AN EXTRA planet in the Solar System could have been the culprit in a massive
series of impacts that battered the Moon nearly 4 billion years ago.
Huge craters were formed on the lunar surface when objects up to 100
kilometres across crashed into it during a 100-million-year period called the
Late Heavy Bombardment. This seems to have come after a long spell of relative
calm in the inner Solar System. So astronomers have been wondering where these
objects were in the 600 million years between the formation of the Solar System
and the moment they began to hit.
John Chambers of the NASA Ames Research Center in California thinks they were
in the main asteroid belt. At the Division for Planetary Sciences meeting in New
Orleans this week, he described computer simulations showing that the Moon鈥檚
ordeal could be explained by an extra planet that originally formed beyond Mars,
just inside the asteroid belt.
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Encounters with Mars would have slowly nudged the planet outwards over
hundreds of millions of years until it strayed too close to Jupiter and Saturn.
The influence of these giants would have stretched the planet鈥檚 orbit into a
long, thin ellipse, reaching from near the Sun to the outer edge of the asteroid
belt, perturbing asteroids 鈥渟o they started flying all over the inner Solar
System,鈥� Chambers told 快猫短视频. Peace would have resumed when the
planet fell into the Sun.
Chambers鈥檚 scenario is only one explanation for the impacts. A second
contender reported in New Orleans is the eviction of Uranus and Neptune to their
present positions from previous orbits much closer to the Sun (see New
快猫短视频, 24 November, p 11).