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.

(More on this later.)

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.