Mars Dunes - Dune Worlds: How Windblown Sand Shapes Planetary Landscapes - page 129

Fig. 12.1 Mars from Earth. In fact, many of the features visible in
this Hubble Space Telescope image in 1997 could be discerned by
19th-century astronomers but can now be captured by amateur
observers with fast digital cameras on modest telescopes. Some of
the broad dark patches change shape over time due to removal and
deposition of dust. It is possible to just make out a dark ring around the
north polar cap, which forms the largest dunefield on the planet:
Olympia Undae. Image credit Space Telescope Science Institute

Fig. 12.2 Proctor Dune Field, among the first dunes to be recognized
on another planet. This is a processed mosaic of two Mariner 9 images
with a resolution of *60 m, acquired in March 1972 (for a raw
Mariner 9 image, see Fig. 1.2 ) . The rectangle indicates an area imaged
by Mars Global Surveyor in 1999 (see Fig. 12.5 ); the dot indicates the
location of a HiRISE image shown in Fig. 12.6 . Credit Malin Space
Science Systems/NASA. Image MOC2-170a

assessing whether life was present. The Viking mission
involved two orbiters and two landers; on July 20, 1976, the
Viking 1 lander became the first spacecraft to return images
from the surface of another planet, at a site in Chryse
Planitia that provided startling evidence of wind-blown drift
deposits (sand shadows, or lee dunes) around some of the
nearby large blocks. Viking Lander 2 showed a few similar
features (e.g., Fig. 16.14 ).
The[50,000 images returned by the two Viking orbiters,
over several years of operation, revealed that dune fields
within large craters were quite common on Mars, and the
largest 'sand sea' (erg) on the planet formed a ring around
the north polar cap (e.g., Cutts et al. 1976; Tsoar et al.
1979). The Viking data prompted a systematic comparison
of Mars dunes with those on Earth (e.g., Ward et al. 1985;
Breed et al. 1979), especially since satellite data on Earth
dunes was becoming comprehensive at similar or better
resolution.
Unfortunately, a gap of nearly 20 years separated the
Vikings from the next successful mission to Mars, but the
sophistication

documented the entire surface of Mars, revealing the pres-
ence of not only impact craters that had dominated the
terrains viewed by the first three fly-bys, but also the
presence of enormous volcanoes (some hundreds of kilo-
meters across), the largest canyon system in the solar sys-
tem (appropriately named Valles Marineris, 'valleys of the
Mariners'), evidence that a flowing liquid carved sinuous
valleys across many portions of the surface, possible evi-
dence of periglacial (ground ice) activity and, most
important to the topic of this topic, the first evidence of sand
dunes on another planet (Figs. 12.2 and 1.2 ).
The dust storm itself demonstrated that dust can be
blown around on Mars, and a number of streaks (see
Figs. 7.1 , and 13.3 ) were seen in the lee of craters that
Sagan et al. (1972) inferred to be wind-generated deposits
of dust or sand. However, the first actual dune fields were
located on the floors of craters, notably the Hellespontus
crater (Cutts and Smith 1973—see Fig. 1.2 ), now called
Proctor crater (Figs. 12.2 , 12.3 and 12.4 ). These first
observations set the longest time interval for dune migra-
tion/evolution studies on Mars (see Chap. 9 ) . Proctor has
been re-observed many times since, e.g., Figs. 12.5 , 12.6 ).
The stunning success of Mariner 9—and the evidence of
past liquids—was strong validation of the NASA decision
to send an armada of spacecraft to Mars, with the goal of

of the spacecraft missions then steadily

increased:
• the Pathfinder landing (on July 4, 1997) included the first
spacecraft (Sojourner) to roam across the surface of

another planet;