Where did the Moon come from? Part 2 - The Big Whack by Joe Leddy

Where did the Moon come from? (Part 2 " The Big Whack")

In my last post we looked at the Origins of the Moon and the "Big Three', theories that tried to answer the question of the Moons beginnings. All three had at least some scientific basis behind them, although they all turned out to fall short in one manner or another. So where does that leave the discussion? If the "Big Three" doesn't answer the question, what does?

Enter the currently accepted theory,"The Big Whack". In the mid 1970’s this new theory started to come together. William Hartman (from the Planetary Science Institute in Tucson, Arizona) and Don Davis (a colleague from the PSI) determined that a roaming planetoid of significant enough size could theoretically knock enough of the mantle off of the Earth if it struck early enough after the formation of the planet.

Meanwhile at Harvard University, Alistair Cameron and William Ward determined independently of Hartman and Davis's work that a planetoid of at least the size of Mars could have provided the needed force to supply enough raw materials for the Moon.

Putting these two pieces of work together, "The Big Whack" Theory was born. It was not without its detractors, scientists tend to frown on outlandish answers to problems. So the theory sat without much work being put into it until 1984. In Kona, Hawaii a conference was held to discuss the origins of the moon, and research started to move forward on the Giant Impact Theory as it was know known. (Apparently Astronomers have a rough life with conferences in Hawaii, as apposed to digging in the middle of Montana like a Geologist)

Robin Canup, an astrophysicist from the Southwest Research Institute, modeled what such an impact would look like. Her calculations arrived at the conclusion that the impacting object would have to have been two to three times the size of Mars. She arrived at this conclusion based on her assumptions that only 20 to 50% of the material ejected from an impact would make it to form the Moon.

The Moon is less dense than the Earth, a moon forming from a cloud of debris from the impactor and the Earth' mantle would explain this. The Big Whack also explains the Moon’s tiny core; models show that the Earth would have absorbed any core within the impactor. And the Big Whack explains while the Earth rotates about an axis on a 23.5-degree tilt.

So far so good, however there are still holes in the Big Whack that need to be worked out. For instance, if the Moon is made up from a combination of an impactor and the Earth, why is there such an oxygen-isotope similarity between the two? Wouldn’t you expect more of a difference? A second problem is the resulting spin of the Earth that would have resulted. If such a large object hit at an angle resulting in all of this debris, the planet would have a significant increase in its rotational speeds. How do we account for the lack of this spin? A second impact slowing the Earth down seems incredibly unlikely.

While there are still questions to be answered, the Big Whack is still the favored theory of lunar development.

For more information on the Moon and its origins
please see the following websites:

www.nasa.gov

www.pbs.org/wgbn/nova (origins by Peter Tyson)

Origin of the Moon by William Hartman published in
1984

www.space.com (Torn Away: The Moon’s Violent Birth
September 1 2000)