THE SOLAR SYSTEM

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THE SOLAR SYSTEM
Our home within the Galaxy Currently not
mysterious We have approached, visited,
photographed and even sampled several bodies of
the solar system. However, in antiquity, it took
some time to understand it. What antiquity
saw Moon and 2 wanderers- phases Prograde
motion, retrograde loops etc. Celestial
sphere Fixed stars Wandering stars
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Geocentric View
Ptolemy- 2nd century B.C. Epicycles- Complex set
of wheels to account for motion of the wanderers-
lasted 1300 years Eventually everything made
sense. Copernicus heliocentric views Tycho
Brahe detailed observations Keplers
Laws Galileo and Newtonian mechanics
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Ancients knew that five wandering stars seemed
to slowly move among the constellations.
These wandering stars, commonly known as
planetes, typically move from west to east,
except during brief periods where they move
backwards or retrograde. The early Greek model of
a celestial sphere did not adequately account for
these retrograde loops.
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Geocentric modelsAncient astronomers
invented geocentric models to explain complex
planetary motions
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Claudius Ptolemy devised the longest used
geocentric model to explain retrograde loops by
putting planets on epicycles and deferents.
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Nicolaus Copernicus devised the first
comprehensive heliocentric (Sun-centered) model

• Copernicus imagined a universe where the Sun was
  at the center instead of Earth.
• He suggested that Earths motion around the Sun
  provided a more natural explanation for
  retrograde loops as Earth passed the other
  planets.

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Heliocentric planetary position terminology is
stated relative to Earth
Heliocentric planetary position terminology is
stated relative to Earth
Heliocentric planetary position terminology is
stated relative to Earth
Opposition Inferior conjunction Superior
conjunction Greatest eastern elongation (appears
east of the Sun in the sky) Greatest western
elongation (appears west of the Sun in the sky)
Opposition Inferior conjunction Superior
conjunction Greatest eastern elongation (appears
east of the Sun in the sky) Greatest western
elongation (appears west of the Sun in the sky)
Opposition Inferior conjunction Superior
conjunction Greatest eastern elongation (appears
east of the Sun in the sky) Greatest western
elongation (appears west of the Sun in the sky)
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Geocentric View
Copernicus Sun is at center of Universe and
the planets travel in circular orbits around it.
Moon is a satellite of Earth Brahe Observed
Mars. Position of Mars departed from predictions
based on Copernicus theory. Kepler 3 laws of
planetary motion 1. Planets move in elliptical
orbits and the Sun lies at one of the
foci 2. The straight line joining the sun
and a planet sweeps out equal areas in equal
intervals of time. 3. The square of the
planets orbital periods are proportional to the
cubes of the semimajor axes of their orbits.
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In this heliocentric model, the planets just
appear to move backwards as the faster moving
Earth laps the more distant planet once each
year when it is at opposition.
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Galileos discoveries of Jupiters moons with his
telescope showed that Earth was not the center of
all orbits strongly supported a heliocentric
model even though Copernicus model was no more
accurate than Ptolemys.
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Galileos discoveries of Venus phases with his
telescope showed that Venus must orbit the Sun
strongly supported a heliocentric model
even though Copernicus model was no more
accurate than Ptolemys.
Venus is clearly smallest when it is at superior
conjunction and largest when it is close to
inferior conjunction.
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Galileos discoveries of Venus phases with his
telescope showed that Venus must orbit the Sun
strongly supported a heliocentric model
even though Copernicus model was no more
accurate than Ptolemys.
Venus can only go through phases if it orbits the
Sun.
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Tycho Brahes astronomical observations disproved
ancient ideas about the heavens.
Brahe constructed enormous instruments to
meticulously record the precise positions of the
planets in the sky to an accuracy never
previously obtained.
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Johannes Kepler proposed elliptical paths for the
planets about the Sun.

• Keplers First Law of Planetary Motion
• The orbit of a planet about the Sun is an ellipse
  with the Sun at one focus.

