(Reprinted from Nature, Vol, 262, No. 5563, pp. 24~27, July I, 1976)

The Viking Mission search for life on Mars

Harold P. Klein

NASA-Ames Research Center, Mountain View, California

Joshua Lederberg

Stanford University, Stanford, California

Alexander Rich

Massachusetts Institute of Technology, Cambridge, Massachusetts

Norman H. Horowitz

California Institute of Technology, Pasadena, California

Vance I. Oyama

NASA- Ames Research Center, Mountain View, California

Gilbert V. Levin

Biospherics Incorporated, Rockville, Maryland

 

On each of two unmanned Viking spacecraft, a number
of investigations are planned to be conducted on the
surface of Mars this summer. Included in the science
payload are instruments designed to determine the state
of chemical evolution, including the possible presence of
living organisms, on that planet.

 

On July 4, 1976, the first of a pair of unmanned spacecraft
is scheduled to touch down on the planet Mars in an area
located at 19.5°N and 34°W, in a region of low elevation,
which seems to be a drainage basin for a large portion of
the equatorial region of Mars (Fig. 1). Two months later,
the second Viking spacecraft should touch down further
north (44.3°N and 10°W) in another region of low eleva-
tion, relatively close to the edge of the northern winter ice
cap, and at a latitude where maximum quantities of water
vapour have been measured in the Martian atmosphere’.

Aboard both of these landers will be identical science
‘payloads’, including cameras, meteorological and seismic
stations, and analytical equipment to conduct analyses of
the surface material. These investigations are now planned
to be conducted over a period of two months on each
lander, but it is possible that the experiments will continue
over considerably extended periods of time—approaching a
full Martian year—if further funding can be made available
for an extended Viking mission. The scientific objectives of
this mission are manifold, but, among these, an assessment
of the organic evolutionary state of the planet including a
search for extraterrestrial life, is of paramount importance.
For the first time, another planet is to be probed directly
for evidence supportive of modern ideas on the origin of
life’. Mars, although not a hospitable planet as judged by
terrestrial standards’, is the one planet, other than Earth,
that most nearly satisfies our theoretical presumptions of
the conditions for life to have evolved‘.

For many years, telescopic evidence of morphological
features, and their temporal changes, inspired speculative
controversies about the presence of life on Mars. These
older controversies no longer bear discussion in the light of
the Mariner Mars-orbiter and fly-by missions (1964-71)
which gave us a complete photographic mapping at resolu-
tions of 1 km or better. It is now clear that water in a form
available to living organisms is a likely limiting factor. The
polar caps seems to contain substantial quantities of ice,
but at temperatures far below those that would support any

active terrestrial life. The temperate latitudes have con-
genial daytime temperatures, but humidities far below those
known for any earthly desert. A number of mechanisms
have, however, been proposed for possible local anomalies
that could furnish moisture at temperatures consistent with
metabolism®. In addition, the Mariner photographs show
many surface features that are most easily explained by
assuming that episodes of weather, precipitation and the
carving of river valleys have occurred in the geological
history of Mars.

These considerations have been taken into account,
together with the engineering constraints, in planning the
selection of landing sites. However, until our first successful
landing, we have no reliable empirical basis for predictions
about the local habitats of Mars. We have then been obliged
to try to outguess Nature over a range of possible realities
in planning these experiments.

Scientific payloads of Viking mission

While all of the experiments are expected to contribute at
least peripheral information pertaining to the local milieu
of the landers (for example, local temperatures, wind speed
and direction, atmospheric composition, elemental composi-
tion and magnetic properties of the surface material), three
of the investigations hold the greatest promise for contri-
buting directly to the question of organic chemical
evolution on Mars.

The first of these is the imaging investigation, which will
utilise two facsimile cameras, capable of stereoscopic imag-
ing, on each lander. These cameras will scan the landing
site area in black and white, colour and infrared, with a
resolution of about 2mm immediately below the landers.
Examination of the surrounding terrain may reveal evidence
of past or present living systems. Furthermore, frequent
scans of this area may show signs (for example, colour
changes) suggestive of living processes*.

Also on board each Viking lander is an instrument that
will perform organic analyses of the Martian surface
material. This is essential not only for obtaining results
which may be relevant to Martian life, but also may pro-
vide valuable information in assessing the state of molecular
complexity if the planet is in a prebiotic state of evolution.
The heart of this instrumentation is a mass spectrometer,
chosen because of its high sensitivity, wide dynamic range,
and broad applicability. For the analyses, samples will be
heated in ovens at 200°C and at 500°C, and the evolved
organic vapour (if any) will first pass through a chromato-
VIKING LANDING SITES

 

ae

 

 

 

LANDER 1: CHRYSE

{19.5°N, 34.0°W; Elevation: - LOkm }

- —

SOuTH

At mouth of Valles Marineris and
several small dry river beds.

