Einsteinhoax wrote:
The Scarcity of Life Bearing Planets
[snip]
Due to its location, Venus receives about twice the heat input
from the
Sun as does the Earth. Its gravitational mass is slightly less than
that of
the Earth and yet it has an atmosphere about 70 times as dense as the
Earth.
In addition, the atmosphere of Venus is alleged to consist of mostly
carbon
dioxide. Since, under the evaporation process, the other normal
atmospheric
gases, having a lower molecular weight, will evaporate before carbon
dioxide
does, the initial Venusian atmosphere must have been significantly
denser
than it is now.
The Earth, on the other hand, has an atmosphere that contains a
negligible quantity of carbon dioxide but is relatively rich in the
lighter
gases. In addition, it is estimated that about 3 billion years ago
the
atmospheric pressure on the Earth was about 20 PSI and has been
reduced to
its current level of 14.7 psi. This means that, for the Earth, 25% of
the
atmosphere has been lost in 3 billion years, probably by a net
evaporation
to space. (Any gas or vapor subject to a vacuum will evaporate, an
atmosphere is no exception.)
Unless the gravitational force of the object is sufficient to keep the
atmosphere together. Jupiter, for instance, is so massive that it
unlikely that its atmosphere has been reduced substantially by solar
evaporation effects.
It seems reasonable to accept that the early history of the
Solar
System approximated the following stages:
The planets were formed by the collision of smaller objects
circling
the Sun in eccentric orbits. The collision process continued until
the Solar
System was virtually cleared of objects in non-circular orbits.
This is generally not consistent with modern stellar evolution models.
Most tend to favour large circumstellar disks. In a collision model,
there is no reason that all of the planets should remain mostly in the
plane of the ecliptic, as we see with our solar system. This model is
also consistent with observations of protostellar objects, even though
we generally don't have the telescope resolution to see planets.
During the planetary formation stage, the planets could not
acquire
atmospheres because the bombardment that was forming them made their
surfaces extremely hot. Atmospheric gases which impacted the planet
from
interplanetary space or from the accreting object might be expected
to boil
away quite rapidly, particularly since they were being added to the
surface
of the planets.
Basically the heaviest chunks of the disks started to conglomerate
through gravitational effects, from the heaviest elements to the
lightest. Most of the molecules were ultimately bound gravitationally,
electrostatically, or magnetically to the disk. Otherwise everything
would just have been blown away. Once the objects were massive enough,
they would be able to start collecting atmospheres, or collecting the
remainder of the disk through other chemical processes.
Once the rate of bombardment forming the planets reduced to the
point
where the planets could cool sufficiently, they were capable of
collecting
atmospheres from gases that remained in the Solar System. (There are
arguments that planetary atmospheres were formed by outgassing. The
writer
doubts this was a major source of atmosphere, but whether it was or
not does
not affect the conclusions.
For Venus and the gas giants to have their present atmospheric
density,
all of the planets, including the Earth, must have initially acquired
enormous (by Earth standards) atmospheres. They gained their
atmospheres by
sweeping up gases from the surrounding interplanetary space (and
possibly by
outgassing) and lost some of that atmosphere by evaporation to that
same
space from the uppermost layer of the atmosphere. In order for a
molecule of
gas to be lost to the planet, it must acquire a thermal velocity
greater
than the planet's escape velocity. This must occur at an altitude at
which
the atmosphere is sufficiently thin so that it does not strike other
molecules while escaping. (This occurs above the altitude where the
effects
of diffusion are significant.) The rate at which atmospheric gases
are lost
to space is determined almost entirely by the rate of energy input
from the
Sun and by the escape velocity of the planet at the top of its
atmosphere
The rate of atmosphere loss is virtually independent of the amount of
atmosphere the planet owns at any instant of time.
