A small lake in Siberia may be a secondary impact feature from the famous 1908 Tunguska event:

http://www.blackwell-synergy.com/doi/full/...2.x?cookieSet=1

Secondary impact crater? I always thought that there was no impact in the first place - being an "airburst" type of event several miles in altitude.

I know -- same here. And the physical effects found at the site all indicate an airburst-type explosion.

Now, one other airburst explosion that we know of has occurred since Tunguska, though it was a much, much smaller bolide that exploded. I'm referring to the Tagish Lake meteor. That meteor did scatter a lot of material on the ground (well, specifically on the surface of a frozen lake) below the airburst. So we have evidence that such airburst-style bolide impacts don't vaporize the entire impactor as they explode.

Perhaps the Tunguska impactor exploded in the air but some of the mass survived the airburst and struck the ground. That impact would have occurred along the ballistic trajectory of the impactor and not in the center of the airburst-generated blast zone, so it would make sense for it to be somewhat offset from the center of that blast zone. Which is supposedly what is being claimed here.

We just don't have a very good concept of how various types of impactors behave under extreme entry heating conditions. We know generally what can happen from looking at the few such events that have occurred on Earth during human history, and looking at the resulting impact effects of even older bolide strikes. But we don't know the exact composition of the various impactors that have been observed; we have a very poor idea of the composition of airburst impactors, especially. I can well imagine that impactors relatively enriched in volatiles would be more likely to explode before they hit the ground, but we don't know how such bodies are organized before they hit our atmosphere. So, if a large cometary fragment should strike us, it's possible that the volatiles would be irregularly distributed within the mass and while a portion of the mass might explode in the air, another portion might be less enriched in volatiles and thus continue on to a ground impact.

We just don't have enough data to be able to model that kind of thing. Yet.

-the other Doug

I couldn't open that original link so went looking and found this:
http://www.blackwell-synergy.com/doi/full/...21.2007.00742.x
Hope that helps others similarly stuck.

Seems like a good piece of science. They have evidence of a buried impactor which they want to drill into. Identification of the impactor would be quite impressive.

Floyd

Yeah, it's hard to give this a proper term. It's a 'secondary' as dvandorn said because it's thought to be an artifact after the main body exploded; this is also why it's displaced 5-10km NW of ground zero (all theoretical, of course).

It would be appropriate if the possible buried mass under the lake were publicly identified as asteroidal in origin on June 30 of next year (the 100th anniversary of the Tunguska explosion).

Some critique also in the BBC story .

The lake may not appear on pre-1929 maps, but surely the people living in the region would have been aware if a large lake suddenly and mysteriously appeared?

Isn't the whole point that the area was uninhabited?

Yeah, I would be cautious as well about the lake having been formed by the Tunguska event, it seems rather large, though it could be a cluster crater (maybe...) However, given its location, it might be good place to look for pieces of the impacting body, so the lake may still be worth investigating.

To me, the whole Tunguska story sure sounded sexier in the ending to The Fire Came By.

(I couldn't access either link, something about a 'spider trap hit' so forgive me if I state something that was already in the articles above)

The authors of a these short papers (two articles; freely accessible here) state that:

“Although the morphology of the lake is compatible with an impact origin, several sedimentological and biological proxies indicate that it’s formation pre-dates the 1908 event.”

(There is also a pretty diagram of Lake Cheko in the text).

-Mike

EDIT: Hmmm, the authors of the papers above are the same authors who wrote the recent Terra Nova article (trying to access the Terra Nova article from their home page still won't work for me). I wonder what changed their minds?

Space.com article out. (Sonar image of lake bottom at space.com site)


http://www.space.com/scienceastronomy/0706...ska_crater.html

-Mike

Strange, both links work for me. Just tested them. I have IE and cookies on.

The area wasn't uninhabited; sparsely inhabited, yes, but there were several eyewitness reports of the blast, and many of the eyewitnesses were interviewed when scientists came in to check out the site. If somebody had said "oh, by the way, there's this remarkable new lake" it would have been taken notice of; one of the scientific teams is supposed to have spent some time draining a lake that they thought might be a crater, so it wasn't exactly the kind of thing they were going to overlook.

I have to wonder just how close eyewitnesses could have been to the blast, though; doesn't seem like anyone in the immediate area would have survived! The paper does mention that the lake was not found on pre-1908 maps, although certainly this could just be due to the fact that it is a truly remote area & may have been simply overlooked.

Is there a limnologist in the house? Hopefully not displaying massive ignorance here, but it seems to me that this debate could be quite easily settled via sediment studies & perhaps radiocarbon dating of the organic components of different strata of same...?

