At least three massive asteroids smashed into the Earth more than 3.2 billion years ago, and caused such destruction, they dramatically changed the structure and composition of the Earth's surface. This is according to new research from scientists at the Australian National University. The team uncovered evidence of major earthquakes, faulting, and volcanic eruptions that were so violent they dramatically changed the way the Earth's surface was forming. This happened during a period that the Moon also suffered heavy bombardment.

“Our findings are further evidence that the seismic aftershocks of these massive impacts resulted in the abrupt termination of an over 300 million years-long evolutionary stage dominated by basaltic volcanic activity and protracted accretion of granitic plutons,” Dr Glikson said.

“The precise coincidence of the faulting and igneous activity with the impact deposits, coupled with the sharp break between basaltic crust and continental formations, throws a new light on the role of asteroid impacts in terrestrial evolution,” Dr Glikson said.

http://www.universetoday.com/am/publish/ea...ids.html?582005

Could one of the geologist interpret this please.

"The Pilbara Craton

in Western Australia has a domainal architecture, which has been interpreted to reflect a history of accretion. The Tabba Tabba Shear Zone is the major division between the East and West Pilbara blocks: this is based on significant differences in the tectono-thermal histories of the bordering terranes.

New laser ablation U-Pb zircon geochronological data, coupled with trace element data for the same core parts of the sampled mineral grains indicate a range of magmatic crystallization ages for representative igneous rocks emplaced before, during or after shearing.

The Tabba Tabba Shear Zone currently forms the eastern bounding fault of the Mallina Basin. The last major activity in the structure occurred during a major phase of oblique sinistral movement, corresponding to closure of the Mallina Basin. Ages of late syn-kinematic granitic intrusions indicate that this occurred at about 2940 Ma.

These unique locations have kept the formations from experiencing the depths that metamorphose rocks, obscuring their original sedimentary and volcanic features. "

Weblink

You can imagine what Ceres and Vesta sized asteroids would do to the surface of the Earth: almost complete resurfacing of the upper crust. That was one heck of a bombardment that occured. Just looking at the Moon you can see the scars left behind. the Earth with its greater mass, would have attracted even more/larger asteroids.

Fascinating prospect!

Why not? With more and more accurate geological data all around the world, we may find more and more unexpected things, such as super large meteorite impacts. But remember the debate about the 64 mllion years transition: at first it was supposed volcanic, but more and more evidences accumulated for an impact. But there are still some points unexplained, and thus still scientists who think about a volcanic origin. So what about 3 billion years?

Remember that there was not one large impact on the Moon, but several, and as far as we know they were not simultaneous. If others are found on Earth, that makes an unusual shower of large bodies with about the same orbit than Earth (The 3 billion years episode seems not to have taken place on other planets).
An explanation was proposed some years ago, I do not remember by who, as what there was an other Earth's satellite which formed at the same time than Earth and Moon. But its orbit was unstable, and it broke in several parts, explaining the shower of large bodies on the Moon (and Earth).

If a body as large as Vesta hit the Earth, I think that all the surface would melt, making life disappear, and even all fossil-bearing layers too. So what happened at that time was not so tremendous, as, as far as we know, life already existed before.

But 3 billion years is a kind of limit: we do not know much older rocks, and thus much older traces of life. So the first billion and a half of the geological evolution of Earth crust and Earth life are still largely unknown. The reason why older rocks are hard to find is that continents were smaller at that time (continents formed progressivelly by accretion of granitic rocks, a process which is still going on). But perhaps most of these ancient rocks still exist deeply buried under the continents.

I thought that Hellas and the other southern basins on Mars were also Late Heavy Bombardment candidates, too, and that even the Northern Plains had been put up as (first) impact basin then (second) ocean?

A second Terrestrial satellite which conveniently scarred both Earth and Moon seems a bit much like special pleading!

I wonder whether the putative second satellite was more-or-less co-planar with the Moon, and if so then perhaps the spatial distribution of (Imbrium and before) basins might give us a clue to any supposed orbital characteristics (or not!).

These three impact layers have been known for quite a while from both Pilbarra in Australia and the Barberton mountain land in South Africa. The new thing is the suggestion that they had large scale tectonic effects. Personally I feel rather skeptic. I think impacts large enough to have large-scale (or even global) tectonic effects would have left much more dramatic traces. The impression one gets from these layers are that the impacts were certainly larger than Chicxulub, but not by orders of magnitude.

Incidentally there is quite a lot of rock older than 3 bya, there is some in nearly every precambrian shield. There is not much supracrustal rock though, which is probably the reason there are rather few traces of life. The "cutoff point" is 3.8 bya, from before which there is virtually nothing, except some detrital zircon crystals.

tty

Well, the copious basaltic outflow known as the Deccan Traps have been implicated with an impact - not local to India, either, but due to globally transmitted shockwaves (as seen on the Moon).

That's a nice theory killed by a set of brutal facts to cite H. L. Mencken. The Deccan eruptions started well before the Chicxulub impact as shown by the Maastrichtian biota found in the intertrappan beds.

There have been some interesting speculations that *very* large impacts might cause large scale melting and volcanism by removing the overburden and/or shock-melting large volumes of rock. It has been suggested that this might be the reason for the vast Siberian traps that erupted near the P-Tr boundary and for the huge mid-Cretaceous Ontong Java plateau in the Pacific. One would expect this to occur preferentially for ocean impacts since oceanic crust is much thinner than continental and more likely to be "punched through" by an impact. The mechanism as such seems quite reasonable but there is little concrete evidence yet.

tty

I bow to your superior knowledge regarding the Deccan Traps - but the antipode to the Caloris Basin on Mercury was, I seem to remember, discovered to have been well-stirred by shockwaves, so presumably we on Earth could also see not just local (local! ha!) volcanic effects from big impacts but global ones.

Thanks all for your precisions.




First the idea as what "something special" happened 3.2 billions years ago is the occurence of large (in the 1000kms) bassins on the Moon, which formed in a relatively short time. This simultaneousity hints at a special event, a group of large bodies with the same origin. This must not be confused with the initial bombing of all the planets at the origin of the solar system, which culminated 4.5 B years ago and decreased steadily since.

If the 3.2 B years event is not local to Earth, we may expect that Mare Caloris (Mercury) Hellas (Mars) and eventually others formed simultaneously. But we do not know the dates of these events.

But the fact that there is a group of large objects crashing on the Moon nearby in the same time makes that the idea of "A second Terrestrial satellite " not so "special pleading" as you say Bob Shaw. Of course other origins are possible, fo instance a large collision in the asteriod belt, and it is even more likely. (At the origin there was only some spherical asteroids, but numerous collisions occured which created the thousands angulous bodies we know today).

