ORIGIN OF THE

LATE HEAVY BOMBARDMENT

OF THE

TERRESTRIAL PLANETS

Morby@obs-nice.fr

Bla bla

BlaTHE LATE HEAVY

BOMBARDMENT

THE MOON

The Moon shows that the bombardment was much heavier in the past, until late after the Moon formation, than at the current time.

Studies of crater densities at sites of known ages (from Apollo samples) give flux data back to ~3.8 Gy ago, and show that the bombardment was ~100 times higher

Problem: What was the evolution of the bombardment before 3.8 Gy ago?

Cataclysmic LHB (Tera, Ryder, Kring, Cohen, Koeberl..)

Slowly fading LHB (Neukum, Hartman..)

?

Evidence for a cataclysm ~4.0-3.8 Gy ago:

The ages of the rocks collected on the Moon cluster at ~3.9-3.8 Gy, and rocks older than 4 Gy are extremely rare.

Suggests a disastrous sudden and short-lived cratering episode about 3.9 Gy ago, which distroyed all primoridal rocks, resetting their ages (Tera et al., 1974)

Counter-argument:

A very heavy, time declining, bombardment, could produce the same effect (Hartung, 1974; Hartmann, 1975, 1980, Grinspoon, 1989)

Evidence for a cataclysm ~4.0-3.8 Gy ago:The ages of many basins (impact features > 200km) cluster in the 3.9-3.8 Gy period (Wilhelms, 1987; Ryder, 1994)

Counter-argument:

Basins datations are fooled because collected samples are dominated by Imbrium ejecta (Haskin, 1998). Only Imbrium is dated.

Evidence for a cataclysm ~4.0-3.8 Gy ago:

The amount of siderophile elements on the ancient highlands suggest that the amount of interplanetary mass accumulated by the Moon in the 4.4-3.9 Gy period is about the same of that required to form the basins in the 3.9-3.8 Gy period (5 1021g), 20 times less than suggested by models with a declining bombardment from the time of formation

Counter-argument:

It critically depends on the assumed composition of the early impactors. Was it the same as that of the current meteorites?

•Causing a cataclysmic LHB requires that a reservoir of small bodies, which have remained stable for ~700 My, suddenly goes `nuts’

•This is possible `only’ if there is a change in the orbital structure of the planetary system

•Planetesimal driven migration provides such a change

Planetesimal driven migration of giant planets sends the required mass to the terrestrial planets to cause the (L)HB but:

In standards simulations it starts and ends too early

WE NEED TO DELAY PLANET MIGRATION

This is possible, at two conditions

I) the planetesimal disk at the started outside of the last planet

…is this plausible ?

Yes, we do expect planetesimals only where their dynamical lifetime is not shorter than the gas disk lifetime

Planet positions

Lifetime of planetesimals

II) the planetary eccentricities were almost zero

Otherwise the planetary system is unstable and `spreads out’

Stability map for the giant planets: Jupiter and Saturn with current eccentricities

Neptune a

Uranus a

18 24

Stability map for the giant planets: Jupiter and Saturn with zero eccentricities

Neptune a

Uranus a

14 18

Having planets on nearly circular, coplanar orbits is consistent with what we know of planet formation

But, how did the planets (J & S in particular) acquire their current eccentricities?

J & S could acquire their eccentricity passing through the 1:2 MMR (Tsiganis, Gomes, Morbidelli, Levison, Nature, in press)

Final e

Among all resonances, only the 1:2 produces the required eccentricities

The final result is a system of planets at the right place with the right eccentricities…..

(Tsiganis et al, in press)

The acquisition of the eccentricity by Jupiter and Saturn destabilizes the planetary system and the disk

End states statistics (Tsiganis et al., in press)

The delay: origin of the LHB (Gomes, Tsiganis, Morbidelli, Levison, Nature, submitted)

Origin of the LHB (Gomes, Tsiganis, Morbidelli, Levison, submitted)

1:2 resonance crossing as a function of disk inner edge

1:2 resonance crossing

What happened to the asteroid belt during the LHB?

