[X] My Assistant

[deglr6328]

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.....
tty

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.

[Guest_Richard Trigaux_*]

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

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...

[tty]

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.

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

[Guest_Richard Trigaux_*]

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.

[elakdawalla]

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

[Guest_Richard Trigaux_*]

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.


--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.

[tty]

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.

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

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.

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.

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.

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

tty

[Bob Shaw]

...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

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

[Bob Shaw]

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.

--Emily

Emily:

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

Bob Shaw

[Guest_Richard Trigaux_*]

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

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

tty

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...

[dvandorn]

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

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

[tty]

Interesting new data on the Late Heavy Bombardment:

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


tty

[edstrick]

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.

[ljk4-1]

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

[Guest_Richard Trigaux_*]

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...