April 11, 2006Astronomy 20101 Chapter 9 Earth-Like Planets: Venus and Mars Venus and Mars resemble...

April 11, 2006 Astronomy 2010 1

Chapter 9Earth-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 Earth’s origins or future.


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.


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.


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 Earth’s.

• 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)!!


Properties of Earth, Venus, and Mars

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

• Compare some properties of these planets.


9.1.3 Basic PropertiesVenusVenus

• 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.

MarsMars• Smaller than Earth• Thin atmosphere• Significant past geological activity

• Probably had a thick atmosphere and liquid water in the past.

• Could have supported life.


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).


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.

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Globe of Venus constructedfrom Magellan radar maps.


Venus Express: Orbiting Venus


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.


Radar Maps of Venus

N

S

1

2


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.


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 don’t make it to the surface, but push up the crust in bulges called coronae.


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).


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.


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.


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, didn’t function).


9.4.2 Global Properties of Mars• Mars is half

Earth’s 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


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


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?).


9.5 Martian Polar Caps and Climate

• Thin atmosphere, like Earth’s 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).


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).


9.5.5 Life on Mars


Search for Life on Mars• Life as we know it needs water.• If Mars had water in the past, life could have developed.

• We’ve 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)


Martian Meteorite


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.

April 11, 2006Astronomy 20101 Chapter 9 Earth-Like Planets: Venus and Mars Venus and Mars resemble...

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