杏吧原创

NAGGING doubts about how the Moon formed may have been laid to rest, thanks
to a new computer simulation.

Astronomers think that the Moon was born about 4.5 billion years ago when a
stray planet walloped into the young Earth. Debris was flung out into space,
forming an orbiting ring around the Earth in which little moonlets clumped
together, eventually forming the Moon.

But computer models of the process have seemed overcomplicated. Some suggest
the initial impact must have been followed by a second giant impact to explain
the total angular momentum of the Earth and Moon today. Others hint that the
impact occurred when the Earth was only partially formed. Yet only impacts on an
almost fully formed Earth can explain the Moon鈥檚 composition.

Now Robin Canup of the Southwest Research Institute in Boulder, Colorado, and
her colleague Erik Asphaug have resolved these problems in a more detailed
computer model. Their complex 3D simulation allows for the effects of heating
and melting during the impact, and the mutual gravitational attraction of all
the debris. This time, all the pieces fit neatly together鈥攊ncluding the
conclusion that a planet the size of Mars hit the Earth when it was almost fully
formed.

These are encouraging results from a challenging study, says Jay Melosh, a
planetary scientist at the University of Arizona. 鈥淐omputer simulation of a
planetary-scale impact is not a task for the faint-hearted.鈥� However, he says
that to simulate events exactly, we need to know more about the physics of the
silicates in the Earth and Moon in their solid, liquid and vapour forms.