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During the last
century and into this one, patient and skilled astronomers carried
out increasingly detailed Mars mapping programs that not only created
excellent classical atlases of Mars but also provided more and more
information about the seasonal changes of light and dark surface
features, of the polar caps, of condensate clouds and dust storms.
With such interesting observations to stimulate them some
imaginations ran wild (that of the US astronomer Percival Lowell in
particular) and ideas were described that conjured up all sorts of
fanciful ideas about martian civilzations past and present. Although
the cold dry nature of the martian surface and the low atmospheric
density had been determined from astronomical measurements even
before the space age began, interest in the possibility of life on
Mars remained high. By that time, however, expectations had been
substantially lowered in the face of discouraging facts and only the
possibility of extremely primitive life was contemplated.
Mariner 4 imaged only a small part
of the cratered martian highlands and the resolution was far too poor
to see the valley networks that we know about today. Immediately we
concluded that Mars was rather a near sister to our own stark Moon.
We continued to believe that until Mariners 6 and 7 flew by Mars a
few years later and sampled some other regions and new kinds of
terrain -- including "chaotic" terrains that we now associate with
the source regions of some massive floods. These exciting new
observations began to make the martian explorers' blood run faster
again.
Not to mention
many craters, different in character from those of the Moon, and huge
basins like Hellas and Argyre that Mars has preserved.
The first effort
to search for life on Mars was conducted by the
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As just described, most carbon dioxide that started out in the Earth's atmosphere has been transformed over time into carbonate rocks. But there is this key difference between Earth and Mars: on Earth subduction (the convective plunging down of surface rocks into the interior of a planet) associated with plate tectonics recycles these sediments (through reheating at depth and subsequent volcanic emission of released gases). |
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Mars, by contrast, is a "one-plate planet" with no mechanism to recycle the carbon dioxide it lost from its atmosphere. So the atmosphere would have become thinner and the climate cold. Presumably the water that flowed once on the surface then became trapped below the surface as permafrost and at the ice caps. Perhaps too, at depths of several kilometers there may still be aquifers charged with liquid water. |
An instructive
approach to investigating the question of life on early Mars is to
compare conditions with those of the early Earth. On Earth there is
definite evidence for life at 3.5 billion years ago -- within a
billion years of planetary formation. This evidence is in the form of
fossilized remains of algae mats (stromatolites) as well as
microfossils. There is even some provocative evidence in the isotopic
signature of sedimentary carbon that life was present on Earth 3.8
billion years ago (biologic carbon has more of the heavy carbon
isotope C14 relative to the common isotope C12). Unfortunately, the
ancient sediments in question have been so altered that the case
cannot be made with certainty. Considering that 4 billion years ago
the Earth was still being blasted by titanic asteroidal impacts
capable of boiling oceans, it is clear that life evolved almost as
soon as the dust settled, so to speak.
Turning now to Mars we note with interest that the period during which life originated on Earth (about 3.8-3.5 billion years ago) is a time when liquid water was active on Mars (in fact, water was still flowing episodically a billion years later). The apparent similarity of Earth and Mars during this early period leads naturally to the possibility of a parallel evolution of the biogenic elements on Mars. With this in mind, scientists have been developing a strategy to systematically determine the early climate of Mars, to understand in detail what geologic processes were taking place that might have created benign locations (for example, thermal springs) for the biogenic elements to evolve toward life, and even to find microfossils that could seal the case.
Where should we search for Life on Mars ?
The ancient lakes:
A lake bed is one ideal location (thermal spring deposits would be another but we have yet to discover such deposits) for searching for fossils because the sediments that accumulated on the lake floor provide the necessary protection to physically preserve organic materials. Without preservation, no fossils! Furthermore, our terrestrial experience suggests that the shores of a martian lake may have provided a suitable habitat for microbial colonies (which on Earth form algal mats) that could have been preserved in recognizable form.
Early martian lakes have another attractive characteristic: they could have provided habitable environments (that is, have been a source of liquid water) long after the rest of the surface of Mars had cooled to intolerable levels. We see this today on Earth in the ice-covered lakes of the Antarctic dry valleys where in spite of average temperatures well below the freezing point there is liquid water all year round under a thick ice cover. These dry valleys are extremely cold and dry and, even before the discovery of the ice-covered lakes, were considered to be the best terrestrial analogs to conditions on Mars.
The ice covered lakes of Antarctica are worth a little side trip here. Let's begin with the mean temperature in the dry valleys, which is about -20 degrees C. Precipitation of snow is highly variable, averaging about 2 cm of equivalent water. Four to six meters of thick perennial ice cover the lakes and act to trap heat within the lake. The liquid water (and its large latent heat of solidification) is refreshed by the trickle of melt water from a nearby glacier associated with the few days each summer that temperatures climb above freezing. These few days, and their "degree-day" product with temperature above freezing, are the key to the persistence of the ice-covered lakes. Simple climate models suggest that on Mars the degree-days above freezing would have been comparable to the situation today in the Antarctic dry valleys for about 500 million years after the mean annual temperature fell below freezing. Of course, we also have to consider the (slow) rate at which the ice cover would have sublimed into the thin cold martian atmosphere. For lakes that began with depths of several hundred meters, similar time scales of hundred's of millions of years are involved. Thus we conclude that ice covered-lakes could have been the ultimate refuge for life on Mars. One of the most exceptional examples of an ancient martian lake is Gusev crater.
Permafrost and ground-water ice:
A second location that may provide evidence for early life on Mars is the permafrost. Near the south pole is it likely that there is the material that have been frozen continuously since the early, possibly biological, period of Mars history. It is observed in the polar regions of Earth that microorganisms are trapped in the active layer and frozen into the permafrost and can be viable after long periods of time, up to 3 million years at -10 degrees. On Mars the timescale for preservation is longer, 3.5 billion years, but the temperature is also much colder (about -70 degrees). Thus, it is possible that well-preserved organically-intact microorganisms, if not viable life, would be found 30 to 50 meters below the martian permafrost. Preserved at this depth, the organisms would be shielded from cosmic and solar radiation (but would still suffer from radioactivity from the martian crust) and would be immune to transient warming events lasting thousands of years.
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