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The Moon

Hello DaG 262 and DaG 400 addicts,

There is an interesting article about the early days of our natural
satellite in the latest Sky & Tel issue that I would like to share with

Good night to  e v e r y b o d y - it is late here!

Best wishes,


---------- snip ---------

A New Twist to Lunar Origins (News Notes, Sky & Tel., Sep 1998, p. 26)

Most planetary geologists agree that the Moon likely coalesced from the
debris of a titanic collision between a Mars-size planetoid and the
Earth. But this theory has unsolved difficulties. How did the Moon get
from a low, equatorial orbit - the kind it probably started out with -
into a 384,000-kilometer-high, steeply inclined one? Jihad Touma
(University of Texas) and Jack Wisdom (MIT) have developed a scenario
that may explain the Moon's orbital evolution. As a bonus, it also
explains the ocean of magma that differentiated and cooled to form the
lunar highlands.
The Moon's tidal friction is gradually slowing the Earth's rotation. For
most of its history, the stolen energy has been slowly expanding the
Moon's orbit. But, by precisely calculating the gravitational
interactions between the Earth, the Moon, the Sun, and the other
planets, Touma and Wisdom found that the Moon may have been twice caught
in resonances that changed its orbit abruptly.
Only 1,000 years after the Moon's tumultuous birth, the first resonance
squeezed the lunar orbit into a narrow ellipse with an eccentricity near
0.5. The intense tidal forces in that elliptical orbit generated
thousands of times as much heat as volcanic Io now derives from its
Jupiter-generated tides. The heat would have melted the surface of the
Moon, creating its iron-poor crust. That melting also used up enough
energy to allow the Moon to escape the resonance and resume its outward
However, immediately thereafter, the Moon was captured into a second
resonance that pushed it out of the Earth's equatorial plane. The Moon
escaped this resonance only after its orbit had been tilted by 12.
Afterward, the Moon's orbit continued to expand and tilt.
Touma and Wisdom did have to fine-tune the model's initial conditions to
reproduce today's Earth-Moon system. Their best model starts with an
early Earth whose days were only 5 hours long and whose spin axis was
already tilted 10 with respect to the ecliptic plane. It also places
the newborn Moon only 22,000 km above the Earth's equator.
Wisdom emphasizes that the study, described in the April Astronomical
Journal, outlines only one possible scenario. The Moon, he says, may
have coalesced inside the first resonance; alternatively, a later impact
may have tilted the Earth's axis.

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