TERRESTRIAL PLANETS
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PChapter 6
The Terrestrial Planets:aEarth, Moon, and their Relatives
g
Mercury, Venus, Earth, and Mars share many
e similar features
Small, compared with the huge planets beyond them, these inner planets also have rocky surfaces
surrounded by relatively thin and transparent atmospheres
Together we call these four planets the terrestrial planets (from the Latin “tera”, meaning earth)
Venus and the Earth are often thought of as sister planets in that their sizes, masses, and densities are
about the same. But in many respects they are very different.
a) The Earth has oceans of water, and atmosphere containing oxygen, and life
b) Venus is hot with temperatures over 750K (900oF), a planet which life seems unlikely to develop
Mars is only 53% the diameter of the Earth and has 10 percent of Earth’s mass.
a) Its atmosphere is much thinner than Earth’s; too thin to breathe
b) Mars two tiny moons are not much more than chunks of rock in orbit
Mercury, the inner most planet is more like our own moon than Earth.
a) Its atmosphere is negligible and its surface is seared by solar radiation
The study of comparative planetology is helping us to understand weather, earthquakes, and other topics
THE EARTH’S INTERIOR
1) The study of Earth’s interior and surface is called geology
2) From seismology and other studies, geologists have been able to develop a picture of Earth’s interior
a) Earth’s innermost region, the core, consists mainly of iron and nickel
i) The central part of the core may be solid, but the outer part is probably a very dense
liquid
b) Outside the core is the mantle, and on top of the mantle is the very thin outer layer called the crust
c) The upper mantle and crust are rigid and contain a lot of silicates, while the lower mantle is partly
melted
3) The layered structure developed from the same material as the Sun and other planets which was a cloud
of gas and dust, and so was surrounded by a lot of debris in the form of dust particles
a)
The young Earth was subject to constant bombardment from this debris.
b)
This bombardment heated the surface to the point where it began to melt, producing
lava
c)
Some of the original heat (though not enough to melt the Earth’s interior) came from
gravitational energy released as particles came together to form the Earth – such energy is
released from gravity between objects when the objects move closer together and collide
4) The major source of the present-day energy in the interior is the natural radioactivity within the Earth
a)
Certain forms of atoms are unstable – meaning they spontaneously change into more
stable forms
b)
In this process they give off energetic particles that collide with the atoms in the rock and
give some of their energy to these atoms. The rock heats up
c)
The Earth’s interior became so hot that the iron melted and sank to the center since it
was denser, forming the core. Eventually other materials also melted.
d)
As the Earth cooled, various materials because of different densities and freezing points,
solidified at different distances from the center. This process is called “differentiation”
5) The rotation of Earth’s metallic core helps generate a magnetic field on Earth
a)
The magnetic field has a north and south magnetic pole that are not quite where regular
north and south geographic poles are – the Earth’s magnetic north pole is near Hudson Bay,
Canada
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CONTINETAL DRIFT
1) Some geologically active areas exist in which heat flows from beneath the surface at a rate much higher
than average
2) The out flowing geothermal energy signals what is below
3) The Earth’s rigid outer layer is segmented into plates, each thousands of kilometers in area, but only
about 50 kilometers thick
4) Because of the internal heating, the top layers float on an underlying hot layer (mantle) where rock is
soft, though it is not hot enough to melt completely
5) The mantle beneath the rigid plates of the surface churns very slowly, thereby carrying the plates around
a) This theory is called plate tectonics, explained the observed continental drift – the continents, over
eons, move at a rate of a few centimeters per year – the same rate at which fingernails grow
6) The continents were connected in a supercontinent called Pangea
a) Over 200 million years the continents have moved apart as plates have separated
b) Evidence for plate tectonics comes from viewing the shapes of landmasses , fossils, and similar rock
types on different continents
c) The coming together and breaking apart of continents have had many cycles in Earth's history
7) The boundaries between plates are geologically active
a) These boundaries between plates are moving apart mark regions where molten material is being pushed
up from the hotter interior to the surface (example: the mid-Atlantic ridge)
i) These boundaries can be traced out by the regions where earthquakes and most volcanoes occur
b) Molten material is forced up through the center of the ridge and is deposited as lava flows, producing
new seafloor
