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Chapter 22
Venus and Mars
Guidepost
The previous chapter grouped Earth’s moon and
Mercury together because they are similar worlds. This
chapter groups Venus and Mars together because we
might expect them to be similar. They are Earthlike in
their size and location in the solar system, so it is
astonishing to see how different they actually are. Much
of this chapter is aimed at understanding how Venus
and Mars evolved to their present states.
Neither Venus nor Mars can tell us much about the
formation of the planets. Both planets have evolved
since they formed. Nevertheless, we find further hints
that the solar system was a dangerous place, with major
impacts smashing the surfaces of the planets, a process
we first suspected when we studied the moon and
Mercury.
Guidepost (continued)
This chapter concludes our exploration of the Earthlike
worlds. In the next two chapters, we will visit planets
that give new meaning to the word “unearthly.”
“Learning is not the result of
an accident, it is the result
of a well-directed strategy.”
Outline
I. Venus
A. The Rotation of Venus
B. The Atmosphere of Venus
C. The Venusian Greenhouse
D. The Surface of Venus
E. Volcanism on Venus
F. A History of Venus
II. Mars
A. The Canals of Mars
B. The Atmosphere of Mars
C. The Geology of Mars
D. Hidden Water on Mars
E. A History of Mars
III. The Moons of Mars
A. Origin and Evolution
Venus and Mars
Two most similar planets to Earth:
• Similar in size and mass
• Same part of the solar system
• Atmosphere
• Similar interior structure
Yet, no life possible on either one of them.
The Rotation of Venus
• Almost all planets rotate
counterclockwise, i.e. in the
same sense as orbital motion.
• Exceptions: Venus, Uranus
and Pluto
• Venus rotates clockwise,
with period slightly longer
than orbital period.
Possible reasons:
• Off-center collision with
massive protoplanet
• Tidal forces of the sun on molten core
The Atmosphere of Venus
UV
UV image
image
4 thick cloud layers ( surface
invisible to us from Earth).
Very stable circulation patterns with
high-speed winds (up to 240 km/h)
Extremely inhospitable:
96 % carbon dioxide (CO2)
Very efficient “greenhouse”!
3.5 % nitrogen (N2)
Rest: water (H2O), hydrochloric
Extremely high surface
acid (HCl), hydrofluoric acid (HF)
temperature up to 745 K (= 880 oF)
The Surface of Venus
Early radar images already revealed mountains, plains, craters.
More details from orbiting and landing spacecraft:
Venera 13 photograph of surface of Venus:
Colors modified
by clouds in
Venus’s
atmosphere
After correction
for atmospheric
color effect:
Radar Map of Venus’s Surface
Surface features
shown in
artificial colors
• Scattered
impact craters
• Volcanic
regions
• Smooth lava
flows
Lava Flows
Young, uneven lava flows (shown: Lava flow near
Flagstaff, AZ) show up as bright regions on radar
maps.
Surface Features on Venus
Smooth
lowlands
Highland Maxwell Montes
regions: are ~ 50 % higher
than Mt. Everest!
Craters on Venus
Nearly 1000 impact
craters on Venus’s
surface:
 Surface
not
very old.
No water on the
surface; thick,
dense atmosphere
 No
erosion
 Craters
appear
sharp and fresh
Volcanism on Earth
Volcanism on Earth is commonly
found along subduction zones
(e.g., Rocky Mountains).
This type of volcanism is not found on Venus or Mars.
Shield Volcanoes
Found above
hot spots:
Fluid magma
chamber, from
which lava erupts
repeatedly through
surface layers
above.
All volcanoes on Venus and Mars are shield volcanoes
Shield Volcanoes (2)
Tectonic plates moving over hot spots producing
shield volcanoes  Chains of volcanoes
Example: The
Hawaiian Islands
Volcanism on Venus
Sapas Mons (radar image)
~ 400 km (250 miles)
2 lava-filled calderas
Lava flows
Volcanic Features on Venus
Baltis Vallis: 6800 km long
lava flow channel (longest
in the solar system!)
Some lava flows collapsed
after molten lava drained away
Aine
Corona
Coronae: Circular bulges formed by
volcanic activity
Pancake
Domes:
Associated
with volcanic
activity forming
coronae
Lakshmi Planum and Maxwell Mountains
Radar image
Wrinkled mountain formations indicate compression
and wrinkling, though there is no evidence of plate
tectonics on Venus.
