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- Suppose that billions of years ago life developed on
Mars. Primitive, tiny organisms thrived deep within rocks and made a living
from water and chemicals seeping through those rocks.
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- Now imagine that a huge asteroid collided with Mars.
Millions of Martian rock fragments were thrown into space by the force
of the impact. Tough Martian organisms hitchhiked on some of these rocks.
Many rocks went into orbit around the Sun, and, after hundreds of thousands
of years some of these collided with the Earth, and a few rocks reached
the surface. Some hardy Martian organisms survived the journey, colonized
the Earth and eventually evolved into the huge variety of life that we
know today.
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- Just a few years ago this scenario would have been dismissed
as wildly imaginative and impossible. It is still highly speculative, but
recent discoveries in several fields of science have shown that it is far
from impossible -- our ancient ancestors might have been Martians.
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- Enter the nanobes
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- It is not every day that a scientist discovers a possible
new life form. Dr. Phillipa Uwins works in the Center for Microscopy and
Microanalysis at Queensland University, Australia. Last year she was asked
to analyze some rock samples taken from several miles under the seabed
in a drilling operation off the coast of western Australia. The temperature
at this location was around 300 Fahrenheit (150 C) and the pressure was
an incredible 2,000 times normal atmospheric pressure at sea level.
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- During an electron microscope examination, Uwins found
what appeared to be tiny, dormant organisms only 20 nanometers across (a
nanometer is one -billionth billionth of a meter). She named them "nanobes."
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- To Uwins' surprise, filaments grew when samples of the
nanobes were given some food and exposed to normal temperatures and pressures.
She was also surprised because the cell walls of the nanobes survived the
intense radiation and vacuum of the electron microscope.
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- Uwins teamed up with two microbiologists to further investigate
the nanobes. A range of chemical tests indicated that the tiny objects
contained DNA. This finding challenged the notion that a "cell"
20 nanometers in diameter was too small to have room for this essential
ingredient of life as we know it (it is also one of the arguments against
the "nanofossils" in Martian meteorites -- discussed below).
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- Here was a tough little critter that was able to survive
the heat and pressure deep underground and, possibly, the vacuum and radiation
of an electron microscope. The Queensland team may have found an organism
that can survive a ride between the planets aboard a meteorite.
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- Creatures don't have to be as small as nanobes to survive
space-like conditions. Several other examples of "extremophiles"
(organisms that can survive very hostile conditions) have been found by
other researchers in recent years, including bacteria that live inside
nuclear reactors.
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- Meteorites from Mars
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- Rocks from Mars have made it to the Earth. Remember the
fuss in 1996 when NASA scientists claimed they had found possible fossil
evidence of ancient life in a meteorite from Mars? (That debate is still
not settled.)
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- After being blasted from the surface of Mars, one small
chunk of rock spent 16 million years in orbit around the Sun. Then some
13,000 years ago it collided with the Earth and landed on the icy Antarctic.
In 1984, scientists searching for meteorites found it and named it ALH84001.
Ten years later researchers figured out that ALH84001 had come from Mars.
This was based partly on an analysis of the Martian atmosphere by the Viking
spacecraft in 1977 -- but that is another detective story.
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- A dozen or so other meteorites are now known to come
from Mars. In 1911 one of them fell to Earth in Egypt and killed a dog,
after which it took almost 80 years to recognize that the unlucky dog had
been killed by a rock thrown from Mars. Planetary scientist and crater
expert Dr. Jay Melosh, from the University of Arizona, has estimated that
about half a ton of Martian material falls to Earth each year.
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- Melosh observed that some of the Martian meteorites had
no evidence of a violent shock when they were blasted into space from the
surface of Mars. This meant "back to the drawing board" for theories
about rocks ejected into space by impacts -- they were supposed to be partly
melted by the intense heat from the shock wave. Melosh worked out that
rocks near the surface could be launched into space without "shock
heating." The effect is something like crumbs being flicked from a
shaken picnic blanket.