Venus' atmospheric conditions are generally attributed to a runaway
greenhouse effect rather than evaporation. Basically, here is the
situation: suppose Earth and Venus atmospheres are initially formed
from the same materials: CO2(g), H2O(g), N2(g) primarily. On Earth, we
end up with large amounts of liquid water. Much of CO2(g) then converts
to CO2(aq), and subsequently undergoes various chemical processes,
mainly resulting in CaC03(s). Because of the presence of liquid water,
Earth ultimately also develops life, that has a net reaction of CO2(g)
-> O2(g). The net result of all of this is that Earth loses most of the
CO2 from its atmosphere.
As you pointed out, Venus receives a great deal more solar input than
Earth, and as a result, had very little liquid water to begin with;
most was either in gaseous form, or was converted to H2 + O2. In this
latter case, the H2 was able to escape, and the O2 reacted with other
molecules (Fe, for instance). The CO2 is unable to escape from the
planet's gravitation even as it heats up. The net result is that Venus
loses virtually none of its atmosphere other than H2.
The Earth-Moon system has two characteristics that are anomalous
compared to the other planets. The first is that it has far too much
angular
momentum (orbital angular momentum, rotational angular momentum of
the Earth
and the Moon, and orbital angular momentum of the Moon around the
Earth). As
pointed out in a text by Dr. Urey, an exponential plot of angular
momentum
vs. total mass for all of the other planets yields a straight line.
The
total angular momentum of the Earth-Moon system lies far above that
line.
The second anomaly is that it contains far too little atmosphere and,
unlike
Mars, the density of that atmosphere has remained almost unchanged. A
satisfactory explanation for both of these anomalies seems to have
been
provided in the 80's by a computer simulation of a glancing impact on
the
Earth by an object having a mass about one sixth of its mass and
which
yields a conclusion for the formation of the Earth-Moon system which
seems
to be currently accepted. The simulation predicted the formation of a
binary
system with a Moon sized object orbiting the Earth an altitude of
about
12,000 miles, with the Earth having a 4 hour day, and with the Earth
having
captured the iron cores of both objects. Since the length of the
Earth's day
was, is, and will remain less than the Moon's orbital period until
the Sun
enters its red giant stage, tidal effects on the Earth will
perpetually
transfer angular momentum from the Earth to the Moon. This transfer
has
lengthened the Earth's day to 24 hours and has caused the Moon's
orbit to
increase to 238,000 miles. More important, such an impact would have
blasted
away most if not all of the atmosphere the Earth had at the time and,
if the
collision occurred late enough in the formation period of the Solar
System,
most of the interplanetary gases would have already been absorbed by
the
other planets and/or lost to interstellar space and not be available
to
reform much of an atmosphere on the Earth. This scenario could easily
allow
the Earth to have the comparatively puny but stable atmosphere
required to
support the evolution of intelligent life.
I think the atmospheric effect I described above may be sufficient to
get the atmosphere to the point where like would be workable. Arguably,
however, there is no reason why a planet with a more significant
atmosphere than Earth couldn't support life, provided that it can
support liquid water.
In order for a planet to support life, not only must it be in
the "life
zone" about a suitable star, it must possess an atmosphere of a
suitable
density for a sufficient period of time for life to evolve. On the
Earth,
life does not seem to prosper above an altitude where the density is
half an
atmosphere. At the other end of the scale, the atmosphere must not be
too
thick or the wavelengths of radiation needed for photosynthesis not
only
will not reach the atmosphere-water interface where life begins, that
interface is likely to be too hot due to the temperature rise of
adiabatic
compression. (This temperature rise is the reason the surface of
Venus is so
hot). Making the optimistic assumption that four and a half
atmospheres is
the highest suitable atmospheric pressure requires that a life
supporting
planet not lose more than four atmospheres of density in the period
required
for intelligent life to evolve. For a planet starting with the
atmospheric
density of Venus to lose 60 PSI of surface atmospheric pressure in 3
billion
years (the time required for intelligent life to have evolved on
Earth), the
existence of such life would require an age of 50 billion years for
the
planetary system. Such a conclusion presents problems. A star similar
to our
Sun will become a red giant about 10 billion years after its
formation and
the apparent age of the Universe is only 15 billion years. On the
other
hand, if a planet such as Mars lost its atmosphere at a sufficient
rate to
reach compatibility with the requirements of life before its star
became a
red giant, it would pass though the "life range" so quickly that
intelligent
life would probably not have had time to evolve. It is the author's
belief
that, without the addition of the 'wild card' implicit in the
postulated
Earth-Moon collision, a planet capable of supporting life cannot
exist. (It
is hoped that this question would be examined further.) It is the
author's
belief that intelligent life is much rarer in the Universe than Dr.
