[meteorite-list] Asteroids Tell Tale of Jupiter Migration

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Tue Nov 2 18:43:38 2004
Message-ID: <200411022314.PAA27997_at_zagami.jpl.nasa.gov>


Asteroids Tell Tale of Jupiter Migration
By Robert Naeye
Sky & Telescope
November 1, 2004

In 1984 astrophysicists Julio A. Fernandez and
Wing-Huen Ip wrote a seminal paper arguing that the outer planets of the
solar system migrated from where they formed to where they are today.

To understand how this could occur, consider gravity assists, such as
the Cassini spacecraft's December 30, 2000, flyby of Jupiter. Like a
table-tennis ball hitting a rotating ceiling fan and speeding up,
Cassini was flung forward an extra 2 kilometers per second by Jupiter's
gravitational field. This speeded up the spacecraft's Saturn arrival by
several months. But tiny Cassini pulled on mighty Jupiter too,
decreasing the planet's orbital momentum by a paltry 1 meter per 6
trillion years. The planet drifted inward by an amount so tiny it would
be impossible to measure with a microscope.

That might not sound like much, but in the early days of the solar
system, the outer planets experienced trillions of close encounters with
small, icy planetesimals. While any one encounter had a negligible
effect on a planet, they added up. Fernandez and Ip showed that Jupiter
must have launched more bodies outward than inward. Some of the
outward-bound objects ended up in the Oort Cloud; others were thrown
into interstellar space. In response, Jupiter must have migrated inward
by perhaps 0.2 astronomical unit (20 percent the average Earth-Sun
distance) over the course of about 100,000 years.

Now Fred A. Franklin and a team at the Harvard-Smithsonian Center for
Astrophysics have found present-day evidence for a migrating Jupiter
long ago. In computer simulations, Franklin and his colleagues
discovered that an inwardly migrating Jupiter reproduces the observed
orbital characteristics of the 700 known Hilda asteroids. These bodies,
which belong to a distinct family with similar orbits and compositions,
are trapped in a 3:2 mean-motion resonance with Jupiter - meaning that
over the long run they go around the Sun exactly three times every time
Jupiter goes around twice. In addition, almost all Hildas have modest
orbital eccentricities between 0.1 and 0.25.

Hilda asteroids

White dots represent the positions of roughly 700 known Hilda-family
asteroids at a particular moment in time. The yellow ellipses show
orbits of three of them. The eccentricities and other characteristics of
Hilda asteroids offer evidence that Jupiter drifted sunward during the
early days of the solar system. S&T diagram by Gregg Dinderman. Source:
Petr Scheirich.

The team's computer simulations show that an inwardly migrating Jupiter
would have shepherded large numbers of asteroids into 3:2 resonant
orbits. These objects would have remained stuck in the relationship even
as Jupiter spiraled inward. The distribution of Hilda orbits requires a
long Jovian migration time scale of at least 100,000 years. Moreover,
Jupiter must have migrated inward by at least 0.35 a.u., and more likely
0.45 a.u., for the process to work. Jupiter currently resides at an
average distance of 5.2 a.u. (484 million miles, or 779 million
kilometers) from the Sun.

This relatively large migration caused low-eccentricity would-be Hildas
to be ejected, which explains the virtual absence of any family members
with eccentricities less than 0.1. Such a migration also successfully
replicates the observed distribution of the Hildas that lie inside the
3:2 resonance itself. The group's work appears in the September 2004
Astronomical Journal.

"Up to now, studies of Jovian migration have relied on computer
simulations of what might have happened in the very early days of the
solar system," says Franklin. "The current work is different in an
important regard: it is observationally based. We are seeking an
explanation of a current solar-system phenomenon that has no other known

Comments dynamicist Renu Malhotra (University of Arizona), "The paper is
significant in that it adds to the support for Jupiter's inward
migration and provides one of the few quantitative constraints on the
magnitude of that migration."
Received on Tue 02 Nov 2004 06:09:25 PM PST

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