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Ellipse
F1,F2 foci 1m constant a semimajor
axis b semiminor exis Planet moves fastest near
perihelion
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Johannes Kepler proposed elliptical pathsthe
planets about the Sun.
Elliptical Eccentricity (e) a number ranging
between zero (for a flat line) and one (for a
perfectly round circle).
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Johannes Kepler proposed elliptical paths for the
planets about the Sun.

• Keplers Second Law of Planetary Motion
• A line joining a planet and the Sun sweeps out
  equal areas in equal intervals of time.

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How flat are the planetary orbits? Eccentricity
e c/a As c/a becomes small, the orbit
becomes more circular For a circle, e 0 For a
line-like orbit, e 1 Eccentricities of
planetary orbits are very small (not so for
cometary orbits)
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• The laws of Kepler could describe (and predict)
  planetary motions with relative high accuracy.
• However, they only described, but did not explain
  why this was so.
• It took the development of mechanics to explain
  this.
• Described by Newtons laws
• An object in motion will retain its motion unless
  acted upon by a force.
• F ma
• Equality of action and reaction
• plus F Gm1m2/ d2

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Discovery of the fainter planets Planets-
wanderers They appear to move with respect to
the stars. But they are faint and
unremarkable Modern technique Blinking -not
available in early days.
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The solar system is populated by 3 families of
bodies (excluding the Sun) Small bodies
(Planetesimals) Rlt1000 km
Mlt1/1000 Ms (a few x 1021
kg) Intermediate-sized planetary Bodies Mercury,
Venus, Earth, Mars, Moon, Galilean Satellites,
Saturns Titan, Neptunes Triton - R between
1500-6500 km - M between 1/200 - 1
MEarth Giant (Jovian) Planets Jupiter, Saturn,
Uranus, Neptune - R between 25,00- 70,000
km - M between 15- 300 MEarth
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Interplanetary Fragments
Still in space- meteoroids Burn up in Earths
atmosphere- Meteors Remnants fallen to the
ground- Meteorites Interplanetary debris with
highly elliptical, Earth-crossing orbits. Large
meteoritic impacts can be devastating. Much
more frequent in young solar system Craters on
Earth- usually lakes Craters on the Moon
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Slide 4
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Barringer crater in AZ - 1.2 km across, 180 m
deep - about 25,000 years old Tunguska
meteorite (June 30, 1908) - Visible 750 km
away - 30 km region scorched Represent
debris of formation of Solar System Volatiles are
lost Once we properly interpret meteorites, they
can tell us a lot about the Solar System - e.g.
AGE
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Age of the Solar System
Very interesting story of the evolution of
scientific thought In 1648, James Ussher,
Archbishop of Armagh, Ireland, pronounced the
creation of the Earth to have occurred
in 4004 B.C. The 6000 year age for the
Earth was actually accepted until the early
1800s Geologists? water erosion ? canyons Since
the amount of soil removed from a canyon can be
estimated, it is clear that 6000 years are not
sufficient to make a canyon In fact, gt 100
million years
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• Naturalist Darwin
• Each generation species change very slightly.
  Fossil evidence shows very different species from
  current ones.
• These substantial changes in animal forms
  required up to millions of generations to occur.
• He concluded that the age of the Earth was many
  hundreds of millions of years.
• Science did not settle the matter there.

