EARTH AS A PLANET Chapter 7

SOME PROPERTIES of EARTH Semi major Axis1.00 AU Period1.00 year Mass5.98 x kg Diameter12,756 km Escape Velocity112. Km/s Rotation Period23h 56m 4s Surface Area5.1 x 10 8 km 2 Atmospheric Pressure1.00 bar

EARTH as a PLANET l Only planet with water in liquid form. l Composition: iron, silicates, oxygen. l Density: highest in Solar System. l Interior: 4 major layers - core, outer core, mantle, crust.

EARTH as a PLANET l Only planet with water in liquid form. l Composition: iron, silicates, oxygen. l Density: highest in Solar System. l Interior: 4 major layers - core, outer core, mantle, crust. l Magnetic Field and Magnetosphere: Earth behaves as if it had a bar magnet inside it.

MAGNETOSPHERE and VAN ALLEN BELTS

The MAGNETOSPHERE SHAPE

EARTH’S STRUCTURE – HOW DO WE KNOW IT? FROM EARTHQUAKES

PLATE TECTONICS l Earth’s crust consists of about 12 plates.

PLATE TECTONICS l Energy escaping from the interior drives the plates (few cm/year). 50 million years from now

PLATE TECTONICS l Where plates interact, dramatic changes occur in Earth’s crust. ä Rift Zones: Plates pulling apart. Material rises from mantle to fill spaces (volcanoes). E.g.. Mid Atlantic Ridge.

PLATE TECTONICS l Where plates interact, dramatic changes occur in Earth’s crust. ä Subduction Zone: Two plates come together, once forced down and melts. Earthquakes and volcanoes common along subduction zones.

PLATE TECTONICS l Coastal BC is on the upper part of a subduction zone.

SAN ANDREAS FAULT

ACTIVE SITES on EARTH’S SURFACE

CONTINENTAL DRIFT

FLASHCARDFLASHCARD AT 2 CM/YEAR, HOW LONG WOULD IT TAKE A TYPICAL PLATE TO TRAVERSE THE PRESENT WIDTH OF THE ATLANTIC OCEAN, ABOUT 6000 KM? A) 3 X 10 6 YR B) 1.2 X 10 7 YR C) 3 X 10 8 YR D) 1.2 X 10 9 YR

< 10 km, most atmosphere here. (N 2 78%, O 2 21%, Ar 1%, traces H 2 O, CO 2 )Troposphere km, cold (-50ºC), cloudlessStratosphere km, O3, absorbs UV radiationOzoneLayer Atmosphere very thin, constant leakage of H and He > 100 km

GREENHOUSE EFFECT l CO 2 content of atmosphere is critical for its role in retaining heat from Sun. l Greenhouse Effect: ä Earth absorbs sunlight and re-emits it as infrared (IR) radiation (Earth acts like a blackbody). ä CO 2 transparent to sunlight but absorbs IR acting like a blanket. ä The more CO 2, the hotter Earth is.

GREENHOUSE EFFECT and GLOBAL WARMING l Greenhouse effect has raised Earth’s average temperature by 23ºC. l Without the greenhouse effect, Earth’s average temperature would be below freezing and Earth would be in a constant global ice age.

GREENHOUSE EFFECT and GLOBAL WARMING l CO 2 levels are increasing. l Prediction: rising CO 2 levels will lead to global warming with uncertain consequences.

THE MOON l 1/4 the size of Earth

THE MOON l Earth’s Moon has no atmosphere.

THE MOON l The Moon is tidally locked to Earth. Non rotating Moon Rotating Moon

THE MOON l Only extraterrestrial object that humans have visited.

APOLLO MISSIONS

MOON’S INFLUDENCE on EARTH l Differential forces on Earth tend to stretch Earth slightly.

TIDAL BULDGES in an “IDEAL” OCEAN

SPRING and NEAP TIDES

MOON’S INFLUDENCE on EARTH l Tidal friction slowly is increasing the length of Earth’s day. (6 hours 4.5 billion years ago, 22 hours 620 million years ago)

LIFE on EARTH l Earth is the only planet known to be harbouring life. l Life arose early on. Fossils date back 3.5 billion years. ä Origin of life is unsure. ä Miller experiment: produced amino acids and other pre-biological molecules from primitive Earth atmosphere - mainly CO 2 (No O 2 ).