A History of Venus
Complicated history; still poorly understood.
Very similar to Earth in mass, size, composition, density,
but no magnetic field  Core solid?
 Solar wind interacts
directly with the
atmosphere, forming a bow
shock and a long ion tail.
CO2 produced during
outgassing remained in
atmosphere (on Earth:
dissolved in water).
Any water present on the
surface rapidly evaporated →
feedback through enhancement
of greenhouse effect
Heat transport from core mainly through magma flows
close to the surface ( coronae, pancake domes, etc.)
Mars
• Diameter ≈ 1/2 Earth’s
diameter
• Axis tilted against
orbital plane by 25o,
similar to Earth’s
inclination (23.5o)
• Seasons similar to
Earth  Growth and
shrinking of polar ice
cap
• Crust not broken into
tectonic plates
• Volcanic activity
(including highest
volcano in the solar
system)
• Very thin
atmosphere, mostly
CO2
• Rotation period
= 24 h, 40 min.
Tales of Canals and Life on Mars
Early observers (Schiaparelli, Lowell) believed to see
canals on Mars
This, together with
growth/shrinking of
polar cap, sparked
imagination and sci-fi
tales of life on Mars.
We know today:
“canals” were optical
illusion; do not exist!
No evidence of life on
Mars.
The Atmosphere of Mars
Very thin: Only 1% of pressure on Earth’s surface
95 % CO2
Even thin Martian
atmosphere
evident through
haze and clouds
covering the
planet
Occasionally:
Strong dust storms
that can enshroud
the entire planet.
The Atmosphere of Mars (2)
Most of the Oxygen bound in oxides in rocks
 Reddish
color of the surface
History of Mars’s Atmosphere
Atmosphere probably
initially produced
through outgassing.
Loss of gasses from a
planet’s atmosphere:
Compare typical velocity
of gas molecules to
escape velocity
Gas molecule
velocity greater than
escape velocity 
gasses escape into
Mars has lost all lighter gasses;
space.
retained only heavier gasses (CO ).
2
The Geology of Mars
Giant volcanoes
Valleys
Impact craters
Reddish deserts of broken
rock, probably smashed by
meteorite impacts.
Vallis
Marineris
The Geology of Mars (2)
Northern Lowlands: Free of craters; probably
re-surfaced a few billion years ago.
Possibly once
filled with water.
Southern Highlands: Heavily cratered; probably
2 – 3 billion years old.
Volcanism on Mars
Volcanoes on
Mars are shield
volcanoes.
Olympus Mons:
Highest and
largest volcano
in the solar
system.
Volcanism on Mars (2)
Tharsis rise
(volcanic bulge):
Nearly as large as
the U.S.
Rises ~ 10 km
above mean
radius of Mars.
Rising magma has
repeatedly broken
through crust to
form volcanoes.
Hidden Water on Mars
No liquid water on the surface:
Would evaporate due to low pressure.
But evidence for liquid water in the past:
Outflow channels from sudden,
massive floods
Collapsed structures after
withdrawal of sub-surface water
Splash craters and valleys
resembling meandering river beds
Gullies, possibly from debris flows
Central channel in a valley
suggests long-term flowing water
Hidden Water on Mars (2)
Gusev Crater and Ma’adim Vallis:
Giant lakes might have drained repeatedly
through the Ma’adim Vallis into the crater.
Ice in the Polar Cap
Polar cap contains
mostly CO2 ice,
but also water.
Multiple ice regions
separated by valleys
free of ice.
Boundaries of
polar caps
reveal multiple
layers of dust,
left behind by
repeated growth
and melting of
polar-cap
regions.
Evidence for Water on Mars
Galle,
the “happy face crater”
Meteorite ALH84001:
Identified as ancient rock from Mars.
Large impacts may have
ejected rocks into space.
Some minerals in this meteorite were
deposited in water  Martian crust must
have been richer in water than it is
today.
The Moons of Mars
Two small moons:
Phobos and
Deimos.
Too small to pull
themselves into
spherical shape.
Typical of small,
rocky bodies: Dark
grey, low density.
