[meteorite-list] Early Solar System May Have Been Chaotic

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Wed May 25 14:04:53 2005
Message-ID: <200505251804.j4PI4FG09502_at_zagami.jpl.nasa.gov>

http://news.yahoo.com/s/ap/20050525/ap_on_sc/cosmic_tale

Early Solar System May Have Been Chaotic
By MALCOLM RITTER,
Associated Press
May 25, 2005

NEW YORK - By using a computer to play a complex game of "what if,"
scientists have developed a tale of chaos in the early solar system that
they say explains several mysteries about why our cosmic neighborhood
turned out the way it did.

"We get so many of the answers right," said Harold Levison of the
Southwest Research Institute in Boulder, Colo. "I never dreamed it would
be so spectacularly successful."

In a single stroke, he and his colleagues say, the tale answers such
questions as:

 - What set off an intense asteroid bombardment some 3.9 billion years ago
   that created huge lava-filled basins on the moon and may have set back
   the development of life on Earth?

 - Why did Jupiter and Saturn leave their circular orbits and take on the
   more oval paths seen today? And how did their orbits became so tilted
   compared to other planets?

 - Why does Jupiter share its orbit around the sun with a swarm of asteroids?

The work is presented in Thursday's issue of the journal Nature by
Alessandro Morbidelli of the Cote d'Azur Observatory in Nice, France,
along with Levison and others. They used computer simulations to study
various scenarios about how the outer solar system may have developed.

Their favorite scenario follows the generally accepted idea that some
4.6 billion years ago, the sun and planets formed from the gravitational
collapse of a cloud of gas, dust and ice. But it adopts what Levison
called the controversial position that the solar system started out as
quite compact. In this scenario, for example, Neptune starts out less
than 15 times as far away from the sun as Earth is now, rather than the
23 times other scientists propose.

So the question was what would happen over eons as the planets followed
circular orbits, surrounded by a huge ring of planetary rubble, chunks
measuring up to hundreds of miles across. As the planets and chunks of
rubble exerted their gravitational tugs on each other, what would change?

Here's what the scenario suggests:

As the planets tugged on the rubble, the rubble tugged back, and that
nudged Uranus, Neptune and Saturn outward from the sun and Jupiter
inward, as previous research has suggested. That in turn affected how
long each planet took to complete an orbit of the sun, since a wider
orbit takes longer. At some point, Saturn started taking exactly twice
as long as Jupiter to complete a lap.

Then all hell broke loose.

Because of their tugs on each other, Jupiter and Saturn began to leave
circular orbits and follow more oval-shaped paths similar to what's
observed today. That wreaked gravitational havoc on the much less
massive Uranus and Neptune, making their orbits "totally nuts," Levison
said. It sent the two planets outward and into the ring of planetary
rubble which, as Levison put it, went "kaplooey."

It's as if Saturn were a bowler, Uranus or Neptune the ball, and the
rubble chunks the pins, he said. The gravity of the intruding planets
scattered the rubble, and one result was the bombardment of Earth and
the moon.

The rubble's gravitational tugs on Uranus and Neptune eventually nudged
those planets into the orbits seen today, according to the scenario,
which also produced the observed tilts of the orbits of all four planets
in the story.

The work also explains the presence and highly tilted orbits of
Jupiter's "Trojans," asteroids that share essentially the same orbit as
Jupiter. While some scientists have suggested they formed near Jupiter,
the scenario suggests they developed far away and were captured in
Jupiter's orbital path just after Saturn and Jupiter hit their crucial
2-to-1 ratio in orbit times.

Renu Malhotra, a professor of planetary sciences at the University of
Arizona, called the work "very interesting and provocative," but said
it's probably not the last word on the subject.

----------------------------------------------------------------------


http://www.nature.com/news/2005/050523/full/050523-6.html

Planetary billiards answer Solar System riddle
Mark Peplow
Nature
May 25, 2005

Scattering rocks moved planets and battered the Moon.

