[meteorite-list] Story from a stone

From: Darren Garrison <cynapse_at_meteoritecentral.com>
Date: Tue, 13 Feb 2007 10:36:06 -0500
Message-ID: <akm3t2le6f2391lnbk8lgt65o9n2rt6f2b_at_4ax.com>

http://www.hinduonnet.com/fline/stories/20070223001109000.htm

PIPLIA KALAN is a small, nondescript village in Pali district in western
Rajasthan. People outside the district would not even have heard its name. Yet,
it is a name familiar to planetary scientists and astronomers, and a casual
search on Google for `Piplia Kalan' will fetch you many entries. It does not owe
its fame to any natural calamity or scandal but to a piece of a meteorite.

A shooting star fell on an uncultivated farm on the outskirts of Piplia Kalan on
June 20, 1996, around 8-30 in the evening. Most villagers were probably enjoying
the summer evening outside their homes, and the meteor that streaked brightly
across the sky did not escape their notice.

The `Piplia Kalan' meteorite was rather small by the standards of famous
meteorites. I t did not even weigh 50 kg - so it was far from being dangerous
like the one that is believed to have brought about the extinction of the
dinosaurs, or even the one that in 1908 exploded over Siberia and destroyed a
forest there. The Piplia Kalan meteorite was tiny in comparison. Yet, the
surviving fragment of this meteorite contained an extraordinary piece of
information, which has changed the way scientists think about the birth of the
solar system. A group of Indian scientists has taken a leading role in the
analysis of this meteorite, and some other meteorites around the world, and in
shaping this new look at the origin of the solar system.

Traditionally, it is believed that the solar system formed roughly 4.6 billion
years ago out of a gaseous cloud in space. Such clouds are abundant in our
galaxy; they hover in the space between stars and are often seen either by the
light falling on them from nearby stars or as silhouettes against a bright
starry background.

Material inside a nebula can, however, begin to contract and decrease in size at
some point of time - either collapsing on its own or being influenced by some
extraordinary event in its neighbourhood, such as being hit by shocks from
stellar explosions. As it contracts, it forms a star (or a number of stars) in
its dense core. Then the leftover material surrounding the central star cools
slowly and forms small grains of solid particles, which gradually coalesce to
form large objects such as planets, asteroids and comets.

It is a straightforward calculation to work out how much time a typical
interstellar cloud takes to contract and form a star. Astrophysicists such as
Frank Shu of the University of California, Berkeley, United States, have
estimated that in the case of a sunlike star this process takes around 10
million years. Then, the residual cloud surrounding the nascent star would need
another 100 million years or so to produce planetesimals and planets. Forming
large planets would take an even longer time.

One can then ask whether the solar system too followed this timetable or whether
it formed in a quicker or in a more lethargic fashion than prescribed in this
scenario. And is there some way one can test this hypothesis? Is there a way of
knowing what the script the solar system followed during its birth? It turns out
there is.

The secret lies in finding radioactive elements in the material that formed soon
after the birth of the solar system. Radioactive materials have atoms that are
markedly different from their normal, run-of-the-mill counterparts. Consider,
for instance, an atom of radioactive aluminium. It has 13 neutrons in its
nucleus, as opposed to the 14 neutrons an ordinary aluminium atom has (both
varieties contain 13 protons). This makes radioactive aluminium change its
identity after a certain amount of time. For instance, if one took a certain
amount of radioactive aluminium, half of it would have changed into a bizarre
form of magnesium after three-quarters of a million years. This particular type
of magnesium is slightly heavier than the normal variety and is easily
identifiable as the odd one out.

