Wednesday, May 17, 2006

"The moon's just right upstairs"

This morning I woke up to a very cool -- and amusingly sound-edited -- NPR report about looking for life on Earth on the Moon. I realize that's a confusing combination of prepositional phrases, but the idea is this: We have a billion-year gap between the birth of the planet and the first chemical traces of life in fossils. This is not necessarily because there was no life on the planet, though -- it's because rocks that old effectively don't exist anymore. We're talking about rocks from before the Earth fully cooled, which means they've been melted and pulverized and folded and spindled and anything else that can happen to you in four and a half billion years or so. Not the best conditions for seeking the very earliest traces of life.

But Earth has, in our only satellite, a nice little attic storage space. Asteroids hitting the earth in these early days -- and they did that a lot -- could easily have kicked up material from the infant earth, splashing bits of it all over the lunar surface. In fact, there are potentially millions of tons of Earth rock on the Moon, according to Dr. Peter Ward at the University of Washington. (I'm not sure why they interviewed him, rather than the two graduate students conducting the study, but I guess that's academia for you.) Mind you, we're not talking about easily recognizable, conveniently labeled chunks of material. For one thing, the Earth-to-Moon meteorites were probably crushed to powder in their fall, with no atmosphere to slow them down. For another, it's not immediately obvious how we would tell terran material apart from lunar material, though according to this report there are several possible chemical parameters for differentiating the two. And there's no obvious Earth material in the lunar rocks we've collected so far. But there's evidence that if we knew what to look for, we'd be able to find pristine (if powdered) Earth rock dating nearly to the birth of the planet.

What would that get us? Well, we often assume right now that life began to emerge after the planet was cool and heavy bombardment by asteroids had tapered off. But that's partly because we don't have any reliable records from before then. If signs of life appear in these relics from Earth's first billion years, then life may happen more easily than we thought. It would indicate that the conditions necessary to support life are not as tricky and specific as we might have thought. After all, we knew our ancestors were hardy little buggers -- otherwise they wouldn't have lived long enough for a lineage -- but hardy little buggers who can live on an unfinished planet under a hail of space debris? That presents a different picture. It makes us even bigger badasses, and perhaps more importantly, it lends credence to the idea that life could easily have arisen on other planets, under other -- maybe tougher -- conditions. Statistically that's pretty much a no-brainer, but if life is easier to produce than we'd imagined, alien life may be not only likely but frequent.

And all we need to do to find out is check upstairs.


Lynne said...

Ooh, a post right up my alley. Go geologist, go!

Actually, I'm not a meteoriticist, and although I've learned all about this, it doesn't stick when you don't use it. So I'm hazy on the details. And I haven't listened to the actual NPR report, so I'm not sure what kind of molecular fossils they might be looking for (but I know that often they look for relatively simple organic molecules that are used in basic microbial processes).

So for the record, to be nitpicky, the Earth has not yet fully cooled; Lord Kelvin was wrong (hooray, radioactivity!). And mostly we do not have evidence of life earlier than, depending on who you believe, maybe as far back as 4ish Ga (giga anni = billion years in geologyspeak) because of plate tectonics. There are not rocks but individual grains in metasediments as old as 4.4 in continental shield areas, but mountain-building and tectonic processes have squished and squeezed and metamorphosed all the rocks anywhere near that old. Most were subducted (or otherwise sank) back into the mantle. The Earth is a giant recycling engine, except for the continents because they're buoyant, but those are a mess because they've collided and ripped apart so many times.

But they do make a good point that the heavy bombardment may have partially - or even fully - remelted Earth, not to mention formed the moon (a very enigmatic body, it turns out), and it may have also added some late chemical material to the planet (the 'late veneer' loved so dearly by some geochemists). But the Earth was not really bombarded 'every day' by planetesimals, though it was surely hit by lots of asteroids and meteorites and *some* planetesimals. Collisions of early pre-planetesimals is what MADE the planet, though. The collisions of early solar system bodies caused the accretions that eventually formed the planets and large asteroids we see today. That these planetoids were probably melted by the process explains their seemingly rapid differentiation into more or less homogenous metal cores and siliceous, rocky mantles/crusts (in the case of terrestrial planets).

