Origins

Where we left off . . .

4.6 billion years ago:  We have a star (the Sun), and planets are forming, although evidence suggests that proto-Jupiter and proto-Saturn's orbits weren't stable, causing other protoplanets to collide, move from one orbit to another, or get flung out of our solar system. 

Formation of the Planets

All the planets looked pretty similar at this stage:

• Thin, hard shell
• Molten inside
• LOTS of volcanic activity
• Outgassing formed the first atmosphere.

Particles still whizzing about the solar system.  These meteor impacts add material and heat to the protoplanets.

Image source: "mercury-venus-earth-mars-jupiter-163610" by Pixabay is licensed under CC BY 4.0

If you look at pictures of the moon, Mars, or many other planets, impact craters dominate their surfaces.  Why don't we see this on Earth?

The Formation of our Moon

AKA:  Giant-Impact Hypothesis / The Big Splash / Theia Impact

The short version of the story?  About 30–50 million years after the Solar System formed , the protoplanet Theia collided with proto-Earth.  But, it's a bit more complicated than that.

Image Source:  "earth_impact_moon_space_art2" by Fahad Sulehria , Nova Celestia is licensed under CC BY 4.0

It can be hard to visualize some processes.  Watch this video, Collision That Formed The Moon by SpaceRip.  As you do, note the following:

• Which of the hypotheses listed above does this video subscribe to?  Or, does it include both?
• Our moon is different from other moons in the solar system is it was made from Earth and Theia and was formed after both protoplanets had started to form.
• Without our moon, Earth's orbit wouldn't have stabilized as quickly as it did, nor would we have moon tides (more about that later in the semester).

What is the evidence for these hypotheses?

1. The Earth and Moon’s spin and orbits similar
2. The Moon has a relatively small iron core & is less dense
3. Moon rock samples indicate:
   • the surface was once molten
   • the composition is similar to that of the Earth

Earlier, you learned about The Scientific Method.  Scientists asked these questions, and then explored possible answers, which lead to the formation of both hypotheses.  We don't have all the answers yet, but continually asking questions will bring us closer to getting them.