[X] My Assistant

[Bob Shaw]

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

[Guest_Richard Trigaux_*]

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.

[elakdawalla]

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!

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

[tty]

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?

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

[Bob Shaw]

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!

[dvandorn]

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

[Bob Shaw]

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

[dvandorn]

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

[Guest_Richard Trigaux_*]

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.

[Guest_Richard Trigaux_*]

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.

[Guest_Richard Trigaux_*]

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

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...

[tty]

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

[tty]

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.

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

[Bob Shaw]

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.

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

[Guest_Richard Trigaux_*]

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

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.