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Lord Kelvin ? mathematical physicist A small
amount of heat flows steadily from the inside to
the surface of the Earth. If the Earth started
molten, and the heat flow remained constant, it
would take 40 million years to reach the present
temperature. Moreover, since the heat flow must
have been larger during the hotter stages, the
age of the Earth must be ? 40 million
years. By 1983, by considering the larger heath
flow on those earlier times, Kelvin had reduced
his estimate of the age of the Earth to 24
million years What was missing? In 1904,
Rutherford discovered radioactivity.
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Radioactive Elements
Thorium, Uranium, Potassium, etc. are radioactive
elements found in the Earths interior. They
release (and re-supply) heat. This discovery
ended up providing a MEASURE of the age of rocks,
asteroids,, the Solar System Radioactive Decay
---- Nuclear Fission Some elements (or particles)
are UNSTABLE They will SPONTANEOUSLY DECAY into
other elements or particles The simplest example
is FREE NEUTRONS (I.e., neutrons not within an
atomic nucleus) n ? p e- ? (it is called ?-
decay because it emits electrons, also known as
?- rays)
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• The most important radioactive elements are HEAVY
  ELEMENTS
• These processes occur at very different rates.
• HALF-LIFE The time required for 1/2 of the
  elements to decay.
• Half-life of the neutron decay is 13 min.
  However, the half-life of the 238U decay is
  4.5 billion years
• To date a rock or whatever, 2 points must be kept
  in mind
• There must be good reasons to believe that in a
  molten state, there is no decay product in the
  rock (either because it is volatile, or because
  it is unstable, etc.)

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2. The method is sensitive when half life
age R.B Boltwood of Yale used the a-decay of
238U To date terrestrial rocks. Boltwood found a
large amount of lead in many rocks. ? several
billion years He found the oldest terrestrial
rocks were 3.3 billion years old Meteorites are
found to be as old as 4.6 billion
years Therefore, this is the age of the Solar
System and also the Sun.
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Planetesimals
The composition of the meteorites and/or their
ancestors, the planetesimals, shows they were
formed by COALESCENCE of minerals, metals, gases
and even organic materials This took place in the
PROTOPLANETARY DISK. Planetesimals are thought to
have formed in 2 ways 1. Origin of the
UNDIFFERENTIATED planetesimals (ordinary and
carbonaceous chondrites) They never grew beyond
a few x tens of km.
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Planetesimals
? NO INTERNAL MELTING ? NO GRAVITATIONAL
SETTLING Differences arising from environment in
nebula Hot near center, cold outside. 2.
Differentiated Planetesimals (achondrites,
strong-iron and iron meteorites) They grew to
large sizes ? partial melting ?
differentiation
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Planetesimals

• Fe and Ni inside
• Silicates and volatiles outside
• Subsequently broken up by collisions
• The age of the Solar System is determined in
  chondrites.
• The best radioactive nucleus used is
• 40K ? 40 Ar
• With a half-life of 1.3 x 109 years
• ? 4.5 - 4.6 x 109 years

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Intermediate Planetary Bodies

• Terrestrial Planets
• Moon
• Galilean Satellites
• Titan
• Triton

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Terrestrial Planets

• Densities
• They contain iron and other heavy elements.
  Uncompressed density decreases with increasing
  distance from sun
• They are differentiated
• Radioactivity? Melting? Settling of heavier
  elements
• The similarities and differences between ? and
  its neighbors tells us much about the development
  of life

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The Moon

• Plays a key role in understanding the origin of
  the solar system
• The only extra-terrestrial body visited by man.
• Pmoon 3.3 g/cm3 (p? 4.2 g/cm3)
• Why?
• The Moon probably has no iron core
• The lunar rocks have no water. (terrestrial
  igneous rocks have 0.2- 0.5 water)
• Lunar rocks have little content of siderophiles,
  or elements which combine with iron (I.e. nickel,
  cobalt, rubidium, iridium, platinum,, etc.)
• Perhaps, (using John OKeefes words), The
  moons iron and siderophiles are in the core of
  the Earth.

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Theories on Origin of Moon

• Accretion? Why no iron core? - Hot moon
• Fission from Earthgt fast-spinning Earth or impact
  of large planetesimal
• ? cold moon
• Apollo 11 results
• 1. Lunar rocks were all very old, some as old as
  4.5 billion years
• The lunar highlands were very old? primordial
  crust of the moon, primordial crust of the ??

