[X] My Assistant

[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