The original theory of the "second satellite of Earth" was in fact more complicated, it was rather a belt of debris: either an unique body never formed, or it broke appart. (Remember that at that time the Moon was much closer from Earth, so there was little place for the accretion of a third body, and matter rather formed a great number of small bodies, like today with the numerous little satellites at the rim of Saturn's ring). So when these bodies may have hit the Moon, due to the loss of mass, angular momentum rearanged the axis of rotation of the Moon, untill the next shock, so Bob Shaw, I think it would be rather difficult to infer orbital data of the impactors from the distribution of today marks.







Thank you tty for your precisions. The date you give (3.8 billion years) is what I was speaking about, but I did not remembered the figure. Perhaps this date is the date when the first continents began to form. Life seems much older, but this is known only from the evolution of DNA which has its own "clock" and we are not sure that this clock moves at a constant rate.






There was a discution about the effect of shockwaves on the opposite side of Mare Caloris, but these effects were purely tectonic, without melting of rock (But mountains were jumping and turned upside down like a pancake in its pan, if was rather better not to be here).




At last very large impact bassins like those we are discussing about (about 1000kms) could have formed in Earth past, but if they were into oceans, all the traces disappeared in 100 or 200 millions years, with the plate tectonics on the ocean floor. And 3.2 billions years ago, there was no large continents like today. So such impacts can be identified only by their ejecta (which may have covered the whole Earth). Are these ejecta basaltic/mantellic, or continental?





A last element is the evolution of life itself. It is now generally accepted that recent (less than Cambian time) impacts modeled the evolution of superior living beings. But 3.2 or 3.8 billons years ago, the skate of the evolution was rather setting the DNA code structure tself. Specialists have an hypothesis as what the genetic code is the result of an evolution in very ancient times. For instance there is a theory as what our three codons code evolved from a two codons code. So very ancien, but very large impacts may have driven the evolution of our DNA well before the appearance of our most ancient bacteria

Oooof record of the longest post to date!!!

I don't know, Richard -- I agree with Bob, I think it's a little bit of a stretch to posit a second terrestrial body as the source of the Heavy Bombardment impactors.

Instead, as has been discussed in the Jupiter board, it's more likely that Jupiter and Saturn moved into a gravitational resonance that disturbed "loose" bodies in the Asteroid Belt. Some of them were probably ejected from the Solar System altogether, but a lot came raining in towards the inner System, pelting the rocky planets with huge impacts. Even so, a lot of the bodies probably simply fell into the Sun, so a *lot* of bodies had to have been tumbled inward.

-the other Doug

The strange thing is that there seem to be no traces of large impact basins even in the oldest archean terranes. There have been a few suggestions (Central Hudson Bay, The Upland structure in Sweden, The Colorado plateau) but the evidence is slender at best. To me it looks as if the Late Heavy Bombardment more or less destroyed all evidence of older crust and that there hasn't been any really big impacts since then.

The only impact where there is strong evidence of global effects on life is Chicxulub which is also by far the greatest known impact during the Phanerozoic.
There is some evidence that there is a threshold at a crater diameter of ca 150 km, above which the energy deposited by fallback ejecta at the top of the atmosphere is sufficient to cause world-wide fires and mass-kill of unprotected land animals.



The Caloris impact was proportionally enormously bigger. The focused energy at the antipodes from Chicxulub is negligible in comparison. This is not the same as saying that the effects of the impact were negligible there. To the contrary the fallback ejecta is greatly concentrated near the antipodal point and India and Madagascar that were nearly antipodal position to Yucatan at 65 Mya were probably more or less sterilized.



That's not quite the way it works. When a major planet migrates its gravitational resonances migrates with it and remove smaller bodies from the space they move through since orbits become unstable near resonances ("resonance sweeping"). I don't think much of the material larger than dust actually hits the Sun. Most of the stuff that is deflected inward would be swept up fairly quickly by the inner planets hence the fairly brief "Late Heavy Bombardment" ca 3.8-3.9 Bya ago.

tty

Hum,
There is also the complication of `coincidence`; with trying to tie in impacts with volcanism.
For example, the chances that asteroid impacts and huge bouts of volcanism coincide randomly to cause mass extinctions, is greater than previously imagined.

I believe that UK researchers conducted statistical tests to determine the probability of such catastrophic events happening at the same time in Earth history.
And they found massive releases of lava and space collisions should have overlapped three times in the last 300 million years. They found the probability of this happening at least once over a period of 300 million years was 57%.
Once the researchers reduced the size of the impact slightly, the probabilities increased sharply.
For craters exceeding 100km, the probability of at least three co-occurrences between flood basalts and impacts was 46%. For craters exceeding 60km, the probability of three or more was 97%!

Although the dates of the proposed impacts are ten times older, the probability that the Pilbara impacts and the volcanic eruptions were coincidences are probably equally high.

But having said that, it would be fair to say that any impact will transfer a tremendous amount of energy to the Earth, which if it were still in the process of `forming` would lead to increased volcanism.

For me, the jury is still out on this one….

Now here's a coincidence worthy of Arthur Koestler: I attended the World Science-Fiction Convention in Glasgow last week, and bought my usual fix of hardish SF.

One such was Ian McDonald's highly acclaimed 'River of Gods', which I began reading this week. Imagine my surprise when, after posting here yesterday, I lay in bed reading a bit more before going to sleep, and the first words I glanced at on p429 of the Pocket Books paperback edition (you'd need a BIG pocket!) were: 'Earth had survived the Chixulub impact and the resulting Deccan melt on the other side of the planet at the cost of twenty-five percent of its species...'.

Now is that spooky, or what?

I do not believe too much in massive volcanic episodes following a large impact, such as the melting of the opposite side of the Earth (On Mercury, the opposite side of Mare Caloris did not melted, but id was severely shattered).

On medium impacts (Moon or Earth as well) we observe lava flows, but without roots, just the fallout of molten rocks in or around the crater. This is visible on Tycho, Moon, or in the Ries crater, Germany.

But what is possible anyway, on a planet with plate tectonics, is that a large impact may remove a part of the crust, and thus change the equilibrium of forces. This may result on the fast opening of distention fractures at a great distance of the original impact, such distention fractures producing large pouring of fluid basaltic lavas. But this is not what happened in Deccan. The Deccan traps started to form several millions years before Chixculub and they are the result of the emergence of a hot spot (diapir of hotter mantellic rock). This hotspot was also involved in the separation of India from Africa and it is now still active, although much weaker, resulting in a volcano in the french island "la reunion", Indian Ocean.

In the case of a very large impact, there may form distention fracturation all around, resulting in the formation of an ocean floor of basalt.


