Freeman, John W.

Vondrak, Richard R.

English

NASA Technical Reports Server

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 '91 - 7i 208
EFFECTS ON THE LUNAR ATMOSPHERE RESULTING FROM LARGE-SCALE MANNED
OPERATIONS; Richard R. Vondrak and John W. Freeman, Stanford Research
Institute, Menlo Park, California 94025 and Department of Space Physics and
Astronomy, Rice University, Houston, Texas 77001.
The present lunar atmosphere is a collisionless exosphere with surface
number densities less than i0? cm-3 and a total mass of approximately 104 kgm.
This tenuous state is maintained by the solar wind which promptly removes the
majority of the ionized gas from the lunar vicinity through the action of
the interplanetary electric field. The mean atmosphere life of an atom or
molecule is the ionization lifetime (typically 106 to 107 seconds). This
process determines the loss rate for all except the lightest atoms for which
thermal escape dominates. The accelerated ions have been directly detected
for both the natural lunar atmosphere and gasses associated with the Apollo
missions (I). For the Apollo exhaust gases decay times of the order of one
month were observed (2).
This solar wind loss mechanism is the dominant process only so long as
the solar wind has direct access to the majority of the atmosphere. As the
atmosphere becomes more dense, newly formed ions of atmospheric origin load
down the solar wind and cause it to be diverted around the moon. Furthermore,
the solar wind can carry away no more mass from the planet than its own mass
flux to the planet. Venus and Mars each lose about 10 gm/sec to the solar
wind. This represents about 1% for Venus and 20% for Mars of the mass flux
of the solar wind through the respective planetary cross-sectional areas.
We wish to explore what happens if the atmospheric source rate from
natural or artificial sources exceeds the solar wind's ability to carry off
the gas. As the atmosphere becomes more dense, the base of the exosphere
will rise above the surface and we expect the exospheric temperature to
become greater than that of the lunar surface. Thermal escape will then
become the dominant loss mechanism for the majority of the atmosphere and the
atmosphere may become long-lived, since thermal escape times are thousands
of years for gases heavier than helium.
The transition from a thin to a thick long-lived atmosphere is difficult
to evaluate quantitatively because of our lack of understanding of exactly
when a bow shock wave will form to begin to deflect the solar wind around the
moon. Also, anomalous ionization may take place via Alfv6n's critical
velocity mechanism (3). We may simplify the problem by assuming the upper
limit of mass loss due to the solar wind to equal the solar wind mass flux
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EFFECTS ON LUNAR ATMOSPHERE
Vondrak, Richard R. and John W. Freeman
intercepted by the lunar disk, or about 50 gm/sec. Assuming source and loss
equilibrium and a mean time to ionization of 107 seconds, this means an atmos-
pheric mass of 5 x i0 S kgm. We may further assume that this represents the
critical mass and source rate beyond which thermal escape can become the
important loss mechanism. Thermal escape will remain small, however, until
the exospheric temperature rises. A quantitative evaluation of these
atmospheric loss mechanisms is shown in Figure i.
Finally, we estimate that for a total atmospheric mass of 108 kgm and an
exospheric temperature of 800°K the thermal escape loss rate would be 60 kgm/
sec. For atmospheres in excess of 108 kgm the loss rate cannot increase sub-
stantially and the atmosphere can grow indefinitely if the source rate
exceeds 60 kgm/sec. Therefore, a constant addition rate of the order of
i00 kgm/sec is required to transform the lunar exosphere into a long-lived
state (4).
Figure 2 shows the ultimate atmospheric masses which result from various
constant gas addition rates, Q. The effect of inducing a transition from
an exosphere with rapid loss to a thick atmosphere with slow loss is illus-
trated by considering the result when the gas source is shut off (Q=O). The
thick atmosphere decays with an exponential lifetime of several hundred years,
whereas the thin exosphere decays in a few weeks.
Each Apollo mission deposited nearly 104 kgm of rocket exhaust in the
lunar environment. A permanent base might be expected to release gas at
the rate of 10 -2 kg/sec.-man assuming supply traffic equal to one Apollo
mission/man-month. Small colonies would not be expected to produce a long-
lived atmosphere. However, even modest exploration releases gas much
faster than the present natural rate of about 20 gm/sec, resulting in a
lunar atmosphere in which the gases of natural origin would be only trace
components.
Vigorous exploration or industrial production could result in gas rates
greater than the I00 kgm/sec necessary to form a long-lived atmosphere.
References
i. Benson, J., Freeman, J. W. and Hills, H. K., Proc. Sixth Lunar Sci.
Conf., pp. 3013-3021, 1975.
2. Freeman, J. W., Jr., Fenner, M. A., Hills, H. K., Lindeman, R. A.,
Medrano, R., and Meister, J., Icarus, 16, p_. 328-338, 1972.
. Lindeman, R. A., Vondrak, R. R., Freeman, J. W., and Snyder, C. W.,
J. Geophys. Res., 79, pp. 2287-2296, 1976.
4. Vondrak, R. R., Nature, 248, pp. 657-659, 1976.
EFFECTSONTHELUNARATMOSPHERESULTINGFROM LARGE-SCALE
Richard R. Vondrak and John W. Freeman
79
IO
161
g
,6'
SOLAR WINO
STAGNATION
_-e-- EXOSPH ERE,O. ,_-- ANO --_-'e- THICK ATMOSPHERE
EXOSPHERiC 800 •
" HEATING _ 60 kQ/sec/
MASS LOST TO /
_ 30gin/No
--- T.E,.,, ESC*PS d s
I * l i I , I ,
IO s IO s 10 7 IO g
TOTAL MASS (kg)
Figure !, Loss rates from an oxygen (mass
16 a.m.u.) atmosphere
i0 ie
i0 m
"_ i0 i4
.1¢
Or) i01_of)
:E
o IoK
n-
W
-I- i0 e
n
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O i0 6
:E
104
i0 _
- F ! atrn.
/ I_p(_.ooo._
-
' _._' !. ,' ,!., ' t I, ' t "
102 104 l_oy 10 Im ly,. I0,0yr. I00_. --
TIME (sec)
Figure 2. Growth curves of the lunar atmosphere.
Dashed lines indicate decay in total
mass if the gas source is shut off.
80
DISCUSSION- (Vondrak and Freeman)
SPEAKERi: This is perhaps a commentrather than a question. Again,
trying to comparea space colony and a lunar surface colony, I'm sure that
in a space colony one would go to enormousefforts not to release gas simply
because it was a valuable commodity and you could recycle it. I don't know
what the economicswould be on the Moon, but I think it's at least suggest-
ible that the recovery of gas might be an equally important value and that
you might turn out not to vent as muchgas as terrestrial or Apollo experience
would indicate.
FREEMAN: Yes. I think that's certainly a valid comment.


Effects on the lunar atmosphere resulting from large-scale manned operations

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