c) Molten material is forced up through the center of the ridge and is deposited as lava flows, producing
new seafloor
d) When two plates come together, one may be forced under the other and the other is raised
e) The great Himalayan chain was produced by the collision of India with the rest of Asia
f) The “Ring of Fire” volcanoes around the Pacific Ocean (which included Mt St. Helens in Washington
State) were formed when molten material made its way through gaps or weak points between the plates
TIDES
a)
b)
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d)
a)
1) It has long been accepted that tides are directly associated with the Moon and to a lesser extent
with the Sun
2) The affect of the Moon can be evidenced because the tides – like the Moon's passage across the
meridian (the imaginary line in the sky passing from north to south, the point overhead) – occur about
an hour later each day
3) Tides result from the fact that the force of gravity exerted by the Moon gets weaker as you get
farther away from it
4) Tides depend on the difference between the gravitational attraction of a massive body at different
points on another body
The high tide on the opposite side of Earth results from the Earth being pulled away from the water
In between the locations of the high tides the water has rushed elsewhere, so we have low tides
Since the Moon is moving in its orbit around the Earth a point on the Earth's surface has to rotate longer
that 24 hours to return to a spot nearest the Moon
Thus a pair of tides repeats about every 25 hours, making 12 ½ hours between high tides
5) The Sun's effect on Earth's tides is only about half as much as the Moon's
6) The Sun exerts a greater gravitational force on Earth than the Moon; the Sun is so far away that its
force does not change very much from one side of the Earth to the other. And it is only the difference in
force from one place to another that counts for tide
We tend to have very high and very low tides when the Sun, Earth, and Moon are aligned, as is the case
near the time of a full moon, because their effects reinforce each other
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b) Conversely, tides are less extreme when the Sun, Earth, and Moon form a right angle, as near the time of
a first quarter moon
THE EARTH'S ATMOSPHERE
1) The layers of the atmosphere are named according to their composition and the physical processes that
determine its temperature
2) The Earth's weather is confined to the very thin troposphere
3) A major source of heat for the troposphere is infrared radiation emitted from the ground, so the
temperature of the troposphere decreases as altitude increases
4) The rest of the Earth's atmosphere, as well as Earth's surface, is heated mainly by solar energy from
above
5) A higher level of the Earth's atmosphere is the ionosphere – where many of the atoms are ionized –
stripped of some the electrons they normally contain – by the higher temperature
a) The free electrons in the ionosphere reflects long-wavelength radio signals
b) When conditions are right, radio waves bounce off the ionosphere, which allows you to tune in to
distant radio stations
6) Winds are caused partly by uneven heating of different regions
7) The rotation of Earth also has a very important effect in determining how the winds blow
a) Comparison of the circulation of winds on the Earth (which rotates in one Earth day) or slowly
rotating Venus (which rotates in 243 Earth days) and on rapidly rotating Jupiter and Saturn (each of
which rotates in about 10 Earth hours) helps us understand weather on Earth
b) Our improved understanding allows forecasters of weather (day to day) and climate (year to year
average conditions) to be more accurate
THE VAN ALLEN BELTS
1) In 1958, the first American space satellite carried into orbit a device to search for particles carrying
electric charge that might be orbiting Earth
2) This device, which was under the direction of James A. Van Allen, detected a region filled with charged
particles having high energies
3) Two such regions – the Van Allen Belts – surrounded Earth, like a small and large doughnut, containing
protons and electrons
4) A more recent discovered inner most belt contains mainly ions of the heavier elements of interstellar
space
5) The particles in the Van Allen Belts are trapped by Earth's magnetic field lines, and not across field lines
6) These particles, often from solar magnetic storms or from Earth's upper atmosphere, are guided by the
Earth's magnetic field toward the Earth's magnetic poles
7) When they interact with air molecules, the cause the atmosphere to glow, which are seen as the
northern and southern lights – the aurora borealis (northern lights) and the aurora australis (southern
lights)
THE MOON
1) The Moon is only 380,000 kilometers (238,000 miles) on average from the Earth
a) At this distance, it appears sufficiently large and bright
The Moon's Appearance
1) Binoculars can reveal that the Moon's surface is pockmarked with craters
2) Other areas, called maria (pronounced mar' ee-a, singular mare, pronounced mar'eyh) are relatively
smooth.
