Inner Planets (Part I)

1) Science2) Intro to Inner Planets3) Four Main Processes4) Planetary Comparisons5) Intro to Atmospheres

Sept. 16, 2002

Announcements

If you are not here today due to Yom Kippur, there will be an opportunity to make up today’s quiz after class on Weds.

An extra credit problem will be available on the course web site tonight or tomorrow morning. It is due Thurs. Sep 19 at 5pm

Review

Pieces of the Solar System Sun (in a few weeks) inner planets (this week) outer planets (next week) other stuff (following week)

Angular momentum angular momentum is conserved

Solar System formation accretion disk, rotation, protostar planetesimals solar wind

Fact vs. Theory Facts are data which has been measured

e.g. the Sun rose this morning Theory is a model which describes/explains

data or predicts future events e.g. the Sun will set tonight a thrown baseball will follow an arc because of gravity and Newton’s Laws the Earth

revolves around the Sun and will continue to do so

It is impossible to prove a theory even Newton’s Laws are a theory

It IS possible to disprove a theory when facts do not agree with the model

Examples Fact: All major (currently observed)

planets/moons/asteroids are revolving around the Earth in the same direction and in a similar plane Theory: The Solar System was created from

a revolving sphere of gas and dust Facts: The inner planets are composed

primarily from refractory materials while the outer planets are mostly volatile materials Theory: The inner region was hotter and the

volatile materials did not survive there, but did in the outer region, this contributed to the planets’ formations

NASA Solar System Missions Flyby missions - satellite to pass by another object

quick look, but cheap examples: Voyager, Mariner, Pioneer, …

Orbiters - satellite in orbit around a planet or moon more detailed studies, but not “hands-on” examples: Galileo, Clementine, Magellan, …

Landers - lander on the surface of a planet or moon get rock samples and direct data, limited area can be

covered examples: Viking, Mars Surveyor, Mars Odyssey, …

Manned missions - humans on the surface of a planet or moon

can do advanced, complicated studies/experiments, but very expensive

examples: Apollo 11 through Apollo 17

Differentiation During planetary formation, the rocks

and planetesimals compress together due to gravity energy is converted into heat material melts and becomes fluid

Differentiation is the process of the heavier materials sinking towards the center of the planet while lighter materials rise to the outer edges materials become separated by type

Outer surface of planet cools fastest and hardens

Planet Interiors Layered

solid inner core liquid outer core solid outer mantle/crust

Hotter inside, cooler outside planet radiates heat into space outer crust cooled and hardened center is hottest and has highest pressure

Melting point depends on temperature and pressure in the center, pressure wins and material is solid farther out, temperature wins and material is

liquid outer edge, both lose and material is solid

Planet Interiors (cont)

The inner planets have similar structure although we don’t have a lot of data on

other planets Data on Earth’s interior comes from

seismic readings of earthquakes

Four Main Processes These processes shape the surfaces of

planets1) Tectonism

movement of pieces of the planet’s crust (plates)

2) Volcanism flow of material (lava) from beneath the

planet’s crust

3) Impact Cratering meteors hitting a planet’s surface

4) Gradation erosion of the surface

The first 3 processes build up structure on the surface (mountains, valleys, etc)

The last process wears the surface down

Tectonics

Major movements of the planets crust create mountain ranges, deep valleys on Earth: tectonic plates rub against each

other other planets: not plates, but major

cracking/shifting (fractures)

Interior Heating Radiative cooling alone should have cooled

the Earth’s interior more than observed Friction adds some of the heat

tidal forces due to the gravitational pull of the Moon and Sun cause pieces of the interior to rub together

this rubbing generates heat (just like rubbing your hands together)

Radioactive decays add most of the heat The interior temperature is a balance

between original heat, radiative cooling and additional heat

As the radioactive material disappears, the Earth’s interior will cool

Volcanism

Fissures in the planet’s crust can allow hot mantle to flow to the top (lava)

the mantle is solid, but after relieving the pressure from the crust, it can turn liquid

Long fissures cause shield volcanoes (large, long mounds of cooled lava) to form over long time periods

Local “holes” can form mounds on Earth, plate movement limits the size and can result

in a chain of islands Large flows of more fluid lava can create great

plains of lava e.g. Lunar mares (seas)

Amount of volcanic activity indicates how active a planet is

Comparative Volcanism Moon

mares are volcanic in nature and indicate the Moon once had a lot of lava flow

Mercury some visual indications of lava flow, not

enough known Mars

largest mountains in the Solar System (up to 25 km high) caused by volcanism

Venus evidence of a lot of complex volcanic activity

Earth lots of current and previous volcanic activity

(Pompeii, Hawaiian Islands, Mt. St. Helen’s)

Impact Cratering The number of collisions between objects

depends on how many objects there are Early in the Solar System there were many more

small planetesimals: more collisions Number of craters can be used to “date” a planet

Craters can be erased by tectonism, volcanism and gradation

occurs on “active” planets (e.g. Earth)

on “dead” planets, craters remain (e.g. Moon)

Formation: heats and compresses material thrown outward surface rebounds

Comparative Cratering Moon

lots of craters in all sizes Mars

craters with impact craters which indicate there might have been water on Mars once

Venus dense atmosphere protects Venus

Earth protected by atmosphere (many meteors

burn up) large oceans leave no impact crater most craters erased by gradation

Moon from the Earth

Theory: the Moon comes from the Earth Mars size protoplanet hit Earth early in its

history this impact showered large amounts of

material into Earth orbit volatile materials were lost remaining materials condensed to form the

Moon Facts which are explained:

Moon composed of same materials as Earth (moon rocks)

Moon has no significant volatile materials (water, air)

Moon is large fraction of Earth’s size

Surface leveling caused by blowing wind, flowing water and

water/ice freezing/melting Moon & Mercury

no atmosphere, possible ice, little gradation Mars

large dust storms observed, evidence of water flow

Venus evidence of blowing wind, no evidence of water

Earth all processes present e.g. dust/wind storms, rain, tides, glacier flow

Gradation

Magnetic field Inner planets all have some magnetic field This magnetic field is not caused only by

magnetized materials At least partially caused by rotation of Earth

spinning electric charges in core create magnetic field

Facts: Earth has a strong magnetic field Earth’s magnetic field moves with time (magnetic

north pole not the same as celestial north pole) the Moon has no or very small magnetic field Mercury has strong magnetic field Venus and Mars have small magnetic field

Gases – Some Basics Lighter gases rise

This is really because heavier gases sink and push the lighter gases upward

Temperature of a gas is really the speed of the molecules Faster gases are hotter

Sunlight and heat from a planet’s interior provide energy to heat atmospheres Sunlight can also break up molecules

Fast atoms/molecules in the outer atmosphere can escape the planet’s gravitational pull

Planets have a hard time hanging onto hydrogen and helium

Primary Atmosphere

A planet’s original atmosphere comes from the gas of the accretion disk It is composed mainly of hydrogen and helium

same stuff the Sun is made of

If a planet’s gravity isn’t strong enough, it can’t hold onto these light gases They escape and leave the planet without an

atmosphere Heating and solar wind help these processes

This happened to the inner planets We will see later it did not happen to the gas

giants