“Planet Earth and Its Moon”
Bob
Malcuit (Department of Geosciences, Denison University, Granville, Ohio,
USA)
Ohio State University at Newark Geology Club
presentation
24 May 2011
The
Moon
has been melted down to 600 km. It has an anorthosite
crust (light-colored) - ~all plagioclase feldspar-rock. It also has
mare with basalts (dark-colored). The Moon is basically black and
white. The astronauts who visited observed that the Moon was “black
and white and gray” - no colors.
Earth’s
composition indicates it is made of chondritic meteorites.
Most
have considered the Moon to have had little significance to the history of
Earth. This is now known not to be the case.
Without
the Moon, Earth’s rotation rate today would be 12-14 hours per day.
Earth’s
actual 24 hours per day rotation rate is due to tidal friction from the
presence of the Moon.
What
is the origin of the Moon? Most advocate the impact model. Here,
Malcuit is presenting a different view.
Earth
is special - it has liquid water at the surface, it has free oxygen in the
atmosphere, it has a highly developed biologic system, it has a strong magnetic
field (Jupiter and the Sun have way stronger magnetic fields), it has a very
large Moon (mass ratio of 1:81), it’s the only planet with true granite,
it’s the only planet with continental crust, it’s the only planet
with operational plate tectonics.
Moon’s
mare deposits (younger) have a weaker magnetic signature than the anorthosite
crustal rocks (older). This means that the lunar magnetic field died out
from 3.9 to 3.6 Ga.
Earth’s
continental crust mostly formed over subduction zones.
Earth
plate tectonics have been operating since the beginning of the Earth, some
say. Others say modern plate tectonics didn’t start until 1
Ga. Before that, Earth was too hot to get subduction. But, there
are volcanic arcs that predate 1 billion years. But, it was probably a
different plate tectonics style.
If
Earth did not have the Moon, Earth’s rotation rate would be ~12
hours/day, there would be no significant tides (only 10-15 cm from solar
tides), there would be no granite and no continents, there would probably be
some ocean water, and Earth would have an all-enclosing basaltic crust.
Earth would probably develop bacterial life, and maybe algal life. It
would have been difficult to have higher forms of life.
If
the Moon was larger than it is, Earth would have 36 hours per day, resulting in
hella-cold nights.
If
the Moon was smaller than it is, Earth would be very different.
Earth
is truly in a goldilocks zone.
Harold
Urey (1893-1981), the discoverer of heavy hydrogen, said this: the Moon was a
very primitive planetoid; it was a “survivor” of a class of
planetoids that did not get consumed by collision with other bodies; the Moon
is a major recorder of solar system events; the Moon is a captured satellite;
the Moon is the “Rosetta Stone of the Solar System”.
Well,
the Moon is as old as the Earth, so it is primitive. The Moon may
be a little older than the Earth.
The
Moon’s maria are in a straight line - the largest is first, the next
largest is next, the next largest is next.
The
origin of the Moon - 4 models.
1)
fission models
2)
co-formation models
3)
capture models - a minority view; calculations demonstrating capture have only
been done at Denison University.
4)
giant impact models - a popular view; there’s actually not much evidence
for it; proponents can’t make a lunar-sized body from the impact debris;
if they can, they can’t get the Moon to be the right composition.
Fission model - Darwin
(1880) proposed the fission model. Early Earth spun so fast that the Moon
pinched off and orbited Earth. See Wise (1963). Problem with the
fission model - how does Earth get spinning so quickly?
Co-formation model -
was popular from 1930 to 1975, after the fission idea died out. Earth and
the Moon formed from the same material. Calculations showed that this
model didn’t provide a Moon in stable orbit. Plus, Earth & the
Moon have different compositions.
Prograde capture model
- calculations have been done by Malcuit and others. The Moon moved from
a Sun-centered orbit to a geocentric orbit. The energy generated by the Moon
capture event was 2.2 x 1028 Joules.