a) Mare comes from the Latin word for sea
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3) The Moon's gravity at the surface is only one-sixth (1/6) that of the Earth
a) Gravity is so weak that any atmosphere and any water (liquid or vapor) that may have once been present
would long since have escaped into space
4) The Moon rotates on its axis at the same rate it revolves around the Earth, thereby always keeping
the same face in our direction
a) The Earth's gravity has locked the Moon it this pattern, pulling on a budge in the distribution of the lunar
mass to prevent the Moon from rotating freely
b) As a result on this interlock, we always see essentially the same side of the Moon from our vantage point
on Earth
c) A full moon is not a good time to observe the lunar surface structure, since any shadows we see are
short and lunar features appear washed out
d) When the Moon is a crescent or quarter moon, the sunlight portion of the Moon facing us is covered
with long shadows. The lunar features there stand in long relief
e) Shadows are longest near the terminator, the line separating day from night
5) The Moon revolves around the Earth every 27 ½ days with respect to the stars
a) During that time, the Earth has moved part way around the Sun, so it takes a little more time for the
Moon to complete a revolution with respect to the Sun
b) The cycles of phases that we see from Earth repeats with the 29 ½ day period
The Lunar Surface
1) Almost all the rocks sampled on the Moon are igneous rocks, mainly basalts, which are formed by the
cooling of lava
2) The Moon and Earth seem to be similar chemically, though differences in overall composition do exist
3) Some elements that are rare on Earth (such as uranium and thorium) are formed in greater abundances
on the Moon
4) None of the lunar rocks contain any trace of water bound inside their mineral
5) As a result of the Apollo missions, we learned that almost all the craters on the moon come from
meteoroids – interplanetary rocks that hit the Moon – which such high speeds that huge amounts of
energy are released at the point of impact
6) One way of dating the surface of the Moon or a planet is to count the number of craters in a given area
a) Since those locations with the greatest number of craters must be the oldest
b) Relatively smooth areas – like maria – must have been covered over with volcanic material at some
relatively recent time (which is still billions of years ago)
c) Crater counts and superposition of one crater on another give only relative ages
d) The absolute ages were determined when rocks were physically returned to Earth
i) The oldest rocks were dated by comparing the current ratio of radioactive forms of atoms to
nonradioactive forms present in the rocks with the ratio that they would have when they were formed
ii)
The oldest rocks that were found on the Moon solidified 4.4 billion years ago
iii)
The youngest rocks solidified 3.1 billion years ago
7) The Moon formed about 4.6 billion years ago
a) From the oldest rocks, we know that at least the top 100 km of the surface was molten about 200
million years later. Then the surface cooled
b) From 4.2 to 3.9 million years ago, bombardment by interplanetary rocks caused most of the craters
seen today
c) About 3.8 million years ago, the interior of the Moon heated up sufficiently (from radioactive
elements inside) that volcanism began
d) Lava flowed onto the surface and filled in the largest basins that resulted from the earlier
bombardment thus forming the maria
e) By 3.1 billion years ago the volcanism was over. The Moon has been geologically quiet since then
f) Active lunar history stopped about 3 billion years ago, while the Earth continued to be geologically
active
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g) About all the rocks on the Earth are younger than 3 billion years old, the oldest single rock ever
discovered on Earth has an age of 4.5 billion years
8) To study the first billion years of the Solar System we must study extraterrestrial bodies that have not
suffered the effects of plate tectonics or erosion which occurs in the presence of water or an atmosphere
The Lunar Interior
1) Most experts believe that the Moon's core is molten, but the evidence is not conclusive
2) The Lunar crust is perhaps 65 km thick on the near side and twice as thick on the far side
3) This asymmetry may explain the different appearances of the sides because lava would be less likely to
flow through the far side's thicker crust
4) One of the major surprises of the human missions of the 1960's, redefined more recently, was the
discovery of mascons, regions of mass concentrations near or under the maria
a) The mascons may be lava that is denser that surrounding matter, providing a stronger gravitational
