The Surface of Mars as seen by Viking
From: http://www.winthrop.dk/
Viking and the Search for Life on
Mars
Despite unresolved controversies concerning evolution
of the Martian environment, the findings of Mariner 9 revived hopes that the
Viking lander spacecraft might detect the existence of life on the planet's
surface. When they reached the Martian surface in July 1976, the two landers
conducted four experiments intended to detect the presence of microbiological
life on the Martian surface
The Gas Exchange Experiment
sought to detect alterations in the composition of the gases in the test
chamber as a result of biological activity. This experiment detected
significant increases in the level of oxygen in the test chamber, which was not
inconsistent with biological activity. But the oxygen release was also
consistent with the reaction of residual humidity in the test chamber with
peroxides and superoxides, produced on the Martian surface by solar ultraviolet
radiation. The existence of such oxidants was anticipated, given the
predominance of CO2 in the Martian atmosphere, since, in the absence
of such an agent, solar ultraviolet radiation would decompose the CO2
to CO and free oxygen. This interpretation was reinforced by the decreasing
level of oxygen release over time, which was inconsistent with biological
activity.
The Labeled Release Experiment
used a liquid nutrient tagged (labeled) with radioactive Carbon-14, to detect
the uptake of the nutrient by micro-organisms. The Martian soil, when wetted by
this nutrient, rapidly released significant volumes of labeled gases over a
period of eight days. This was initially interpreted as indicative of the presence
of biological activity. But the high rate of gas release slowly plateaued in a
manner that was inconsistent with either chemical or biological activity.
The Pyrolytic Release Experiment
would detect the existence of organic materials in the soil. Following a
five-day incubation period under a xenon-lamp, during which it was exposed to
radioactive CO2, surface samples were heated to high temperatures to
determine if any of the radioactive CO2 had been incorporated into
molecular compounds. Seven of the nine runs of this experiment seemed to
indicate very small concentrations of micro-organisms. These results were
subsequently discounted on the basis that indigenous Martian life-forms would
have been killed by the relatively high temperatures they were exposed to in
the initial incubation period.
The Gas Chromatograph -- Mass
Spectrometer heated a soil sample to determine the chemical
composition of the Martian surface. While primarily of interest for geological
investigations, this experiment could also be used to detect concentrations of
carbon compounds that would be the constituents of Martian micro-organisms.
Although these tests evolved a surprising amount of water, they failed to
detect organic compounds. This not only strongly suggested the absence of life
on Mars, it also suggested the presence of some mechanism that was destroying
the carbon compounds in meteorites that were reaching the Martian surface.
These results were generally taken to
indicate that Mars was not habitable
"Viking
not only found no life on Mars, it showed why there is no life there.... the
extreme dryness, the pervasive short-wavelength ultraviolet radiation... Viking
found that Mars is even dryer than had previously been thought... The dryness
alone would suffice to guarantee a lifeless Mars; combined with the planet's
radiation flux, Mars becomes almost moon-like in its hostility to life."
However, not everyone was convinced. Several
lines of argumentation were advanced to suggest that the Viking results were
not inconsistent with the presence of life on Mars.
First, it was suggested that while the
Viking findings did not provide proof of life on Mars, they did at least
provide evidence suggesting the existence of life. Gilbert Levin, who
was the principal investigator for the Labeled Release (LR) experiment,
maintains that.
"...it
is now more than probable than not that the LR experiment did in fact detect
life on Mars."
This conclusion is based on a number of
observations concerning the results of the Labeled Release Experiment:
Analysis of a sample retrieved in the
morning (prior to extensive exposure to solar ultraviolet radiation) that had
been covered by a rock which provided long-term protection from solar
ultraviolet radiation, produced results that were only slightly weaker than
those from exposed surface samples.
A sample which was exposed to a temperature
of 50oC, which was regarded as sufficient to destroy Martian
life-forms but not chemical reactants, produced 65% less activity than the
sample tested at the control temperature of 170oC, suggesting that
the activity of Martian life-forms had been attenuated at the lower
temperature.
