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 MARTIAN
METEORITES
Meteorites are the only martian samples we have on Earth (as
of 2006). There are over 30 different martian meteorites. The
smallest weighs only 12 g and the largest, Zagami, which fell
in Nigeria in 1962, totaled 18 kg. These meteorites were first
identified as martian in the 1980's. They were ejected from
the surface of Mars following large impacts.
[above
left] Two pieces of the martian meteorite, Shergotty.
Each piece is about 2 cm across. Even with the naked eye it
is easy to see that there are many long, thin crystals in this
rock. Shergotty is a basalt that contains the minerals pyroxene
and olivine.
[left] The
Zagami martian meteorite is a basalt made of the minerals pyroxene
and plagioclase. If it were not for the striking black fusion
crust, it would be very difficult to notice that this is a meteorite.
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|  ATMOSPHERE
Martian meteorites contain gases from the martian atmosphere.
Mars has an atmosphere, although it is so thin that people could
not breathe it. It would also be poisonous to us, since it is
mostly made of carbon dioxide. In 1977, the Viking lander measured
the composition of the martian atmosphere, which also contains
nitrogen, argon, xenon, and other gases.
In some martian meteorites, small pockets of glass contain atmospheric
gases. The glass was produced when the rock melted, following
a large impact. The gas trapped in the glass can be extracted
and analyzed in the laboratory. It is an exact match to measurements
made on Mars. This is the strongest evidence that these meteorites
come from Mars.
| [above
left] Martian meteorite EETA79001, found in Antarctica,
is also a basalt. On the cut surface it is easy to see several
dark spots. These are patches of glass that cooled from
impact melts. Gases from the martian atmosphere got trapped
in these glasses when they formed. This sample is 13 cm
across. Courtesy of Johnson Space Center.
[above right]
Impact glass pocket in martian meteorite EETA79001. The
white circles are bubbles where gas escaped. Tiny crystals
grew in the glass as it cooled. This is a light microscope
image, 1 mm across. |
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VOLCANOES
Martian meteorites are igneous (melted) rocks, and many are
volcanic.
Mars has volcanoes that are similar to many volcanoes on Earth.
Lava from deep in the martian mantle erupts onto the surface as
flows of basalt. Martian volcanoes were active for a long time
in the same spots, building up volcanic cones many times larger
than volcanoes on Earth. Several martian meteorites are cumulate
rocks that formed when the molten rock solidified before it reached
the martian surface.
The oldest martian meteorite is 4.5 billion years old, and the
youngest is 180 million years old. In terms of geological ages,
180 million years is very recent. Large areas of Mars do not have
many craters, which means that they are covered by young lava
flows. Volcanoes may still be active on Mars today.
| [above
left] Basaltic martian meteorites consist of pyroxene
and plagioclase. In this light microscope image, taken in
cross-polarized light, the pyroxene is brightly colored
and the plagioclase is black. Plagioclase would not normally
be black in this kind of image. It has been changed into
a glass by the high shock pressure that lifted the rock
off the surface of Mars. The shock also made a lot of cracks
in the pyroxene. This is the Zagami meteorite, and the image
is 2 mm across.
[above center]
Martian meteorite Y980459 is a basalt. It has large crystals
of olivine (the big, light grey crystal in the bottom right),
and smaller crystals of pyroxene (the dark grey crystals).
The rest of the rock is glass with tiny crystals of plagioclase.
Glass means that the rock cooled quickly when it erupted
as a lava. This is a back-scattered electron (SEM) image.
The photographed area is approximately 600 micrometers across.
[above right]
Governador Valadares is a cumulate rock, made of pyroxene
crystals that settled to the bottom of a magma chamber.
Most of the crystals in the image are a type of pyroxene
called clinopyroxene. The image is taken in cross-polarized
light and it is 2 mm across. |
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|  WATER
Martian meteorites record a time when water flowed through
the rocks.
Photos of Mars show ancient river valleys, lakes, and maybe oceans.
All the water is frozen today, but some minerals in Martian meteorites
could only have been created with water. Clay minerals formed
when water reacted with the mineral olivine. Salts such as carbonates,
sulfates and sodium chloride formed when water evaporated.
Isotopic ratios of elements such as oxygen in water extracted
from martian meteorites help us understand how conditions of the
planet have changed over time. Mars used to be much warmer and
wetter than it is today.
| [pictured
above] The Governador Valadares meteorite is a clinopyroxene
cumulate rock. There are also some grains of olivine in the
rock. Olivine crystals are partly altered to an orange material
that contains clay minerals. This provides evidence that water
once flowed through the rock. This is a light microscope image,
2 mm across. |
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| LIFE
ON MARS?
Martian meteorites are good places to look for evidence of
life on another planet.
People have wondered for many years whether there was ever life
on Mars. There is no evidence that life exists there today. But
billions of years ago, Mars was warmer, wetter, and had a thicker
atmosphere. These conditions might have allowed life to arise.
In 1996, a team of scientists thought they had found fossils
and other evidence of ancient life in a Martian meteorite, ALH84001.
Since then, many scientists have studied this rock and Earth rocks
like it and concluded that there is not fossil life in this meteorite.
Scientists continue to study meteorites looking for what traces
life, either terrestrial or extraterrestrial, might leave behind
for us to recognize.
| [above
left] ALH84001, found in Antarctica, is the only
rock of its kind. It is a cumulate rock made of pyroxene
known as orthopyroxene. It is also much older than all the
other martian meteorites, 4.5 billion years old. The meteorite
is covered in shiny fusion crust. The black cube in the
photo is 1 cm across. Courtesy of Johnson Space Center.
[above center]
Cracks in ALH84001 contain veins of orange carbonate minerals
that were deposited in this rock on Mars, 3.9 billion years
ago. There has been a lot of discussion about whether there
is evidence that tiny organisms once lived in these veins.
Most scientists do not currently believe that there is sufficient
evidence to make this claim. This is a light microscope
image of the carbonate minerals, 1 mm across. The black
spots are spots where analyses were made to measure the
isotopes in the carbonates.
[above right]
This is a an SEM image (using back-scattered electrons)
of the carbonate minerals in ALH84001. In this kind of image,
the shade of grey is a measure of the composition. Carbonate
minerals are banded with different compositions being deposited
as the composition of the fluid flowing through the rock
changed. The photographed area is approximately 300 micrometers
across. |
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