If extra large impacts happened into the 3.8-3.2 Billion years time, they had most chances of hitting oceanic plates, which are now absorbed from long ago into the depths of Earth mantle. Only small islands of continental plates existed at this time, so if a large impact occured right on one, it may have been completelly destroyed. If the margin of a crater intersected a continental plate, this margin was further extended in a way similar to continental margins when today an ocean opens into a continent. In both cases, no traces of the crater itself remain, only ejecta blankets or traces of far fallout. On the other hand, such fallout are visible everywhere (Chixculub's far fallout are findable all around the Earth). So infering large or extra-large craters from ejecta blankets or fallout layers is consistent. Just more accurate studies are needed to find the magnitude of the impact, and perhaps other data such as where the blanket rocks came from. (two Chixculub's rocks layers were dated and identified all around the globe).


But I shall abstain of very positive statements about corellations of volcanism with other phenomenon. There are large scale cycles in Earth magnetic field (>200 Myears) which are not yet completelly understood and which also resulted into variations of volcanism. And recently it was postulated a volcanic cycle of 25000 years in corellation with a climatic cycle!

In volcanism, there are things we can say we understand, and others we cannot.

Richard:

I think the Chixculub/Deccan Traps link always *was* rather speculative, although interesting. The facts-as-they-are-known appear not to support the idea, so let's leave it to the SF writers!

A comment made earlier, however, about the antipodal clustering of secondary impacts remains intriguing. Anyone know the figures for velocities of ejecta from big whacks? And, as Tycho's rays stretch so far round the Moon, is there any evidence for secondaries around it's antipode?

Bob Shaw

I do not see any reason so that secondary impact may gather in the antipodes. But I cannot say definitively no, this would deserve a study, for instance about Tycho. But the antipodes of Tycho are out of reach of amateurs telescopes, they are in the databases of images... Where?



A common phenomena is that there are often secondary impactors. For instance the 20kms Ries crater in Germany has a secondary 1km crater with the same date. The Chixculub too has a secondary in the USA, although there is no evidence it was simultaneous. There is a Moon crater (Copernic, I guess, but I am not sure) surrounded by a hundred of small craters all around, as if the main impactor was followed by a swarm of debris.

I've been trying to track down a paper I wrote for an impact cratering class in grad school on this topic, antipodal effects of major impacts. I'm having no luck finding the paper, but dredging my memory, I think I remember a couple of interesting factoids: computer modeling of this question shows that there could indeed be huge shock pressures at the exact antipode of a major impact. And, in fact, now that there is very good coverage of the Earth with seismographs, there are now some actual data supporting the idea, seismographic records showing unexpectedly high shock pressures at the precise antipode to very large earthquakes. However, the key thing in the models and in the data is that the focusing really does take place only within less than one degree of the antipode. There are a lot of proposed pairings -- Deccan traps is just the famous one -- of impact sites with antipodal volcanism, but few of them can satisfy that criterion of being exactly antipodal, even when you take continental drift into account. And, as Mr. Trigaux pointed out, the physical mechanisms for how high shock pressures could cause a long-lasting volcanic episode are poorly defined.

As for where the ejecta falls, there are concentrations having to do with antipodal focusing, but the Coriolis effect also does some strange stuff, making really interesting patterns in where the ejecta falls, depending on the latitude the initial impact took place. Hopefully a little deeper trenching into my files will eventually recover the papers on this topic that I know I spent hours in the library photocopying!

But as far as I can tell, antipodal effects aren't what the Australian National University researchers were talking about. Basically they say that a bunch of impacts 3.2 billion years ago caused the Earth's volcano-tectonic style as a whole to change from a place where there was mostly ultramafic volcanism (which would look more like the Moon, Venus, and Mercury than it does now) to a place where you had continents (totally different rock composition) and plate tectonic activity. But I can't figure out whether or not they have any mechanism in mind by which major impacts could have caused that change, which makes it hard to assess the quality of the paper (which has been submitted for publication, but not reviewed or accepted, again as far as I can tell). And all the impact experts I know have been in Rio for a week at the IAU Symposium on Asteroids, Comets, and Meteors, where I am sure they have been poring over the Deep Impact results -- or just enjoying Rio -- and not paying attention to the claims of upstart Australian researchers!

Emily

Actually this is a well-known and well-documented phenomenon, and actually rather obvious if you consider the trajectories. For more details see e. g.

http://www.lpi.usra.edu/meetings/lpsc97/pdf/1252.PDF

http://www.lpi.usra.edu/meetings/lpsc2001/pdf/1447.pdf

However I'm not sure how it would work on the Moon where the escape velocity is only 2.3 kms-1. Most of the high-energy ejecta would probably escape altogether or be greatly perturbed by the Earth. Mars would be a better place to look.

tty

Although the Moon's escape velocity is low, it also means that the impact velocities could be lower, too... ...so perhaps the mechanism would still work. No Coriolis, though, not with a rotation period that's so long!

The Moon rotated a heck of a lot faster 3.8 billion years ago than it does now. It was also a heck of a lot closer to the Earth. And it may well not have become tidally locked with Earth yet, so it may well have rotated faster than it revolved around its primary.

-the other Doug

other Doug:

True, but... ...Tycho was formed just a few hundred million years ago, as were the other ray craters. That's why the rays are still visible - they've not been 'gardened' into the ground.

Bob Shaw

Tycho was created something like 109 million years ago, according to clues from the Apollo 17 landing site.

But yeah, I was thinking a lot more of the basin-forming impacts, since that's the size of impact that is being discussed by the authors of the article that triggered this thread. Tycho was a large impact, but nowhere near the size of a basin-forming impact. However, I suppose it would be more analagous to the Chixculub event than a basin-forming impact would have been.

And yes, by the time of the Tycho impact, the Moon was close to where it is now, and was therefore taking several weeks to rotate on its axis. My bad.

-the other Doug

Thanks elakdawalla for your detailed discution on antipodal focusing of shock waves in the antipodes of a large impact.


I would like to add that:
The today antipodes of Chixculub is not Deccan, but Coco islands, south of Sumatra. and 64 MILLIONS YEARS AGO Deccan was about the today place of Madagascar, and Chixculub was at rough guess the place where is today Porto Rico (if you take Africa as a fixed reference). It is 120° appart, not 180°, so it was really far from antipodal!!

So in geological past events we must alway REASON WITHIN HE EPOCH CONTEXT!!!!! And the idea of Deccan antipodal volcanism remains the strict domain of (undocumented) scifi, or worse of archaeology fiction, a genre of litterature that scientists do not like very much.

True antipodal effect is reliably documented in only one case: Mare Caloris on Mercury. It has two distinctive features:
-It is a very large impact, although smaller than Moon bassins
-It is on Mercury.

Mercury is the densest planet, with a huge iron core. So focusing could occur only with the geometry of inner Mercury, and not on other planets. For instance on Mercury the iron core would act as a magnifying glass, focusing the waves on a point which happens to be on the surface. Similar phenomenon could take place on Earth, but the focusing point (if one) would be into the depth of the mantle, or in outer space.

At a pinch, this special geometry of Mercury worked only at the epoch of Mare Caloris, and knowing this epoch and the today size of the Mercury iron core, would perhaps allow to infer the diametre of the liquid and solid part of Mercury core at this epoch.

