Asteroid explosions

[Auk]

When a meteor enters the atmosphere most people think that the whole rock is extremely hot. But that is not the case the outer crust is extremely hot. But if you go about half an inch further into the rock you'll find that it is still cold. I found this interesting since I always thought otherwise.

[Martin]

Ophie - can you estimate the megatonnage of the Chesapeake Bay impactor based on the following info?

 

I have spoken with the scientists that are doing this research and I asked them what the best guess was on the size of the impactor - they told me about 3 miles in diameter. They don't know the incoming speed - somewhere between 20 and 70 km/sec.

....

 

hello coquina, havent spoken with you in a while

I can estimate you the Gigajoules

I cant remember the conversion from GJ to tons of TNT but it cant be all that hard

 

how many metric tonnes is a a sphere 5 km in diameter if it is 3 times density of water?

 

it 65 cubic kilometers volume and water is a billion metric tonnes per cubic kilometer.

 

So if it were same density as water the mass would be 65 billion metric tonnes.

 

And e.g. moon rock is about thrice water density so lets guess

200 billion metric tonnes.

 

========

Now I will estimate the kinetic energy of one tonne going 1 km/s.

 

It is 1/2 of a GJ (that is just the usual 1/2 m vee-square)

 

if it is going 20-70 km/s then one has to multipy by 400-4900, so

it is going to be 200 to 2450 GJ.

=========

So the thing has a mass of 200 billion metric tonnes and each tonne of it is carrying a kinetic energy of 200-2450 Gigajoules.

=========

so it is going to deliver around 40,000 to 490,000 billion GJ

 

I just multiplied the 200-2450 by 200 billion.

==========

 

So that boils down to 40-490 trillion GJ.

 

===========

Coquina I had to look up in the handbook that a ton TNT is about 4.2 GJ.

 

So to convert my estimate I have to divide by about 4

 

and so my estimate becomes 10-100 trillion tons TNT

 

if you like megatons then it is 10 to 100 million megatons TNT.

 

because a trillion is a million million or a million mega.

this is just back of envelope

someone else can work it out nice and proper in powers of ten notation.

[coquina]

Thanks Martin -

 

It was one helluva bang - was it not? What is amazing is the residual effects after 35 million years. The land is sinking and settling. Sea level is rising faster here than anywhere else on the east coast - that never made sense - now we know the ground here is sinking. Contractors were sued because houses settled and cracks developed in the sheet rock. Now we know that they didn't build inferior homes, it is all because of the fact the 400' down the ground is mush and the top level is subsiding.

 

What is different about the CBIC is that although it is buried, it is "reachable" for study. I really believe that new inroads will be made in Geology as a result of what we learn here.

[Ophiolite]

Drat, Coquina, I missed your original query and an opportunity to prove geologists can do maths too! I came up with the same figures as Martin, but I cheated. I used Excel. I've made a nice little spreadsheet where you input the size, velocity and density of the incoming bolide and voila. If anyone is dumb enough to want a copy pm me.

[Martin]

...I came up with the same figures as Martin, ....

 

I am glad to get some confirmation. it is easy to make a mistake of arithmetic when one is just winging it.

 

10 million Megatons TNT seems like a lot of energy doesnt it?

 

I tried to think of some way that a substantial amount might be released off into space and not have to be absorbed by the planet. but i could not see how much could. I think the flash of light would not represent a large percentage of the energy. So i was left with the idea that most of it has to be absorbed in the earth somehow-----in the rock, in the seawater, and the atmosphere----and most probably just goes into making the crater.

 

Whoever was living in St. Louis at the time must have been quite surprised.

[coquina]

There is a hypothesis that really big blasts may cause turbulence in the earth's mantle. It is thought that the Chicxulub impactor which eradicated the dinosaurs was about twice as large as the Chesapeake Bolide, and that the heat was so extensive that most of North America burned. (There was an article about that in Discover Mag. a couple of years ago.)

 

Some scientists believe that Chicxulub may have caused the eruption and formation of the Deccan Traps in India - on the opposite side of the globe. The theory is that the pressure was so enormous that it caused the earth to split and lava to flow out.