Phobos
Very close to Mars; orbits around
Mars faster than Mars’ rotation.
Probably captured from outer
asteroid belt.
Deimos
New Terms
subsolar point
composite volcano
shield volcano
corona
outflow channel
valley network
Discussion Questions
1. From what you know of Earth, Venus, and Mars, do
you expect the volcanoes on Venus and Mars to be
active or extinct? Why?
2. If humans someday colonize Mars, the biggest
problem may be finding water and oxygen. With plenty
of solar energy beating down through the thin
atmosphere, how might colonizers extract water and
oxygen from the Martian environment?
1. Why do larger and more massive planets take longer to cool?
a. Larger planets have a greater mass-to-surface area ratio and therefore cool
more slowly.
b. The more mass a planet has, the greater is its heat of formation.
c. The more mass a planet has, the more heat-producing radioactive elements it
has.
d. The more mass a planet has, the thicker is its insulating blanket.
*e. All of the above.
2. Why might we expect Venus and Earth to be similar?
a. Both planets are about the same size and density.
b. Both planets have about the same chemical composition.
c. Both planets have about the same atmospheric composition.
*d. Both a and b above.
e. All of the above.
3. Why might Venus have the slow retrograde rotation we see today?
a. A large off-center impact may have set Venus to spinning backwards.
b. The long-term effect of solar tides on its dense atmosphere may be
responsible.
c. The tidal effect of Earth on Venus may have spun Venus backwards.
*d. Both a and b above.
e. All of the above.
4. What evidence do we have that Venus once had more water than at present?
a. We find dry lakebeds on the surface of Venus.
b. We find dry riverbeds on the surface of Venus.
*c. The deuterium hydrogen ratio in the atmosphere of Venus is about 150 times
that of Earth.
d. Both a and b above.
e. All of the above.
5. Which gas is most abundant in the atmospheres of Venus and Mars?
a. Oxygen (O2).
b. Nitrogen (N2).
c. Argon (Ar).
*d. Carbon dioxide (CO2).
e. Methane (CH4).
6. Why is the surface temperature of Venus higher than that of any other planet?
a. Its period of rotation is longer than that of any other planet.
b. Its orbit is more nearly circular than that of any other planet.
*c. It has an extreme greenhouse effect.
d. It is the closest planet to the Sun.
e. It has a retrograde rotation.
7. Why are the surface temperatures of Venus and Earth so very different?
a. Venus is too close to the Sun to have liquid water oceans.
b. Earth is far enough from the Sun to have liquid water oceans.
c. The ozone layer of Earth shields the surface from ultraviolet radiation.
*d. Both a and b above.
e. All of the above.
8. How do we know what the surface of Venus looks like?
a. Large Earth-based optical telescopes have photographed the surface.
b. The high resolution of the Hubble Space Telescope reveals the surface details.
*c. Radar mapping at radio wavelengths allows us to determine surface elevations.
d. Orbiting spacecraft have imaged the surface at ultraviolet and infrared wavelengths.
e. Two spacecraft that entered the atmosphere of Venus released balloons that floated
just beneath the cloud deck and mapped the surface.
9. Why do you suppose that the smallest impact craters on the Moon are microscopic,
the smallest on Earth are dozens of meters in diameter, and the smallest on Venus
are more than one kilometer in diameter?
a. The Moon has no atmosphere and Venus has an atmosphere much more dense
than Earth's.
b. Atmospheric gases easily slow the speed of smaller incoming meteoroids.
c. Venus is farther away from the asteroid belt.
*d. Both a and b above.
e. All the above.
10. All of the dormant volcanoes on Venus and Mars are the shield type, and many are
much larger than any shield volcano on Earth. What does this tell us about Venus and
Mars?
a. Venus and Mars both have plate tectonics
*b. Neither Venus nor Mars has plate tectonics.
c. Their interiors are at a higher temperature than Earth's interior.
d. Their interiors are at a lower temperature than Earth's interior.
e. Venus and Mars both have carbon dioxide atmospheres
11. Planets that have strong magnetic fields also have rapid rotation and convective
fluid interior zones that are good electrical conductors. The magnetic field of Venus is
so weak that it has not yet been detected, which makes it at least 25,000 times weaker
than Earth's magnetic field. Examine the Venus data file on page 465. What
information here may indicate why Venus is lacking a strong magnetic field?
a. The eccentricity of orbit = 0.0068.
b. Inclination of orbit to ecliptic = 3 degrees, 23 arc minutes, 40 arc seconds.