The last chance to have an antipodal effect on Earth would be considering surface waves, an usual component of seismic waves. Theoretically they focuse in the antipodal point, but they propagation is not homogenous, and thus they would be scattered by continents and geological features before reaching the antipodes. So noticeable effect would occur only with very large impacts, and not so accuratelly as 1° of the antipodes.

Hi all,


about the three fallout layers in the 3.2-3.8 Billion years which started this thread, of course they could have been formed by huge bassin-forming impacts on Earth itself, impacts of which few or no trace remain today.

But they could also be FALLOUT FROM THE MOON IMPACTS. As a matter of facts these impacts may have ejected from the Moon huge ammounts of matter, which, once passed the Lagrange point, could easily fall on Earth, very much in the way where, in binary star systems, matter from one star can fall on the other once passed the Lagrange point. This may be checked with isotopic analysis of these fallout layers, which may reveal their Earth or Moon origin.

Anyway if there was a swarm of very large impacts on the Moon at this epoch, Earth surely received its share, unless we have a very local origin for the impactors, such as a third body ot debris belt into the Earth/Moon system.

May the Earth functionning have changed at a given epoch? Likely, as there is this "cutoff point at 3.8 Billion years, where we start to find continental rocks. So likely continents started to gather only at this epoch (and may be still gathering matter today).

Specialists of the Earth early stages dated (with radioactive clocks) the separation of the iron core from the rocky mantle at only some millions yeras after accretion. This suppose that the overal temperature was hotter than today, to allow for iron to sink in a soft mantle. (Today large items such as crustal pieces seem able to sink down to the core in several tens of million years, but I think the mantle today is too rigid to allow the un-mixing and gathering of billions of small iron pebbles lef by accretion).

This estimate of Earth temperature tends to say that plate tectonics could have started at once. Dit it really started some millions years after accretion? Or is water necessary for the working of plate tectonics, as some theoritians stated (looking at Venus, which is hot enough but has no plate tectonics).

So plate tectonics may have started only when there was enough water available on the Earth surface, (from volcanoes or comets?) somewhere between 4.4 and 3.8 billion years. Incidentally life may have started at a similar date, when there was enough standing water with enough amino-acids (from cometary origin? or formed in the atmosphere by UV and Lightning?)

But it was surely operating 3.8 billion years ago, as it began to create lighter rocks (from the granite series), first some islands, then terranes which agregated to form the today continents.


So all this explain that plate tectonics may have started at a given epoch, but does not let any place for impacts, even very large, to start it. Plate tectonics is driven by cold chunks of crust falling into the mantle (it is why it may need water to work, as only deep water is able to cool the crust fast enough) and this cannot be changed by impacts: even impacts creating 2000 kms wide bassins are only pin holes into a planet. So bassin impacts may not have started plate tectonic, only they could influence it locally by removing pieces of crust.


Things may have hapened int he same way on early Venus, with a plate tectonic forming the 'continental" highlands we see today, and eventually forming life. But when the atmosphere became too thick, greenhouse effect inflated, water was absorbed into the clouds, and plate tectonics stopped. If this theory is true, from the size of venus "continents", we can guess that plate tectonics operated in a rough guess 1 billion years (until roughly 3 billions years ago). And life may have evolved into bacteria, of which rare fossils may still remain today into the highlands. Test: are these highlands containing granite or sedimentary rocks? If yes, the theory is true. The thrill would be to find coal or similar materials...

I've been reading up on earlier studies of the impact layers from the Barberton Mountain Greenstone Belt (BGB) in South Africa. A number of interesting points emerge:

1. To judge from the thickness of the spherule beds these were big impacts. Not Imbrium-size, but definitely Clavius-size and perhaps even Crisium-size.

2. The composition of the spherules suggests impact on oceanic crust in all four layers (ergo, little if any hope to find any trace of the craters).

3. No coarse ejecta, only spherules, so the impacts must have been at least a couple of thousand kilometers away.

4. There is evidence of tsunami activity in three cases suggesting more-or-less open water between the impact sites and BGB (at the time both Pilbarra and Barberton Mountains are thought to have been part of Ur, the first major continent we have evidence of, and there might not have been any other major landmasses). This of course also excludes a lunar origin for the spherules.

5. The isotope data from the spherules are best compatible with a carbonaceous chondrite composition for the impactors

6. The coincidence of the largest impacts (S2-3) and a shift from mafic/ultramafic (Onverwacht series) to felsic (Fig Tree series) volcanism and a change in tectonic style has been noted before and a causal connection suggested, so the announcement that started this thread is actually not a new idea.

tty

It is true that the antipodal point at 65.5 Ma BP was well east of India. However when considering impacting secondaries the minimum transit time to the antipodal point is about an hour during which time Earth rotates 15 degrees eastwards. So the antipodal focusing of secondaries starts east of, but not very far from, India and then sweeps west across India, Madagascar and Africa.

If You want to check it out have a look at:

http://pgap.uchicago.edu/KMApgeoglarge.html


tty

Tty:

With a shorter day, I'd expect the rotational effect to be slightly greater - and certainly by the time corals etc were being laid down there's ample evidence for that shorter (about 2/3rds, as I recall) rotation period (though obviously by 65.5 Ma BP it must have been close enough to present values). In the period of the LHB (and subsequent) presumably the effect was even larger!

Now, let's factor in the presence of a big fat Moon in a much lower orbit...

Bob Shaw

15°east, not the 40° required to reach India in its Madagascar-position at this epoch (after your map). And I am not sure that earthquake waves are submitted to Coriolis force, as they do not move in free space but relative to the rock, they have a fixed speed relative to the rock and rotate with it.

When I read that second paper I kept scratching my head at the graph showing power deposition in the atmosphere at the antipode....1 HOUR?!! The impact ejecta was travelling at >20,000 Km/h ?? Wow.

Good work, with such evidences it is now fairly sure that three or four large impacts occured during this period (around 3.2 billion yeas) and that they were on Earth, not on the Moon, but simultaneous to the ultra-large impacts which created Moon bassins.

As for the volcanic modification, they may be local, and not a consequence of the impacts. And it was not the starting of the plate tectonics, as it already started before to create the Ur continent (Today south Africa and Australy). Local variations in lava composition or volcanic styles are common, all the history of geology is made of alternances of tectonic styles so there is no reason to link this one specially to the impacts.


But, on the Moon, there was also a surge of high lava activity following the bassins creations. It is also tempting to see here a consequence of the bassin formations, but I also heard that the volcanism surge at this epoch was a consequence of the inner magma core of the moon cooling and finishing to solidify, a phenomenon which is very generally the cause of emission of lavas (maturated lavas, at least basalt and preferably acid lavas).

What would be interesting is a stratigraphy of the fallout layers. I suppose they did not occured in the same time, but in a time lapse of millions of years.


