Winds and Atmospheres - Dune Worlds: How Windblown Sand Shapes Planetary Landscapes - page 25

Table 3.1 Properties of planetary atmospheres

Body Venus Earth Mars Titan

Planetary radius (km) 6052 6370 4470 2575

Gravitational acceleration (m/s 2 )

8.9 9.8 3.7 1.35

Rotation period (sidereal day) (days) 223 1 1.04 16

Diurnal period (solar day) (days) 119 1 1.04 16

Surface atmospheric pressure (bar) *90 1 0.006 1.46

Surface atmospheric temperature (K) 740 283 200 94

Dominant gas species CO 2 N 2 ,O 2 CO 2 N 2 ,CH 4

Atmospheric density (kg/m 3 )

64 1.25 0.02 5.4

Dynamic viscosity (10 -6

Pa-s) 35 17 13 6

Planetary boundary layer (km) 0.2 ? 0.3-3 [10 2-3

density (i.e., the total atmospheric mass) remains constant,
as it should—the atmosphere just puffs up.
It is important to note that on Mars in particular the
atmospheric parameters are not global constants. The
topographic range on Mars is large compared with the scale
height, such that the pressure and, more importantly, density
of the atmosphere can vary significantly. A notable example
is that on the Tharsis volcanos, moving from the base where
the pressure is about 6mb to the peak around 22 km, the
pressure and density drop by almost a factor of 6, which is
reflected in the size of the bedforms observed there (Lorenz
et al. 2014).
Atmospheric pressure on planets may change through
geologic time, depending on how volatiles are delivered to
and lost from the planet (although variations on Earth have
probably been quite small). Variations may be dramatic if
the dominant atmospheric component can condense, as is
the case at Mars. Indeed, over the course of a Martian year,
the atmospheric pressure varies by about 30 % as part of the
thin atmosphere freezes as a seasonal polar cap of CO 2
frost. The Martian atmosphere may well have been a factor
of several times thicker in the deep past than at present.
The other static property of a gas that is of interest is
viscosity (l), which is a measure of how well momentum
diffuses through a fluid. This is a function both of compo-
sition and temperature. The property is the ratio of shear
stress to strain rate (or velocity gradient) and has SI units
of Pa-s. Confusingly, there is also a unit named the Poise
(P, with 0.1 Pa-s = 100,000 lPa-s = 1P= 100 cP).
Further confusion arises because in some fields it is com-
mon to refer to a 'kinematic viscosity', which is just
dynamic viscosity divided by density and has units of its
own; generally, if unspecified then dynamic viscosity is
meant, but to be safe one should state explicitly which is
referred to. In air, l = 18 uPa-s, whereas on Titan (also
mostly nitrogen, but rather colder) the viscosity is almost
three times lower. On the other hand, carbon dioxide has a

slightly higher viscosity than nitrogen. Liquid water has a
much higher viscosity, 900 lPa-s, and liquid hydrocarbons
on Titan may be a factor of 2 lower or higher than that,
depending on composition.
The viscosity is a property that affects how thick a layer
of fluid is slowed down as it flows across or around a body.
We will discuss its implications, particularly via the Rey-
nolds' number (the ratio of viscous to inertial forces in a
fluid) in Chap. 4 .
A final property that occasionally is important is the
mean free path, the distance an air molecule may statisti-
cally travel before it hits another air molecule. Generally,
this distance is very short (in sea level air on Earth it is
about 70 nm, much smaller than any object we are con-
sidering). The distance increases at lower pressures (den-
sities), and can be several microns at Mars' surface
conditions, which is comparable with the size of fine dust
particles.

3.2 The Planetary Boundary Layer

A distinct feature in the vertical structure of a world's
atmosphere is the planetary boundary layer (PBL). One
must avoid confusion between this (a meteorological
structure typically 300 m-10 km deep) and the friction
layer (which aerodynamicists might term a boundary layer)
of a few centimeters to a few meters near the surface where
the windspeed falls: the friction layer is discussed in Sect.
4.6 on the mechanics of sand. The PBL is the layer which is
well-mixed, and so has a uniform profile of certain ther-
modynamic properties, notably the potential temperature.
The PBL typically grows in thickness throughout the day, as
solar heating stirs the air near the surface.
The PBL is of profound importance for dune construc-
tion, in that the top of the PBL acts as a cap; it is ener-
getically demanding to push the PBL upwards. Thus, in