 

http://volcano.und.nodak.edu/vwdocs/volc_images/europe_west_asia/india/deccan.html

[JohnB]

Ophiolite, hmmmm.

 

Maybe I just have trouble getting my head around it. Do you happen to know of any other similar occurrences? Devastation on that scale would surely leave a record somewhere. Of course, 70% of the Earth is covered by water, so there would be no evidence in 70% of cases.

[Ophiolite]

Coquina is more switched onto the details of these impactors than I am, so I imagine she may post something in a day or two. (She lives on top of the remains of one.) You might try checking her earlier posts for links.

 

There is plenty of evidence of the small (Tunguska size) and medium events. I think around seventy impact craters of various ages have been identified globally. There is an excellent site that has illustrations of all of them. I'll try to refind it an post it here.

 

There is a suspicion that the Permo-Triassic extinction, bigger than the KT boundary event, may have been the consequence of a strike.

Nobody was really looking for this kind of evidence until the last couple of decades, so a lot of the data was there, but had been misinterpreted.

 

Why did nobody look for this? Geologists used to be schooled (or, if you weren't very bright, indoctrinated) in Lyell's Theory of Uniformity, the notion that the condition of the Earth today is the result of the processes we observe on the Earth today, acting slowly over long periods of time. This was hugely important when introduced because it helped overturn the then prevalent notion of catastrophism that was associated with literal biblical interpretations and a 6000 year old Earth. (In fact I think Darwin took Lyell's book with him as bedisde reading on the Beagle.) It was therefore difficult for some geologists to accept that catastrophic events could play a role in Earth history, especially since this involved listenting to those suspect characters called Astronomers.

 

Am I rambling again? Yes. Best stop.

[coquina]

http://www.unb.ca/passc/ImpactDatabase/index.html

 

There is an enormous amount of imformation here, but if you want to learn a lot about impact cratering, it is the best source I know of on the web.

 

At this time, there are about 160 known impact sites - more are being discovered each year. This site has a list that can be sorted alphabetically, by size, and by location.

 

This is the image reference page, http://www.unb.ca/passc/ImpactDatabase/images.html - you can see actual pictures of some craters - others have graphic images based on seismic, gravity, and magnetic studies.

 

There are three problems associated with detecting old craters.

1. Many of them are under the ocean.

2. Because of plate tectonics, many of them have been subducted and "recycled".

3. The ones that are on land have suffered from the effects of erosion and sedimentation.

 

One of the most fascinating to me is the Sudbury crater in Ontario...

http://www.unb.ca/passc/ImpactDatabase/images/sudbury.htm

 

It is 250 km in diameter and is 1850 +/- 3 million years old.

Here's another page about it:

http://ottawa.rasc.ca/astronomy/earth_craters/sudbury/

 

This impactor was big enough to melt a significant amount of the earths crust:

The Sudbury Igneous Complex (SIC)

Thick sheets of melted rocks line the bottom of many large meteor craters. Some of these impact melts are derived from the release of kinetic energy at bolide impact that is converted to heat. Also, rocks lying kilometers deep within Earth are often on the verge of melting but are prevented from doing so by the immense pressure from the weight of the material lying above them. A large bolide would blast away this weight, releasing the pressure on the buried rocks and causing the underlying minerals to melt.

 

The impact melts may not fully cool for hundreds of thousands of years. In the meantime, water from the environment and the heat from the newly exposed rocks can combine to form hydrothermal systems in the heavily fractured rocks in and around the crater. Scientists believe such warm mineral-rich venues could have played a role in the early development of life on Earth. (Science News: 3/9/02, p. 147) Evidence of the hydrothermal systems is documented in my ground tour.

 

The SIC is this type of large melt sheet produced from crustal melting resulting from a bolide impact. The target rocks, which remained within the crater after the impact, ponded to form a sub horizontal sheet of magma and differentiated as it cooled. It is currently exposed as an elliptical 60 x 30 km, 2.5 km thick remnant of the original impact melt sheet and consists, from bottom to top, of inclusion-rich, in places ore-bearing, quartz diorite sub layer, norite, quartz gabbro, and granophyre layers, and, within the target rocks surrounding the SIC, the quartz dioritic offset dikes.