At last the carbonaceous chondrite composition of the impactor excludes a third body in the Earth-Moon system. It also excludes the asteroid belt as the source of the impactors. (Collision between asteroids, change in the orbit of the great planets modifying resonnances...)

So only remain large comets or Pluton-like bodies. But why a surge of such bodies at this precise epoch???? Two possible explanatons:
1) It is at this epoch that may took place the modifications of orbits between Neptune-Pluton-Charon Triton, eventually with a collision
2) At this epoch the solar system may have been in a dense zone of the galaxy, with frequent close approaches with other suns, resulting in a massive emptying of the Oort cloud. This situation may result from the movement of the Sun into the Galaxy, or from the dynamics itself of our Galaxy (massive star formation event, or absorption of another galaxy...)


And comet-like bodies may also have brought much water, together with carbon. There was water before, but this comet shower may have improved conditions for life on Earth.


And we can guess this simply in looking ar dirt on the ground?? Wouaoouuuw these geologists are really...

S1 >3445, <3475 Mya
S2 ca 3256 Mya
S3 slightly less than 3243 Mya
S4 not dated but only 8 meters above S3, so probably just a few million years younger

tty

Thanks tty for the dates.

What would be interesting now is if somebody has the dates for the Moon large bassins. It is roughly the epoch they were lava filled, but I do not know when they formed.

These dates are in a relatively short span of time, suggesting a special event. But do we have a complete series of geological layers in the 3.8 - 3 Gyears ? It could happen that such large impacts were common at this epoch, but we do not notice them as we do not have the complete series, only a sample. After all, 4 ejecta layers in 200 Myears, it is not much more than today. Only the thickness of those layers would point at much larger events than the -64 Myeras Chixculub and the -225 Myears event.

If I remember well, into the South Africa green rocks belt, there is also a large impact. I have seen this recently in a science review.

OK, I finally dug up that paper I wrote for professor Peter Schultz's cratering class a couple of years ago on antipodal focusing of seismic energy from impacts; download it here.

Also, here's a more recent LPSC abstract by Pete and one of his other students that shows some really interesting patterns that would have resulted from specific Earth impacts (Chesapeake, Popigai, and Manicougan):

THE EFFECT OF ROTATION ON THE DEPOSITION OF TERRESTRIAL IMPACT EJECTA, K.E.
Wrobel and P.H. Schultz, 2003.

--Emily

Right, thank you Emily for this work related to our discution here.

It result of it that I was false in saying than antipodal effect was observed only on Mercury with Mare Caloris, as it was also observed on the Moon and several icy satellites of Jupiter and Saturn.

Also, contrarily to what I wrote, there can be antipodal focusing of seismic waves on Earth, and it was even actually observed.

This work going on is interesting, as it allows to infer the inner structures of bodies from the presence of absence of antipodal terrain effects (eventually focusing can be an annulus and not a spot).

Although you deem this unconclusive, you discuss the major effect of wave focusing of very large impacts on Earth. Direct surface effects are not observed, or were destroyed with time; but also you discuss the heating of mantellic rocks by focused waves to form hot spots, especially symetrical hot spots (one under the impact, one antipodal). The search for symmetrical pairs was not very conclusive (perhaps because such pairs formed only billions years ago, and are now disappeared).

So, contrarily to what I wrote, the effect of a very large impact on a planet is not exactly a pin hole; it can induce volcanism at a great distance. Did this occured on Earth in the 3.8-3.2 Gyears period?

At last a last point: the focusing point is not necessarily on the surface; it can be in the depths of the mantle. There can even be several chained points, or caustic hyperboloides. Hot spots such the ones we see today are expected to have roots down to the bottom of the mantle. Finding a hotspot with no root will suggest a focusing of seismic waves by a very large impact, especially if the calculus of the wave "optical" propagation shows that the focusing is at this depth.

I recall that about 50 hot spots are known today on Earth, that their upward movement seems independent of the general convection pattern of plate tectonics, and that they are supposed to form from a very thin layer of hotter mantle directly in contact with the core, by bulging and upward move of a mushroom shaped bubble of hotter rock. The arrival of a "head" on the surface generally produces large lava flow (Deccan, Siberia...) but the tail can remain several hundred Myears to form isolated vocanic regions. This sheme seems also working on Venus which has no plate tectonics.

They are thought to be mostly 3.8-4.0 bya old, though the lava filling is much younger



There are indeed few good sedimentary sections that old. They have mostly been eroded long ago. Pilbarra is really a quite remarkably old and stable area. There are even places in Pilbarra where there are still traces of glacial morphology from the Carboniferous-Permian glacial ages some 300 mya.



That's the ones we have been talking about! They are from the Barberton Mountais Greenstone Belt.

tty

Emily:

Thanks for that! .PDFs are a joy!

SW England is famous for Tin mines; I wonder whether the pools and outflows below the strange white spoil tips would be worth looking at for Manicougan material...

Bob Shaw

Emily:

Fascinating stuff - any more items like that which you'd like to casually drop into our discussions, please feel free!

Bob Shaw

Yes, Emily's article was very interesting, and it settled many points in a discussion which otherwise was dying.




Thank you again for the dates tty!

So the Earth impact series we are speaking about (Pilbara-Barbenton) is much younger: 3.4 to 3.2 Gyears counter 3.8-4.0 Gyears for impacts bassins on the Moon. So we cannot be sure it is the same event. Anyway such large impacts as Barbenton mountains were common at that times, as we can see on most Earth-like planets. And the apparent grouping of these impacts would result only of the fact that it is a sample limited in time.

Remains that strange series of extra-large impacts which formed the Moon bassins. We cannot apply to them your previous results as what they were formed by carbon chondrites (comets - Kuyper belt objects) so all the previous conclusions are void. There was indeed some special cause which acted to form a series of extra-large impacts on the Moon, and seemingly nowhere else. Was Earth hit by the same series of bodies? Difficult to say, as we have no remains of this epoch. Perhaps it is such impacts which formed the "cutoff point" in destroying most of the continents already formed, but we shall perhaps never know.


I though again to that theorie as what hot spots were formed by impacts. This is not proven, and hot spots could form from other reasons, they are diapirs of hotter mantle rocks really able to grow upward by themselves, there are even some which seem young (The islandic hot spot seems 200-250 Myeears, the Deccan -La Reunion seems 70 Myears, the "french" hotspot responsible of volcanism in the Massif Central would be 200 Myears old, and its peak activity only 50Myears old, etc...) and anyway weakened hot spots channels may be twisted and blown away by the overall convection movement, until new diapirs dig new channels right upwards. But the idea of hot spots being remnants of very old very large impacts is somewhat fascinating...

