[meteorite-list] The Sky Is Falling ... More Than We Thought

From: Michael Groetz <mpg4444_at_meteoritecentral.com>
Date: Mon, 31 Jan 2011 19:55:39 -0500
Message-ID: <AANLkTikSmjFKqkGwm=G_wtbR2dT9_ddpd43dAZZQTajq_at_mail.gmail.com>

http://news.sciencemag.org/sciencenow/2011/01/the-sky-is-falling-more-than-we-.html?ref=hp

The Sky Is Falling ... More Than We Thought
by Sid Perkins on 31 January 2011, 4:50 PM

Sometime in the past 5000 years, a small iron meteorite slammed into
the southwest corner of Egypt, punching a 45-meter-wide crater into
the rock and sand. A new study suggests that small meteorites like
this may survive their plunge through Earth's atmosphere intact much
more often than previously suspected. And that means these objects
could pose a greater danger than once believed.

Although small impact craters are common features on the moon and
other airless bodies in our solar system, they're rare on Earth: Only
15 of our planet's 176 or so known craters measure less than 300
meters across. But that rarity doesn't stem from a lack of objects
whizzing through our cosmic neighborhood. Instead, two other factors
are at play. Many small objects break apart as they plummet through
the atmosphere, either disintegrating entirely or surviving to leave
multiple craters. Also, the craters blasted by those objects that do
reach the ground are quickly masked by erosion or other geological
processes not present on other worlds. But the Egyptian impact is
causing scientists to rethink their notions of how often iron
meteorites fall to Earth in one piece.

The so-called Kamil crater, named for the nearest mountain, was
discovered in autumn 2008 during a low-altitude aerial survey
conducted for Google Earth. A field expedition to the site in February
2009 recovered more than 5000 fragments of nickel-rich iron that
together weighed more than 1.7 metric tons?a sure sign that the
meter-deep crater had been blasted by an iron meteorite. In next
month's issue of Geology, Massimo D'Orazio, a geochemist at the
University of Pisa in Italy, and his colleagues estimate that at
impact the meteorite weighed about 9.1 metric tons, which suggests
that the object weighed between 20 and 40 metric tons before it
entered the atmosphere.

Previous studies of Earth's small craters indicate that objects
weighing less than 3000 metric tons typically break apart to form
multiple craters on impact. As of a decade ago, only three of the 12
iron meteorites in that class had survived their fiery fall to make a
single crater. But the Kamil finding, along with a little-studied
crater in Alberta, Canada, that was also formed by the impact of a
small iron meteorite, boosts the proportion of such objects that
struck Earth intact from one-fourth to more than one-third.

The apparent rise in survival rates may be due simply to having a
larger sample size that better represents the objects slamming into
the planet, says Philip Bland, a planetary scientist at Imperial
College London. "The more of these craters we find, the better our
statistical model of meteorite survival can be," he notes.

Whether an object survives its plunge through the atmosphere depends
on its material properties and its history, Bland adds. Objects that
have suffered repeated collisions in space may be highly fractured and
readily break up during their plunge to Earth, whereas those made of a
relatively intact mass of fine-grained iron would have a better chance
of landing in one piece.

As is often portrayed in movies such as Armageddon, an
extraterrestrial object that strikes Earth in one piece does more
damage than one that lands in several smaller fragments. Thus, the new
findings suggest that earthlings may face somewhat more danger from
meteorites than previously recognized.
Received on Mon 31 Jan 2011 07:55:39 PM PST