 

 

This area has the single largest magmatic nickel source in the world. The Creighton Deep Project is currently mining and actively exploring well below the 7500-ft. level, maintaining its status as the deepest working mine in the western hemisphere. The size of the underground workings at Creighton dwarfs all man-made structures on the surface of the Earth. The No. 9 vertical shaft is between 4-5 times higher than the CN Tower!

 

The Sudbury Neutrino Observatory is housed in a cavern as large as a 10-story building, in the deepest section of the Creighton Mine.

 

The 200-m-thick impact melts found within the Sudbury Crater are a treasure trove of minerals. More than $1 billion of metal ores including those bearing nickel, platinum, and copper are mined from the melts each year. Isotopic analyses show that the metals come from Earth's crust, not from the meteorite that fell from space. Before the impact melt solidified, the deep, thick blend of light silicates and dense metal ores—which didn't mix well with each other—separated into two layers, according to density, just like oil and vinegar do. This ancient segregation makes mining today much easier. (Note 2)

 

The hydrothermal system created by the Sudbury impact also dissolved minerals containing copper and other metals from a broad area and then concentrated them in rich veins. (Richard Grieve, Natural Resources Canada in Ottawa)

 

 

I will post some more links later - I read recently that a crater had been found that might have caused the Permian Extinction.

 

As Ophie said - these craters were not taken seriously for a long time. Certainly, impacts don't occur as often as they did during the early formation of the solar system. One of the things that really got everyone's attention was the broken up asteroid that hit Jupiter in '94.

 

Gene Shoemaker was a pioneering geologist in the study of impact craters and he, his wife and Levy were the discoverers of Shoemaker-Levy. Sadly, he was killed in an automobile accident in Austrailia several years ago.

[coquina]

Here is the crater I read about earlier:

 

http://beckeraustralia.crustal.ucsb.edu/

 

from the website:

EVIDENCE OF METEOR IMPACT FOUND OFF AUSTRALIAN COAST

An impact crater believed to be associated with the "Great Dying," the largest extinction event in the history of life on Earth, appears to be buried off the coast of Australia. NASA and the National Science Foundation (NSF) funded the major research project headed by Luann Becker, a scientist at the University of California, Santa Barbara (UCSB). Science Express (Paper Reprint), the electronic publication of the journal Science, published a paper describing the crater today.

Most scientists agree a meteor impact, called Chicxulub, in Mexico's Yucatan Peninsula, accompanied the extinction of the dinosaurs 65 million years ago. But until now, the time of the Great Dying 250 million years ago, when 90 percent of marine and 80 percent of land life perished, lacked evidence and a location for a similar impact event. Becker and her team found extensive evidence of a 125-mile-wide crater, called Bedout, off the northwestern coast of Australia. They found clues matched up with the Great Dying, the period known as the end-Permian. This was the time period when the Earth was configured as one primary land mass called Pangea and a super ocean called Panthalassa.

 

During recent research in Antarctica, Becker and her team found meteoric fragments in a thin claystone "breccia" layer, pointing to an end-Permian event. The breccia contains the impact debris that resettled in a layer of sediment at end-Permian time. They also found "shocked quartz" in this area and in Australia. "Few Earthly circumstances have the power to disfigure quartz, even high temperatures and pressures deep inside the Earth's crust," explains Dr. Becker.

 

Quartz can be fractured by extreme volcanic activity, but only in one direction. Shocked quartz is fractured in several directions and is therefore believed to be a good tracer for the impact of a meteor. Becker discovered oil companies in the early 70's and 80's had drilled two cores into the Bedout structure in search of hydrocarbons. The cores sat untouched for decades. Becker and co-author Robert Poreda went to Australia to examine the cores held by the Geological Survey for Australia in Canberra. "The moment we saw the cores, we thought it looked like an impact breccia," Becker said. Becker's team found evidence of a melt layer formed by an impact in the cores.

