Fremlin, J. H.

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REMOTE-CONTROLLED STUDY OF THE HISTORY OF LUNAR VOLATILES; J. H.
Fremlin, Department of Physics, University of Birmingham, England.
Since the formation of the moon, very large quantities of volatile
materials must have been evolved from the interior. If the moon was origin-
ally formed nearer the sun as its high refractory content would suggest, most
of these must be permanently lost. This could also be true if it formed in
association with the earth, but had for a long time a high surface temperature
internally generated. For most of the time since it was established in its
present orbit, however, it must have had a surface temperature determined as
now by the dynamic equilibrium between the input of energy from sunlight and
the loss from thermal radiation. During this period some further loss of
volatiles, including small quantities of water derived from interaction
between solar wind protons and surface oxides must have continued.
It is well known that the gravitational field of the moon is inadequate
to hold an atmosphere at its average surface temperature of around 250°K, but
if areas exist on the moon sufficiently cold to freeze out volatiles, some of
these may still persist. Such areas may exist near the poles. If the moon
were perfectly smooth an area near one of its poles would be calculably
cooler than the equator, but not cool enough. The axis of the moon's orbit
and of its rotation is inclined at just over 5° to the axis of the earth's
orbit round the sun and hence in the polar 'summer' there will be six months
insolation rising to a maximum with the sun just over 5° above the horizon.
The rate of input of solar energy will then be reduced to about i/ii of
that at midday at the equator and, since thermal radiation varies as T_ the
maximum temperature reached will be only about 210OK. At this temperature
the vapour pressure of water is still high enough for it to be lost quite
quickly but 210OK is the temperature that would be reached if the moon were
perfectly smooth. It is not in fact perfectly smooth. Even a micro-roughness
in which some dust particles projected to reach near-equatorial temperatures
and hence lost thermal radiation at eleven times the local average rate would
enable temperatures well below 210°K to be permanently maintained by much of
the surface, although estimation of the magnitude of the effect would be
difficult. The moon's surface, however, is rough on a major as well as on a
minor scale. A range of hills at a latitude just under 85° (5° from the pole)
and with a poleward slope of over I0° would leave an area in permanent
shadow; hills of only 200 metres high would shadow a strip a kilometre wide.
Near the pole craters or parallel ranges of hills with internal slopes over
5° will give permanently shaded areas. Thermal conduction from the sunlit
areas, themselves having average round-the-year temperatures of only 170°K
or so, would be negligible over even a few hundred metres - much less than
https://ntrs.nasa.gov/search.jsp?R=19910073670 2020-03-24T07:04:17+00:00Z
REMOTE-CONTROLLEDSTU Y:
Fremlin, J. H.
HISTORY OF LUNAR VOLATILES
151
the thickness of the hills or crater walls.
We can, therefore, confidently expect that there will be areas of signif-
icant size near the lunar poles in which the surface temperature has for
extended periods been almost entirely determined by the amount of heat from
the interior reaching the lunar2surface, q_nis has been found from Apollo
studies to be 2 x 10-6 watts/cm . At the equator we have a daytime absolute
temperature of about 390°K with a solar heat input of 1.4 kw/m 2. The equi-
librium temperature on a dark area near the pole should then be in the region
of 390 ÷ (0.14/2 x 10-6) 0.25 or 26 ° K.
This would be low enough to hold water, ammonia or carbon dioxide
indefinitely, probably together with oxygen and nitrogen. Only a fraction
of volatile gases evolved from the interior would, of course, reach such
areas; most on the day side may be ionised at once and swept away by the
solar wind, but any large emissions, especially on the dark side, should
have left frozen layers in the polar craters. In the long term it is even
possible that there could be enough water ice to be of practical value to
a lunar colony, but my present concern is with the possible scientific value
of the "sedimentary" layers in offering us a record of volatile emissions
over a very long period of lunar history. Careful analysis of a core from
such sediments, on the lines of the examination of deep ocean cores, should
enable us to reconstruct this history.
My specific proposal is that a mobile unmanned remote-controlled unit
should be landed at a suitable point at polar midsummer when the permanently
dark areas could readily be identified. The unit would carry a core-digger,
an evaporator and a recording or on-line mass spectrograph which could
transmit results periodically to a lunar satellite in polar orbit. The unit
should have sensors capable of measuring surface temperatures so that it
could first survey the surface temperature distribution in the chosen area
and then be directed to the coldest point for its analysis of volatiles as a
function of depth. If more convenient, the motion of the unit could be self-
controlled to follow surface temperature gradients downwards until it
reached a minimum.
If the results were interesting, a later Version of the unit could
include a recoverable module either to return a complete core suitably
cooled or at least a sample of the underlying rock which might be dated by
fission-track analysis to determine the time at which the crater itself was
formed and hence the length of time over which the sediments were collected.


Remote-controlled study of the history of lunar volatiles

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