'Mild' collision spawned Earth's moon
- From: "George" <george@xxxxxxxxxxxxxxxxxxxxx>
- Date: Mon, 09 Jan 2006 15:31:56 GMT
http://www.newscientistspace.com/article.ns?id=dn8550
11:58 09 January 2006
NewScientist.com news service
Maggie McKee
The collision that spawned the Earth's moon was relatively mild, reveals
the longest and most detailed computer simulation ever done of the impact.
The research puts limits on the size and velocity of space rocks that can
lead to the formation of satellites in cosmic smash-ups.
Computer models suggest the Moon formed after an object the size of Mars
(just over half the diameter of Earth) crashed into Earth about 4.5 billion
years ago. Debris from the impact formed a disc around Earth that
eventually coalesced to become the Moon.
But modelling the process realistically is extremely difficult, and
researchers have tried a variety of approaches. Most have used single
particles in the models to represent some larger number of real particles,
a method called Smoothed Particle Hydrodynamics (SPH).
But the best of these models use just a few thousand particles in the
debris disc, and therefore can not reveal detailed disc structures. As a
result, the models can only recreate conditions for less than a day after
the impact.
"Extreme" simulations
Now, researchers led by Keiichi Wada at the National Astronomical
Observatory of Japan in Tokyo have used another approach to model the disc
for about four days. They divided the disc into a three-dimensional grid of
boxes - each with its own properties, such as temperature and density - and
evolved the boxes over time. They ran two "extreme" simulations - one in
which the disc was made mostly of hot gas, and another where it was mostly
liquid and solid.
Both simulations behaved similarly for the first 10 hours after the initial
impact, with the damaged impactor circling back and hitting Earth a second
time, when it is destroyed. This accords with SPH models as well,
suggesting gravity is the dominant force in the early formation of the
disc.
But the two models begin to diverge after that. If the impactor vaporises
when it is destroyed, spiral shock waves are created that slow down the
disc's rotation. This allows the disc material to fall onto the Earth and
prevents the formation of a moon.
Trouble with models
In contrast, if the impactor produces mostly liquid or solid debris, the
shocks cannot slow the disc down enough to make it fall to Earth, and the
Moon is formed. The researchers suggest that any impact powerful enough to
vaporise the impactor would not form a satellite.
In the case of the Earth, they estimate the Mars-sized object must have
been travelling at less than 15 kilometres per second. In more general
terms, they conclude that if an impactor is more than a few times the mass
of Earth, then "the giant impact never results in forming a large
satellite".
Scott Kenyon, an astronomer at the Smithsonian Astrophysical Observatory in
Cambridge, Massachusetts, US, says the conclusion is reasonable. "The
gaseous disc would most likely collapse faster than a solid or liquid
disc," he told New Scientist.
But he points out that astronomers have long struggled with modelling the
viscosity of gas in rotating discs. He says all models have this problem,
but that the 3D grid approach may be more vulnerable to it because the
viscosity must be chosen by the researchers, and the value selected could
affect the timescale over which the disc falls to Earth.
Journal reference: Upcoming issue of the Astrophysical Journal
.
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