Late Heavy Bombardment - Geological Consequences On Earth

Geological Consequences On Earth

If a lunar cataclysmic cratering event were truly to have occurred on the Moon, the Earth would have been affected as well. Extrapolating lunar cratering rates to Earth at this time suggest that the following number of craters would have formed:

• 22,000 or more impact craters with diameters >20 km (12 mi),
• about 40 impact basins with diameters about 1,000 km (620 mi),
• several impact basins with diameter about 5,000 km (3,100 mi),

Serious environmental damage would occur about every 100 years, although life is not known to have existed on Earth at this time.

Prior to the introduction of the Late Heavy Bombardment theory, it was generally assumed that the Earth had remained molten until about 3.8 Ga. This date could be found in all of the oldest known rocks from around the world, and appeared to represent a strong "cutoff point" beyond which older rocks could not be found. These dates remained fairly constant even across various dating methods, including the system considered the most accurate and least affected by environment, uranium-lead dating of zircons. As no older rocks could be found, it was generally assumed that the Earth had remained molten until this point in time, which defined the boundary between the earlier Hadean and later Archean eons.

Older rocks could be found, however, in the form of chips off asteroids that fall to Earth as meteorites. 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. The Hadean, then, was the period of time between the formation of these early rocks in space, and the eventual solidification of the Earth's crust, some 700 million years later. This time would include the accretion of the planets from the disk and its slow cooling into a solid as the gravitational potential energy of this collapse was released.

Later calculations showed that the rate of collapse and cooling was dependent on the size of the body, and applying this to an Earth-sized mass suggested this should have happened quite quickly, as quickly as 100 million years. The difference between measurement and theory was something of a mystery at the time.

The Late Heavy Bombardment is now offered as an explanation of this oddity. Under this model, the rocks dating to 3.8 Ga represent those that were solidifying after much of the crust was destroyed by the Bombardment. The Acasta Gneiss in the North American cratonic shield and gneisses within the Jack Hills portion of the Narryer Gneiss Terrane in Western Australia are, collectively, the oldest continental fragments on Earth and do not predate the late heavy bombardment. The oldest mineral yet dated on Earth, a zircon from Jack Hills, predates this event but may simply be a fragment of crust left over from this event, contained within a much younger (~3800 Ma old) rock.

This has led to something of a revolution in the understanding of the earliest stages of Earth's history during the Hadean. Older references generally show the Hadean Earth having a molten surface with prominent volcanos. The name referred to the "hellish" conditions assumed on Earth for the time. It is now theorized (although still controversial) that the Hadean surface may have been solid, temperate, and water covered (albeit acidic). This is due to the presence of several particular isotopic ratios which suggest water-based chemistry took place at some point prior to the formation of the oldest rocks.

Of particular interest, Manfred Schidlowski argued in 1979 that the carbon isotopic ratios of some sedimentary rocks found in Greenland were a relic of organic matter. There was much debate over the precise dating of the rocks, with Schidlowski suggesting they were about 3800 Ma old, and others suggesting a more "modest" 3600 Ma. In either case it was a very short time for abiogenesis to have taken place, and if Schidlowski was correct, arguably too short a time. The Late Heavy Bombardment and the "re-melting" of the crust that it suggests provides a timeline under which this would be possible; life either formed immediately after the Late Heavy Bombardment, or more likely survived it, having arisen earlier during the Hadean. Recent studies suggest that the rocks Schidlowski found are indeed from the older end of the possible age range at about 3850 Ma, suggesting the latter possibility is the most likely answer.

More recently, a similar study of Jack Hills rocks shows traces of the same sort of potential organic indicators. Thorsten Geisler of the Institute for Mineralogy at the University of Münster studied traces of carbon trapped in small pieces of diamond and graphite within zircons dating to 4250 Ma. The ratio of carbon-12 to carbon-13 was unusually high, normally a sign of "processing" by life.

Three-dimensional computer models developed in May 2009 by a team at the University of Colorado at Boulder postulate that much of Earth's crust, and the microbes living in it, could have survived the bombardment. Their models suggest that although the surface of the Earth would have been sterilized, hydrothermal vents below the Earth's surface could have incubated life by providing a sanctuary for heat-loving microbes.