Late Heavy Bombardment
The Late Heavy Bombardment (LHB), or lunar cataclysm, is a hypothesized astronomical event thought to have occurred approximately 4.1 to 3.8 billion years (Ga) ago,[1] at a time corresponding to the Neohadean and Eoarchean eras on Earth.
According to the hypothesis, during this interval, a disproportionately large number of asteroids and comets collided into the terrestrial planets and their natural satellites in the inner Solar System, including Mercury, Venus, Earth (and the Moon) and Mars.
Isotopic dating showed that the rocks were last molten during impact events in a rather narrow interval of time, suggesting that a large proportion of craters were formed during this period.
[3] Other researchers doubt the heavy bombardment, arguing for example that the apparent clustering of lunar impact-melt ages is a statistical artifact produced by sampling rocks scattered from a single large impact.
Such objects are rare in the current asteroid belt but the population would be significantly increased by the sweeping of resonances due to giant planet migration.
[13] Studies of the highland crater size distributions suggest that the same family of projectiles struck Mercury and the Moon during the Late Heavy Bombardment.
[15] While the cataclysm hypothesis has recently become more popular (in the last fifty years), particularly among dynamicists who have identified possible causes for such a phenomenon, it is still controversial and based on debatable assumptions.
Additional criticism also argues that the age spike at 3.9 Ga identified in 40Ar/39Ar dating could also be produced by an episodic early crust formation followed by partial 40Ar losses as the impact rate declined.
As no older rocks could be found, it was generally assumed that Earth had remained molten until this date, which defined the boundary between the earlier Hadean and later Archean eons.
Nonetheless, in 1999, the oldest known rock on Earth was dated to be 4.031 ± 0.003 billion years old, and is part of the Acasta Gneiss of the Slave Craton in northwestern Canada.
Like the rocks on Earth, asteroids also show a strong cutoff point, at about 4.6 Ga, which is assumed to be the time when the first solids formed in the protoplanetary disk around the then-young Sun.
This picture derives from the presence of particular isotopic ratios that suggest the action of water-based chemistry at some time before the formation of the oldest rocks (see Cool early Earth).
The original Nice model simulations by Gomes et al. began with the Solar System's giant planets in a tight orbital configuration surrounded by a rich trans-Neptunian belt.
This jumping-Jupiter scenario quickly increases the separation of Jupiter and Saturn, limiting the effects of resonance sweeping on the asteroids and the terrestrial planets.
[40] According to one planetesimal simulation of the establishment of the planetary system, the outermost planets Uranus and Neptune formed very slowly, over a period of several billion years.
[41] Harold Levison and his team have also suggested that the relatively low density of material in the outer Solar System during planet formation would have greatly slowed their accretion.
calculations of gas-flows combined with planetesimal runaway growth in the outer Solar System imply that Jovian planets formed extremely rapidly, on the order of 10 My, which does not support this explanation for the LHB.
[44][45] A hypothesis proposed by Matija Ćuk posits that the last few basin-forming impacts were the result of the collisional disruption of a large Mars-crossing asteroid.
Then, roughly 3.9 billion years ago, a catastrophic impact disrupted the Vesta-sized asteroid, significantly increasing the population of Mars-crossing objects.
Ćuk points to the weak or absent residual magnetism of the last few basins and a change in the size–frequency distribution of craters which formed during this late bombardment as evidence supporting this hypothesis.
[53] Planetesimals left over from the formation of the terrestrial planets were shown to be depleted too rapidly due to collisions and ejections to form the last lunar basins.
[54] The long-term stability of primordial Earth or Venus co-orbitals (trojans or objects with horseshoe orbits) in conjunction with the lack of current observations indicate that they were unlikely to have been common enough to contribute to the LHB.
[55] Producing the LHB from the collisional disruption of a main belt asteroid was found to require at minimum a 1,000–1,500 km parent body with the most favorable initial conditions.