The lake is 10 mile from the epicenter of destruction and in a swampy area that no one may have traveled through between the time of impact and early investigations. Our mind set comes from today's conditions with roads within a few miles of almost everywhere and human densities of 10 to 10,000 people/square mile. The investigators may have missed talking to the one or two trappers who were famaliar with the area and noticed that the lake was new. This was the Siberian wilderness with very few people wandering around.

The words of Gareth Collins in the article are funny. He says that the researchers have not provided any conclusive evidence that the lake is a consequence of the Tunguska event. In fact, I have not seen any such definitive sentence in the original article. I think the researchers have done a good speculative work, that will need to be verified, and at this stage they can only provide a tentative identification.

Presuming the lake pre-dates the impact and is not impact related, the lake *may* contain a better-than-previously-obtained sedimentary record of before, immediately after, and long-term after the impact.

Good, modern non-marine coring techniques may produce a really significant record related to the event, and with modern analytical methods, might yield information from more or less altered micro-residue from the impact.

Can someone explain what would make a meteor explode? As it enters the atmosphere it is being heated from the outside, so shouldn't it simply ablate itself away? Maybe if it were hollow and filled with water, I suppose the water might flash to steam and cause an explosion, but would even this account for a blast comparable to that of a nuclear weapon? I think it's unlikely. Most of that explosive energy must be kinetic energy, not depending on the chemical properties of the impactor but just on its mass, so what are the conditions necessary to liberate it in a high-altitude blast? How is it related to the angle and speed of the strike? If people saw the Tunguska object streaking across the sky, then it must have entered the atmosphere at a pretty shallow angle. But the conditions needed for explosively vaporizing a rock would seem to require a straight-down impact at very high speed so that hitting the atmosphere would most closely resemble hitting a solid object. Like jumping off a high enough bridge - the water might just as well be a pavement by the time you hit it. Anyway if there is an expert who can explain how these Hiroshima-like energies can be liberated in the atmosphere without impacting solid ground, I'd be curious to understand it better.
-Peter

The answer is sort of "all of the above." The factors that contribute to a bolide explosion include:

Angle of Attack: A shallow entry allows for a lot more heating time. A more direct trajectory straight towards the ground creates a higher heating pulse, but for a far shorter amount of time. On a steep-angle trajectory, the impactor reaches the ground very quickly after encountering the upper atmosphere and is usually still "frozen" in the middle when it strikes. But shallow-angle impactors have time to heat through far more effectively, even though the peak heat pulse is less.

Composition: It's likely that only bodies containing volatiles will actually explode dramatically in the air prior to impact. A large stony body will simply ablate as you suggest. No matter how long it is heated, the worst that will happen to such a stony body is that it will come apart at maximum heat load and/or aerodynamic stress. Such a break-up can look a little like an explosion, but the energy is all kinetic. If the impactor is a cometary fragment, however, with frozen volatiles within, those volatiles can heat up as the bolide travels through the atmosphere. Let's say a lot of the impactor is made up of methane ices and clathrates -- and in the lower atmosphere, the body begins to break up as hundreds of tons of now-flammable methane and other hydrocarbon products are released into a white-hot plasma trail surrounded by fire-feeding oxygen. It goes kablooey... As evidence, we have the Tagish Lake meteor fragments, which are some of the few examples of what (we think) is a cometary fragment that have been recovered (the pieces were actually fine-grained clays, not stone or metals like most meteorites). Not coincidentally, the Tagish Lake body exploded in mid-air.

Speed: This isn't as important of a factor, in that a shallow entry angle will usually slow a bolide to relatively slow speeds by the time it heats enough to explode. But a higher-energy entry will actually be less likely to cause an explosion in that it can reduce the heating time, as compared to the heating time endured by a slower impactor traveling along the same trajectory. A lot of this depends on whether the body's vacuum perigee is a positive or negative number at the time it hits the upper atmosphere.

Now, I'm not a meteorite expert, but I play one on the Internet... However, I think I have the basics correct, here.

-the other Doug

I believe that the kinetic energy of a meteoroid is in the range of several hundred times the chemical energy of an equal mass of high explosives.

The "explosion" of a large meteoroid, as I understand the process, would be the result of a relatively fragile cometary or unconsolidated asteroidal body, being hit with massive aerodynamic loads -- resulting in multiple gees of deceleration -- which cause the body to break up into smaller fragments, increasing the total surface area of the fragmenting body, which in turn causes even greater aerodynamic loads, in a positive feedback loop that results in almost all the huge kinetic energy being turned into heat in a very short time. Most of that heat would have been deposited into a sheath of air around the object, turning it into plasma -- and from there, the superheated plasma would expand in the usual fashion, in an explosion.