A collision between the Earth and a very large planetesimal - Mars-sized, perhaps - is the most poopular going theory for how the moon formed. A couple people out there still believe the moon was captured, but they're regarded about the same way as the anti-plumists (subject for another day, but trust me - crazies!). Pretty impressive computer models show that for the most part, the molten material that blobs off and forms the moon is made up of the foreign body's material, not the Earth's. This explains some of the big chemical differences between the Earth and the moon. It fails to explain at least one highly spooky chemical similarity (identical oxygen isotopes). Some scientists think that this late collision remelted much of Earth and formed a magma ocean through the entire mantle, causing mantle stratification and the formation of some bizarre melts only found in the oldest parts of continents (like the ones I studied for my senior thesis at Smith), among other things. There is also some weirdness on the moon - rocks that make no sense unless there was a magma ocean there, too, and then that ocean completely turned itself upside down.

Anyway. Moon-Earth trivia aside, there are some problems with their study if you buy all of this. If life started before the moon formed, they won't see it, unless of course there was already some debris around the planet and the newly formed and cooled moon was able to sweep it up - probably not all that likely, because there might not have been much, things were all melty, and the material would be so statistically minor. Could be wrong about that, though. But I suspect they are only going to find molecular fossils - should they exist - that were blasted off of Earth after the possible magma ocean cooled back down. I'm just not clear on the timing, maybe because I thought the heavy bombardment had already started to taper by moon formation.

But I suppose that doesn't matter - if they find a rock with evidence life, they can date it and *know* when it's from, whether or not it fits with suggested scenarios for moon formation. It's the question of setting up the study that I'm not getting - i.e. I might not fund their grant unless they convinced me better.

But the idea is totally cool, and if they find molecular fossils everyone would shit themselves.

5/18/2006 9:22 AM  
Lynne said...

popular, not poopular!

(are you unpoopular? do you pop out at parties?)

5/18/2006 10:55 AM  
jess said...

Well I didn't say planetesimals hit the earth every day! After all, there weren't even days then, because God hadn't created night and morning. Honestly, Lynne.

Thanks for all this extra info. I guess if the earth had "fully cooled," both you and Dan would be out of a job... but I got the impression that it was pretty wet and melty at the time they're talking about.

Dan brought up the fact that the moon split off from the Earth (I never know how to capitalize) and so how would we be able to tell the difference between lunar and Earth material, so thanks for answering that! Is it possible that moon rocks that make no sense are Earth rocks? (Probably not, since most Earth material was probably smashed to dust.)

I think molecular fossils is probably what they're looking for, since they seem pretty confident that they can find useful information in small particles of rock. Plus, molecular is really as big as you're going to get in that time period. "Chemical traces of life" is what they said... that gives me flashes of the CHON movie from the Natural History Museum, but not a lot else.

You should definitely listen to the report, though. It's very funny. You'll pee. (And be poopular.)

5/18/2006 12:08 PM  
Lynne said...

oops, should reference things right. the article you linked referred to daily collisions with planetesimals, which is pretty crazy.

there is a lot of chemistry and isotope work that shows earth and moon rocks are similar but different, and those things can be used to fingerprint the origins of any rocks they find. they can also determine how long a rock has been exposed and undisturbed on a surface like the surface of the moon (bombardment by cosmic radiation causes certain detectable isotopic changes). the power of geochemistry!

5/18/2006 1:11 PM  
Jess said...

Dang, no wonder it only took one geology class to turn people into majors! Rock chemistry is pretty cool.

5/18/2006 2:21 PM  
Lynne said...

pretty cool!! so you just tell me if you need some input so you can post about lava chemistry on earth and what it tells us about the structure and evolution of the whole planet, because then holy shit am i your woman.

5/18/2006 3:27 PM  

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