Another sample, which had been stored in the
Viking lander for several months, produced no activity when exposed to the
nutrient. This result was consistent with the death of the Martian
micro-organisms after long-term confinement in the dark of the sample chamber,
but perhaps more difficult to reconcile with the disappearance of reactive
chemicals. Terrestrial tests displayed a similar response, with Antarctic
micro-organisms dying within eight days of confinement to a sample chamber.
The failure of the Gas Chromatograph -- Mass
Spectrometer to detect Martian organic matter could have resulted from the
relative insensitivity of this instrument. In laboratory tests it failed to
detect the existence of biological activity in Antarctic rock samples. In
addition, it is relatively insensitive to some large organic molecules.
Levin further argues against the existence
of chemical reactions that could mimic biological activity:
"...
the Mariner 9 IRIS instrument found no trace of hydrogen peroxides on Mars...
Other non-biology theories involve: minerals that catalyze the reactions with
the LR nutrients, ultra-violet radiation... ionizing radiations of various
kinds, finely divided and desiccated oxygen-rich minerals, and large surface
areas of fine particulate generating heat of hydration upon wetting. We and
others have tested all of these proposals, and none of them could provide a fully
satisfactory explanation..."
Other experiments on Earth seemed to
contradict these conclusions. Although their presence on the Martian surface
has not been conclusively established, tests with Iron (Fe) rich clays were.
"...
able to reproduce accurately the Viking labelled release results. Fe-rich clays
were also used successfully to simulate the gas exchange and pyrolitic release
experiments."
Similar results were claimed for oxidizing
agents formed by the interaction of atmospheric water vapor and surface
minerals..
Although this debate continues, the
preponderance of opinion, if not evidence, is against the notion that the
Viking biology experiments detected compelling evidence of the existence of
life on Mars.
Second, it was suggested that, while the
surface samples that were tested might not have contained Martian organisms,
life-forms might reside in Martian surface materials not sampled at the Viking
lander sites. Indeed, Viking may have merely sampled Martian dust, which would
have been exposed to extreme environmental conditions, rather than compacted
Martian regolith, which might be expected to have been subjected to somewhat
less stressful conditions.
Levin has suggested that an analysis of
images of Martian rocks indicates the existence of greenish patches whose
appearance changes with time. He suggests that these should be interpreted as
analogous to Terrestrial lichen or algae, which can live several centimeters
below the surface of porous rocks. When samples of such biologically active
rocks are subjected to analysis in the Labeled Release Experiment, they produce
results strikingly similar to those produced by Viking on Mars. Whether r not
the Viking images have detected Martian lifeforms, this model of lifeforms
protected by rocks from the harsh Martian environment is not without merit.
Third, it was suggested that conditions at
the Viking lander sites might not be typical of the planet as a whole.
Almost all of the remaining water on Mars is
trapped below the Martian surface, at depths of up to several kilometers. In
the polar regions, the ice may lie near the surface. But in the region within
30o to 40o of the equator ice deposits are unlikely to be
found at depths up to several hundred meters. In this equatorial region,
subsurface ice would be warmed by the internal heat of Mars and turn into water
vapor, which would slowly diffuse to the surface where it would either be
disassociated by solar ultraviolet radiation and escape into space, or else
precipitate in the colder polar regions.
The Viking 1 landing site in Chryse Planitia
was at 22.4o north latitude, where extensive depletion of sub-surface
ice would be expected. The Viking 2 lander came to rest at 48o north
latitude, in Utopia Planitia, which was sufficiently far north that sub-surface
ice would not have been extensively depleted. Paradoxically, the positive
results from the Pyrolytic Release Experiment were obtained at the presumably
drier Chryse site.
Earlier discussions of the possibilities for
life on Mars suggested that life forms might not be distributed over large
areas of the planet, but rather concentrated in small locales that provided
particularly clement conditions. Obviously, such unique environments were not
sampled by Viking. Moreover, such candidate surface locations for such isolated
paradises are difficult to establish.
However, Martian life forms might not be
confined to the Martian surface. These suggestions are based on the.
