EAS 105-THE PLANETS
Prof. Robert L.
Nowack
Lecture 9
Moon Rocks (or is the Moon made of green cheese?)
One
of the scientific accomplishments of the Apollo program was the return of about
300 kilograms of Moon rocks back to Earth for study and absolute age
dating. Before looking at Moon rocks,
let’s review different types of matter as well as rocks.
There
are 5 general types of matter:
(1) Gas
The
gas and giant planets formed of gaseous hydrogen and helium
(2) Plasmas
Dilute hot gases composed of
electrically charged electrons and ions.
Ex) the Solar wind from the Sun
(3) Ice
Hydrogen, oxygen, carbon and nitrogen all form simple
compounds that freeze at temperatures of the outer Solar System
Water ice - H2O
Dry ice - CO2
Ammonia ice - NH3
Methane ice - CH4
These
compounds make up the outer planet’s satellites and comets.
(4) Rock
Mixture of compounds composed in
part by elements of silicon, oxygen, magnesium, calcium, sulfur, carbon and
iron. These are the building blocks of the
outer surfaces of the inner planets, as well as many asteroids.
(5) Metal
Most metallic elements will form
compounds with oxygen to form rocky material.
However, there are places where metals exist naturally in a pure
state. Ex) central cores of the inner planets.
The theory is that nickel and
iron are more dense and tend to migrate toward the center as the inner planets
formed.
Note: Metals
will quickly oxidize at the Earth’s surface - iron rusts, silver
tarnishes. Gold is one of few metals
that does not oxidize; one reason why it’s so valuable!
Examples of Common Rock Forming Minerals
Olivine – A dense greenish mineral composed of silicon,
magnesium, and iron oxide.
This is a common element so we
would expect it to be a common mineral of rocky planets [the “chemical
formulas” would be Mg2SiO4 or Fe2SiO4.]
Pyroxene – This is similar in
composition to olivine except with more silica and oxygen.
Feldspar - This rock is a general class of silicate
minerals rich in aluminum.
Much of the Moon's highlands are
made from a calcium rich feldspar in a rock called Anorthosite.
Feldspar tends to be less dense
than some other minerals (because of the lighter aluminum content.
Quartz - SiO2
Quartz, as well as sodium and
potassium rich feldspar, tends to make up granites forming the continental
crust on the Earth.
From
the geology on Earth, rocks can be categorized in 3 general areas:
(1) Igneous Rocks - directly cooled from a molten state
Intrusive - (large crystals)
granite - quartz, potassium and sodium-rich
feldspar, mica
gabbro - dark rock of
calcium-rich feldspar, pyroxene
peridotite - dark rock, mostly
olivine
Extrusive - (volcanic, fine-grained)
basalt - fine-grained equivalent
of a gabbro
Much of the Moon’s Maria are made of
basalt flows.
(2) Sedimentary Rocks (common on an active planet like the Earth)
- formed
by depositions or settling of eroded bits of igneous rocks or dead organisms by
the action of water, ice, and wind.
- common
varieties on Earth - sandstone, shale, limestone
(3) Metamorphic Rocks
- Chemical or physical alteration of igneous or sedimentary rocks by
pressure and temperature.
On
the Moon, we are mainly dealing with Primary Igneous Rocks. However, on the Moon's surface, conglomerates
called Breccia are prevalent; presumably formed by impacts during the period of
heavy bombardment.
Examples of Moon
Rocks
Mare Basalt from Apollo 15 Hadley-Apennine Site
How is the age date determined
for an igneous rock? One must identify
the time since the igneous rock solidified from a molten state. Prior to the discovery of radioactivity, in
the 1890’s, no absolute way existed to determine this.
A
“radioactive” nucleus is unstable and will eventually change to another more
stable non-radioactive nucleus. This
decay is, to some degree, random for any particular nucleus. But for a large collection of atoms, there is
a specific time - The Half Life – during which ½ of all the atoms in a sample
will have decayed to the more stable nuclei.
After 3 half-lives, only 1/ 8 of
original number of radioactive nuclei remain.
Radioactive elements provide accurate “nuclear clocks” if one can
measure the relative abundance of the original radioactive element (“the
parent”) and the element it decays to (“the daughter”). Most rocks contain several radioactive
elements of differing half-lives.
Ex)
Potassium-40 > Argon - 40
19
protons 18
protons
with a half-life
of 1.3 b.y.
Ex)
Rubidium-87 > Strontium-87
37
protons 38
protons
with a half-life
of 47 b.y.
Ex)
Uranium-238 > Lead-206
92
protons 82
protons
with a half-life
of 4.5 b.y.
