Results of the Imager for Mars Pathfinder windsock experiment

Abstract

Abstract. The Imager for Mars Pathfinder (IMP) windsock experiment measured wind speeds at three heights within 1.2 m of the Martian surface during Pathfinder landed operations. These wind data allowed direct measurement of near-surface wind profiles on Mars for the first time, including determination of aerodynamic roughness length and wind friction speeds. Winds were light during periods of windsock imaging, but data from the strongest breezes indicate aerodynamic roughness length of 3 cm at the landing site, with wind friction speeds reaching 1 m/s. Maximum wind friction speeds were about half of the threshold-of-motion friction speeds predicted for loose, fine-grained materials on smooth Martian terrain and about one third of the threshold-of-motion friction speeds predicted for the same size particles over terrain with aerodynamic roughness of 3 cm. Consistent with this, and suggesting that low wind speeds prevailed when the windsock array was not imaged and/or no particles were available for aeolian transport, no windrelated changes to the surface during mission operations have been recognized. The aerodynamic roughness length reported here implies that proposed deflation of fine particles around the landing site, or activation of duneforms seen by IMP and Sojourner, would require wind speeds >28 m/s at the Pathfinder top windsock height (or >31 m/s at the equivalent Viking wind sensor height of 1.6 m) and wind speeds >45 m/s above 10 m. These wind speeds would cause rock abrasion if a supply of durable particles were available for saltation. Previous analyses indicate that the Pathfinder landing site probably is rockier and rougher than many other plains units on Mars, so aerodynamic roughness length elsewhere probably is less than the 3-cm value reported for the Pathfinder site.

Giant saltation on Mars

“…Direct entrainment of dust by air shear requires much stronger winds than those required for moving sand (36), but once saltation initiates, the new threshold for dust transport is the impact threshold for saltation, u ‫ء‬t . Although the threshold for sand entrainment is very infrequently exceeded (2), values comparable to the impact threshold u ‫ء‬t Ϸ 1.0 m/s obtained in our calculations may occur often on the Martian surface (16). This frequency leads to a new view of aeolian transport on Mars.…”

“…1). However, two mechanisms compete for making saltation on Mars very different from what it is on Earth: (i) Martian conditions require 10 times higher wind velocities than Terrestrial ones (13,14), being seen only a few times a decade during gusts of extreme aeolian activity (2,3,5,(15)(16)(17)(18)(19); and (ii) the lower gravity and stronger winds of Mars allow particles, once in the air, to fly higher and to remain longer in the atmosphere, being accelerated by the wind more than they are on Earth (8,20). From wind tunnel experiments under low-pressure conditions that reproduce the density of the Martian air, we could already gain some flavor of the efficiency of such high-speed saltating particles in dislodging sand grains or raising dust when colliding onto Martian soils (4).…”

The Modern Near-Surface Martian Climate: A Review of In-situ Meteorological Data from Viking to Curiosity

“…However, wind direction could be retrieved with an uncertainty of ∼15° ). In addition, eight sequences of measurements were made using the windsocks and several of these measurements were used to deduce the wind profile (Sullivan et al 2000). The MPF pressure sensor also had calibration problems (Haberle et al 1999).…”

Telltale wind indicator for the Mars Phoenix lander

“…It was furthermore clear that such type of wind indicator required only a very simple mechanical interface to the lander. Previous mechanical wind sensors operating on Mars [Sullivan et al, 2000] were not designed to monitor the winds at the landing site, but used to monitor surface roughness.…”