1. The Earth-Moon System The Earth’s Atmosphere
Unique among terrestrial planets in its composition
78% Nitrogen (N2)
21% Oxygen (O2)
0.9% Argon (Ar)
0.1% other gases including 0.03% Carbon Dioxide (CO2)
The atmospheres of the other terrestrial planets are mostly CO2.
The Earth and Moon as seen from Mars!

2. Formation of Earth’s Atmosphere
There are two theories about the origin of Earth’s atmosphere:
Through volcanic eruptions or from impacts gases were released from rocks on Earth.
Impacts from comets brought gases to Earth.
Scientists are testing volcanic emissions and studying comets for answers to these questions.
Either way the original atmosphere was likely very different from what we have today.
Changes came from the breakdown of molecules by sunlight and Earth’s weak gravity allowing certain gases to escape.
Plants provided most of the Earth’s oxygen.
Mt. St. Helens volcano

3. Earth’s Interior
The thin crust of Earth rides on an elastic layer of rock called the mantle.
Below the mantle lies the liquid outer core composed of iron and nickel
At the center is the solid inner core also composed of iron and nickel
Motions within the mantle cause the crust to be dragged along. The crust is broken up into “plates” that shift around causing earthquakes, volcanoes and forming mountain ranges

4. Earth’s Magnetic Field
The motion of molten iron beneath the Earth’s crust combined with the spin of the Earth is thought to be the cause of Earth’s magnetic field
Like a magnet Earth has a magnetic north pole and a magnetic south pole
These magnetic poles are offset from the geographic poles by about 11 degrees. A compass therefore does not point to “True North”.
The strength and orientation of the
Earth’s magnetic field changes very
slowly over time. In the past the
magnetic poles have even reversed.

5. Earth’s Magnetosphere
Earth’s magnetic field extends into space. The region of space enclosed by Earth’s magnetic field is called the magnetosphere.
It protects Earth from dangerous high-energy particles streaming from the Sun. This stream is called the Solar Wind.
Some high-energy particles are trapped in Earth’s field and form the Van Allen Belts
The Solar Wind warps Earth’s field into a tear-drop shape.
see
for the Tsyganenko model of the magnetosphere

6. The Aurora
Near the Earth’s North and South magnetic poles glowing arcs and streamers in the sky are often seen at night. This is the Aurora.
They are caused by energetic electrons and protons moving along the magnetic field and striking air molecules causing them to glow.
When the density or speed of the solar wind increases the Earth’s magnetosphere can get compressed.
During this time of compression and expansion the location and intensity of Aurora can change.
IMAGE-FUV animations
Space Weather

7. The Moon The Moon is about 1/4 the size of Earth.
The surface is covered in impact craters. Crater sizes range from a few inches to several hundred miles across.
The Moon has some dark, smoother areas and other rough brighter areas.
Over billions of years impacts have pulverized the surface into a fine powder called regolith.
Footprint of Apollo 11 astronaut
in soft lunar regolith

8. Near Side and Far Side
The Moon is locked into a 1:1 spin orbit resonance with the Earth. The Moon spins 1 time on its axis for every 1 orbit around the Earth.
Because of this the Moon always has one side facing the Earth. This is the “Near Side”.
The side not facing the Earth is called the “Far Side”.

9. Types of Lunar Surfaces
There are two major types of lunar surfaces:
Maria- dark, relatively smooth areas
Highlands - light colored, very rough cratered areas
Highlands are made of less dense rock and are older.
Maria are made of more dense rock and are younger

10. Formation of Maria and Highlands
As the Moon formed, denser material sank towards the core while lighter material floated to the surface.
The Highlands were formed after the Moon’s crust cooled.
This surface was constantly being impacted during the early years of the solar system.
Much later, but before the Moon’s interior had cooled, very large impacts broke through the crust. Denser rock flowed into the cracks and filled the large craters that formed. This dark dense material formed the Maria.

11. The Moon’s Interior
The Maria are located on the near side of the Moon.
The crust of the moon is thinner on the near side of the Moon than on the far side.
The Moon’s core is thought to be shifted slightly in the direction towards the Earth. This shift may have caused the crust on the far side of the Moon to cool more and become thicker than the crust on the near side

12. Formation of the Moon
Several theories have been proposed to explain the origin of the Moon.
The one that seems to explain the observed properties of the Moon the best is that a very large object struck the Earth and the debris from this impact was blasted off of Earth and slowly coalesced into the Moon.
This theory explains the composition of the Moon and its orbital inclination.