Models show that the asteroid belt was excited/depleted in a few My after Jupiter formation (Petit et al., 1999)

Consistent with the survival of Vesta’s 60 km-thick basaltic crust after 4.5 Gy of collisional evolution, which provides a key constraint on the amount of impact processing that has occurred (Davis et al., 1985, Icarus 62, 30-35)

The very massive phase could last only ~10 My

During Jupiter-Saturn migration at the time of the LHB, the secular resonances nu6 and nu16 should have swept through the asteroid belt.

If the asteroid belt was pre-excited, about ~10% of the asteroids should have survived such sweeping.

This argues for an asteroid belt ~ 5x10-3 ME

prior to the LHB, contributing ~1022g of impact on the Moon. Large uncertainty!

Asteroidal contribution to the LHB: consistent with recent findings by Cohen and Kring and by Tagle

Comet contribution (3-20My)

Asteroid contribution (10-30/ 50-150My)

The passage of Jupiter and Saturn through their mutual 1:2 MMR explains the current orbital architecture of the giant planets system.

But, is there an independent evidence that this really happened?

Yes, the Trojan asteroids of Jupiter

The Trojan region becomes fully unstble when Jupiter and Saturn cross the 1:2 resonance.

Thus, `primordial Trojans’ would leave, but new Trojans can be captured from the planetesimal disk that is driving the migration

The captured Trojans have a final orbital distribution that is remarkably similar to that of the observed Trojans.

So far, this is the only model of Trojans’ origin that explains so well the observed distribution

Morbidelli, Levison, Tsiganis, Gomes, Nature in press

Given the mass of the planetesimal disk required to move the planets over the instability range, our simulations predict the capture of 4x10-6 – 3x10-5 M, depending on migration speed.

Jewitt et al. (2000) estimated the mass of the Trojans to be almost 10-4 M

Problem?

Jewitt et al., 2000

Observed distribution

Jewitt et al. observations determined the slope of the CLF, but calibration was a problem.

We think that it is more reasonable to `branch’ Jewitt et al. Slope on the observed distribution at H~11

Indeed more recent SDSS observations suggest that Trojans are complete up to H~11

Trojans CLF

Using the new H-distribution and updated estimates for

1) Bulk density (1.3 g/cm3 for Patroclos)

2) Albedo (0.056, Fernandez et al., 2003)

we estimate that the Trojans mass is ~1.1x10-5 M

(capture model predicts 4x10-6 – 3x10-5 M)

If our model is correct, Trojans are cometary nuclei (they came from the same population that generated the scattered disk and the Oort cloud), which became extinct (on average they spent >10,000y at q<3 AU before capture)

CONCLUSIONS

We have developped a model, based on planetesimal driven migration that:

1) Reproduces the current orbital architecture of the 4 giant planets

2) Explains the LHB: cataclysmic nature, mass delivered to the Moon, duration

3) Is more or less consistent with what we see in the Kuiper belt (caveat with Levison-Morbidelli push-out scenario)

4) Explains the origin of Jupiter Trojans: mass and orbital distribution

Relevance for extra-solar worlds

IMPLICATIONS ON THE GAS-DISK PHASE

If all this is right, then at the end of the gas disk phase:

1) The system of the 4 giants was very compact (5.5-17 AUs)

2) Jupiter and Saturn were not in the 1:2 MMR; the ratio of their orbital periods was smaller than 2

3) Orbital eccentricities and inclinations were very small

4) A massive disk of planetesimals (35 ME) extended from a few AUs beyond the 4th planet to 30-35 AU.

5) Does this argue that gas-driven migration was never substantial?

IMPLICATIONS ON THE PRIMORDIAL SCULPTING OF THE SMALL BODY RESERVOIRSIf all this is right, then:

1) The Kuiper belt & SD were `born’ late

2) The asteroid belt was re-shuffled at the time of the LHB and lost ~90% (again) of its mass. Consistent with the absence of families at LHB time.

3) The Oort cloud formed in two stages. The first stage was very early. Formed by planetesimals in the planets region. Ejected in a gaseous, stellar dense environment. The second stage was 700 My late, formed by trans-Neptunian planetesimals.

4) Regular satellites of giant planets are primordial, but irregular satellites had to be captured at the LHB time