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The BBC has a report that evidence has been found in lunar rock samples of a planetismal (called Theia) that was thought to have crashed into the Earth to form the Moon. The conclusion is based on a difference in oxygen isotope ratios detected in lunar rock samples returned from the Apollo space missions versus terrestrial samples.
The report is published in the journal Science [abstract]; report is paywalled.
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Evidence Of Planet That Formed Moon May have been Found In Rock Samples
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Fascinating is a word I use for the unexpected.
-- Spock, "The Squire of Gothos", stardate 2124.5
(Score: 3, Interesting) by Alfred on Friday June 06 2014, @03:07PM
I never could get how if the moon came from the earth after some impact, how could they both be round? There isn't any real erosion force in a vacuum. I would expect a random asteroid shape for both.
I suppose it could be possible if the earth was a hot ball of lava before impact, if the gravity of the lava would pull itself into a round shape before it cooled, but no one presents that as possible.
(Score: 1, Informative) by Anonymous Coward on Friday June 06 2014, @03:13PM
(Score: 2) by Alfred on Friday June 06 2014, @04:38PM
Gravity is the only force inherent of the planet itself I can think of. Do you think there is a size threshold where under a certain size an object will just never round off? I think time is a factor too. Granted astronomical times are really long but how long gravity has had to act must be a factor. That clock would start after enough mass clumped together. Do we see any young planets or bodies that haven't gone full round yet?
>see all the other planets for examples.
This is not good. The statement assumes that there is only one mechanism for rounding and it applies to all planets. There is insufficient fact to rule out all other possibilities. it would be like saying that mountains only come from volcanoes or only from plates crashing together when there is more than one possible mechanism for making mountains.
>Who is this "no one" you speak of?
They are an overgeneralization of people I have not met. But I am open to correction about the ideas of others.
(Score: 4, Informative) by DrMag on Friday June 06 2014, @05:23PM
> Do you think there is a size threshold where under a certain size an object will just never round off?
Yes. Statement by Dr. Derek Sears in Scientific American [scientificamerican.com]:
> The statement assumes that there is only one mechanism for rounding and it applies to all planets. There is insufficient fact to rule out all other possibilities.
But that is the theory; there are no other theories I know of that are sound enough to explain it. Plus, it's in the IAU definition of a planet [wikipedia.org] (emphasis added):
> it would be like saying that mountains only come from volcanoes or only from plates crashing together
Again, that's the theory. I've not heard of any other mechanism for mountain formation. (Technically, volcanoes are very much related to plates crashing together (or pulling apart), so there's really only one mechanism, at least in the simplified sense of "mountains are created by ____".) Sediment deposit in the oceans doesn't create mountains; it creates stratified layers that are later pushed up into mountains by plate tectonics.
Point is, planet formation is a well-studied (albeit not perfectly understood) process; it is generally accepted that planets become round through self-gravitation. The moon isn't considered a planet because it doesn't fit the classification of "not a satellite", even though it is sufficiently large to undergo isostatic adjustment.
* Hopefully my tone is not condescending in all of this--just trying to share the knowledge that already exists. Do some reading; it really is a fascinating topic.
(Score: 3) by Alfred on Friday June 06 2014, @06:49PM
No condescension sensed. This is becoming quite the informative discussion. I appreciate your use of the word theory, most people take the first theory they hear as fact and run with it. And if I had mod points you would win for fantastic participation, or maybe informative I guess.
Self gravitation makes sense for bodies that are sufficiently fluid for their size. I am suspecting that an asteroid would go round if it were fluid enough even though it much smaller than a planet. The contrasting, really big and really solid, brings in interesting questions and the need to go draw some force-body diagrams.
If only I had time for reading enough. You are right this is a fascinating topic.
(Score: 3, Informative) by DrMag on Friday June 06 2014, @07:16PM
Keep in mind, though, what is actually meant by the word theory. Colloquially, it's usually interpreted as a synonym for hypothesis, but that's not the traditional definition. Scientifically, theory means a collection of concepts that brought together explain a particular phenomenon. Relativity, evolution, quantum mechanics, electrodynamics--these are all theories. That doesn't mean they're not well understood or are controversial (though it doesn't necessarily imply the antithesis either). Similarly, in science parlance, a "law" doesn't mean "that's the way it is", it just means we have a mathematical equation that describes a particular observation. "Newton's law of gravity" is wrong; very useful, but wrong. "Law" should never be conflated with truth, as that's not the intent of scientific law, just as "theory" should not be equated with "guess". A theory will be made of a number of hypothesis, observations, and laws. The three parts should be self-consistent, and also be testable.
