ÒPlanet Earth and Its MoonÓ
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 Òblack and white and grayÓ - no colors.
EarthÕs 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, EarthÕs rotation rate today would be 12-14 hours per day.
EarthÕs 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), itÕs the only planet with true granite, itÕs the only planet with continental crust, itÕs the only planet with operational plate tectonics.
MoonÕs 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.
EarthÕs 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 didnÕt 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, EarthÕs 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 ÒsurvivorÓ 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 ÒRosetta Stone of the Solar SystemÓ.
Well, the Moon is as old as the Earth, so it is primitive. The Moon may be a little older than the Earth.
The MoonÕs 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; thereÕs actually not much evidence for it; proponents canÕt make a lunar-sized body from the impact debris; if they can, they canÕt 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 didnÕt 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 doesnÕt 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 (Òbanana indexÓ) - 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 wasnÕt 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 canÕt 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 doesnÕt); 2) it accounts for the angular momentum of the Earth-Moon system (actually, it doesnÕt); 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. ThereÕs lots of investment in the impact model.
Earth canÕt 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? DidnÕt know, originally.
When Malcuit retired in 1999, he still didnÕt 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 EarthÕs orbit. If so, would get a Moon with the same composition as Earth. (which it isnÕt)
2) in the inner part of the Asteroid Belt. If so, would get a Moon with ices/water. (which it doesnÕt)
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 canÕt 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, itÕs been thought. LetÕs 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.
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 wasnÕt the case.
The Moon capture event occurred at 3.95 Ga. The older lunar maria rocks came into existence then.
EarthÕs 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 MercuryÕs 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 - ÒNo NamesÓ.
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 Òbenign estrangementÓ scenario.
Capture models in general - captured bodies can enter a retrograde orbit or a prograde orbit - thereÕs 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 - itÕs 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 Òfatal attractionÓ 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 oneÕs 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 MoonÕs 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 doesnÕt 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 - itÕs 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 ÒbandwagonÓ 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 ÒrenaissanceÓ 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. BobÕs current research is in Planetary Geology and one of the themes of several of his projects is that ÒTHE MOON IS THE ROSETTA STONE OF THE SOLAR SYSTEMÓ. He inherited this concept from Harold Urey (American chemist) and Zdenek Kopal (Czech astronomer). In other words, Bob thinks that the EarthÕs 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.