“Origin of the Earth and Moon”, G Jeffrey Taylor
“Time to Solidify an Ocean of Magma”, G Jeffrey Taylor
(Seacrest School Moon Crew, Naples, Fl., 2011, “if there are mistakes here, We made ‘em!”)
Summary Notes:
“Origin”:
This article is very readable for anyone with any background. It’s date 1998, puts it in the
early descriptions of planetary formation, but a theory that is still current. Note that the early solar
system is a busy place, with lots of projectiles in non-circular orbits capable of colliding with anything,
especially those objects, like the new Earth, that ARE in circular orbits. Big impacts fit into our picture of
planetary formation. A brand new Earth “half built”, colliding with a smaller abject (1/2 its size) could
have created the present Earth-Moon system with all of its current properties. During the collision
much of the debris rebounds into orbit around the Earth, forming the moon.
Fun Fact: This collision and orbiting of the debris takes about 30 minutes! Watch the simulation!
Page 5: Note the use of radiometric dating using aluminum 26 and hafnium 182 to mark the age of the
solar system at 4.566 billion years. The dating is on rocks in meteroites that appear the be the first
forming solids in the solar system.
Giant Impact hypothesis is not proven to everyone’s satisfaction, but it explains a lot about the earth
and moon and continues today to be the main theory of planetary system formation.
“Time to Solidify an Ocean of Magma”
This article from 2009 requires a bit of translation but
is also very readable.
The main idea: the moon was a molten blob when it formed. See page 2: Diagram: Lunar magma Ocean
The important rocks names here are:
Plagioclase (feldspar) and anorthosite are lighter and will float to the surface
Pyroxene and Olivine contain heavier elements and will sink (somewhat) and be lower in the moon’s
interior.
Page 3: see the Diagram:
Plag = plagioclase feldspars (sodium, aluminium, Calcium, silicon, oxygen compounds) HIGHLANDS
Pig, cpx, ilm = pigeonite, ilmenite, etc. ( Ca, Mg, Fe, Si, compounds, denser than Plag) MANTLE
OPX, Olivine: Ca, FE, Mg, Zn compounds. (Olivine = MgFeSiO4) denser compounds deeper in the moon
Time to Solidify Ocean of Magma (cont)
Radiometric dating shows that the earliest the moon could have formed is about 50 million years after
the Solar System formed, 4.517 billion years ago.
Here is a great example of the dating of rocks from Apollo. Apollo 17 collected samples from boulders
in the Taurus Littrow valley in 1972. Look at the photos Page 4,5. Using rock hammers, the astronauts
broke off chunks of rock, they numbered the samples and returned them to earth where they are still
kept in an oxygen free environment to prevent any chemical changes to the rocks. The samples are
breccias – rocks that have been shattered and melted and ejected by meteor impacts, but still retain
some of their original materials unchanged.
Radiometric dating of the zircon grains (see pictures pg 5,6) those parts that were not changed by later
impact history is from 4.417 billion years ago . Some of the harder to date surface crust rocks
(anorthosites) are slightly older, 4.456 billion years (chart page 7).
Conclude based on rock ages: (page 7)
Moon forms: 4.517 billion years ago
Crust of lighter compounds: 4.456 billion years ago
Solid moon: 4.417 billion years ago (takes a total of 100 million years)
Take away idea:
The Moon forms a blob of liquid rocks after the collision of a wandering planetoid with the new Earth.
the Moon blob takes about 100 million years to solidify. The moon’s oldest surfaces should be from
that time, 4.456 billion years ago.