"...
discoveries of functional and complex microbial ecosystems in the deep
subsurface at a variety of locations in terrestrial oil fields, solfatara, and
(more recently) aquifers. Work from as long ago as the 1920s has shown that the
deep subsurface itself is not inhospitable to microbes, at least on Earth....
microbial populations found in the deep aquifers of the Savannah River Plant
may have arrived during the late Cretaceous period as the sediments were being
laid down... sulfur reducing bacteria... have a wide distribution in
oil-bearing rocks and underground waters of the Earth.... we are conservatively
considering only anaerobic chemolithoautotrophs... The deep sea hydrothermal
vents are aerobic systems with primary production based on the oxidation of
sulfide and therefore not directly relevant to the anaerobic systems we
hypothesize for Mars.
"Chemolithoautotrophs
are those microbes capable of fixing inorganic carbon, CO2, into
living material by means of energy produced from an oxidation reaction of an
inorganic compound. On Earth, there are many groups of such microorganisms.
Some respire aerobically using O2, but many operate anaerobically
using NO3 as a terminal electron acceptor in respiration instead of
O2. Other kinds of bacteria can also use SO4, CO2,
CO, or iron as terminal electron acceptors in respiration.
"If
Mars currently possesses some geothermal activity, then such activity in the
near or deep subsurface could provide both liquid water and energy sources to a
microbial ecosystem.
"Volcanic
gases could include H2, H2S, SO2, CH4
CO, CO2, and others. Gases percolating from below would easily
dissolve to the limits of their solubility and be available for biological use.
In such a system, the direction of flow of the "rain of nutrients"
would be from below to above, as we see, for example, in rift zones, on the
floor of the modern ocean...
"We
envision any microbes present in such a system to have been present from
antiquity. As conditions on the Martian surface worsened, organisms that had
migrated or been carried to deeper levels would enjoy a measure of protection
not experienced by surface dwellers..."
Despite the mass of data on Mars that has
flowed from space probes over the past three decades, we are ultimately little
better off than the astronomers of prior centuries. Today, as then,
observational data places some constraints on speculation concerning life on
the Red Planet, but the data neither proves nor disproves whether Mars is
indeed the abode of life. Certainly today's data places much more explicit
constraints on the nature of potential Martian lifeforms, probably excluding
the existence of multi-cellular organisms. What remains is essentially a
theological debate, in which the central thesis remains, in the tradition of
Scottish jurisprudence, not proven. What is proven, however, it that the search
for life on Mars will require a level of effort that far exceeds that
envisioned in previous centuries.
References
14. Gold, Michael, "Through the Veil of
Venus," Science 82, January/February 1982, page 12.
15. Arrhenius, Svante, Kosmos, 1910,
pages 123-128, quoted in Ley, Willy, and von Braun, Wernher, The Exploration
of Mars, (New York, Viking, 1956), page 74.
16. Horowitz, Norman, To Utopia and Back:
The Search for Life in the Solar System, (New York, W.H. Freeman, 1986),
page 97.
17. Burgess, Eric, "There Are
"Canals" on Mars," Spaceflight, 1965, pages 46-47, 74.
18. Anders, Edward, "Mars and Earth:
Origin and Abundance of Volatiles," Science, vol 198, 4 November
1977, pages 453-464.
19. Haberle, Robert, "The Climate of
Mars," Scientific American, May 1986, pages 54-62.
20. This conclusion is reached on the basis
of the analysis of the ages of meteorites that are presumed to have derived
from Mars in: Laul, J.C., "The Shergotty Consortium and SNC Meteorites: An
Overview," Geochimica Cosmochimica Acta, vol 50, 1986, pages
875-888.
34. Ezell, Edward, and Ezell, Linda, On
Mars -- Exploration of the Red Planet 1958-1978, (Washington, NASA, 1984),
NASA History Series SP-4212.
35. Horowitz, Norman, "The Biological
Question of Mars," in Reiber, Duke, editor, The NASA Mars Conference,
(San Diego, Univelt, 1988), American Astronautical Society Science and
Technology Series, vol. 71, pages 177-185.
36. ibid.
37. Levin, Gilbert, and Staat, Patricia,
"A Reappraisal of Life on Mars," in Reiber, Duke, editor, The NASA
Mars Conference, (San Diego, Univelt, 1988), American Astronautical Society
Science and Technology Series, vol. 71, pages 187-208.
