[meteorite-list] Article - It Came from Outer Space?

[ken newton]

http://www.americanscientist.org/template/AssetDetail/assetid/37224;_QqdrDyc#37436


American Scientist Online
SCIENCE OBSERVER  November-December 2004
It Came from Outer Space?
David Schneider

In 1981, I was a beginning graduate student taking a course in field 
geology at UC Berkeley. This was only a year after Berkeley physicist 
Luis Alvarez, his geologist son Walter and two colleagues published what 
was then a startling (and not-much-believed) theory suggesting that the 
impact of an asteroid or comet caused, among other things, the 
extinction of the dinosaurs. If these Berkeley luminaries could offer up 
such patently absurd ideas, the students figured we were entitled to do 
the same. So whenever our professor queried us about some puzzling 
geologic structure, we had a ready response: "Must have been an 
asteroid." Alvarez's theory ultimately triumphed, and appreciation of 
the importance of impact events grew enormously within the geological 
community. Now earth scientists are far more ready to accept the 
validity of extraterrestrial influences. But a recent episode suggests 
that the pendulum might have swung too far.

In 2002, Jens Ormö, Angelo P. Rossi and Goro Komatsu, working at the 
International Research School of Planetary Sciences in Pescara, Italy, 
reported evidence for what they claimed was a relatively recent 
meteorite strike: a field of craters located in the Abruzzi Apennines, 
roughly 100 kilometers east of Rome. The largest feature of the field is 
a 100-meter-diameter circular basin, situated in the Prato del Sirente 
plain, close to the town of Secinaro. Associated with the main basin are 
17 nearly circular depressions, which presumably formed at the same time 
because the extraterrestrial object responsible for them broke up in the 
atmosphere just before hitting.

Ormö's team was unable to locate any definitive markers of an impact, 
such as meteoritic material emplaced below a crater or telltale grains 
of shocked quartz in the target rock. But these signs could be missing 
for good reason: Quartz is almost absent from the limestone-rich 
sediments found in the area, and perhaps the group's 4.5-meter-deep 
excavation of one of the craters had been too shallow to reach the 
meteorite they believed to be buried below. Ormö and his colleagues did 
find one line of evidence that they found very compelling—curious 
magnetic anomalies associated with many of the smaller craters, which 
they interpreted to mean that remnants of meteorites (which are quite 
often highly magnetic) were indeed buried there.

In 2003, Ormö and his two coworkers, joined by Roberto Santilli, used 
radiocarbon dating to argue that the meteorite that formed this crater 
field might have done more than just that, publishing their ideas in the 
journal Antiquity. Their finding that the impact took place in the 4th 
or 5th century A.D. fit well with a locally preserved legend that 
describes people seeing a star falling to earth, an event that was 
seemingly important to their conversion from paganism to Christianity. 
These authors also proposed ties with the conversion of Emperor 
Constantine himself, which took place at very roughly the same time and 
place and was said to have been preceded by notable celestial phenomena.

Not surprisingly, this intriguing story garnered the attention of the 
popular press. For example, last year New Scientist published a piece 
entitled "Crater find backs falling star legend." It seems the glib 
answer I gave to my geology professor a quarter-century ago had become 
mainstream.

My graduate student career was not long enough to see this shift in the 
attitude of the scientific community through, but it was long enough to 
introduce me to Pierre Rochette, a French rock magnetist who later 
became a close friend. So I was quite interested to learn that earlier 
this year he and two Italian colleagues, Fabio Speranza and Leonardo 
Sagnotti, published a challenge to the notion that the circular 
depressions on the Sirente plain are impact craters at all, much less 
ones that have anything to do with Constantine's conversion to 
Christianity. (I should note that, having personal connections with one 
of the players in this debate, I harbored some bias toward his position 
from the outset.)

Rochette, who normally works out of the University of Aix-Marseille, 
became interested in the topic while on a sabbatical at the Istituto 
Nazionale di Geofisica e Volcanologia in Rome, where he discovered that 
one of his new colleagues was very skeptical of the crater theory. 
Speranza, a structural geologist, explains the source of his initial 
doubt: "I have a house about 10 kilometers away. I've known this place 
since I was a child," adding, "I knew that the landscape of Abruzzi was 
full of similar shapes." Could they all be impact craters? Surely not, 
he thought.

Speranza points out another difficulty with the impact-origins theory. 
Large blocks of limestone sit within the boundaries of the Sirente 
"crater." Such limestone would not have survived an impact. So if Ormö's 
theory is correct, one must surmise that somebody set these giant chunks 
of rock in place since the crater formed. To Speranza, that just didn't 
make sense. Speranza and colleagues further argue that Ormö's 
radiocarbon dating gave one age for the main feature (placing it in the 
4th or 5th century a.d.) and a completely different age for a nearby 
"crater" called C9, a date in the 3rd millennium B.C.

Indeed, to Speranza, the only suggestive evidence for an impact origin 
seemed to be the magnetic anomalies that Ormö and his colleagues had 
measured over some of the smaller depressions. But according to 
Rochette, even those anomalies are easy enough to understand. One needs 
simply to realize that these pockets are "dolines," places where the 
limestone has dissolved and the hole has filled in with sediments that 
are slightly more magnetic. Careful measurements of the magnetic 
properties of these materials showed that this mechanism is sufficient 
to account for the magnetic anomalies.

If not an impact crater, what is the large circular depression found in 
the Sirente plain? Speranza, Sagnotti and Rochette give a plausible 
answer: It is a reservoir made by human hands for the purpose of 
watering herds of sheep. They describe how this area of Italy was one of 
the main wool-producing regions of Europe between the 12th and 16th 
centuries, although locals have been involved in the activity since 
before Roman times. The great permeability of the underlying rock, 
however, does not allow rivers or even large springs to form, which 
creates a problem for shepherds trying to maintain millions of sheep 
there through the summer. The logical answer to this problem, they 
posit, was to dig reservoirs at the low points of these plains, where 
water tends to accumulate.

http://www.americanscientist.org/content/AMSCI/AMSCI/Image/MediumImage_200492910720_846.jpg  

Ormö, Rossi and Komatsu have refused to be questioned about this recent 
challenge to their theory. In a written reply to my request for an 
interview, Ormö states: "It is not possible for us to comment in [the] 
media on the work done by other scientists and on our own unpublished 
results." Fair enough. Curiously, the short written remarks these 
authors shared with me appear far more tentative than the statements 
given in their published papers. They say: "As long as the structure is 
not a proven impact crater field, it is impossible to draw any 
conclusions about historical consequences." This tone is in stark 
contrast to almost the entire body of their Antiquity paper, which is 
all about linking the structures seen in the Sirente plain with 
historical events.

Speranza notes that officials in the nearby town of Secarino are now in 
a bit of a quandary. After Ormö's papers were published, they began 
promoting the site as a crater park, hoping to make it a local tourist 
attraction. In August of last year they held a meeting on the "crater," 
which brought together many of the local dignitaries. But now that a 
significant scientific challenge has been published, it is hard to see 
how officials of the community of Secarino can in good conscience go 
ahead with those plans. After all, what tourist would want to visit a 
"Crater or Just-Plain-Watering-Hole Park"?—David Schneider