source COSMOS+
Agençe France-Presse
6 September 2007
PARIS: The extinction of the dinosaurs 65 million years ago has been
traced to a collision between two monster rocks in the asteroid belt
nearly 100 million years earlier.
The smash drove a giant sliver of rock into Earth's path, eventually
causing the climate-changing impact that ended the reign of the
dinosaurs and enabled the rise of mammals. Other asteroid fragments
smashed into the Moon, Venus and Mars, pocking their faces with mighty
craters, U.S. and Czech researchers now believe.
Mixing skills in time travel, jigsaw-making and carbon chemistry,
the scientists carried out a computer simulation of the jostling among
orbital rubble left from the building of the Solar System. The sleuths
were guided by an intriguing clue – a large asteroid called (298)
Baptistina, which shares the same orbital track as a group of smaller
rocks.
Turning the clock back
As they report today in the U.K. journal Nature, they turned
the clock back on the simulation and found that the Baptistina bits not
only fitted together, they were also remnants of a giant parent
asteroid, around 170-kilometres-wide, that once cruised the innermost
region of the asteroid belt.
Around 160 million years ago – the best bet in a range of 140 to 190
million years – another giant some 60-kilometres-wide whacked this
behemoth. From this soundless collision was born a huge cluster of
rocks, they said, including 300 bodies larger than 10 km and 140,000
bodies larger than one kilometre.
Over aeons, the fragments found new orbits with the help of
something called the Yarkovsky effect, in which energy from the Sun
gives a tiny yet inexorable push to orbiting rocks. As the family
gradually split up, a large number of chunks – perhaps one in five of
the bigger ones – crept their way out of the asteroid belt and became
ensnared by the gravitational pull of the inner planets.
Around 65 million years ago, one 10-kilometre-wide piece crunched
into Earth, unleashing a firestorm and kicking up clouds of dust that
filtered out sunlight. In this enduring winter, much vegetation was
wiped out and the species that depended on them also became extinct.
Only those animals that could cope with the new challenge.
The trace of the great event, called the Cretaceous/Tertiary (or
K-T) extinction event, can be seen today in the shape of a
180-kilometre-diameter impact crater at modern-day Chicxulub, in
Mexico's Yucatan peninsula.
Rock sampling
The trio of researchers – William Bottke and David Nesvorny of
Southwest Research Institute in Colorado, U.S. and David Vokrouhlicky
of the Czech Republic's Charles University in Prague – took their
theory a stage further and checked out sediment samples from the
Chicxulub site. They found traces of a mineral called carbonaceous
chondrite, which is only found in a tiny minority of meteorites, as the
Earthly remains of plummeting asteroids are called. Most asteroids can
be excluded from the Chicxulub event, but not Baptistina-era ones, they
contend.
Putting simulation and chemical evidence together, the team rule out
theories that a comet was to blame rather than an asteroid, and say
there is a "more than 90 per cent" probability that the killer rock was
a refugee from the Baptistina family.
The investigators also argue that there's a 70 per cent chance that
a four-kilometre-wide Baptistina asteroid hit the Moon some around 108
million years ago, forming the 85-kilometre crater Tycho.
The probability is lower than for Chicxulub because it is based only
on a simulation. The peak of "Baptistina bombardment" was probably
around 100 million years ago but is not over yet, the paper cautions.
Many of the asteroids that skim dangerously close to Earth today owe
their orbits to that great collision in the deep past, according to the
authors. "We are in the tail end of this shower now," says Bottke. "Our
simulations suggest that about 20 per cent of the present-day
near-Earth asteroid population can be traced back to the Baptistina
family."
An important point raised by the study "is how severe the
repercussions of cataclysmic collisions in the asteroid belt can be for
the Earth–Moon system," commented geologists Philippe Claeys and Steven
Goderis of Vrije University in Brussels, Belgium, in an accompanying
commentary also in Nature. "The terrestrial impact record needs
to be scrutinized more closely to identify and understand these periods
of more intense bombardment, and to link them to the huge and dangerous
game of billiards continuously being played out between the orbits of
Mars and Jupiter," they said.
source COSMOS+
HemuZ Planetary
HemuZ Space
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