller then a single thermal inertia pixel. So, the thermal inertia value combines all the wind streak and a bunch of surrounding terrain instead of having several values inside the wind streak like you'd need. Second, if a wind streak is thin enough the thermal inertia value of the underlying surface is also detected. While a very thin layer of surface dust might cool quickly, perhaps the rock underneath received enough sunlight to heat up and will take a long time to cool down. So, the received thermal inertia value is a combination of both the dust and the rock.

Even though wind streaks are a relatively easy feature to identify, it is possible to get it wrong. When you study a region always think about how the sun is illuminating the scene. When you're trying to identify a small feature, keep the surrounding context in mind. Your scientific reputation depends in part on how correctly you identify features.