From: cain@geomag.gly.fsu.edu (Joe Cain)
Subject: Summary of planetary evolution
Followup-To: alt.sci.planetary
Summary: elementary summary for comment
Organization: Florida State University Geology Dept.
Date: Tue, 29 Sep 92 14:38:01 GMT

I posted a note yesterday asking those interested in commenting on an
elementary outline of the evolution of our planetary system from the
big bang to the differentiation of planets send me a note asking for a
copy. I was not prepared for the massive response so have to post the
three pages below. It contains some improvements from the first dozen
or so who had comments. I will be happy to hear from others though
with such suggestions, though many have sent items which would be too
deep for this elementary group of some 80 non-scientists to be (unless we can
convert some).

                  The Universe, and everything 
            From the standpoint of planetary geology

-according to the hot big bang theory our universe began as a small
kernel of energy about 16 billion years ago (most agree on 15 to
20).

-the temperature and density dropped from the beginning and is
continuing to decrease as the universe expands. The more distant
the object, the faster it appears to be moving from us (about 70
km/sec per parsec distance. A parsec is about 200,000 AU, the same
as the number of arcseconds in a radian). Light from such moving
objects is lower in frequency and appears redder.

-no theory covers the time prior to about 10-43 sec. During the
first 1/100 of a second it was only filled with exotic particles
due to the extremely high temperature and pressure. After .01
seconds it was a soup of radiation and matter including the protons
and electrons that make up our universe today.

-the nuclei of hydrogen, helium, and a tiny amount of 7Li (lithium
isotope) were formed after the first 3 or 4 minutes. The theory has
about 12 atoms of hydrogen forming for one of helium, which leads
to a mass percentage of 75% H to 25% He, and matches that observed
in the universe. (The helium formed in stars over the next 15
billion years is only 1/200 of that formed initially).

-up until about 700,000 years after the bang the universe was
opaque because photons could not travel very far in such a high
density of electrons. Afterwards, its temperature had dropped to
about 3000 K and the electrons could combine with the nuclei to
make elements. The photons were then able to be released and travel
great distances. When we now look out into space we are actually
looking back into time. The "edge" of the universe we see is thus
the time when it became transparent. Its evidence is long
wavelength radiowave hiss which is the cosmic background radiation
whose temperature curve at 2.7 K matches that which you would
expect from radiation that had been red shifted from a distance of
some 16 billion light years.

-a gigayear (billion years) later the stars and galaxies began to
form with the young stars composed only of hydrogen and helium.
This coming together is caused by gravitational collapse in volumes
where it is cool and dense enough to begin. 

-nuclear fusion began to take place in stars creating heavier
elements: carbon, oxygen, ... up to the atomic number of about iron
for very massive stars. This process is possible in stars as the
pressure and temperature needed are very high, and each of these
fusion reactions give off energy, keeping the interior hot enough
to keep the star inflated against gravity.

-Having burned all possible fusion material in the last reaction
from silicon to iron, these massive stars go into a "refrigerator"
phase whereby the gravitational force is no longer balanced by the
pressure of the gases, and the star implodes very fast, perhaps up
to a third of the speed of light. The reason this occurs is that
the maximum atomic binding energy is found in iron. Fusion
reactions for elements with higher atomic numbers absorb energy.
(Fission of heavier atoms gives off energy as the heat being
produced in planets such as Earth from radioactive decay of
elements like 40K).

-Upon collapse, such stars can have several possible outcomes
depending on size. Those that are sufficiently large rebound with
a cataclysmic explosion blowing debris far into the surrounding
space as supernovas. Most important is the yet not well understood
processes whereby nucleosynthesis takes place during such an event.
The several possible processes allow atoms of lower atomic number
to be loaded with neutrons, and by various paths of decay, become
stable heavier elements. Even so, hydrogen and helium remain as 98%
of the visible mass.

-Not all is known, however. For example, there is a major unsolved
problem in astronomy in that galaxies behave as though there should
be about 10 times as much matter as is observed. The dilemma of the
presence of this unseen "dark matter" has not been solved.