force on satellites passing overhead
b) If the Moon were a perfect, uniform sphere, then space craft orbits around the Moon would be
perfect ellipses, but this is not the case
The Origins of the Moon
1) The leading models for the origin of the Moon that were considered at the time of the Apollo missions
are as follows
a)
Fission – The Moon was separated from the material that formed the Earth; the Earth
spun up and the Moon somehow spun off
b)
Capture – The Moon was formed far from the Earth in another part of the Solar System
and was later captured by the Earth's gravity
c)
Condensation – The Moon was formed near to (and simultaneously with) the Earth and
the Solar System
2) Recent work has all but ruled out the first two and made the third less likely. The model strongly
considered now is:
a) Ejection of a Gaseous Ring
i) A plantesimal (perhaps the size of Mars) hit the proto-Earth, ejecting matter in gaseous
(and perhaps some liquid or solid) form
ii) Although some of the matter fell back to Earth and part escaped entirely, a significant
fraction started orbiting the Earth, probably in the same direction as the incoming
plantesimal
iii) The orbiting material eventually coalesced into the Moon
iv) Comparing the chemical composition of the lunar surface with the composition of the
Earth's surface has been important in narrowing down the possibilities
v) The mean lunar density of 3.3 grams /cm is close to the average density of the Earth's
mantle, and the Moons seems especially deficient in iron (Fe)
iv) However, detailed examination of the lunar rocks and soils indicate abundances of
elements on the Moon and Earth's mantle are sufficiently different from each other,
providing evidence that the Moon did not form directly from the Earth
MERCURY
1) Mercury is the innermost of our Sun's eight planets
2) Its average distance from the Sun is 4/10 of the Earth's average distance (0.4 AU)
3) Except for distant Pluto, its elliptical orbit around the Sun is the most elongated
4) Since on Earth we are outside Mercury's orbit looking in at it, Mercury always appears close to the Sun in
the sky
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a)
At times Mercury rises just before sunrise and at time it sets just after sunset, but it is
never up when the sky is really dark
b)
The Sun always rises or sets within an hour or so of Mercury's rising and setting
c)
As a result, whenever Mercury is visible, its light has to pass obliquely through Earth's
atmosphere. d) This long path leads through turbulent air leads to blurred images
5) Astronomers have never really had a clear view of Mercury from Earth
a) Even the best photographs taken from Earth show Mercury as a fuzzy ball with indistinct markings
b) On rare occasions, Mercury goes into transit across the Sun – that is we see it as a black dot crossing
the Sun
The Rotation of Mercury
1) Early observations from Earth based telescopes lead astronomers to conclude that Mercury rotates
in the same amount of time it takes to revolve around the Sun
2) They thought that one side always faces the Sun and the other side always faces away from the Sun
– similar to our Moon
3) When the first measurements were taken of Mercury's radio radiation the planet turned out to be
giving off more energy than expected.
4) The dark side of Mercury was too hot for a surface that was always in the dark
a) Later RADAR (radio detection and ranging) was able to be transmitted to Mercury
b) Since one edge of the visible edge of Mercury is rotating toward Earth while the other edge is
rotating away from Earth, the reflected radio waves were slightly smeared in wavelength according
to the Doppler effect
c) This measurement allowed astronomers to determine Mercury's rotation
d) Knowing the rotation speed and Mercury's radius, the rotation speed could be determined
5) Mercury rotates every 59 days. Mercury's 59 day period of rotation with respect to the stars is exactly 2/3
of the 88 day orbital period, so the planet rotates three times for every two times it revolves around the
Sun
6) Mercury's rotation and revolution combine to give a value for the rotation of Mercury relative to the Sun
– a mercurian solar day – that is neither 59 nor 88 days
a) If you lived on Mercury, you would measure each day (each day/night cycle) to be 176 Earth days
long
b) We would be alternatively fried for 88 Earth days and then frozen for 88 Earth days
c) The hottest temperature is about 700K (800oF) and the minimum temperature is about 100K (280oF)
Mercury Observed from Earth
1) The details of the surface of Mercury can't be seen very well from Earth, but other properties can be
observed
2) Mercury's albedo – the fraction of the sunlight hitting Mercury that is reflected – can be measured