The impact basins range from 3.55 to 4.2 billion years old, if memory serves, with Imbrium being the youngest of the large basins (even younger, apparently, than Orientale). Not all basins are mare-filled -- especially those on the far side. But of those that do have a mare fill, the basalts we've sampled are on the order of 3.1 to 3.8 billion years old (though there is some support for the concept of areas of western Procellarum being slightly less than a billion years old, and the lavas at the Surveyor 1 site may be less than two billion years old).

In general, though, I wouldn't call the mare basalts "much younger" than the basins themselves -- in most cases, the maria are less than a half-billion years younger than the basins they occupy. Over a 4.5-billion-year lifespan, that makes both of them similarly ancient -- nothing on the Moon can really be fairly called "young," I think. As of about three billion years ago, the Moon had undergone about 98% of all of the major activity it would ever see.

They've dated the basins directly by dating samples of the impact melt created by several basin-forming impacts. I believe they are pretty certain about having dated impact melts from the Imbrium, Serenatatis and Nectaris basins, and have tentatively dated others (such as the 4.2-billion-year-old South Pole-Aitkin Basin) from a few isolated samples returned from Apollo 16, plus analysis of crater counts and degradation of basin structures. IIRC, Imbrium is dated at about 3.55 billion years old, Serenatatis in the 3.8 billion range, and Nectaris being a little older than both, in the 3.9 billion range. (I'm speaking entirely from memory here, but I think I'm close...)

I recall that the lavas in Imbrium were apparently erupted over a period of around 300 million years, beginning hundreds of millions of years after the basin was formed. This effectively killed the theory that lunar lava eruptions were triggered by the basin-forming impacts, though the lava may well have seeped to the surface through cracks formed by basin-forming impacts. It just took hundreds of millions of years for the lavas to start to flow.

While all of the visible maria are no older than about 3.8 billion years, there are pieces of basalt found as clasts in some of the highland breccias that have been dated as old as 4.2 billion years -- so some of the basin-forming impacts must have wiped out very old maria that simply no longer exist.

-the other Doug

Interesting new data on the Late Heavy Bombardment:

http://www.spaceref.com/news/viewpr.html?pid=17817


tty

Elakdawalla: "paper I wrote for professor Peter Schultz's cratering class"

Did you see Pete's reaction during the Deep Impact mission coverage as he got his first look at the hirez cam's pic of the impact plume?

He stands there gaping and puts both hands to his face in a totally classic "Oh My God!" sort of reaction.

Peak moment of his entire career! Pete's a great guy.

COSMIC COMPONENT DISCOVERED IN BEDOUT BRECCIA

Luann Becker et al., Lunar and Planetary Science XXXVII (2006)

http://www.lpi.usra.edu/meetings/lpsc2006/pdf/2321.pdf

ET Extraterrestrial Chromium at the Graphite Peak P/Tr boundary and in the
Bedout Impact Melt Breccia

Luann Becker 1), Alex Shukolyukov 2), Chris Macassic 2), Guenter Lugmair 2), and
Robert
Poreda 3)

1) University of California Santa Barbara, Dept. of Geology, Santa Barbara, CA,
93106
lbecker@crustal.ucsb.edu
2) Scripps Institution of Oceanography, University of California, San Diego, San
Diego
CA, 92093;
3) School of Earth and Environmental Sciences, University of Rochester,
Rochester New
York 14627

Introduction: Any major impact structure should include an extraterrestrial
chemical signature such as platinum group elements (PGEs). The concentration of
iridium (Ir) and other noble metals in K/T boundary sediments worldwide was key
to the interpretation that an impact (asteroidal or cometary) occurred 65 myr
ago (1,2). For instance, some researchers have argued
that excess Ir and noble metals can be explained by enhanced volcanic activity
(3). Extensive volcanism could provide a transport of mantle-derived metals
that, like meteorites, have high concentrations of noble metals. However, the
discovery of the Chicxulub crater, coincident with the K/T boundary suggests an
ET source for Ir and noble gas metals in K/T sediments worldwide.

Several isotopic systems have also been used to search for an ET signature in
K/T boundary sediments (e.g. osmium), the most diagnostic being the chromium
(Cr) isotopic systematics (i.e. unlike osmium, Cr isotope values cannot be
confused with terrestrial signatures;(4). Isotopic compositions of Cr in several
K/T boundary sediments indicate an ET signature that is consistent with a
carbonaceous-type impactor. Thus, chromium isotopes not only make a good ET
signature, but it can also serve as a diagnostic tool for determining the type
of impactor that collided with the Earth. This method has also been applied to
Archaen impact deposits, impact melt samples and Late Eocene deposits (5).

Graphite Results: We have now measured the Cr isotopes in some of the isolated
magnetic fractions (MF) found in the Graphite Peak P/Tr boundary. Our group and
others have reported on the detection of Fe-Ni-Si-rich metal grains and impact
spherules that accompany the meteorite fragments in the Graphite Peak P/Tr
boundary section (6,7). We studied the Cr isotopic composition in the bulk
magnetic fraction (MF) for Graphite peak (8). The concentrations of major and
minor elements in the bulk MF are surprisingly similar to chondritic, with the
exception of Ca. The isotopic data in Table 1 are presented in epsilon (?)
units, where 1? is 1 part in 10^4 and terrestrial ratios of 53Cr/52Cr are
defined as ? = 0. For high precision, in our method of data reduction we use a
'second order' mass fractionation correction based on the 54Cr/52Cr ratio (9).
This correction assumes no excess or deficit of 54Cr, which is the case for most
meteorite classes. Carbonaceous chondrites, however, have excess 54Cr causing
second order corrected ?(53) values to be negative. This is a convenient and
precise way to distinguish carbonaceous chondrites from the other meteorite
classes. Bulk MF reveals a clearly non-terrestrial Cr isotopic signature:
?(53)corr= -0.13 ±0.04? and falls outside the range of previously studied
carbonaceous chondrites ( -0.3 - to -0.4?). In other words, this isotopic
signature has never been measured before and cannot be attributed to
contamination. The most striking feature is the presence of a large excess of
54Cr in the MF residue: ?(54)raw= +8.10±0.78? (the subscript 'raw' designates
that the second order fractionation correction has not been applied). 54Cr
excesses of a comparable magnitude have been reported in the acid resistant
residues of CI and CM chondrites. It is important to note, however, that the
?(54)raw in the MF residue is intermediate between values measured in the Ivuna
(+13.2±0.20?) and Murchison (+5.35±0.29?) meteorites.

Prelimary results on Bedout: We have also evaluated the chromium isotopic
compositions in the
Bedout impact melt breccia. Previous investigations of the Yax-1 and Yucatan-6
cores have indicated only slightly elevated levels of chromium and iridium
despite the elevated levels found in some K/T boundary sediments (10,11). This
may be due to the nature of the samples (e.g. bulk powders containing an
abundance of crustal material that would greatly dilute the ET signature). In
order to concentrate a potential cosmic component for the Bedout breccia, we
applied a differential dissolution. A 10-gram sample of Bedout breccia was first
treated with HF. The residue was additionally treated with an HF/HNO3 mixture at
room temperature. This dissolution procedure left behind a minute (a few ?g)
acid-resistant residue enriched in Cr. This residue was dissolved in an HF/HNO3
mixture at 180°C in a bomb. The Bedout residue revealed an extraterrestrial Cr
isotopic composition. The corrected (see above) 53Cr/54Cr ratio is ~ -0.25?.

