az

“2

Reprinted from Sumner, dune 27, 1958, Vol. 127, No. 3313, pages 1473-1475.

Moondust

The study of thiscovering layer by space vehicles

may offer clues to the biochemical origin of life.

Joshua Lederberg and Dean B. Cowie

Lacking an appreciable atmosphere,
the moon may scem to offer little op-
portunity for biological research. How-
ever, astronomers suppose that the moon
is covered by a layer of dust of great
antiquity (7). This dust is cosmic mate-
rial captured by the moon’s gravitational
field and presumably left undisturbed by
atmospheric and biological alteration. It
should therefore contain a continuing
record of cosmic history as informative
with respect to the biochemical origins
of life as the fossil-bearing sediments of
the earth’s crust have been in the study
of its later evolution.

For the biologist, this dust may fur-
nish two striking opportunities: (i) to
assess the prebiotic synthesis of organic
compounds and (ii) to make an empir-
ical test of cosmic dissemination of bio-
spores {Arrhenius pan-spermia hypothe-
sis (2)]. The scope and uniqueness of
these opportunities demand a cautious
approach to the planning of space mis-
sions lest they be prejudicial to later
scientific study.

Prebiotic Synthesis of

Organic Molecules

The traditional picture of the nonliv-
ing world is colored by the composition
of the earth’s crust with its predominance
of siliceous rocks. A comparison of the
relative abundances of the elements in the
earth’s crust and the cosmic abundances
(Table 1) reveals that the earth’s crust
is not a fair sample of total matter. In

fact, carbon, nitrogen, and oxygen make
up about three-fourths of the total mat-
ter of the universe, apart from hydrogen
and helium. The formation of the earth
therefore involved a fractionation which
left it relatively impoverished with re-
spect to the lighter elements which make
up organic compounds (3, 4), and the
richness of organic complexity on our
own planet was formed out of the mere
dregs of cosmic distillation, However, the
data on cosmic abundance are based
mainly on the atomic spectra of the stars,
and we have only the most rudimentary
information on the molecular chemistry
of the universe (5).

The overabundance of H suggests that
the most prevalent molecules will be
more or less saturated hydrides cf car-
bon, nitrogen, and oxygen (CH, OH,
CH,, H,O, NH,, ...). Many proposals
for the prebiotic appearance of complex
organic molecules are based on the pho-
toactivation of these simple precursors
by ultraviolet light (6, 7). Furthermore,
since the elements must have been
formed initially at very high tempera-
tures (8), they are already “activated”
by their incidence as free atoms in space.

Indeed, the bulk of cosmic matter is
still dispersed among the stars and
galaxies as atoms and molecules and
larger particles which are condensates
of these, which constitute the cosmic
dust. The details of these condensing
processes are still most obscure. As sum-
marized by Dufay (9): “Although the
densities of the CH and CH* molecules
formed by atomic and molecular reac-

tions are very small, they are perhaps
large enough to initiate the condensation.
If this point is granted, it would then
be necessary to examine the capture of a
second atom of hydrogen or of carbon
by the CH molecule. Because of the
abundance of hydrogen, the first is more
probable but the calculation of the proba-
bility of formation of the CH, molecule
is very difficult. It is possible that some
more hydrogen atoms attach themselves
to the CH, molecule (CH, CH, CH, ?)
but before long it is mainly atoms of
much larger mass (C, N, O,...) which
are captured because the large molecule
formed is not sufficiently cold to prevent
the evaporation of hydrogen. Thus, little
by little, due to a mechanism which is
difficult to evaluate in detail, minute
grains which scem to serve as centers of
condensation are built up.” The scatter-
ing of light from distant stars indicates
that the grains may grow to sizes of a few
tenths of a micron. Furthermore, the ob-
served polarization of the light from dis-
tant stars following its passage through
galactic clouds speaks for the asymmetric
shape of the grains and their orientation
presumably by magnetic fields (9, 10).
The grains, being condensed from a
pool of reactive free atoms among which
hydrogen predominates, might be ices
(crystals) of the simple hydrides— CH,,
NH,, and H,O—or more complex mole-
elements,
Which composition predominates will de-
pend on the extent of discrimination
against H which was quoted in the pre-
vious paragraph. One basis for further
speculation is the occurrence of spectra!