LIFE on EARTH l Development of plants: l Formation of the ozone (O 3 ) layer ä Use up CO 2 and produce O 2 - photosynthesis. ä Free O 2 in atmosphere 2 billion years ago. ä Protects Earth from UV. ä This allowed life to leave protective oceans and colonize the land. Not possible earlier.

COSMIC INFLUENCES on EARTH’S EVOLUTION l Moon is heavily cratered - caused by impacts. ä Where are Earth’s craters? ä Most lost by erosion and geological activity.

COSMIC INFLUENCES on EARTH’S EVOLUTION l More than 150 impact craters are still recognized on Earth’s surface.

COSMIC INFLUENCES on EARTH’S EVOLUTION l Concern is that a large object, ≥10 km, will collide with Earth releasing a few billion times as much energy as Hiroshima bomb.

COSMIC INFLUENCES on EARTH’S EVOLUTION l Concern is that a large object, 10 km, will collide with Earth releasing a few billion times as much energy as Hiroshima bomb. l One such object crosses Earth’s orbit every 100 million years.

ORBITS of the 100 LARGEST KNOWN NEAR-EARTH ASTEROIDS

IMPACT FREQUENCY Impactors on the surfce of the space shuttle 1  m, 30  sec Shooting stars 1 mm, 30 sec Meteroites 1m, 1yr Arizona Crater 100 m, 10 4 yr Sudbury, Ontario 10 km, 10 8 yr

COSMIC INFLUENCES on EARTH’S EVOLUTION l More than 150 impact craters are still recognized on Earth’s surface. METEOR CRATER ä 0.1 km meteor ä Crater size = 10 x size of impacting object 1.2 km

TUNGUSKA EVENT Siberia June 30, 1908 Projectile exploded before impacting Earth

Projectile’s mass estimated to be 100,000 tons Caused widespread damage.

TUNGUSKA EVENT

SHOEMAKER-LEVI COMET

IMPACTIMPACT Earth’sViewVoyager’sView

SHOEMAKER-LEVI IMPACT

DEVELOPMENT of DUST CLOUD Impact

IMPACT DUST CLOUDS

IR IMAGE of IMPACTS Io

COSMIC INFLUENCES on EARTH’S EVOLUTION con’t

EFFECTS of a MASSIVE IMPACT

ä Huge tidal waves. ä Dust high up in the atmosphere. ä Block out Sun for about one year, kills photosynthesis. ä World-wide fires, forests and grasslands destroyed. ä Highly acidic rains. ä Animals depending on plants die (dinosaurs?), rise of mammals. ä Global earthquakes.

EXTINCTIONS, the SOLAR SYSTEM and our GALAXY l Past 100 million years many large scale extinctions (~ 10) identified on Earth. äIn these, a significant fraction of existing species were extinguished. l Best known is extinction of the dinosaurs 65 million years ago after flourishing for 100 million years.

EXTINCTIONS, the SOLAR SYSTEM and our GALAXY ä äImpact of ~ 20 km asteroid. ä äEvidence: crater in Gulf of Mexico and Iridium layer. l What caused the extinctions?

EXTINCTIONS, the SOLAR SYSTEM and our GALAXY l What caused the extinctions? ä äImpact of ~ 20 km asteroid. ä äEvidence: crater in Gulf of Mexico and Iridium layer. ä äNearby supernova (depletes O 3, forming NO 2 ). Crab Nebula

EXTINCTION of the DINOSAURS

ALTERNATE DINOSAUR EXTINCTION THEORY

EXTINCTIONS, the SOLAR SYSTEM and our GALAXY l Supernovae and impacts more likely when Sun passes through spiral arm in our Galaxy äOort cloud comets get perturbed.

SPIRAL GALAXY

OORT CLOUD GETS PERTURBED