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HARVARD COLLEGE OBSERVATORY

CAMBRIDGE 38, MASSACHUSETTS
30 November 1965

Dr. Philip Abelson

Geophysics Laboratory

Carnegie Institution of Washington
Washington, D. C.

Dear Dr. Abelson:

Many thanks for letting me see a prepublication copy of
your paper "Abiogenic Synthesis in the Martian Environment."
Several of the points touched, such as the low mean water vapor
abundance, and the high surface ultraviolet flux have been dis-
cussed many times before. Since reasonable physical mechanisms
to protect hypothetical Martian organisms against these potential
hazards can be envisioned easily, I do not see how they can be
used as an argument against indigenous life on Mars. Your deduc-
tion that the Mariner IV observations exclude the presence of hot
springs on Mars seems to me very dubious, considering the diffi-
culties in interpretation of the Mariner IV pictures and because
of the assumed terrestrial analogy.

a_i uas 17 ey

The main new point which you introduce is, I believe, the
contention that Mars could, at any rate, have retained a reducing
atmosphere only for very brief periods of time in its early his-
tory, and that the origin of Life there is correspondingly un-
likely. I believe the same argument implies that life on Earth is
impossible. Rubey's discussion of excess volatiles in the Earth
neglects the outgassing of hydrogen, because of its rapid escape
from the Earth. Due to the high rate of escape of hydrogen from
the base of the terrestrial exosphere, the net rate of escape of
hydrogen from the Earth is governed not by the exospheric tempera-
ture but by the supply rate to the base of the exosphere, which
is in turn determined by the diffusion properties of the lower
atmosphere, The net e-folding time for the escape of hydrogen
Dr. Philip Abelson
Page 2.

from the Earth's exosphere is ~ 1000 years. If Chamberlain's
estimate of the Martian exos phere temperature, ~900°, is valid,
the e-folding time for the escape of hydrogen from Mars is of
the same order of magnitude. In this case the actual hydrogen
escape rate is not determined by the fraction of outward-
directed molecules with velocities in excess of the escape
velocity, but instead, as on Earth, is diffusion~limited. t£
the much lower temperatures implied by the Mariner iv occul=
tation experiment are applicable to the Martian exosphere,
hydrogen will have substantially more difficulty escaping from
Mars than from Earth,

) :
Thus, any argument which uses contemporary parameters for the

escape processes and which purports to show that Mars could not
retain a primitive reducing environment must also imply that the
Earth could not have retained a primitive reducing environment,
and that life never developed on this planet, contrary to the
prevailing evidence. The existence of a primitive reducing
terrestrial atmosphere for a significant period of time implies
the existence of a similar atmosphere on Mars. This conclusion
might be avoided if we were to assume much less outgassing of
reduced compounds in early Martian history, but there is no
compelling evidence for such a conclusion. In discussions of
exospheric energy balance in primitive reducing atmospheres, it
would be preferable to use, instead of contemporary parameters,
the more germane primitive values, Unfortunately, the theory
required to compute exospheric temperatures in reducing atmospheres
is in an extremely primitive state, I am enclosing a related
discussion of atmospheric evolution for your comments.

Sincerely,

a a ~~
¥
Carl Sagan\__ _/

cs /ttb
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