Those big oceans out there. Where did they come from?
This is a question that had never crossed my mind before. I mean much of the time I take the water that comes out of my tap for granted. I had certainly never wondered any farther up the supply chain than a groundwater aquifer. But recently I ran into a comet scientist and Trekkie named Karen Meech who set me straight on a few misconceptions.
It’s actually a bit of a puzzle, Meech said, because during the time that the Earth was forming, roughly 5 billion years ago, it was too hot for water to exist as a liquid. And as a gas it was unlikely to respond to Earth’s gravitational tug. Cosmologists have actually calculated the solar system’s “snow line” during its early life – apparently you had to head out to the asteroid belt before you could find any water.
So how did Earth end up with vastly more water than any other planet in the solar system? (Witness Mars’s feeble attempts to keep up.) Meech thinks most of it arrived when either asteroids or comets slammed into the planet early in its life. She and others in her field have even measured hydrogen isotope ratios in comets to see whether they match ocean water. The trouble with that approach is it’s hard to build your sample size. That’s one reason NASA sent up Deep Impact to pitch a fastball at comet Tempel 1, to read the spectral signature of the comet’s ice (watch a cartoon of it here).
Sci-fi novels aside, Meech says, water is probably a prerequisite for life – or advanced life, at least – on other planets. Liquids are great places for life, much better than solids at transporting nutrients and food yet without the unfortunate tendency to waft away that gases have. And water, thanks to those dear old hydrogen bonds, has a far greater temperature range at which it is liquid than other naturally occurring compounds.
This leads Meech to think that as long as we’re looking for life in other solar systems, we might want to look for ones with a belt of asteroids and comets, to boost the chances that that world is wet like ours. Of course, that assumes our water did come from comets or icy asteroids, and the jury is still out on that. It reconvenes at the Bioastronomy 2007 conference in July, in Puerto Rico. Mark your calendars.
(Thanks to NASA for an actual, real-life picture of Deep Impact smacking into Tempel 1. The yellow arrow is to aid the lay person in deducing the location of the strike.)
What sort of volume are we talking (i.e. how much agua in your average comet), and how many comets are thought to have hit the earth over the last, oh, 4 billion years? Seems like numbers that would be handy in figuring out whether this theory, ah, holds any water…
Ah – I knew someone would call me out on this nearly number-free post. The answer is “It’s complicated.”
But just for mucking about, it would take an iceball with a radius of roughly 434 km to supply all of the ocean’s water. That’s more than 30 times the radius of Tempel 1, but a bit smaller than Ceres, the largest asteroid (excuse me, dwarf planet) in the Asteroid Belt. And the belt contains more than 200 other chunks larger than 100 km across. (And as long as we’re wheedling, those are just the chunks that are LEFT after the ones that, so the argument goes, have already splatted into us.)
Of course those numbers are only directly comparable if the thing that hits us is composed entirely of ice. And then there’s all that water stored in Earth’s interior. But it gives you some numbers to hold up the tent of your imagination while you construct hypothetical galactic smashups.
The real problem is that so far it doesn’t look like comets have similar isotopic ratios to ocean water. But that’s complicated in both directions: we haven’t sampled many comets, and lots of things have happened to Earth’s water that could have changed its isotopic composition since it got here.
As for the rest of the figures and probabilities, I guess we’ll have to wait until Meech googles herself and swoops in to rescue us!