The Earth and Its Moon

The Earth Solid inner core, liquid outer core atmosphere - 50km thick magnetosphere - charged particles caught in Earth’s magnetic field The Moon Moon has no hydrosphere, atmosphere or magnetosphere same basic interior regions as Earth

The Tides Earth is the only planet with large quantities of liquid water on the surface Most coastal locations experience 2 low and high tides each day what causes this? The moon pulls the water The moon also pulls the Earth This causes two bulges - one on the side facing the moon and one on the opposite side where the water is “left behind”

Earth M M M M tidally locked The moon is tidally locked to the Earth - the same side of the moon is always facing us (moon rotation period is the same as its orbital period) Is there a side which is always dark? Is there a side which we (on Earth) never see?

Earth’s Atmosphere nitrogen (78%) oxygen (21%) argon (0.9%) carbon dioxide (0.03%) Convection occurs here Ozone absorbs UV light Protects the surface Regulates temperature

Convection Warmer air travels up and cooler air comes down to take its place Results in convection cells which heat the atmosphere creates surface winds and is responsible for most types of weather

The Greenhouse Effect Sunlight not reflected by clouds reaches the Earths surface (about 50%) The heated earth re-radiates this light in the form of infrared radiation Infrared light is partially blocked by the Earth’s carbon dioxide (and water vapor) So, only part of the IR light goes into space, part goes back to earth The average Earth temperature is about 40K hotter because of greenhouse effect than it would be without it! Why is this important?

Why does Earth retain its atmosphere (and the moon has none)? Gravity Gas molecules are in constant motion - hotter gas, faster motion The fact that the atmosphere is heated keeps it from falling onto Earth Escape speed is the speed an object (in this case, a molecule) must travel to escape another object’s (Earth, the Moon) surface If a planet’s escape speed is at least 6 times greater than the molecules’ velocity, molecules of that type will not “escape” in significant quantities Earth’s escape speed = 11.2 km/s Oxygen and nitrogen molecular speed = 0.6 km/s Moon escape speed = 2.4 km/s

Seismology Earth’s interior structure is probed by studying how waves travel through it (we can only drill so far! - 10km) Earthquakes generate seismic waves Certain types of waves reflect off different materials and travel through these materials at different speeds (higher density - faster)

 Mantle km thick (80% of planet volume)  Crust - 15 km thick (8 km under ocean km under continents)  Density and temperature increase with depth  High central density suggests the core is mostly nickel and iron  Density “jumps” between mantle and core – sudden change in material  No jump in density between inner and outer core - material is the same and just goes from liquid to solid

Evolution of the Solid Earth Accretion- material comes together to make the planet 4.6 Billion years ago (age of Sun). Earth was bombarded by interplanetary debris which made it hot. Differentiation - different densities and compositions to the earth - Earth was molten, allowing higher-density material to sink to the core (this core material still has temperatures like that of the Sun!) Crustal Formation - cooling and thickening of crust about 3.7 Billion years ago

The Surface of the Earth The Earth is still active today: earthquakes, volcanoes… Sites of activity outline surface plates - plate tectonics Continental drift - few cm/year Plates collide head on (mountains) or shear past (earthquakes) Some plates are separating (under Atlantic) - new mantle material wells up between them Sites of earthquakes or volcanoes in the past 100 years

What causes the forces that move the plates? Convection! Each plate is crust+mantle Warm mantle rock (softened by high temps) moves up - cools - moves down

Lunar Surface - lack of atmosphere and water preserves surface features Maria – mantle material “seas” - darker areas resulting from earlier lava flow Basaltic, iron rich, high density (3300 kg/m 3 ). Highlands – crust material elevated many km above maria Aluminum rich, low density (2900 kg/m 3 ).

Craters Caused by meteoroid impacts pressures to the lunar surface heats the rock and deforms the ground explosion pushes rock layers up and out The ejecta blanket surrounds the crater A 1-kg object hitting the Moon’s surface at 10 km/s would release as much energy as the detonation of 10 kg of TNT! Craters can be up to 100km in diameter A new 10km crater is formed every 10 million years A new 1m crater is formed each month A new 1cm crater is formed every few minutes!

The rate of cratering on the moon is determined from the known ages of the highland and maria regions. The Moon (and solar system?) experienced a sharp drop in the rate of meteoritic bombardment ~ 3.9 billion years ago. The rate of cratering has been roughly constant since that time. What happened then? The end of the accretion process

The Earth’s Magnetosphere – space influenced by Earth’s magnetic field Magnetic field lines run from the south to north magnetic poles Magnetic poles are close to (but not the same as) the axis poles The field is distorted by the solar wind

The Van Allen belts make up the inner magnetosphere 2 belts at 3000km and 20000km above Earth Highly charged particles (electrons and protons) from the solar wind become trapped by the magnetic field lines

Aurora Borealis Northern Lights – caused when the charged particles escape the magnetic field and collide with Earth’s atmosphere near the poles What causes the Earth’s magnetic field? The rotation of the planet coupled with the electrically conducting liquid metal core = dynamo effect

Formation of the Moon - theories 1.The moon was a sister planet - formed together with Earth But, the moon has a very different chemical composition 2.The moon formed somewhere else and was captured But, the moon’s mantle is too similar to the Earth Impact Theory Mars-sized body hit the molten Earth Parts of the mantle blew off and later formed the moon Earth had already differentiated so little iron was in the mantle