More measurements are underway to confirm this result, however, it appears that
a cosmic component has been detected in the Bedout breccia. This Bedout value
differs slightly from the Graphite Peak value, probably due to our method of
concentrating the Crbearing component in the acid-resistant residue, which is
enriched in a meteoritic chromite-spinel phase. The apparent deficit of 53Cr in
the Bedout breccia implies a carbonaceous chondrite projectile and is consistent
with the data obtained earlier for the Graphite Peak P/Tr sediments. If these
data are confirmed then the previous measurements of the Graphite P/Tr sediments
can be directly linked to the Bedout structure.

=============
(2) THE BEDOUT STRUCTURE

Wikipedia

http://en.wikipedia.org/wiki/Bedout

Bedout or Bedout High, (pronounced "Bedoo") is about 25 km off the northwestern
coast of Australia in the Roebuck basin. It is a large circular depression in
the ocean basin approximately 200 km across, with a central uplift that is a
distinguishing feature of impact craters (see Chesapeake Bay impact crater for
comparison). It was noted in 1996 by Australian geologist John Gorter of Agip in
currently submerged continental crust off the northwestern shore of Australia.
The geology of the area of continental shelf dates to the end of the Permian.

Some scientists speculate that Bedout might be the result of a large bolide
impact event that occurred around 250 million years ago; a large impact event
during that time frame, incurring other factors, could account for the
Permian-Triassic extinction event. Geologist Luann Becker, of the University of
California, found shocked quartz and brecciated mudstones and other
mineralogical evidence of impact conditions at the site [1]. Several
Permian-Triassic boundary sites have produced evidence of impact material prior
to the Bedout discovery: shocked quartz from sites in Antarctica and Australia,
glassy spherules at sites in China and Japan, fullerenes with evidence of
extra-terrestrial gases in P-Tr sites in Japan and southern China (Becker et al,
2001).

Sediment samples appear to match the date of the extinction event. 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 added in a news release [2].

Significant erosion has affected the structure, and differences in subsidence
have tilted it. Skeptics contend that the shape of the depression is
inconsistent with bolide impacts; instead, the depression might be explained by
other scenarios, such as an oddity in the earth's structure. In addition,
iridium anomalies, a feature associated with other massive bolide impacts, have
not been found. Continuing research could yield more clues in the years to come.

===============
(3) THE GREAT DYING

Science@Nasa, 28 January 2002

http://science.nasa.gov/headlines/y2002/28jan_extinction.htm

250 million years ago something unknown wiped out most life on our planet. Now
scientists are finding buried clues to the mystery inside tiny capsules of
cosmic gas.

Some perpetrator -- or perpetrators -- committed murder on a scale unequaled in
the history of the world. They left few clues to their identity, and they buried
all the evidence under layers and layers of earth.

The case has gone unsolved for years -- 250 million years, that is.

But now the pieces are starting to come together, thanks to a team of
NASA-funded sleuths who have found the "fingerprints" of the villain, or at
least of one of the accomplices

The terrible event had been lost in the amnesia of time for eons. It was only
recently that paleontologists, like hikers stumbling upon an unmarked grave in
the woods, noticed a startling pattern in the fossil record: Below a certain
point in the accumulated layers of earth, the rock shows signs of an ancient
world teeming with life. In more recent layers just above that point, signs of
life all but vanish.

Somehow, most of the life on Earth perished in a brief moment of geologic time
roughly 250 million years ago. Scientists call it the Permian-Triassic
extinction or "the Great Dying" -- not to be confused with the better-known
Cretaceous-Tertiary extinction that signaled the end of the dinosaurs 65 million
years ago. Whatever happened during the Permian-Triassic period was much worse:
No class of life was spared from the devastation. Trees, plants, lizards,
proto-mammals, insects, fish, mollusks, and microbes -- all were nearly wiped
out. Roughly 9 in 10 marine species and 7 in 10 land species vanished. Life on
our planet almost came to an end.

Scientists have suggested many possible causes for the Great Dying: severe
volcanism, a nearby supernova, environmental changes wrought by the formation of
a super-continent, the devastating impact of a large asteroid -- or some
combination of these. Proving which theory is correct has been difficult. The
trail has grown cold over the last quarter billion years; much of the evidence
has been destroyed.

"These rocks have been through a lot, geologically speaking, and a lot of times
they don't preserve the (extinction) boundary very well," says Luann Becker, a
geologist at the University of California, Santa Barbara. Indeed, there are few
250 million-year-old rocks left on Earth. Most have been recycled by our
planet's tectonic activity.

Undaunted, Becker led a NASA-funded science team to sites in Hungary, Japan and
China where such rocks still exist and have been exposed. There they found
telltale signs of a collision between our planet and an asteroid 6 to 12 km
across -- in other words, as big or bigger than Mt. Everest.

Many paleontologists have been skeptical of the theory that an asteroid caused
the extinction. Early studies of the fossil record suggested that the die-out
happened gradually over millions of years -- not suddenly like an impact event.
But as their methods for dating the disappearance of species has improved,
estimates of its duration have shrunk from millions of years to between 8,000
and 100,000 years. That's a blink of the eye in geological terms.

"I think paleontologists are now coming full circle and leading the way, saying
that the extinction was extremely abrupt," Becker notes. "Life vanished quickly
on the scale of geologic time, and it takes something catastrophic to do that."

Such evidence is merely circumstantial -- it doesn't actually prove anything.
Becker's evidence, however, is more direct and persuasive:

Deep inside Permian-Triassic rocks, Becker's team found soccer ball-shaped
molecules called "fullerenes" (or "buckyballs") with traces of helium and argon
gas trapped inside. The fullerenes held an unusual number of 3He and 36Ar atoms
-- isotopes that are more common in space than on Earth. Something, like a comet
or an asteroid, must have brought the fullerenes to our planet.

Becker's team had previously found such gas-bearing buckyballs in rock layers
associated with two known impact events: the 65 million-year-old
Cretaceous-Tertiary impact and the 1.8 billion-year-old Sudbury impact crater in
Ontario, Canada. They also found fullerenes containing similar gases in some
meteorites. Taken together, these clues make a compelling case that a space rock
struck the Earth at the time of the Great Dying.

But was an asteroid the killer, or merely an accomplice?

