Wednesday April 21th 1999
Announcements:

Lecture notes:

Lecture 42.
Earth's Moon

Mean distance = 384000 km.  Period relative to stars (sidereal) = 27.3 days.  Period as seen from Earth (synodic) = 29.5 days.
1/4 radius of Earth = 1740 km.
1/80 mass of Earth = 7.35 E22 kg.

A bit less dense than Earth = 3360 kg/m³

When looking at Moon see dark and light patches.  Lunar lowlands are smooth dark.  Highlands are battered by craters.

The flat dark plains of the lunar lowlands are known as maria = "seas" in Latin.

Early in Moon's history large meteor impacts blasted out craters lava rose up from inside Moon to fill these somewhat leading to darkness and lack of craters (since cratering heavier early on).

History of Earth's Moon.  Dominant fact is that Moon is small size.
When moon formed it was molten and heavy elements sank to the middle.   (4.1-4.5  billion years ago.)   Today may be slightly molten in the middle but not too much.

Also studies of Moon's density and magnetic field rule out the presence of a large iron/nickel core - only a small one - Moon is
iron poor as we shall discuss.  This may be a clue about how the moon formed.  Also small size allowed atmosphere to escape.

Heavy cratering - was very heavy during the first 0.5 billion years.  Creation of maria as discussed before by lava that was
around in early Moon .  Maria made 3.8 - 3.2 billion years ago.  Lava after heavy cratering leads to low cratering in maria.

Slow surface evolution.  No atmosphere means little erosion - so we can learn about early solar system from moon's surface.  Only micrometeorite impacts and a few larger ones to make erosion.

Origin of Earth's Moon.
Many ideas over the last 200 years but most of these are now falsified.

Fission hypothesis fails to explain angular momentum - can't get Earth spinning fast enough.
Condensation hypothesis (Moon and Earth from same gas cloud) fails to explain different Earth/Moon densities and compositions.
Capture hypothesis - (Moon captured later on) fails since Moon would be ripped apart in capture process - also very improbable.

Don't know precisely how the Moon formed but leading theory is large impact hypothesis.
Moon formed when planetesimal as large as Mars smashed into molten proto-Earth - glancing impact not head on.  Planetesimal and Earth had already differentiated so ejected material from Earth was iron poor.  Iron core of impactor would have fallen back to Earth leaving only iron poor material to make Moon.

Rotation of Moon.
Moon rotates with the same period as it orbits the Earth - 27.3 days.  Called synchronous rotation.  This means that the Moon
keeps the same side facing Earth at all times.

This synchronous rotation is not an accident!

Just as Moon raises tides on Earth Earth raises tides on Moon. In fact since Earth more massive tidal force on Moon is about
20x greater.

This tidal force acting over time has caused the Moon's rotation period and orbital period to become tidally locked together.
many of the other moons in the solar system also have synchronous rotation for the same reason.

Mercury and Venus have no moons.
Moons of Mars.
Mars has 2 moons.  We think they did not form with Mars but instead are asteroids that were captured.  They both are
irregularly shaped sort of like potatoes and heavily cratered.  And they both move in nearly circular equatorial synchronous
orbits.

Phobos = "fear" - 16 km long by 10km wide.  Orbits at 9378km from planet center ( less than 3 planetary radii).  7h 59m orbital period.

Deimos = "panic" - 28 km long 20 km wide.  Orbits at 23459km. 30h 18m orbital period.