  
   

LANDER 2: CYDONIA

(44.3°N, 10.0°W, Elevation: -0.5km}

   

Covered seasonally by polar hood:
high atmospheric water content.

Fig. 1 Map of Mars, constructed from Mariner 9 photographs, showing the two prime landing sites for the Viking landers.

graphic column for separation. A complete gas chromato-
gram takes 84 min and, during this period, the mass spec-
trometer will record a complete mass spectrum from mass
12 to mass 200 every 10.3 (ref. 7).

The biological payload supports three separate experi-
ments which are based on different assumptions about the
nature of a possible Martian biology. In the face of our
relative ignorance of the local environmental conditions on
Mars—for example, up to the present, there is no positive
indication that nitrogen is present on Mars*—there is ample
reason to approach the question of life detection on Mars
with a broad approach. The combination of biological
experiments on Viking is thus designed to create a diverse
set of environments within which to elicit evidence of
metabolism. Three kinds of metabolic question will be
asked. Is there evidence for building up of molecules con-
taining carbon starting with “CO or “CO,? Can small radio-
active organic molecules be broken down to “CO.? Are
gases other than CO: produced or consumed? These broad
experiments try to measure anabolic and catabolic reactions

on Mars. The conditions vary from dry to wet; include the
presence of simple and complex nutrient sources, as well as
the absence of added nutrients; and include incubation in
the light or dark.

For the biological experiments samples will be analysed
several times during each mission. They will be obtained
from the upper 4cm of surface material and will be in-
cubated for different periods of time in each experiment.
Incubation temperatures are expected to run at 15+10°C.
Furthermore, each of the experiments is provided with the
capability to perform a control experiment with samples
preheated to 160°C for 3h, in the event that any of the
experiments gives a presumptive positive result.

Surface samples (approximately 6cm*) will be delivered
into the biology instrument by an extendable boom at the
end of which a jaw-like scoop digs into the surface to
acquire samples (Fig. 2).

The biology instrument contains four modules, one for
each of the three experiments, as well as a common services
module, which contains a helium source used to raise and
lower the incubation cells, purge incubation chambers, and
for other operations as well as a system of vents for gases and
for liquid wastes. A detailed description? of the instrument is
beyond the scope of this communication, but a_ brief
account follows.

The pyrolytic release (or carbon assimilation) experiment
is designed to measure either photosynthetic or dark fixa-
tion of CO. or CO into organic compounds'*". Both of
these gases are known to be present in the atmosphere of
Mars, and the experiment is designed to simulate, as closely
as conditions permit, the actual Martian environment. The
experiment hardware includes a xenon arc lamp with a
spectral range and distribution quite similar to the solar
spectrum, and with an intensity of about 20% of the maxi-
mum Martian solar irradiation at the surface. The light
source is provided with a filter to remove ultraviolet radia-
tion below 340nm to prevent the non-biological fixation
of carbon monoxide into organic compounds, as has been
reported by Hubbard ef a/.”'*. For this experiment, 20 nCi
of a mixture of “CO: and “CO (in a ratio of approximately

BADD SUERTE

CANERRS| METEOROLOGY

PHYSICAL ANG
MADNENE DROPLWTES:

 

Fig. 2 Viking lander. Surface sampler is seen extended at left. Biology

sample processor and distribution assembly is seen midway between

the two cameras. Biology instrument is beneath this component and
not visible in this figure.

95:5) is injected into the headspace (containing Mars
atmosphere) above a 0.25-cm* surface sample that has been
sealed off in one of the three incubation chambers provided.
Incubation proceeds for the next five days in the light.
While the incubation is proceeding, a long background
count is taken. Following this period, the original incuba-
tion atmosphere is automatically vented, and the sample is
pyrolysed by heating to 625°C. During this stage of the
experiment sequence, any residual initial radioactive CO
or CO; will be driven out of the sample, through a column,
the organic vapour trap (OVT), and into a “C detector,
while organic fragments will be adsorbed on the OVT.
After counting at this point in the sequence, the radioactive
gases in the detector are automatically vented, and the
detector is then heated to remove adsorbed gases. Following
this step, and several additional helium purges of the OVT
and the detector, the organic matter that had been trapped
on the OVT is now eluted by heating it to 650 °C for 3 min.
During this heating, any trapped organics are released from
the column and are simultaneously oxidised to CO: by copper
oxide present in the packing of the OVT. The effluent from
this procedure is purged into the detector and counted. A
radioactive peak at this time would constitute presumptive
evidence for biological activity, to be followed by a control
experiment using heat-sterilised soil.

