Download 66 The Terrestrial Planets - Mercury Diameter = 0.38 x Earth`s

The Terrestrial Planets - Mercury
Diameter = 0.38 x Earth’s
Density = 5.4 x water
Expect nickel-iron core and silicate mantle.
• Orbital period = 88 Earth days
• Rotation about axis = 59 Earth days
• Solar day = 176 Earth days Figs. Z9.23 & K8-10
• Radius of orbit = 0.39 AU
Very hot: 700K by day, 425K by night complex molecules unstable at this temp.
High temp. + low mass = no atmosphere
Mariner 10 detected traces of hydrogen &
helium at one thousand million millionths of
Earth’s atmospheric pressure.
Surface is heavily cratered, fewer large craters
than on the Moon and no mountains. Caloris
Basin shows outflow of lava from impacts.
Scarps due to cooling and wrinkling.
Iron core gives magnetic field 1/100th of Earth’s.
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The Terrestrial Planets - Venus
Diameter = 0.95 x Earth’s
Density = 5.2 x water = 0.95 x Earth’s
No magnetic field detected.
Surface temperature 740K (lead melts)
Atmospheric pressure = 90 x Earth’s
• Orbital period = 225 Earth days
• Rotation about axis = 243 days retrograde
• Radius of orbit = 0.72 AU
Clouds obscure surface, clouds from 30km
to 65km altitude are sulphuric acid, other
sulphur compounds & some water vapour.
Atmosphere is 96% carbon dioxide, 3%
nitrogen and 1% other gases.
Clouds reflect 76% of sunlight back into
space - Venus is a bright object in the
morning or evening sky.
Carbon dioxide blanket gives an extreme
greenhouse effect - heat is retained.
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Venus - Surface Conditions
Venus is 500K warmer than it would be with a
transparent atmosphere (Earth is 35K hotter
than it would be, Mars 5K).
Where is the water?
Venus may have begun with hot water
oceans with water vapour a major component
of the atmosphere.
Ultraviolet light from the Sun split up water to
hydrogen and oxygen. The hydrogen escaped
into space. Water reservoir in oceans used up.
Surfaced mapped by Magellan spacecraft in
1990 using radar - Venus is fairly flat. The
north is mountainous with upland plateaux.
The south is rolling lava plains with cratering.
No evidence of plate tectonics or active
volcanoes.
Potential for life… poor.
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The Terrestrial Planets - Mars
Diameter = 0.53 x Earth’s
Density = 3.94 x water = 0.71 x Earth’s
Very weak magnetic field suggests a small iron/
iron sulphide core and a rocky mantle.
• Orbital period = 1.88 Earth years
• Rotation about axis = 24.6 hours
• Radius of orbit = 1.52 AU
Surface temperature range 310K to 200K.
Atmospheric pressure = 0.007 x Earth’s. Thin
atmosphere = greater heat loss.
Thin atmosphere is 95% carbon dioxide,
2-3% nitrogen, 1-2 % argon, some oxygen and
a little water vapour near poles.
2004: Mars Express detected atmospheric
methane (10 parts in a thousand million)
http://www.esa.int/esaMI/Mars_Express/
Mars rovers Spirit and Opportunity found
sedimentary rocks - patterned layers suggest
they formed beneath water.
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Mars - Surface Features
Water ice is present on the surface at the
poles and in clouds. There may possibly be
permafrost beneath the surface.
See polar caps of H2O ice and CO2 ice; no
free flowing water.
Southern hemisphere: flat, older surface,
heavily cratered.
Northern hemisphere: lava flows, younger,
huge volcanoes.
Near equator: Valles Marineris, 5000km
long by 500km wide.
Viking Lander found rock covered surface
with gravel, sand and silt, primarily basalt.
Rocks have tiny holes - probably volcanic.
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Mars - Continued
Prominent features:
outflow channels; direct evidence for water
erosion in the past, very similar to those on
Earth. Suggest flow downhill with meanders,
tributaries, sand bars etc.
Volcanoes:
shield type; flat in outline, gentle slopes.
Olympus Mons 600km across base, 27km
high. So massive it must be supported on
crust twice as thick as Earth’s.
Thick crust = no global plate tectonic system.
Lava flows from Tharsis ridge have wiped
out impact craters.
Satellites:
Mars has two moons, Phobos and Deimos,
which look ellipsoidal, very much like
captured asteroids.
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Evolution Of The Terrestrial Planets
1. Formation by accretion of planetesimals,
heating of interior  molten rock, crust
formation.
2. Crust solidifies, intense bombardment impact cratering of surface.
3. Basin formation and flooding, lowlands
form.
4. Low-intensity impacts, atmosphere forms
through outgassing and impacts. Mars
5. Volcanoes, crustal movement
continent formation.
Venus
& Earth
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Our Moon
Geologically quiet.
Synchronous rotation. Moving outwards at
4cm a year; must have been closer to Earth
in past.
No significant atmosphere as surface
gravity is 1/6 th of Earth’s.
Temperature 375K to 100K.
Surface features:
Maria - smooth solidified lava flows, mostly
in northern hemisphere facing the Earth.
Craters - impact craters with central peak
and shock rings.
Rocks brought back by Apollo missions
have ages up to four thousand six hundred
million years, mare rocks are three
thousand five hundred million years old.