 

In the paper, Becker documented how the Chicxulub cores were very similar to the Bedout cores. When the Australian cores were drilled, scientists did not know exactly what to look for in terms of evidence of impact craters. Co-author Mark Harrison, from the Australian National University in Canberra, determined a date on material obtained from one of the cores, which indicated an age close to the end-Permian era. While in Australia on a field trip and workshop about Bedout, funded by the NSF, co-author Kevin Pope found large shocked quartz grains in end-Permian sediments, which he thinks formed as a result of the Bedout impact. Seismic and gravity data on Bedout are also consistent with an impact crater.

 

The Bedout impact crater is also associated in time with extreme volcanism and the break-up of Pangea. "We think that mass extinctions may be defined by catastrophes like impact and volcanism occurring synchronously in time," Dr. Becker explains. "This is what happened 65 million years ago at Chicxulub but was largely dismissed by scientists as merely a coincidence. With the discovery of Bedout, I don't think we can call such catastrophes occurring together a coincidence anymore," Dr. Becker adds.

 

Becker's paper was originally published in Science Mag

 

http://www.sciencemag.org/cgi/content/abstract/1093925?ijkey=s6ut2p.yjJGq.&keytype=ref&siteid=sci

 

Site detailing Luann Becker's Antarctic Expedition

http://beckerantarctica.crustal.ucsb.edu/

 

 

http://www.astrobio.net/news/modules.php?op=modload&name=News&file=article&sid=969

[JohnB]

Ophiolite, the problem here is that Tunguska was not an impact. For your figures to work, the bolide would have to have run into the equivalent of the ground at 10 miles up. (Air pressure perhaps?) Also, in the case of impacts, there is usually a crater and the remains of the meteorite somewhere under the ground even after the liberation of energy.

 

Tunguska has neither of these things. There is no crater and virtually no debris. Hence my question about other occurrences.

 

As the majority of meteors do not make it to the surface, one would expect that air burst explosions would be relatively more common, yet they don't seem to be. ( This could be because the Tunguska bolide was of unusual composition and therefore exploded with unusual violence.)

 

I've read many theories about the cause of the blast, from the "Black Hole" one to the latest fringe "Ancient Planetary Defense" one. None of them seem to be able to fit all the known facts and witness observations without ignoring some of the factors.

 

This to me makes Tunguska a real anomaly and therefore extremely interesting.

 

Cochina, thanks for the links, I'm having a great read.

[Mokele]

one would expect that air burst explosions would be relatively more common, yet they don't seem to be.

 

Actually, from what I've read, they are common, but often occur so high in the atmosphere that we never see them due to the typically small size of the objects involved. Iirc, some government agency that actually watches stuff up there (NORAD, I think) was reported as seeing about 800 such events a year.

 

These are what mostly interests me, more than just hitting the ground, and were actually why I started the thread

 

Mokele

[coquina]

I played around with this site quite a bit:

 

http://www.lpl.arizona.edu/impacteffects/

 

Impact Effects

Robert Marcus, H. Jay Melosh, and Gareth Collins

Your Inputs:

Distance from Impact: 0.01 km = 0.01 miles

 

Projectile Diameter: 100.00 m = 328.00 ft = 0.06 miles

 

Projectile Density: 1500 kg/m3

 

Impact Velocity: 45.00 km/s = 27.95 miles/s

 

Impact Angle: 20 degrees

 

Target Density: 2500 kg/m3

 

Target Type: Sedimentary Rock

 

Energy:

Energy before atmospheric entry: 7.95 x 1017 Joules = 1.90 x 102 MegaTons TNT. The average interval between impacts of this size somewhere on Earth during the last 4 billion years is 6.3 x 103years

 

Atmospheric Entry:

The projectile begins to breakup at an altitude of 88100 meters = 289000 ft

The projectile bursts into a cloud of fragments at an altitude of 19800 meters = 64800 ft. The residual velocity of the projectile fragments after the burst is 16.9 km/s = 10.5 miles/s

 

The energy of the airburst is 6.83 x 1017 Joules = 1.63 x 102 MegaTons.

