[BLANK_AUDIO] Of all the planets in our solar system
there's little doubt in saying That Mars has attracted the most Optimism about the possibilities for
extra-terrestrial life. And it reaches right back to Percival Lowell and his speculations about Martian
canals. He drew detailed maps of Martian canals
that he thought had been constructed by Ancient Martian civilizations seeking to
channel water from the polar icecaps down To the equator to their beleaguered,
desiccated cities. But Mars has also permeated science
fiction, Stories of warring Martians appearing from
Mars to Try and lay claim to the Earth in H.G.
Well's, War of The Worlds, for instance. And even in recent times, the possibility
of Civilizations on Mars has captured the
human imagination. Here's a rather nice example.
On the left-hand Side, you can see a hill that was Photographed by the Viking orbiters in the
mid-1970s. The hill has a distinctly monkey-like
appearance, some Sort of primitive primate and those people
who saw This image in the 1970s began to believe That it was some monument constructed by
Martian civilization. Well, when you take this photograph again
with higher resolution images taken by Martian orbiters, that you can see on the
right-hand side, You can see there is no monkey face on
Mars. This is just a hill, a meter, outcrop of
rock That really has no resemblance to a face
at all. This is just a trick of the low resolution Imagery which this hill was photographed
in the early 1970s. It's a very good example of how, if you
stare long enough at an image, You can see what you want to see. Percival Lowell stared at the surface of
Mars through His telescope and he saw what he thought
were canals. An optical processing trick of the human
brain as it Tries to create lines between images that
it's trying to connect.
He saw features on Mars. His brain created lines between them, and
he thought they were canals. In this way, people look at images on the
surface of Mars from orbiters and They think they see monkey faces, whereas
in fact when we look at them At higher resolution, we see that there
are no images of faces at all. Here's another example. This is an impact crater on the surface of
Mars. Is it a monument built by a happy Martian
civilization? No, it's just an impact crater, but if we
look at it, In particular ways, we can see all sorts
of things in these images. These are lessons from The history of astrobiology. Lessons about how one should be cautious
about gathering data, but also Cautious about making interpretations on
things that one might want to believe. One might want to believe that there is
life on Mars even though the data is telling you
something very different. The first attempt to seek life on Mars Using a real experiment were the Viking
biology experiments. These were launched to the surface of Mars
on The VIking landers that landed on Mars in
1976. The Viking biology experiment consisted of
three Sub experiments and these three
experiments Were the following: first of all, there
was a labeled release experiment. And here, in nutrients were placed into
the soil that had radio-labeled carbon. The idea is the nutrients would be used by Microbes, that would produce waste
products and these waste products Would contain radioactive carbons which is
carbon dioxide, which Could be picked up by the instrument as a
demonstration There are metabolically active microbes in
Martian soil. The second experiment was the gas exchange
experiment and here, nutrients were added To the soil to see if any gases were given
off that might be The products of biological activity, such As methane, hydrogen, oxygen, that might
demonstrate
That microbes were actively using those
nutrients And producing gases as a waste product. And the third experiment was the pyrolytic
release experiment, and Here carbon dioxide with radio-labeled
carbon was added to the soil. The idea is that the carbon dioxide would
be taken up By autotrophs, microbes that use carbon
dioxide as a source of carbon. This radio-labeled carbon would be
incorporated Into the cells and then the cells Would essentially be heated up, burnt up,
inside the experiment apparatus, and this Radioactive material, radioactive carbon
that had been fixed in the cells by The cells taking up the carbon dioxide
would be released and detected. The experiment also had a heat sterilized
control where Martian soil was Collected by the Viking lander and heated
to kill any possible biology. And then the same experiments were
performed on this heat sterilized control. So what were the results of the Viking Biology experiments? Well, the labeled release experiment did
give off gas, but This was not repeated in later attempts at
the same experiment. So that was inconclusive. The gas exchange experiment did result in
the Production of oxygen, but the problem was
that Oxygen was also produced in the
non-biological control That had been heated to kill any potential
microrganisms. And then finally, In the pyrolytic release experiment,
radioactive Carbon was found in gases released In that experiment, but it was also found
in the non-biological controls. Taken together, these results suggest that
there Was a reactive component to Martian soil. We now know that Martian soil contains
perchlorate, an Oxidizing compound, and it may contain
other oxidants as well. And we think that those oxidants were
reacting with the nutrients, Other compounds added in the Viking
experiments and Causing the release of these gases in the
experiment.
