Project Cyclops, A Design... - Department of Earth and Planetary ...

intooceans, andwiththesmallerUVandX-rayfluxat 1
AU,theearlytectonicatmosphere of theEarthwould
havebeenstable.However, evenEarthcouldnothave
engendered life,givenasmuchCO2buildupasoccurred on
Venus.Mostof thecarbonin theEarth'scrustislocked
upin limestone. Calciumandmagnesium silicatesreact
with CO2in waterto form silicaandcalciumor
magnesium carbonates. OnVenus,thelackof oceans
prevented thisreaction.
A greatdealoflimestone aswellasallcoalandoil are
the fossil remnants of early life. It may well be that
life appeared quite early and, through photosynthesis,
started removing CO2 from the atmosphere before
tectonic activity had released anywhere near the amount
that has accumulated on Venus. We must remember that
volcanic activity is due to heat produced by the decay of
long-lived istopes and, while it may have been greater
initially, it has continued for billions of years. Vegetation
and plankton are responsible for the extremely
small percentage of COz in the present
atmosphere despite all our burning of fossil fuels. Living
systems have certainly assisted in the CO2 removal and
may have been the decisive factor. If so, then life may be
what saved Earth from the heat death of Venus, and is as
much responsible for our fleecy skies and blue seas as it
is beholden to them. Let us hope this symbiosis will
continue!
ECOSPHERES AND GOOD SUNS
The ecosphere is the region surrounding a star within
which planetary conditions can be right to support life.
Too close to the star, any planet will be too hot; too far
away, any planet will be too cold. We do not know
precisely the limits of insolation that define the ecosphere.
Dole, in his study of the probability of planets
being habitable by man (ref. 4), choses the limits at 65
percent and 135 percent of the solar flux on Earth on
the basis that 10 percent of the surface would then be
habitable. If we consider the adaptability of life and do
not require man to survive the environment these limits
may be pessimistic. On the other hand, the factors
responsible for appropriate atmospheric evolution may
be more limiting than the temperature that would later
exist on an Earthlike planet at various distances.
We can say that for an Earth-size planet, the inner
ecosphere limit in the solar system lies somewhere
between Earth and Venus. Rasool feels that the limit is
not closer to the Sun than 0.9 AU, corresponding to ! 23
percent of Earth's insolation. On the other hand, Mars,
at i.5 AU and with 43 percent as much solar flux, is
probably barren, not because of too little sunlight, but
because it is too small to have had enough tectonic
activity to give it much of an atmosphere. With a heavier
atmosphere than ours, the greenhouse effect could make
conditions on Mars quite livable.
The greater the ratio of the outer to inner radius of
ecosphere the greater the probability of finding one or
more planets within it. Dole has computed these
probabilities using the data on planetary spacings of the
solar system. Taking (rmax/rmin) = 1.5, his curves show
a 66 percent probability of one favorably situated planet
and a 5 to 6 percent probability of two. Since we do not
have statistics for other planetary systems, all we can do
is fall back on the assumption of mediocrity and say that
something over half of all planetary systems should have
at least one favorably situated planet.
As we consider stars of increasing luminosity, the
ecosphere moves out and widens. As Cameron observes,
if Bode's law holds generally-if the planetary spacing is
proportional to distance from the primary-this exactly
compensates for the widening of the ecosphere and gives
a constant probable number of planets per ecosphere,
regardless of the size of the primary star. Assuming that
the more luminous stars were able to dissipate their
nebulae to proportionately greater distances so that
terrestrial planets could evolve, the upper limit of
luminosity for a good sun is reached when the stellar
lifetime becomes too short. If intelligent life typically
requires the 4-1/2 billion years it has taken to evolve on
Earth we see from Figure I-7 that we should exclude all
stars hotter than F4 stars.
As we consider stars of decreasing luminosity, the
ecosphere shrinks and a new difficulty arises, if the tidal
braking becomes great enough to stop the planet's
rotation, the entire atmosphere will probably freeze out
on the dark side and all life will die. To estimate the
limit imposed by this effect, let us divest the Earth of its
moon and bring the Earth closer to the Sun, decreasing
the Sun's luminosity (and mass) to keep the insolation
constant, until the solar tide equals the present combined
solar and lunar tide. Since the solar tide is 46
percent of the lunar tide we can afford to increase the
solar tide by the
factor:
k =
x/! + (0.46) 2
- 2.4 (6)
0.46
without changing the tidal drag Earth has experienced.
The tide raising force is proportional to M/r 3, while the
insolation is proportional to M3"S/r 2. Thus if we keep
the insolation constant, M will vary as r2/3"s and the
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