In
practice, a number of mother-daughter pairs can be used to date a rock. The result is usually accurate within about 1
to 5% of the total age. This age is
usually thought of as solidification age after which rock has stayed
undisturbed.
Using
radioactive age dating, the ages of rocks returned from the Moon were found to
be:
Mare Basalt: - 3.1 to 3.8 billion
years old
Highland Anorthosite - proved
difficult for absolute dating; but are estimated to be 4.4 billion years or
older.
Thus, in contrast to Earth, the
basaltic lava flows on the Moon are all old; greater than 3 billion years
old. In fact, no major geologic activity
has occurred on the Moon in the last 3 billion years other than from uniform
low-level impact craters.
The
Lunar Highlands are the oldest regions on the Moon dating back to the early
differentiation of the Moon. The idea is
that the Moon had an early molten surface layer in which the lighter Ca-feldspar
crystals stayed at the surface while the denser olivine crystals sank. The result was an Anorthosite feldspar layer
on the Moon’s early surface with a thickness of about 60-130 km for the
original
Cross-section of the Lunar Crust
The
following events then resulted for the Moon’s surface:
(1) The original
(2) The basins formed by
impacts were flooded by basaltic lava from below.
(3) Uniform cratering and
small impacts formed the soil-like regolith and the fewer craters on the Maria.
3-Dimensional Cross-section of Lunar Surface
What about the interior of the
Moon? Apollo astronauts placed
seismometers at different landing sites to monitor “Moonquakes”.
Seismograms Comparing Seismic Events on the Moon
First result: The Moon has much less seismic activity than
the Earth, indicating the Moon to be a geologically dead planet. However, when the Moon is closest to the Earth
at perigee, tiny infrequent Moonquakes do occur from tidal stresses caused by
the Earth.
The
Moon in turn induces tides on Earth - both ocean and solid Earth tides.
These tides vary on a 12-hour
interval basis. Additional fluctuations
also occur twice a month as well as in the fall and spring depending on the
alignment of Sun and Moon. A few years
ago, someone attempted to predict a large earthquake near
In
addition, tidal friction affects the Earth-Moon system over long periods of
time:
- There is gravitational phase locking of the Moon’s rotation and
revolution period.
- The
Moon is slowing down and consequently moving further away from the Earth.
- Earth’s
rotation is slowing down.
- Over
geologic time the eventual result will be that the day and month in the
geological future will lock at 47 present Earth days (theory was proposed by
- In
the past geologic history of the Earth-Moon system, the day may have been as
short as 6 hours and the month maybe as short as 1 week.
Moonquakes
have been detected in the lunar interior at depths of about 800 to 1100
kilometers. Moreover, a rough model of
the lunar interior can be constructed from analyzing the seismic records.
Interior Model of the Moon
The outer crust of the Moon
appears to be thinner on the near side than the far side. Consequently Mare deposits form more easily on
the near side since the crust is thinner.
Also, there is only a small iron core (if any). This is confirmed by the Moon’s low average
density. Also, the Moon has no global
magnetic field.
However,
some lunar rocks returned to Earth had slight remnant magnetism as if they
formed when the Moon did have some form of a magnetic field.
Water on the Moon?
During
its passing over the polar regions of the Moon, the Lunar Prospector satellite
provided indirect evidence of frozen molecular water in shadowed polar regions
of the Moon. This could have resulted
from impacts since the Moon is otherwise dry.
If this is true, it has important implications for future exploration of
the Moon.
Theories of Lunar Origin
(1) Daughter Theory - The
Moon split off from the Earth, theorized by
-
consistent with tidal theory but poses other problems such as how can it
happen?
(2) Sister Theory -
Earth and Moon formed simultaneously.
- Why doesn’t
the Moon have an iron core?
- Why are
there differences in composition between the Moon and Earth?
Moon rocks are
lacking in volatiles.
(3) Capture Theory -
Moon formed elsewhere.
- How did a stable orbit develop?
Hybrid Theory of (1) and (3):
Catastrophic
collision impact hits the Earth ejecting a massive amount of the Earth’s mantle
deficient in iron content. Energy from
the impact heats up ejected mantle to a temperature high enough to allow the
volatile matter to escape. Material
aggregates creating the Moon.
This
impact model is presently becoming more accepted. It assumes a Mars-size body striking the
Earth. Fragments from this monumental
collision form a ring of matter orbiting the Earth from which the Moon
aggregates into a planetary body.
The Birth of the Moon?
An object perhaps the size of the
planet Mars could have collided with the Earth and thrown enough material into
orbit to create the Moon.
