13 March 2008

Ask not where Earth got its water…

Posted by melissa

For decades, planetary scientists have been arguing about how the Earth acquired its oceans.  Maybe the water came from volcanic outgassing, or was delivered by icy comets, or came from a bombardment of asteroids over geologic time.  Models of the Earth’s formation show that our planet didn’t form with enough water to account for its present oceans – that means the water came from somewhere else. 


But maybe our models are wrong.  Maybe the Earth formed with more than enough water to explain its oceans.  That’s what Dr. Joseph Nuth from NASA’s Goddard Space Flight Center proposed at LPSC yesterday morning.  He said, “You shouldn’t ask where the Earth got all its water, you should be asking, ‘where did all the Earth’s water go?’


The problem with the current models, Nuth said, is that they use the composition of asteroids and meteorites to model the composition of the early Earth.  Asteroids and meteorites are the most “primitive” rocks around, so they’re a good estimate of what the rocks that formed our Earth looked like.  They’re just an estimate, however – these rocks have been hanging out in space for 4.5 billion years, and that’s a lot of time for their composition to evolve.  In particular, they could have lost a lot of volatiles (like water) over that time.


Nuth’s models show that the rocks that ultimately made our Earth must’ve formed in the chilly outer reaches of the solar system (past the “snow line,” where water will be an ice instead of a gas).  These “planetessimals” would’ve had a lot of ice mixed with the rock (they would’ve been more than 30% water), and when they started accreting together, they would’ve formed a proto-Earth that was saturated with water.


Nuth’s point is this: asteroids are dry now, but were chock full of water back when the Earth was forming.


But he said it better than I can: “Using meteorites and asteroids to model the composition of Earth is like using raisins to model the origin of champagne.”


I’ll drink to that.