Thursday, February 17, 2011


Ed Lu gave a talk at Google today on his B612 foundation.

He mentioned that the asteroid that caused the K-T extinction was probably 10 miles in diameter, um hummm.... which meant that the top had not yet entered the bulk of the atmosphere when the bottom hit the ocean. That image really got me.

The speed of sound through granite is 5950 m/s, which is substantially less than the speed of an incoming asteroid. Things in low earth orbit go at about 7800 m/s, and Ed said incoming asteroids are around 3-4 times that. So that means that when the asteroid smacks into the earth, there is a really good chance that the back of the asteroid will hit the earth before the shock wave gets to it -- it'll punch all the way into the Earth surface. Koeberl and MacLeod have a nice table here which shows a granite on ice impact needs only 6 km/s vertical velocity to punch all the way in (they neglected water as a target material, an odd oversight since the majority of the earth is covered in water, more or less a solid at these velocities). If the incoming velocity is 25 km/s, which is on the low side of what Ed suggested, then anything striking within 76 degrees of vertical is going all the way in. It seems to me that most impacts would be like that.

So after the impact, most of the energy is added to stuff below the ground surface. That's 1e25 joules for the K-T asteroid. Enough to melt 5e18 kg of rock, which is 100 times as much mass as the asteroid itself. Figure a small fraction of that will vaporize and the whole mess goes suborbital.

For the next hour you have thousands of trillions of tons of glowing molten lava raining down on everything. Everything that can burn (6e14 kg of carbon in the world's biomass) does so, promptly.

And this asteroid impact thing has actually happened, many times. As Ed says, it's like pushing Ctrl-Alt-Del on the whole world.

Side notes: There is 1e18 kg of oxygen in the atmosphere, far more than necessary to burn every scrap of biomass on earth. The story I read was that the oxygen in the atmosphere came from plants. If so, there must be a lot of carbon buried somewhere: 8.6e14 kg of known coal reserves are less than 1%.

Another interesting point, vis-a-vis ocean fertilization: there is about as much carbon in the atmosphere as in the world's biomass. We'd have to boost the productivity of 1/10 of the world's ocean by a factor of 8, from existing productivity (125 gC/m^2/year), to fix the CO2 problem in the atmosphere in 15 years. Ocean CO2 would take a century or more. That productivity boost is like converting that much ocean into a coral reef! This seems like a lot to me.


  1. Suddenly, I'm no longer quite so stressed over the quality of the product I'm putting out to customers next week.

  2. Cool post! I was a doubter about how fundamental asteroid impacts were to Earth's history, they sound so zany, until I read Wally Broecker's "How to make a habitable planet", sort of an overview of planetary science with special reference to Earth. Turns out we've been bombarded all the time, for all our 4.5 BYR history! And you can find "circular features" all over the earth's crust, according to a soviet map I one saw. I suppose it should be obvious just by looking at the moon, right? Anyhow, this was a cool analysis. Adam

  3. You say "there must be a lot of carbon buried somewhere." The amount of carbon in carbonate rocks far exceeds the amount as coal. --Carl Feynman

  4. Nice blog. Do you accept guest posts? I've got an idea for a piece you might be interested in.

    Please contact me if you like this idea.

    - J

  5. Yeah, sure I'd accept a guest post. Try