1 Where should we start our study of EARTH SCIENCE? The most logical choice is to start with the origin of our planet, EARTH. Since our Earth is part of a bigger family, whose members circle the Sun; we should begin with the origin of the Solar System.

2 Six known facts…  There are many hypotheses regarding the origin of the Solar System. Each of the hypotheses must explain six known facts:  1. all planets revolve around the sun in the same direction  2. the orbits of the planets are nearly circular

3 Six known facts…continued  3. most of the orbits are in nearly the same plane  4. the sun rotates on its axis in almost the same plane as the planets and in the same direction that the planets revolve

4 Six known facts…continued  5. most of the planets rotate in the same direction as the sun  6. six of the eight known planets have moons (Mercury & Venus do not) and most of the moons revolve around the planets in the same direction that the planets revolve around the sun

5 FYI: Rotation vs. Revolution  Rotate means to move around an axis inside the object (internal axis). The axis can be real or imaginary.  Examples:  Revolve means to move around an axis outside the object (external axis). The axis can be real or imaginary.  Examples:

6 One hypothesis is widely accepted.  The protoplanet hypothesis is favored by most astronomers because it best explains the six facts.

7 A simple explanation of the protoplanet hypothesis:  5 billion years ago there was a large cloud of gas and dust rotating in space; time passed and the cloud shrunk; shrinking made it rotate faster, compressing the interior; the interior became so hot that atoms began to fuse and the cloud became the sun; 10% of the material formed a plate-like disk around the sun, reaching into space; that material formed protoplanets; the protoplanets formed planets and moons; the leftover material formed comets, meteors and asteroids.  SLOW DOWN AND READ THIS OVER A FEW TIMES!

8 OK, we have a planet but it is nothing like the Earth we know today. The story continues…  When the Earth first formed it had no ocean or atmosphere. As the protoplanet changed to Earth, it grew hotter from three sources.  1. compression-squeeze molecules tightly under pressure and heat is generated  2. radioactive minerals-natural sources of energy, most of which becomes heat  3. bombardment by showers of meteorites-produce heat and friction as they move through the atmosphere, as well as upon impact

9 The story continues…oceans  As the Earth became hotter, iron melted and sank towards the center of the Earth, forming the dense core. As it sank, it partially melted other materials which released water and gas. The molten materials separated into layers allowing steam and gas to escape in volcanic eruptions. The steam condensed to water that accumulated into the oceans.

10 The story continues…atmosphere  Today’s atmosphere is much different than the original. Originally the atmosphere came from volcanoes so it would have been a mixture of water vapor, carbon dioxide, and sulfur gas. The mixture contained no free oxygen.  So where did the oxygen come from?

11 The story continues…oxygen  Free oxygen likely came from water molecules that broke apart by sunlight in the upper atmosphere.  As simple green plants came into existence they added more free oxygen by photosynthesis.

12 Today’s atmosphere  Today our atmosphere is:  78% free nitrogen  21% free oxygen  Remaining 1% is argon, carbon dioxide, and helium  The amount of water vapor in the air varies with weather and climate

13 The story continues…continents  One hypothesis suggests that the molten iron and nickel sank to the core, forcing out enough light rock to form one huge continent (Pangaea?).  Another hypothesis suggests that the continents were formed by great lava flows from volcanoes  Either way, today’s continents are much different than those that were first formed. We will talk on this more later.

14 OK…time to summarize  We have Earth!  Tomorrow we will start looking at the structure of our planet.  Bring colored pencils!  Read pages 7-11 in your text!