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ASTR100 (Spring 2008) Introduction to AstronomyEarth as a Planet
Prof. D.C. Richardson
Sections 0101-0106
The Greenhouse Effect
Certain molecules let sunlight through but trap escaping infrared photons.
(H2O, CO2, CH4)
The Greenhouse Effect
Greenhouse Effect: Bad?
The Earth is much warmer because of the greenhouse effect than it would be without an atmosphere……but so is Venus.
Why is the sky blue?
The atmosphere scatters blue light from the Sun, making it appear to come from all directions.
Sunsets are red because less of the red light from the Sun is scattered.
Planetary magnetic fields
Moving charged particles create magnetic fields.
So can a planet’s interior, if the core is electrically conducting, convecting, and rotating, i.e., hot!
Earth’s Magnetosphere
Earth’s magnetic fields protects us from charged particles from the Sun.
The charged particles can create aurorae (e.g., the “Northern lights”).
If a planet’s core is cold, do you expect it to generate magnetic
fields?
A. Yes, refrigerator magnets are cold, and they have magnetic fields.
B. No, planetary magnetic fields are generated by moving charges around, and if the core is cold, nothing is moving.
If a planet’s core is cold, do you expect it to generate magnetic
fields?
A. Yes, refrigerator magnets are cold, and they have magnetic fields.
B. No, planetary magnetic fields are generated by moving charges around, and if the core is cold, nothing is moving.
ASTR100 (Spring 2008) Introduction to AstronomyThe Other Terrestrial Worlds
Prof. D.C. Richardson
Sections 0101-0106
Was there ever geological activity on the Moon or
Mercury?
Moon
Some volcanic activity 3 billion years ago must have flooded lunar craters, creating lunar maria.
The Moon is now geologically dead.
Impact Flooding Mare Humorum
Before
Cratering of Mercury
Mercury has a mixture of heavily cratered and smooth regions like the Moon.
The smooth regions are likely ancient lava flows.
Tectonics on Mercury
Long cliffs indicate Mercury shrank early in its history.
Mercury is now geologically dead.
Shrinkage not to scale!
Messenger at Mercury
• First flyby Jan 14, 2008.
• Will enter orbit Mar 19, 2011.
• First views of unseen side.
Mars versus Earth
50% Earth’s radius, 10% Earth’s mass.
1.5 A.U from the Sun. Axis tilt about the same as Earth.
Similar rotation period. Thin CO2 atmosphere: little greenhouse.
Main difference: Mars is smaller.
Seasons on Mars
Seasons on Mars are more extreme in the southern hemisphere because of its elliptical orbit.
Storms on Mars
Seasonal winds on Mars can drive huge dust storms.
What geological features tell us water once flowed
on Mars?
Surface of Mars appears to have ancient river beds.
Closeup of a crater perhaps eroded by water.
Volcanoes…as recent as 180 million years ago…
Past tectonic activity…
Low-lying regions may once have had oceans.
Low-lying regions may once have had oceans.
OpportunitySpirit
• 2004: Opportunity Rover provided strong evidence for abundant liquid water on Mars in the distant past.• How could Mars have been warmer and wetter in the past?
The Rovers
Today, most water lies frozen underground (blue regions)…
Why did Mars change?
Climate Change on Mars
Mars has not had widespread surface water for 3 billion years.
The greenhouse effect probably kept the surface warmer before that.
Somehow Mars lost most of its atmosphere…
Climate Change on Mars
Magnetic field may have preserved early Martian atmosphere.
Solar wind may have stripped atmosphere after field decreased because of interior cooling.
Is Venus geologically active?
Cratering on Venus
Impact craters, but fewer than Moon, Mercury, Mars.
Volcanoes on Venus
Many volcanoes, including both shield volcanoes and stratovolcanoes.
Tectonics on Venus
Fractured and contorted surface indicates tectonic stresses.
Erosion on Venus
Photos of rocks taken by landers show little erosion.
Does Venus have plate tectonics?
Most of Earth’s major geological features can be attributed to plate tectonics, which gradually remakes Earth’s surface.
Venus does not appear to have plate tectonics, but its entire surface appears to have been “repaved” 750 million years ago.
Why is Venus so hot?
Why is Venus so hot?
The greenhouse effect on Venus keeps its surface temperature at 470˚C.
But why is the greenhouse effect on Venus so much stronger than on Earth?
Atmosphere of Venus
Venus has a very thick carbon dioxide atmosphere with a surface pressure 90 times that of Earth.
Thick atmosphere produces an extremely strong greenhouse effect.
Rocks—like limestone (and some in plant life and in the ocean).
Why did this happen on Earth and not on Venus?
Venus lacks oceans to dissolve the carbon dioxide and lock it away in rock on the seafloor.
And why is that?
Where is Earth’s CO2?
What would happen if…
What would happen if Earth were moved to Venus’ orbit?
It would get hot!
How would the initial heating affect the oceans and the greenhouse effect?
It would get hotter!
Runaway Greenhouse Effect
Eventually H2O breaks down and H escapes into space, as apparently happened on Venus.