Mars Dunes - Dune Worlds: How Windblown Sand Shapes Planetary Landscapes - page 132

Fig. 12.7 Mars draws its own
wind roses. Dark dust plumes
indicate the prevailing winds as
the underlying surface begins to
defrost. Such features are seen
widely in the Olympia Undae
dunes and elsewhere. Defrosting
dune surfaces are shown in
Fig. 12.18

Earth, yet at those low abundances the thin atmosphere
holds nearly all the water vapor that it can (that is, the
Martian atmosphere is almost saturated with water vapor,
making the relative humidity near 100 %, even though the
total abundance remains incredibly small). Faint clouds of
water ice sometimes form, and water ice is present in the
near-subsurface (as dramatically shown by the Phoenix
lander—see Fig. 16.16 )—an important effect of the migra-
tion of water vapor is its possible deposition in the sub-
surface, indurating dunes and freezing them in place.
Polar temperatures during winter in both hemispheres get
low enough to condense carbon dioxide to form seasonal
polar caps on Mars (visible telescopically from Earth). The
Viking landers measured variations in atmospheric pressure
of *30 % through a Martian year, indicating that almost
one-third of the atmosphere freezes out at each pole in turn,
only to be released again as the seasonal caps later shrink
and eventually disappear completely. The deposition of
CO 2 frost on polar dunes is evident when the dunes are
observed closely from orbit—avalanches of dark sand are
visible against the bright frost (and in fact sand and dust are
sometimes sprayed out into ephemeral dark spidery patterns
when CO 2 gas builds up under the frost cover in spring and
escapes suddenly through cracks in it; see Fig. 12.7 ).

The low pressure of the Martian atmosphere causes it to
respond quickly to temperature variations that take place
throughout the day; several landers have documented day-
night atmospheric temperature swings of up to 100 C. One
consequence of such dramatic temperature changes is that
strong gradients can be produced quickly in the lower
atmosphere, driven by rapid changes in the temperature of
the Martian surface as the solar insolation varies through the
course of a day—these gradients can drive strong slope
winds. Another consequence of the low density is that the
atmospheric boundary layer (see Chap. 3 ) can grow to be
many kilometers thick. Thus Mars dunes, in many cases
larger than Earth's, may be limited in size by how quickly
they can grow rather than by how large they may ultimately
grow to, capped by the boundary layer.
Insolation varies through the Martian year as well as
through the day; the orbit of Mars is five times more
elliptical than that of the Earth, causing large seasonal
variations in the energy derived from sunlight (perihelion
presently occurs during summer in the southern hemisphere
of Mars, at which time the planet receives 45 % more solar
energy per unit area than when the planet is at aphelion).
This arrangement of the seasons changes dramatically over
astronomical time, see below.