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Theories on Origin of Moon

• Rocks found in marias were few hundred million
  years younger
• Assume the planetesimal impact origin is correct
• The heat of impact melted much of the ?, besides
  the planetesimal
• A cloud of hot rock vapor was ejected, and formed
  a ring about ?
• T several x 103 K
• Lost water and volatiles to space

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Theories on Origin of Moon

• Cooling? condensation into grains or solids? grow
  into the Moon
• All the lunar rocks were basalts? congealed lava
• Origin of melting?
• 1. Intense meteoritic bombardment of young Moon?
  fast
• 2. Radioactive heating (substantial amounts of
  Uranium etc., were formed in the lunar rocks) ?
  slow
• ANSWER Probably both
• Meteoritic bombardment? rocks found in highlands
• Radioactive heating? rocks found in marias

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Current Picture

• Young Moon relatively cold
• Covered with a global ocean of molten rock due to
  meteoritic bombardment
• As the meteoritic bombardment died down, the
  surface cooled and became solid
• But now, the lunar interior becomes hot due to
  radioactive decay
• ? pockets of molten rock leak through weakest
  parts of crust to form marias

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Volcanism

• Rocks in marias range in age between 3.1 and 3.9
  billion years
• Therefore, volcanism lasted about 800 million
  years
• Ended about 3 billion years ago? geologically
  quiet moon
• Difference between ? and moon
• The ? has currently molten core because larger
  objects cool more slowly

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Volcanism

• Remember that heat melts the interior due to
• Heating? mass? volume ? R3
• Cooling? surface area? R2
• Heating/Cooling ? R3/R2 ? R
• Larger objects stay hot inside longer

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Nemesis

• A companion star of the Sun?
• There is some evidence for the occurrence of mass
  extinctions on ? every 26 million years
• Maybe due to a comet collision with ?
• Nemesis a small solar companion orbiting the
  (earth?) with a 26 million year period
• (a 2.8 ly)
• Perihelion distance beyond Pluto

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Nemesis

• If during each perihelion passage, it disturbs
  the Oort cloud, many comets might be thrown
  towards us
• Those that collide with the Earth raise enormous
  dust clouds that obscure the Sun, interrupt plant
  growth and cause calamity!
• Interesting but,
• No evidence
• Stability of orbit problem

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Comparative Planetology The Rocky Worlds

• 4 interesting features of intermediate-sized
  planetary bodies
• Impact craters
• Geological activity
• Presence of H2O
• Atmospheres

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Craters

• Most heavily cratered
• Moon
• Mercury
• Callisto
• Ganymede
• Southern Hemisphere of Mars

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Craters

• Light cratering
• Earth
• Venus
• Northern Hemisphere of Mars
• Europa
• Not cratered at all
• Io

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Venus

• Currently very dry, with some water vapor in
  atmosphere
• Probably at one time, there was about as much
  water as on Earth
• However, because of the high temperature, it was
  in vapor form. Because of the copious solar UV
  radiation, water decomposed into H and O
• Since O reacts very easily, it quickly became
  locked up into the surface of Venus, whereas the
  H escaped into space.

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Venus

• This scenario is supported by the finding of
  Pioneer-Venus of a ratio of 2H/1H about 100 times
  as large as found on Earth.
• This is because 1H, lighter, could escape more
  easily than 2H from the gravitational bond of
  Venus
• Also, in recent years, Venus has been mapped out
  be space-based radars
• Pioneer Venus
• Magellan

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Venus

• Maps with resolutions of 50-100 m.
• 2 continents have been found
• In the Northern Hemisphere Ishtar Terra
• (about the size of Australia) with a mountain
  range including Maxwell Mons, a mountain which
  rises about 12,000 m (7 miles) above a reference
  level (higher than Mt. Everest-- 8848 m)
• In the Southern Hemisphere Aphrodite Terra
• (about half the size of Africa)

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Venus

• The size of Venus (similar to Earth) suggests
  that it should have volcanism
• The existence of high mountains confirms this
  since, in the dense atmosphere of Venus, wind
  weathering is much more severe than on Earth. A 1
  mph wind on Venus is comparable to a 90 mph wind
  on Earth.

