Hadean Outline
From Palaeos
| Precambrian 4,567-542 | Phanerozoic 542-0 | ||
|---|---|---|---|
| Hadean 4,567-3,800 | Archean 3,800-2,500 | Proterozoic 2,500-542 | Phanerozoic 542-0 |
The Hadean Eon of Precambrian Time:
4567 to 3800 million years ago
| Up: Geological Timescale |
| Up: Geological Timescale | Next: Archean |
Hadean was the eon of massive solar system catastrophes. Hadean, coined after the greek God of underworld, was part a true hell, part mystical calm ages when planet Earth evolved in a way vaguely similar to the modern conditions. A combination of purely geological clues and solar system clues, make us believe that Hadean was subdivided into three distinct eras, which in typical astronomical fashion are three letter abbreviated: first the truly hellish EHB, then the calm and cool CEE, and lastly: the hot and wet LHB!
From a geological standpoint, the eon Hadean (after the greek god Hades) is everything before the first geological record. This was how the geologist Preston Cloud described Hadean, when he coined the term in the 1970:ies. The term catched, since it stimulated our fantasy and associated nicely to what everybody expect the formation of the entire planet Earth to be. However such a term is also apt to describe something shrinking before our eyes, so far as science is alive and kicking, so in order to save the vision, the discussion of Hadean turned direction from geology towards astronomy to catch missing clues. Everybody can see, without binoculars, that something gigantic have happened on the face of the Moon, |
Before the 1960:ies, the geology science was characterized by old time conflicts against fantastic creation and flooding theories based on Bible interpretations. From that standpoint geology developed the (mostly) sound philosophy of slow gradual changes, and applying such models of thought, geology established a proper understanding of geological strata. Except - the world of geology was just two things: very slow gradual creation of igneous and sedimentary rock - interspersed with fast but very local volcanic activity... and nothing else, no wandering continents, no wicked tough rock boulders from space, no nothing dramatic at all! Alfred Wegener's theory of wandering continents was rejected just for the lack of a plausible drive mechanism for the slow movements.
The static philosophy of geology slowly turned around since the discovery of plate tectonics, which part gave geology a more robust insight between tectonic forces and the strata, and part explained that things, over time, actually can change quite dramatically. Next the Moon landings falsified the volcanic theory of Moon craters - the impact theory explained most smaller features of the surface of Moon to be due to meteorite impacts. Next thing was the immense popular interest of the Luiz and Walter Alvarez impact theories about the extinction of the dinosaurs. This forced the geologists into debate with astronomers - and while the debate was hot, the geologists were well able to defend their positions from foreign hairy non-geologists! This allowed the geology/astronomy amalgamation necessary to be able to map the conditions about the eon of subject here: Hadean.
Contents |
Hadean outline
Now what happened? In very very short: first a huge supernova exploded as a dramatic prelude, then in fast succession our Sun (still what's called a protostar), a protoplanetary disk and a number of protoplanets emerged. Now everything became complicated, and things here and there collided in explosions of fire and magma, while the sun flared up hundredfold about a dozen times, ending with a set of, after 40 million years, eight planets. This was the era of EHB - the Early Heavy Bombardment era, and the hottest period, when most collisions occurred, is called the Accretion.
Next the solar system stabilized into a state pretty similar to what we know from recent. There were at least four inner planet, Mercury, Venus, Earth with the Moon and Mars. There were two, or much more probably, four outer giant planets, Jupiter, Saturn, and then probably Uranus and Neptune orbiting the Sun. During this time, the meteorite impact flux onto Earth was not very high, well it was about 10 times the impact frequency of today, but compared to earlier bombardment it was just cool, so that a solid crust could form, and cool liquid water flow in lakes and oceans. Thence the era name CEE, Cool Early Earth.
Now back to the face of the Moon: the Moon landings of the 1970:ies brought home lots of lunar rocks, which were radiometrically dated and otherwise thoroughly analysed. ...INSERTME...
Major Questions
The Time Line of Events
| Hadean eon 4,565-3,840 | ||||||
|---|---|---|---|---|---|---|
| EHB era 4,565-4,450 | CEE era 4,450-3,900 | LHB era 3,900-3,800 | ||||
| Accretion 4,565-4,555 | Protogean 4,555-4,530 | Lava Cryptic 4,530-4,450 | Wet Cryptic 4,450-4,150 | Basin Groups 4,150-3,900 | Nectarian 3,900-3,850 | Lower Imbrian 3,850-3,800 |
The "Formation" Period AKA EHB
The "Calm Period" AKA "Cool Early Earth (CEE)"
The "Late Heavy Bombarment (LHB) Era"
However: what we for absolute certain know about LHB is that it didn't affect the element composition of the atmosphere, and very little of its mass left Earth for space. It didn't contribute any important part of neither atmosphere nor ocean matter. It did increase the Earth temperature, but over any lengthy period just with an amount of 20-30 K not 200 K nor higher numbers, as was believed in the past.
Conclusions
External Links
- NASA planetary remote sensing tutorial, chapter 19.
- A Cool Early Earth, Geology & Geophysics Dept. of University of Wisconsin-Madison.
