Lecture 3 – Planetary Migration, the Moon, and the Late Heavy

Bombardment

Abiol 574

Planetary migration

• Planets can migrate within the solar nebula for a variety of reasons

1. Gravitational interactions with the gas/dust disk (important for “hot Jupiters”)

2. Gravitational interactions with smaller planetesimals3. Near collisions between large planets

• Migration may have played a role in determining planetary orbits in the outer Solar System (Nice model)

• A key avenue of inquiry comes from the Moon..

• Many clues about the origin of the Earth may be gained by studying

the Moon. One of the big questions concerns the origin of its craters.

The big ones are called maria, which means “seas”.

1200 km

Imbrium

Serenitatis

Tranquillitatis

Tycho

Copernicus

Procellarum

MareOrientale

(as seen from NASA’s Lunar Orbiter 4, 1967)

• 930 km in diameter• Three separate rings• Requires a 100-km diameter impactor

Energeticsof largeimpacts

N. Sleep et al.,Nature (1989)

Chiron

Orientale

The Late Heavy Bombardment

• The Moon shows us that there was a flux of large (100-km diameter or more) objects in the inner Solar System, after the Moon’s formation

• Many (most) Moon rocks have age dates of around 3.8-3.9 Ga

• Was this a “pulse” of bombardment, or did this represent the tail end of a long period of heavy bombardment?

Other facts about the Moon(from Wood, 1986)

1. The Earth-Moon system has high angular momentum

2. The Moon is depleted in volatile elements and compounds (including moderately volatile elements like Zi, Bi, Pb

3. The Moon has only about 25% of Earth’s Fe and is depleted in other siderophile elements as well

4. The Moon has the same O isotope composition as Earth’s mantle

5. The Moon shows evidence of having a magma ocean at its surface early in its history

Moon-forming impact(painting by William Hartmann,

PSI)• All the evidence is

consistent with the Moon having been formed by a glancing impact from a Mars-sized planetesimal (0.1 Earth masses or larger)

There is a new theory of outer Solar System evolution – the “Nice model” -- that can explain the clustering of lunar age dates

Orbital evolution of the giant planets

• Simulated evolution of outer planets interacting with a ‘hot’ disk of planetesimals • Note that Uranus and Neptune flip positions as Jupiter and Saturn pass through the 2:1 mean motion resonance• Today, Saturn and Jupiter’s periods are near the ratio 5:2

Tsiganis et al., Nature, 2005

Sem

i-maj

or a

xis

(AU

)

Time (yrs)

Tsiganis et al. (2005), Fig. 2

• This model successfully explains the observed eccentricities and orbital inclinations of the giant planets

• The crossing event also sends swarms of comets (and probably asteroids as well) into the inner Solar System

ObservedSimulated

Question: Can one delay the crossing event long enough to explain the late heavy bombardment at ~3.8 Ga?

Question: Can one delay the crossing event long enough to explain the late heavy bombardment at ~3.8 Ga?

Answer: Yes, provided that one assumes the right distribution of icy planetesimals.– But this is not unreasonable…

Gomes et al., Nature, 2005, Fig. 3

• Calculated semi-major axes of the giant planets vs. time (perihelion and aphelion shown)

• Cumulative mass of comets and asteroids accreted by the Moon

J

S

N

U

• In the latest version of the Nice model, Jupiter and Saturn migrate inward, then back outward, early during the accretion period

• This removes most of the planetesimals beyond 1.5 AU, thereby explaining the low mass of Mars

Nature (2011)

Walsh et al. results• Large filled circles are the giant planets• Small unfilled circles at the bottom are the terrestrial planets from a different simulation• Red dots are S-type asteroids• Blue dots are C-type asteroids (water-rich)