Hot Steam, Hard Rain, and Icy Wastes in the Hadean

Details Hot Steam, Hard Rain, and Icy Wastes in the Hadean Hot Steam, Hard Rain, and Icy Wastes in the Hadean

Dec 2001

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001agufm.p21c..07z&link_type=abstract

American Geophysical Union, Fall Meeting 2001, abstract #P21C-07

Statistics

1630 Impact Phenomena, 5455 Origin And Evolution

Scientific paper

In the Moon one looks on the face of the Hadean. The basins and ranges of the Moon were carved by huge impacts, hundreds and thousands of times greater than the K/T impact. The youngest ( ~ 3.8-3.85 Ga) and best preserved of them are 1000-km wide multi-ringed records of the impacts of bodies 100 km across. At minimum the Moon suffered tens of such collisions before 3.8 Ga. The largest survivor is the far-side 2500 km diameter Shoemaker basin. Any more ancient record is long since battered into dust. Asteroids and comets also struck the Earth. While the Moon was struck by tens of basin-forming impactors, the Earth was struck by hundreds of similar objects, and by tens of objects much larger still. The largest would have been big enough to evaporate the oceans, and the ejecta massive enough to envelope the Earth in 100 m of rock rain. It is reasonably likely that such an event occurred on Earth between 3.8 and 3.9 Ga. In this sense the Hadean Earth was a more dangerous place, with only the best protected niches, e.g., the mid-ocean ridge hydrothermal vents or deep aquifers, being continuously habitable. Smaller impacts were also more frequent. Cratering rates were 102-10^3 times higher ca. 3.9 Ga than they are today. On average, a Chicxulub fell every 105 years. It is natural to assume that rocks from this time would be rich in platinum-group elements of extraterrestrial origin. However, this assumption is not necessarily a good one. Most of the mass striking the Earth was in the largest bodies to hit. This is so over every time scale >104 years. Thus on all scales we are confronted with the statistics of small numbers, since it is usually the single largest body to strike in any interval that has the greatest net effect. The same mass distribution also implies that most short time intervals are quiet. Most days were quiet days, most millenia were quiet millenia. Only on time scales longer than 104 years are global effects to be noticed, and only on scales of 105 years would one expect to see a boundary clay. Thus, although impacts dominated the Hadean in many ways, especially through the production of abundant highly damaged mafic and ultramafic impact ejecta that controlled atmospheric and oceanic chemistry and on ten million year scales may have dominated the Hadean rock cycle, they didn't control daily life. The Hadean was in principle perfectly habitable, and it is in all likelihood the Eon in which life became established on this Earth.

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