Giant impact model -
first seriously proposed in 1984 at a Hawaii conference. It has been
favored ever since. It doesn’t relate to the geology of Earth or
the Moon very well. The model has a Mars-mass body smashing into
Proto-Earth. The impact event melts Proto-Earth completely. The
debris coalesced into the Moon, which originally formed at 3 Earth radii
distance, according to the model. It was 24 hours between the impact and
the embryonic Moon. Earth initially rotated ~5 hours per day. No
one has made a Moon from the debris, though.
Capture
calculations have been done at Denison University since 1987.
A
viable Moon origin model has to explain the following:
1)
anhydrous nature of the Moon
2)
K index for Solar System bodies (“banana index”) - potassium
content of planetary bodies decreases in a regular way in the Solar System
3)
volatile element depletion patterns for solar system bodies - water and other
volatile element contents of planetary bodies decrease in a regular way in the
Solar System
4)
body density differences - the Moon has a 5 g/cc density; the Earth has a 3.3
g/cc density
5)
lunar crust & mare rock dates
6)
maria origin
7)
asymmetry of lunar mass distribution
8)
temporal patterns of lunar rock magnetization
The
discovery of Moon water was much hyped - it wasn’t quite a hoax, but it
was very overstated - wishful thinking. The very thin, scattered coating
of water ice on the Moon was mostly implanted by solar wind. People
can’t live on the amount of water there.
The
giant impact model talking points: 1) it accounts for the masses of the Moon
and the Earth (actually, it doesn’t); 2) it accounts for the angular
momentum of the Earth-Moon system (actually, it doesn’t); 3) it accounts
for the iron depletion of the Moon (yes, it does).
The
tidal capture model talking points: 1) it accounts for the masses of the Moon
and the Earth (yes); 2) it accounts for the angular momentum of the Earth-Moon
system (yes, it does) - a 10 hours/day rotating Earth that captures the Moon
results in an angular momentum that we have now; 3) it accounts for iron
depletion of the Moon, relative to Earth (this is now explained by the
capture model since 2003 understandings).
The
Earth-Moon system is unique. Moon origin models require an unusual
explanation. Uncommon objects, like the Moon, require an uncommon origin.
Leaning
toward the capture model - it was the default explanation from 1975 to 1984.
Capture
vs. collision models - a David vs. Goliath scenario.
Tens
of millions of dollars have been spent on computer simulations of the giant
impact model, mainly at Los Alamos. There’s lots of investment in
the impact model.
Earth
can’t dissipate the high amounts of energy generated by the capture event
over short periods of time. The Moon can.
If
capture happened, where did the Moon come from? Didn’t know,
originally.
When
Malcuit retired in 1999, he still didn’t have an answer.
The
capture scenario turns out to be far more complex and fascinating than realized
15 years ago.
Compare
the Moon capture model with plate tectonics and Milankovitch climate
cyclicity. It took 60 years for plate tectonics to be accepted (1912 to
1972) - a long incubation period for the concept. It took 64 years for
the Milankovitch model of Ice Ages to be accepted (1912-1976) - also a long
incubation period.
The
capture model is simple in principle but complex in the details.
Where
did the Moon come from? Possibilities:
1)
near Earth’s orbit. If so, would get a Moon with the same
composition as Earth. (which it isn’t)
2)
in the inner part of the Asteroid Belt. If so, would get a Moon with
ices/water. (which it doesn’t)
3)
in the inner part of the Solar System, near the Sun. A near-Sun origin for
the Moon was first suggested in the 1970s.
Relevant ideas -
X-Wind Model, Cool Early Earth Model, K Index for Solar System Bodies,
Calculations by Evans & Tabachnik (1999)
The
X-Wind Model allows for an iron-poor Moon forming near the Sun. Calcium-aluminum
inclusions (CAIs) are also explained by the X-Wind Model. The giant
impact model can’t explain CAIs.
The
most stable orbits in the Solar System have been found to occur at 0.1 to 0.2
AU (astronomical units). Orbits at this distance from the Sun are stable
for up to 1 billion years. Objects that orbited the Sun at this distance
(= closer than Mercury)
have been called Vulcanoids. No one has seen a Vulcanoid,
it’s been thought. Let’s start the Moon in a Vulcanoid orbit.
