Radon and its decay product polonium can be used as tracers to search for lunar volatiles. One effective technique to look for them would be by using alpha-particle spectrometers from lunar orbit. Alpha spectrometers were flown in the Apollo Service Modules during the Apollo 15 and 16 missions, and did observe Rn-222 and its decay product Po-210 on the lunar surface from orbit. This demonstrates that radon and polonium can be observed from orbit; what must next be shown is that such observations can reveal something about the locations of volatiles on the Moon

Friesen, Larry Jay

English

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28 Neiu Technologies /or Lunar Resource Assessment
This research was also supported by NASA grant NAGW-28.
Image analysis facilities generously provided by the W. M. Keck
Foundation.
References: [ I ] Belton M. j. S. et al. ( 1992) Science, 255, 570.
[2] Adler I. et al. (1972) Proc. LSC 3rd, 2157. [3] Adler 1. et al.
(1973) Proc. LSC 4th, 2783. [4] Adams J. B. and McCord T. B.
(1973) Proc. LSC 4th, 163. [5] Adams J. B. and Charette M. P.
(1975) Moon, 13, 293. (6) Bielefeld M.J. et al. (1977) Proc. LSC
8th, 901. [7] Wildey R. L. (1977) Moon, 16, 231. [8] Hawke B. R.
et al. (1984) LPSC XV, 350. (9| Spudis P. D. et al. (1984) Proc.
LPSC 15th, in JGR, 89, C197. [10J Pieters et al. (1992) JGR, in
preparation.
RADON AS A TRACER FOR LUNAJR VOLATILES. Larry
Jay Friesen, Lockheed Engineering and Sciences Company, P.O.
Box 58561, Houston TX 77258, USA.
Introduction: Radon and its decay product polonium can be
used as tracers to search for lunar volatiles. One effective tech-
nique to look for them would be by using alpha-particle spec-
trometers from lunar orbit.
Alpha spectrometers were flown in the Apollo Service Modules
during the Apollo 15 and 16 missions, and did observe 222Rn and
its decay product 21(:>Po on the lunar surface from orbit [ 1 ,2J. This
demonstrates that radon and polonium can be observed from orbit;
what must next be shown is that such observations can reveal
something about the locations of volatiles on the Moon.
How Radon and Polonium Can Act as Tracers for Volatiles:
What must first be understood is that radon released in the lunar
regolith has a very low probability of reaching the surface before
it decays, if it must do so under its own power, by random walk.
Under the extreme vacuum of the Moon, the mean free path
of a radon atom's random walk is not determined by collisions
with other gas molecules, but rather by collisions with soil grain
surfaces. The problem the atom faces is that it has such a high
heat of adsorption that when it collides with a soil grain surface,
it tends to stick very tightly to that surface, and thus remain there
for a comparatively long time before it takes the next hop. This
slows down the diffusion process so much that almost all of the
radon atoms random walking in this way will decay before they
reach the lunar surface [3]. On top of that , radon atoms may
have difficulty getting "unstuck" from their production sites into
the intergrain volume for even the first hop [2].
For these reasons, if radon or its decay product polonium are
seen on the lunar surface in significant amounts, the radon was
almost certainly swept out by some other outflowing gas or gases.
The Apollo 15 and 16 orbital alpha spectrometer results indicate
another reason to suspect that radon is brought to the surface
by sweeping rather than by random diffusion: the radon and
polonium decay rates 'observed were not in equilibrium with each
other [1,2]. At Aristarchus Crater during Apollo 15, the decay
rate of "2Rn was in excess of what would be in equilibrium with
''°Po 11). At other locations, such as Grimaldi Crater and the
edges of circular maria, the decay rate of ''°Po was in excess.
The half-life of 222Rn is 3.8 days. 1 he time to reach decay
equilibrium between 227Rn and 210Po is latgely controlled by the
22.3-year half-life of the intermediate isotope 2l°Pb. Thus the
disequilibria between decay rates of radon and polonium indicate
that outflow rates of radon at these locations on the Moon have
varied on rimescales of days to years. Indeed, given the short half-
life of 2nRn, the radon observed at Aristarchus during Apollo 15
must have been coming out of the Moon during the time of the
mission, or within a few days before. This is a further argument
in favor of a sweeping mechanism for bringing the radon to the
surface, rather than a steady diffusion. In this connection, it is
worth noting that the sites where higher-than-average radon and
polonium decay rates were observed—Aristarchus, Grimaldi, and
edges of circular maria—are among those most frequently cited
as locations for lunar transient events (LTE), as summarized by
Middlehurst and Moore [4,5]. For these reasons, it has previously
been suggested [6] that radon and polonium can act as a tracer
system for recent geologic activity on airless planetary bodies.
But radon and polonium activity can trace not only where
activity is taking place, but where volatile deposits are. For in
order for a "sweeping" mechanism to bring radon to the surface,
