Download Lecture 7

“Between impulse and action, there is a
realm of good taste begging for your
acquaintance.”
The Doctor, Star Trek Voyager
HW1 on-line now and due in 1 week.
http://sdbv.missouristate.edu/mreed/CLASS/A113Sp16
Cell phones put away and clickers turned on
when class begins please.
The Earth-Moon system.
The Earth's structure: Density=5.5g/cc
The Moon's structure
Also crust,
mantle, and
core, but the
density is
lower
(3.3 g/cc).
There are multiple ways to make/get
moons.
Each has consequences, mostly
related to
Composition and Orbit
An atmosphere is a layer of gas that surrounds
the central bodyAtmosphere
(planet or moon). Even if
that atmosphere is evaporative.
Atmospheres are determined by:
1) Temperature
2) mass of the gas particles (heavier moves
slower)
3) mass of the object (planet/moon/etc.) escape velocity
What
determines
an
atmosphere?
Tides
The Moon's gravity pulls
the Earth. At the middle,
it's pulled straight, but at
the poles, it's pulled
slightly inward.
This squeezes the Earth
and makes tides.
The same occurs for some
other planets/moons.
But the Earth spins faster than the Moon
orbits, so the bulge does not point
directly at the Moon.
Consequences:
The bulge is a forward pull on the Moon's orbit and
a backwards pull on the Earth's spin.
Consequences:
The bulge is a forward pull on the Moon's orbit and
a backwards pull on the Earth's spin.
The Moon's orbit is getting larger (the Moon's
distance increases about 3 cm/year) and therefore
longer.
Laser ranging
the Moon
Consequences:
The bulge is a forward pull on the Moon's orbit and
a backwards pull on the Earth's spin.
The Earth's spin is slowing, so days are getting
longer (1.5 msec/century).
Consequences:
The bulge is a forward pull on the Moon's orbit and
a backwards pull on the Earth's spin.
Eventually, the Earth will be tidally locked to the
Moon, just as the Moon is tidally locked to the
Earth. Then, 1 Earth day will equal 1 Moon orbit
(about 30 current Earth days).
The Moon is tidally locked to the Earth: It spins at the
same rate of its orbit. So 1 side always faces the Earth.
Consequences:
Working backwards in time.
The original Earth day (post-Moon creation) was
about 10 hours long.
For dinosaurs, the day was about 22 hours long.
(Confirmed from shellfish.)
The Earth-Moon system is dynamic.
It seems like it is constant, but it is
always changing.
Similarly, our solar system seems like the
same ol' thing. But in fact it is in the process
of changing all the time!!!!
Water on the Moon?
There are areas on the Moon that never receive
direct sunlight. IF ice can get in there, it will not
evaporate.
Eureka!
Experiments on Clementine and Lunar Prospector
detected the signature of water on both the North
and South Poles of the Moon.
Each pole possibly contains at least a small-ish
lake's worth of water-ice.
L-Cross then impacted into this icy soil in 2010.
Where did all that water come from?
Possibly from comets in the early solar system.
Quiz 2:
Our Moon is getting...
A) closer to us.
B) farther from us.
C) free from our orbit.
D) lighter in weight (less massive).
The Earth: Colorful, solid rocky planet with a thin
blue-water ocean and white-cloud atmosphere.
Blue/brown land masses, white polar caps (spin axis
nearly aligned with orbital axis).
Water/wind weathering,
plate tectonics, volcanoes.
Life!
Round, so massive.
The Moon: Not colorful massive, round, rock in
space. Solid, heavily cratered surface. One side
(tidally) locked to the Earth. No obvious atmosphere.
Surface age based on cratering
1) Smothered with craters; the surface is 4+ billion
years old. (e.g. Lunar Highlands)
2) medium (-heavy?) cratered; 3.5 billion years old.
(e.g. Lunar maria)
3) lightly cratered; ~200-500 million years old. (e.g.
Earth's surface)
4) no craters; <few million years old.
Color Variation
1) Composition (different colors made of different
stuff)
2) Temperature (solid, liquid, or gas can change
color)
3) Altitude (shading)
Tides: Objects want to be tidally
locked. It is the lowest energy state.
Smaller things will tidally lock to
larger things.
Now that we have our baseline.
It's time to explore our solar
system!!
Starting with the Terrestrial planets.
Mercury
What do you see and what does it mean?
Altitude map
Mercury



Old, heavily cratered surface. Probably of similar age
as the highlands on the Moon: about 4 billions years
old.
Some smooth areas which were caused by later
volcanism (lava flows, like the mare on the Moon).
Globally cracked surface indicates that Mercury's
surface solidified first (as expected) and as the mantle
solidified, Mercury shrank slightly (by up to 7 km in
radius!), cracking the crust.
Mercury

Density: 5.4 gm/cc

Structure

Thin cracked rocky crust

Rocky mantle
Iron core- 85% the size of the planet. Some of the core is
molten (liquid).
Temperature: Daytime 441F, Night -279F
Atmosphere (yes, it has one!): Thin atmosphere
created by solar wind blasting the surface (made
of sodium mostly). It evaporates into space. 1
trillionth of Earth's pressure
Mercury, Unusual Spin/Orbit 3:2
Spin/Orbit resonance.



One sidereal day is 59
Earth days.
One solar day is 176
Earth days.
One orbital period is
88 Earth days.
This is a form of
tidal locking!
Orange areas (and
white arrows) are irondeficient lavas. Black
areas are explosive
volcanoes. Blue
regions are ironbearing minerals.
Information from
Mercury Messenger.
Mercury Messenger: 2011-2015
Results:
1) Mercury formed with less oxygen and more
sulfur (10x) than Venus, Earth, or Mars.
2) Mercury's magnetic field is 100X smaller than
the Earth's.
Mercury Messenger Results:
3) Mercury has water-ice at the poles in craters.
Mercury's
structure
Still cooling and
shrinking