[meteorite-list] Meteorites Are Made Up Of Evidence About How The Solar System Was Born

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Tue, 19 Dec 2006 16:28:30 -0800 (PST)
Message-ID: <200612200028.QAA27039_at_zagami.jpl.nasa.gov>

http://www.economist.com/science/displaystory.cfm?story_id=8345618

Fire from heaven
The Economist
December 19, 2006

Meteorites are made up of evidence about how the solar system was born

AT HALF past six on the morning of December 14th 1807, the folk of
Weston, Connecticut, were woken up by a loud bang. Shortly afterwards,
it rained rocks. In earlier times such hard rain might have been seen as
a sign of the gods' displeasure. The folk of Weston, however, saw it as
an opportunity.

"Strongly impressed with the idea that these stones contained gold and
silver, they subjected them to all the tortures of ancient alchemy, and
the goldsmith's crucible, the forge, and the blacksmith's anvil, were
employed in vain to elicit riches which existed only in the imagination."

That was part of the report of Benjamin Silliman who, together with
James Kingsley, went to Weston from Yale University to investigate. The
following March, Silliman presented what they had found to the American
Philosophical Society. Only Thomas Jefferson was sceptical. On reading
the report he is supposed to have said, "I would more easily believe
that two Yankee professors would lie than that stones would fall from
heaven."

Meteorites fascinate scientists because they are the smashed-up remnants
of asteroids - the tiny wannabe planets that orbit between Mars and
Jupiter. Because asteroids never got it together to form a larger
planet, a lot of what they are made of was formed in the solar system's
earliest days. So meteorites are tangible evidence of what was happening
when the solar system was born.

About 90% of meteorites are classified by the successors of Silliman and
Kingsley as chondrites. That means they contain spherical nodules a few
millimetres across, known as chondrules. They also contain a lot of
cosmic crud, mostly in the form of dust-sized grains.

It was the study of chondrites that allowed researchers to work out how
old the solar system is. Chondrules are frozen droplets of once-liquid
rock. Such isolated droplets must have formed in free space, rather than
as part of a larger body, or else they would have merged into a more
conventional igneous rock. That chondrules formed in such quantities
suggests that the heating which created them was widespread. In free
space, such heat could come only from a star - in this case, presumably,
the youthful sun. Work out how old the oldest chondrules are, and you
know when the sun ignited.

The way to do that (and much else that is needed in order to read the
history written in meteorites) is to look at the exact mixture of atomic
isotopes in them. Isotopes are different atomic versions of a particular
chemical element. They have the same number of protons in their nuclei
(this is the defining characteristic of an element), but different
numbers of neutrons. For the scientific detectives who study meteorites,
this variability is invaluable. For example, radioactivity (which is a
fancy name for the way that unstable atomic nuclei break up) depends
crucially on the number of neutrons. Particular radioactive isotopes
break up into other, non-radioactive isotopes at regular and
well-understood rates called half lives (the amount of time it takes
half the atoms in a sample to change from one sort to the other). Most
famously, uranium-238 decays into lead-206 with a half life of 4.5
billion years, though radioactive rubidium and samarium are also useful
for dating things billions of years old.

Looking at these isotopes allows the chondrules to be dated, and they
were formed 4.56 billion years ago. That, then, is the age of the solar
system. But isotopes can do more. They can reach back before the solar
system was created, and forward to record the creation of the planets.

To reach back, you need to look in the dust grains in chondrites, rather
than at the chondrules. Like the chondrules themselves, most dust grains
were created in the early solar system - in this case by bigger objects
grinding against each other. Modern telescopes can see clouds of dust
created in this way around several of the solar system's stellar
neighbours. But a few grains have survived from the primitive nebula
that the solar system condensed from. This time, it is carbon isotopes
that give the game away.

To have survived the chaos in which the solar system was born, a grain
of dust has to be tough. The toughest materials around are diamond (a
type of crystalline carbon) and silicon carbide. Carbon has two
non-radioactive isotopes, and in material from the solar system,
including most meteoritic minerals, these are mixed in a well-known
ratio. The carbon in diamond and silicon-carbide grains from meteorites
usually has ratios very different from this.

The silicon carbide is thought to have come from red giants. These are
stars that have swelled up in old age and are nearing the ends of their
lives. Each carbon ratio represents a different parent star. The
diamonds, by contrast, are thought to be the products of supernova
explosions. Again, many carbon ratios are known, each from a different
supernova. Dozens, if not hundreds, of red giants and supernovas seem to
have contributed to the primitive solar nebula. Unfortunately, the
grains examined do not carry the sort of isotopes that would allow them
to be dated.