there must be volatile reservoirs as well as activity to bring it up.
So observing major amounts of radon and/or polonium at some
location on the surface would be a strong indicator of a source
of volatiles beneath that location.
Where might such sources of volatile materials lie, and how
might they have gotten there? One possibility is that, even though
analysis of returned samples indicates that the near-surface layers
of the Moon are extremely depleted in water and other volatiles,
deposits of volatiles may exist at greater depths. Our present
understanding of the Moon's formation and history is not yet
complete enough to rule this out. Another concept, mentioned
by Smith 17] and Feldman et al. [8], is that some fraction of the
water from comets impacting the Moon may be driven down into
the regolith and megaregolith by the impact process, to be trapped
in pores and fractures or to form hydrated minerals, and remain
buried for geologically significant periods of time.
The latter idea, if correct, would make understandable the
association of many LTE with young bright ray craters, such as
Aristarchus.
Such potential pockets of buried cometary volatiles represent
another place to search for useful volatile materials, in addition
to the frequently mentioned idea of ice deposits in permanently
shadowed craters at the lunar poles |9). We should keep in mind
that no one presently knows for certain that such polar ice deposits
exist.
We don't know, of course, whether any pockets of cometary
volatiles exist elsewhere, either. And in fact, Smith's suggestion
runs counter to estimates by Chyba [ 10] and others that volatiles
from objects impacting the Moon would be almost completely lost.
The point is that any significant lunar source of, for example,
water or carbon dioxide or nitrogen would have enormous value
for life support at a manned lunar settlement, so much value that
we should use every reasonable search technique, and look every-
where we can think of, in our efforts to find sources of useful
volatiles on the Moon. Even if the likelihood of success is small,
the potential payoff justifies a significant search effort.
One attraction of using radon as a tracer is that it doesn't matter
if the idea of cometary implantation of volatiles is wrong. This
technique does not depend on any assumptions about how volatiles
got into place. If volariles are present, and if they are coming to
the surface, they will bring radon with them. The radon—or its
polonium descendant—can then be observed from lunar orbit.
The Apollo 15 and 16 alpha spectrometer results already give
us strong reason to suspect that deposits of volatiles do exist at
some locations within the Moon, from which "burps" occasionally
come to the surface in an episodic fashion.
https://ntrs.nasa.gov/search.jsp?R=19930008059 2020-03-24T07:24:33+00:00Z
LP1 Technical Report 92-06 29
One caution: Detecting radon or polonium from lunar orbit
can reveal the existence and selenographic locations of volatile
reservoirs within the Moon; however, such detections will not
reveal the amount of volatile material within a reservoir, its
composition, how-deeply it is buried, nor how difficult it will be
to gain access to the material. Thus this search technique may
be of more value for the long term, for permanent human set-
tlement of the Moon, than for an initial base.
Proposed Method of Search: It will surprise no one who has
read this far that I am proposing to look for radon and polonium
with an alpha spectrometer from lunar orbit, just as was done
on Apollo 15 and 16. One important difference is that this time
it should be done from lunar polar orbit in order to get a complete
global survey of any significant radon sources.
It would be useful to narrow the selenographic resolution of
detection, if feasible. The maps of2 '°Po distribution produced from
Apollo 15 and 16 data show 2lcPo activity variations for blocks
of the lunar surface 10° wide [11]. It would be very helpful to
be able to locate volatile flows more precisely. By integrating over
several orbits, it should be possible to improve the signal-to-noise
ratio enough to do this [12].
Information of the sort I am proposing may be provided by the
proposed Lunar Prospector mission [12], if and when it flies. That
mission has an alpha spectrometer among its planned suite of
instruments.
References: [1] Bjorkholm P. J. et al. (1973) Proc. LSC 4th,
2793-2802. |2] Gorenstein P. et al. (1973) Proc. LSC 4th, 2803-
2809. [3] Friesen L. j. and Adams J. A. S. (1976) CCA, 40, 375-
380. (4) Middlehurst B. M. (1967) Rev. Geoprryy., 5, 173-189.
[5] Middlehurst B. M. and Moore P. A. (1967) Science, 155, 449-
451. [6] Friesen L J. (1975) Bull. Georgia Acad. Sci., 33, 77.
[71 Smith H. (1990) LP1 seminar. [8] Feldman W. C. (1991) GRL,
18, 2157-2160. [9| Arnold J. R. (1979) JGR, 84, 5659-5668.
[lOlChyba C. F. (1987) Nature, 330, 632-635. [ 1 1 ] Proc. LPSC
4th (1973), Plate V. [ 12] Binder A. (1992) personal communication.
N 9 3 " 17
ECONOMIC GEOLOC)LOGY OF THE MOON: SOME CON-
SIDERATIONS. Stephen L. Gillett, Department of Geology,
