Download Planet Development

Document1 1 of 7 pages
Understanding our Solar System through solar nebula theory and evolutionary planet development.
Planet building
The four stages of evolutionary planet development are:




differentiation,
crating,
flooding, and
slow surface evolution.
Differentiation is the process of heavier elements sinking to the core of a planet
while its interior is molten.
Cratering occurs by impacts with asteroids or planetesimals. There appears to have
been an era of intense cratering in the inner solar system in the past.
Flooding occurs when magma from under the surface oozes from cracks or craters in
the planet's surface. This molten material forms relatively smooth plains over the
surface.
Slow surface evolution can include plate tectonics, volcanism, and wind and water
erosion.
Terrestrials
Differentation
Differentiation is a natural process that will take place when the interior of a planet is
molten. This is clearly the case for Earth. The mean densities of the other terrestrial
planets also indicate that they are differentiated, lending support to differentiation as
a natural process in planetary evolution.
Our earthquakes occur when the tectonic plates move. Old crust is destroyed where
one plate moves into and under another plate, as is the case along the eastern Pacific
Ocean just off the coasts of North and South American. New crust is formed where
molten material rises though rifts. The largest of these crust forming rifts is located
near the middle of the Atlantic ocean.
Cratering
Moon
Our Moon is an example of cratering. Meteoritic bombardment is its active form of
erosion.Small meteorites are constantly hitting the surface of the moon. This causes
tiny fragments of large rocks, hills, and mountains to be broken off to form dust. This
process is very slow. It is not an effective form of erosion on Earth because Earth's
atmosphere slows these small meteors down and destroys many of them through
friction.
Document1 2 of 7 pages
Venus
The cratering record on Venus suggests that its surface is significantly younger than that of the
moon, but older than that of Earth. Since the cratering rate between these three bodies should
not be significantly different, Venus must have eroded or covered many of its older craters. The
cratering record suggests that this probably occurred within the last 300 million years.
Earth
Earth doesn't have as many craters as the moon or Venus because Earth's surface is
active and hence younger. Plate tectonics and erosion are continually resurfacing
Earth.
Craters impact each of these objects at roughly the same rate. The atmospheres of
Earth and Venus do help protect them to some extent from some of these impacts.
However, Earth has the ability to resurface itself through erosion and plate tectonics,
thereby covering and/or reshaping the impact craters, While this process takes a great
deal of time on the scale of a human life, on the scale of the life of a planet, it is rather
rapid.
Atmospheres of Terrerstrials
CO2
Earth
The Earth's primeval atmosphere was rich in carbon dioxide (CO2). First it appears that the
terrestrial planets formed in a hot environment. This means that only fairly heavy atoms and
molecules would have been held to the planet by gravity. The light weight elements like
hydrogen and helium would have been moving so fast that they would easily escape from the
surface. Secondly, Earth grew from accreting planetesimals which were rich in volatiles like
water and carbon dioxide. Therefore CO2 was added to the atmosphere during this phase of
evolution.
The Greenhouse Effect due to CO2
The greenhouse effect occurs when the atmosphere of a planet is capable of trapping infrared
radiation emitted by the planets surface. Carbon dioxide is very good at doing this.
Earth was able to avoid an extreme greenhouse effect because Earth formed oceans and
carbon dioxide is easily soluble in water. Carbon dioxide is also very easily dissolved in water. As
Earth cooled, the temperature and pressure of the atmosphere was sufficient to allow liquid
water to form and eventually oceans developed. The water quickly dissolved large amounts of
carbon dioxide from the atmosphere and made the atmosphere more transparent to the
infrared radiation that was emitted by Earth's surface.
Venus as an extreme example of the Greenhouse effect
The atmosphere of Venus is rich in carbon dioxide because it outgassed a great deal of carbon
dioxide from rocks while it was fairly hot. Since Venus was fairly close to the sun and never
formed extensive liquid water oceans, the carbon dioxide remained in the atmosphere. The
Document1 3 of 7 pages
surface termperature is around 482 oC ( 900 oF). Iron (Fe) melts at 2100 oF and aluminum at 122
o
F.
Mars
Mars, like Earth and Venus, probably outgassed a significant amount of carbon dioxide, as well
as oxygen and nitrogen.