Nevertheless, other isotopes suggest a supernova did go off just as the
solar nebula was forming. That is because meteorites contain a lot more
of an isotope called magnesium-26 than would be expected. Magnesium-26
is the decay product of aluminium-26. And aluminium-26 is produced in
supernovas. Whether this supernova somehow triggered the collapse of the
primitive solar nebula and thus the formation of Earth is not clear
(though the theory was popular in the 1950s, before the evidence for
aluminium-26 was found). But meteorites can certainly illuminate the
processes that formed the planets. That is because, among the 10% that
are not chondrites, there is a group that is composed almost entirely of
metal.

The metal in question is an alloy of iron and nickel. Or, rather, it is
two alloys that have different ratios of the two metals. These alloys
are called kamacite and taenite, and when cut, polished and etched with
acid they produce an attractive criss-cross called a Widmanst?tten
pattern. But the really attractive thing about metallic meteorites, from
a scientific point of view, is that they provide the best evidence
available of what Earth's interior is like.

The process of planetary formation, as deduced from meteorites and
confirmed as plausible by computer models, went like this. First, dust
particles clumped together to form cosmic dustballs. Bursts of heat from
the primitive sun melted the dustballs, which solidified into
chondrules. Local concentrations of chondrules were drawn together by
gravity and, when they encountered each other, often merged. Once a
merged mass of chondrules and dust got big enough, things started to
happen. Radioactive decay generates heat. In larger bodies, that heat
gets trapped. The temperature rises and the rock melts. At this point,
heavy elements sink towards the centre and light ones rise to the
surface. The most abundant heavy elements are iron and nickel, and it is
these that form the cores of what can now reasonably be referred to as
small planets.

Smash one of these small planets open and the fragments from the centre
will form metallic meteorites. (The outer, non-metallic layers of the
planet form what are known as achondrite stony meteorites, and there is
a separate class of stony-irons that come from the boundary between
inner core and outer layer.) Analysis of the chemistry of metallic
meteorites suggests they come from more than 60 different small planets
that have broken up over the solar system's history. But most small
planets met a different fate. They merged to form larger ones, still
with iron-nickel cores, culminating in those seen today.

Isotopes can even indicate the order in which the planets formed. The
decay products of a short-lived isotope called hafnium-187, also
suspected of being formed in the supernova that brought aluminium-26 to
the solar system, are rare on Earth. On Mars they are more abundant.
This indicates that Mars formed before Earth, trapping hafnium-187 while
there was still some around.

And how is it known that Mars had hafnium-187? Because a few dozen of
the meteorites that have fallen to Earth come not from the asteroid belt
but from Mars. They were blasted off that planet's surface when it was
hit by huge meteorites. That trapped bits of the Martian atmosphere
within them, as a telltale of their origin.

One of these Martian meteorites was once thought to harbour signs of
life, in the form of carbon-containing compounds and objects that the
eye of faith interpreted as fossil bacteria. Alas, few researchers now
believe that story. But Martian meteorites do have one other tale to
tell - that planets are sometimes hit so hard that rocks can escape from
them completely.
    
Hell's kitchen

At a quarter past seven on the morning of June 30th 1908, the folk of
Tunguska in Siberia heard a rather larger bang than the one that had
woken the burghers of Weston just over a century previously. Tunguska is
far more remote from centres of academic excellence than Weston, and it
took until 1927 for a scientific team to reach the site. When Leonid
Kulik and his colleagues got there they found an area of devastation
60km across. At the centre the trees remained upright but were stripped
of bark and branches. Around it the taiga was flattened, with the broken
trees pointing outward from the middle like the sticks in a game of
spillikins.

The meteorite which devastated Tunguska is now estimated to have been
about 50 metres across. The explosion, at an altitude of between six and
eight kilometres, was 50 megatonnes. That is more powerful than the
largest hydrogen bomb ever detonated. The meteorite that excavated
Barringer crater in Arizona (see top) about 50,000 years ago was
smaller - about 40 metres across - but it made it all the way to the Earth's
surface. Barringer crater is 1.2km in diameter.

Events of this size are rare, but not so rare that they can be ignored.
They occur perhaps once a century. And bigger impacts happen, too.
Several huge craters seem to coincide with the disappearance of
previously well-established groups of animals - the most famous being the
Chicxulub crater in Mexico, which was formed 65m years ago, at the time
the dinosaurs vanished, by an explosion estimated at 100,000 gigatonnes.

The search is now on for space-rocks large enough to cause serious
devastation if they hit Earth. Given enough warning, it should be
possible to push a threatening boulder out of the way. Only a slight
nudge would be required to change its orbit, and that nudge could be
provided by rocket motors no more powerful than ones that have already
been built.

The chances of needing to do that anytime soon are slim. But if the
search does turn up something nasty, then perhaps the billions of
dollars spent so far on spaceflight might look like a wise investment
rather than money down the drain.
Received on Tue 19 Dec 2006 07:28:30 PM PST