Transcript Discussion Notes from October 17th

Viking Missions 1976
•Gas Exchange
Experiment
•Pyrolytic Release
Experiment
•Labeled Release
Experiment
Also carried a Gas
Chromatograph Mass
Spectrometer
Gas Exchange
Water and Organic Compounds (19 amino
acids and over a dozen growth factors, plus
numerous inorganic salts )
Mars soil
The GEX experiment partially submerged a 1cc 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).
Gas Exchange Results
This gas exchange experiment (GX) was based on the assumption that life
consisted of heterotrophic micro-organisms requiring the presence of or
direct contact with organic substrates and possibly various organic growth
factors for their metabolism, and that metabolism would involve the uptake or
release of metabolic gases. Therefore, this experiment called for a thick mixture
consisting of many organic compounds both in the solution and in the vicinity of the
solution and tested for the evolvement of gas from the sample. This experiment
was also designed to distinguish between those gases that emerged from nonbiological processes from those that emerged form biological processes.
The GC results that every gas except O2 exhibited the same level as the
control indicating that no change had occurred. The increase in O2 seemed
to indicate that some heterotrophic organism produced it upon contact with the
thick nutrient medium.
Problems
Another viable reason for the evolution of more O2 was the presence of
peroxides and superoxides in the soil. Upon contact with moisture, these
compounds could have produced O2 and are therefore not evidence of life. In
addition, the rapidity of the gas evolvement seems indicative that the reaction was
a chemical one and not a biological one.
Pyrolytic Release
Mars soil
Of the three Viking biology experiments, this was the only one to
attempt to detect signs of life in the complete absence of water
and organic nutrients. It was assumed that any organisms on
Mars would have developed the ability to assimilate carbon
dioxide and carbon monoxide from the atmosphere and convert
these, in the absence of water, to organic matter. Therefore, the
PR experiment exposed a small sample of Martian soil to
quantities of these two gases which had been labeled with
radioactive carbon-14 for detection purposes. After 120 hours of
incubation under an artificial sun (a xenon arc lamp), the soil
chamber was heated to about 625°C to break down (pyrolize) any
organic matter and release the volatile organic products for
subsequent testing by a radiation counter. Since any organisms
present would be expected to carry out metabolic processes
during which they would take in carbon-14 from the gas in the
chamber, detection of carbon-14 would be a positive result. It
would not, however, be conclusively biological, since a first peak
of radioactivity might equally be due to purely chemical processes.
In order to rule out this possibility, other samples, serving as
controls, were sterilized by heating before the carbon source was
admitted
PR Results
The data show that a fixation of atmospheric carbon occurs in the surface material
of Mars under conditions approximating the Martian ones. Although the amount of
material was small relative to that which organisms on Earth produced, it was still
more significant than the margin of error.
Problems
Due to the lack of organic material as detected by the GC/MS, the probability of
there existing such organic material was slim and made the results hard to
believe. In addition, the argument was brought up that any Martian organisms
would have been killed under the lamp energy as it provided heat that greatly
exceeded its normal climate while the sample tested positive after being heated.
Therefore, the material detected must have been the product of the nonbiological
reactions.
Another explanation is related the existence of peroxides and superoxides; the CO
could have been reduced by H2O2 to produce the organic material that the
detector was testing for.
Labeled Release
Water and Organic Compounds (formate,
glycolate, glycine, D-alanine, L-alanine, Dlactate, and L-lactate)
Mars soil
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).)
Labeled Release Results
1)
There appeared a uniform production of gas issues when the nutrient was placed on
Martian soil.
2)
The reactant in the Mars soil is completely inhibited by heating the soil to 160C
Therefore, one would suggest that this experiment did indeed find life on Mars.
Success?? Did we detect life!
Labeled Release Problems
Many discrepancies were detected between the experiment
and other known facts about Mars and the assumptions the
experiment was based on:
•No organic compounds were found in Martian soil analyzed by the Viking Gas
Chromatograph Mass Spectrometer (GCMS).
•H2O2 , chemically formed in the upper atmosphere, was thought to descend to the
soil and, directly or through forming complexes or compounds, to oxidize the LR
substrates to evolve labeled gas. For example, the reaction of formic acid (present in
nutrient) and hydrogen peroxide (H2O2) produces water and carbon dioxide which
would be detected as radioactive.
