THC, atropine, morphine, human chorionic gonadotrophin, cocaine, psilocybin, bufotenin, solanine, bungaroo toxin. Earth is a rich source of peculiar chemicals that might not be found anywhere else in the galaxy. The real pirates of Boskone may find us to be quite a a little treasurehouse.
A dangerous question to ask an Igneous Petrologist/Trace-Element Geochemist, Podkayne! I’ll try not to put you to sleep.
The primary geochemical mechanisms for concentrating various elements in magmatic systems are fractional crystallization and liquid immiscibility. Plate Tectonic processes have a lot to do with magma generation in the first place, and the effects of Plate Tectonics (e.g., over-thickened crust) have a lot to do with how differentation processes proceed, but Plate Tectonics itself has little to do with how trace-elements concentrate in natural systems.
Sushi, pizza, and beer.
Bungaroo toxin?!?
Used primarily to kill inflatable kangaroos, with a possible application in the cosmetic field. No known antidote, but its LD50 is one of the most variable known to man.
Brittany Spears, although except to look at her, her value is dubious.
Sedimentary rocks and clays. Unless you’ve had liquid water running over stone you won’t get much.
There are all kinds of things here that extraterrestrial aliens might find useful. I get scads of e-mail messages about them every day. I even got one today for cast-iron Christmas tree stands.
RR
Spam! No way there’s spam anywhere else.
Interesting, the Earth is obviously special place.
Then again, how about Io?
There might be some of these concentrating processes going on there.
And don’t the asteroids contain most minerals and elements in a less concentrated but more accessisble state- i.e. not trapped on a 1-gee planet?
SF worldbuilding at
http://www.orionsarm.com/main.html
Penn & Teller created a brilliant short movie for TV which addresses a question very similar to the OP. I think it’s brilliant, but if you haven’t seen it don’t read the spoiler box.
I haven’t seen it for a long time so my recall will be imperfect, but it goes something like this. Aliens are threatening to wipe out Earth unless we can prove that we Earthlings are a worthwhile species and we should be allowed to survive. We try to prove this in various ways, none of which impress the aliens. Then someone does a magic trick, and says it’s done with ‘invisible thread’, and makes the claim that we should be allowed to survive because we’ve advanced to the stage where we can invent invisible thread.
We, the viewers, know that the trick is not, actually, done with invisible thread and that there’s no such thing anyway (this is all explained in the movie in typical P&T style). However, the hope is that the aliens will be impressed by our amazing invention of ‘invisible thread’.
In the end, the aliens crush our plans by revealing they are well aware that the trick was done by other means, and that all the stuff about ‘invisible thread’ was just bogus. However, they decide to let us live because we’re the only species in the universe that would have the imagination to come up with a concept as ludicrous as ‘invisible thread’ and the audacity to expect anyone else to believe it.
The shaving cream atom? Buxom Hollywood starlets? Earth girls are easy you know.
Oops, that’s lab-slang. The intergalactic drug-lords would actually plunder the earth for our rich supply of bungarotoxin.
The atmospheres of Uranus and Neptune are primarily methane, as is that of Saturn’s moon Titan. Jupiter has methane, hydrogen and ammonia. All the fuel our would-be conquerers would ever need, assuming they could get a suitible oxidizer.
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Mangetout, is there a reason why you (and a lot of other scientist) think that communication will be by radio waves? Isn’t it possible that extraterrestrials will have different technologies and/or sensitivities to the electromagnetic spectrum? Is there a reason to think that they don’t “see” in the infrared range and communicate in UV or something like that?
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I could be wrong, but I believe that radio waves are much less vulnerable to insterstellar absorption than UVs or opticals are. Why do you think we communicate using radio waves?
Marble… aliens would get crazy about it, would trade antimatter weight to weight for it… at least if they have any artistic or aestetic sense 
I, too have seen that movie about the “Magic Trick.” It had Dan Akroyd in it. We impressed the aliens and saved humanity with a booger, if I recall. It had some pretty spooky parts to it.
-M
I think Pantellerite is on to something. Earth is the most massive rocky body in our solar system, although Venus comes close. So there are bound to be some unique geologic processes here. Of course, orbiting Sirius A is Jinx, which is a rocky body 2.5 times as massive as earth. If the aliens were smart they’d go there instead.
Volatiles are much much more easily obtained in the outer solar system. You wouldn’t want to ship liquid water anyway. Just let it stay below 0C and it’s as easy as transporting rocks.
There won’t be much free O2 anywhere except on planets with ecosystems. But oxygen itself is very common. After hydrogen and helium, carbon, oxygen, and nitrogen are the most common elements in the universe. Which means methane, ammonia, and water are the most common compounds in the universe. Nobody is coming to earth looking for water or natural gas.