cules containing the same

lines identifiable with nonhydride mole-
cules such as C,, CN, and CO in the
emission from comets and in the inter-
stellar absorption (5, 1/0). The possibility
of extensive macromolecular organic syn-
thesis by this mechanism is a new point
of convergence of biochemical and astro-

 

Professor Lederberg is chairman of the Depart-
ment of Medical Genetics, University of Wiscon-
sin, Madison; Dean B. Cowie is a member of the
staff of the Department of Terrestrial Magnetism,
Carnegie Institution of Washington, Washington,
D.C. This paper was delivered at a symposium on
“Possible Uses of Earth Satellites in Life Science
Experiments,” in Washington, 14-17 May, under
the sponsorship of the National Academy of Sci-
ences, the American Institute of Biological Sci-
ences, and the National Science Foundation.
physical research. The problem is ex-
tremely complex both in theory and in
the construction of laboratory models;
many factors, such as local fluctuations
in atomic abundance, radiation flux and
polarization, magnetic orientation, tem-
perature, and the catalytic role of nuclei
of condensation, all have to be taken into
account,

In favorable circumstances the chemi-
cal evolution of the growing grain will be
determined in the first place by the ran-
dom sequence of atomic collisions. The
accretion of a colliding atom will depend
in part on its kinetic energy and on its
composition and that of the incipient
grain. The composition of the grain be-
tween impacts will also determine its
stability—that is, its temperature in the
radiation field, in accord with its absorp-
tion spectrum. Thus, despite the random
input of precursor atoms, the evolution
of the grain will also be determined by
a sort of natural selection, those agglom-
erates that are maladapted being va-
porized, and the constituent atoms will
be returned to the precusor pool. The
grains rarely grow larger than 0.5 mi-
cron in diameter, perhaps due in part
to evaporative collisions of such large
grains with one another (9).

A serious difficulty in the theory of
interstellar grains has been the ineffi-
ciency of diatomic reactions in free space
without the participation of preformed
grains to serve as nuclei of condensation
(11). It would seem to be necessary to
invoke either an independent source of
nucleation in the emanation from sooty
“carbon stars,” as Hoyle has suggested
(72) or a mechanism of fission of spon-
taneously formed nuclei so that these
may increase in number as well as ac-
crete in mass. The fission of grains might
occur either by internal exergonic rear-
rangement, by photochemical dissocia-
tion, or by splitting from colliding atoms.
The potentiality for fission is therefore
another element in the chemical evolu-
tion of the grains.

Apart from the scope of its own evo-
lutionary development, the system of or-
ganic synthesis in space is important to
the origin of life in two respects: as a
model of some of the reactions accom-
panying the formation of the planet and
as a continual source of replenishment
of organic precursors for life. From an-
other viewpoint, Hoyle (12) has invoked
the possibility of a large budget of or-
ganic material in the initial condensation
of the earth. This budget would be re-
plenished by photochemical reactions in
the atmosphere and by the accretion of

2

Table 1. Relative abundance of the ele-
ments (expressed in atomic abundances).
Scale: Elements other than H and He
total 1.)

 

Relative abundance

 

 

Terres-
trial
Ele- atmos- ;
ment — Cosmos* pheret Earth's
and crust}
hydro-
spheret
H 1600 2 0.03
He 160
O 0.378 0.978 0.623
0,269 0,003
Ne 0.168 +
Cc 0.135 0.0001 0.0005
Si 0.017 0.211
Mg 0.015 0.018
Fe 0.012 0.019
S 0.003 0.0005
Ca 0.002 0.019
A 0.002
Al 0.002 0.064
Na 0.001 0.008 0.026
Ni 0.0005
Others 0.002 0.011 0.020

 

* After Urey (3).
} After Hutchinson, in The Earth as a Planet (4).
t After Mason, in The Earth as a Planet (4).

cosmic dust by meteoric infall. The im-
portance of the initial budget depends on
the outcome of controversial questions
about the temperature of the earth dur-
ing its early history (3, 4).

