Monday, January 9, 2012

Asteroid Impact Physics

Here's a bit more fascinating grain to the asteroid impact issue; this question was asked of me by a fellow scientist in the US Geological Survey after HE tried to answer an asteroid impact question as another volunteer on Ask-a-Geologist. For context, he asked me what was the typical velocity these incoming asteroids are bringing with them. Keep in mind that kinetic energy goes as mass times velocity squared, so double the speed and you quadruple the energy delivered. The velocities we are talking about are orders of magnitude faster than a rifle bullet. They are so fast that the atmospheric air stacks up in front of them until all but the nickel-iron meteors are blown apart from the accumulating atmospheric stress - this typically happens at about 8 km to 12 km altitudes.

This is what happened at Tunguska in 1908: the (probably stony asteroid) object broke apart from the growing stress on it, abruptly increasing the atmospheric ablation (burning) hundreds of times as more surface area was suddenly exposed to the air. This sudden flash was so powerful that it set the forest beneath it on fire. Seconds later the blast-wave blew out the fires, and stripped the branches off the trees in the center of the blast zone - it is littered with standing, burned poles with roots to this day. All the trees outside of this core zone beneath the blast were flattened in a radial pattern. A man in a trading post at Yeravan, 40 kilometers to the south-southeast, had the back of his wool shirt set on fire - then he was bowled over and knocked across the clearing of his trading post.


This stuff is all mind-blowing to me.
I don’t suppose anyone has done any wind-tunnel tests of FeNi objects traveling at 25 km/s, which can be used to calibrate the model. 
Larry M.


Hard to imagine a wind tunnel that wouldn't detonate at a tiny fraction of those velocities.

In the 1970's and 1980's, Dave Roddy (USGS-Flagstaff) participated in some NASA-funded experiments using bullets against metal plates to better understand the impact physics.

A couple of things they figured out included the fact that a crater is roughly 20 times the diameter of the incoming object... it's unclear to me how much the velocity controls the crater-diameter/bolide-diameter ratio, however.

Another take-away: the impact craters are always circular until the incoming angle drops to 15 - 22 degrees from the horizontal surface plane. The reason for this is that the object is going so fast that it first buries itself. Then the huge residual kinetic energy from the very high velocities has to go somewhere, so it blows up for lack of a more descriptive process term. There is a rare elongated "skip" crater on Mars that is now believed to have brought the Nakhlite meteorites to Earth. Nakhlites are firmly believed to be of Martian origin by meteorite specialists (from their isotopic content); I bought a small piece of one for my wife as a birthday gift; she worked for the Mars Society and participated in several habitat simulation efforts, including one on Devon Island next to the huge Haughton asteroid impact crater in the high Canadian arctic.

Incidentally, the burial process is an amazing thing in itself. My all-time prized rock sample lies in a shelf in my office for all to see. It consists of loose sand converted instantly into laminated sandstone from the impacting shockwave of the Wabar asteroid. It was then blown up into the "jetting" curtain - the ever-widening cone of high-temperature mixed material spurting out from around the impact point. This jet material Gene Shoemaker and I called "Glass" - from chemical analysis, it's very uniformly 90% local sand and 10% FeNi asteroid, and probably represents temperatures greater than 10,000 degrees Centigrade. My "Instarock" sample was then "painted" by the jet-cone Glass, and for all the world it looks like a piece of white sandstone with black lumpy paint spattered over a large part of it. Other samples I have include Instarock that was completely covered with the Glass - the sandstone quickly turned to molten glass - but the heat was so high that it then bubbled and turned to a vitreous white glass froth inside the paint "shell". These samples will actually float in water!

The largest of the Wabar craters in the Empty Quarter of Saudi Arabia is 116 meters in diameter. In most ways it is indistinguishable from the Sedan atomic crater blasted out of the desert at the Nevada Test Site in 1962 - save one. It has a circular crater shape, overturned surface materials, shocked quartz minerals including coesite and stishovite - but no radioactivity.

Craters typically have little or no contents of the original bolide in them - just fall-back material from an eruption cloud that can reach typically the stratosphere. Only about 15 of the known 182 proven impact craters on Earth have any remnants of the original bolide associated with them. That's because the huge kinetic energy associated with the impact blows everything out of the crater - and a long distance away. I have found molten glass beads that rained down as far away as 850 meters from the Wabar craters. If you had been present during the impact and survived the blast, you would have still died in the rain of molten glass that reaches out at least 9 football fields away.

Daniel Berenger was a wealthy 19th Century engineer who hoped to mine the iron asteroid at the bottom of Meteor Crater in Arizona - early explorers had found bits and pieces of iron long distances away all around the crater. After drilling and sinking a shaft, he came up with nothing - and threw away his family's fortune after refusing to give up.

At Wabar the craters are circular, but there is an asymmetric ejecta field, with Instarock preferentially distributed on the southeast side of the craters. This was a key clue to determine the incoming direction. I was later able to verify this from historical records. I have Henry St. John ("Abdullah") Philby's book on my book shelf; he was the first European to visit the Wabar site in 1932. Only after spending several days at the site did he finally figure out that Wabar wasn't volcanic in origin, but instead asteroidal in origin. The object was actually seen as a fireball in the sky passing south of Riyadh, heading into the deep desert in a south-southeastern direction. Pieces fell out of the flight path about 25 kilometers northwest of the final impact zone, and they are chemically identical to rare metal fragments found near Wabar. Bedouin apparently visited the site shortly afterwards; they probably could see the stratospheric-height, atom-bomb-like mushroom cloud from 100's of kilometers away.


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