a. We can measure the albedo because we know how much sunlight hits Mercury (we know the
brightness of the Sun and the distance of Mercury from the Sun)
b. Then we can easily calculate at any given time how much light Mercury reflects
c. Comparing albedos of materials on the Earth and the Moon can teach us something about what the
surface of Mercury is like
Mercury from Mariner 10
1) We learned most of what we know about Mercury during a brief time in 1974
a. The 10th in a series on Mariner spacecraft, launched by NASA, flew by Mercury
b. The most striking overall impression is that Mercury is heavily cratered and at first glance, looks like
the Moon
c. There are several basic differences between the Mercurian surface and the lunar surface
i) Mercury’s craters seem flatter than those on the Moon, and have thinner rims
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ii) Mercury’s higher gravity at the surface may have caused the rims to slump more
iii) Mercury’s surface may have been softer (more plastic-like) when most of the cratering occurred
iiii) The craters may have been eroded by any number of methods including
1) impacts of meteorites or micrometeorites
2) erosion may have occurred during a much earlier period when Mercury may
have had an atmosphere
3) undergone internal activity or have been flooded by lava
2) Most of the craters seem o have been formed by the impacts of meteorites
a) The Caloris Basin is the site of a major impact
b) The secondary craters, caused by materials ejected as primary craters formed, are closer to the
primaries on the Moon, presumably because of Mercury’s higher surface gravity
c) In many areas, the craters are seen superimposed on relatively small plains which are so extensive
they are probably volcanic with ages estimated to be 4.2 billion years, the oldest features on
Mercury
d) Smaller, brighter craters are sometimes superimposed on the lager craters and thus must have been
made afterward
i) Some craters have rays of higher albedo emitting from them (just as some lunar craters
do)
3) Lines of cliffs hundreds of miles long are visible on Mercury and, as on Earth, such lines are called scarps
a) The scarps are particularly apparent in the region of Mercury’s south pole
b) On Mercury there are no signs of faults (geologic tension like rifts or fissures nearby)
c) The scarps are global in scale and not just isolated
d) The scarps may actually be wrinkles in Mercury’s crust
e) Part of Mercury’s surface is very jumbled, probably the result from the energy of an impact
4) Mercury’s average density is about the same as Earth’s
a) It’s core is probably iron and takes up about 50% of the volume or 70% of the mass, a much
greater portion than Earth’s core
b) Perhaps it was molten and shrank by 2 km (1 mile) as it cooled causing the buckling, causing
the scarps now seen
c) Data from Mariner 10 indicate that the surface of Mercury is covered with a fine dust, like
the surface of the Moon
5) Mariner 10 detected a magnetic field in space near Mercury
a) The field is weak, only about 1 percent of Earth’s surface field
b) It had been thought that magnetic fields were generated by the rapid rotation of molten iron
core in planets, but Mercury is so small that its core should have quickly solidified
c) So the magnetic field is probably not now being generated – perhaps the magnetic field has
been frozen into Mercury since the time when its core was molten
Mercury Research since Mariner
1) About a dozen years after Mariner 10, Earth based telescopes detected a thin atmosphere Mercury
a. Mercury’s atmosphere contain more sodium than any other element – 150,000 atoms per cubic
centimeter compared to 4500 of helium and smaller amounts of oxygen, potassium, and hydrogen
b. The latest evidence suggests that Mercury’s atmosphere may have diffused up through Mercury’s
crust
Mercury’s History
1) Mercury may be the fragment of a giant early collision that nearly stripped it to its core
2) The stripping would account for the large proportion of iron in Mercury relative to Earth
3) Early on the core heated up and the planet’s crust expanded, the expansion opened paths for molten
rock to flow outward from the interior, producing intercrater plains
4) As the core cooled, the crust contracted, and scarps developed
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5) Solar tides slowed Mercury’s original rotation until its rotation period became 2/3 of the orbital
period
VENUS
1) Venus orbits the Sun at a distance of 0.7 A.U.
2) It comes closer to us than any other planet, but we did not know much about it until recently
because it is always shrouded in highly reflective clouds
3) Venus will be in transit in 2012 for the first time since 1874 and 1882.