Like Earth, Mars did form liquid water on its surface. large bodies of water did not exist for a
significant amount of time and the carbon dioxide did not dissolve out of the atmosphere as it
did on Earth.
Ultraviolet radiation from the sun would have broken up water molecules eventually, because
Mars did not form an ozone layer to absorb the ultraviolet radiation. Once water was broken
into hydrogen and oxygen atoms or molecules, These lighter molecules and atoms moved more
rapidly than water molecules and could easily escape the gravity of Mars. Therefore,
The best evidence of variations in the Martian climate is the variation in the rate at which layers
of dust form near the permanent portion of the polar caps. As the CO2 portion of the polar cap
melts each spring, dust accumulates on the H2O portion. Photography suggests that the rate at
which material builds up in these layers changes over periods of thousands of years. This
suggests that the wind speeds, and possibly the temperature, varies over these time scales as
well
Martian Crust
Mars has a much thicker crust than the Earth’s. Olympus Mons’s extreme size, more massive
and taller than any mountain on the Earth, and other mountains on Mars implies that the
Martian Crust must be thicker or stronger than the Earth’s. Because it would quickly push
down into the crust and spread out on Earth and Mars is a much smaller planet. That isn't
happening rapidly on Mars.
There are two types of erosion on the surface of Mars that indicate the presence of liquid
water at some point in Mars' past. The first consists of large food plains that appear to be
created by the large scale flooding of a region. The second is the presence of meandering
drainage valleys. These valleys show long-term erosion due to flowing liquid.
The Origin of the Moon – four hypotheses.
Fission
The fission hypothesis suggests that the moon broke from a rapidly spinning protoEarth. This break would have had to have occurred after Earth differentiated or the
moon would have about the same mean density as Earth, which it does not. One
prediction of this hypothesis, regardless of when the fission took place, is that lunar
rocks should have the same composition as Earth rocks, and they do not.
Condensation
Document1 4 of 7 pages
The condensation hypothesizes that the Earth and the moon forming simultaneously in
the solar nebula as sister planets. This predicts that the composition and mean density
of the two should be identical, and they are not.
Capture
The capture hypothesis proposes that the moon was formed somewhere else in the
solar system and was captured by Earth's gravitational field. However, if the moon
got close enough for Earth to capture it, Earth's gravitational field would have torn
it apart.
Large-Impact
The large-impact hypothesizes that the moon formed as a large planetesimal and
collided with Earth. The impact was glancing and shattered the planetesimal and
put much of the debris from the collision in orbit around Earth.
From computer simulations of such an impact we find that the iron core of the
planetesimal should sink into Earth's iron core very soon after the collision and
that the debris that forms the moon would be primarily rocky material from the
planetesimal and Earth's crust.
If the planetesimal formed at a different distance from the Sun than Earth did, the
condensation sequence tells us that the composition of the planetesimals rocks
would be somewhat different than those of Earth which it is.
Four processes cleared the solar nebula of material and ended the period of
planet building.




Radiation pressure from the sun once it became luminous,
the solar wind once the sun became luminous,
the sweeping up of debris by the planets, and
the ejection of material from the solar system by close encounters with planets.
Radiation pressure and the solar wind would have removed very small particles like individual
atoms and small dust grains, while impacts with planets and ejection by encounters with
planets would have removed larger planetesimals.
Jovian (meaning Jupiter like)
Direct comparison between terrestrial and gaseous is not realistic.
The Jovian planets have such thick atmospheres and vastly different compositions than the
terrestrials that it is impossible to make a direct comparison between the two.
Document1 5 of 7 pages
The major points are
 the structure of the interior and atmosphere,
 the strength of the magnetic field, and
 rings and satellites of each planet.
Common characteristics of the gaseous planets --Jupiter, Saturn, Uranus, & Neptune
Jupiter, Saturn, Uranus, and Neptune formed much further from the sun than Earth so the
planetesimals that came together to form the proto-planets were rich in icy particles. Once
these planetesimals grew fairly large it began to accrete gas directly from the solar nebula.
Hence, their composition is fairly close to that of the solar nebula. Most of the gas in the solar
nebula was hydrogen and helium.
Jupiter
Jupiter, the massive of all of the planets, is large enough to hold on to even the lightest of
molecules and atoms and was able to retain much of the hydrogen and helium that it drew in
from the solar nebula.
