Terrestrial Planets

Planetary size and heat loss

Large planets retain internal heat longer

Small planets cool off quickly

Geologic processes

1) Impact cratering

2) Volcanism

3) Tectonics

4) Erosion

Discussion

The Moon has lots of impact craters, while the Earth has very few. Why do you think there is a difference?

Surface age

The more impact craters on a surface, the older that surface is.

The age of a surface refers to when that surface solidified.

Discussion

Which area on the Moon is older, the light region to the left or the dark region in the center of the picture?

Discussion

Why do you think a more heavily cratered planetary surface is older than a smooth one?

Why?

All the terrestrial planets probably receive about the same number and size distribution of impacts. All the other geologic processes (volcanism, tectonics, and erosion) tend to erase impact craters on the surface.

Discussion

Rank the terrestrial planets (include the Moon) in terms of the age of their surfaces from youngest to oldest to try and predict which planets will have the most craters.

1. Earth2. Venus3. Mars4. Mercury5. Moon

Smaller planets retain less heat and therefore have less geologic activity.

Discussion

Almost all impact creators are round. Why do you think there are not many elliptically shaped craters?

Almost all impact craters are round

In general, impactors will hit with a velocity greater than the escape velocity. The high speed collision results in the almost complete vaporization of the impactor. This explosion of super-heated, vaporized rock is what creates the crater.

Moltke Crater (7 km)

Bessel crater (16 km)

Euler Crater (25 km)

King Crater (77 km)

Copernicus Crater (95 km)

Schrodinger (320 km)

Mare Orientale (930 km)

Mare Imbrium (1100 km)

Meteor Crater (1.2 km)

Arizona from the Shuttle

Wolf Creek (0.85 km)

New Quebec Crater (3.4 km)

Clearwater Lakes (26 km)

Chichxulub Crater (250 km)

Manicouagan Crater (100 km)

Discussion

Why is the Manicouagan crater inverted, with a lower rim and higher center?

Hint: It is at high northern latitude and is rather old.

Radar images of Venus

Bright areas in radar images indicate rough terrain, while dark areas are smooth. Typically, rough terrain is younger than smooth terrain.

Discussion

Why are rough areas younger?

Adivar Venus (29 km)

Meitner basin Venus (150 km)

Discussion

What does the fact that there are so few eroded craters tell you about Venus?

Plate Tectonics

No plate tectonics on Venus

Venus may not have an asthenosphere, due to the lack of water in Venus’s mantle.

Alternatively, the crust of Venus may be too plastic to form large-scale faults.

Colaris basin Mercury (1340 km)

Hellas basin Mars (2000 km)

Discussion

Notice anything unusual about this crater on Mars? What can you conclude about the surface of Mars where this crater formed?

Why are craters important

• Numbers of craters provide relative ages

• Transition size between simple and complex craters as well as ejecta blanket size and distribution provide clues to the strength of the surface.

Liquid Water

All life on Earth depends on liquid water.

Earth is the only body in the solar system with liquid water on its surface.

Atmospheres

All the terrestrial planets and the Moon have some atmosphere.

The Moon and Mercury have very thin atmospheres while Venus has the thickest.

Venus at inferior conjunction

Effects of atmospheres

Atmospheric pressure keeps liquids from boiling

Protects the surface from solar wind and cosmic rays and, depending on composition, ultraviolet and x-ray radiation

Moderates surface temperatures

Discussion

How do you think an atmosphere can moderate a planet’s surface temperature?

As the Sun shines on the planets surface, it heats the soil and rocks. Without an atmosphere that heat is radiated away mostly in the low-energy infrared region of the spectrum.

If the planet has an atmosphere, the heat can be transferred to the gas molecules and stored, acting as a blanket.

Discussion

Rank the terrestrial planets (including the Moon) in terms of their daily temperature swings from most extreme to the least extreme?

Surface temperatures

Mercury – day: 425 C night: -175 CMoon – day: 125 C night: -175 C Mars – day: -3 C night: -96 CEarth – day: 15 C night: 3 CVenus – day: 470 C night: 470 C

Discussion

Although Venus is closer to the Sun than Earth, its cloud cover reflects nearly 60% of the light that falls on it. The surface therefore receives less energy from the Sun that the Earth’s surface does.

So why is Venus so much hotter than Earth?

The greenhouse effect

Carbon dioxide (as well as water vapor and methane) absorb infrared radiation. Sunlight can pass to the surface of Venus and heat it. But the infrared radiation it emits can not escape back into space.

Exam next Tuesday

Covers chapters 1-4 in the text + Seasons (p265) and all lecture notes including this week

4 essay questions in which you will be asked to explain concepts

Allowed one sheet of notes with writing on one side only