Fragmento do meteorito Allende
Allende é um
meteorito caído no estado
mexicano de
Chihuahua. A sua queda ocorreu às 01.05 horas do dia 8 de fevereiro de 1969, e a bola de fogo originada pela sua entrada na
atmosfera terrestre foi testemunhada por milhares de pessoas.
O meteorito Allende é o maior
condrito (tipo de meteorito primitivo) já descoberto
. Como resultado de uma pesquisa neste meteorito, foi descoberto um novo
óxido de titânio e esse mineral foi batizado de
panguite.
Composição
Este mineral possui a fórmula química (Ti
4+,Sc,Al,Mg,Zr,Ca)
1.8O
3. Os elementos encontrados na panguite são
titânio,
escândio,
alumínio,
magnésio,
zircónio,
cálcio e
oxigénio. Nas amostras retiradas do meteorito, também foram encontrados
zircónio enriquecido. A panguite foi encontrada associada com um outro mineral identificado como
davisite e com
olivina agregada.
Origem e Propriedades
A panguite, está na classe dos minerais
refratários,
que se formaram sob altas temperaturas e pressões extremamente altas, o
que ocorreu há mais ou menos 4.500 milhões de anos atrás, no inicio do
nosso
sistema solar. Isso faz com que a panguite seja um dos minerais mais antigos de nosso Sistema Solar. O
zircónio é um dos elementos determinantes para se saber as condições de antes e durante a formação do nosso sistema solar.
Descoberta
Allende meteorite - image by Matteo Chinellato; cube = 1 cm
The
Allende meteorite is the largest
carbonaceous chondrite ever found on
Earth. The
fireball was witnessed at 01:05 on February 8, 1969, falling over the
Mexican state of
Chihuahua. After breaking up in the
atmosphere, an extensive search for pieces was conducted and it is often described as "the best-studied
meteorite in history". The Allende meteorite is notable for possessing abundant, large
calcium-aluminium-rich inclusions, which are among the oldest objects formed in the
Solar System.
Carbonaceous chondrites comprise about 4 percent of all meteorites
observed to fall from space. Prior to 1969, the carbonaceous chondrite
class was known from a small number of uncommon meteorites such as
Orgueil, which fell in France in 1864. Meteorites similar to Allende were known, but many were small and poorly studied.
Fall
The original stone is believed to have been approximately the size of an
automobile traveling towards the Earth at more than 10 miles per
second. The fall occurred in the early morning hours of February 8,
1969. At 01:05 a huge, brilliant
fireball
approached from the southwest and lit the sky and ground for hundreds
of miles. It exploded and broke up to produce thousands of fusion
crusted individuals. This is typical of falls of large stones through
the atmosphere and is due to the sudden braking effect of air
resistance. The fall took place in northern Mexico, near the village of
Pueblito de Allende in the state of Chihuahua. Allende stones became
one of the most widely distributed meteorites and provided a large
amount of material to study, far more than all of the previously known
carbonaceous chondrite falls combined.
Path of the fireball and the area in northern Mexico where the meteorite pieces landed (the strewnfield)
Strewnfield
Stones were scattered over a huge area – one of the largest meteorite
strewnfields
known. This strewnfield measures approximately 8 by 50 kilometers. The
region is desert, mostly flat, with sparse to moderate low vegetation.
Hundreds of meteorites were collected shortly after the fall.
Approximately 2 or 3 tonnes of specimens were collected over a period of
more than 25 years. Some sources guess that an even larger amount was
recovered (estimates as high as 5 tonnes can be found), but there is no
way to make an accurate estimate. Even today, over 40 years later,
specimens are still occasionally found. Fusion crusted individual
Allende specimens ranged from 1 gram to 110 kilograms.
Study
Allende is often called "the best-studied meteorite in history." There
are several reasons for this: Allende fell in early 1969, just months
before the
Apollo program
was to return the first moon rocks. This was a time of great
excitement and energy among planetary scientists. The field was
attracting many new workers and laboratories were being improved. As a
result, the scientific community was immediately ready to study the new
meteorite. A number of museums launched expeditions to Mexico to
collect samples, including the
Smithsonian Institution and together they collected hundreds of kilograms of material with
CAls. The CAls are billions of years old, and help to determine the age of the solar system. The CAls had very unusual
isotopic
compositions, with many being distinct from the Earth, Moon and other
meteorites for a wide variety of isotopes. These "isotope anomalies"
contain evidence for processes that occurred in other stars before the
solar system formed.
