Mars Dunes - Dune Worlds: How Windblown Sand Shapes Planetary Landscapes - page 133

Fig. 12.8 Dust streaks in the lee of craters (e.g., Fig. 8.2 ) provide another set of indications of Mars winds. This formidable compilation is
courtesy of Peter Thomas of Cornell University

Both daily (diurnal) and yearly (seasonal) variations of
several factors affect the winds on Mars, which essentially
represent the global mechanism for redistributing sensible
heat across the planet; it is the transfer of warm air from near
the equator toward the poles, along with the transfer of cold
air from the poles toward the equator, that drives global
winds. Global Circulation Models (GCMs) are mathematical
representations of this heat transfer process for a planet (see
Chap. 19 ) . GCMs indicate that the intensity and direction of
global winds change through the course of a Martian year,
and these patterns can be greatly affected by the strength and
duration of seasonal dust storm activity (Fig. 12.8 ).
Recently, GCMs have been used to investigate how the
Martian atmosphere and surface are influenced by changes
in the orbital parameters induced by the gravitational effect
of the other planets (primarily Jupiter and Saturn). The lack
of a large moon at Mars allows such perturbations to alter
greatly the conditions on the planet, primarily with regard to
the tilt of the rotational axis relative to the orbital plane
(called obliquity). While the obliquity of Earth ranges by
about one degree around the present value of 23.5, the
obliquity of Mars can range from zero to near 80, large
deviations from the present value of 25, which along with
variations in orbital conditions (such as eccentricity) can
cause enormous changes to the overall climate of the planet,
all of which can affect wind patterns and their intensities
(and thus dune locations, morphologies and orientations) as
well as the stable distribution of ground ice (which may
therefore cause dunes to be immobilized). For example,

when the obliquity exceeds about 45, the polar regions will
on average over the year receive more sunlight than the
equator!

12.3 Sediments on Mars

Mechanical weathering is the primary mode for the gener-
ation of sand-sized particles on Mars, mainly because of the
dearth of water under present Martian conditions. The large
temperature swings experienced daily and seasonally will
act (albeit slowly) to break apart rocks exposed at the sur-
face, perhaps enhanced by intermittent films of water
molecules when conditions allow localized condensation of
the trace amount of water vapor present in the atmosphere.
Impact craters also provide an intense but very localized
means to break up near-surface rocks into smaller constit-
uent pieces. Still, when billions of years are available to
carry out the mechanical weathering, it is not surprising that
collections of sand-sized material can be found across much
of Mars. When winds of sufficient intensity occur, the sand-
sized material eventually tends to concentrate into proto-
dunes or dune fields, usually within topographic depressions
that act to trap the particles. The Viking, Pathfinder, and
Phoenix landers, along with both MERs, all returned evi-
dence of fine-grained material wherever these explorers
looked (see Chap. 16 ), although only the last three space-
craft carried cameras that could resolve sand from the silt to
clay-sized dust particles that settle ubiquitously out of the