[meteorite-list] 'Mild' Collision Spawned Earth's Moon

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Tue Jan 10 16:41:46 2006
Message-ID: <200601101723.k0AHJkZ12633_at_zagami.jpl.nasa.gov>

http://www.newscientistspace.com/article/dn8550--mild-collision-spawned-earths-moon.html

'Mild' collision spawned Earth's moon
Maggie McKee
New Scientist
09 January 2006
 
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
Received on Tue 10 Jan 2006 12:18:37 PM PST