Selection of a particular
compound was made after its metabolism was confirmed in each of a wide
variety of microbial species in pure and mixed cultures and in soils.
Designated VM1, the LR nutrient solution was an aqueous solution of 2.5 x
10-4 molar concentrations each of formate, glycine, D-alanine,
L-alanine, D-lactate, L-lactate, and glycolate, each uniformly labeled
with 14C. The LR experiment moistens a 0.5 cc soil sample with
0.115 ml of VM1 for a total of 3.91 uCi per application. Evolution of
14C gas from the moistened soil indicates microbial activity.
Following such a positive response, a duplicate of the original sample is
heated to impair or destroy any organisms present. After cooling, that
sample is tested exactly as was the one that produced the positive result.
A positive response from the second sample indicates that the response of
the first was non-biological in nature, while a negative response,
demonstrating that the causitive agent had been deactivated by heat,
completes the evidence for life. The temperature regimen selected by the
Viking Biology Team to distinguish a biological response from a
non-biological one was 160o C for 3 hr.
Thousands of soils and microbial cultures of a wide
variety of species were tested. No sample with demonstrated living
organisms failed to respond. The controls readily confirmed biological
activity. (Only several of the hundreds of soils were sterile.) The
metabolism of as few as 50 cells in lag phase was readily detected. In a
final test of an LR flight instrument, 0.5 ml of a California soil was
placed under the experimental conditions to prevail on Mars. After 3 days
of the reduced atmospheric pressure, humidity and temperature, the LR
substrate solution was injected. Radioactive gas immediately evolved and
increased through the standard 8-day LR test period (see SN103, FIG. 6). A
second injection of nutrient then produced an increased response. A
duplicate of the test soil was heated to 160o C for 3 hours and, after
cooling, was tested. Essentially no
response from the heated sample occurred, completing the
evidence that there had been living organisms in the first sample and that
they had survived exposure to the martian test conditions.
2.2 The LR on Mars
The results from the first LR experiment at the Viking
1 Site on Mars are shown in FIG. 1. At the Viking 2 Site, some 4,000 miles
away, the soil produced a similar positive response. After eight sols (one
martian sol = 24.67 Earth hr) of continuing, but slowing, evolution of
gas, the soil samples at both sites were injected with nutrient a second
time. Each sample promptly reabsorbed approximately 20% of the gas evolved
to that point. A duplicate of the active sample was heated at 160o C for 3
hr and, upon testing, gave a nil response. In additional controls, 60%
less gas was evolved after heating a duplicate sample to 46.0░C. After
another duplicate sample was heated to 51░C, the response became erratic
and produced 90% less gas than did the active sample. Active Mars soils,
enclosed in the Viking sample distribution box and held at approximately
10░C for 2 months at one site and 3 months at the other, showed no
activity upon testing. In all, 9 tests were run. All of the Viking LR
results6 at both sites are summarized in
Table 1.
Taken alone, the LR results were a strong positive
indication of living microorganisms in the soil of Mars. The lability of
the active agent in the soil to mild temperatures is difficult to explain
otherwise. The difference between the responses after the heatings at
46.0░C and 51.5░C are reminiscent of laboratory microbiology analyses,
such as the standard method7 for distinguishing fecal coliforms
from the coliform group, which relies on small temperature
differences.
2.3 Defects in non-biological interpretations of LR
results
Updating the last review8 with more recent
information, each of the above enumerated arguments against a biological
interpretation of the LR Mars data will be addressed.
1. No Organics: The Viking GCMS, designed to
identify organic matter in the martian soil, failed to detect any at
either Viking Lander site9. The presence of organic matter on
Mars had been generally anticipated as a result of: photochemical
synthesis from primitive atmospheric constituents10, synthesis
from current martian atmospheric gases11,12 by UV, infall from
comets and meteorites13 and from interplanetary dust
particles14 (estimated15 at some 105 kg
of organic carbon per year).
The report of organic matter, including amino acids,
inside the two SNC meteorites presumed from Mars, if verified, would
confirm this anticipation. A report16 on EETA79001 found
L-amino acid concentrations exceeding those for D-amino acids. Such chiral
preference is found only in biological material. The finding17
of low molecular weight organic compounds being solar-stripped from the
Hale-Bopp comet supports the widespread formation of organic
matter.
Nonetheless, the Viking GCMS negative results were
widely accepted as persuasive "evidence" that the Viking samples contained
some exotic chemistry, but no life18,19.
However, the LR's finding20 of living
microorganisms, as incorporated into FIG. 6, in Antarctic soil #726, in
which the GCMS concurrently had detected no organic matter, provided a
direct comparison of the two instruments. Moreover, a wet-chemistry
analysis of Antarctic #726, performed as a control, found21
organic matter. Recently, the same soil, maintained in bonded storage by
NASA, was re-examined22. The sample was subjected to prolonged
digestion in concentrated HF/HCl and then pyrolyzed at 500░C (the highest
pyrolysis temperature applied by the Viking GCMS).