So it wasn't caused by biology but rather
by chemistry. This is consistent with what we now know
about the Martian surface. It's a very extreme surface. There's no liquid water. There's high levels of ultraviolet
radiation, ionizing radiation. It's not a very good place for biology and So we might expect that the Viking biology
experiments Would be inconclusive in the discovery of
biology That might suggest the possibility of
reactive soils. Since the Viking experiments, there have
been other papers that Have brought evidence suggesting life on
Mars and perhaps the most Famous one, is a paper published in 1996
in Science Magazine about the discovery of potential
life in ancient Martian meteorites. And the meteorite at the focus of these Studies is ALH84001, a Martian meteorite
launched from Mars Many millions of years ago, and discovered
in Antarctica. It sat in Antarctica for over 13,000 Years, and then was discovered by
scientists. We know it's a Martian meteorite because
its composition matches Up with the composition of the Martian
surface, and it contains Within it, bubbles of Martian atmosphere
trapped within the meteorite that Have the same composition, chemical and
isotopic, with the Martian atmosphere. What did scientists Find in this meteorite that led them to
suggest the possibility of life? Well, first of all, they found shapes that
they claimed looked Like microorganisms, long, tubular
structures On the surfaces inside the meteorite. The scientists also found magnetite in the
meteorite. Now magnetite can be produced by Non-biological processes, but it's also
produced By bacteria, magnetotactic bacteria that
use little grains Of magnetite to align themselves with
magnetic fields. And one of the characteristics of these
bacteria is they produce chains of Magnetite, and chains of magnetite were
claimed to be found in the meteorite.
The scientists also found PAHs,
polyaromatic hydrocarbons. These are complex organic compounds that
could be associated with life. They were Found inside the meteorite and associated
with, with globules Of carbonate rocks, inside carbonate
minerals, inside the meteorite. The scientists claim that the association
of these PAH's with a carbonate grain Suggest possibility that these came from
life Rather than from contamination outside the
meteorite. Each one of these lines of evidence is
contestable. Shapes of microbes may not be real
microbes. They could be non-biological processes
depositing Filimentous-like minerals on the surfaces
of meteorites. Chains of magnetite could just be chains Of magnetite minerals formed in
non-biological reactions. And, of course, organic compounds such as
PAHs, We know are formed in the interstellar
medium. They're not necessarily associated with
biology. But the authors claim that each line of
evidence when put together Suggests the presence of biology, even if
individually, they can be contested. The question remains open. Is this evidence of early life on Mars? Astrobiologists debate this evidence
vigorously. Another problem greatly debated amongst
biologists is whether the shapes of Microbes found in these meteorites are too
small to be microbes. They're much smaller Than typical microbes found on the Earth,
maybe They're even too small to be biological
entities. These are also questions that
astrobiologists are attempting to address. So, what have we learned? We've learned that Mars has always been a
focus in the search for life. We've learned that the first serious
experiments Sent there were the Viking biology
experiments. The Viking experiments can be explained by
non-biological Processes, but more recent evidence has
come from
Martian meteorites, but even this remains
highly controversial. Really, the question of life on Mars can
now only be solved with Robotic and human explorers and by
studying new samples of the surface and Subsurface of Mars to get more evidence,
more data points that might help Us to understand whether Mars was ever Habitable and whether it ever harbored
life. [BLANK_AUDIO]