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Mars

• Mountain ranges and evidence of lava flows
• Olympus Mons rises to 26 km altitude
• Some canyons are hundreds of km long and up to 7
  km deep
• There is evidence for wind, water, and glacier
  erosion
• The volcanic activity probably lasted longer than
  on the Moon, but it has already ceased
• Mlunalt Mmars lt Mv Me

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Mars

• At Mars temperature of 150- 290K and atmospheric
  density of .007 atm., water either freezes or
  boils
• So, it is either in the form of polar ice or
  water vapor
• However, at one time, liquid (water, probably)
  must have been plentiful, cutting canyons and
  flood-basins. Most of it must have been lost,
  and some of it may be below the surface.
  Currently, no liquid water on the surface.
• Catering is inversely correlated with volcanism

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Geological Activity

• Caused by forces on the planetary surfaces, which
  cause movements of the crust.
• Gravity
• Convection in the interior
• Weathering on the surface
• Tidal interaction with another body
• Strong- crust is thin and weak interior is
  molten
• Otherwise, the planet is geologically INERT or
  INACTIVE

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Volcanism

• Remember that heat that melts the interior is due
  to
• Gravitational forces during formation
• Radioactivity minus cooling
• Heating? mass? volume ? R3
• Cooling? surface area? R2
• Heating/Cooling ? R3/R2 ? R
• Larger objects stay hot inside longer

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Earth

• Lithosphere (100 km thick) made up of rigid
  tectonic plates floating on the denser
  semi-molten asthenosphere
• The most heavily cratered objects
• The crater density is close to SATURATION
• With some actual overlap

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Earth

• A large variety of scales of craters
• Huge ? Microscopic
• Age? up to a few billion years (4) for light
  geologic activity
• At the other extreme
• Io is so geologically active that any new crater
  is quickly erased by new lava flows
• Similarly, northern hemisphere of Mars has more
  volcanic activity than the southern hemisphere

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Plate Tectonics

• Causes volcanism
• Shapes mountains, continents, ocean floor
• Weathering
• Venus ? we cannot see the surface because of
  permanent clouds of sulfuric acid. Landers are
  quickly destroyed on the surface
• However, we can map the surface by means of radar
• From Arecibo (when Venus is at inferior
  conjunction)

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Europa, Ganymede, Callisto, Titan Triton

• All have retained large amounts of water, forming
  as much as 50 of each satellites mass (the
  outer portion because of differentiation).
• At 150 K or less (-120 C), the ice is very hard
  and rigid, capable of retaining the shapes of
  impact craters for long periods of time.

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Earth

• Over 2/3 of the surface of Earth is covered by
  water. Despite this, plus all the water locked
  into rocks, etc., much less than 0.1 of the mass
  of Earth is water.
• It follows that slight differences in the
  evolution of Earth may have removed from is this
  substance so essential for life.

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Other I-S Bodies

• Europa may have tectonic activity, but the crust
  is made up of ices and the heat of the interior
  may be due to tidal interactions with Jupiter.
• For the other intermediate bodies, it seems that
  volcanism was only an early activity that ceased
  long ago, and there is no evidence of plate
  tectonics.

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Water

• Water is present today on all intermediate-sized
  planetary bodies excepting
• Mercury and Io? Hot, low velocity of escape
• Moon ? Perhaps because of the way it formed

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Atmospheres

• The atmospheres of Earth and Venus are vastly
  different, even though the planets are very
  similar to each other.
• Understanding what produced these different
  atmospheres will tell us a lot about their
  formation.
• In particular- sources and sinks

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Venus vs. Earth vs. MarsA study in contrasts

• Suppose they did not have atmospheres, Energy
  absorbed energy emitted, since
• Put equations from slide 1 of VenusEarthMars
  here!!!!

