[X] My Assistant

[Guest_BruceMoomaw_*]

...in Antarctica. This was the biggest mass extinction event in the history of multicellular life -- it wiped out something like 95% of all existing marine species as opposed to only about 50% for the impact that ended the dinosaurs -- and it has been one of the biggest remaining geological mystery stories. An even bigger giant impact has always been a serious suspect -- but because no crater could be found, other suspects have also been high on the list, ranging from the titanic flood volcanic outburst of the "Siberian Traps" region to a huge buildup of CO2 in the deep sea that was then suddenly released due to geological events (a planet-wide version of the Lake Nyos CO2 eruption that smothered hundreds of people in a matter of seconds in Cameroon). Indeed, there has been a recent comprehensive book on the mystery which ended by concluding that the mystery is still utterly unsolved.

Well, we seem finally to have found a crater that is both gigantic enough, and of just the right age, to be the culprit -- in the only place on Earth it could have hidden for so long: beneath the Antarctic ice layer.

http://www.space.com/scienceastronomy/060601_big_crater.html

It isn't quite settled yet, but this looks like an impact crater fully 500 km wide -- more than twice as wide as Chiczulub -- and thus quite big enough to have done the job, if it really IS an impact crater.

[NMRguy]

What evidence has been provided that would suggest that this large round-shaped structure should lie at the Permian-Triassic (P-T) boundary? Granted, the P-T extinction was a momentous event in the history of life on this planet. But has any evidence been cited other than a giant ringed structure that resembles a crater has been found under half a mile of Antarctic ice?

More specifically, has any sediment been found anywhere in the strata of the Permian-Triassic boundary (about 251 million years ago) that would suggest extraterrestrial activity? My understanding is no. Antarctica shared borders with Australia, India, and southern Africa in the Permian time, and these lands are not buried under thick layers of polar ice. For a collision of this magnitude, one might expect shocked quartz or perhaps rare earth metals (eg. iridium) to be at least as common as that of the Cretaceous-Tertiary boundary. I just haven’t seen that evidence show up in the rocks.

[Guest_BruceMoomaw_*]

The lack of iridium might be explained if the impactor was a comet nucleus rather than an asteroid, but I'll admit that the lack of shocked quartz is suspicious. Obviously we'll have to wait and see more.

[tty]

There is some limited evidence of an impact layer at the P/Tr boundary. The best evidence is actually from Antarctica (Graphite Peak), see e. g.

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

http://www.lpi.usra.edu/meetings/lpsc2004/pdf/1875.pdf

tty

[Guest_Richard Trigaux_*]

There was already the Bedout crater associated with the permian extinction. It is as big as Chixculub, and, depending on geochemical/biological/ecological conditions, it could have caused much more extinctions than the Chixculub, for instance if, in the epoch, there was less biodiversity.

The newly discovered circular structure in the Antarctic is a good candidate too, provided there will be drills into it to really assess its meteoritic origin.

By the way, 500kms large craters are rare into the solar system, and they all are ancient (3.3 -3.7 billions years for the Moon. I strongly suspect that the bodies which created them belong to a special class of large meteorites, which resulted from some event in the solar sytem into this time. Most of large craters are caused by comets, which seem to have a sharp size limit.

We also are lucky to still see it. since 250 millions years, all the oceanic crust was absorbed into Earth, that makes about only 30% of the surface of that epoch which is still observable.

[NMRguy]

My position was not to be overly pessimistic, but I find it misleading when the headline of a news story is "Antarctic crater linked to big die-off". Blame it on media sensationalism, I guess. But these scientists now have a great hypothesis, and they must now go dig up evidence (literally) to prove their position. Greenland and Antarctica are areas of the Earth that still have secrets locked away under all of that ice.

[ljk4-1]

By the way, 500kms large craters are rare into the solar system, and they all are ancient (3.3 -3.7 billions years for the Moon. I strongly suspect that the bodies which created them belong to a special class of large meteorites, which resulted from some event in the solar sytem into this time. Most of large craters are caused by comets, which seem to have a sharp size limit.

Maybe the body that made this crater - assuming all this is true - was one of
the last of the great planetesimals that first formed Earth? It just got delayed
for whatever reason. But ouch for the creatures living then!

Or maybe it was a protoplanet offset by the Jovian worlds migrating?

[tty]

Please note that there is absolutely no evidence linking this (probable) crater to the P/Tr boundary. In my opinion it may well be too large to explain the P/Tr extinction. An impact of that size would probably kill almost everything except some taxa living at great depth in the oceans or deep cave systems.

tty

[Guest_Richard Trigaux_*]

My position was not to be overly pessimistic, but I find it misleading when the headline of a news story is "Antarctic crater linked to big die-off". Blame it on media sensationalism, I guess. But these scientists now have a great hypothesis, and they must now go dig up evidence (literally) to prove their position. Greenland and Antarctica are areas of the Earth that still have secrets locked away under all of that ice.

