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

tideraisingforcesasr--_ where ,v = 3 -(2/3.5). We can
therefore reduce r until (r/ro) --a = k or
and
¥
ro
M
Mo
- k-l/ct = k -7/17 (7)
- k -2/3'5Ot = k -4/17 (8)
Taking k = 2.4 we find r/r o = 0.7 andM/M o = 0.81. This
corresponds to being at the orbit of Venus around a KO
star.
So long as we restrict ourselves to the same pattern of
seas, continents, and basins, it is reasonable to suppose
that tidal drag varies as the square of the tidal
amplitude-that is, as k 2 . However, differing continental
configurations and coastal features can radically affect
the total drag. There is evidence that the tidal friction on
the Earth has varied widely during its history. If
Gerstenkorn's theory (refs. 12, 13) of the evolution of
the Moon is correct, the Earth underwent a period of
enormous tides on the order of 10 9 years ago and was
slowed greatly by the Moon at that time. Without the
Moon, the Earth might well have been able to have
endured solar tides two or four times as great as the
present total tide and still have the present length of
day. Also, a planet with less water and perhaps isolated
seas would experience far less tidal drag.
If we let k = 5 we find r/ro = 0.51 and M/M o = 0.68,
which corresponds to a body 0.5 AU from a K5 star;
while if we let k = 10 we find r/ro = 0.387 and M/M o =
0.58, or a body in the orbit of Mercury around a K7
star. We concede that, as tidal forces increase, we are
restricting more and more the planetary configuration
that can survive. But we see no reasons to drop the
probability of surviving tidal drag to zero at K2 as Dole
has done.
Some current theories predict that the UV and X-ray
flux of a star, which depend on coronal activity, may
actually increase as we go from G to K to M stars. The
effect of this on atmospheric evolution may well place a
more stringent lower limit on likely main sequence stars
than tidal braking. For the present, we feel that all F, G,
and K main sequence stars should be included in a
search, with G stars given priority.
THE ORIGIN OF LIFE
It is now believed that chemical evolution leading to
life began on the Earth between four and four and a half
billion years ago. At that time the atmosphere was
probably composed primarily of a mixture of hydrogen,
nitrogen, carbon dioxide, methane, ammonia, and water
vapor. Ultraviolet radiation from the Sun was able to
penetrate the Earth's atmosphere because ozone had not
yet been formed in its upper layers. The Earth's crust
was undergoing early differentiation. Earthquakes and
volcanic activity were probably intense and frequent.
The primitive oceans had formed, and contained, in
contrast to the oceans of today, only small quantities of
dissolved salts. Most scientists believe that life began in
the sea.
A major requirement for the development of living
systems is the presence of organic molecules of some
complexity. Until recently it was thought that these
molecules could be produced only by the activity of
living systems, and there was no satisfactory explanation
of where the living systems came from in the first place.
In 1938, the Russian biochemist Oparin (ref. 14) put
forward his theory of chemical evolution, which proposed
that organic compounds could be produced from
simple inorganic molecules and that life probably originated
by this process. In 1953, Miller (ref. 15) working
in Urey's laboratory, showed that organic molecules
could indeed be produced by irradiating a mixture of
hydrogen, ammonia, methane, and water vapor. Chemists
began to experiment with all the different forms of
energy thought to have been present early in the Earth's
history. Ponnamperuma and Gabel (ref. 16) soon
showed that ultraviolet radiation, heat, electric discharges,
and bombardment by high energy particles also
worked. Recently sonic cavitation produced in water by
wave action has been shown by Anbar (ref. 17) to be
effective in producing organic molecules.
A number of the organic compounds produced in this
way are identical with those found in the complex
biochemical structures of present-day organisms. The
first compounds synthesized in the laboratory by Miller
were amino acids, necessary for the formation of
proteins.
Later experiments, including those of Ponnamperuma,
have produced sugar molecules. Specifically, he
isolated among others the two sugars with five carbon
atoms: ribose and deoxyribose. These are essential
components of the deoxyribonucleic acid (DNA) and
ribonucleic acid (RNA) molecules, which carry the
genetic code of all Earth-based life. The purine and
pyrimidine bases, whose locations in the DNA molecule
represent the information carried by the gene, were also
synthesized in the same simple apparatus. Finally,
Hodgson has recently shown (ref. 18) that porphyrin
compounds can be isolated. These substances are an
18