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Instead, Te 10 C Why? Greenhouse
effect Water vapor CO2 Methane Ozone
etc. A simplified formula EQUATION Warm but
not terrible ? poles
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Radio Telescopes Space Probes- US and
USSR Showed insert equation from slide 3 of
venusearthmars.pdf Melts lead Destroys any form
of (familiar) life ? DEAD, INHOSPITABLE
BODY Why the difference? Same
origin Similar size Similar
distance from Sun
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Primitive atmosphere of both planets H, He,
Methane, Ammonia, N, Ne, and a small amount of
Ar. Present atmospheres are very different in
composition. ? The primitive atmosphere must
have escaped. Support Noble gases, which do not
combine, should be far more abundant than
observed. Today, substantial atmospheres blanket
both planets. Where do they come
from? Outgassing from the planetary
interior Volcanic eruptions contain large
amounts of water vapor, N, CO2
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If volcanoes have remained constant throughout
Earths history, and all material remained,
enough water vapor would have leaked to fill the
oceans, enough N would have leaked to become the
important constituent of the atmosphere it is
today. BUT Enough CO2 would have leaked to
constitute 90-95 of the atmosphere (like Venus)
instead of the 0.03 it is today. The density of
the atmosphere would also have been 90 times
denser. Venus behaves as expected. Earth does
not! Why?
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Rocks on Earths crust can absorb CO2 CO2
rocks ? carbonates CO2 CaSiO3? SiO2
CaCO3 Ca Quartz Ca Silicate
Carbonate So, CO2 in Earths Atmosphere became
solid compounds of the Earths surface.
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Venus wasnt so lucky The ability of rocks to
absorb CO2 decreases rapidly with increasing
T. The early Tearth 248 K -25 C The early
Tvenus 48 C Sufficiently larger to stifle CO2
combination ALSO At recent times on Earth, after
life developed, marine animals continue to
transform additional CO2 (from volcanoes) into
CARBONATES (e.g. seashells).
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What about Mars? Recall (equation from slide 7
of venusearthmars.pdf) Mars albedo is 0.15
(about 4x more absorbing than Venus
clouds) Mars-Sun distance 1.5 AU (about 2x the
distance from Sun as Venus) With nearly perfect
offsetting penalties, Teff 305 K 32 C Actually
a bit cooler (273 K 0 C) when Sun is out and
falls rapidly at night As with Venus, 90 of
atmosphere is CO2. Interestingly, atmospheric
pressure varies 25 when it precipitates onto
South pole! P lt 1 of Earths atmosphere? only
ice and vapor However, in valleys/ lowlands can
get fog-- possibly meta-stable water flows in
lowlands
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Water on Mars Estimates from outgassing
models cover Mars to 10 - 100 m Estimates
from fluvial pattern models also suggest Mars
has/had sufficient H2O to be covered www.discovery
.com/news/features/marswater/marswater.html Likely
in permafrost, perhaps hundreds of meters thick
(but not near equator) All this outgassing would
have led to several bars of CO2 atmosphere! Where
did it go?!? On Earth, carbonaceous rocks are
melted in subduction zones, and CO2 is returned
to atmosphere. Once plate tectonics stopped on
Mars, the atmosphere got trapped in the ground!
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• Main Lessons
• Too much greenhouse gases are bad (look at Venus)
• Too little greenhouse gases are bad (look at
  Mars)
• If we had no greenhouse effect, the oceans would
  freeze over!
• NEW!!! Hot of the NASA press releases Ozone
  depletion in the arctic enhanced by greenhouse
  gases!
• Extra GH gases trap heat low in atmosphere
• Stratosphere colder
• Big crystals (gt 20 microns) of nitrogen form and
  precipitate
• Less nitrogen allows chlorine to remain as active
  catalyst longer
• More O3 destroyed
• Balance is the name of the game.

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