Prebiotic world redirects here but there is as yet no subdivision of this Eon to
reflect recent developments The Hadean is a geologic eon of the
Earth, and lies before the Archean. It began with the formation of the Earth
about 4.6 billion years ago and ended, as defined by the ICS, 4 billion years
ago. The name "Hadean" comes from Hades, the ancient Greek god of the underworld,
in reference to the hellish conditions on Earth at the time: the planet had
just formed and was still very hot due to high volcanism, a partially molten
surface and frequent collisions with other Solar System bodies. The geologist
Preston Cloud coined the term in 1972, originally to label the period before
the earliest-known rocks on Earth. W. Brian Harland later coined an almost
synonymous term: the "Priscoan period". Other, older texts simply refer to the
eon as the Pre-Archean. Nonetheless, in 2015, "remains of biotic life" were
found in 4.1 billion-year-old rocks in Western Australia. According to one of
the researchers, "If life arose relatively quickly on Earth ... then it
could be common in the universe." Etymology
The Hadean was a violent period in the history of Earth. Hence, it receives its
name from Hades, the ancient Greek god of the underworld.
Subdivisions Since few geological traces of this eon
remain on Earth, there is no official subdivision. However, the Lunar geologic
timescale embraces several major divisions relating to the Hadean and so
these are sometimes used in a somewhat informal sense to refer to the same
periods of time on Earth. The Lunar divisions are:
Pre-Nectarian, from the formation of the Moon's crust up to about 3,920 million
years ago Nectarian ranging up to about 3,850
million years ago, in a time when the Late Heavy Bombardment, according to
that theory, was in a stage of decline. There is a recently proposed alternative
scale that includes the addition of the Chaotian and Prenephelean Eons preceding
the Hadean, and divides the Hadean into three eras with two periods each. The
Paleohadean era consists of the Hephaestean and the Jacobian periods.
The Mesohadean is divided into the Canadian and the Procrustean periods.
The Neohadean is divided into the Acastan and the Promethean periods.
Hadean rocks In the last decades of the 20th century
geologists identified a few Hadean rocks from Western Greenland, Northwestern
Canada, and Western Australia. The oldest dated zircon crystals,
enclosed in a metamorphosed sandstone conglomerate in the Jack Hills of the
Narryer Gneiss Terrane of Western Australia, date to 4.404 ± 0.008 Ga.
This zircon is a slight outlier, with the oldest consistently-dated zircon
falling closer to 4.35 Ga—around 200 million years after the hypothesized
time of the Earth's formation. Atmosphere and oceans
A sizeable quantity of water would have been in the material that formed the
Earth. Water molecules would have escaped Earth's gravity more easily when
it was less massive during its formation. Hydrogen and helium are
expected to continually escape due to atmospheric escape.
Part of the ancient planet is theorized to have been disrupted by the impact
that created the Moon, which should have caused melting of one or two large
areas. Present composition does not match complete melting and it is hard to
completely melt and mix huge rock masses. However, a fair fraction of
material should have been vaporized by this impact, creating a rock vapor
atmosphere around the young planet. The rock vapor would have condensed within
two thousand years, leaving behind hot volatiles which probably resulted in a
heavy CO 2 atmosphere with hydrogen and water
vapor. Liquid water oceans existed despite the surface temperature of 230
°C because of the atmospheric pressure of the heavy CO
2 atmosphere. As cooling continued, subduction and dissolving in ocean water
removed most CO 2 from the atmosphere but levels
oscillated wildly as new surface and mantle cycles appeared.
Study of zircons has found that liquid water must have existed as long ago as
4,400 million years ago, very soon after the formation of the Earth. This
requires the presence of an atmosphere. The Cool Early Earth theory covers a
range from about 4,400 to 4,000 million years ago.
A September 2008 study of zircons found that Australian Hadean rock holds
minerals that point to the existence of plate tectonics as early as 4,000
million years ago. If this is true, the time when Earth finished its transition
from having a hot, molten surface and atmosphere full of carbon dioxide, to
being very much like it is today, can be roughly dated to about 4.0 billion years
ago. The actions of plate tectonics and the oceans trap vast amounts of carbon
dioxide, thereby eliminating the greenhouse effect and leading to a much
cooler surface temperature and the formation of solid rock, and possibly
even life. See also
Chaotian Formation and evolution of the Solar
System History of the Earth – the first
sections describe the formation of the Earth
Oldest rock Timeline of natural history
Phanerozoic Eon References
Further reading Hopkins, Michelle; Harrison, T. Mark;
Manning, Craig E., "Low heat flow inferred from >4 Gyr zircons suggests
Hadean plate boundary interactions", Nature 456: 493–496,
Bibcode:2008Natur.456..493H, doi:10.1038/nature07465, PMID 19037314 .
Valley, John W.; Peck, William H.; King, Elizabeth M., "Zircons Are Forever", The
Outcrop for 1999, University of Wisconsin-Madison, retrieved January 10,
2006 – Evidence from detrital zircons for the existence of continental crust
and oceans on the Earth 4.4 Gyr ago. Wilde, S. A.; Valley, J. W.; Peck, W. H.
& Graham, C. M., "Evidence from detrital zircons for the existence of continental
crust and oceans on the Earth 4.4 Gyr ago", Nature 409: 175–178,
doi:10.1038/35051550, PMID 11196637 . Wyche, S.; Nelson, D. R. & Riganti, A.,
"4350–3130 Ma detrital zircons in the Southern Cross Granite–Greenstone
Terrane, Western Australia: implications for the early evolution of the Yilgarn
Craton", Australian Journal of Earth Sciences 51: 31–45,
doi:10.1046/j.1400-0952.2003.01042.x . Carley, Tamara L.; et al., "Iceland is
not a magmatic analog for the Hadean: Evidence from the zircon record", Earth
and Planetary Science Letters 405: 85–97, doi:10.1016/j.epsl.2014.08.015 .
External links Palaeos.org: Hadean eon
Peripatus.nz: Description of the Hadean Era
Astronoo.com: Hell of the Hadean