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The Origin Of The Moon
Several lunar formation models have been
suggested, most likely are:
Binary Accretion
Earth & Moon formed simultaneously out of
the same type of planetesimals. Their similar
mixture of oxygen isotopes supports theory.
Doesn’t explain why Moon has no iron core.
Giant Impact
Figs. Z9.21 & K6-30
a Mars-sized body strikes the young Earth their iron cores merge, rocky mantle material
forms a ring of debris. This accretes to form
the Moon beyond the Roche limit and the
crust of the Earth within the Roche limit.
Gives correct compositions and Moon’s orbit
can be accounted for if the blow was a
glancing one.
Moon is formed devoid of volatiles but ice is
found there - could have come from comets?
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Meteor- Material
Meteoroid - in space
Meteor - vaporises in atmosphere
Meteorite - hits ground
Some of these are debris from comets;
meteor showers occur when Earth passes
through the orbit of a broken up comet.
Types of material:
Iron - 90% iron, 9% nickel, high
density, metallic appearance.
Stony - Earth-like rocks - many
contain silicate spheres called chondrules,
these rocks are categorised as “chondrites”.
Stony - Iron - a mix of the above.
Carbonaceous chondrites: some stony
meteorites which contain about 2% carbon,
10% water and other volatiles. As you would
get if you condensed some solar material suggests this type have suffered no major
change since the Solar System formed.
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Meteorites
Most meteorites are too dense to be from
comets and more closely resemble asteroids.
Their parent orbits are similar to those of
asteroids and they may arise from
asteroid/asteroid impacts.
Iron meteorites: these contain a crystal
structure that only occurs if a nickel/iron
mixture cools very slowly (at 1K per million
years), so they must have been made inside
a much larger body.
The age of meteorites, determined from
radioactive dating techniques, is the same as
that of the Solar System - four thousand six
hundred million (4.6 x 109) years.
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Asteroids - Minor Planets
Mostly irregular, cratered chunks of rock.
Largest is Ceres at 1000 km diameter.
Mainly in orbits between Mars and Jupiter;
average distance from Sun = 2.8 AU known as the Asteroid Belt.
Some are in highly eccentric orbits and pass
near Earth.
• S-type - stony, light coloured
• C-type - contain carbon compounds, dark
• M-type - show evidence of metals
S-type near to Mars, C-type further out.
Similar types to meteorites in fact.
A collision with an average asteroid could
be disastrous for life on Earth - cause of
extinction of dinosaurs?
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Comets - Observations
Dirty snowballs - ices and rocky material
surrounded by a thin rocky shell.
Figs. Z12.5 & K10-12
Best observations:
Halley’s Comet - Giotto probe fly-by in 1986.
Shoemaker-Levy 9 - impact on Jupiter 1994.
Temple I - “Deep Impact” mission hit it with a
projectile on 4th July 2005.
Features:
Coma (head of comet) is a spherical cloud of
gas and dust, 100,000 km wide, with a small,
bright nucleus ~10 km in diameter.
Approaching Sun, head heats up and material
vaporises to form two tails; a dust tail of
neutral material and a plasma or gas tail of
ions blown outwards by the solar wind.
Gas tail, 10 to 100 million kilometres long, is
bluish due to emission from carbon monoxide.
Dust tail is yellowish due to reflected sunlight contains similar size particles to smoke.
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Comets - Composition & Fate
Giotto observed streams of gas escaping from
the nucleus of Halley’s Comet.
Nucleus made up of 80% water, 10% carbon
monoxide, 3.5% carbon dioxide, polymerised
formaldehyde (H2CO)n plus traces of other
organic material and a vast range of atoms
from hydrogen to titanium.
Halley’s Comet is periodic, returning every 76
years. Many short period comets have periods
of less than 200 years. They orbit in the same
direction as the planets, in the plane of the
ecliptic. Long period comets have highly
eccentric orbits at any angle to the ecliptic.
Possible cometary fates:
• Dissipates energy and breaks up near Sun
• Ejected from Solar System by close encounter
with a planet.
• Collides with planet - e.g. Shoemaker-Levy 9
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Comets - Origin
Short period comets exist for a very short
time compared with the age of the Solar
System, so there must be a reservoir.
Long period comets lie about 100,000 AU
from the Sun at aphelion, having an orbital
period of about 10 million years.
Jan Oort suggested a cometary cloud at
this distance - the comets only reach the
inner solar system if their orbits are
perturbed by a passing star or gas cloud.
Problem: comets presumably formed at
same time as the Solar System but density
of material at 100,000 AU would have been
far too low. Maybe formed closer and were
ejected e.g. by gravitational kick of Jupiter.
In 1992 the Hubble Space Telescope found
a store of comet-like objects beyond Pluto’s
orbit - The Kuiper Belt - possible origin of
short-period comets.
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Comets - Close Encounters
Comets may have provided the atmosphere
for terrestrial planets, and possibly also the
complex organic molecules required for life.
Halley’s Comet has the same deuterium to
hydrogen ratio as Earth’s oceans.
Shoemaker-Levy 9
Discovered in March 1993, the “string of
pearls” comet had already broken up into 21
bright pieces, probably in an encounter with
Jupiter in July 1992.
Orbital calculations confirmed it would hit
Jupiter in July 1994.
Fragments hit Jupiter at 216,000 km/hour.
Some 3-4 km diameter fragments released
energy equivalent to 50 million nuclear
bombs.
Should we be worried?
See
http://www.nearearthobjects.co.uk
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