Wind Streaks on Mars February 2004
Some of the most prominent aeolian features on the surface of Mars are wind streaks.
They appear in numerous images of the surface and are the result of wind erosion and deposition. As surface wind markers these streaks are very important in helping to understand the surface circulation patterns on Mars. Consequently researchers have been gathering data about the orientations and characteristics of wind streaks as early as the Mariner 9 Mission.
The most common type of streaks are bright and dark streaks, which are almost always observed in the lee of topographic features such as craters, hills and scarps. High resolution images taken by the Mars Global Surveyor show that these streaks change over a short period of time. The bright streaks in particular are the most common of all surface aeolian features.
Bright streaks are considered to be depositional features consisting of fine, bright dust. Veverka et al. (1981) proposed that the bright streaks form during dust storms when the atmosphere is more stable than normal, creating a shadow zone behind craters where dust can settle out. The bright streaks occur in a zone from 30° N to 30° S and their orientation correlates well with the circulation patterns during southern summer.
By contrast, the rarer dark streaks are interpreted as erosional features which are created by the removal of the bright dust. In Viking images these dark streaks have been seen to disappear at the onset of a global dust storm and reappear again when the dust settles. Veverka et al. (1981) propose that as the atmosphere clears after the dust storm, surface heating during the afternoon creates an unstable atmosphere which destroys the protective shadow behind topographic features, enabling the dust to be eroded in their wake.
Fenton et al. (2001) have taken these studies further by applying a Mars Global Circulation model to the problem. Their conclusion is that bright streaks form in a thermally stable (i.e. the temperature increases with altitude) but nearisothermal state (an isothermal state forms the boundary between an unstable and a stable state) atmosphere, when low winds diverge around obstacles, leaving a quiet zone in its wake. This condition occurs during large dust storms, when the winds are often low. Although strong winds also occur during dust storms, the near isothermal state of the atmosphere prevents strong interaction of the winds with the surface flow, inhibiting most erosion.
Dark streaks are formed when higher winds in the stable atmosphere flow down the lee side of obstacles. This occurs mostly after large dust storms and in the evening, as at these times the winds are strongest and the clearing atmosphere is very stable at night.
Another approach to understand the formation of streaks was used by Pelkey et al. They used thermal-inertia and albedo data from the Mars Global Surveyor in order to estimate the physical properties of the streaks and thus provide an insight in the way they are formed.
In a study of crater-related streaks, they have identified three major types. Type I bright streaks are optically distinct, but thermally indistinct from the surrounding terrain. The lack of a thermal signature indicates that the bright deposits have a low thermal inertia and thus are probably very thin (< 3 mm). Type I dark streaks are optically and thermally indistinct from the surrounding terrain, which indicates they are formed in brighter deposits which themselves obscure darker underlying material.
Both Type I bright and dark streaks probably formed by the modification of regional windflow by the craters’ topography.
In contrast, Type II dark streaks are both optically and thermally distinct from the surrounding terrain. They are thermally thick (more than a few cm). Their range of thermal-inertia values corresponds to sand-size particles. They are therefore thought to have formed from intracrater material transported via saltation or short-term suspension.
Paul De Schutter