[meteorite-list] New Study Suggests Uranus Got Sideways Tilt From Multiple Impacts

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Mon, 10 Oct 2011 09:38:50 -0700 (PDT)
Message-ID: <201110101638.p9AGcokY000666_at_zagami.jpl.nasa.gov>

http://www.space.com/13231-planet-uranus-knocked-sideways-impacts.html

Planet Uranus Got Sideways Tilt From Multiple Impacts
space.com
10 October 2011

The giant planet Uranus was tipped on its side by a succession of
punches rather than a single knockout blow as previously thought, a new
study suggests.

The finding sheds light on the early history of Uranus and its many
moons. It could also force astronomers to rethink their ideas about
how the solar system's giant planets formed and evolved, researchers said.

"The standard planet formation theory assumes that Uranus, Neptune and
the cores of Jupiter and Saturn formed by accreting only small objects
in the protoplanetary disk," said study leader Alessandro Morbidelli, of
the Observatoire de la Cote d'Azur in Nice, France, in a statement.
"They should have suffered no giant collisions."

"The fact that Uranus was hit at least twice suggests that significant
impacts were typical in the formation of giant planets," Morbidelli
added. "So, the standard theory has to be revised."

An oddball planet

Uranus is a real oddball in our solar system.

Its spin axis is tilted by a whopping 98 degrees, meaning it essentially
spins on its side. No other planet has anywhere near such a tilt.
Jupiter is tilted by 3 degrees, for example, and Earth by 23 degrees.

Scientists have long suspected that some manner of violent impact
knocked Uranus off kilter. The accepted wisdom had been that a single
object several times more massive than Earth did the damage, slamming
into Uranus long ago, researchers said.

After performing a series of computer simulations, Morbidelli and his
team may have found a better explanation.

The research was presented Thursday (Oct. 6) at a joint meeting of the
European Planetary Science Congress and the American Astronomical
Society's Division of Planetary Science in Nantes, France.

Multiple impacts?

The researchers began by modeling the single-impact scenario. They found
that the collision likely occurred in the solar system's very early
days, when Uranus was still surrounded by the disk of dust and gas that
would eventually form its moons.

After a monstrous collision, the disk would have reformed around Uranus'
new, highly skewed equatorial plane. The moons would share Uranus' tilt,
which they indeed do.

So far, so good, but then the simulations offered up a surprise,
researchers said. If there was just one collision, Uranus' moons
would display retrograde motion, orbiting in the opposite direction than
that which astronomers observe today.

To account for the discrepancy, the researchers tweaked their
simulations' paramaters a bit.

They found that a series of at least two smaller collisions can explain
the moons' motions much better than a single giant impact, researchers
said. The early solar system thus may have been a more volatile and
violent place than previously thought, they added.
Received on Mon 10 Oct 2011 12:38:50 PM PDT