Inner Planets (Part I)

1. Inner Planets (Part I) Sept. 16, 2002 Science Intro to Inner Planets Four Main Processes Planetary Comparisons Intro to Atmospheres

2. 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

3. 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

4. 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

5. 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

6. 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

7. 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

8. 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

9. 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

10. Four Main Processes • These processes shape the surfaces of planets • Tectonism • movement of pieces of the planet’s crust (plates) • Volcanism • flow of material (lava) from beneath the planet’s crust • Impact Cratering • meteors hitting a planet’s surface • 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

11. 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)

12. 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

13. 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

14. 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)

15. 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

16. 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

17. 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

18. Gradation • 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

19. 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

20. 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

21. 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