Why does the Solar System look like it does? The question has teased
astronomers for centuries, but researchers have now come up with a
single theory that they hope can explain three of the most mysterious
features of our corner of the Universe.

The giant planets Jupiter and Saturn have unusually elliptical orbits
that are significantly tilted out of the plane occupied by the smaller,
rocky planets close to the Sun. And Jupiter itself is accompanied by
small asteroids called Trojans, which fan out for millions of kilometres
ahead of and behind the planet, speeding along the same orbital path.

Most perplexing of all is the 'late heavy bombardment' about 3.8 billion
years ago, which peppered the Moon with chunks of rubble left behind
from the planets' formation some 700 million years earlier. No
convincing explanation has so far been put forward for why this sudden
battering happened so long after the Solar System's violent early years.

But according to Hal Levison of the Southwest Research Institute in
Boulder, Colorado, all of these quirks can be explained by a game of
planetary billiards that began when Jupiter and Saturn fell into a
specific orbital pattern just a few million years after they formed. "We
have really explained a lot of the structure of the Solar System with
this model," says Levison. He and his colleagues present their computer
simulations of the Solar System's history in this week's Nature.

Junk theory

Levison singles out his colleague Alessandro Morbidelli, of the
Observatory of Nice in France, as the prime architect of their idea.
They argue that the tiny gravitational effects of junk left over from
the formation of Saturn and Jupiter gently nudged the planets around
until Saturn fell into an orbit that sent it around the Sun exactly once
for every two orbits completed by Jupiter.

This resonance meant that twice every saturnian year, at almost exactly
the same two points in space, Jupiter's gravity tugged at its ringed
colleague and forced it into an ever more elliptical and tilted orbit.

The migration had a knock-on effect for Saturn's outer neighbour,
Neptune, flinging it beyond Uranus into the outer Solar System where it
now resides. Neptune's headlong crash into the rubble that had
accumulated in the system's outer reaches sent thousands of these
planetesimals spinning towards the Sun.

Some were trapped around Jupiter, falling into line with its orbit to
form the trail of Trojans. And the rest battered the inner planets and
their satellites, including the Moon, leaving its face scarred by impact
craters. "This is the first fully self-consistent model of the late
heavy bombardment," says Levison.

Hot topic

The theory has had a warm reception at recent presentations to
astronomers, says Levison. But some remain wary. "The fact that a
simulation of planet formation produces an end-state in good agreement
with the observed Solar System does not prove that the simulated events
actually happened," says Joe Hahn, an astronomer at St Mary's University
in Halifax, Canada, who also studies the Solar System's evolution.

Hahn is most sceptical about the explanation for the late heavy
bombardment. Planetesimals from the outer Solar System should have been
scattered towards the Moon very soon after the planets began their
wanderings, rather than taking 700 million years to get there.

Levison argues that they were slowed down by the gas and dust left over
from planet formation. But Hahn is doubtful that they could have been
delayed for so long. "These models are all a bunch of fairy tales
anyway," he says. "I won't be convinced by any of them until astronomers
can observe planet formation around other stars."

Further support for Levison's theory could come from the surface of
Jupiter's Trojans, concedes Hahn. If they came from the outskirts of the
Solar System, they should look like the icy Kuiper belt objects found
there today.

Levison himself thinks that establishing the orbits of Kuiper belt
objects should help to refine his theory, and plans to spend the next
couple of years checking their effects on his model.

References

   1. Tsiganis, K., Gomes, R., Morbidelli, A. & Levison, H. F. Nature
      435, 459-461 (2005).
   2. Morbidelli, A., Levison, H. F., Tsiganis, K. & Gomes, R. Nature
      435, 462-465 (2005).
   3. Gomes, R., Levison, H. F., Tsiganis, K. & Morbidelli, A. Nature
      435, 462-465 (2005).
Received on Wed 25 May 2005 02:04:14 PM PDT