In addition to the sequence just described, the experi-
ment can be modified by the introduction of water vapour
into the incubation atmosphere by commands from Earth.
Another commandable option in this experiment is to turn
off the lamp to conduct a dark incubation.

The labelled release experiment” will test for metabolic
activity during incubation of a surface sample that has
been moistened with a solution of “C-labelled simple organic
compounds, consisting of a mixture of one-, two-, and three-
carbon compounds, all uniformly labelled and in dilute
aqueous solution. For this experiment, one of the four in-
cubation cells provided in this module will receive 0.5 cm’
of surface material, and will be sealed with Martian atmos-
phere trapped in the headspace above the sample. Following
this, a background count, lasting approximately 24h, is
taken, after which 0.115cm* of nutrient is automatically
injected onto the sample. Incubation proceeds for the ensu-
ing 8d. A second injection of labelled nutrient then occurs,
and incubation continues for 2d. During the entire time,
the atmosphere above the sample is monitored by a “C
detector to determine the kinetics of released labelled gas
following each injection. Incubation is ended by a termina-
tion sequence to bring the background count down to pre-
incubation levels. Following this ‘clean-up’ operation, the
experiment is ready for another analysis cycle.

The gas exchange experiment” will be conducted in two
different modes using the same equipment. In one mode (the
‘humid’ mode), Martian surface samples will be incubated
in the presence of CO, and water vapour. This mode is
based on the assumption that substrates may not be limit-
ing on Mars, but that biological activity is dormant in these
samples until enough water becomes available in the envi-
ronment. The second mode (the ‘heterotrophic’ mode)
assumes the presence of frankly heterotrophic organisms in
the Martian surface.

For both of these experimental modes, it is further
assumed that metabolism of active organisms in the samples
will cause the disappearance or release of one or more gases
known to be involved in terrestrial metabolic processes. For
an analysis, 1cm*® of sample is delivered into the single
incubation cell. A mixture of CO. and Kr (in He) is then
added to serve as the incubation atmosphere. After this
step, 0.5cm* of nutrient is delivered to the bottom of the
incubation cell. During incubation, the sample is retained
in an inner-cup within the incubation cell so that the
nutrient does not come into direct contact with the sample.
The inner cup is porous and allows equilibration between
the sample and the incubation atmosphere. Gas samples
(100 ul) are taken from the headspace above the sample
immediately after the nutrient has been injected and at
specified times thereafter for the next 7d. The gas samples
are swept through a chromatographic column and the com-
ponents detected by a thermal conductivity detector. If no
command to the contrary is given after 7d in the ‘humid’
mode, the instrument will automatically inject additional
nutrient bringing the total aqueous volume now to 2.5 cm’.
With this amount of nutrient, the sample will be wet by
the nutrient and incubation now will proceed in the ‘hetero-
trophic’ mode. (The nutrient solution contains both p- and
L-amino acids as well as other organic substrates, growth
factors and inorganic compounds.) Incubation continues in
this mode with gas samplings at stipulated intervals for the
next 9d. At this point, options exist to proceed in various
directions, depending on the data obtained. One is to drain
the nutrient from the bottom of the cell and to introduce
fresh atmosphere and fresh nutrient, and to incubate the
original sample for an additional 19d. Or, the initial incu-
bation conditions can simply be extended for an additional
19-d period (that is, without draining the original nutrient
or changing the gas atmosphere). If circumstances warrant
it, the nutrient can be drained, the original sample dried,
and a new sample introduced on top of the first one for a
second analysis in the ‘heterotrophic’ mode.

In addition to the “C data from the two radioactivity
detectors, and the gas chromatographic data, several other
measurements will be telemetered periodically from the
spacecraft during the course of these experiments. These
include the status of the xenon lamp, the position of each
incubation cell, temperature measurements of each incuba-
tion cell being used, and temperature measurements of both
nuclear detectors, of the OVT and of the gas chromato-
graphic column and detector. In addition, we will receive
data on the pressure of the critical helium reservoir in the
common services module.

Figure 3 shows one of the flight instruments in its final
stages of assembly. It contains, in addition to the incubation
cells and sample distribution system, four different gas sup-
plies, two different nutrients, two “C detectors, a thermal
conductivity detector and chromatographic system, 43 dif-
ferent heaters, 4 thermoelectric coolers, 39 miniature latch-
ing solenoid valves, a xenon arc lamp, 22,000 transitors,
and 18,000 other electronic parts, All of this is packaged
into a volume somewhat greater than 1 foot’, and weighs
35 pounds.