There are many such radioactive elements, but there is something special about
radioactive aluminium. Aluminium happens to be an element that requires very
high temperatures to turn into its gaseous phase - in the jargon of science, it
is called a `refractory' element. If it needs very high temperatures to turn
gaseous, then it follows that if a hot mixture of gaseous substances is allowed
to cool aluminium (and other refractory elements) would also be among the first
to turn back into the solid phase. As an analogy, consider high-rise buildings
in a flooded city. As the flood water rises, the tallest buildings are the last
ones to drown - they are like the refractory elements in a gas that is being
heated. By the same token, when the flood water subsides, the tallest buildings
are also the first ones out of the water - in our case, refractory elements such
as aluminium becoming cool and solid again. In other words, one would expect
aluminium and other refractory elements to form the first solid particles as the
pre-solar nebula slowly cooled down. One would also expect a small, but
perceptible, amount of radioactive aluminium to be mixed with the normal
aluminium in such material. Since meteors that haunt the dark reaches of the
solar system and occasionally dart inside the earth's atmosphere are actually
leftovers from the process of the formation of planets, they provide scientists
with a method of pinning down the story of the birth of the solar system - they
act as a sort of fossil to track its birth history.

The idea then is to try to detect traces of radioactive aluminium, or its decay
product - the odd form of magnesium - in meteorites, the surviving fragments of
meteors that reach the earth's surface, and to determine when the meteorite
formed. Since one knows the time taken by radioactive aluminium to change its
identity, it is easy to "date" the formation epoch of ancient meteorites.

This is exactly what a group of Indian scientists attempted to do when they came
by the Piplia Kalan meteorite, and they were rewarded with an interesting
result. As one can imagine, the method involves sifting through slices of
meteoric rock for tiny, almost elusive, quantities of a particular element, and
it requires extremely accurate measurements.

One research institute in India, the Physical Research Laboratory (PRL) in
Ahmedabad, has instruments to measure minute quantities of different elements in
small volumes of matter. The institute was founded by Vikram Sarabhai 60 years
ago and is currently celebrating its diamond jubilee year. A group formed in the
institute in the 1970s around Devendra Lal (now Professor of Nuclear Physics,
Scripps Institution of Oceanography, Geosciences Research Division, University
of California, San Diego) specialised in studies of the history of the objects
of the solar system, particularly that of the earth, moon and meteorites. Among
the scientists who have helped this group flourish over the years are Dr.
Jitendra Goswami and Dr. Narendra Bhandari. It is not surprising that this
institute has gained importance in the wake of the mission to the moon being
planned by the Indian Space Research Organisation.

When the PRL scientist G. Srinivasan along with Goswami and Bhandari measured
the amount of the odd form of magnesium in the meteorite from Piplia Kalan, they
estimated that in the beginning the amount of radioactive aluminium was a mere
one part in a million compared with ordinary aluminium. In other words, when the
rock that fell in Piplia Kalan formed, the ratio of radioactive to ordinary
aluminium was one in a million.

This measurement in itself does not reveal much. But when one compares this
ratio with the ratio found in refactory phases in ancient meteorites - such as
the famed Allende meteorite that crashed into Mexico in 1969 - one can get an
idea of the formation epoch of the Piplia Kalan meteorite. The ratio of
radioactive to ordinary aluminium in the Allende meteorite - which is thought,
from other measurements, to be among the most ancient pieces of material in the
solar system - is hundred times larger than that for the Piplia Kalan meteorite.
This means that the Piplia Kalan meteorite formed slightly later than the
Allende meteorite, by which time only a fraction of the radioactive aluminium
had changed into magnesium. Scientists at the PRL pegged their estimate of the
formation era at within five million years of the solar system's formation.

By this time, the PRL scientists had gathered another piece of information from
the piece of rock. Its composition clearly hinted at its origins. They found
that the meteorite was a piece broken off from a well-known asteroid named Vesta
- which is the brightest asteroid and the second largest (although it will now
be the largest asteroid since Ceres, which was the largest asteroid to date, has
been promoted to the status of a "dwarf planet").

These two pieces of information taken together boggle the mind. Vesta is a large
object, with a radius of around 530 km, and it certainly must have taken a
considerable time to form such a large object in the early solar system -
certainly longer than the estimate of five million years.

But this is too rapid compared with the theorists' estimate of how long it takes
a gaseous nebula to form a star like the sun, let alone forming solid particles
in it after cooling. Recall that the theoretical estimate of a cloud contracting
on its own to form a star is approximately 10 million years.

