Unit 5: Galaxies, Asteroids, and the Moon

Galaxy and Milky Way

What is a Galaxy?

A Galaxy is a massive, gravitationally bound system of stars, stellar remnants (like black holes and neutron stars), interstellar gas, dust, and dark matter.

Galaxies come in different shapes: spiral (like ours), elliptical, and irregular.

The Milky Way: Our Galaxy

• Origin: Formed from the collapse of giant clouds of gas and dust shortly after the Big Bang, approximately 13.6 billion years ago.
• Physical Properties:
  • Type: It is a Barred Spiral Galaxy.
  • Size: Approximately 100,000 light-years across.
  • Contents: Contains 100-400 billion stars.
  • Our Location: Our Solar System is not at the center. We are located in one of the minor spiral arms (the Orion Arm), about 27,000 light-years from the galactic center.

Asteroids

Origin

Asteroids are rocky, airless remnants (planetesimals) left over from the early formation of our solar system 4.6 billion years ago. They are essentially "failed planets" that were prevented from clumping together to form a large planet by the immense gravitational pull of nearby Jupiter.

Composition

They are classified based on their composition (what they are made of):

• C-type (Chondrite): Most common. Composed of clay and silicate rocks. Dark in appearance.
• S-type (Stony): Composed of silicate materials (rocky) and nickel-iron.
• M-type (Metallic): Composed of nickel-iron. These are thought to be the cores of larger, differentiated planetesimals that were shattered.

Most asteroids are found in the Main Asteroid Belt, a region of space between the orbits of Mars and Jupiter.

Meteorites

Origin and Geological History

Meteorites are fragments of other celestial bodies. They can come from:

• Asteroids: The vast majority of meteorites are pieces of asteroids.
• Planets: We have found meteorites that were blasted off of Mars and the Moon by large impacts.

Geological History: Meteorites are our most valuable "fossils" of the early solar system. Many (the chondrites) are primitive, unchanged samples of the original solar nebula, allowing us to date the age of the solar system (4.6 billion years) and know its starting chemical composition.

Classification and Composition

1. Stones (or Stony Meteorites):
   • Composition: Composed of silicate minerals (like rock on Earth).
   • Chondrites: The most common type. They are primitive and are defined by chondrules—small, spherical grains that were "flash-melted" in the solar nebula.
   • Achondrites: Igneous rocks from space (e.g., basalts). They come from differentiated (melted) asteroids or planets.
2. Irons (Iron Meteorites):
   • Composition: Composed almost entirely of iron-nickel alloy.
   • Origin: They are the metallic cores of large, ancient planetesimals that were shattered by collisions.
3. Stony-Irons:
   • Composition: A mix of silicate minerals and iron-nickel metal. They represent the "core-mantle boundary" of a shattered planetesimal.

Lunar (Moon) Geology

Origin of the Moon

The leading theory is the Giant-Impact Hypothesis.

1. About 4.5 billion years ago, the very young Earth was struck by a Mars-sized object named "Theia".
2. The cataclysmic impact ejected a massive amount of debris (mostly from the mantles of both Earth and Theia) into orbit around Earth.
3. This ring of debris quickly tiny-img{width:20px} (stuck together) under its own gravity to form the Moon.

Lunar Surface Environment

• No Atmosphere: The Moon has no air, which means...
  • There is no weather, wind, or erosion.
  • The sky is always black.
  • Temperatures are extreme (from >120°C in sunlight to <-150°C in shadow).
• Regolith: The entire surface is covered in a thick blanket of fine, grey dust and pulverized rock called regolith, formed by billions of years of micro-meteorite impacts.

Lunar Features and Rocks

The Moon's surface has two main, distinct regions:

1. Lunar Highlands (Terrae)

• Appearance: The bright, light-colored, rugged, and heavily cratered regions.
• Lunar Rocks: Composed of a light-colored, low-density igneous rock called Anorthosite (made mostly of feldspar).
• Age: Very old (4.0 - 4.5 billion years). They represent the Moon's original crust.

2. Lunar Maria (singular: Mare)

• Origin: "Maria" means "seas" (Latin), but they are *not* bodies of water.
• Appearance: The dark, smooth, flat plains that make up the "face" of the Man in the Moon.
• Lunar Rocks: ComPOSTed of a dark, dense volcanic rock: Basalt.
• Origin: They are giant impact basins (craters) that were later flooded by lava (magma) from the Moon's mantle, which then cooled.
• Age: Younger than the highlands (3.0 - 3.8 billion years).

Lunar Craters

• Origin: Formed almost exclusively by the impact of asteroids and meteoroids.
• Because the Moon has no atmosphere or erosion, craters are perfectly preserved, providing a record of impacts over billions of years.