The Atmosphere of Venus
1) Studies from Earth show that the clouds on Venus are primarily composed of droplets of sulfuric
acid, H2SO4, with water droplets mixed in
2) Observations from Earth show a high concentration of carbon dioxide in the thick atmosphere of
Venus – with carbon dioxide making up of 96% of the mass of Venus’s atmosphere
a)
By comparison, the Earth’s atmosphere is mainly hydrogen, with a fair amount of oxygen as
well – carbon dioxide makes up less than 1 percent of the terrestrial atmosphere
3) Because of the large amount of carbon dioxide in its atmosphere, which leads to the atmosphere being
so massive, Venus’s surface pressure is 90 times higher than the pressure of the Earth’s atmosphere.
a)
Carbon dioxide on Earth dissolved in seawater and eventually forms some types of terrestrial
rocks (limestone for example)
b)
If the carbon dioxide were released from the Earth’s rocks along with other carbon dioxide
dissolved in seawater, our atmosphere would become as dense and have as high a pressure as that
of Venus
c)
Slightly closer to the Sun than Earth, and thus hotter, had no oceans in which carbon dioxide
could dissolve to help take it up from the atmosphere and therefore the carbon dioxide remains in
Venus’s atmosphere
The Rotation of Venus
1) Venus rotates in 243 days to the stars in the direction opposite from the other planets
2) Venus revolves around the Sun in 225 Earth days
3) Venus’s periods of rotation and revolution combine so that a solar day/night cycle on Venus
corresponds to 117 Earth days – that is the Sun returns to the same position in the sky every 117 days
4) Nobody definitely knows why Venus rotates “the wrong way”
a) One possibility is that when Venus was forming, a large clump of material struck it at an angle
that caused the merged resulting planet to rotate backward
5) The slow rotation of Venus’s solid surface contrasts with the rapid rotation of its clouds
a) The tops of the clouds rotate in the same sense as the surface rotates but about 60 times more
rapidly, once every four days. Lower parts of the atmosphere, however, rotate more slowly
The Temperature of Venus
1) The surface of Venus is very hot, about 750 K (900oF), even on the night side
2) The high temperatures from radio measurement indicate that Venus traps much of the solar energy that
hits it
3) The process by which the surface of Venus is heated so much on Venus is attributed to the greenhouse
effect
a. The situation is so extreme on Venus that we say a “runaway greenhouse effect” has taken place
there
4) Venus may have begun its existence with a pleasant climate, perhaps even with oceans
a) Perhaps rain and other processes were not quick enough in removing the atmospheric CO2 out gassed
by volcanoes
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b) As CO2 accumulated, the greenhouse effect increased, and temperatures rose, oceans evaporated faster,
ad a runaway greenhouse effect was in progress
c) The water vapor was gradually broken apart by ultraviolet radiation from the Sun, and hydrogen (being
so light) escaped from Venus
d) Eventually there were no oceans and no rain – but continued CO2 out gassing of CO2 led to more
greenhouse warming and extremely high temperatures
5) Studies of Venus are important for an overall understanding of Earth’s climate and atmosphere
6) It is very unlikely that Earth will ever experience a runaway greenhouse effect that makes the planet
unfit for all forms of life, but our understanding of Venus’s atmosphere and our own has shown that the
Earth’s atmosphere is a very complex system
Missions to Venus
1) Venus was an early target of both American and Soviet space missions
2) During the 1960’s American spacecraft flew by Venus and Soviet spacecraft dropped through its
atmosphere
a) In 1970, the Soviet Venera 7 spacecraft radioed 23 minutes of data back from the surface of
Venus before it succumbed to the high pressure and temperature
b) In 1972, Venera 8 survived on the surface for 50 minutes
c) Both landers confirmed the Earth-based results of high temperatures, high pressures, and high
carbon dioxide content
3) In 1990 the Galileo spacecraft observed Venus’s clouds.
a) Structure in the clouds can only be seen in ultraviolet light and the clouds appear as long,
delicate streaks, looking like terrestrial cirrus clouds
4) Studies of Venus’s weather have practical value
a) The principles that govern weather on Venus are similar to those that govern weather on Earth
b) The better we understand the interaction of solar heating, planetary rotation, and chemical
composition in setting up an atmospheric circulation, the better we will be able to understand
our Earth’s atmosphere
c) The potential financial return from such knowledge is enourmous; it would be many times the
investment we have made in planetary exploration
5) None of the spacecraft (form 1970 – 2001) deteceted a magnetic field
a) The absence of a magnetic field may indicate either that Venus does not have a liquid core or
does not rotate fast enough
6) Spacecraft have provided evidence that volcanoes may be active on Venus.
