I read someplace else that this "Late Heavy Bombardment (LHB) that occurred 3.9 billion years ago could have also been caused by the instability in
the orbit of a fifth "long-lost" rocky planet (our solar system has four rocky planets) that is commonly referred to as "Planet V" by some
astronomers.
As the theory goes, this fifth rocky planet orbited between Mars and the asteroid belt. Somehow, its orbit became perturbed enough for it to gain a
highly elliptical orbit that sent it through the asteroid belt then eventually back toward the Sun -- where, perhaps after a few million years in this
eccentric orbit, the Sun then devoured it.
This path of its orbit through the asteroid belt is what some theorize may have caused the LHB.
Here's an article on the subject:
www.space.com...
and another article from the same source cited by the OP:
www.cosmosmagazine.com...
[edit on 10/21/2009 by Soylent Green Is People]