|
|
The goal of this project is to map Martian global wind patterns by mapping Martian wind streaks. On Mars are there large regional wind patterns or is it all driven by local events?
In my small backyard, it seems like the wind direction is pretty random. Sometimes it is from one direction, sometimes another. However, there are place on Earth where I know the winds are more regular. A good example is the Atlantic Trade Winds. Just north of the equator the wind generally blows east to west from the "old world" to the new. By the mid-atlantic states the winds blow west to east all the way to Europe. When ships where powered by sail the Trade Winds where the key to travel. Does Mars have Trade Winds?
If you look hard, you can find wind streaks on Mars.
This image has a number of wind streaks. They are all bright tear-drop shaped features to the southwest of the circular impact craters. Wind streaks tell us several important things. First, if there is wind then there must be an atmosphere. You won't find wind streaks on the Moon. Second, there must be some material to streak. Something small and light like dust or sand must exist. This further assumes there must be a source of such material, such as extensive volcanic emissions or some way to break rock down into dust. Finally, there must be enough temperature variation in the atmosphere to cause wind.
In the above image crater rims are disturbing the wind causing bright streaks to form on the leeward (downwind) side. There are two ways to form streaks. Either the disturbed wind can deposit material or it can scour away material. On Mars both happen. On Mars some streaks are bright and some are dark.
Here's a close up image of a wind streak:
In this image I greatly increased contrast by creating a second Brightness And Contrast Filter for the Global Imagery layer.
PEP has a tool to help map directional fields like wind streaks or water flow. Click on the Tools menu option and select "Draw Arrows Panel". Use this panel to create an arrow showing the direction of the wind streaks.
Now, zoom out to see a large region of the planet. Naturally you won't be able to see the wind streaks any more because they'll be must to small. However, the arrow remains showing the wind direction.
If you search the Martian globe for wind streaks and mark each field you'll end up with record of the global winds. Do you see the equivalence of Trade Winds. Are the streaks formed by local events that don't suggest any global patterns?
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.
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.
|
