[meteorite-list] Small Mass of Mars Could be Due to Planetary Orbital Migration

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Sun, 5 Jun 2011 20:21:50 -0700 (PDT)
Message-ID: <201106060321.p563Lot4015555_at_zagami.jpl.nasa.gov>

Small Mass of Mars Could be Due to Planetary Orbital Migration
Planetary Science Institute
June 5, 2011

A long-ago inward migration by Jupiter during the formation of our Solar
System could explain why Mars is small in relation to Earth and Venus,
according to a paper published in Nature.

Researchers have long sought to explain the small mass of Mars, which
has remained an outstanding problem in terrestrial planet formation,
said David P. O'Brien, a Research Scientist at the Planetary Science
Institute and co-author of "A low mass for Mars from Jupiter's early
gas-driven migration" that appears in Nature.

"This work not only solves a difficult problem in Solar System
formation," O'Brien said, "it shows that the solution lies in the giant
planets of our Solar System undergoing significant early migration,
which was generally thought to only have occurred in extrasolar
planetary systems."

Simulations of the formation process of the four inner planets in the
Solar System - Mercury, Venus, Earth and Mars - generally produced a
version of Mars far more massive than the real planet.

"We tried a large variety of simulation parameters to solve this
problem, but nothing seemed to work," O'Brien said.

A 2009 paper by Brad Hansen from UCLA offered a new clue: Hansen showed
that if the initial distribution of solid material in the solar system
was assumed to have an outer boundary at 1 Astronomical Unit (1 AU being
the current distance from the sun to Earth), a smaller Mars could form.

The presence of a sharp outer boundary at 1 AU required in Hansen's
work was hard to explain, given the existence of the asteroid belt
between 2 and 4 AU, the giant planets between 5 and 30 AU and the Kuiper
Belt beyond that.

However, it has been shown in numerical simulations over the past
decade that Jupiter and Saturn could migrate in the early Solar System
when gas was still present, and in some cases could move inwards and
then back outwards to roughly their current locations.

"Rapidly the pieces of the story came together," said Kevin J. Walsh,
lead author of the paper who began work on the project at the
Observatoire de la Cote d'Azur in Nice, France and is now at the
Southwest Research Institute in Bounder, CO. "If Jupiter had moved
inwards from its birth place down to 1.5 AU from the sun and then had
turned around because of the formation of Saturn, eventually migrating
outwards towards its current location, it would have truncated the
distribution of solids in the inner Solar System at about 1 AU, as
required to explain the small mass of Mars."

Jupiter now orbits the sun at 5.2 AU.

"The problem was to understand whether the inward and outward migration
of Jupiter through the 2-4 AU region could be compatible with the
existence of the asteroid belt today," Walsh said. "So we started to do
a huge number of simulations."

"The asteroid belt, which was a priori our main problem, turned out to
be the main strength of our model," said O'Brien.

"The result was fantastic," Walsh said. "The simulations showed that
the migration of Jupiter was consistent with the existence of the
asteroid belt, but it also explained properties of the belt never
understood before."

The passage of Jupiter depleted then re-populated the asteroid belt
region, with inner-belt bodies originating between 1 and 3 AU and outer
belt bodies originating in a very distinct region between and beyond the
giant planets, naturally producing the significant compositional
differences existing today across the belt.

The model was called the "Grand Tack Scenario" with Jupiter's motion
similar to a sailboat tacking around a buoy.

Other authors are Alessandro Morbidelli (Observatoire de la Cote
d'Azur, France), Sean N. Raymond (Observatoire de Bordeaux, France) and
Avi M. Mandell (NASA Goddard).

O'Brien's work was funded by a grant to PSI from NASA's Planetary
Geology and Geophysics research program.

CONTACT:

David P. O'Brien
Research Scientist
520-547-3977
obrien at psi.edu
Received on Sun 05 Jun 2011 11:21:50 PM PDT