Interstellar and Interplanetary Dust
Efforts to make detailed evalua-
tions of the composition of cosmic
matter are hindered by the discrepan-
cies between interstellar and inter-
planetary dust, since the latter must re-
flect in part the fractionation of the
elements in the formation of the solar
system and also the local intensity of
the gravitational and radiation fields of
the sun. The interplanetary dust will in-
clude solar emanations, interstellar mat-
ter swept out by the proper motion of
the sun, and (possibly most abundant)
the debris of asteroids and comets. The
former, comprising most of the meteor-
ites, will have a stone and iron compo-
sition like that of the earth; the cometary
debris, which, according to Whipple
(13), is mainly responsible for visual
meteors and for micrometeorites, is of
special interest in the present context.
Being formed at great distances from the
sun, the comets most nearly, among all
the interplanetary objects, represent the
composition of interstellar matter.

The density, sources, and composition
of the interplanetary material represent
some of the most challenging objectives
of satellite research. Published estimates
of the rate of infall of this material vary
widely and have dealt almost exclusively
with nonvolatile constituents which can
be observed in meteorites. Some of the
higher estimates can be extrapolated so
as to suggest that infall during geological
history might have made a significant
contribution to the composition of the
earth’s surface. For example, Petterson
(14) has estimated an infall of 3.5 x 1012
g of Ni per year. If this is multiplied by
the cosmic C:Ni ratio (Table 1), we ob-
tain for C alone 10*4 g per year, or 108
g for a billion-year interval of geological
history. This figure may be compared
with an annual photosynthetic turnover
of 10*’g; a biosphere of 1018 to 1079 g;
102° g of C available to the biosphere in
the atmosphere and oceans; and 107° g
of fossil C, including that trapped in the
sedimentary rocks (4). The extrapolated
estimate is almost certainly exaggerated.
However, a precise knowledge of infall
is certainly necessary in order to evalu-
ate its contribution to geochemical evo-
lution, on our concept of which the anal-
ysis of biochemical evolution must be
based.

Terrestrial observations of infall are of
limited value with respect to the quan-
tity and quality of interplanetary carbon,
since whatever organic matter fails to be
oxidized while falling through the atmos-
phere is likely to be degraded by or con-
fused with the existing biosphere. Some
exceptional opportunities invite more
detailed analysis by modern methods; for
example, the “Cold Bokeveld” meteorite
is reported to contain appreciable
amounts of organic acids, as well as or-
ganic compounds containing S, N, and
Cl which have not been explicitly iden-
tified (75). Barring such exceptional
finds we must look to the moon for sam-
ples of undegraded and uncontaminated
dust with which to check these specula-
tions. Owing to the loss of smaller
(lighter) molecules from the weak gravi-
tational field of the moon, our assess-
ment of this fraction should be bolstered
by whatever samples can be filtered from
interplanetary space.

Panspermia

The earth and perhaps other planets
might serve as sources for the cosmic dis-
semination of life. Because the hypothesis
of panspermia does not solve the basic
problem of the origin of life, but merely
transfers it to another site, it has been
discounted by most commentators. How-
ever, gene flow among the plancts would
alter the diversification of evolutionary
patterns, and the transport of even a
fragment of an organism might short-cir-
cuit an otherwise tortuous history of
evolutionary progress. It should be con-
ceded that planetary biology will have
greater fundamental interest if it does
reflect wholly independent origins of life
so as to furnish an opportunity for com-
parative study of wholly different bio-
chemical systems. On this basis, the most
intercsting questions will not be the ex-
pected convergent evolution of extrater-
restrial organisms towards superficial re-
semblances in parallel habitats, but the
details of their biochemical make-up. As
far as we know, all terrestrial organisms
have their genetic basis in nucleic acids.
If, say, the Martian flora likewise proves
to contain deoxyribonucleic acids, we
will face a difficult problem: Have these
compounds evolved recurrently as a
unique solution to the requirements of
genetic replication, or do we share a
common ancestor? The dilemma is no
less pointed if we find another genetic
material: Is it an independent solution or
an evolutionary divergence conditioned
by a unique habitat? Thus, a definitive
conclusion on Arrhenius’ hypothesis will
be an indispensable element of cosmic
biology.

