Field Studies - Dune Worlds: How Windblown Sand Shapes Planetary Landscapes - page 187

Fig. 16.14 The fluted texture and pyramidal shape of this rock (named Jake Matijevic, after the architect of the Sojourner rover) is believed to
have been caused by abrasion by saltating grains. The rock is 25 cm tall and 40 cm wide, imaged by the rover Curiosity in late 2012. Image
NASA/JPL

16.7 In Situ Spacecraft Observations

Mars—an extreme example being the Phoenix lander
(Fig. 16.16 ) in the polar regions, which cleaned off *10 cm
of regolith to expose a layer of solid ice underneath.
(Thruster erosion has also been documented on the moon,
too, but that body is of little interest in aeolian studies.)
Some landers (e.g., Surveyor, Viking, Phoenix) have
been equipped with arms; in addition to acquiring samples
for on-board instruments, these arms can be used for soil
mechanics investigations,(e.g., Fig. 16.17 ). Moore et al.
(1987) give a very detailed report on the mechanical prop-
erties of the Martian regolith from Viking measurements.
The arm can be used to dig a trench—whether the walls of
the trench collapse will indicate the degree of cohesion in the
material. Soil may be dug and drizzled into a pile (e.g.,
Fig. 5.3 ), the shape of the pile indicating the angle of repose
of the material (and thus indicating the friction between
grains). Figure 2.5 shows a microscopic imager picture of
basaltic sand, with evidence of very low cohesion.
Spacecraft surfaces can become coated with windblown
material, and that material can be detected by imaging.
Although fine airborne dust can be deposited on lander
surfaces, and is responsible for the typically steady decline
of the output of solar panels (e.g., the solar panel on the
Sojourner rover lost about 0.3 % of power per day), the size
of some observed particles on the MER panels (Fig. 16.18 )

We live in an age when robot explorers are traversing ripples
(and, at the time of writing, declining to risk traversing dunes!)
on another world, Mars. Field imaging of various types is
performed by landers and rovers on other worlds—panora-
mas, stereo pairs, and timelapse sequences (e.g., Fig. 16.15 ,
also Fig. 5.4 ) just as discussed above. In this situation, how-
ever, there is a much higher cost per image (if nothing else, an
opportunity cost) in that, unlike the memory card of a modern
camera, the data downlink capacity to the Earth is limited, so
only a finite number of pictures can be taken.
The act of landing on a planetary surface can itself pro-
vide some information on the properties of the surface. The
landing loads on legs or the underside of the vehicle can be
inferred from accelerometer measurements and can be
influenced by the bearing strength and cohesion of the sur-
face material. The penetration of feet, or even of ejected
items such as restraint pins or covers, can be assessed by
imagery to allow similar insights. Loose granular material
can also be mobilized at landing, either by the turbulent wake
of a probe in an atmosphere (e.g., landers on Venus kicked
up clouds of dust that could be detected by their blocking
sunlight for many seconds after landing), or by erosion by the
exhaust from rocket motors used to control the descent on