Geology Reference
In-Depth Information
is not a true velocity that one would measure but is a
function of the boundary layer pro
le.
As outlined in the classic work by the British army
engineer Brigadier Bagnold (
1941
), wind
“
threshold
”
curves (
Fig. 3.36
)de
ne the minimum wind speeds
required to initiate movement of different particles and
show that the particle size moved by the lowest speed
wind is about 100 micrometers (
μ
m) in diameter, or
ne
sand. The ability of wind to attain threshold is a function
primarily of atmospheric density. Thus, the very-low-
density atmosphere on Mars (
Table 1.1
) requires wind
speeds that are about an order of magnitude stronger than
on Earth for particle motion, while relatively gentle winds
can move grains in the dense atmosphere of Venus. Once
they have been set into motion (
Fig. 3.37
), wind transports
sediments by suspension (mostly silt and clay particles, or
“
dust,
”
smaller than about 60
μ
m in diameter), saltation
(mostly sand-size particles, 60
-
2,000
μ
m in diameter),
and surface creep (particles larger than about 2,000
μ
m
in diameter).
Winds can redistribute enormous quantities of sediment
over planetary surfaces, resulting in the formation of land-
forms large enough to be seen from orbit and deposition of
windblown sediments that can be hundreds of meters
thick. One of the most useful types of features for inter-
preting wind processes is the dune, a depositional land-
form (
Fig. 3.38
). Dunes form by sediment transport in
saltation and signal the presence of sand-size particles.
Both the planimetric shape and the cross-sectional pro
le
of dunes can re
ect the prevailing winds in a given area
(
Fig. 3.39
). Thus, if certain dune shapes or slopes can be
determined, local wind patterns can be inferred for the
time of their formation.
On Earth, great quantities of silt and clay are trans-
ported in dust storms and eventually deposited as loess.
However, even where they are relatively young and well
exposed on the surface, loess deposits are nearly impos-
sible to identify in remote sensing data. Yet, identi
cation
of such deposits could be very important in understanding
planetary surfaces, especially on Mars.
Wind erosional features include pits and hollows
(called blowouts) that form by de
ation (the removal of
loose particles) and wind-sculpted hills called yardangs
(
Fig. 3.40
). Yardangs have been likened to inverted boat
hulls because of their streamlined shape, the orientation of
which indicates the prevailing wind direction at the time
of their formation.
Observations of active dust storms, dust devils, and
other aeolian features provide direct information on the
atmosphere. For example, variable features are surface
Figure 3.36. Wind friction velocities needed to set particles into
motion (termed threshold wind speed) as a function of grain size.
Friction velocity is not a true wind speed, but rather is a
characterization of the wind velocity pro
le in the turbulent
boundary layer near the surface. Note that the grain size moved
by the lowest wind is about 100 micrometers in diameter (about
thesizeof
fine sand), regardless of planet. Values for Titan
are offset because the grains are likely to be ice, rather than
higher-density silicate minerals, such as quartz (from Greeley and
Iversen,
1985
).
Figure 3.37. Commonmodes of transport of grains
by the wind; saltation applies primarily to sand-size
particles, which move in a series of short
“
hops;
”
suspension involves
finer material such as dust,
some of which can be ejected into the atmosphere
by the impact of saltating grains; larger materials,
such as granules and small gravels, move by surface
creep, which can be enhanced by saltation impact.
Windblown materials can abrade rocks, forming
ventifacts (sand-blasted rocks), and can be broken
into
finer grains upon impact.
