Encyclopedia of Evolution.pdf - Online Reading Center

0 Mars, life on
• The spacecraft Opportunity and the rovers that it delivered
to the Martian surface sent back evidence in 2004 that the
chemistry and structure of some Martian rocks probably
resulted from percolation by water in the distant past.
• Satellite photographs of Martian gullies, published in 2006,
showed that over a four-year time span, a flow of liquid
water had filled a gully with ice and mineral deposits.
What happened to the water on Mars? Being a smaller
planet than Earth, Mars cooled down more quickly from
its initially molten condition. With less gravitation, it was
unable to hold onto a significant atmosphere. The Martian
atmosphere is about one percent as dense as Earth’s. Mars
apparently no longer has liquid magma circulating in its interior;
therefore, it has no magnetic field that can deflect solar
particles. The solar particles have scoured away much of the
atmosphere, including the water vapor.
In 1996 scientists from NASA and several universities
announced direct evidence that there may once have been life
on Mars. They obtained the evidence from one of more than
50 meteorites that apparently came from Mars: a rock called
ALH84001, so named because it was specimen number 1
found in 1984 in the Allen Hills of Antarctica. Meteorites
from Mars are the next best thing to actually bringing rocks
back from Mars.
How could scientists conclude that the meteorite came
from Mars?
• Scientists had to confirm that the rock really was a meteorite.
It was found on top of an ice field in Antarctica, far
from rock outcrops. Scientists therefore concluded that the
This photograph taken in 00 by the Mars Global Surveyor shows gullies
coming from the sides of an impact crater. These gullies were most likely
formed by melting water, either from groundwater or from snow. The
scale bar is about 0 feet ( 00 m). (Courtesy of NASA)
rocks at this location were meteorites. At the location where
the rocks were found, the ice slowly rises and the water
molecules enter the atmosphere, leaving the meteorites
behind. Therefore, over thousands of years, meteorites have
accumulated at this location in relatively large numbers.
• Scientists had to confirm that the meteorite really came from
Mars. Most meteorites are small asteroids (see asteroids
and comets). Some meteorites are volcanic in origin, which
means they must have come from a planet. ALH84001 is of
volcanic origin. Could it have come from a volcano on the
Earth? An analysis of gas bubbles trapped within the rocks
closely matched the atmosphere of Mars. The rocks were
therefore identified as meteorites from Mars.
How could volcanic rocks from Mars have reached the
Earth? The most likely explanation is that a large meteoric
impact on Mars about 15 million years ago blasted some
volcanic rocks up into space. Some of these volcanic rocks
crossed the orbit of the Earth and became meteorites. The
ALH84001 meteorite fell to the Earth about 13,000 years
ago.
What evidence of life can be found in these meteorites?
NASA scientists presented two types of evidence: chemical
and fossil.
Chemical evidence. ALH84001 contains organic compounds
known as polycyclic aromatic hydrocarbons (PAHs),
similar to some of the compounds found in petroleum.
Although terrestrial PAHs are usually produced by biological
activity (as in petroleum), they are also found in interstellar
space (see origin of life).
Is there any direct evidence that the PAHs in the Martian
meteorite were formed by biological activity? The PAHs were
found closely associated with carbonates in the meteorite.
Carbonates (for example, in limestone and the shells of invertebrates)
are often produced by biological processes—but can
also be produced inorganically as dissolved CO2 reacts with
calcium in water. The carbonates have been dated to be 3.6
billion years old, younger than the 4.5-billion-year-old rock
itself. The carbonates, and PAHs, may have entered the rock
after it formed, perhaps from prolonged soaking in water that
contained microbes.
The scientists considered the possibility that the PAHs
could be contaminants from petroleum fumes in the Earth’s
polluted atmosphere. But, they concluded, if this were the
case, there would be more PAHs on the outside than the
inside of the meteorite. Since the outside of the meteorite had
less PAH than did the inside, the scientists concluded that the
PAHs could not have come from a relatively brief exposure
to atmospheric PAHs on earth.
Fossil evidence. Scanning electron micrographs of the
meteorite revealed structures that look like microorganisms
(see photo on page 261). Although it is possible that they are
crystal-like structures formed under lifeless conditions, they
resembled Earth’s microorganisms so much that some observers
could not resist believing them to be the fossils of once
living cells.
Since the initial announcement of the possible Martian
microbes, enthusiasm for the discovery has diminished. The