Geoscience Reference
In-Depth Information
1 Planet Earth and Earth
systems
1.1
Comparative planetology
1.1.1
Lateral thinking from general principles
discovered, by judicious use of experiment, field
observation, and theory, the immutable physical laws that
governed the transport of sand and silt particles in the
Earth's atmosphere, especially in the concentrated layers
close to the ground surface during sandstorms. NASA
asked Bagnold advice on how to modify his earthbound
physical laws for application to the Red Planet. Bagnold
and his collaborator C. Sagan had to take due account of
the Martian atmosphere, surface, and rock properties,
such as were then known: they had to find accurate values
for gravitational acceleration, air density, rock density, and
surface wind velocity. Then they had to calculate the likely
extent and severity of sand blasting, dust transport, and
possible effects on the landers. The results are of contin-
ued interest in view of plans to land humans on Mars early
this millennium.
Physical processes on Earth and other planets must obey
the same basic physical laws, depending in detail on the
nature of the particular planetary environment, for example
physical composition and gravity. While this topic is obviously
concerned with Earth processes, it would be narrow-
minded of us not to pause for a moment right at the start
and make some comparisons between Earth and our three
nearest neighbor rocky planets. This turns out to be the
beginning of a stimulating intellectual and practical exer-
cise. Why so? An anecdote will help explain our point.
In the early 1970s, the desert explorer, soldier, and
hydraulics engineer R. A. Bagnold, who helped create the
scientific discipline of loose-boundary hydraulics , was con-
tacted by NASA to undertake consultancy regarding
ongoing orbital and future lander missions to Mars. The
background to this strange request from the world's most
prestigious space outfit to a retired brigadier of engineers
was that NASA scientists had been appalled and intrigued
by the enormous planetary dust storm that covered the
planet for the first 2 months of the Mariner 9 mission.
Although the storms died down in late January 1972,
revealing a fabulous dune-covered landscape, like the
Sahara in places, NASA wondered if the planetary winds
were so severe that ground conditions would be inimical
to survival of the planned lander mission. This was an espe-
cial worry in the face of the failure of contemporary
Russian Mars 3 orbiter and lander missions: the latter had
arrived in the middle of sandstorms and never transmitted
more than a few seconds of data back to Earth.
Bagnold's work was the key here. Working from first
physical principles and making use of breakthroughs in
fluid dynamics achieved in the 1920s and 1930s, he had
1.1.2
Earth in context
How to characterize a planet (Fig. 1.1)? There are intrinsic
properties of solid size (diameter d ) and mass ( m ) from
which we can compute mean planetary density (
p ) and
gravitational acceleration ( g ). Then the nature of any
atmospheric envelope, its surface pressure ( p s ), and tem-
perature ( t s ). Also its mass, composition, and thickness.
Astronomical information includes distance from the Sun,
rate of planetary spin (length of day L d ), rate of revolution
about the Sun (length of year L y ), and inclination of the
equator with respect to orbit ( I e ). The regularity and
eccentricity of the orbit are of additional interest. We wish
to know the mean chemical compositions of the solid
and gaseous components and whether the planet has inter-
nal layering that might separate distinctive functioning
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