Environmental Engineering Reference
In-Depth Information
of continents and mobilization of needed plant nu-
trients.
The Earth's uniqueness goes deep below its surface.
The planet's lithosphere is constantly recycled into
the underlying mantle as the incessant motions of giant
tectonic plates create new ocean floor, form mountain
belts, and generate earthquakes, volcanic eruptions, and
tsunamis.
The uniqueness of these tectonic arrangements results
in a giant material cycle: an internal heat engine in the
mantle extrudes magma to form laterally rigid plates that
are pushed away from mid-ocean ridges, and the floor
is eventually subducted back into the mantle in order to
be reprocessed into new magma (Eiler 2003). Without
these tectonic processes there would be no re-creation
of continents and oceans, and the Earth's surface would
be flattened by erosion. Such an undifferentiated world
would not have been conducive to evolutionary diversifi-
cation of life. Accounts of planetary energy balances and
of thermal and kinetic energy flows in the atmosphere,
hydrosphere, and lithosphere could thus be seen as set-
ting the grand stage for all subsequent inquiries into the
energetics of life and civilization.
rests on photosynthetic conversion of solar radiation to
phytomass while precipitation, ice, and wind—all just
thermal and kinetic transformations of solar radiation
that was absorbed by the atmosphere, land, and waters—
keep reshaping the planetary surfaces and are the key
determinants of the food-producing potential of every
civilization.
The source of this radiation, the Sun, is one of roughly
100 billion radiant bodies in our galaxy, a star located in
one of the spiral arms of the Milky Way galaxy, about
33,000 light-years (ly) from its center and near the galac-
tic plane (Phillips 1992). The Sun's closest neighbors are
the binary Alpha Centauri (4.3 ly distant) and Epsilon
Eridani (10.8 ly distant), and it is visible with naked eye
from a distance of 20 parsecs, that is, about 620 Pm, or
roughly 4.1 million astronomical units (AU, the mean
distance between the Earth and the Sun). Its size (class
V) makes it a dwarf star (like Sirius or Vega); its spectrum
is G2, yellow stars with characteristic lines of ionized cal-
cium and metals. Its mass is 1
:
991
10
33
g(5OM
larger than that of Earth); the radius of its visible disk
spans 696.97 Mm (compared to Earth's @6.5 Mm); its
average density is 1.41 g/cm
3
; and its steady radiation
corresponds to a surface temperature of nearly 5800 K.
The Sun is almost perfectly in the middle of the
main sequence of the Hertzsprung-Russell diagram,
which plots the distribution of absolute visual magnitude
against the spectral class for stars of known distance (fig.
2.1). Hydrogen makes up about 91% of its huge mass,
helium all but about 0.1% of the rest; C, O, and N are
the most abundant elements among the minor constitu-
ents. All of the Sun's energy is produced within the
innermost quarter of its radius, which encloses a mere
1.6% of its volume, and most of it (nearly 80%) comes
from the proton-proton (p-p) cycle, the net result of
2.1 Sun: The Star and Its Radiation
Not everything on the Earth is energized by solar radia-
tion: it does not keep the planet on its orbital path, it
does not drive the plate tectonics, the process that con-
stantly reshapes oceans and continents (see section 2.5),
and it does not power the metabolism of chemoautotro-
phic bacteria, most notably those that live in total dark-
ness at the bottom of deep ocean near hot vents, where
they oxidize H
2
S and support many larger organisms,
including white clams, crabs, and giant tube worms.
But chemoautotrophs aside, the whole pyramid of life