The word theory is also used in many other fields, including art and music. I suspect the mix-up in definitions came from well-meaning people (many of whom were probably even scientists) who didn't really understand the scientific method. A quick dictionary look-up [reference.com] shows 4/5 definitions match what I'm describing--the other probably has become generally accepted because that's what happens in language; people start using a word wrong, and eventually the definition is adjusted to reflect common usage.
As for asteroids, in the absence of other bodies you're probably correct. It's surprising how much influence the various planets in a solar system can have on things, though, and with all the gravitational perturbations that are present, you might find that, at best, an asteroid becomes "round-ish" if sufficiently fluid, and not just in the oblate spheroid sense like the Earth is.
(Score: 3, Interesting) by TrumpetPower! on Saturday June 07 2014, @12:42AM
Liquidity is not required. Ever see a landslide? Even just a few pebbles rolling down a slope? That's the Earth's gravity causing the Earth to become even more spherical. Similar processes would work equally well to shape anything massive enough into something basically spherical.
b&
All but God can prove this sentence true.
(Score: 3, Interesting) by JeanCroix on Friday June 06 2014, @03:21PM
(Score: 2) by Alfred on Friday June 06 2014, @05:53PM
Thank you for not pointing to a long video, my lunch break isn't really long. Two minutes I can do.
Breakdown:
0:08 to 1:00 is stuff floating around in space and some things collide. Our feature subject, presumably the earth, is established as mostly molten.
1:00 A second body with some lines of lava smashes into the first
1:06 to 1:18 shows the smaller sinking into the larger
1:19 to 1:22 is some kind of filler, a crusty/molten surface and a debris ring?
1:23 to 1:53 is the moon orbiting and sucking up debris while it looks like the dropping lava out the south pole.
The most important part, how the two separate, that time from collision to orbiting moon, is entirely missing. The three second filler shows/explains nothing when it should have been the core of the feature. This is what I want explained.
I think the consensuses is things in space collide and there are molten things. What happens otherwise has yet to be explained.
(Score: 0) by Anonymous Coward on Friday June 06 2014, @04:17PM
Imagine a meteorite impact when the 'meteorite' is so big. After the impact, both were a hot ball of lava.
(Score: 2) by Alfred on Friday June 06 2014, @05:11PM
This is an interesting idea. You get me to thinking...
Assuming that enough kinetic energy turns to heat then things could melt. And not everything would have to melt, just enough to ease the action of going to a round shape. And even though debris would be flying everywhere our scenario only worries about what is left.
However:
I think that it would take a lot of energy to melt all those solids into liquids. The fracturing of solids would take considerably less energy and thus be more likely. Kinda like hitting a rock with a hammer, the rock will break before it melts.
hmm...
/HypotheticalRambling
(Score: 0) by Anonymous Coward on Friday June 06 2014, @10:47PM
Do you have any idea just how big space is, how big planets are, or how fast things go in space? The earth itself is moving around the sun at about 30 kilometers per second. Thats somewhere around 66,500 miles per hour. For the sake of simplicity, lets just use that as the average speed for anything in orbit around the sun. The earth's mass is something like 6 x10^24 (a 6 followed by 24 zeros), and its a small planet. So if earth were to impact something else, the kinetic energy involved from just the earth alone (Force = 0.5 x Mass x Velocity x Velocity) is a mind-bogglingly huge number, more than enough to melt a whole lot of rock.
I dont mean to sound condescending, but this is some pretty basic stuff, and its incredibly fascinating if you start looking into it (which I really encourage you to do!)
(Score: 3, Informative) by gman003 on Friday June 06 2014, @05:37PM
The impact generated a lot of heat - even if they were already solid rock (they weren't - they had cool surfaces but molten interiors), they would have melted.
Space is a good insulator. It takes time to radiate that kind of heat away, especially when gravity has pulled you into a minimal-surface-area-for-volume sphere.