Environmental Engineering Reference
In-Depth Information
F
¼
sT
4
;
the first 2 or perhaps even 3 Ga of the Earth's evolution,
yet the oldest sedimentary rocks can be reliably dated to
as far back as 3.8 Ga before the present time. Enhanced
greenhouse gas effect is the best solution of this paradox,
and Sagan and Mullen (1972) proposed that this was
achieved by about 10 ppm of atmospheric ammonia.
This assumption was questioned owing to the short life-
time of NH
3
, caused by photochemical decomposition,
and Sagan and Chyba (1997) suggested that UV absorp-
tion by high-altitude organic solids produced from CH
4
photolysis may have shielded ammonia from such de-
composition. But higher atmospheric CO
2
levels (equili-
brated by feedbacks within the carbonate-silicate cycle
and later amplified by biota) provide a much more likely
explanation (Kasting and Grinspoon 1991).
After some 5 Ga of radiation, hydrogen will have been
depleted from 75% to 35% of the Sun's core mass. As the
core continues to contract, the star's radiation will in-
crease, and its diameter will expand. Eventually the heat
will evaporate the oceans as the lifeless Earth continues
orbiting the brightening Sun. Then, after some 5 Ga, the
yellow star will leave the main sequence and be trans-
formed into a red giant, whose diameter will eventually
be 100 times larger than it is now (the star's photosphere
will reach beyond the orbit of Mars) and whose luminos-
ity will be about 1,000 times greater than today's. Fi-
nally, after it has swallowed the four inner planets, the
Sun will become a brilliant, Earth-size, white dwarf. But
it is the Sun's remarkably stable past, due in large part to
the star's fortuitous but fortunate galactic location, that
has made life possible.
The Sun's orbit around the galactic center is less ellip-
tical than those of similar nearby stars, and this prevents
the solar system's passage through the inner galaxy with
its many destructive supernovas. A small inclination of the
with s (Stefan-Boltzman constant) equal to 5
:
67
10
8
W/m
2
/K
4
. The Sun's isotropic radiation thus produces
an average flux of 63.2 MW/m
2
of the photosphere's
surface, corresponding to the effective temperature of
5778 K. Total luminosity is the product of the star's sur-
face and total flux:
F
¼
4pr
2
sT
4
:
The Sun's immense energy flux of 3
:
85
10
26
W
is transported outward through the radiation zone
(@45% of the star's radius) and then through the outer-
most convection zone to the photosphere. Temperature
declines from 10
7
K in the core to 10
6
K in the radia-
tive zone and 10
4
K in the outer layer of the convection
zone. The thin (@200 km), finely granulated, and opa-
que photosphere radiates the bulk of both visible and
infrared wavelengths (fig. 2.1). The photosphere has nu-
merous sunspots, regions of reduced temperature that
persist for hours to weeks and whose frequency is subject
to an 11-year cycle during which their locations converge
toward the solar equator (Radick 1991). The chromo-
sphere separates the photosphere from an extremely hot
corona (@2 MK, and
a
29 MK in flares), which emits
X-rays (1-10 nm) and extreme (10-100 nm) ultraviolet
(UV) radiation. Coronal flares, marked by terrestrial
magnetic disturbances and high-latitude auroral displays,
follow the 11-year cycle.
Because the conversion of H to He increases Sun's
density, and hence the star's core temperature and its
rate of thermonuclear reactions, luminosity of the young
Sun was about 30% lower than the present rate. This
faint young Sun would not have allowed liquid water for