 

No crater is formed, although large fragments may strike the surface.

 

Major Global Changes:

The Earth is not strongly disturbed by the impact and loses negligible mass.

The impact does not make a noticeable change in the Earth's rotation period or the tilt of its axis.

The impact does not shift the Earth's orbit noticeably.

 

Air Blast:

 

What does this mean*

 

 

The air blast at this location would not be noticed. (The overpressure is less than 1 Pa)

 

*What is the air blast?

The energy due to the impact causes a distortion in the air. This distortion travels in the form of a wave. If the energy of the impact is very high, the wave may initially be a shock wave, travelling at a velocity greater than the speed of sound in air. The wave eventually decays into a sound wave travelling at 300 m/s (671 mph).

 

Peak overpressure is a measure of how much the pressure in the blast wave exceeds the atmospheric pressure of 105 Pa (1 bar). The air blast caused by the impact can cause a great deal of damage.

 

The damage due to the air blast may include the following:

Multistory wall-bearing buildings will collapse.

Multistory wall-bearing buildings will experience severe cracking and interior partitions will be blown down.

Wood frame buildings will almost completely collapse.

Interior partitions of wood frame buildings will be blown down. Roof will be severely damaged.

Multistory steel-framed office-type buildings will suffer extreme frame distortion, incipient collapse.

Highway truss bridges will collapse.

Highway truss bridges will suffer substantial distortion of bracing.

Highway girder bridges will collapse.

Cars and trucks will be largely displaced and grossly distorted and will require rebuilding before use.

Cars and trucks will be overturned and displaced, requiring major repairs.

Glass windows will shatter

About 30 percent of trees blown down; remainder have some branches and leaves blown off

Up to 90 percent of trees blown down; remainder stripped of branches and leaves.

 

I was not able to arrive at a combination that gave a damaging air blast, but I don't have all day to play with it either.

 

I do believe I heard an object explode in the atmosphere a couple of years ago. I was outside in the early morning when it was just daylight - there were no clouds in the sky. Suddenly, there was a very sharp explosion high in the air. I have heard the sonic booms from planes before, and this sounded similar, but not quite the same.

 

The explosion was very short and sharp. More like a very loud POP! It did not shake the ground. I did not see any smoke in the air. I talked to neighbors, people at Langley AFB, and the police department, and no one else heard it, or knew anything about it. It was so short and quick I doubt anyone inside would have taken much notice. But it definitely came from overhead - it couldn't have been a gunshot (I know what they sound like - people hunt around here all the time) or an automobile backfire.

 

I believe a small meteorite or a piece of space junk (deorbiting satellite) exploded in the atmosphere almost directly overhead, but I have no way to prove it.

[JohnB]

Thanks Coquina.

 

From that I would conclude that the Tunguska event was unusual only in that the actual detonation occurred low enough for the air blast to affect the surface.

 

While there may have been similar events in the past, if it was older than say 5,000 years, I dare say that there would be no evidence left of that fact. Trees rot and new forests cover the evidence.

[coquina]

John,

 

I don't know whether you've run across this site before, but I just found it and think it is quite interesting it is titled: 1908 Siberia Explosion, Reconstructing an Asteroid Impact from Eyewitness Accounts:

 

http://www.psi.edu/projects/siberia/siberia.html

 

The man made several paintings from various distances and locations based on eyewitness accounts. What I found particularly interesting was how close the paintings are to the descriptions at the impact calculator site.

[Mokele]

I found a tidbit in The Straight Dope (which I consider pretty reliable, given it's myth-busting theme):

 

"Between 1975 and 1992 military satellites detected 136 objects between 30 and 50 meters in diameter exploding in the upper atmosphere."

 

Mokele

[Ophiolite]

JohnB

I agree that Tunguska could seem anomalous. One my fabvourite explanations was that it was an alien spacecraft that was in trouble and heading for Lake Baikal (the largest fresh water volume on the planet) to refuel!!!! I've always felt that a combination of airblast and scant witnessess could likely explain away the anomalies. Coquina's two sites appear to confirm this.