Bill

Any body that is not absolutely solid will disintegrate under the forces that it will be subjected to as it enters the atmosphere. As it disintegrates the surface area increases and that leads to increased conversion of kinetic energy to heat. This leads to further disintegration and so on. This effect is likely to happen very fast indeed if the basic makeup of the meteor is relatively weak (say a friable cometary surface or a bunch of conglomerate like that seen on Itokawa).

There is no real nead to invoke any chemical interaction between volatiles "burning" off the meteor and the atmosphere - all you need to do is convert the kinetic energy into heat to cause a very, very large explosion.

Explosive disintegrations of meteoric bodies in the upper atmosphere (30-40km) in the kiloton+ range are not uncommon so it is not a huge stretch to get a larger body (50-100m diameter iceball) to yield a couple of 10's of megatons at an altitude of 7-10km provided it comes in slow enough (say 20-30km/sec) and is made of the right stuff (ice ball).

Now it may not have happened that way but it's certainly plausible.

Ah beaten to it!

I don't think that the internal heat transfer idea into a solid body is all that practical. Thermal conductivity is a slow process even when the temperatures differential is in the 10's of thousands of degrees range - the heat pulse will only travel a few centimeters to maybe a few tens of cm in the 10-30 seconds that even a shallow entry trajectory allows. The dominant effects that is needed in order to get complete disintegration has to be caused by shock\dynamic pressure effects leading to physical breakup of the body.

BTW, is the Tunguska impactor still tentatively thought of as a fragment of P/Comet Encke? I remember buzz about this some years ago...orbital similarities, IIRC. If so, it would presumably have been quite rich in volatiles and hit with comparatively low relative velocity for a cometary body, which would therefore make it more likely to go kaboom in midair as Helvick described...

"Can someone explain what would make a meteor explode? ..."

The fundamental answer for larger objects is that as it rams at hypervelocity into the atmosphere, the dynamic pressure on the forward side increases till it's more than the crushing strength of the meteor's material. A chunk of nickel-iron has a very high crushing strength, solid rock (like a well metamorphosed chondrite or hard stony meteorite) will have some 10 times lower crushing strength, a carbonaceous chondrite like the Tagash Lake fireball will have some 10 times further less crushing strength (like a clod of garden dirt) and comet "fluff" will have much less strength than that.

Dr. Sekanina, at JPL some 15 years ago, did calculations for Tunguska and inferred from the probable trajectory and speed that it most likely had crushing strength of solid rock and could NOT have been cometary in origin. I think that conclusion remains valid.

Seems as if the estimated impact velocity places some significant constraints on the object's composition given the fact that it blew up real good, then. Is the impact speed known to any degree of certainty?

EDIT: After reading the eyewitness reports, it also seems that there were multiple 'events', at least in the sense that more than one kaboom was heard. To me, this argues for a shredded cometary fragment, and most of these sub-fragments blew up at high altitude yielding the 'artillery fire' comparison.

Topographic echoes?

Maybe; the surrounding area looks pretty flat, but there sure could have been some ground/air/shockwave/thermal boundary reflections.

While we're celebrating the 100th anniversary of this event, several magazines (Science, Sky_At_Night, etc...) bring articles in which they speculate that the remains of the asteroid (thought to be a piece of the comet Encke) are at the bottom of Lake Cheko, a lake north of the crash site probably formed when a meter-size fragment survived the airburst explosion and streaked into the ground!

I might speculate that a body entering the earth's atmosphere would not necessarily be 'hermetically' sealed around it's surface. Any crack or fissure in the body when on the leading face of the object will pressurize with super heated air from the oncoming slipstream. A fissure inside a body might have a surprisingly large surface area allowing the compressed entrained atmosphere to exert an enormous cleaving force from inside the body, and then fracturing it catastrophically when the tensile forces can no longer be resisted by the mechanical strength of the materials making up the incoming body.

This effect would seem operative even on seemingly tough materials like nickel/iron if the fractures were extensive.

Interesting way to 'blow up' something.

I personally knew one of the authors of the article (Giuseppe Longo) when I was studing at University of Bologna. I recall they were already making scientific expeditions to Tunguska, it was 17/18 years ago... Hope they will be able to recover impactor fragments in the lake (if any).
Here an elaboration of CG south view of Cheko, with improved contrast (reduced fog effect):
Click to view attachment

One question...

Was any material from the blasted body ever recovered (leave aside the supposed Cheko remnant). I mean, there must have been some dusty remnant of the thing.
It couldn't have just vanished

I'd always thought that some sort of residue had indeed been found many years ago, but seemingly not. This is the only reputable paper I googled on the subject.

Some "superb" Tunguska web resources:

http://www-th.bo.infn.it/tunguska/

http://news.bbc.co.uk/2/hi/science/nature/7470283.stm

the other Phil