Many scientists believe that life was already struggling when the putative space
rock arrived. Our planet was in the throes of severe volcanism. In a region that
is now called Siberia, 1.5 million cubic kilometers of lava flowed from an
awesome fissure in the crust. (For comparison, Mt. St. Helens unleashed about
one cubic kilometer of lava in 1980.) Such an eruption would have scorched vast
expanses of land, clouded the atmosphere with dust, and released
climate-altering greenhouse gases.

World geography was also changing then. Plate tectonics pushed the continents
together to form the super-continent Pangea and the super-ocean Panthalassa.
Weather patterns and ocean currents shifted, many coastlines and their shallow
marine ecosystems vanished, sea levels dropped.

"If life suddenly has all these different things happen to it," Becker says,
"and then you slam it with a rock the size of Mt. Everest -- boy! That's just
really bad luck."

Was the "crime" then merely an accident? Perhaps so. Nevertheless, it's wise to
identify the suspects -- an ongoing process -- before it happens again.

Editor's note: Becker's colleagues include Robert Poreda and Andrew Hunt from
the University of Rochester, NY; Ted Bunch of the NASA Ames Research Center; and
Michael Rampino of New York University and NASA's Goddard Institute of Space
Sciences. Funding for the research was provided by NASA's Astrobiology and
Cosmochemistry programs and the National Science Foundation.

==============
(4) MASS EXTINCTION IMPACTS MAY HAVE SPREAD MICROBIAL LIFE TO OTHER WORLDS

BBC News Online, 18 March 2006

http://news.bbc.co.uk/1/hi/sci/tech/4819370.stm

By Paul Rincon
BBC News science reporter, Houston , Texas

Terrestrial rocks blown into space by asteroid impacts on Earth could have taken
life to Saturn's moon Titan, scientists have announced.

Earth microbes in these meteorites could have seeded the organic-rich world with
life, researchers believe.

They think the impact on Earth that killed off the dinosaurs could have ejected
enough material for some to reach far-off moons such as Titan.

Details were unveiled at a major science conference in Houston, US.

The theory of panspermia holds that life on planets like Earth and Mars was
seeded from space, perhaps hitching a ride on meteorites and comets.

To get terrestrial, life-bearing rocks to escape the Earth's atmosphere and
reach space requires an impact by an asteroid or comet between 10 and 50km
across. Only a handful of recorded strikes in geological history fit the bill.

Million-year journey

One of them is the asteroid strike 65 million years ago, which punched a crater
between 160 and 240km wide in what is today the Yucatan Peninsula, Mexico.

Brett Gladman, from the University of British Columbia (UBC) in Vancouver, and
colleagues calculated that about 600 million fragments from such an impact would
escape from Earth into an orbit around the Sun.

Some of these would have escape velocities such that they could get to Jupiter
and Saturn in roughly a million years.

Using computer models, they plotted the behaviour of these fragments once they
were in orbit. From this, they calculated the expected number that would hit
certain moons of Jupiter and Saturn.

The principal targets they chose, Titan and Europa, are of considerable interest
to astrobiologists, the community of researchers who study the origin of life on
Earth and its implications for the habitability of other planetary bodies.

Titan is rich in organic compounds, which provide a potential energy source for
primitive life forms, Europa is thought to harbour a liquid water ocean under
its thick crust of ice.

Hitting at speed

Dr Gladman's team calculated that up to 20 terrestrial rocks from a large impact
on Earth would reach Titan. These would strike Titan's upper atmosphere at 10-15
km/s. At this velocity, the cruise down to the surface might be comfortable
enough for microbes to survive the journey.

But the news was more bleak for Europa. By contrast with the handful that hit
Titan, about 100 terrestrial meteoroids hit the icy moon.

But Jupiter's gravity boosts their speed such that they strike Europa's surface
at an average 25 km/s, with some hitting at 40 km/s. Dr Gladman said other
scientists had investigated the survival of amino acids hitting a planetary
surface at this speed and they were "not good".

"It's frustrating if you're a microbe that's been wandering the Universe for a
million years to then die striking the surface of Europa," Dr Gladman mused.

Asked after his presentation by one scientist whether he thought microbes would
be able to survive Titan's freezing temperatures, Dr Gladman answered: "That's
for you people to decide, I'm just the pizza delivery boy."

The UBC researcher gave his presentation at the astrobiology session held at the
Lunar and Planetary Science Conference in Houston, Texas.

Copyright 2006, BBC

Info: a meteorite which hit the Earth in the 19th century, known as the Dhurmsala meteorite (Dharamsala) was COLD when hitting the ground. More exactly the fragments of this light grey stone chondrite were cold, so cold that they dumbed the fingers and gathered frost still half an hour after fall.

This seemingly unexplainable event was even quoted by Charles Fort. But it can be explained very well, if we consider that the original rock was cold from space (equilibrium temperatures can be far below zero for clear bodies in space near the Earth). Then it entered the atmosphere, a very short event which set its surface burning and melting, but don't allowed the heat to reach the core. Then the rock exploded from this unbalanced heating, and the fragments had no time to heat again.

This may be relatively frequent, and eventually if there are microbes aboard, they may reach the ground safely, provided that the target world has an atmosphere (even Mars would do). But on airless worlds like Europa, the rock entirely turns to a ball of fire...

Review: Cosmic Collisions
---

The universe is replete with gigantic collisions, from impacting
asteroids to merging galaxies. Craig Remillard reviews a new movie
showing at the American Museum of Natural History that helps people
better visualize some of the violent aspects of the cosmos.

http://www.thespacereview.com/article/600/1

New Insight into Earth’s Early Bombardment

Researchers examined about 50 different melted rock samples collected by astronauts during the Apollo missions in the late 1960s and 1970s. Using radiometric dating techniques, they found that all but a few of the rocks were between 3.8 and 4 billion years old. Earth itself is about 4.5 billion years old.

The central conclusion:

Furthermore, many of the samples displayed different chemical "fingerprints," which suggests that they were formed from different meteorites and lunar rocks.

"The evidence is clear that there was repeated bombardment by meteorites," said study team member Robert Duncan from Oregon State University.

The paradox of meteorites:

Another intriguing possibility, say Duncan and others, is that rather than being vehicles of death and destruction, meteorites carried life, or molecules important for the emergence of life, to Earth.


Rodolfo

Big Bang In Antarctica: Killer Crater Found Under Ice

Planetary scientists have found evidence of a meteor impact much larger and earlier than the one that killed the dinosaurs -- an impact that they believe caused the biggest mass extinction in Earth's history.
An ancient mega-catastrophe paved the way for the dinosaurs and spawned the Australian continent, new research suggests.
The 300-mile-wide crater lies hidden more than a mile beneath the East Antarctic Ice Sheet. And the gravity measurements that reveal its existence suggest that it could date back about 250 million years -- the time of the Permian-Triassic extinction, when almost all animal life on Earth died out.

http://www.sciencedaily.com/releases/2006/...60601174729.htm