Late Heavy Bombardment
Apollo 15, 16, and 17 landing sites were chosen for their proximity to the Imbrium, Nectaris, and Serenitatis basins. Astronauts collected impact melt rocks from these locations during missions in the early 1970s. Radiometric dating of these samples revealed a clustering of ages between about 3.8 and 4.1 billion years ago. This narrow interval suggested that a large proportion of lunar craters formed during this specific period. Scientists named this phenomenon the lunar cataclysm. The hypothesis proposed a dramatic increase in bombardment rates around 3.9 billion years ago. If these melts derived from those three basins, then many other basins must have formed simultaneously based on stratigraphic evidence. At the time, this idea was considered highly controversial. Later data from lunar meteorites supported the general trend but showed ages spanning from 2.5 to 3.9 billion years without clustering at 3.9 billion years. These meteorites likely originated from regions far from the Apollo landing sites, including the lunar far side. Studies of HED meteorites from the asteroid belt revealed numerous ages from 3.4 to 4.1 billion years. Computer simulations using hydrocode models show that impact volume increases 100 to 1,000 times when velocity rises from 5 km/s to 10 km/s.
The cluster of impact melt ages near 3.9 billion years could be an artifact of sampling ejecta from a single basin. A significant portion of samples collected at Apollo sites might derive from the Imbrium basin rather than multiple distinct events. Quantitative modeling indicates that substantial amounts of ejecta from the Imbrium event should exist at all Apollo landing locations. This alternative hypothesis suggests the age spike reflects material from one large impact instead of several. Another criticism concerns the lack of impact melt rocks older than about 4.1 billion years. One explanation posits that old melt rocks existed but their radiometric ages were reset by continuous cratering over the past 4 billion years. It is also possible these putative samples were pulverized into sizes too small for standard radiometric methods. Scientists continue studying the Moon's bombardment history to clarify inner solar system dynamics. Some argue the age spike identified in 40Ar/39Ar dating could result from episodic early crust formation followed by partial argon losses as impact rates declined. These critiques challenge the assumption that the lunar cataclysm was a real, singular event.
Extrapolating lunar cratering rates to Earth suggests thousands of craters formed during this period. Before the LHB hypothesis, geologists assumed Earth remained molten until about 3.8 billion years ago. This date appeared as a strong cutoff point beyond which no older rocks could be found. Uranium-lead dating of zircons confirmed this boundary across various methods. In 1999, the oldest known rock on Earth dated to 4.031 ± 0.003 billion years old within the Acasta Gneiss of northwestern Canada. Older rocks exist as asteroid fragments falling to Earth as meteorites. Asteroids show a strong cutoff at about 4.6 billion years when first solids formed around the Sun. The Hadean eon represents the time between these early space rocks and Earth's solidification roughly 700 million years later. Later calculations showed cooling depends on rocky body size. Scaling to Earth mass suggested rapid cooling requiring only 100 million years. Under the LHB model, rocks dating to 3.8 billion years solidified after much crust destruction. The Jack Hills zircon from Western Australia predates this event but is likely a fragment contained in younger rock. Manfred Schidlowski argued in 1979 that carbon isotopic ratios in Greenland sedimentary rocks indicated organic matter processing. These rocks were dated to about 3.85 billion years by a 2002 study. Three-dimensional computer models developed in May 2009 suggest hydrothermal vents below Earth's surface could have incubated life during bombardment.
The Nice model attributes the Late Heavy Bombardment to dynamical instability in the outer Solar System. Initial simulations began with giant planets in tight orbits surrounded by a rich trans-Neptunian belt. Objects from this belt strayed into planet-crossing orbits causing planetary migration over several hundred million years. Jupiter and Saturn drifted apart until crossing a 2:1 orbital resonance. This increased their orbital eccentricities and destabilized Uranus and Neptune onto wider orbits. Scattered objects disrupted the outer belt causing comet bombardment as they entered planet-crossing paths. Interactions between objects and planets drove faster migration of Jupiter and Saturn. Recent modifications show giant planets beginning in multi-resonant configurations due to early gas-driven migration. Encounters between an ice giant and Saturn propelled the ice giant onto a Jupiter-crossing orbit. This jumping-Jupiter scenario quickly increased separation limiting resonance sweeping effects on asteroids. While required to preserve low terrestrial planet eccentricities, it reduces asteroid removal fractions. The ice giant is often ejected following its encounter with Jupiter. Some propose the Solar System began with five giant planets. Recent works found impacts from the inner asteroid belt insufficient to explain ancient impact spherule beds. The asteroid belt was probably not the source of the Late Heavy Bombardment.
Harold Levison and his team suggested Uranus and Neptune formed very slowly over several billion years. Low material density in the outer Solar System during formation greatly slowed accretion. However recent calculations imply Jovian planets formed extremely rapidly within 10 million years. The Planet V hypothesis posits a fifth terrestrial planet caused the bombardment when its unstable orbit intersected the inner asteroid belt. This hypothetical planet had mass less than half of Mars and orbited between Mars and the asteroid belt. Perturbations from other inner planets made its orbit unstable causing many asteroids to enter Earth-crossing orbits. Numerical simulations showed uneven asteroid distribution concentrated toward the inner belt was necessary for this mechanism. An alternate version suggests lunar impactors were debris from Planet V impacting Mars forming the Borealis Basin. Matija Cuk proposed the last few basin-forming impacts resulted from collisional disruption of a large Mars-crossing asteroid. This Vesta-sized remnant initially belonged to a much larger population. A catastrophic impact disrupted this asteroid roughly 3.9 billion years ago increasing Mars-crossing object populations. Many evolved onto Earth-crossing orbits producing a spike in lunar impact rates. Weak or absent residual magnetism in the last few basins supports this hypothesis. Other potential sources include additional Earth satellites, planetesimals left from terrestrial planet formation, and breakup of main belt asteroids.
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Common questions
What is the Late Heavy Bombardment and when did it occur?
The Late Heavy Bombardment refers to a hypothesized astronomical event where impact rates increased dramatically around 3.9 billion years ago. Radiometric dating of lunar samples collected during Apollo missions in the early 1970s revealed clustering between about 3.8 and 4.1 billion years ago.
How does the Nice model explain the cause of the Late Heavy Bombardment?
The Nice model attributes the Late Heavy Bombardment to dynamical instability in the outer Solar System involving giant planets. Jupiter and Saturn drifted apart until crossing a 2:1 orbital resonance which destabilized Uranus and Neptune onto wider orbits causing comet bombardment.
When was the oldest known rock on Earth dated to 4.031 ± 0.003 billion years old found?
In 1999, the oldest known rock on Earth dated to 4.031 ± 0.003 billion years old was discovered within the Acasta Gneiss of northwestern Canada. This finding confirmed a boundary across various methods before the LHB hypothesis suggested thousands of craters formed later.
What is the Planet V hypothesis regarding the source of the Late Heavy Bombardment?
The Planet V hypothesis posits that a fifth terrestrial planet with mass less than half of Mars caused the bombardment when its unstable orbit intersected the inner asteroid belt. Perturbations from other inner planets made this hypothetical planet's orbit unstable causing many asteroids to enter Earth-crossing orbits.
Why do some scientists argue the age spike near 3.9 billion years could be an artifact?
Some scientists argue the cluster of impact melt ages near 3.9 billion years could be an artifact of sampling ejecta from a single basin like Imbrium rather than multiple distinct events. Quantitative modeling indicates substantial amounts of ejecta from the Imbrium event should exist at all Apollo landing locations.