Earth-Like Planets: Venus and Mars

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Chapter 9

• Earth-Like Planets Venus and Mars

• Venus and Mars resemble Earth more than any other
  planets.
• Is it possible that life exists or did exist on
  either?
• One day, will we be able to establish a human
  presence on Venus or Mars?
• Can studying these planets give us clues to
  Earths origins or future.

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9.1.1 Appearance of Venus

• Venus gets closer to Earth than any other planet.
• It appears as a bright object near the Sun after
  sunset or before sunrise.
• Venus goes through phases, like the Moon.
• Venus is shrouded in thick clouds making it
  impossible to view the surface from Earth.

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9.1.1 Appearance of Mars

• About every 24 months the Earth and Mars are at
  their closest.
• Mars is reddish due to iron oxides (rust) in the
  soil.
• Around 1900, Percival Lowell thought he saw
  canals on Mars!
• Mars has polar ice caps, thin clouds, and dust
  storms.

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9.1.2 Rotation of the Planets

• The rotation of Mars has been measured by
  observing features on the planet over a very long
  time, about 200 years.
• Mars sidereal rotation period is 24 h 37m 23s,
  about 40min longer than Earths.
• The rotation of Venus is measured using radar,
  but unlike Mercury, the radar is used to observe
  surface features and watch them rotate.
• Venus sidereal rotation period is 243 days!
• Venus rotates backwards (retrograde)!!

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Properties of Earth, Venus, and Mars

• Compare some properties of these planets.

Earth Venus Mars
Semi major axis (AU) 1.00 0.72 1.52
Period (Earth years) 1.00 0.61 1.88
Mass (Earth 1) 1.00 0.82 0.11
Diameter (Earth 1) 1.00 0.95 0.53
Density (g/cm3) 5.5 5.3 3.9
Surface Gravity (Earth 1) 1.00 0.91 0.38
Escape velocity (km/s) 11.2 10.4 5.0
Rotation period 23.9 hours 243 days 24.6 hours
Surface Area (Earth 1) 1.00 0.90 0.28
Atmospheric Pressure (bar) 1.00 90 0.007
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9.1.3 Basic Properties

• Venus
• Very similar to Earth
• High geological activity
• Thick atmosphere of CO2
• Surface temperature of 730K (over 850 F).
• Atmospheric pressure of 90 bar, equivalent to 1km
  under the ocean.

• Mars
• Smaller than Earth
• Thin atmosphere
• Significant past geological activity
• Probably had a thick atmosphere and liquid water
  in the past.
• Could have supported life.

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9.2 The Geology of Venus

• Being of similar size and composition to Earth,
  we might expect Venus to have similar geology.
• More spacecraft have visited Venus than any other
  planet.
• 1962 U.S. Mariner 2 flyby
• 1970 Soviet Venera 7 lands on Venus and sends
  back pictures for 23 minutes (heat).

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More Missions to Venus

• 1970s Venera craft measure soil and atmosphere.
• 1970s U.S. Pioneer Venus -- radar map.
• 1980s Venera 15 and 16 radar orbiters.
• 1991-3 Magellan radar maps with 100m resolution.

Globe of Venus constructed from Magellan radar
maps.
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Venus Express Orbiting Venus
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Venus Express

• Built by the European Space Agency (ESA)
• Launched on a Russian Soyuz rocket in November
  2005.
• Arrived at Venus April 11, 2006.
• Will study Venus for 2 to 5 years, especially to
  understand the greenhouse effect.

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Radar Maps of Venus
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S
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9.2.3 Venus Craters and Surface Age

• Count craters to estimate surface age.
• Largest crater called Meade, 275km in diameter.
• Thick atmosphere stops only small projectiles.
• Almost no craters smaller than 10km in diameter.
• Count larger craters and estimate surface age at
  500 million years.

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9.2.4 Volcanoes on Venus

• Significant volcanism.
• Largest volcano, Sif Mons, is wider but shorter
  than Mauna Kea.
• Some volcanoes produce pancake domes.
• Many volcanoes dont make it to the surface, but
  push up the crust in bulges called coronae.

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9.3 The Massive Atmosphere of Venus

• Atmosphere is 96 CO2, 3.5 nitrogen and trace
  amounts of H2O, H2SO4.
• The CO2 traps heat on the planet via the
  greenhouse effect.
• Runaway greenhouse effect.
• The surface temperature is more than 700 Kelvin
  (850F).

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The Birth of Venus

• Backward rotation could mean that Venus suffered
  a giant impact early in its history.
• The runaway greenhouse effect explains why Venus
  has such a massive atmosphere and high surface
  temperatures.

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9.4 The Geology of Mars

• Humans could survive on Mars, making it more
  interesting.
• Spacecraft have visited and landed on Mars.
• 1965 Mariner 4 fly by
• 1971 Mariner 9 orbited
• 1976 Vikings 1 and 2 landed
• 20 years pass with 2 failed missions to Mars.

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9.4.1 Spacecraft Exploration of Mars

• 1997 Pathfinder lands on Mars and Global
  Surveyor orbits.
• Several more failed space missions.
• 2004 4 spacecraft to Mars, 2 U.S., 1 ESA, 1
  Japan. (Japanese craft failed to orbit, ESA
  rover, Beagle, didnt function).

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9.4.2 Global Properties of Mars

• Mars is half Earths diameter.
• Small metal core.
• No magnetic field.
• We have good maps of Mars showing
• Olympus Mons highest peak in the solar system.
• Vallis Marineris largest canyon in the solar
  system.
• Old highlands and younger lowlands.

Olympus Mons
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9.4.3 Volcanoes on Mars

• The lowland plains seem to be formed 3 to 4
  billion years ago
• Lava flows
• Largest volcanoes on the Tharsis bulge.
• 3 shown at right
• Olympus Mons is largest, larger than Mauna Loa in
  Hawaii

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9.4.4 Martian Cracks and Canyons

• Valles Marineris is the largest canyon in the
  solar system.
• Formed by cracking in the Tharsis bulge
• L.A. to N.Y.
• Landslides in valley are evidence of erosion
  (water?).

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9.5 Martian Polar Caps and Climate

• Thin atmosphere, like Earths 30km up.
• Mostly CO2.
• Clouds of dust, H2O, and CO2.
• Seasonal ice caps of CO2 (dry ice).
• Permanent polar caps (south at left) have H2O.
• Ice in the soil (permafrost).

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9.5.4 Climate Change on Mars

• Mars suffers from the runaway refrigerator
  effect.
• Mars might have had a much thicker atmosphere and
  milder climate in the past.
• Mars has smaller surface gravity than Venus or
  Earth.
• Atmospheric gases can escape into space, cooling
  off the planet (evaporative cooling).

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9.5.5 Life on Mars
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Search for Life on Mars

• Life as we know it needs water.
• If Mars had water in the past, life could have
  developed.
• Weve tried looking for signs of life in a
  variety of ways, so far with negative results.
• It could be that UV light has sterilized the
  surface -- could microbes be underground?
• Some scientists claim that a meteorite from Mars
  shows signs of microbial life. (next)

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Martian Meteorite
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Discussion Question

• This is to review some of the material just
  presented.
• First answer the following question
• Venus, Earth, and Mars have mountains. What are
  two factors that may explain why Mars has the
  largest mountain, though it is the smallest of
  the three planets.
• Second pass your answer to another person for
  discussion and grading. Please sign your name at
  the bottom of the answer you are grading.