K
depletion trends: carbonaceous chondrite meteorites (CI, CM, CV chondrites)
have high K contents. From there, in order of decreasing K content: Mars, Earth, Vesta
(parent body of eucrite
meteorites), Moon, Angra (parent body of angrite
meteorites - not yet identified, but probably in the Asteroid Belt now).
The
Moon is chemically associated with Vesta and Angra (eucrites & angrites).
Primitive Meteorites -
have chondrules, calcium-aluminum inclusions (CAIs), and matrix. Chondrules
are 1-5 mm spherical structures in primitive meteorites. Calcium-aluminum
inclusions (CAIs) are 1-3 mm spherical structures in primitive
meteorites. They are rare, except in a meteorite that fell in 1969.
The matrix is fine dust - it has element ratios very similar to the Sun.
Compositions of Planets & Planetoids - Mars is ~90% matrix material. Earth and Venus
are a combination of chondrules and matrix + some CAI material. The Moon,
Vesta, and Angra have ~90% CAI compositions.
When
the Sun formed
(very hot), it pushed water and all other volatiles out to the snowline, at 5
AU. Jupiter
picked up that material and grew like mad.
The
iron line is at 0.4 AU, where Mercury orbits. ~1200° C. Get a
concentration of iron at 0.4 AU - why Mercury is so Fe-rich (huge Fe core,
compared to planet size).
~0.15
AU is where the Moon formed - inside the iron line, resulting in an Fe-depleted
Moon (which it is). ~0.15 AU is a stable orbit zone.
The
X-Wind Model explains CAIs - they formed at the reconnection ring shown in an
X-Wind diagram and then they got swept elsewhere in the Solar System.
Cool Early Earth -
proposed by John Valley et al. (2002). 4.4 Ga zircons from Jack Hills,
Australia were discovered in 1986. The zircons indicate Earth was cool at
4.4 Ga - cool enough to have surface water. Many Earth geologic time
scales start with the Hadean - Earth was supposed to have been hotter-than-hell
back then. 4.4 Ga zircons indicate that wasn’t the case.
The
Moon capture event occurred at 3.95 Ga. The older lunar maria rocks came
into existence then.
Earth’s
primitive crust got recycled at 3.95 Ga - seen in Australia, Greenland, South
Africa.
Earth
was moonless for 600 million years after its formation. Lunar
capture started at 3.95 Ga.
Looking
at the relationship between Mercury, the Moon, and Vesta. The spacecraft Dawn
will orbit Vesta soon and eventually land and sample.
SPZ
- the stable planetoid zone inside Mercury’s orbit. Angra was
closest to the Sun, Luna (the Moon) was a bit farther out, and Vesta was a bit
farther than that - all inside the Mercury orbit. The reconnection ring
(CAI formation locality) is inside the Angra orbit. The area had
fluctuating ~1600° K temperatures. There were other bodies in the SPZ
- “No Names”.
Why
did the Moon get melted to a depth of 600 km? The same thing happened to
Mercury.
The
Sun entered a T-Tauri stage, after the X-Wind stage. X-Wind involved
generation of x-rays and more powerful radiation. The T-Tauri stage was a
slow burn - microwave radiation. Luna (Moon), Vesta, and Angra were
heated & baked from the outside-in by the T-Tauri event. This melted
the top 600 km of the Moon. Luna then had a stronger electromagnetic
field.
Luna formed at 0.15 Au
and ended up as a Moon of the Earth at 1 AU by prograde gravitational capture -
a “benign estrangement” scenario.
Capture
models in general - captured bodies can enter a retrograde orbit or a prograde
orbit - there’s a 50-50 chance for each. Happily for us, the Moon
was captured in a prograde orbit - it has been getting farther away from Earth
through time. If it was captured in a retrograde orbit, the Moon would be
getting closer to Earth through time - bad.
Vesta was born at 0.19
AU and ended up at 2.4 AU, in the Asteroid Belt. Vesta was tossed around
Mercury and Venus. Vesta is too small to have been captured.
Options for Vesta - collision with larger bodies or passed by larger
bodies. The latter is what happened - now in the Asteroid Belt.
Angra was born at 0.1
AU - it’s current location is unknown, but probably in the Asteroid Belt.