38. Adelman, Benjamin, "The Question of
Life on Mars," Journal of the British Interplanetary Society, vol.
39, 1986, pages 256-262.
39. Carr, Michael, "Martian
Geology," Nature, vol. 294, 26 November 1981, page 307.
40. Chaikin, Andrew, "The Case for Life
on Mars," Air & Space, February/March 1991, pages 63-71.
41. Banin, A, and Navrot, J., "Chemical
Fingerprints of Life in Terrestrial Soils and Their Possible Use for the
Detection of Life on Mars and Other Planets," Icarus, vol. 37,
1979, pages 347-350.
42. Levin, Gilbert, and Staat, Patricia,
"A Reappraisal of Life on Mars," in Reiber, Duke, editor, The NASA
Mars Conference, (San Diego, Univelt, 1988), American Astronautical Society
Science and Technology Series, vol. 71, pages 187-208.
43. Thomas, David, and Schimel, Joshua,
"Mars After the Viking Missions: Is Life Still Possible?" Icarus,
vol. 91, 1991, pages 199-206.
44. Lederberg, Joshua and Sagan, Carl,
"Microenvironments for Life on Mars," Proceedings of the National
Academy of Sciences, vol. 48, num. 9, 15 September 1962, pages 1473-1475.
45. Boston, Penelope, et al, "On the
Possibility of Chemosynthetic Ecosystems in Subsurface Habitats on Mars," Icarus,
vol 95, 1992, pages 300-308.
The
Viking Biology Experiment (2)
The Viking biology experiment weighed 15.5 kg (34 lbs) and
consisted of three subsystems: the Pyrolytic Release experiment (PR), the
Labeled Release experiment (LR), and the Gas Exchange experiment (GEX).
Viking Biology Experiment
In addition, independent of the biology
experiments, Viking carried a Gas Chromatograph/Mass Spectrometer (GCMS) that
could measure the composition and abundance of organic compounds in the martian
soil. (It should be noted that
Labeled
Release
The LR experiment moistened a 0.5-cc sample
of soil with 1 cc of a nutrient consisting of distilled water and organic
compounds. The organic compounds had been labeled with radioactive carbon-14.
After moistening, the sample would be allowed to incubate for at least 10 days,
and any microorganisms would hopefully consume the nutrient and give off gases
containing the carbon-14, which would then be detected. (Terrestrial organisms
would give off CO2, carbon monoxide (CO), or methane (CH4).)
Gas
Exchange
The GEX experiment partially submerged a
1-cc sample of soil in a complex mixture of compounds the investigators called
"chicken soup". The soil would then be incubated for at least 12 days
in a simulated martian atmosphere of CO2, with helium and krypton added. Gases
that might be emitted from organisms consuming the nutrient would then be
detected by a gas chromatograph -- this instrument could detect CO2, oxygen
(O2), CH4, hydrogen (H2), and nitrogen (N2).
Pyrolytic
Release
Of the three Viking biology experiments,
only the PR experiment approximated actual martian surface conditions and did
not use water. In this experiment, a 0.25-cc soil sample was incubated in a
simulated martian atmosphere of CO2 and CO labeled with carbon-14. A xenon arc
lamp provided simulated sunlight. After 5 days, the atmosphere was flushed and
the sample heated to 625 degrees C (1157F) to break down, or pyrolyze,
any organic material, and the resulting gases were passed through a carbon-14
detector to see if any organisms had ingested the labeled atmosphere.
The
Results
The most important result for the detection of life came not
from the biology experiment, but from the GCMS. It found no trace of any
organic compound on the surface of Mars. Organic compounds are known to be
present in space (for example, in meteorites), so this result came as a
complete surprise. The GCMS was definitely working, however, because it was
able to detect traces of the cleaning solvents that had been used to sterilize
it prior to launch.
The total absence of organic material on the
surface made the results of the biology experiments moot, since metabolism
involving organic compounds were what those experiments were designed to
detect. However, the results from the biology experiments were sufficiently
confusing to be worth examining.
