[meteorite-list] Craters on Vesta and Ceres Could Tell Jupiter's Age

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Tue, 15 Sep 2009 08:47:37 -0700 (PDT)
Message-ID: <200909151547.n8FFlb1C024982_at_zagami.jpl.nasa.gov>

http://www.spaceflightnow.com/news/n0909/14jupiterage/

Craters on Vesta and Ceres could tell Jupiter's age
EUROPEAN PLANETARY SCIENCE CONGRESS NEWS RELEASE
September 14, 2009

Crater patterns on Vesta and Ceres could help pinpoint when Jupiter
began to form during the evolution of the early Solar System. A study
modeling the cratering history of the largest two objects in the
asteroid belt, which are believed to be among the oldest in the Solar
System, indicates that the type and distribution of craters would show
marked changes at different stages of Jupiter's development. Results
will be presented by Dr. Diego Turrini at the European Planetary Science
Congress in Potsdam, Germany, on Monday 14 September.

The study, carried out by scientists at the Italian National Institute
for Astrophysics in Rome, explored the hypothesis that one or both
objects formed during Jupiter's formation by modeling their cratering
histories during the birth of the giant planet. Their simulation
described Jupiter's formation in three stages: an initial accretion of
its core followed by a stage of rapid gas accretion. This is, in turn,
followed by a phase where the gas accretion slows down while the giant
planet reaches its final mass. During the last two phases Jupiter's
gravitational pull starts to affect more and more distant objects. For
each of these phases, the team simulated how Jupiter affected the orbits
of asteroids and comets from the inner and outer Solar System, and the
likelihood of them being moved onto a collision path with Vesta or Ceres.

"We found that the stage of Jupiter's development made a big difference
on the speed of impacts and the origin of potential impactors. When
Jupiter's core approaches its critical mass, it causes a sharp increase
in low-velocity impacts from small, rocky bodies orbiting nearby to
Vesta and Ceres which lead to intense and uniform crater distribution
patterns. These low-speed collisions may have helped Vesta and Ceres
gather mass. Once Jupiter's core has formed and the planet starts to
rapidly accrete gas, it deflects more distant objects onto a collision
course with Ceres and Vesta and the impacts become more energetic.
Although rocky objects from the inner Solar System are the dominant
impactors at this stage, the higher energies of collisions with icy
bodies from the outer Solar System make the biggest mark," said Dr.
Turrini.

The third stage of Jupiter's formation is complicated by a period known
as the Late Heavy Bombardment, which occurred around 3.8 to 4.1 billion
years ago. During this time a significant number of objects, rich in
organic compounds, from the outer Solar System were injected on
planet-crossing orbits with the giant planets and may have reached the
Asteroid Belt. In addition, Jupiter is thought to have migrated in its
orbit around this time, which would have caused an addition flux of
impactors on Vesta and Ceres.

The team will have an opportunity to confirm their results when NASA's
Dawn space mission reaches Vesta in 2011 and then flies on for a further
rendezvous with Ceres in 2015. Dawn will gather information on the
structure and the surface morphology of the two asteroids and send back
high-resolution images of crater patterns.

"If we can see evidence of an underlying intense, uniform crater
pattern, it will support the theory that one or both of these minor
planets formed during the final phases of Jupiter accretion, provided
that they aren't obliterated by the later heavy bombardment. Dawn will
also measure concentrations of organic material, which may give us
further information about the collisional history with organic-rich
objects from the outer Solar System," said Dr. Turrini.
Received on Tue 15 Sep 2009 11:47:37 AM PDT