The three biology experiments differ in their sensitivity,
as measured by their responses to terrestrial organisms.

 

Fig. 3 Biology flight instrument in final stages of assembly.

Both the gas exchange and labelled release experiments
have, by now, been tested against a wide variety of soils
and they readily ‘“‘detect” the presence of microbial flora
in these samples when they are incubated in ambient terres-
trial conditions. When these soil samples are incubated
under the conditons in which the experiments will be
operated on Mars, the responses are delayed by prolonged
lag periods, but qualitatively similar responses are seen. In
contrast, the pyrolytic release experiment does not respond
to terrestrial soils at all when incubated in ‘“‘Martian” con-
ditions. Only when water is included in the incubation test
cell is terrestrial biological activity demonstrable in this
experiment. Accordingly, during the long test programme
that preceded the manufacture of the flight instruments,
several ‘standard’ soils were used to challenge the two ‘wet’

experiments. For the pyrolytic release experiment, soil was
preincubated in the light in the presence of water to pro-
vide fixed radioactive carbon compounds, dried, and then
used in this form to test the equipment. Since the final
flight hardware is designed to work on Mars, and not on
Earth, and because the design of the biology package pre-
cludes repetitive incubations (beyond four per experiment),
and because of the rigid requirements to prevent terrestrial
contamination, the flight instruments were not, and could
not be, tested with terrestrial soils. Precursor instruments,
built to flight specifications (with minor exceptions), did go
through at least one prolonged test in simulated Martian
conditions using one of our ‘standard’ soils, (containing
110° aerobic, and 5x10* anaerobic organisms per g;
with a pH of 4.8 and an organic content of 0.8%). In addi-
tion, a flight-like instrument was installed on a lander and
tested as part of a complete system, but these tests did not
include analysis of any soils. Rather, known mixtures of
gases were processed through the gas exchange hardware,
and “CO, was used to challenge the other two experiments.
These tests’®, while not completely satisfying, in that only
a part of the complete system was tested, gave results
entirely consistent with extensive prior testing at the module
level. Finally, at the component level (that is, detectors,
chromatograph columns, and so on), many of the actual
flight components were extensively tested for their scien-
tific as well as their engineering characteristics. In these
tests, no significant anomalies were uncovered. In summary,
the testing programme was less than optimal, being limited
by time and fiscal constraints, but met the minimum stan-
dards for confidence that the instruments will perform their
norninal functions after landing on Mars.

The team of scientists selected by NASA, in 1969, to
plan and conduct the biological investigation on the Viking
spacecraft consisted of the authors and the late Dr Wolf
Vishniac (University of Rochester, Rochester, New York).
The following individuals are collaborators in these investi-
gations: Bonnie Berdahl, Glenn Carle and Richard D.
Johnson (NASA, Ames Research Center, Mountain View,
California); George Hobby (California Institute of Tech-
nology, Pasadena, California); Jerry Hubbard (Georgia
Institute of Technology, Atlanta, Georgia); and Patricia
Straat (Biospherics Incorporated, Rockville, Maryland).

Farmer, C. B., and LaPorte, D. D., Icarus, 16, 34 (1972).

Horowitz, N. H., and Hubbard, J. S., Ann. Rev. Genet., 8, 393 (1974).

Glasstone, S., The Book of Mars, NASA SP-179 (1968).

Horowitz, N. H., Acc. chem. Res., 9, 1 (1976).

Sagan, C., and Lederberg, J., Icarus (in the press).

Mutch, T. A., et al., Icarus, 16, 92 (1972).

Anderson, D. M., et al., Icarus, 16, 111 (1972).

Barth, C. A., Ann. Rev. Earth planet. Sci., 2, 333 (1974).

Report No. 21020-6003-RU-00, August 1975 (TRW Systems Group, Redondo
Beach, California 90278).

Hubbard, J. S., Hobby, G. L., Horowitz, N. H., Geiger, P. J., and Morelli, F. A.,
Appl. Microbiol., 19, 32 (1970).

il Horowitz, N. H., Hubbard, J. S.. and Hobby, G. L., Icarus, 16, 147 (1972).

12 Hubbard, J. S., Hardy, J. P., and Horowitz, N. H., Proc. natn. Acad. Sci. U.S.A.,
68, 574 (1971).

Hubbard, J. S., Hardy, J. P., Voecks, G. E., and Golub, E. E., J. molec. Evol.,
2, 149 (1973).

14 Levin, G. V., Icarus, 16, 153 (1972).

15 Oyama, V. I., Icarus, 16, 167 (1972).

16 Reports Nos MMK 1124 and MMC 1141 (TRW Systems Group, Redondo Beach,

California 90278).

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