Or how do we determine when earth was “earth”? I realize that earth was formed through a process of swirling gas clouds being pulled together by gravity, it’s a fluid process. But is there a point towards the end where it snaps together?
This article about an asteroid got me thinking. The claim is that, “based on the type of meteorite, it is expected to have formed 4.56 billion years ago, making it roughly 20 million years older than Earth”.
I know these are big numbers but they still seem pretty definite. So the earth is 4.543 billion years old. What is it about the planet that sets that date. What would the earth have looked like 5 million years earlier than that, for example?
The age of the Earth is primarily estimated using radiometric dating using ratios of isentropic composition of heavy minerals and particularly of lead (which is an end stage decay product of 232Th, 235U, 238U) versus primordial lead (204Pb). The clustering of these products in zircon-containing strata versus the more even distribution of primordial lead gives a good estimate of the age at which the crust solidified.
However, Patterson get getting inconsistent results, and after investigation discovered that this was due to pervasive lead contamination in the environment from various sources but predominately tetraethyl lead, an anti-knocking additive to automotive and aviation gasoline. Patterson went to great lengths to create a lead-free measurement facility and to get ‘clean’ samples. He also needed to use meteorites to create a non-terrestrial baseline to adjust the for unknowns in the age of formation of terrestrial samples. For reasons unclear, he was never nominated for a Nobel Prize for this accomplishment even though this was a major accomplishment of experimental science requirement years of dedication to process control and application of metrology to get a validated answer.
Note that 4.543 billion years ago is a million year period. That’s only short compared to the total life of the Earth, but still a long time in human terms. So there’s no one point where you can say “it’s now a planet” whereas before it wasn’t.
The Earth didn’t form from gas clouds, but rather from planetesimals. Planetesimals are basically asteroids of roughly 1 km in size. The asteroids we have now are leftovers that didn’t get incorporated into a large planet. But then they’ve been playing bumper cars with each other for 4.5 million years, so there’ve been many collisions that have broken some up and left most of them with a surface that’s mostly unconsolidated rubble.
The error bound is actually ±50 mya. When we say, ‘forming’, that means a stratification of the mantle from the core and formation of a quasi-permanent solid crust such that solid minerals can form.
First, the Earth being fully formed is kind of an arbitrary distinction. The vast majority of the collisions leading to an accumulation of mass happened in the first 10 to 100 million years of the solar system, with a possible slight uptick a few hundred million years later called the Late Heavy Bombardment. But the same process of collisions that “formed the Earth” brought in the Georgia meteorite last month (and a probably pretty good one in Australia yeaterday that hasn’t been found yet) so you could legitimately say that the Earth is still forming at a much slower rate.
Second, the Georgia meteorite has not been tested for age. It has just been determined to be an L (for Low metal) chrondrite, and that is the previously determined age fir the L chrondrite parent body. (Some versions of the news release mention a break-up date. That is also only referring to previously known science.)
Third, the guy who released this does not have a sparkling reputation in the meteoritics community, as you can see in the comments in this Facebook post:
If the Giant Impact Hypothesis is correct, the Earth was somewhat smaller before the impact with Theia, which was a roughly Mars-sized planet that collided with proto-Earth about 4.5 billion years ago. These two colliding objects swapped mass to a certain extent, and the result was a slightly larger Earth and a relatively small cloud of rocks that eventually coalesced into the Moon. I think some of the debris was lost for ever when it reached escape velocity, or evaporated due to the phenomenal energies involved.
Well, yes, this last collision was a very sudden event, but it was the last and (perhaps) largest of the innumerable impacts that occurred before to create the Earth from orbiting rubble. What was ProtoEarth like before the Great Impact? It might have had mountains, seas and continents, but it was probably very volcanically active and prone to continual smaller impacts. I doubt it would be worth visiting.
Do we know on what side of the Earth Theia impacted? Or is it impossible to know or irrelevant because everything got more or less homogenized after the impact?
Not really a meaningful question because there was nothing on the Earth’s surface at the time that could be considered a landmark today. Even if the whole surface hadn’t been remelted (which it probably was) there would have probably been none of what we call continental crust formed yet
All continental crust is ultimately derived from mantle-derived melts (mainly basalt) through fractional differentiation of basaltic melt and the assimilation (remelting) of pre-existing continental crust. The relative contributions of these two processes in creating continental crust are debated, but fractional differentiation is thought to play the dominant role.[11] These processes occur primarily at magmatic arcs associated with subduction.
There is little evidence of continental crust prior to 3.5 Ga.[12] About 20% of the continental crust’s current volume was formed by 3.0 Ga.[13] There was relatively rapid development on shield areas consisting of continental crust between 3.0 and 2.5 Ga.[12] During this time interval, about 60% of the continental crust’s current volume was formed.[13] The remaining 20% has formed during the last 2.5 Ga.
Over geological time, the ‘side’ of the Earth is an ambiguous designation. The impact would have dislodged and for the most part re-liquified whatever crust had been formed so the only residual evidence would be denser portions of the inner mantle which have migrated over the intervening few billions of years.
That’s for sure. But I suspect that the Great Impact that formed the Moon was so disruptive that much or most of the continental crust that had already formed by that date (if any) would have been shattered and deformed, and any volatiles that were temporarily resident on the surface or in the atmosphere would have been ejected into space.
Some, but not all, of this ejected material might have been recaptured, but Earth would have been very different to ProtoEarth.
Last I heard, this was the leading hypothesis for why Earth’s atmosphere is so thin. We could support a much thicker atmosphere: Venus’s is two orders of magnitude thicker, despite being a bit smaller and much hotter.
Mars’ atmosphere is also thinner than it could be, but in that case, it’s probably because of the lack of magnetic field, so the solar wind gradually blew it away.
There really isn’t a ‘leading’ hypothesis for why the atmosphere is what it is but the explanation for why Earth’s atmosphere only has traces of carbon (mostly carbon dioxide) is straightforward; most of it is locked in the lithosphere through some combination of weathering and absorption by photosynthetic organisms. Venus (which also has essentially no dynamo-powered magnetosphere, although the ionosphere is charged by the solar wind and does provide some protection against loss) doesn’t have living organisms, could not support photosynthesis on the surface, and lacks plate tectonics to subduct carbon rich surface layers and expose ‘virgin’ carbon-absorbing minerals, so there is nothing to regulate the atmosphere from becoming thick with chemically stable carbon dioxide.
Mar’s atmosphere is thin because after a relatively wet period with a carbon-rich primordial atmosphere it stopped outgassing from volcanic atmosphere and more volatile gases including nitrogen-bearing compounds escaped or were absorbed into the subsurface. As the surface became too cold to sustain liquid water, the atmosphere thinned and became incapable of holding onto heat from what insolation passed through the atmosphere even though the Sun was getting progressively hotter.
The mystery of Earth’s atmosphere is why we have such a high proportion of nitrogen in comparison to anything else, which both helps the atmosphere be transparent enough to regulate temperature but which is also relatively geochemically inert and biochemically important (at least, for life as we know it). Venus has a lower percentage of nitrogen in its atmosphere but by molar mass it is almost double the quantity on Earth. No other rocky planet or body other than Titan has a significant amount of nitrogen in its atmosphere (also a mystery) or in large concentrations in surface compounds. (Uranus and Neptune are believed to have significant amounts of ammonia in their lower atmospheres but not in the upper atmosphere.) In general, the Solar System appears to have a low abundance of nitrogen relative to cosmic abundance but Earth’s atmosphere is extremely rich in it.
