we
(eo tb = Magna
4

February 14, 1961

br. Edward Anders“
Associate Prof. of Chemistry
Phe Enrico Fermi Institute for

Nuclear Studies
University of Chicage
Chicago 37, Illinois
Dear DOr. Anders;

Thank you for your letter and manuscript on "The Moon as

a Collector of Biological Material." I am sorry I have delayed
in véplying but I have only just returned from ten days in
PaSadena anc Santa Monica on business of the Planetary

Btmospheres study Group of the Space Science Board.

Since our conversation in New York in December, I nave
also been thinking about the integration of cosmobiota con-
trisutions from the distrioution of stars in the Galaxy, and
nave Ootained an expression slightly different from yours.
The Galaxy can be considered a Shpere of radius RB, surround-

dng the Sun, imbedded in a disk of radius Re and thickness Rie
In this way tne contribution of biological material from the
solar neighborhood is explicitly taken into account. MfTranserib-
imy to your notation, I obtain

Bor
2eR, +fin (R/R,)

if

 

where is the expected Lunar surface Gensity of cosmobiota, and

(ay om et oD pd

—
fis the volume number density of stars in the solar neighbor-
hood. Withf =O, the expression essentially reduces to your

eq. (2). If we take Rae tne thickness of the galaxy at the
Sun's distance from the galactic center, to be of the order of
iUS pees, the two terms in the denominator are of equal magni-
tude, This decreases your value of #W by at most a factor of 2.
sowever, it does point out that if the stars in the solar
neighborhood were ever very much closer together than they are
today, W can nave been much Smaller. Many current ideas on

Star formation suygest that there was a sudden condensation of
clusters of Stars in the remote past. The clusters then dissipate;
open clusters and multiple star Systems are thought to be the
remnants of this process. If for 10° y@ear8, the stars in the
solar neighborhood were ten times closer together, the effective
Ww would be smaller by @ factor of 100. Life would have to

arise very rapidly in these early times, but this may not be
impossiple. There is some evidence that life arose very rapidly

on the primitive Earth.

ri we set & = 0, ,ftake as the mean density of stars in the
universe, and Ry aS the effective radius of the universe, we ob-

“hE 9728 x 10733 x 10% = a x 1947 gm, or about

tain W= 4x 10
a factor of ten less tnan the previous estimate. I£ the big-bang
cosmogony is correct, then f/f waS once very much greater. it

therefore seems that if appropriate ejection and acceleration
and Survival mechanisms exist, the lunar cosmobiota population
~~ if any -- may be predominantly intergalactic, rather than

interstellar in origin.

Z wonder if the values of W are as prohibitive as you suggest.

a ls 3 . . -
= we Lo gm, tnis corresponds to an average ejection flux

during geoloyical time, for a terrestrial planet, of about

~ TG ~ -— . .
= gm com 7 sec a, Current eStimates for the infall flux

3

LO
ck meteorites on the Earth are 10” to 10° times larger. put
anotner way, the escape flux would be about 1 micro-organism

per Square centimeter per year. Suppose a one-ton meteorite
ejects into interplanetary space upon impact a mass of surface
material equai to 1% of its mass {is this a reasonable
agSumption?). One ky of soil may contain about rot? micro-
organisms, so the meteorite ejects 10° gm x 107? x 1075 = 197! gm
of micro-organisns = zot+ micro-organisms into Space. There-
fore, to give the required escape flux, 107* of the Barth's
Surtace must have been hit by a one-ton meteorite once during

geological time. Isn't it plausible that Such is the case?

Finally, I would object to your revised statement of the
need for sterilization of lunar impact vehicles, as contained
in the copy of your reply to Dr. Lederbery, which you kindly
sent to me. As Zi attempted to suggest in my paper on biological
contamination of the Moon, a copy Of which i8 enclosed, there

are four other reasons for sterilization besides the possible
confusion of terreStriai micro-organisms with cosmobiota.
These are:
(1) a confusion between deposited terrestrial micro-organisms

2

ang rélics of primitive Lunar indigenous organisms

(2) a confusion between deposited terrestrial organic

matter and prebiological Lunar organic matter

we
Sut

an @xplosive reproduction of deposited terrestrial micro-

oe

organisms in prebiological lunar organic matter, and

{4} interaction or confusion between deposited terrestrial
micro-organisms and indigenous contemporary Lunar

organisms.

TO this can now be added the very exciting suggestion of Dr.
Tuvkevica, which you quote, namely,
(5} interaction or confusion between deposited terrestrial
micro-organisms anc ancient terrestrial organisms, or

their remnants, ejected to the Moon in early times.

In my paper, Z conclude that the likelihoods of (1) and {2)
ocuurring are very Small, but the probabilities of {3), (4},

and (5), while remote, are non-negligible. Even if it were
certain that the Moon nad been previously contaminated through
meceoritic ejection, I would argue for sterilization of impacters
tO keep the Moon safe for paleomicrobiologists. But as you point
out, we are not certain. Since the information to be gained

from an investigation of Lunar prebiological organic matter,
indigenous lunar organism, or ancient terrestrial micro-organisms,
are 60 important, despite the fact that the probabilities of
making these discoveries are low, I feel that rigorous steriliza-
tion should be supported. The engineers are anxious to be rid
o£ the burdensome chore of sterilization, anc I know they will seize
upon your Statement. i would therefore urge you to reconsider
vour comments on lunar sterilization.

Thank you again for sending me your faScinating manuscript.
I hope we will have the opportunity of discussing these topics
again soon.

Sincerely,

Carl Sagan

Cs:mr
Enclosure - 1
cc: Dr. Lederberg