*c. Period of rotation = 243.01 days (retrograde).
d. Average density = 5.24 grams per cubic centimeter.
e. Surface temperature = 745 K (472ºC or 882ºF).
12. What determines whether an atmospheric gas can be retained by a planet, rather
than be lost to space?
a. The escape speed of the planet.
b. The mass of the gas atoms or molecules.
c. The temperature of the planet's upper atmosphere.
d. Both a and b above.
*e. All of the above.
13. What evidence do we have that the Martian atmosphere was denser in the past than
it is today?
a. The abundance of argon by mass is 1.6%.
b. The observed dry streambeds and outflow channels.
c. Mars is a small body and light gases can easily escape.
*d. Both a and b above.
e. All of the above.
14. Evidence suggests that Mars had a greater abundance of atmospheric carbon
dioxide in the past than it has today. How could Mars have lost some of its atmospheric
carbon dioxide?
a. Some carbon dioxide solidified and is part of the polar ice caps.
b. The solar wind can ionize and strip gas molecules from the upper atmosphere.
c. The weak gravity field of Mars cannot easily hold onto carbon dioxide molecules.
*d. Both a and b above.
e. All of the above.
15. Measurements from orbiting spacecraft suggest that Mars had a magnetic field in its
past. How could Mars lose its magnetic field?
*a. Mars has cooled to the point that it no longer has a fluid interior.
b. The rotation of Mars was once retrograde and now is in the prograde direction.
c. Mars’ rate of rotation has slowed such that it now has tidal coupling with the Sun.
d. Both a and b above.
e. All of the above.
16. What evidence do we have that Mars had much more liquid water at its surface in
the past than it has today?
a. The deuterium-hydrogen ratio is 5.5 times as high as on Earth.
b. Martian meteorites must have formed from lava that was nearly 2% water.
c. We see large, dry outflow channels and valley networks on the surface of Mars.
d. Both a and b above.
*e. All of the above.
17. How do we know that the slope gullies found in recent images are younger than
both the outflow channels and the valley networks?
a. These gullies did not appear in the Viking images of 1976, and thus must have
formed in recent years.
*b. The outflow channels and valley networks have a higher density of impact craters.
c. The gullies are darker, indicating that the soil along their floors is still wet.
d. The gullies are smaller than the outflow channels and valley networks.
e. The outflow channels and valley networks are larger than the gullies.
18. What do we now believe is responsible for the seasonal changes that were seen on
Mars through Earth-based telescopes over the past century or two?
a. These changes are optical illusions.
*b. These changes are due to seasonal global dust storms on Mars.
c. These are seasonal plant life cycles in the plains and forests of Mars.
d. These are long linear faults that break open then close due to the freezing and
thawing of ground water.
e. These changes are due to global volcanic activity that creates new shield volcanoes
and fresh lava plains.
19. A large high-mass terrestrial planet cools more slowly than a small low-mass
terrestrial planet. The outward flow of heat drives surface activity for a longer period of
time on larger bodies. Thus, today we expect to find a lower density of impact craters
on larger bodies. Use this information to predict the order of the terrestrial planets from
lowest density of impact craters to the highest density of impact craters.
a. Mercury, Venus, Earth, Mars.
b. Mercury, Mars, Venus, Earth.
c. Earth, Mercury, Mars, Venus.
d. Mars, Earth, Mercury, Venus.
*e. Earth, Venus, Mars, Mercury.
20. Why are Phobos and Deimos not spherical?
a. They are the two halves of a once spherical body.
*b. Their gravity fields are too weak to pull their material into a spherical shape.
c. They were spherical at one time and have accreted a few large rocky blocks that hide
their original shape.
d. They were originally spherical and have suffered many small random collisions that
give them a chiseled appearance.
e. The non-spherical shape suggests that they skimmed the atmosphere of Mars during
the capture event and were shaped by the encounter.
Demos----Panic. (pandemonium)
Phobos-----Fear. (phobia)