Liquid Metal Hydrogen
Jupiter's interior is liquid because the pressure is great enough to condense hydrogen into its
liquid form. At greater heights, the pressure slowly decreases and at some point the pressure is
low enough that hydrogen exists as a gas. At the height of the visible surface, the pressure is so
low that hydrogen can only exist in its gas phase. By contrast, Earth is not able to hold onto
atomic hydrogen because its temperature is greater and its gravitational force is weaker.
Large magnetic fields due to liquid metallic hydrogen
Jupiter and Saturn are both massive and create a large enough pressure to cause some of the
hydrogen in their cores to be in the form of liquid metallic hydrogen. Liquid metallic hydrogen is
an excellent conductor. Saturn and Jupiter also rotate very rapidly. The rapid rotation causes
currents to flow in the liquid metallic hydrogen deep in the interior of each planet. These
electrical currents produce the magnetic fields that have been measured for each planet.
Jupiter's field is believed to be larger because the larger mass of Jupiter should allow a larger
region of Jupiter's interior to be composed of liquid metallic hydrogen.
Even Uranus and Neptune have magnetic fields larger than their size would suggest compared
to the Earth’s.
Belts and rings
Document1 6 of 7 pages
A moon can produce a gap in rings through resonance. Particles in the rings that have a period
that is an even multiple of the period or in resonance with the moon will be drawn into elliptical
orbits and accreted producing a gap.
Saturn’s, Uranus’s, & Neptune’s rings
The width of Saturn's F ring and the rings of Uranus and Neptune are controlled by shepherd
moons. Shepherd moons are a pair of moons that orbit such that one is just inside the orbit of
the ring and the other moon is just outside the orbit of the ring
Jupiter’s moons Io, Europa, Ganymede, and Callisto
Io
The greatest geologic activity is on lo, the closest satellite, and decreases with increasing
distance from Jupiter. Io has the active volcanoes.
Europa
Europa is about the same size of our Moon and the brightest of the Galilean moons. It may
have a global ocean of water warned by Jupiter induced tides.
Ganymede
If stored internal heat produced the geologic activity, then the largest satellite, Ganymede,
should have the most geologic activity, and it has very little geologic activity
Callisto
Callisto's heavy cratering and lo’s lack of craters imply that the surface of Callisto is much older
than that of Io. If the surface of each planet were of the same age, we would expect Io to be
more heavily cratered than Callisto because of gravitational focusing by Jupiter. Io is probably
being hit by meteorites much more frequently than Callisto, but because Io is geologically very
active, the impact craters on to are quickly covered by fresh material from the molten mantle
Jovian planets except Uranus produce more heat than they receive from the Sun.
Atmospheric activity among the Jovian planets appears to be driven by heat flowing from the
interior of the planet. Uranus radiates an amount of energy that is almost exactly equal to what
it receives in solar energy. Neptune and Saturn, as well as Jupiter, radiate noticeably more
energy than they receivefrom the sun.
Since Uranus has little heat flowing from its interior, convection does not drive the atmosphere
as energetically as it does on Neptune, Saturn, or Jupiter.
Uranus and Neptune are blue primarily because they contain small amounts of methane in their
atmospheres. Methane is very efficient at absorbing red light.
Uranus rotates 98o to axis of revolution about the Sun.
Document1 7 of 7 pages
The best guess for this axis of rotation is the planet was hit by another planet size object that
tipped it over.
Triton – a satellite of Neptune
Few craters mark the surface of Triton suggesting that it has filled recent craters. Currently,
astronomers are considering a theory that proposes that many small icy planetesimals may
have formed just beyond the orbit of Neptune. Most of these bodies were either destroyed in
collisions or ejected to the outer reaches of the solar system (i.e. hundreds of AU from the sun).
It is possible that Pluto is the largest of these ancient objects and captured another of these
objects to form the Pluto-Charon system.
Dwarf Planets
To be defined as a planet an object
 must revolve around the Sun,
 shaped by gravity meaning a round or nearly round shape
 by gravity clear out debris in its orbit.
We know of Pluto whose orbit actually brings it inside of Neptune’s in one part, but there are
many others. Two commonly known ones are Ceres, in the asteroid belt between Mars and
Jupiter, and Eris goes out to 97 AU even further than Pluto at 50 AU at its further point from the
Sun.
THE END
.