Age of the earth (before 4.6by)

[KnightWhoSaysNi]

Here's something I'm a bit curious about. The scientific consensus is that the earth is roughly 4.6 billion years old. So what did the earth look like 4.8 billion years ago, or perhaps 5 or 6 billion years ago? Was it basically a Mars sized molten proto-planet? Overall, what are the intervening steps from a clump of asteroids to a full-fledged planet?

[Ezkerraldean]

it looked like part of a big cloud of gas and dust, i guess. whichever nebula it was that birthed the sun would only have begun condensing into proplyds more recently than 5 billion years ago.


there's another question. can we tell which other local stars came from the same nebula as the sun?

[rnrstar]

[quote=""Ezkerraldean""]it looked like part of a big cloud of gas and dust, i guess. whichever nebula it was that birthed the sun would only have begun condensing into proplyds more recently than 5 billion years ago.


there's another question. can we tell which other local stars came from the same nebula as the sun?[/quote]That's a question I have often asked myself. Can we determine our sister stars or even the remnant of the star that gave birth to the nebula we formed from.

The only answer I have ever seen is that we have orbited the center of the galaxy many times since and that our solar sisters have probably wondered off to the point where we wouldn't be able to distinguish them from any other star in our neighborhood.

As to the question in the OP, the Earth started out as a single grain of dust that eventually coalesced with other grains of dust to become the Earth.

[Straight Hate]

[quote=""KnightWhoSaysNi""]Here's something I'm a bit curious about. The scientific consensus is that the earth is roughly 4.6 billion years old. So what did the earth look like 4.8 billion years ago, or perhaps 5 or 6 billion years ago?[/quote]

Didn't exist in any appreciable form. My understanding is that ~4.6 Ga figure is an estimation of the beginning of our solar system based on meteorite dating. I'm also under the impression that the period of accretion for the Earth was relatively short, and largely post-dates the 4.6 Ga figure (perhaps overlapping by a small margin in the initial accretionary process).

Was it basically a Mars sized molten proto-planet? Overall, what are the intervening steps from a clump of asteroids to a full-fledged planet?

The Moon is hypothesized to result from a Mars-sized object colliding with the 'proto' Earth, so while I don't have a specific answer, I assume that the proto-Earth was of a smaller mass than the Earth today.

As for the steps, in simplistic terms the model speaks of accretion via gravity. A detailed and specific model, no doubt, would be far more complex. Massive bombardment of solar system bodies pre-dates 3.8 Ga (based on the formation of the Lunar maria), and is typically referred to as the Hadean eon of Precambrian time (4.6-3.8 Ga).

If we make the assumption of a short period of initial accretion - say, 150 million years (pulling numbers out of my ass) - that still gives a decent amount of time for a few broad events:

a) accretion of proto-Earth
b) Moon-forming collision
c) secondary accretion forming present Earth.

Zircon grains in Australian sediment have been dated to ~4.4 Ga, so if that figure is accurate, we can infer that igneous processes bearing at least some resemblance to what occurs today were happening at and/or before that date.

(Perhaps someone will correct/clarify me on any of the above.)

[Hyndis]

http://en.wikipedia.org/wiki/History_of_Earth




They even have a really neat picture in the article showing relative timespans of various eras.

[Late_Cretaceous]

Probably, if you went back in time 5 billion years, all the matter that is the earth and other solar system planets was in the core of a giant star (and none of the current atoms existed still only H and He).

From hyrodgen to humans in 5 billion years (to steal something from the creationists)

[skepticalbip]

[quote=""KnightWhoSaysNi""]Here's something I'm a bit curious about. The scientific consensus is that the earth is roughly 4.6 billion years old. So what did the earth look like 4.8 billion years ago, or perhaps 5 or 6 billion years ago? Was it basically a Mars sized molten proto-planet? Overall, what are the intervening steps from a clump of asteroids to a full-fledged planet?[/quote]If you go back far enough then the solar system started out as a nebula that was the remains of an earlier generation star the went supernova. The nebula coalesced into a disk (gravity is funny that way) with the lion's share eventually forming the sun. What was to become the Earth was dust, gas, small and larger "rocks" orbiting the center of this disk which were accreted into a growing mass eventually becoming the Earth. 5 or 6 billion years ago, there were no sun or planets only a disk of dust and gas slowly becoming denser.

[ahdenai]

The 4.6 BY age comes from Ur -> Pb and K -> Ar dating. The decaying isotope is bound into a crystal which contains none of the daughter isotope, and today we measure the accumulation of daughter isotope in undisturbed crystals. So the actual age measurement is of the time of crystalization; e.g., the age of the oldest rocks we can find reflects the time since those rocks cooled from a molten state. So there was an "earth" before that - mostly hot, and none of the rocks present then have survived. Much earlier than that, and it was unconsolidated dust, as mentioned above. To my understanding, we don't have any good way to measure the time interval between dust and first surviving cooled rock.