The Moon
Chapter 7
Read pages 183-197
Satellite: A body orbiting a planet (moons are known as natural satellites)
The Moon is ¼ the diameter of Earth
Diameter: 2,159.2 miles
Equatorial Circumference is 6,783.5 miles
Within the Solar System it is the largest satellite in relation to its parent planet
Surface Features
Mare/Maria – Large, smooth, dark areas on the surface
Came from Latin word for “seas”
Maria are made of basalt; a dark, congealed lava which filled in low lying areas (rich in iron, magnesium, and titanium silicates)
Highlands – Bright, heavily cratered, mountainous areas on the surface surrounding the Maria
Made mainly of anorthosite (rich in calcium and aluminum silicates)
Highland rock is less dense and considerably older than mare rock
Far side of the Moon
It is thought the maria are impact sites that were filled with lava.
The maria show younger dates than the highland on the Moon’s surface.
Why do the mare areas show fewer craters than the highland areas?
The mare are typically younger, and the lava filled in and erased any trace of previous impact craters.
Rays – long, white streaks of pulverized rock ejected from impact craters
At bottom are craters Aristarchus (left) and Kepler. Copernicus is at the top.
With time the radiation in sunlight may darken lunar dust. Ejecta rays expose dust and rock beneath that has been shielded from sunlight but how long material has been exposed is not the only thing that will affect the brightness of the rays.
Rilles – Lunar canyons
Those winding away from crater may have been carved out by ancient lava flows or be collapsed lava tubes
Straight rilles are probably a result of crustal cracking or fault lines
When objects strike the surface, craters may be created
The size and type of crater depends on the mass, the velocity, and angle of the impacting object
An impact may compress the under lying rock of the crater enough that it rebounds and creates a central peak
Multiple, mountainous rings surrounding a crater are signs of a major impact.
Ex. Mare Orientale
These impacts result in the formation of regolith, which blankets the surface of the Moon.
Regolith: made up rock chunks and fine powder
Lunar Reconnaissance Orbiter was launched in 2009 to help create a map of the Moon’s surface.
Due to gravity and an elliptical orbit the speed at which the Moon orbits will vary
The object has to travel faster when closer and slower when farther away
The Moon has an average distance from the Earth of about 238,857 (250,000) miles.
Perigee: When the Moon is closest to the Earth in its orbit.
Apogee: When the Moon is farthest from the Earth in its orbit.
Supermoon: When a full moon coincides with the perigee, the moon appears larger and brighter than usual.
Micromoon: When the full moon coincides with the apogee, the moon appears smaller and more dim.
A Supermoon can be up to 14% bigger and 30% brighter than a normal full moon.
A Micromoon appears about 12-14% smaller than a full supermoon and about 7% smaller than an average full moon.
The distance between the Earth and the Moon are routinely measured using LIDAR (Light Detection and Ranging) stations.
Apollo astronauts placed 5 reflecting panels (mirrors) on the Moon. Laser pulses bounce off these mirrors and can measure the distance to within a few millimeter.
The Moon is moving away from the Earth at a rate of 1.5 inches (3.8 cm) per year.
Sidereal Period – The time it takes a body to revolve once around a central body, as measured with respect to the stars
The Moon’s sidereal period is 27.3 days.
Synodic Period – The time between repeated configurations (or alignment) of a planet or moon
The synodic period of the Moon is 29.5 days (aka – Lunar month)
The Moon rises 50 minutes later each evening.
The synodic period corresponds with the lunar month and the moon’s phases.
Why don’t we have a lunar or solar eclipse every month?
The Moon’s orbit is at a little more than a 5° tilt from the Ecliptic plane
Synchronous Rotation of Moon
Synchronous Rotation – When a body’s rotation period is the same as its orbital period
This means the same side of the Moon is always facing the Earth.
When Apollo astronauts landed on the Moon, they set up seismometers and recorded the seismic activity.
We received this seismic data for roughly 8 years, and they recorded more than 12,000 events.
These seismic waves allowed us to study the interior of the Moon just like we did the Earth’s.
Four categories of Moonquakes
1. Very deep quakes in the lower mantle
Made up about ½ of the quakes – Repeated every 27 days – Caused by tidal forces of the Earth on the Moon
2. Very shallow quakes in the upper crust
At most a few miles deep – caused by thermal stress along fracture plane due to the great temperature changes
3. Meteorite impacts
About 1,700 or the 12,000 events recorded
4. Medium depth quakes
28 quakes with a depth of less than 100 miles and magnitudes of up to 5 on the Richter scale
Molten moon went through differentiation by density, like the Earth did.
There is a crust, mantle, and core.
(Earth’s is 5.5 g/cm3)
A lack of magnetic field indicates that at most the core is partially molten.
The Moon has a low density of 3.3 g/cm3, indicating the core is small.
The core is rich in iron mixed with small amounts of nickel and sulfur.
The mantle is believed to be made of dense olivine, which is what composes most of Earth’s mantle.
The Moon’s mantle appears too cold and rigid to be moved by the Moon’s weak inner heat – Geologically inactive.
The rocks on the crust are silicates, much like those found on the Earth’s crust, but have more aluminum and titanium.
The Apollo missions brought 842 pounds of Moon rock back to the Earth.
NASA once valued moon rock at over $300,000 a gram in today’s dollars based on the cost of sending a spacecraft to the Moon and back.
The crust varies in thickness for an average thickness of (100 km) 60 miles
Near side – 65 kilometers (40 miles)
Far side – 150 kilometers (93 miles)
Probably from the pull of Earth’s gravity on the Moon’s core
Why are the near and far sides of the Moon so different?
Why are the Moon and the Earth so Different?
The Moon is much smaller, so it cooled off much more quickly
The smaller mass also accounts for a smaller gravitational force
Size!
Astronauts set up seismic detectors on the Moon, which show that its interior is essentially inactive but there are moonquakes
Moon has a mantle, but it is too cold and rigid for material to be stirred like it is in the Earth
The Moon has no magnetic field
What can we infer from this information?
Atmosphere
Essentially has no atmosphere
Tiny amounts of helium – by product of radioactive decay of Moon’s interior
Traces of hydrogen – from frozen water on the Moon’s surface brought by colliding comets
Atmosphere (continued)
which emits gases
(Has an escape velocity ¼ that of Earth)
Why no atmosphere?
Atmosphere (continued)
No atmosphere means there is no weather and no greenhouse effect to modify temperatures
Origin and History of the Moon
1. Capture Theory
2. Twin Formation Theory
3. Fission Theory
4. Collision Theory
Theories before Apollo Missions
Theory after Apollo Missions
Capture Theory
Twin Formation Theory
“twins”, forming side by side
from a common cloud of
dust and gas
composition of the Moon
would be nearly identical
to that of the Earth’s
Fission Theory
bulge of matter formed and
was ripped away from the
Earth to become the Moon
composition of the Moon
should be the same as the
Earth’s crust
Collision Theory
A large body collided with the newly formed Earth, causing crustal material to be hurled into space. The debris cooled and gravity pulled it together to form the Moon.
Spring Tide
Neap Tide
Highest of high tides, and lowest of low tides. Happens during the new and full moon phases.
Lowest of high tides and highest of low tides. Happens during the 1st and 3rd quarter moon phases.