Cumberland Times-News

Columns

September 10, 2011

Where did the moon come from?

It has been nearly 40 years since the last human walked on the moon. Analysis of our Moon’s rocks has given rise to a very interesting theory as to how our Moon came to be.

Moon rocks have the same basic chemical composition as Earth rocks except for the near absence of volatile elements (would be vaporized at lower temperatures).

Could the moon’s material have come from Earth? William Hartmann, an astronomer at University of Arizona was first to propose that our Moon formed from debris of a planetary collision.

In this collision, the Earth was dealt a glancing blow by another planet about the size of Mars. The intruding planet was shattered and a large amount of the Earth’s mantle was hurled into space.

Some of this material went into orbit about the Earth, forming a ring of debris. Within this ring was a concentration of material that formed our moon.

The early moon was much closer to the Earth than it is now. The mechanism to drive the moon out further was the friction of our moon’s tides against the ocean floor. This friction slowly lengthened the Earth’s rotation period.

But to conserve rotational motion of the Earth-Moon system, the moon was pushed faster around its orbit, increasing its distance. This has resulted in our Moon now about 60 Earth radii from us.

The best indications that our moon resulted from a planetary collision is to simulate this event on a supercomputer. Different sized planets were hurled at the Earth in computer simulations.

At certain impact velocities, the smaller planet would be destroyed and large amounts of Earth middle layer (the Mantle) were ejected. The Earth’s iron-nickel core would have survived. This core gave the Earth enough mass to build itself back up.

Some of the ejected mantle material would have gone into a near Earth orbit, of which about 1/2 congealed into the moon.

Can this theory of our moon’s origin be proven? No, as you can’t prove a theory; it can only be disproven if you have observations that go against a theory.

But over the past two decades, most astronomers have accepted the “Giant Impact Theory” as being the most likely way for the moon to have formed.

The other proposed theories don’t explain the chemistry of moon rocks, or are inconsistent with physical laws. (One flawed explanation is that the Earth spun so fast that a chunk flew off; the ‘wound left behind’ is the Pacific Ocean basin.)

Today our Frostburg State Planetarium reopens for our final fall season. Early in 2012, Tawes Hall will be torn down and our programs will then be presented on Sundays in Compton Science Center on a large flat screen. A new planetarium will be put into the new building (called CCIT), slated to open in 2014.

Today’s program is “Moon Curiosities,” all about the most unusual moons in our solar system. We first cover the evening sky sights for the entire fall season and acquaint our audience with the brightest evening sky sights. There are free sky charts available in our rack as you leave.

Our free showings are at 4 p.m. and 7 p.m. in Tawes 302. The Planetarium is just off Tawes’ front lobby that faces the Compton Science Center.

The easiest way to reach the Planetarium is to take Exit 33 off Interstate 68 and drive North towards Frostburg. You will then be on Braddock Road, passing by the new housing project (Braddock Green) and some private homes on the right.

Look for FSU’s main entrance on the left, turn in and then turn to your right. After driving several hundred feet, you will see the large Performing Arts Center (PAC). Park near the PAC building and walk around it on the right. You will see Tawes Hall further down the street and the Compton Science Center on your right.

“Moon Curiosities” starts with the usual regularities of moons, then considers our moon. Our moon is the nearest moon to the sun as both Mercury and Venus are moonless.

Tonight, by coincidence, the moon is full. This is the Harvest Moon, the full moon closest to the first day of fall. You will notice much early evening moonlight for the next five evenings (weather permitting).

Returning to our program, we next take up the pygmy moons of Mars that scurry about Mars from 7.3 to 30.3 of our hours. The smaller of the two moons, Deimos is about as long as the island of Manhattan, the best known part of New York City. 

Jupiter has four big moons, each with different surface features; one moon is covered with active volcanoes while the outer big moon is covered with icy craters that have flattened out over the billions of years since they were formed.  

Saturn has a huge moon with an atmosphere thicker than Earth’s and is covered with lakes of hydrocarbons. The medium sized moons closer to Saturn cause gaps in the rings; two small moons orbit on either side of Saturn’s narrow F ring. 

Uranus rolls on its side as it orbits the sun. Uranus’ major moons also orbit side wards around the planet. Uranus’ bigger moons are named after characters in Shakespeare’s play, “The Tempest.” 

Neptune in many ways is a twin to Uranus, but without Uranus’ side wards tilt. Neptune has a big moon (Triton) orbiting backwards; instead of slowly drifting outwards (as our moon does), Triton is spiraling in! In the far future, Triton will be torn apart by Neptune’s tides, giving Neptune a magnificent ring.  

The dwarf planets also have moons. (Dwarf planets orbit the sun, are roughly spherical but have a significant amount of debris across their orbits.) Pluto is the most famed dwarf planet; recently a fourth moon about Pluto was discovered. Another dwarf planet (Eris) is slightly larger than Pluto and also has a moon. 

Enjoy the Harvest Moon both tonight and tomorrow night (actual time of full moon is 5:26 a.m. tomorrow). On Thursday and Friday evenings the moon will be near the bright planet Jupiter, shining steadily low in the east.

Bob Doyle invites any readers comments and questions. E-mail him at rdoyle@frostburg.edu . He is available as a speaker on his column topics.

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