Depletion and excitation of the asteroid belt by migrating

planets

Kevin J. Walsh, Alessandro Morbidelli (SwRI,OCA-Nice)

Sean N. Raymond (Obs. Bordeaux), Dave P. O’Brien (PSI), Avi M. Mandell

(GSFC)

Motivation: a solution to the Mars problem?

• Problem: Mars analogs are 5-10x larger than Mars in standard simulations.

Raymond et al. 2009

Mars analogs are bad

A solution to the Mars problem? Mars analogs are great

Hansen 2009

• Solution: Hansen (2009) solved this problem with ad-hoc initial conditions, a narrow annulus of material between 0.7—1.0 AU.

• Question: Is there a mechanism to create these initial conditions?

• Problem: Mars analogs are 5-10x larger than Mars in standard simulations.

Migration of Jupiter and Saturn in a gas-disk

3:2 res

Masset and Snellgrove, 2001, Morbidelli and Crida, 2007; Pierens and Nelson, 2008

• For a wide range of possible gas-disk parameters Jupiter will open a gap and migrate inwards via type II migration

• Saturn migrates inwards, getting captured in resonance with Jupiter.

• Saturn in resonance with Jupiter can halt and reverse the inward migration of Jupiter.

Saturn

Jupiter

Jupiter’s migration - truncating the disk• Jupiter migrates

inward to ~1.5,

• Saturn migrates inward, getting captured in the 3:2 resonance with Jupiter, while increasing in mass,

• Saturn reaching near full mass halts their migration, and reverses it.

• They migrate out together as the gas-disk dissipates.

Semimajor axis

?

Problem? The Asteroid Belt

• Jupiter’s outward migration scatters bodies into the asteroid belt

• Thus, seeking to produce taxonomic distributions, we envision reservoirs of primitive bodies between and beyond the giant planets.

• The asteroid belt provides strict constraints in its taxonomic and orbital distribution.

Gradie and Tedesco 1982

Jupiter’s migration - truncating the disk• Jupiter migrates

inward to ~1.5,

• Saturn migrates inward, getting captured in the 3:2 resonance with Jupiter, while increasing in mass,

• Saturn reaching near full mass halts their migration, and reverses it.

• They migrate out together as the gas-disk dissipates.

S-type C-type

Semimajor axis??

scattered S-types

X,Y movie

Repopulating the Asteroid Belt

•The “S-type” bodies from the inner disk are scattered back roughly where they originated.

•This means that they largely repopulated the inner part of the asteroid belt region a<2.8 .

Semimajor axis (AU)

AsteroidsGradie and Tedesco 1982

• Bodies are implanted in the asteroid belt

• ~10-3 efficiency,

•~10x current asteroid belt mass for an initial MMSN,

• ~Taxonomic distributions largely recreated

• Orbital distribution matches pre-LHB expectations

e = 0-0.3i=0-25°

We

•We are not done. There is ~500 Myr until the LHB

•We have a component of high-e bodies that will accrete onto planets or could collide with each other.

Asteroid Belt implications• Separate parent populations

– 0.5-3.0 AU and ~6-13 AU – Requires diversity in both populations to explain

the significant observed diversity among asteroids.

– Suggests that our primitive asteroids may originate closer to comets than our more metamorphosed asteroids

• Pre-Depleted asteroid belt– The asteroid belt was depleted rapidly before the

gas-disk had fully dissipated.• Pre-Excited asteroid belt

– Asteroid belt gets its inclination distribution at this early time,

– Eccentricities will be re-shuffled later (LHB)• Chondrules/CAIs

– Need to be formed/transported to ~ 13 AU and beyond?

Conclusions

• Conclusions: Jupiter migrating to 1.5 AU can solve some outstanding problems– Small mass of Mars– Physical dichotomy

of the asteroid belt– Freedom for Jupiter

to form very near the Snow Line

• Implications:– Jupiter and Saturn

migrated significantly in the gas-disk: Jupiter reached 1.5 AU

– The asteroid belt was repopulated from two distinct parent populations

AsteroidsGradie and Tedesco 1982

• Bodies are implanted in the asteroid belt

• ~10-3 efficiency,

•~10x current asteroid belt mass for MMSN,

• ~Taxonomic distributions recreated

• Orbital distribution matches pre-LHB expectations

e = 0-0.3i=0-25°

Asteroid Distributions: e and i

• The Grand Tack is not the last event to alter the orbital distribution in the asteroid belt.– The orbital instabilities related to the LHB will

happen 500 Myr later.

• The sweeping of resonances across the asteroid belt when the giant planets migrate will– Deplete the population 2-5x,– Not change a distribution substantially,– Not change i distribution substantially,– Likely change the e distribution substantially,

Eccentricity

Asteroids post-Grand TackAverage e = 0.2

Current-Day Asteroid belt H<10.8Average e = 0.15

Minton & Malhotra did this for us!

This analytical work found a good match for a rapid, and smooth, sweeping of resonances in τ < 1 Myr.

The post-Grand Tack distribution is similarpost-Grand Tack

We don’t trust Minton, so we test this numerically….

• What is the parameter space for giant planet migration?– Differing smooth migration rates, exponential with τ < 0.5 Myr

• τ = 0.5 Myr – match Minton et al. 2009

• τ = 0.2, 0.1, 0.05 Myr as a proxy for even more rapid migrations (e.g. jumping Jupiter)

– “Jumping-Jupiter” migration, using the rapid and non-smooth evolution of the giant planets ->

“jumping-Jupiter” Morbidelli et al. 2010

“jump”

Smooth Migration τ=1e5 yr