Depletion and excitation of the asteroid belt by migrating planets

1. 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)

2. Motivation: a solution to the Mars problem? Mars analogs are bad • Problem: Mars analogs are 5-10x larger than Mars in standard simulations. Raymond et al. 2009

3. A solution to the Mars problem? Mars analogs are great • Problem: Mars analogs are 5-10x larger than Mars in standard simulations. • Solution: Hansen (2009) solved this problem with ad-hoc initial conditions, a narrow annulus of material between 0.7—1.0 AU. Hansen 2009 • Question: Is there a mechanism to create these initial conditions?

4. Migration of Jupiter and Saturn in a gas-disk • 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 3:2 res Jupiter Masset and Snellgrove, 2001, Morbidelli and Crida, 2007; Pierens and Nelson, 2008

5. 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. ? Semimajor axis • They migrate out together as the gas-disk dissipates.

8. 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

9. 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. C-type S-type scattered S-types ? ? Semimajor axis • They migrate out together as the gas-disk dissipates.

13. X,Y movie

15. Repopulating the Asteroid Belt Semimajor axis (AU) • 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 .

16. Gradie and Tedesco 1982 Asteroids • 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 = 0-0.3 • i=0-25°

17. 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.

18. 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?

19. 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

20. Gradie and Tedesco 1982 Asteroids • 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 = 0-0.3 • i=0-25°

21. 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,

22. Eccentricity Current-Day Asteroid belt H<10.8 Average e = 0.15 Asteroids post-Grand Tack Average e = 0.2

23. 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.

24. The post-Grand Tack distribution is similar post-Grand Tack

25. 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”

26. Smooth Migration τ=1e5 yr