I somewhat agree. If we look at the images in the article, we' see that the structure is VERY ROUGHLY circular. Many other circles of similar diametre also appear on the gravimetric map, as the poor resolution create a blur of this size around any punctual object. So it is a bit fast to claim that it is the big new hypothesis. With such a aproximate identification, the Hudson bay in Canada, or the northern plain of Italy, would be candidates too.

[dvandorn]

...With such a aproximate identification, the Hudson bay in Canada, or the northern plain of Italy, would be candidates too.

I thought shocked rock beds had been found radial to Hudson Bay, indicating that it is, indeed, an ancient impact crater.

Is my memory failing me again?

-the other Doug

[edstrick]

Hudson's bay is an "epicontinental sea", a cold version of the many shallow seas that have flooded over parts of continents during much of earth's history. Not an impact.

[blobrana]

Hum,
i too seem to recall that it was once speculated that the Bay was formed by an impact.
However it may be that they hadn't found the Chesapeake bay impact structure.

(35 million years ago, a meteorite smashed a 83 kilometre hole in Earth's surface.)

But i agree that it is just speculation as to what the gravity anomaly is. If it is an impact crater then it could date from 545 million years ago, (an upper limit of 100 million years is dictated by a tectonic fault line that cuts through the feature). Personally i think it`s an old caldera.

BTW Another intriguing `crater` worth looking at in Canada (with say google earth) is at
Position: Latitude 76.628177° Longitude -109.037557°

[Guest_BruceMoomaw_*]

I believe the references are not to Hudson Bay as a whole, but to that uncannily neat semicircular bite out of its southwestern shore -- which really does look, at least to the untrained eye, like a possible impact crater.

[Bob Shaw]

I believe the references are not to Hudson Bay as a whole, but to that uncannily neat semicircular bite out of its southwestern shore -- which really does look, at least to the untrained eye, like a possible impact crater.

Bruce:

Yup. Canada is well-served by the Crater Gods! The Laurentian Shield has some very ancient craters on it, too - and Sudbury's metal ores are assumed to be associated with an impact. I suspect, though, that gravimetric surveys are of less value there than in many other parts of the world, due to the large and homogenous lump of continent which forms the landscape.

Bob Shaw

[tty]

Bruce:

Yup. Canada is well-served by the Crater Gods! The Laurentian Shield has some very ancient craters on it, too - and Sudbury's metal ores are assumed to be associated with an impact. I suspect, though, that gravimetric surveys are of less value there than in many other parts of the world, due to the large and homogenous lump of continent which forms the landscape.

Bob Shaw

I once happened to fly across Ungava in spring when the snow had melted but the lakes were still ice-covered and noted at least four perfectly circular lakes that looked mighty suspicious crater-wise.

Incidentally the Sudbury crater is particularly interesting since it is a counter-example to the often repeated "fact" that a meteor impact cannot cause volcanism.


tty

[Bob Shaw]

I once happened to fly across Ungava in spring when the snow had melted but the lakes were still ice-covered and noted at least four perfectly circular lakes that looked mighty suspicious crater-wise.

Incidentally the Sudbury crater is particularly interesting since it is a counter-example to the often repeated "fact" that a meteor impact cannot cause volcanism.
tty

Apart from pseudo-vulcanism, impacts could, on Mars, play a major part in creating life-friendly oases. A really good whack takes a helluva time to cool down to ambient (tens of thousands of years, in some cases IIRC) and breaks up the subsurface layers allowing percolation of groundwater etc.

I wonder if THEMIS can see such hot spots (assuming any currently exist)?

Bob Shaw

[Guest_BruceMoomaw_*]

Looking for hydrothermal warm spots was one of THEMIS' prime goals from the start, and they've been doing so VERY thoroughly, using its nighttime IR images. Unfortunately, not a trace of one has been discovered, and the feeling is now that -- if MRO's super-detailed inspection using HiRISE and CRISM fails to turn up evidence of either current or fossil hydrothermal spots -- that particular proposed "Pathway" for the possible design of the Mars program beyond 2013 will be dropped.

[Phil Stooke]

I don't pay much attention to terrestrial impacts, but my limited understanding was that the big bite out of Hudson Bay showed no evidence at all of being the result of an impact. The impact suggestion was only based on shape, but nothing else was found there.

Phil

[Guest_Richard Trigaux_*]

I don't pay much attention to terrestrial impacts, but my limited understanding was that the big bite out of Hudson Bay showed no evidence at all of being the result of an impact. The impact suggestion was only based on shape, but nothing else was found there.

Phil

In my post 9 I quoted the Hudson bay as an exemple of circular feature which was NOT an impact. I did not expected this discution about it (that some had supposed that it was one, based on fuzzy evidences, such as the presence "all around" of crater ejecta, but caused by another smaller impact).