Mercury
Mercury
is the closest planet to the Sun and has a very elliptical orbit. Minimum and maximum distances from the Sun vary
from 0.3 to 0.46 Au. (Average distance =
0.38 Au). Mercury’s diameter is slightly
larger than the Moon. The Moon has a
diameter of 3476 km compared to Mercury which has a diameter of 4878 km. Mercury is an airless planet like the Moon
due to Mercury’s small size. Its surface
looks similar to the Moon by having a heavy distribution of surface
craters. In fact, Mercury has a similar
ratio of small to large craters indicating a similar bombardment history.
The Planet Mercury
Based on crater densities,
Mercury’s surface is old, similar to that of the Moon.
Mercury’s Orbit and Spin
Mercury rotates on its axis once
every 58 2/3 Earth days (sidereal period). Mercury orbits about the Sun once every 88
Earth days. Hence, Mercury rotates on
its axis 3 times (3 x 58 2/3 = 176) for every 2 orbits about the Sun (2 x 88 =
176).
This
strange tidal coupling is due to Mercury’s highly eccentric orbit. It takes Mercury 176 Earth days to go from
sunrise to sunrise - a “Solar Day” or 2 Mercury years!
Mercury
rotates on its axis three times while orbiting the Sun twice. This synchronous rotation can be followed in
schematic diagram by observing how the dot changes position. A dot represents a fixed point on Mercury’s
surface as the planet moves from position 1 to 2, 2 to 3, 3 to 4,………25 to
position 1 again.
Important Differences Between Mercury and the Moon
(1) Mercury is much closer to the Sun resulting
in
gravitational phase locking with
the Sun
10 times as much heat and light
than the Moon receives
(2) Mercury’s average density of 5440 kg/m3
is much higher than the Moon's 3340 kg/m3 and is more similar to
that of Earth at 5520 kg/m3.
However, adjusting these average
densities for the pressures due to overburden pressure, we get even more
contrast.
|
Planet |
Measured Density |
“Uncompressed” Density (kg/m3) |
|
Mercury |
5440 |
5300 |
|
Venus |
5300 |
4400 |
|
Earth |
5520 |
4500 |
|
Moon |
3340 |
3300 |
|
Mars |
3940 |
3800 |
Thus, Mercury has the highest
"uncompressed" density. In
addition, Mercury was found to have a magnetic field by the satellite Mariner
10 in 1974. (Although small, less than
1% the strength of Earth’s.)
The
most likely model for the interior of Mercury is a large iron-nickel core
extending to within 700 kilometers of the surface.
However, the surface layer of
Mercury is rocky and we assume that it is similar to the Moon. However, the material filling the basins
isn’t as dark on Mercury as the Maria material on the Moon. This difference implies a slightly different material
on the Moon.
The
Caloris basin is the largest on Mercury at over 1300 km across and was caused
by a large impact.
The
presence of large scarps are another prominent surface feature on Mercury. These scarps are cliffs several kilometers
high and thousands kilometers long.
One model for this is that when
the iron of the planet Mercury was settling into its core, heat was released
causing expansion. As the planet cooled,
it contracted forming the cracks on the surface we see as scarps.
Schematic Showing Possible Evolution of Mercury
These schematic
drawings show a possible evolutionary process of Mercury. Drawing labeled “top” is a warm planet
Mercury perhaps heated by impact energy from meteoroids striking the striking
the surface making craters. Mercury
starts melting on the inside.
The “Middle”
drawing is the stage that the iron migrates to the center of Mercury releasing
pulses of heat as it descends. The whole
planet warms causing it to expand.
The “Bottom” drawing has Mercury cooling,
shrinking and creating scarps seen as surface cracks.
One
of the most surprising features of Mercury is the presence of polar caps as
revealed by earth based radar. A polar
cap is a frozen deposit of water or other volatiles in the cool polar regions
of a planet. Radar reflectivity suggests
water ice.
Upper pair of images shows radar-bright
deposits that most likely are caused by ice near the poles of Mercury. These deposits were first observed in 1991. The circular patches coincide with shadowed
floors of large, fresh craters seen in photographs taken by the Mariner 10
space probe (bottom pair of images). The
match becomes nearly perfect if the position of the poles are allowed to shift
from their Mariner-based locations (grid centers) to the new ones marked by
asterisks.
Radar-bright
deposits were first spotted in 1991 near the poles of Mercury. These places have been interpreted to be
water ice. This was surprising since
Mercury is so close to the Sun. This may
result since Mercury spins in its orbit plane and little direct sunlight falls
on the polar regions. Many questions
still exist concerning the occurrence of polar caps on Mercury, including
whether the ice is primordial, and if not, where did it come from?