Mackay School of Mines, University of Nevada, Reno NV 89557,
USA.
Supporting any but the smallest lunar facility will require indi-
genous resources due to the extremely high cost of bringing
material from Earth [1]. The Moon has also attracted interest
as a resource base to help support near-Earth space activities,
because of the potential lower cost once the necessary infrastruc-
ture has been amortized. Obviously, initial lunar products will be
high-volume, bulk commodities, as they are the only ones for which
the economics of lunar production are conceivably attractive.
Certain rarer elements, such as the halogens, C, and H, would
also be extremely useful (for propellant, life support, and/or rea-
gents), and indeed local sources of such elements would vastly
improve the economics of lunar resource extraction.
However, early scenarios for lunar resources, based on extracting
many common elements from ordinary regolith [2], are probably
unworkable. A survey of terrestrial mining experience indicates
that the overwhelming criterion of a potentially economic deposit
is its recoverable concentration of the desired element [3). This
results because separating elements is difficult and costly, and is
indeed why "ores"—i.e., anomalous concentrations of the desired
element—are worth seeking in the first place. In fact (and some-
what paradoxically), a high-grade ore is most critical for large-
volume commodities, as those would be prohibitive to purify on
the scale required. Hence, identifying the most concentrated
sources of desired element(s) for local resource extraction should
be an element of future lunar missions, and certainly should be
a factor in siting a lunar base.
Few lunar geologic studies to this point, however, are directly
relevant to exploration for potential ores, as they have been
concerned with identifying large-scale, global processes. This focus
has been appropriate, of course, given our a priori ignorance of
the Moon. Moreover, quite apart from the Moon's scientific
interest, serious concern about its resource value would also have
been highly premature, for at least two reasons: (1) Useful con-
straints would be impossible without first knowing the basic geo-
logic framework and (2) potential resources cannot be evaluated
in a vacuum; their value depends on their possible uses. However,
the broad-brush approach of planetological studies is opposite that
of economic geology, because economic geology deals with the
products of rare events. Ore deposits typically occur where by
happenstance a rare process, or a fortuitous combination of pro-
cesses, has run to an extreme. Such deposits are rare by their
very nature; indeed, almost by their very definition. Nonetheless,
they are worth seeking because of the extreme expense of separ-
ating elements, so that it is highly cost effective to let natural
processes carry out as much separation as possible first. Hence,
global characterizations such as typify traditional lunar studies are
not immediately relevant for economic geology investigations.
They merely set the geologic context, whereas economic geology
lies in the unusual—and extremely local—variations from the
norm.
It has been argued, especially in the popular literature, that
the anhydrous nature of lunar materials precludes any ore deposits
like those on the Earth. This is overstated. For one thing, the
Moon is vastly more heterogeneous than had been thought even
until relatively recently, as has been demonstrated by both ongoing
studies of lunar petrology [4] and by remote sensing [5]. The Moon
underwent protracted igneous activity, spanning hundreds of mil-
lions of years, during its early history, and this led to varied and
large-scale differentiation and fractionation.
Second, the mere waterlessness of the lunar environment does
not preclude all ore-forming processes. Although it is true that
water is vital in forming many terrestriql ores, anhydrous magmatic
differentiation is also capable of generating substantial element
concentrations, and indeed has formed some terrestrial ore de-
posits. Such purely magmatic processes include partial melting,
fractional crystallization, and phase separation (liquid unmixing
or crystal settling). Examples of such magmatic ores on Earth
include Cu-Ni sulfide ores from sulfide liquid immiscibility, chrom-
ite from cumulate settling, and magnetite ores crystallized from
late-stage magmatic fluids extremely enriched in Fe [6). Similar
processes may occur on the Moon [7]. Yet other potential lunar
processes may have no terrestrial analogues, as with the possibility
of cold-trapped volatiles in permanently shadowed regions at the
lunar poles [8J.
To be sure, the lunar geologic environment is very different
from the terrestrial, and the possible consequences of the Moon's
low oxidation state and waterlessness for generating useful con-
centrations of elements must be addressed. For-example, consid-
ering the behavior of the rarer elements under lunar conditions
with a view to how (or whether!) they can become concentrated


Radon as a Tracer for Lunar Volatiles

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