Moreover, the very existence in meteorites of radioactive nuclei that "live" for
a short time is hard to understand. Any short-lived radioactive atom from the
original gas cloud would have decayed by the time the solid particles formed in
the solar system. How is it that these atoms hung around when the meteorite
formed?

There are only two processes that can explain their existence in that epoch.
They were either produced in a nearby star and then transported to the site of
the formation of the solar system by shockwaves, or wind, from an explosion, or
they could have been produced by bombardments of energetic particles produced by
the baby sun.

But when Goswami and his collaborators worked out the details of the particle
bombardment case, they found that this scenario could not explain the observed
abundances of the radioactive elements such as aluminium, and especially calcium
(with a "lifetime" of around a tenth of a million years) that coexisted with
radioactive aluminium in the refactory phases of ancient meteorites. The only
alternative then is to ascribe their presence to a nearby star. In that case,
the short lifetime of these radioactive nuclides would also mean that the
collapse of the pre-solar nebula took place within a million years or so, much
too rapidly to be compatible with the scenario of the unassisted collapse of a
cloud.

Astronomers have long been suspecting that a nearby cataclysmic stellar event
may have triggered the formation of the solar system. Perhaps, the pre-solar
nebula did not begin to collapse on its own, and perhaps, it was nudged and
prodded into its fateful collapse. The rest was, as they say, history, and
eventually, the sun was born and, later, the planets. A view has emerged in the
last decade that the sun owed its birth to a stellar accident of some sort. And
a few strands of support for this concept have come their way in the last few
years.

Stars usually shine and produce light with the help of nuclear reactions in
their cores. In the case of the sun, for instance, four protons combine to form
a helium nucleus and a fraction of the total energy leaks out in the form of
radiation. The "lives" of stars then depend crucially on this supply of nuclear
material in the core, and stars undergo monumental changes when this supply runs
dry. The sun will one day (after five billion years) bloat and engulf the orbits
of Mercury and Venus and shine menacingly red in sky. Stars more massive than
the sun are doomed to a more fiery finale: they explode and regurgitate the
material of their innards into space at high speed. Shock waves from such
explosions tear into nearby gas clouds and can trigger one into collapsing and
contracting under its weight. Such an explosion is called a supernova.

In addition to the triggering by shock waves, there can be material strewn
around in space from such explosions. This strewn material includes elements
such as nitrogen and oxygen and others, which are created inside stars and which
then fly out inspace after a supernova - next-generation stars form out of this
stellar ash. The calcium in our bones and the iron in the haemoglobin of our
blood were all created inside stellar furnaces eons before being expelled in
supernova explosions.

But present in the ash raining from these explosions were some exotic elements
that were created during the cataclysmic event. In fact, a supernova can produce
some radioactive elements that cannot be produced otherwise or anywhere else in
the universe.

Radioactive iron, for instance, is one such element. Existence of such telltale
material points surely towards a supernova near the pre-solar nebula.
Radioactive iron - with 60 particles in its nucleus compared with the 56 in the
normal iron nucleus - only lives for 1.5 million years, and so its existence
also indicates the time when the supernova could have happened.

Interestingly, a group of scientists led by Gary Huss of the Arizona State
University, Tempe, U.S., analysing a piece of another Indian meteorite,
Semarkona, found trace amounts of a decay product of this radioactive iron in
it. An odd from of iron, remnant of a heavier counterpart of chlorine
(chlorine-36), which, like radioactive iron, could have been produced in the
aftermath of a supernova (and only lives for 300,000 years), has also been
detected in ancient meteorites.

It then adds another piece to the jigsaw puzzle. First, one finds evidence that
a nearby supernova could have triggered the birth of the solar system, and then
one finds a bit of the stellar ash that must have rained on the early solar
nebula. One then wonders whether this was an extraordinary event or whether this
is more or less a rule in the universe - that the death of a star foretells the
birth of another.
Received on Tue 13 Feb 2007 10:36:06 AM PST