a) The abundance of sulfur dioxide they found varied at different times by a factor of 10
b) The effect could come from eruptions at least 10 times greater than that of even the largest
terrestrial volcanic erruptions
c) Lightning that was detected is further evidence that those regions are sites of active volcanoes
7) Probes that penetrated the atmosphere and went down to the surface found that high-speed winds at
the upper levels are coupled to other high speed winds at lower altitudes
a) The lowest part of Venus's atmosphere is relatively stagnant
b) The probes detected three distinct layers of venusian clouds, separated from each other by
regions of relatively low density
c) One of the probes measured that only about 2 percent of the sunlight reaching Venus filters
down to the surface, making it like a dim terrestrial twilight
d) Most of the Sun's energy is absorbed or reflected by the clouds, and Venus's clouds are more
important than Earth's for controlling weather.
e) Most of the light that reaches the surface is orange, so photographs taken on the surface have an
orange cast
Venus's Surface
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1) From radar maps we know that 60 percent of Venus's surface is covered by a rolling plain, flat to withing
plus or minus 1 km
2) Only about 16 percent of Venus's surface lies below this plain, a much smaller fraction that the two
thirds of the Earth covered by the ocean floor
3) Two large features, the size of small Earth continents, extend several km above the mean elevation
a) The northern continent is called Terra Ishtar – is about the size of the continental United States
b) The giant chain of mountains on it known as Maxwell Montes is 11 km high, which is 2 km higher
than Mount Everest on Earth
c) Ishtar's western part is a broad plateau, about as high as the Tibetan plateau but twice as large
d) A southern continental Aphrodite Terra, is about twice as large as the northern continent and
much rougher in terrain
4) From radar maps, it appears that Venus is apparently made of only one continental plate
a) Venus may have such a thick crust that any plate tectonics that existed in the distant past was
choked off
b) Therefore, venusquakes are probably much less common than earthquakes
5) Perhaps hot spots force mountains to form on Venus in addition to causing volcanic erruptions.
a) The rising lava may also make the broad, circular domes that are seen.
b) Venus lost its internal energy to volcanism, and apparently virtually the whole planet was
resurfaced with lava about 500 million years ago
6) The Soviet spacecraft found that the soil resembles basalt in chemical composition and density in
common with the Earth, the Moon, and Mars
a) They measured temperatures of about 750 K and pressures over 90 times that of the Earth's
atmosphere
7) Recent space results coupled with ground based knowledge show the following differences between
Earth and Venus
a) Venus's rotation is slower than that of Earth
b) Venus has a one plate surface
c) The absence of a satellite (moon)
d) The extreme weakness or absence of a magnetic field
e) The lack of water in the atmosphere
f) Venus's high surface temperature and pressure
MARS
Characteristics of Mars
1) Mars is a small planet, 6782 km across, which is only about half the diameter and one-eighth the volume
of Earth and Venus, although Mars is somewhat larger than Mercury
2) It has two tiny (about 20 km in diameter) irregularly shaped moons named Phobos and Deimos
3) Mars's thin atmosphere is thin, only 1 percent of Earth's, but could possibly support certain forms of life
4) The orbit of Mars is outside that of the Earth, so Mars can be seen in the late night sky
5) Mars revolves around the Sun in 1.9 Earth years
6) The axis of its rotation is tipped at 25% from the perpendicular to the plane of its orbit, nearly the same
as the Earth's 23 ½o tilt
a) Because the tilt causes the seasons, we know that Mars goes through its year with four seasons
just as Earth does
b) In the martian winter, there is a polar cap. As the martian spring comes into the northern
hemisphere, the north cap shrinks and material at more temperate zones darkens
7) The surface of Mars appears mainly reddish-orange when seen from Earth against dark space, probably
due to the presence of iron oxide (rust) with darker areas that appear blue-green for physiological
reasons (the human eye and brain misjudge the color contrast.
a) The changes over time apparently result from dust that is either covering surface rock or is blow
off by winds that can be hundreds of miles per hour.
