LIFE ON MARS (??)
For the curious:
Meteorites
from Mars! Information about martian meteorites, provided by NASA's Johnson
Space Center.
The
War of the Worlds, by H. G. Wells. A classic of science fiction.
Key Concepts
- Liquid water existed on Mars in the past.
- Searches for life on Mars have yielded no evidence that
life is currently present.
- A meteorite from Mars contains structures which might
be microscopic fossils.
(1) Liquid water existed on Mars in the past.
Currently, there is no liquid water on the surface of Mars.
There is frozen water on the surface (in the ice caps near the
north and south poles). There is frozen (and perhaps liquid) water underneath
the surface. However, the surface of Mars is currently a bone-dry desert, with
no oceans, no lakes, and no rivers.
In the past, however, liquid water existed on the surface
of Mars. We have evidence that this is so, in the form of features on the
surface of Mars that have been cut by running water. Narrow dry
riverbeds are found on Mars, meandering across the surface of the
planet like rivers on Earth. These features are found in the southern,
volcanically inactive, hemisphere of Mars, so it is implausible that they were
cut by lava instead of water. The dry riverbeds are (approximately) 3 billion
years old, judging from the density of impact craters on top of them. A typical
Martian dry riverbed is shown below.
Also, broad flash-flood channels are
found on Mars, apparently the result of catastrophic floods rushing across the
surface. The outflow channels are often hundreds of kilometers across. On
average, the broad channels scoured by sudden catastrophic floods are younger
than the dry riverbeds cut by stably flowing rivers. A typical outflow channel
is 2 billion years old.
(2) Searches for current life on Mars have yielded
negative results.
Life on Earth is based on complicated carbon compounds
suspended in water. Since terrestrial life is the only kind of life we know
about, let's restrict our search to lifeforms based on carbon compounds and
liquid water. Complex carbon compounds are ubiquitous in the universe, but
liquid water requires fairly unusual conditions. On the Earth's surface, the
temperature dropped below the boiling point of water about 4.3 billion years
ago; the first traces of life have been dated to 3.8 billion years ago. On
Earth, at least, life started several hundred million years after liquid water
was first present.
On Mars, liquid water was present for a billion years or
more, judging from the ages of riverbeds and outflow channels. Therefore, it is possible
that life evolved on Mars.
The Viking 1 and Viking 2 spacecraft, which landed on the
Martian surface in the year 1976, tested for the presence of life. Aboard each
of the two spacecraft a soil sample, scooped up by a mechanical arm, was
subjected to a battery of three tests. The bottom line for the experiments: No
evidence for life was found. (There is still a loophole, however, for those who
are rooting for life on Mars. The Viking landers sampled only two locations on
the planet - it is remotely conceivable that life might exist at other locations
on Mars.)
(3) A meteorite from Mars contains structures which might
be microscopic fossils.
There is now evidence that life currently
exists on Mars. Could life have existed on Mars in the past, becoming extinct
when the liquid water dried up? Please note that the notorious ``Face on Mars'',
depicted below, does not constitute proof that life has existed on Mars.
The ``face'' was claimed by some individuals to have been artificially created
by intelligent life on Mars. Alas, the truth is frequently more mundane than
fiction; the face is nothing more than a wind-sculpted mesa, about a mile
across. Personally, I am rather fond of the hypothesis that Mars is inhabited by
Muppets; after all, there is a geological formation on Mars that looks
suspiciously like Kermit the Frog raising his arm to wave hello to us:
A Martian fossil would provide strong evidence that life once
existed on Mars. Although humans have not yet traveled to Mars for a
paleontological dig, some Mars rocks have actually arrived on Earth, in the form
of meteorites. Meteorites on Earth are most easily collected in central
Antarctica. On an ice field hundreds of miles across, a dark meteorite is simple
to spot. One meteorite, collected in the Allan Hills of Antarctica in
1984, and
given the name ALH 84001, proved to have the same chemical composition as Mars
rocks studied by the Viking and Pathfinder spacecraft. In addition, small
pockets within the meteorite contain gas identical in composition with the
Martian atmosphere. The age of ALH 84001 is about 4.5 billion years. The most
plausible hypothesis is that ALH 84001 was blasted away from the Martian surface
by the impact of a large asteroid. After millions of years orbiting the Sun as a
meteoroid, its path intersected that of the Earth, and it landed in Antarctica.
A picture of ALH 84001 is given below (click on the small image for a larger
view). The meteorite is roughly the size of a potato, and weighs four pounds.
Having a chunk of Mars to play with certainly excited the
planetary scientists at NASA. One of the things they did with ALH 84001 was to
make thin slices of it and look at them under an electron microscope. To their
surprise, the scientists found a number of small, elongated objects, as shown in
the picture below (click on the image for a larger view).
The elongated objects are about 100 nanometers across, and
up to 1000 nanometers long. Some of the objects are nearly round instead of
being elongated. The objects (whatever they are) are similar in shape to
bacteria. Chemical analysis shows that they contain crystals of magnetite (a
mineral which is also found in some bacteria). The objects also contain `polycyclic
aromatic hydrocarbons' (a class of carbon compounds found in Earth organisms).
The similarity in shape to bacteria, combined with the presence of materials
associated with life, has led some to claim that the objects are microscopic
fossils.
On the other hand...
- Magnetite and polycyclic aromatic hydrocarbons can also
be created by nonbiological processes. Their presence is not proof of life.
- The possibility of contamination AFTER the meteorite
reached Earth cannot be ruled out. Polycyclic aromatic hydrocarbons have
been detected in the ice of Antarctic, and may have seeped into the
meteorite as it rested on the Antarctic ice cap.
- The Martian ``fossils'' are smaller than typical
micro-organisms on Earth. Can all of the mechanisms needed for life be
packed into a tube only 100 nanometers across? It's not clear that it can be
done. (see Nanobacteria
)
The jury is still out. In the words of a NASA spokesman,
``It's too early to come to a final conclusion about life on Mars. No one knows
the truth yet.''
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