Transcript Building the Terrestrial Planets: Constraining Accretion

Open problems in terrestrial planet
formation
Sean Raymond
Laboratoire d’Astrophysique de Bordeaux
…with audience contributions welcome!
How did the Solar System form?
• Simulations can roughly reproduce the
masses and orbits of Earth and Venus
(O’Brien et al 2006; Kenyon & Bromley 2006; Chambers 2001; Agnor et al 1999; Raymond et al 2006)
• Biggest problem: Mars’ small size
• Accretion process strongly dependent
on giant planets
(Wetherill 1991)
(Levison & Agnor 2003; Raymond et al 2004)
• Goal: Reproduce inner solar system
– Constrain Jup, Sat’s orbits at early times
– Test relevant physics
Constraints
– Mars’ small mass is a mystery (Wetherill 1991, Chambers 2001)
– Very low eccentricities (O’Brien et al 2006)
• Structure of asteroid belt
– Separation of S, C types
– No evidence for remnant embryos (gaps)
• Accretion timescales from Hf/W, Sm/Nd
– Earth/Moon: 50-150 Myr (Jacobsen 2005; Touboul et al 2007)
– Mars: 1-10 Myr (Nimmo & Kleine 2007)
• Water delivery to Earth
– Asteroidal source explains D/H (Morbidelli et al 2000)
– Other models exist (Ikoma & Genda 2007; Muralidharan et al 2008)
Stronger Constraints
• Masses, orbits of terrestrial planets
Gas
giants
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Late-stage
accretion
Runaway
gas accretion
Earthsized
planets
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Runaway
growth
Cores
Embryos
Planete
-simals
(~km)
dust
sticking
Oligarchic
growth
Grav. collapse
(cm - m)
Dust
(µm)
104-5 yrs
105-7 yrs
107-8 yrs
Initial conditions for late-stage accretion
Planetary embryos (aka
protoplanets) form by runaway
and oligarchic growth: ~MoonMars sized (~105-6 yrs) (Kokubo & Ida
1998, Leinhardt & Richardson 2005)
•
Late-stage accretion starts when
local mass in embryos and
planetesimals is comparable
Eccentricity
•
(Kenyon & Bromley 2006)
(Giant planets must form in few
Myr, so they affect late stages)
Semimajor Axis (AU)
Kokubo & Ida 2002
Key factors for accretion
1. Giant Planets
(Levison & Agnor 2003)
– Formation models predict low eccentricity
– Nice model: Jup, Sat closer than 2:1 MMR
during accretion (Tsiganis et al 2005; Gomes et al 2005)
• Perhaps in chain of resonances (Morbidelli et al 2007)
2. Disk Properties
(Wetherill 1996, Raymond et al 2005)
– Total mass ~ 5 Earth masses inside 4 AU
(Weidenschilling 1977; Hayashi 1981)
– ∑ ~ r-1.5 (MMSN) or perhaps more complex
(Jin et al 2008; Desch 2007)
Nice model 2 (J, S in 3:2 MMR)
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Nice model 2 (J, S in 3:2 MMR)
• No Mars analogs
• Embryos in asteroid
belt
– Inconsistent with
observed structure if
embryo Mars-mass
or larger
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Nice model 2 (J, S in 3:2 MMR)
• No Mars analogs
• Embryos in asteroid
belt
– Inconsistent with
observed structure if
embryo Mars-mass
or larger
Eccentric Jup, Sat (e0=0.1)
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Eccentric Jup, Sat (e0~0.1)
• Strong secular
resonance (6) at 2.2 AU
• Mars consistently forms
in correct configuration
• Earth and Venus are dry
Inconsistent with Kuiper
Belt structure
–no migration of giant
planets possible (Malhotra 1995,
Levison & Morbidelli 2003)
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Influence of giant planets
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Raymond, O’Brien, Morbidelli, & Kaib 2009
Influence of giant planets
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Hard to form low-e, highly concentrated terrestrial
planet systems
Raymond, O’Brien, Morbidelli, & Kaib 2009
Mars
• Small Mars forms
naturally if inner disk is
truncated at 1-1.5 AU
(Agnor et al 1999; Hansen 2009)
• Can reproduce all 4
terrestrial planets if
embryos only existed
from 0.7-1 AU (Hansen 2009)
Hansen 2009
Other effects
• Gas disk effects:
– Type 1 migration (McNeil et
al 2005; Morishima et al 2010)
– Secular resonance
sweeping (Nagasawa et al 2005;
Thommes et al 2008)
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• Collisional
fragmentation (Alexander &
Agnor 1998; Kokubo, Genda)
Morishima et al 2010
Jin et al (2008) disk
• Assume MRI is
effective in inner, outer
disk but not in
between
• At boundary between
low, high viscosity, get
minimum in density
• Occurs at ~1.5 AU
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– Explanation for Mars’
small mass?
Jin et al (2008)
Summary
• No tested configuration of Jup, Sat reproduces
all constraints (Raymond et al 2009)
– Closest is eccentric Jup, Sat but Earth is dry and JS
not consistent with Kuiper Belt
• Including gas disk effects doesn’t solve the
problem (Morishima et al 2010)
• Hard to reproduce Mars’ small size
– Strong constraint on Jup, Sat’s orbits at early times
– Was there just a narrow annulus of embryos? (Hansen
2009)
• What’s missing?
– Secular resonance sweeping during disk dispersal
(Nagasawa et al 2005, Thommes et al 2008)
– Something else?
Recent progress
•
•
•
•
•
•
•
•
•
•
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Morishima et al 2008, 2010
Raymond, O’Brien, Morbidelli, Kaib 2009
Hansen 2009
Thommes, Nagasawa & Lin 2008
O’Brien, Morbidelli & Levison 2006
Raymond, Quinn & Lunine 2006
Kenyon & Bromley 2006
Nagasawa, Thommes & Lin 2005
Kominami & Ida 2002, 2004
Chambers 2001
Agnor, Canup & Levison 1999
Initial conditions
• Start of chaotic growth
phase (Wetherill 1985; Kenyon & Bromley 2006)
• Equal mass in 1000-2000
planetesimals and ~100
embryos (5 ME total)
– Embryos is Mars’ vicinity
are 0.1-0.4 Mars masses
• Integrate for 200 Myr +
with Mercury (Chambers 1999)
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Current JS
Eccentric JS
Nice model 1
Mars
Lowecc.

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Ast.
belt
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Nice 1
eccentric
Form.
time
Earth
Water

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
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
Nice model 2

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Jin disk
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Cases
• Current Jup, Sat
• Jup, Sat with e0~0.1
– e ~ current values after
accretion
• Nice Model 1: Jup
5.45 AU, Sat 8.12 AU,
e0=0
• Nice Model 2: Jup, Sat
in 3:2 MMR, low-e
• Disk: ∑~r-1 and r-1.5
– Little difference
• Disk from Jin et al
(2008)
– Dip in ∑ at ~1.5 AU
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