How do we know the asteroids formed at the same time as earth?

Question

In continuation of the question Why is Earth's age given by dating meteorites rather than its own rocks?, what evidence do we have that the asteroids indeed formed at the same time as earth? Is there any physical evidence, or is it a conclusion reached mostly through simulation?

Answer 1

what evidence do we have that the asteroids indeed formed at the same time as earth?

It depends on what is your definition as "the same time". The formation of the solar system and Earth did not happen at a particular second in time but was rather a continuous process. It also depends on what you define as "asteroids".

I'll try to put some things in order.

1. The oldest known material from Earth is a zircon from the Jack Hills in Australia. Its age has been recently refined to be 4382 million years. We know that because we measured it.⁽¹⁾
2. The moon forming event occurred sometime between 4348 and 4413 million years ago.⁽²⁾ Now, that zircon should be younger than the moon (because the moon forming event would destroy any already-existing minerals on Earth). Yet, there is a slight overlap between the ages. This is due to the fact that analytical capabilities have their uncertainties and error.

This means that the Earth has been here at least ~4.4 billion years ago, because you need the Earth to have a moon. The Earth didn't just pop into existence, but it formed over a period of time, in which small planetesimals collided with each other and accreted to form a larger body.

The question now is, what is the age of the planetesimals? We can't know for sure because they are all integrated into Earth, but we can look at asteroids and see their age. We do that by dating the meteorites that fall on the Earth, which in some cases are blasted off asteroids by (even more) collisions and impacts.

When you look inside a specific type of meteorites called chondrites, you can see objects that are called CAIs (calcium-aluminum inclusions). These things are solids condensed from vapourised gas that existed in the solar nebula before any planets and planetesimals formed. These are basically the first solid to form in the solar system, and they define the birth of the solar system. We know that they formed 4568^(3,4) or 4569.5⁽⁵⁾ million years ago.

Planetary bodies (which for this discussion will be considered as km-sized chucks or rock with the ability to melt and differentiate to mantle and crust, and accrete to form proper planets) began forming around 4566.2 to 4567 million years ago^(5,6,7). We know that because we can date meteorites that we know originated in asteroids. This is just 2 million years after the formation of the first solid droplets in the solar system. Now, while 2 million may seem like a long time for your daily commute, it is not too long for planetary processes (considering Earth is 4.5-4.4 billion years old).

Now there is a time gap - what happened between 4566 (formation of planetesimals and asteroids) and 4413 (formation of the moon)? There are about 150 million years that I haven't talked about yet. Was the Earth gradually growing in a linear fashion during that time or did it form rather quickly? Luckily, there are answers to that as well. Based on some measured geochemical data it was shown that most of Earth's mass actually accreted ~10 million years after the formation of the solar system(8). That's rather quick! So the Earth has been sitting there, all by itself, around 100 million years until the moon formed. As to how the Earth itself formed, it could be just amalgamation of small planetisimals, collision of larger bodies, or something completely else. Chemical evidence and physical models do not always agree, and it's all highly debated.

This is a field which is rapidly evolving and discoveries are made all the time due to better analytical capabilities and better models. Just look at the years of the papers below. This is all cutting edge and a highly exciting field of study. Everything that I wrote here can be inaccurate or even plain wrong, but that's the fun in science.

Is there any physical evidence, or is it a conclusion reached mostly through simulation?

So it is a combination of both. You find physical evidence: rocks from Earth and parts of meteorites and their minerals. You measure their isotopic composition to find their age and other characteristics. Then you make a model (or a simulation) that tries to see what has to happen in order for the physical properties to agree.

Further reading:

An introduction to Meteorites and the origin of the Solar System - a very accessible and interesting read.

Chronometry of Meteorites and the Formation of the Earth and Moon - a more technical review, may be paywalled.

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Refs:

^{1 Valley, J. W., Cavosie, A. J., Ushikubo, T., Reinhard, D. A., Lawrence, D. F., Larson, D. J., … Spicuzza, M. J. (2014). Hadean age for a post-magma-ocean zircon confirmed by atom-probe tomography. Nature Geoscience, 7(3), 219–223. doi:10.1038/ngeo2075}

^{2 Carlson, R. W., Borg, L. E., Gaffney, A. M., & Boyet, M. (2014). Rb-Sr, Sm-Nd and Lu-Hf isotope systematics of the lunar Mg-suite: the age of the lunar crust and its relation to the time of Moon formation. Philosophical Transactions. Series A, Mathematical, Physical, and Engineering Sciences, 372(2024), 20130246. doi:10.1098/rsta.2013.0246}

^{3 Bouvier, A., & Wadhwa, M. (2010). The age of the Solar System redefined by the oldest Pb–Pb age of a meteoritic inclusion. Nature Geoscience, 3(9), 637–641. doi:10.1038/ngeo941}

^{4 Burkhardt, C., Kleine, T., Bourdon, B., Palme, H., Zipfel, J., Friedrich, J. M., & Ebel, D. S. (2008). Hf–W mineral isochron for Ca,Al-rich inclusions: Age of the solar system and the timing of core formation in planetesimals. Geochimica et Cosmochimica Acta, 72(24), 6177–6197. doi:10.1016/j.gca.2008.10.023}

^{5 Baker, J., Bizzarro, M., Wittig, N., Connelly, J., & Haack, H. (2005). Early planetesimal melting from an age of 4.5662 Gyr for differentiated meteorites. Nature, 436(7054), 1127–31. doi:10.1038/nature038825}

