[meteorite-list] A Metallic Asteroid May Have Coincided With The Fall Of Rome

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 10:18:25 2004
Message-ID: <200302090418.UAA10854_at_zagami.jpl.nasa.gov>

http://www.guardian.co.uk/online/science/story/0,12450,889308,00.html

A metallic asteroid may have coincided with the fall of Rome, says Duncan Steel

The Guardian (United Kingdom)
February 6, 2003

In the early fifth century, rampaging Goths swept through Italy.
Inviolate for 1,100 years, Rome was sacked by the hordes in 410 AD.
St Augustine's apologia, the City of God, set the tone for Christians
for the next 16 centuries.

But the Rome of that era came close to suffering a far worse calamity.
A small metallic asteroid descended from the sky, making a hypervelocity
impact in an Apennine valley just 60 miles east of the city. This
bus-sized lump of cosmic detritus vaporised as it hit the ground. In
doing so, it released energy equivalent to around 200 kilotonnes of TNT:
around 15 times the power of the atomic bomb that levelled Hiroshima in
1945.

Pescara is on the Adriatic coast, located across the Italian peninsula
from Rome. Housed there is the International Research School of Planetary
Sciences, where staff and students study topics ranging from planetary
geology to astrobiology. In 1999, a young impact cratering specialist
from Sweden, Jens Ormö, arrived to take up a three-year position
funded by the European Union.

Ormö, it happens, is keen on hill walking, and just inland from Pescara
are some of the most spectacular mountains in the Apennines. He decided
that some hiking in the area of the Sirente Massif was in order, and so
he consulted a local guidebook. As he thumbed its pages, Ormö came
across a photograph of something that amazed him. What he saw, labelled
as a natural lake, was surely an impact crater.

An expedition to the site of the putative impact, on the Sirente plain,
was hastily organised. Colleagues confirmed Ormö's initial suspicion.
Here was an impact crater about 140 metres wide, previously unrecognised
despite lying only a short distance from a busy road, and visible from
miles away. It has appeared on maps for centuries, and in guidebooks
for decades - but no one had recognised its significance.

Natural lakes are common in the area. But this one has a raised rim, now
about two metres high, but originally rather thicker. This was produced
by the asteroid throwing material out from the impact zone, as it crashed
at a speed of around 20km per second, producing a huge explosion. Later
filled with rainwater, the crater is now only a few metres deep, and
occasionally dries up during hot summers. But it was more than 30 metres
to the bottom when first formed. Centuries of weathering has eroded its
bank and gradually filled it in.

Relatively modest craters like this are unusual, because small asteroids
can only reach the ground intact if they are metallic, and thus strong
enough to withstand the physical shock of slamming into the atmosphere
at such speeds. The best guess at present is that the asteroid was about
10 metres across, and had a composition similar to nickel-iron meteorites.
If it had been stony in composition, as most asteroids are, it would have
shattered in flight and released all of its energy in a phenomenal
explosion. This is what happened when a 50-metre rock blew up over
Siberia in 1908, leaving no crater.The expectation of a metallic impactor
is backed up by the identification of rust grains in the surrounding soil.

Confirmation of the impact origin comes from 17 smaller craters,
typically 10 metres wide, scattered around the Sirente plain. These are
due to fragments of the asteroid that separated in flight through the
atmosphere. A magnetic survey shows that most are associated with
anomalously high fields, indicating sub-surface metallic lumps.

Crater fields like this are not unusual. In central Australia, 120km
south of Alice Springs, the Henbury craters were formed in a similar way.
What is peculiar about the Sirente crater is where it occurred, and its
youth. Dozens of ancient craters are known in northern Europe,
geological stability allowing their long-term preservation. Two examples
are the Ries and Steinheim basins in Germany. Many others are known in
Scandinavia. But these are all huge, and millions of years old. There
is a small, recently formed crater in Estonia, but the Sirente crater
is of far greater interest: it was excavated around the time of the
fall of the Roman Empire, and close to Rome itself.

The crater has been dated through radiocarbon analysis of a drill core
cut down through the bank. The uppermost material, having been thrown
out of the cavity, contains organic matter older than the impact. At
the original ground level the radiocarbon ages minimise, and then deeper
down the material is older again.

The data indicate that the crater was formed in about 412 AD, with an
uncertainty of 40 years in either direction. Additional sampling may
allow this spread to be reduced, but it is clear that the event
occurred close to the fall of Rome: some time between 370 AD and 450
AD, when the city was again under attack, this time by the Vandals.

No matter what the trajectory of the asteroid entry, it would have
been a phenomenal sight from Rome, and scarier still for those closer
to ground zero. The fireball produced would have only lasted 10 seconds
or so, but would have been brighter than the sun, and so visible
even in daytime. The smoke trail left in the atmosphere would have
been visible for some hours.

Another remarkable aspect of the event is that the main crater sits
squarely in the middle of the Sirente plain, which is only about a
mile long, and half that wide, being surrounded by mountainous
terrain. It could be that this is just luck. Alternatively, the
array of craters now identified might represent only a tiny
fraction of the havoc wreaked, with many other impacts on the
mountainsides having long since eroded or been hidden by tree growth.

Even considering simply the energy involved in forming the known
crater, it is sobering to ponder what might have happened should
the impact zone have been on the flat coastal plains nearer Rome,
rather than in the mountains. Scaling from nuclear bomb tests
indicates that a 200 kilotonne surface explosion would devastate an
area of 100 square
kilometres.

A frequently used aphorism says that Rome was not built in a day.
That's true. But it did come awfully close to being destroyed in
seconds.
Received on Sat 08 Feb 2003 11:18:41 PM PST