To reduce the chance of false positives, the
biology experiments not only had to detect life in a soil sample, they had to fail
to detect it in another soil sample that had been heat-sterilized (the control
sample). Had terrestrial life been tested with the Viking biology instrument,
the following results would have been expected:
response for response for sample heat-sterilized control GEX oxygen or CO2 emitted noneLR labeled gas emitted nonePR carbon detected none
If life
was completely absent from Mars, as the GCMS results suggested, these should
have been the results from the biology experiments:
response for response for sample heat-sterilized control GEX none noneLR none nonePR none none
In highly
simplified form, these were the actual results from Mars:
response for response for sample heat-sterilized control GEX oxygen emitted oxygen emittedLR labeled gas emitted nonePR carbon detected carbon detected
The fact
that both the GEX and PR experiments produced positive results even with the
control sample indicates that non-biological processes are at work. Subsequent
laboratory experiments on Earth demonstrated that highly-reactive oxidizing
compounds (oxides or superoxides) in the soil would, when exposed to water,
produce hydrogen peroxide. Oxidized iron, such as maghemite, could act as a
catalyst to produce the results seen by the PR experiment.
Only the LR experiment appears to have met the
criteria for life detection, and it does this rather ambiguously. When the
nutrient was first injected, there was a rapid increase in the amount of
labeled gas emitted. Subsequent injections of nutrient caused the amount of gas
to decrease initially (which is surprising if biological processes were at
work) but then to increase slowly. No response was seen in the control sample
sterilized at the highest temperature (160C, 320F.) While there is still some
controversy, the consensus opinion is that the LR results can also be explained
non-biologically.
Extinct
Life
Most
researchers now believe that the results of the Viking biology experiments can
explained by purely chemical processes that do not require the presence of
life, and the GCMS results completely rule out life in any event. Thus, there
is no detectable life at the two Viking landing sites, which were widely
separated and different in character (the Viking 2 landing site was
specifically chosen because of its high latitude, since it was closer to polar
water sources.) While the possibility of "oases" of more favorable
conditions for life cannot be eliminated, for example in subsurface permafrost
layers or in geothermal vents near volcanoes, the chances that life exists on
Mars at the present time do not seem good.
However, we have seen evidence that Mars
may have been significantly wetter, perhaps with a denser atmosphere,
earlier in its history. If so, there is the possibility that life arose on
Mars, only to die out as conditions on the planet worsened. Therefore, some
researchers have suggested that future searches for life on Mars be shifted to
focus on extinct, rather than extant, life.
On Earth, such extinct life can be found in the form
of microfossils and stromatolites. Such forms, as found in
western Australia, are the oldest evidence of life on Earth, dating from 3.5
billion years ago. Microfossils are individual fossilized organisms (typically
algae), as much as a few millimeters in diameter. Stromatolites are formed when
layers of microbial organisms in shallow lakes or pools are covered with
sediments. The organisms migrate toward the light after being covered, and the
remaining organic material forms a characteristic layered or domed structure.
Stromatolites are important because they may be large
enough to be seen by lander (or perhaps even high-resolution orbiter) cameras,
and so some researchers have suggested searching for them near features that
appear to be ancient lakes or bays. While definitive proof of biological origin
would require microscopic imaging or sample return, the discovery of such
features would lend credibility to the idea of extinct life.
Conclusions
The
question of whether life is common or rare in the universe has deep
philosophical implications. It is uncertain exactly how life arose on Earth, so
it is difficult to determine how common such mechanisms are. But if life also
arose on Mars, this would show that those mechanisms operated not just once,
but twice, arguing that life may well be common elsewhere.
However, the search for life on Mars thus far has
been unsuccessful. Some portion of the scientific community feels that further
searches are a waste of time, while another portion remains neutral or
guardedly optimistic. In principle, it's simple to prove that there is
life on Mars -- all one need do is find an example. Proving there isn't life on
Mars is much harder. Even a prolonged negative search can be countered with the
suggestion of yet another, more inaccessible place in which to look.
In the case of Mars, the issue has been complicated
by the emotional belief in an Earthlike Mars, which has largely been shown to
have been a myth. Mars is a spectacular place, and will remain so even if it is
finally proved to be lifeless. Today, we don't know for sure if there is or
ever was life on Mars. But one thing is certain -- one day, there will be.