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-Short Attention Span Theater-
2.5 Megaton Blast over Arizona reported
2005-03-13
Researchers have long wondered why there is not more melted rock at Arizona's Barringer Meteorite Crater between Flagstaff and Winslow.

At an impact speed of 34,000 to 44,000 mph, the massive space rock should have melted substantial quantities of the white Coconino geological formation. One possible explanation has been that the meteor contained large amounts of water, which would have lessened the force of the impact that created the 570-foot-deep, 4,100-foot-wide crater.
 
But new calculations suggest the rock, after it was broken up in the atmosphere, was going slower than previously believed.

The Barringer site "is probably the most studied impact crater on Earth," said astronomer H. Jay Melosh of the University of Arizona. "We were astonished to discover something entirely unexpected about how it was formed."

Using computer models for how such objects would interact with the atmosphere, Melosh and astronomer Gareth Collins of Imperial College London concluded that the 300,000-ton, 130-foot-diameter meteor fractured before it hit the ground, with about half of it dispersing into small fragments.

The remaining half struck the ground at a speed of 26,800 mph, about 10 times the velocity of a bullet fired from a high-powered rifle, but not fast enough to melt large quantities of rock, the scientists reported this week in the journal Nature.

The intact fragment exploded with the energy of at least 2.5 megatons of TNT, they said.
Posted by:Mrs. Davis

#10  "Hmmm, AC. Are you forgetting the extra heat needed for the state change from liquid to vapor? "Water, for example, needs about 500 times more heat to vaporize than to change its temp 1 degree C. Too lazy to look up silicon!"

I probably should know that but I don't. I do know that water has a very high specific energy of state change compared to other substances, which is one reason steam power is so effective. A rock will not actually vaporize under these conditions since it is essentially a gross mixture of different compounds in the form of crystalline minerals. What will happen is that some constituent compounds will vaporize while others remain liquid or even solid, producing a tremendous explosion.

"I thought thermal was inverse r-cubed, with shock waves inverse r-squared."

The first is true for a purely thermal phenomenon such as expanding steam. In the case of a meteor impact, most thermal effects are a conversion of shock effects and follow the same energy density profile.
Posted by: Atomic Conspiracy   2005-03-13 10:40:24 PM  

#9  What's the issue here? Meteors make big holes. I've seen one of the biggest existant - Wolfe Creek. Man thats a big hole.
Posted by: phil_b   2005-03-13 10:36:57 PM  

#8  Anything emanating from a point source will form a sphere. The area of the sphere is 4 pi r^2. so the energy is inversely proportional to the square of the distance.

But actually, pie are round, cornbread are squared.
Posted by: Alaska Paul   2005-03-13 9:49:02 PM  

#7  Latent heat of vaporization
Posted by: Bobby   2005-03-13 9:37:33 PM  

#6  Giant rocks from space: why do they hate us?
Posted by: Seafarious   2005-03-13 9:28:13 PM  

#5  Big Rock hit Arizona made big hole :)
Posted by: djohn66   2005-03-13 9:24:27 PM  

#4  Man, youse guys take all the fun out of a perfectly good cataclysm.
Posted by: Shipman   2005-03-13 8:15:09 PM  

#3  the difference between the melting point, about 1500 C, and the boiling point, about 2800 C, is not at all enormous.

Hmmm, AC. Are you forgetting the extra heat needed for the state change from liquid to vapor? Water, for example, needs about 500 times more heat to vaporize than to change its temp 1 degree C. Too lazy to look up silicon!
Posted by: SteveS   2005-03-13 6:57:52 PM  

#2  I thought thermal was inverse r-cubed, with shock waves inverse r-squared. That makes the melted region even narrower.

To my untrained eye, Barringer looks an awful lot like Sedan.
Posted by: Dishman   2005-03-13 6:17:52 PM  

#1  I have disagreed with Dr. Melosh about this. Keep in mind that he is not a geologist.
One problem with this scenario is the apparent expectation of large amounts of melted rock. At these energy levels, dissipating into a massive solid, the difference between the melting point, about 1500 C, and the boiling point, about 2800 C, is not at all enormous.
Keep in mind as well that the energy density in any particular part of the affected area is inversely proportional to the square of the distance from the center.
This means that the zone in which we could expect the rock to melt, but not vaporize, is comparatively narrow. Much of the liquified material could be well within the zone of dislocation by blast, since this component of the energy yield would be enormously high as a percentage of the total.
If so, a large part of the molten material would be blown into the atmosphere by blast and dislocation effects, where it would condense and re-crystallize as ordinary rock and dust.
Posted by: Atomic Conspiracy   2005-03-13 5:52:12 PM  

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