Lecture 5 6/8/07 AST1001. Geology Basics Can probe the interior of a planet with seismic waves Core of the planet is dense, made out of metals Mantle.

Lecture 5

6/8/07

AST1001

Geology Basics

• Can probe the interior of a planet with seismic waves

• Core of the planet is dense, made out of metals

• Mantle is moderate density

• Crust is very thin, contains low density rock

The Core• The Earth’s core consists of

two parts: the molten outer core and the solid inner core

• The Earth is almost entirely rock

• Other planets might be similar, but no seismic data

• Differentiation causes the density structure of the planets

Other Planets

• The lithosphere is the outer, thin layer of rocks

• Mars, Mercury, the Moon have very thick lithospheres that extend almost to their cores

• Earth, Venus have very large mantles

How Interiors Get Hot

• Accretion– Planetesimals collide, bring energy to planet

• Differentiation– Heavy stuff moves inward, reducing potential

energy

• Radioactive Decay– Lots of radioactive elements in core, and

radioactive decays produce energy

How Interiors Cool Off

• Convection– Hot things rise, cool things fall

– Examples: weather and boiling water

• Conduction– Contact allows heat transfer

• Radiation– Light is radiated into space

Group Work

• The text does a example surface area to volume calculation on page 255. Repeat the calculation but now calculate the ratio using Mars (radius = 3402 km). What does this tell you about why Mars is no longer very geologically active?

Magnetic Fields• Magnetic fields occur

due moving charged particles

• You need 3 things for a planetary magnetic field– An interior region of

electrically conduction fluid

– Convection in that fluid– Moderately rapid rotation

• Earth is the only terrestrial planet to have these things

Shaping Planetary Surfaces

• There are 4 ways of shaping planets surfaces– Impact Cratering– Volcanism– Tectonics– Erosion

Cratering

• Impacting objects typically travel at speeds of 250,000 mi/hr

• Collisions convert that kinetic energy into moving rock to make craters

• Craters can give us information about the planet’s past surface

Craters and Surface Aging

• Craters should be equally spaced and equally dense on all planets if there is no weathering

• Craters can be erased by erosion, lava, etc

• We can tell where activity happened by looking at crater density

• Smaller planets are less geologically active

Volcanism

• Refers to molten lava coming to the surface

• Magma rises because its less dense and is squeezed to the surface

• Lava creates volcanoes and volcanic plains

• Also creates rocks like basalt• Outgassing can bring different

gasses to the surface

Tectonics

• Refers to the stretching, compression or any other force on the lithosphere

• Usually occurs due to mantle convection

• We have massive pieces of the crust here on Earth, which leads to plate tectonics

Erosion

• Processes that break down or transport rock

• Can also create things like sand dunes, river deltas, deposits, sedimentary rock

Small Bodies

• Maria have formed on the Moon due to lava outflows

• Today the surface of the Moon is pretty much unchanging

• Mercury is very similar, but probably more volcanically active– Huge feature called Caloris

Basin

Mars• Surface has many dramatic differences

– Southern hemisphere has high elevation with lots of craters, including Hellas Basin

– Northern Hemisphere is low, has few craters

Volcanism on Mars

• Lots of volcanoes on northern plains

• Towering shield volcanoes

• The Tharsis Bulge is a continent sized feature

• Valles Marineris– Huge canyon– Might have formed due to tectonic cracking

Water on Mars• Lots of features that look like dry riverbeds

• No liquid water on the surface of Mars today– Water is not stable on the surface

• Spirit and Opportunity have found lots of minerals that would only form in water

Venus

• Magellan mapped the surfaces, down to features 100 meters in size

• Lots of volcanoes on Venus– Probably active

• Lots of contorted features, probably formed by tectonics

• Virtually no erosion

Map of Venus

Tectonics on Earth

• Ample evidence to suggest that plates drift around

• Results in a wide range of unique effects– Earthquakes– Hot spots– Pangaea

Atmosphere Basics

• An atmosphere is just the gasses surrounding the solid part of a planet– Atmosphere always makes up a tiny fraction of

the planet’s mass

• Planets have a wide range of atmospheres– Vary widely in composition, density

More Basics

• Atmospheres very thing– 2/3 of air on Earth lies within 10 kilometers of

surface

• Air pressure occurs due to particles running into things

• Air is held to the planet with gravity, so there is more air near surface

• Bars are used to measure pressure

Atmospheric Effects

• Creates pressure– Important for liquid water

• Scatters and absorbs light

• Creates wind and weather

• Creates magnetosphere

• Can cause greenhouse effect

Greenhouse Effect• The greenhouse effect is a natural effect• Earth absorbs some light from the Sun, reemits it as

infrared light• Greenhouse gasses absorb the IR light, delay its

escape• Too much of the greenhouse effect is a bad thing

Atmospheric Properties and Height

• The atmosphere has structure– Troposphere is closest to

the surface– Stratosphere– Thermosphere– Exosphere

Magnetosphere• The Sun emits a low

density breeze of charged particles called the solar wind

• The Earth’s magnetic sphere influences the particles

• Particles can get trapped along magnetic field lines– For Van Allen Belts– If they are in the

magnetosphere, then you get aurora

Mercury and the Moon

• Very thin atmospheres

• Created by “surface ejection” of particles

• There might be water ice in polar craters

Mars Today

• Present day surface of Mars looks a lot like a desert on Earth

• Atmosphere is very thin: 1% of pressure as that on Earth

• Most UV radiation gets through the atmosphere

Martian Seasons• Mars has seasons similar to those on Earth

– Twice as long

• Eccentricity of orbit is important– South has shorter, hotter summers and longer, colder winters

than North

• Can have strong winds

Mars in the Past• For its first billion years,

Mars might have been wet and warm

• Atmosphere was probably lost once its magnetic field went away– Particles left Mars and

went into space

– Rocks could have also taken oxygen

Group Work

• The escape velocity on Mars is about 5 km/s. The average daytime temperature is 230K. Using this information, compare the peak thermal velocity of the atoms in Mars’s atmosphere to the escape velocity. How does this relate to potential atmosphere loss by Mars?

Venus Today

• Surface is searing hot, very high atmospheric pressure

• Almost no wind or weather

• Temperature is uniform and no seasons

• Sulfuric acid in the atmosphere

• Mysterious high speed upper winds

Venus and Its Clouds

• Earth gets rid of its carbon dioxide with water– Where did Venus's water go?– Leading theory involves UV rays breaking

down the water

• Because Venus is slightly closer to the Sun, water evaporates more, creates runaway greenhouse effect