Sagan
suggested.
You will recall that there is life on Earth that survives in conditions
much more harsh than a mere 4 atmospheres of pressure. While arguably
there is little chance of intelligent life evolving under such
circumstances, were are aware of strains of bacteria that live in
volcanic vents with extreme temperatures that use unconventional means
of gaining energy, and certainly the majority of marine life can
survive pressures in excess of 4 atmospheres.
The asteroid belt exists as a ring of stony and iron rocks in
orbit
about the Sun between the orbits of Mars and Jupiter. The radius of
that
orbit coincides with the anticipated location of a planet under the
conventional theory of planetary formation. If one examines the
objects in
the asteroid belt, the moons of Mars, and the meteorites that strike
the
Earth, one finds that, unlike comets, many if not most of them
composed of
stone or of iron. Unlike the flimsy comets, such objects cannot form
by
accretion, they can only be formed within a planet-sized object that
has
already accreted. One must conclude, therefore, that initially a
planet did
form at the radius of the asteroid belt and was later shattered by a
collision. Such a collision would drive away most of the planetary
material
and leave a residue of rocks from the planet's upper layers and iron
objects
from the planet's core. That collision is a reasonable candidate as
the
source for the object that impacted the early Earth to form the
Earth-Moon
system, the meteorites which strike the Earth, and the moons of Mars.
The more prevalent theory is that the asteroid belt is remains from the
protostellar disk that was not able to form a planet because there was
not enough mass (asteroids consistute less than 1/4 the mass of the
Moon). It is highly unlikely that after the *glancing* collision with
the Earth you suggest, that the pieces of this object would remain in
the ecliptic. Furthermore, it is highly unlikely that after this
collision these objects would coalesce in the area between Mars and
Jupiter, rather than, for instance, the area between Earth and Mars.
Furthermore, assuming these objects did not have the energy to escape
the Sun's gravity, and were not captured by something else's gravity,
then energy considerations indicate that if these objects are to orbit
the Sun, they MUST pass through the collision point with the Earth.
The writer is of the strong opinion that, unless a planet that
is
located around its star and sized to be suitable for the retention of
an
atmosphere, undergoes such a history at the appropriate time in the
planetary system process a planet suitable for the evolution of
intelligent
life cannot evolve. It would seem, therefore, that in addition to the
probability factors now considered for the existence of life bearing
planets
that yield the possibility of perhaps 100 civilizations within our
galaxy,
an additional factor must be considered for each candidate planetary
system.
This factor is the probability of AN EVENT occurring at the right
time in
the planetary formation process to drive off the excess atmosphere
from a
planet that is large enough to retain a stable atmosphere. When added
to the
already tabulated probabilities assumed for the SETI observations, it
seems
quite probable that instead of a civilization occurring about 100
times in a
galaxy as is currently hoped, civilization would occur once in a
hundred or
a thousand galaxies. If this were the case, the SETI project would
seem to
be doomed to failure.
Incidentally, in spite of my arguments, I generally do agree with your
conclusions, if for different reasons. I would also contend, that
regardless of whether or not there are other civilizations out there
that are sending out signals we could detect, if the speed of
information that we can receive is limited to the speed of light, then
it offers only a small window of time for which we could encounter
other civilizations. If SETI has been broadcasting for 50 years, that
means that our earliest signals have only been able to travel a mere 50
lightyears away--that is, only the very closest stars would actually be
able to receive it.
A.
.