[edstrick]

Bruce commented: "...believe the references are not to Hudson Bay as a whole, but to that uncannily neat semicircular bite out of its southwestern shore ..."

I don't know the name of the bite out of Hudson's bay's southeast (not southwest), but the archipelago of islands within the bite are the Belcher Islands. I did a quick google on it and found a nice summary

http://www.nationmaster.com/encyclopedia/Belcher-Islands

with a link to a nice picture of the rather spectacular fold-belt of the islands. These and I suspect materials under the "bite" are later precambrian sedimentary or metamorphosed sedimentary rocks superimposed on older precambran metamorphic rocks of the Canadian shield. There's a link to a geologic summary of a big chunk of the Canadian north for those that can stand a concentrated dose of "geologese" geek speek.

[silylene]

In my post 9 I quoted the Hudson bay as an exemple of circular feature which was NOT an impact. I did not expected this discution about it (that some had supposed that it was one, based on fuzzy evidences, such as the presence "all around" of crater ejecta, but caused by another smaller impact).

Another point of view:

Hudson Bay Could Hold Giant Crater
Larry O'Hanlon, Discovery News


Oct. 25, 2006 — Lessons being learned from massive impact craters on Earth and beyond could help settle the question of whether such a crater exists in the eastern Hudson Bay.

Any map of Hudson Bay shows a suspicious, semi-circular coastline in northwestern Quebec. The Belcher Islands, just to the west of that curved coast, might be the remnants of a central peak — a common feature of impact craters on the moon and elsewhere.

What's more, past work has suggested that when recent rifting of the land near the crater site is accounted for, even more of the circle — about two-thirds — would be visible.



But while the idea of a 280-mile-round (450-km diameter) Hudson Bay crater has been around for 40 years, there has been no conclusive evidence of its existence.

If such a crater exists, it would be the largest known on Earth. The current record-holder is the 112-mile-wide Chicxulub Crater on Mexico's Yucatan Peninsula.

Until recently, no one has been quite sure what evidence would point to the existence of a crater so big, or so old, in this case at least half a billion years.

Any telltale minerals for such a big, old crater would be scarce or at least hard to find, explained geologist Michael Brookfield of Guelph University. What's more, the region's remote location and harsh climate make it a difficult research site.

But recent studies of impact craters on Earth and other planets are helping zero in on what to look for in Hudson Bay — so much so that he visited the area on a recent "on the cheap" research excursion.

Brookfield summarized the trip and his ideas for further exploration in a poster presented this week at the Geological Society of America meeting in Philadelphia.

The crater may have multiple concentric rings, said Brookfield, as is the case in the recently discovered Vredefort Crater of South Africa, the famous Chicxulub Crater of the Yucatan, and on large craters on the moon and elsewhere in the solar system.

If so, there could be evidence in the form of minerals that have been altered by sudden, violent fault movements. There also ought to be jumbles of older, broken rocks called breccia.

Finding evidence of such rings would no doubt bolster the theory that an impact crater exists in Hudson Bay. But just what causes an impact crater to have multiple rings is not exactly clear, said Jay Melosh, a crater researcher at the University of Arizona's Lunar and Planetary Laboratory.



"The moon has multi-ring craters and Mercury has none," observed Melosh. The reason may be that the moon has a more rigid crust with a weak zone underneath.

A multi-ringed crater on Earth would indicate that the impact was felt as deep as the upper mantle — the softer rock zone below the crust. The moon's ringed craters could be from billions of years ago when the moon still had a hot, soft interior.

Jupiter's icy moons also have an abundance of multi-ringed craters and are suspected to have soft interiors under hard crusts, said Melosh.

As for the chances that Hudson Bay holds the largest impact crater on Earth, Melosh said the definitive evidence would be "shocked" minerals that prove extreme pressures, far beyond what any volcano can produce. But without such evidence, he said, it's far-fetched to suspect a crater in Hudson Bay.

Still, "it wouldn't be surprising if there were large impact craters on Earth that we don't know about," said Melosh. That's because the moon, right next door, retains many large craters because it has no weather or plate tectonics to erase them. Based on the moon, said Melosh, Earth should have 6,000 craters larger than 300 miles (500 km) in diameter.

"There's got to be some evidence of the big ones on Earth," said Melosh. "But so far there's no proof."

http://dsc.discovery.com/news/2006/10/25/h...=20061025170030

[nprev]

Hmm. Just for fun, check out this map of Alaska with particular attention to the Seward Peninsula at 9 o'clock (look south of the peninsula on the mainland coast). If this map is accurate, that is a VERY pronounced and fairly large semicircular feature. However, I have no idea how old the landmass is; certainly it must be younger than the Canadian Shield.

Alaska Map