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8) From Mars’s mass and radius we can calculate that it has an average density of about that of the Moon,
substantially less that the density of Mercury, Venus and Earth
a) The difference indicates that Mars’s overall composition must be fundamentally different from
that of these other planets. Mars probably has a smaller iron-rich core and a thicker crust than
does Earth
Mars Surface
1) Mars has been the target of several series of space craft. Mostly from the US, including Mariner 9 in
1971, a pair of Vikings that started orbiting in 1976 and which dropped two landers onto the martian
surface; Mars pathfinder which landed on July 4, 1997, and placed Sojourner rover on Mars’s surface and
Mars Global Surveyor which is circling the planet.
2) Surface temperatures measured from the Viking landers ranged from a low of 150 K (-190oF) at the
northern site to over 300 K (80oF). The temperature varied each day by 35 to 50oC (60 to 90oF)
3) The surface of Mars can be divided into four major types: volcanic regions, canyon areas, expanses of
craters, and terraced areas near the poles
4) A surprise found on Mars were widespread areas of volcanism
a) The largest volcano which corresponds in position to the surface marking known as Nix Olympia
(the snow of Olympus) is named Olympus Mons (Mount Olympus)
b) It is a huge volcano, 600 kilometers at its base and about 25 kilometers high. Its crater is so wide
Manhattan Island could be dropped inside
c) Perhaps the volcanic features on Mars can get so huge because continental drift is absent there
d) If molten rock flowing upward causes volcanoes to form, then on Mars the features just get
bigger and bigger for hundreds of millions of years since the volcanoes stay over the sources and
do not drift away
5) A system of canyons was also discovered on Mars
a) One tremendous canyon – about 5000 km long – is as big as the continental United States and
comparable with the Rift Valley in Africa, the longest geological fault on Earth
6) The most amazing discovery on Mars was the presence of sinuous channels
a) Some of the channels show some characteristic features of streambeds on Earth
7) Even though liquid water cannot exist on the surface of Mars under today’s condition, it is difficult to
think of ways to explain the channels satisfactorily other than to say that they were cut by running water
in the past
a) Data from Mars pathfinder and the high-resolution Mars Global Surveyor found additional signs,
such as terracing, showing that had flowed
b) The indication that water most likely flowed on Mars is interesting to biologists who feel that
water is necessary for the formation and evolution of life
c) The presence of water on Mars, even in the past may indicate that life could have formed and
may even have survived – but where did the water go
d) Most of the water is probably in a permafrost layer – permanently frozen subsoil – beneath the
middle latitudes and polar regions, although a substantial fraction may have escaped from Mars
in gaseous form
e) Some of the water is bound in the polar caps
i) The large polar caps that extend to latitude 50o during the winter are carbon dioxide, but
when the cap shrinks during its hemisphere’s summer, a residual polar cap of water remains
in the north, while the south has a residual polar cap of carbon dioxide, probably with water
ice below
Mars Atmosphere
1) The martian atmosphere is composed of 95% carbon dioxide with small amounts of carbon monoxide,
oxygen, and water
2) The surface pressure is less than 1 percent of that near Earth’s surface
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3) The current atmosphere is too thin to significantly affect the surface temperature, in contrast to the
effect of the atmospheres of Venus and Earth have on climate
4) Long ago, Mars had a thicker atmosphere and a hospitable climate
a) Perhaps partial loss of the atmosphere as it escaped into space, lead to colder overall
temperatures and the freezing of CO2 and water
b) There would consequently be less greenhouse heating and hence colder temperatures
5) The lengthy observations of Mars’s atmosphere made by the Viking orbiters, led to the discovery of
weather patterns on Mars
6) With its rotation period similar to that of Earth, some features of Mars’s weather are similar to our own
a) Studies of Mars weather have already led to better understanding of windstorms in Africa that
affect weather as far away as North America
7) Studies of martian dust storms on the planet led to the idea that the explosion of a nuclear weapon on
Earth would lead to a “nuclear winter”
a) Dust through into the air would shield the Earth’s surface from sunlight for a lengthy period, with
dire consequences on Earth
b) The effect, at present, seems smaller than first feared, so it is better described as a “nuclear fall”
which should still be avoided