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- The discovery meant that organisms hiding within ejected
rocks could survive the blast from a nearby asteroid impact. There are,
however, many other hazards in a flight from Mars to Earth. Melosh investigated
these hazards and was able to show that some organisms had a fighting chance
of making the trip between the planets.
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- Surviving space flight
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- In his classic 1865 novel From the Earth to the Moon,
science fiction writer Jules Verne wrote of a piloted space capsule being
launched by a gigantic cannon. Verne was aware of the scientific error
with this idea -- the astronauts would be crushed by forces thousands of
times greater than the pull of gravity (1 G or an increase in speed of
32 feet per second every second). Verne knew that the steady and comparatively
gentle rocket was the way to reach space, but his audience of the day was
more likely to believe the cannon story.
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- For most creatures on Earth, exposure to several hundred
Gs would be fatal. They have no hope of surviving the Jules Verne cannon
blast, or a ride on a rock blasted into space by an asteroid impact. But
some very primitive forms of life are so small and simple that they can
survive enormous accelerations -- 10,000 G or more.
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- One way that scientists can test the ability of organisms
to survive very high G forces is to fire them out of a cannon. This was
recently done in Sweden and a substantial proportion of dormant organisms
survived the launch conditions. Jules Verne had the right idea but the
wrong organism.
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- Of all the rocks blasted from the surface of Mars into
orbit around the Sun, about 1 in 15 will eventually collide with the Earth.
Some make it in thousands of years. Others may take millions of years.
For organisms aboard these rocks the journey would be extremely hazardous,
with freezing temperatures, deadly cosmic rays and ultraviolet radiation.
But the rock spaceship provides some protection from radiation and cosmic
rays, and the deep freeze may actually help some organisms survive the
tough conditions.
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- Next, the organisms would have to survive the rigors
of colliding with the Earth. The outside of a meteorite glows white-hot
as it plunges into the atmosphere at 25,000 mph or more. Many burn up completely
and never reach the ground. However, a small percentage survive and fall
to the surface.
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- The inside of a meteorite is protected from the heat
of re-entry because rock is a very good heat insulator. People who have
come across a freshly fallen meteorite sometimes report that a layer of
frost has formed on its surface. The inside remained at the freezing temperature
of space even though the outside glowed white hot during re-entry. Any
organisms within the meteorite could therefore, in theory, survive to reach
the surface of the Earth.
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- Finally, the organisms would have to make a home for
themselves on their new planet. Billions of years ago the conditions on
Earth might have been similar to those on Mars, so colonizing the Earth
may not have been difficult, compared with the hazards of getting there.
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- Of course, this scenario all depends on Mars having some
tough forms of microscopic life billions of years ago. Maybe the exciting
space missions to Mars planned over the next few years tell us whether
this was the case or not.
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- Lifeboats in space
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- Another intriguing possibility is that meteorites may
have acted as lifeboats ("escape pods" for Star Wars fans).
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- Giant asteroids and comets bombarded the planets up until
the time that life is first thought to have arisen. Following some of these
impacts the surface of the Earth would have been sterilized by temperatures
much hotter than an oven, and any oceans would have boiled away. Perhaps
the only escape for organisms was to be blasted into space and the really
lucky ones returned to the Earth when things cooled down. The same rescue
system could have worked for any life on Mars.
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- Maybe Martians came from Earth
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- Even if the scenario of Mars life seeding the Earth is
not correct, the reverse seems quite likely. Rocks all over the surface
of the Earth have been found to contain microscopic life. It is hard to
find places that don't have life. Hundreds of huge asteroid impacts have
occurred on Earth since life first began. Without doubt some of these impacts
would have launched rocks bearing microscopic life.
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- In his book "The Fifth Miracle," physicist
Paul Davies discusses the origins and development of life on Earth. He
pays great attention to the possibility of life being exchanged between
Earth and Mars and concludes "It is therefore inevitable that life
from Earth has reached Mars ... that is why I am certain that there was
life on Mars in the past, and may well be life there today".
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- Paul Davies also notes that, 4 billion years ago, Mars
may have been more suitable than Earth for the development of life. He
argues that we should be prepared for the possibility, remote though it
may seem, that we are descended from Martians.
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