•The amplitude and kinetics of the LR response from Mars was thought to be "too much too
soon" for any putative Martian biology.
•Although not a pre-mission criterion for life, a second injection of LR nutrient onto positive
samples failed to re-invigorate the evolution of gas, as generally occurs with terrestrial soils.
Instead, some of the gas evolved after the first injection appeared to be reabsorbed into the
soil.
•UV light from the sun was thought to activate soil particles, which then disrupted the LR
nutrient upon contact, releasing labeled gas. Also, UV light's destructive effect was held to
account for the reported lack of any organic matter on Mars.
•Clays on Mars were proposed to react with the LR nutrient to release labeled gas.
GCMS
The Gas Chromatograph/Mass Spectrometer (GCMS) was designed to measure
organic compounds in the martian soil. Organic compounds are present in space
(for example, in meteorites), but the GCMS found no trace of them on the surface
of Mars. Gilbert Levin believes, however, that the GCMS instrument sent to Mars
could easily have missed biologically significant amounts of organic matter in the
soil, as it had in a number of tests on Earth.
GCMS 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.
ALH84001: The Case for Past Life on Mars?
Image below shows possible elongated fossilized Martian cell on the surface of a
clay mineral which fills veins or cracks in the meteorite.
4 basic issues associated with claim meteorite had indications
of past life.
1. Polycyclic aromatic
hydrocarbons (PAHs);
2. Carbonates (compounds with both carbon and
oxygen present);
3. Magnetite (including iron sulfides) found; and
4. Microfossils (size scale of above features
around 1 micron).
Evidence consistent with life, but not proof of it.
1) PAHs
•PAHs are called
“Kerogen” if they are the
result of biological organic
molecules
•PAHs are also the
insoluble “leftover”
products of non-biological
organics.
2) Carbonates
•Age of carbonate formation us
disputed (>3.5 Ga or closer to 1.5
Ga??)
•Temperature of formation disputed.
Some results show these formed in
temps greater than 650° C.
The blebs are about 200 µm in diameter.
The orange centers are relatively calcium
and iron-rich, and the white rims are
nearly pure magnesium carbonate. The
black rim contains minute iron oxides and
sulfides.
3) Magnetite
The iron sulfide and magnetite particles interspersed in carbonate which seems
to be partially dissolved present an interesting problem. To produce these
particles chemically, you need very alkaline conditions -- but you need acidic
conditions to dissolve the carbonate. Thus it seems unlikely that the origin of
these particles is entirely chemical.
Some terrestrial bacteria, known as magnetosomes, are known to produce
iron sulfide and magnetite particles, either inside themselves or (for some
anaerobic bacteria) outside. The fossil remains of ancient magnetosomes,
known as magnetofossils, are very similar in structure and composition to the
particles of iron sulfide and magnetite found in the carbonate in ALH84001.
Magnetite and iron sulfide particles have been found in the carbonate globules.
There are complicated inorganic explanations for the presence of these mineral
grains, but as Kathie Thomas-Keprta explains, "The simplest explanation is that
these are products from microorganisms on Mars." The magnetite particles are
similar in shape and composition to the magnetite particles (magnetofossils)
produced by terrestrial bacteria.
However, one puzzling aspect of the magnetite crystals is that whereas certain
Earth bacteria use them as a compass to navigate through sediments, the martian
magnetic field is only 0.2% that of the Earth's.
Also it has been demonstrated that similar grains can be abiogenically produced.
Shock heating of carbonate??
4) Size
•Metabolism is dependent upon a cell’s S/V ratio
•Small cells with a 0.5 micron diameter have a S/V ratio 100-1000>Euk.
Cell
•Bacteria can energetically outcompete Eukaryotes.
•Smaller than 0.5 microns and most cells don’t have more than a few free
protons
•Liquid phase chemistry begins to break down smaller than 20nm (1E9m) there is just no room left for basic cell functions (DNA/RNA, ribosomes,
enzymes, etc.)
• A ribosome is 20-25 nm thick
•ALH84001 structures were 10-100nm thick. TOO SMALL