^{6 Greenwood, R. C., Franchi, I. A., Jambon, A., & Buchanan, P. C. (2005). Widespread magma oceans on asteroidal bodies in the early Solar System. Nature, 435(7044), 916–8. doi:10.1038/nature03612}

^{7 Amelin, Y., Kaltenbach, A., Iizuka, T., Stirling, C. H., Ireland, T. R., Petaev, M., & Jacobsen, S. B. (2010). U–Pb chronology of the Solar System’s oldest solids with variable 238U/235U. Earth and Planetary Science Letters, 300(3-4), 343–350. doi:10.1016/j.epsl.2010.10.015}

^{8 Rudge, J. F., Kleine, T., & Bourdon, B. (2010). Broad bounds on Earth’s accretion and core formation constrained by geochemical models. Nature Geoscience, 3(6), 439–443. doi:10.1038/ngeo872}

Answer 2

The formation of a T-Tauri star spells the beginning of the end of the protoplanetary disk from which planets and asteroids can form. The end is nigh when that star ignites. The large solar winds and solar radiation pressure sweep the disk clean of all small objects. Some spirals inward, some outward. There's no dust and no gas from which new planets and asteroids can form. All that can be done is a mopping-up of the not-so-small objects that escaped the solar wind and radiation pressure.

Answer 3

The age of 4.55Ga for the Earth is actually a date of "hard stuff" based on Iron-Nickel meteorites, dated using radio-isotope dating.

If you had enough meteorite samples it should be possible to identify those with isotopes that imply a different age. For example, when plotted, their Pb isotope ratios will have different gradients. We do not see this.

Answer 4

The fact that the overwhelming majority of celestial objects within our solar system orbit in a like manner (same direction as the Sun's rotation) is observational evidence that they formed at about the same time from the same processes. A rogue planet/proto-planet/asteroid captured by our solar system's gravitational footprint in space time would have a 50/50 chance of developing an orbit in the same direction. If that was a retrograde orbit, its chances of survival would be considerably less. Think of a single NASCAR driver deciding to race in the wrong direction.

I'm certainly not saying that odd things in orbital mechanics don't happen and while our solar system has calmed down quite a bit from its earlier catastrophically chaotic days, it is currently believed by a growing number of scientists that Neptune and Uranus swapped orbital paths due to a harmonic gravitational tug from Jupiter and Saturn and an exo-planet with a retrograde solar orbit (WASP-17b) was either a rogue planet captured by its solar system or its orbit was flipped into a counter solar spin direction from a near collision with another object.

Answer 5

Perhaps a little step back to get a bigger point of view is required. Take note of this line in the answer provided by Michael - "it might be plain wrong".

The question uncovers some very glaring assumptions: 1. We know how the earth was formed. 2. We know when the earth was formed.

The current theory of accretion doesn't hold much water - both metaphorically and literally speaking. There just is no way that dust or rocks in space can accrete all by itself to form a body such as the earth. Even basic physics disagrees with this. The simulations have to ignore this basic fact and instead introduce fudge factors to make rocks and dust "stick" together when the collide. You just go out and try making rocks stick together when they smash into each other - confirm for yourself whether it will work in space where there's even less resistance to flying apart. Furthermore, even if one allows for some miraculous hot sludge of radioactive material to stick together, one immediately creates another problem: the exclusion of water because of heat. There is no acceptable way for all the water on earth to arrive here from outer space. Why would the water only strike earth [ and where would it come from in the first place ] in such a way that Mars, Venus and the moon do not have any evidence of a watery bombardment?

Then there's the question of how does one measure the age of anything?

Age is unfortunately not a physical property that we can measure directly like length, volume, density, etc.

In order to know the age of anything one needs to have been present when it came into existence and then to have some way by which to assign some kind of measure of passage of time to it. In short - age is an abstract construct that cannot be measured physically.

What can be measured is the proportions of the number of atoms of mother and daughter products of radioactive material. But having established that proportion and applying a rate of decay to determine age doesn't give a true age. Have you ever found a record of calibration that confirms that the radioactive methodology does indeed deliver the correct age? Did someone witness and record the creation of basic radioactive rock and then track it's age using the radioactive decay method? To my mind, it hasn't been done and in fact where some measurements have been made of newly created basalt in St Helen's eruption, the ages produced are plain incorrect. The best that it can produce is an "age" based on some very questionable assumptions.

Since we do not know the history of the earth we need to make some assumptions regarding its creation. We need some form of "clock" by which we can measure how old the earth is. That clock has to be independent of the earth itself, hence why people use meteoroids, under the assumption that it must be at least as old as if not older than the earth itself.

The main problem is that we need to know when that clock started ticking and we just don't have any means to establish an objective way to determine that. No one was there to witness and record that event. There-in lies the problem.

Even if you assume that the radio-active decay happened at a constant rate[ which we have seen is now no longer true ], you still have to make assumptions about the quantities involved. But therein lies another problem - you don't know the origin and history of the original material - how long did IT exist before it started its radioactivity? One can go on ad-infinitum.

So in spite of the highly "scientific" answer provided by Michael, the fact is that one cannot determine the age of the earth without making some highly metaphysical assumptions about the origin of the universe. You have to make some questionable unsubstantiated assumptions as to how the whole universe came into being. It cannot be determined scientifically, in spite of using the latest and greatest technology in cosmology today. We simply weren't there. We have no eye-witness account. We have no record. The best we can do is make assumptions of the starting point.

You may not like this answer. You may even get angry at seeing it but unfortunately for you, you cannot push it's validity out of the way.