The problem of expulsion of particles
no smaller than He* from the gravita-
tional field of a planet like the earth by
natural agencies already poses formid-
able difficulties for the hypothesis of
panspermia. These are compounded by
the radiation hazards of extra-atmos-
pheric space (6), although we need more
information on the potency of these haz-
ards under the actual conditions of space.
To defend Arrhenius’ thesis on the basis
of present information is difficult, but the

3

thesis is too important to be rejected by
prior reasoning short of an empirical test.

In fact, we can point to one mecha-
nism which meets the requirements of
ejection and safe transport from the
earth: the interplanetary missile. Unless
specific precautions are taken, such space
craft are likely to carry terrestrial or-
ganisms to the moon or other targets.
Such contaminants might make an ex-
plicit test of Arrhenius’ hypothesis diffi-
cult to interpret. The surface area of the
moon is 4 x 102° m?. Microbial popula-
tions can easily reach 10'* microorgan-
isms per kilogram of contaminated ma-
terial.

The same considerations apply even
more strongly to other destinations. Arti-
ficial contamination might distort the
microbiology of more hospitable planets
and might even perform the function
posited by Arrhenius of seeding a pre-
viously sterile planet.

A given level of contamination, be it
biological or chemical, is significant in
relation to the sensitivity of currently
available methods of analysis, Advances
in analytical chemistry are likely to make
meaningful levels of occurrence or con-
tamination which are beyond the scope
of present techniques. Present methods
are already sensitive enough that radio-
chemical analysis of, say, the moon’s sur-
face might be perturbed by the fallout
from a single atomic missile.

Since the sending of rockets to crash
on the moon’s surface is within the grasp
of present technique, while the retrieval
of samples is not, we are in the awkward
situation of being able to spoil certain
possibilities for scientific investigation for
a considerable interval before we can
constructively realize them. However,
our assessment of the validity of these
risks may become more reliable with the
accumulation of information on the phys-
ical parameters of extra-atmospheric
space and with the astrophysical data

that can be collected to great advantage
from artificial satellites. There are, in
addition, many model experiments in
microbiology and biochemistry that can
be performed in the terrestrial labora-
tory, and some information on the sur-
vival of spores might be collected from
telemetered experiments in satellite de-
vices.

At the present pace of missile devel-
opment we urgently nced to give some
thought to the conservative measures
needed to protect future scientific objec-
tives on the moon and the planets. We
are pleased to note that at the instance
of the National Academy of Sciences of
the United States, the International
Council of Scientific Unions has estab-
lished a special committee under the
chairmanship of M. Florkin of Liége,
Belgium, to review the problems of con-
tamination of extraterrestrial objects.

References and Notes

1. D. H. Menzel, Natl. Geographic Mag. 113,
277 (1958).

2. 8. Arrhenius, Worlds in the Making (Harper,
New York, 1908).

3. H. C. Urey, The Planets: Their Origin and
Development (Yale Univ, Press, New Haven,
Conn., 1952).

4. G. P. Kuiper, Ed., The Earth as a Planet
(Univ. of Chicago Press, Chicago, 1954).

5. “Les particules solides dans les astres,” Col-
log. intern, d’astrophys. No, 6; “Les molécules
dans les astres,” Colloq. intern. d’astrophys.
No, 7 (Inst. d’Astrophys. Univ. de Lidge,
Cointe-Sclessin, Belgium, 1954; 1956),

6. A. I. Oparin, The Origin of Life on the Earth

(Academic Press, New York, ed. 3, 1957).

S. Miller, in Reports on the International

Symposium: The Origin of Life on the Earth

(Akad. Nauk. 5.5.8.R., Moscow, 1957), pp.

73-85.

8. E. M. Burbidge e¢ al., Revs. Modern Phys.
29, 547 (1957).

9. J. Dufay, Galactic Nebulae and Interstellar
Matter (Philosophical Library, New York,

1957).

10. J. A. Hynek, Ed., Astrophysics, a Topical
Symposium (McGraw-Hill, New York, 1951).

ll. D. R. Bates and L. Spitzer, Astrophys. J. 113,
441 (1951).

12. F. Hoyle, Frontiers of Astronomy (Mentor,
New York, 1957).

13. F. L. Whipple, Astrophys. J. 113, 464 (1951).

14, H. Petterson, Nature 181, 330 (1958).

15. G. Nueller, Geochim et Cosmochim. Acta 4,
1 (1953).

~