Adonis - a 0.5
Moon-mass body born at ~0.22 AU. Was gravitationally captured by Venus
(0.7 AU) into a retrograde orbit. Adonis approached Venus through
geologic time, and eventually coalesced with Venus at ~1 b.y. to ~500 m.y.
ago. This is why Venus
is a basalt cauldron - a smoldering mass. This is a “fatal
attraction” scenario - retrograde capture + coalescence half a billion to
a billion years ago.
Earth
would have been like Venus & Adonis if the Moon was captured in retrograde
orbit.
The
Moon as a vulcanoid. Vulcanoids were named in 1978. No
one’s seen a vulcanoid. Well, we have been seeing a
vulcanoid all this time.
The
Moon has a blow-out hole (Mare
Orientale) - material necked out by tidal disruption and fell back to the
Moon’s surface to form the maria in a great circle pattern. ~18
orbits after Moon capture, there was possibly a solar perturbation that pushed
the Moon closer to Earth - got maria splashes.
The
X-Wind Model well explains CAIs and dehydration events. It doesn’t
explain chondrules. The T-Tauri phase microwaved the Moon.
Where
did chondrules form? After the dehydration event, the entire inner
Solar System must have been chondrules. Chondrules formed by shock waves
in the inner Solar System. The shock waves may have originated during
Jupiter planetesimal coalescence events. Jupiter formed from large balls
of hydrogen and helium and solid material. If two of these balls collided,
would get a shock wave. With another ball collision, another shock wave
was generated. Chondrules formed by melting of material by the shock
waves, like a lightning strike, resulting in droplets in space. They
cooled quickly into glass and later crystallized. Very rapid formation
for chondrules.
The
K index pattern developed before chondrules formed.
Earth
and Venus are ~the same.
Moon,
Adonis, Vesta, Angra - 4 vulcanoids.
Capture
is simple in principle. Capture is complex in detail.
Earth
has a large Moon - prograde capture.
Triton,
a moon of Neptune, was captured for sure - it’s going the wrong way,
orbit-wise.
Most of the energy generated by the
Moon capture event was absorbed by the Moon.
Summary provided by Professor Malcuit: The origin of the Moon is still an unsolved problem
in the natural sciences. In recent years many investigators have jumped
onto the “bandwagon” to espouse the merits of the Giant Impact
Model. This model proposes that the Moon was formed as a result of a collision
of a mars-mass body with the primitive earth about 30 million years after the
formation of the Earth. Although the Giant Impact Model appears to be
physically possible, at least in part, the model does not relate very well to
the rock records of the Earth or the Moon.
The other physically possible model is the
Gravitational Capture Model. Most of the recent work, 1972 to present,
has been done at Denison University as a combined geology and physics
project. Our Capture Model is now undergoing a “renaissance”
in light of (1) the Potassium Index for solar system bodies (~1995), (2)
the discovery of a multitude of stable planetoid orbits between the orbits of
planet Mercury and the Sun (~1999), and (3) the Cool Early Earth Model (~2002).
In other words, the Gravitational Capture Model does relate to a number of
features of the rock records of both the Earth and the Moon.
This presentation will summarize some of the
scientific evidence in favor of the Gravitational Capture Model and compare and
contrast this information with the main features of Giant Impact Model.
Biographical information on Professor Malcuit: Bob Malcuit received his Bachelor and Master degrees
in Geology from Kent State Univeristy in 1968 and 1970 and his Ph.D in Geology from
Michigan State University in 1973. Bob’s current research is in
Planetary Geology and one of the themes of several of his projects is that
“THE MOON IS THE ROSETTA STONE OF THE SOLAR SYSTEM”. He
inherited this concept from Harold Urey (American chemist) and Zdenek Kopal
(Czech astronomer). In other words, Bob thinks that the Earth’s
Moon is one of the most important recorders of scientific information in the
Solar System (a minority view at the present time). He is also promoting the
view that without a large satellite like our Moon, planet Earth would be very
different from what it is today. For example, without the Moon and the
associated rock and ocean tidal action, the Earth would probably not be
habitable for life forms higher than bacteria and algae.