Mokele

I have also read about the spy satellite sightings and seen a documentary on them. Are you sure you dont have their size out by an order of magnitude? That just looks like too many of that size - but then intuition is not the same as facts!

[coquina]

There is evidence of 2 blasts similar to Tunguska in South America during the 30's.

 

http://www.meteor.co.nz/feb96_2.html

 

The first occurred in the Rio Curacá area of Brazil on August 13, 1930.

 

The second occurred in the Rupununi area of British Guyana in 1935.

 

Here's more about them from a 1996 Science Frontiers article:

http://www.science-frontiers.com/sf103/sf103g08.htm

Target: south america

August 13, 1930. Upper reaches of the Brazilian Amazon. In SF#102, we provided a short notice of a probable large bolide impact near Brazil's border with Peru. Apparently, this event resembled the much more famous 1908 Tunguska blast. More details have now been provided by M.E. Bailey et al in the Observatory, as based on old accounts that appeared in the British Daily Herald and the papal newspaper L'Osservatore Romano. Bailey et al write:

 

 

"The Daily Herald report [March 6, 1931] describes the fall of 'three great meteors...[which]...fired and depopulated hundreds of miles of jungle...The fires continued uninterrupted for some months, depopulating a large area.' Unfortunately, although the fall is said to have occurred around "8 o'clock in the morning" and to have been preceded by remarkable atmospheric disturbances (a 'blood-red' Sun, an ear-piercing 'whistling' sound, and the fall of fine ash which covered trees and vegetation with a blanket of white), few details are provided that constrain the time and place of the event. Nevertheless, the story refers to an article in the papal newspaper L'Osservatore Romano [March 1, 1931], apparently written by a Catholic missionary 'Father Fidello, of Aviano', and it is to this that we now turn.

Apparently, there were three bolides or fireballs seen. Father Fidello wrote:

 

 

"They landed in the centre of the forest with a triple shock similar to the rumble of thunder and the splash of lightning. There were three distinct explosions, each stronger than the other, causing earth tremors like those of an earthquake. A very light rain of ash continued to fall for a few hours and the sun remained veiled till midday. The explosions of the bodies were heard hundreds of kilometres away." (Ref.1)

M.E. Bailey singles out two puzzling features of the Brazilian event: (1) the fall of dust before the fireballs were observed; and (2) the lack of any mention of a blast wave. Further, the L'Osservatore Romano account does not say anything about extensive forest fires. (Ref. 1; see Ref. 2 for a synopsis.)

 

Circa December 11, 1935. British Guiana (now Guyana). Only five years after the Brazilian event, a large bolide apparently smashed into the jungle of Guyana. Buried in the library stacks, we found a mostly forgotten trio of reports on the 1935 event in a 1939 issue of The Sky, predecessor of Sky & Telescope. The articles suggest that the devastated area "may equal or exceed that of the great Siberian meteor of 1908." The bolide and apparent impact area were observed by a gold prospector, a Dr. G. Davidson. Davidson testified:

 

 

"About 10:30 in the morning we climbed to the top of the mountain in order to get a panorama of the surrounding country. We could see some areas that had been swept down by some great force, trees twisted off some 25 feet above the ground. We tried to enter one of these areas but the bush was in such a tangle that we had to give it up." (Ref. 3)

 

 

From the same Science Frontiers source, there was also a strike in Brazil in 1995 that was not as large, but left a 16' crater:

 

1995. Northeastern Brazil.

 

"Scientists in Brazil's northeastern state of Piaui are baffled by a crater that was punched into the tropical rain forest shortly after witnesses reported seeing a bright light streak across the sky. Researchers are uncertain whether the crater, 16 feet wide and 32 feet deep, was left by a meteorite or a piece of a comet. Physicist Paulo Frota of the University of Piaui believes it was caused by a block of ice from a comet because the surrounding vegetation is not burned and the crater's rim is not raised." (Ref. 4

 

As has been previously stated, since so much of the earth's surface is covered by water, there must be more explosions there.

 

I wonder if some of the "rogue waves" mariners encounter aren't caused by these blasts?