Allende contains chondrules and CAls that are estimated to be 4.567 billion years old,
the oldest known matter (other carbonaceous chondrites also contain
these). This material is 30 million years older than the Earth and 287
million years older than the
oldest rock known on Earth,
Thus, the Allende meteorite has revealed information about conditions
prevailing during the early formation of our solar system. Carbonaceous
chondrites, including Allende, are the most primitive meteorites, and
contain the most primitive known matter. They have undergone the least
mixing and remelting since the early stages of solar system formation.
Because of this, their age is frequently taken as the "age of the solar
system."
Structure
The meteorite was formed from nebular dust and gas during the early
formation of the solar system. It is a "stone" meteorite, as opposed to
an "iron," or "stony iron," the other two general classes of meteorite.
Most Allende stones are covered, in part or in whole, by a black,
shiny crust created as the stone descended at great speed through the
atmosphere as it was falling towards the earth from space. This causes
the exterior of the stone to become very hot, melting it, and forming a
glassy "fusion crust."
When an Allende stone is sawed into two pieces and the surface is
polished, the structure in the interior can be examined. This reveals a
dark matrix embedded throughout with mm-sized, lighter-colored
chondrules, tiny stony spherules found only in meteorites and not in earth rock (thus it is a
chondritic
meteorite). Also seen are white inclusions, up to several cm in size,
ranging in shape from spherical to highly irregular or "amoeboidal."
These are known as
calcium-aluminum-rich inclusions or "CAls", so named because they are dominantly composed of calcium- and aluminum-rich
silicate and
oxide minerals. Like many chondrites, Allende is a
breccia,
and contains many dark-colored clasts or "dark inclusions" which have a
chondritic structure that is distinct from the rest of the meteorite.
Unlike many other chondrites, Allende is almost completely lacking in
Fe-Ni
metal.
Composition
The matrix and the
chondrules consist of many different minerals, predominantly
olivine and
pyroxene. Allende is classified as a CV3 carbonaceous chondrite: the chemical composition, which is rich in
refractory elements like calcium, aluminum, and titanium, and poor in relatively
volatile
elements like sodium and potassium, places it in the CV group, and the
lack of secondary heating effects is consistent with petrologic type 3
(see
meteorites classification). Like most carbonaceous chondrites and all CV chondrites, Allende is enriched in the oxygen
isotope
O-16 relative to the less abundant isotopes, O-17 and O-18. In June
2012, researchers announced the discovery of another inclusion dubbed
panguite, a hitherto unknown type of titanium dioxide mineral.
There was found to be a small amount of carbon (including graphite and
diamond), and many organic compounds, including amino acids, some not
known on Earth. Iron, mostly combined, makes up about 24% of the
meteorite.
Subsequent reserch
Close examination of the chondrules in 1971, by a team from
Case Western Reserve University,
revealed tiny black markings, up to 10 trillion per square centimeter,
which were absent from the matrix and interpreted as evidence of
radiation damage. Similar structures have turned up in
lunar basalts
but not in their terrestrial equivalent which would have been screened
from cosmic radiation by the Earth's atmosphere and geomagnetic field.
Thus it appears that the irradiation of the chondrules happened after
they had solidified but before the cold accretion of matter that took
place during the early stages of formation of the solar system, when the
parent meteorite came together.
The discovery at
California Institute of Technology in 1977 of new forms of the elements
calcium,
barium and
neodymium
in the meteorite was believed to show that those elements came from
some source outside the early clouds of gas and dust that formed the
solar system. This supports the theory that shockwaves from a
supernova
- the explosion of an aging star - may have triggered the formation
of, or contributed to the formation of our solar system. As further
evidence, the Caltech group said the meteorite contained
Aluminum
26, a rare form of aluminum. This acts as a "clock" on the meteorite,
dating the explosion of the supernova to within less than 2 million
years before the solar system was formed. Subsequent studies have found isotopic ratios of
krypton,
xenon,
nitrogen
and other elements that are also unknown in our solar system. The
conclusion, from many studies with similar findings, is that there were a
lot of substances in the presolar disc that were introduced as fine
"dust" from nearby stars, including novas, supernovas, and
red giants. These specks persist to this day in meteorites like Allende, and are known as
presolar grains.