-Over the past 10 gigayears this cycle been repeated several times.
Stars form, die or explode, and the universe has been spread with
the dust and debris of such processes. The largest stars burn out
the most quickly.

-about 5 gigayears ago gravitational collapse started in a nebula
we call the Solar Nebula. The composition of this nebula was about
the average observed with the greatest amount of Hydrogen and
Helium, but included grains and gas representing all other natural
elements and some simple compounds from the past recycling of
material. It is not clear whether this collapse was helped by shock
waves from a nearby supernovae or whether it just happened by
chance.

-When the Solar Nebula began to form

     -it began to rotate faster to conserve its angular momentum.
     (Most nebular material has some rotational momentum)
     - It began to flatten due to collisions of the particles 
     - and it began to heat up as the particles lost their
     gravitational potential energy

-the next stage of formation was that of the gas and dust
condensing to form grains of various chemicals and minerals
depending on the distance from the "protostar" which was to become
the Sun. The innermost regions were very hot (perhaps 10,000K)
with the temperature decreasing outward. The freezing point dropped
below 300 beyond the distance of the present asteroid belt. These
grains began to accrete by electrostatic and gravitational forces
to form planetesimals of increasingly large size.

-at some time the Sun's nuclear fuel ignited when its inner
temperature reached about 8 million degrees K with a density above
100 g/cm3. At some time it began to go though a violent "T-Tauri"
stage of rapidly varying intensity with a large magnetic field
which rapidly changed. This helped slow down the rapidly spinning
young Sun as the magnetic field was gripped by the now ionized
remnants of the nebula. 

-meanwhile, the planetesimals continued to grow into protoplanets
with the larger ones accumulating more material. These protoplanets
could have included gases surrounding even the inner members. Then
the T-Tauri stage also began to clear the Solar nebula and the
atmospheres of the inner planets of gases and fine dust. Beyond the
"freeze line" much of the abundant hydrogen and helium was
collected by what would become the giant planets. The rapidly
varying field also acted like a microwave oven and melted the
surfaces of some of the small bodies. 

-in the first 100 million years bodies of 100 km size were very
numerous, leading to large impact craters now visible on the inner
planets. The elements began to combine into various minerals and
compounds according to their abundance and temperatures. The
"Impact-Capture" model theorizes that during this early stage a
body about the size of Mars (NOT Mars!), hit the newly forming
Earth and peeled off much of its outer surface, and the debris
later accreted in Earth orbit to become our Moon.

-even while the major collisions and accretion continued, the inner
planets began to cool by radiating their heat into space. Initially
there may have been rapidly decaying radiogenics that heated these
bodies from below and helped differentiation or core formation to
proceed. The continued bombardment led to at least the surfaces
being molten.

-more heat was generated as the differentiation continued as the
denser iron (in the inner planets) sunk inward and converted its
gravitational potential energy to heat.

-the ultimate composition of the planets was thus one whereby the
inner ones were composed of more refractory material (minerals that
freeze at high temperatures) gradually changing to those outer
planets who captured a greater proportion of the volatiles, gases
and ices, as we see them today. The present state is that the
greatest amount of angular momentum was not left with the now
slower rotating Sun, but with Jupiter, the largest planet which
retained the major amount of gases. As we will see later in the
course, Jupiter is similar to an unfired star with its own mini-
solar system.

-the heat due to the energy of impacts has now decreased as the
number of available bodies has decreased, and most of the core
formation of the inner planets has been completed. Over the past
4.6 billion years the rotational periods of the planets has been
altered by interactions and collisions and there is some
significant heating of a few bodies by the friction resulting from
tidal forces. With the draining of the primordial heat, radiogenic
heating remains the main internal heat source in the inner planets
in addition to the surface and atmospheric radiation received from
the Sun.

(to be continued with more details on atmospheres, asteroids and
the outer planets)
                               42
Joseph Cain		cain@geomag.gly.fsu.edu
cain@fsu.bitnet		scri::cain
Newsgroups: alt.sci.astro.aips