Mississauga Centre RASC

79th Meeting

                                                                                                 Speakers’ Night       

Day:                Friday February 23, 2007

Chair:                Randy Attwood

Speaker:          Tom Bolton

The Sky Tonight

Roy Swanson spoke about sky happenings for the following month. He discussed the ellipticity of orbits.

The Origin of the Moon

Dr. Tom Bolton, professor at U of T and discoverer of the first black hole spoke about the origin of the Moon. Some important work on the origin has been done at the University of Toronto using rocks gathered by the Apollo astronauts.  The Apollo mission to the Moon had the political purpose of getting there before the Russians as well as to try to understand the origin of the Moon.

In the 1950’s we did not know about asteroid satellites or about the Kuiper Belt, or about Charon. The Moon was unique in that it had the unusually large mass ratio to the earth of 1:81 as opposed to the satellites of other planets. No other terrestrial planet had a large moon. The Moon had an old cratered crust, large circular basins and light coloured highlands. Two arguments pre-Apollo for crater formation were volcanic vs. impact. In addition, Thomas Gold postulated that the surface of the Moon was 1 km thick in dust, a fact disproved when the Ranger spacecraft landed on the lunar surface. Lunar orbiters surveyed the Moon for safe landing spots.  Known facts pre-Apollo were that the lunar orbit oddly enough is inclined to the ecliptic, that the angular momentum is large and density small, the highlands are older than the maria from crater counts and chemically different, the Moon is tidally distorted and is gradually moving away from the earth, and the backside lacks large lava-filled basins as shown by soviet spacecraft. 

At that time there were 3 models of formation of the Moon:

 i) capture wherein the Moon formed elsewhere in the solar system and was captured by the Earth. This predicts a primitive meteoritic composition and is also difficult to produce dynamically unless 3 bodies are involved.

ii) fission wherein a rapidly spinning Earth broke apart into 2 bodies. The Pacific Ocean was viewed as the likely site for the Moon’s fission but the theory fails to explain the large angular momentum and inclination of the Moon’s orbit to the ecliptic.

iii) binary accretion at the Langrangian point and gradual drift towards the Earth. Problems with this scenario are the orbit not in the equatorial or ecliptic planes and the low density of the Moon (one would expect a similar composition to the Earth).

Apollo Mission data included rocks and soil yielding composition and chemistry, natural and impact seismic data, and magnetic field measurements from orbit and from surface rocks. Work at U of T concentrated on rock magnetism.

“Geo”chemistry of the Moon showed that the crust was strongly differentiated after formation, that volatile elements (<1300K melting) are strongly depleted compared to Earth, that sideroblastic elements (iron-like) are depleted compared to Earth’s crust even though the Moon’s core is very small, and that oxygen isotopes are identical to Earth’s suggesting that both bodies formed at the same distance from the Sun.  Conclusion from this is that the Moon is not made of primitive material, is not like the Earth but that it formed at the same distance.

Seismic data show that moonquakes occur near the core-mantle boundary, and that the core is partly molten. The long attenuation time for waves implies a dry Moon with a rubble surface with a lot of gaps. Thus a cross-section of the Moon would show a small iron core, solid mantle and semi-solid outer crust which is thinner towards the Earth.     

Magnetism studies show that all lunar rocks have a remnant magnetism implying that there was a lunar magnetic field when the rocks became molten.  Magnetic fields have been disrupted by impacts and are randomly oriented.  Studies done U of T at Mississauga show that the magnetic field strength varies with the age of the rocks which would be expected if the crust was magnetized by an internal dynamo like an iron-sulfur core. There was a dynamo that died out. Craters of different ages have a different field orientation. Big mare (impact basins) lie along the magnetic equator for that age.  The rotational pole was changing because of impacts.

From all this, the “Big Whack Theory” can be derived. A Mars-sized body struck the Earth off centre within 100 million years of the Earth’s formation blasting off a great deal of debris which is depleted of iron and similar elements and lacking volatiles because it is molten.  The lunar core formed outside the  Roche limit and grew by accretion.  The Moon’s core formed within hours and the Moon itself within 10 days from the outer regions of the Earth and the “whacker”.

Eclipse of the Moon

Randy Attwood described the upcoming eclipse of the Moon of March 3. The Moon will rise while deep in the Earth’s shadow. He described the appearance of the Earth from the Moon and the colours of the moon during a total eclipse

The rest of the evening was taken up by the Annual Meeting of the Mississauga Centre RASC

Submitted by Chris Malicki, Secretary  Chris Malicki, Secretary                               back to Miss Centre. meeting reports page
Mississauga Centre RASC