Lunar and Planetary Science XXXIII (2002)
1835.pdf
MAPS OF LUNAR RADON-222 AND POLONIUM-210. S. L. Lawson1, W. C. Feldman1, D. J. Lawrence1,
K. R. Moore 1, S. Maurice 2, R. D. Belian 1, and A. B. Binder3. 1Los Alamos National Laboratory, Los Alamos, NM;
2
Observatoire Midi-Pyrénées, Toulouse, France; 3Lunar Research Institute, Tucson, AZ. ([email protected])
Introduction: The Lunar Prospector Alpha Particle
Spectrometer (LP APS) searched for lunar surface gas
release events and mapped their distribution by
detecting alpha particles produced by the decay of
gaseous radon-222 (5.5 MeV, 3.8 day half-life), solid
polonium-218 (6.0 MeV, 3 minute half-life), and solid
polonium-210 (5.3 MeV, 138 day half-life, but held up
in production by the 21 year half-life of lead-210).
These three nuclides are radioactive daughters from the
decay of uranium-238. Radon reaches the lunar
surface either at areas of high soil porosity or where
fissures release the trapped gases in which radon is
entrained. Once released, the radon spreads out by
“bouncing” across the surface on ballistic trajectories
in a random-walk process. The half-life of radon-222
allows the gas to spread out by several 100 km before
it decays (depositing approximately half of the
polonium-218 recoil nuclides on the lunar surface) and
allows the APS to detect gas release events up to
several days after they occur. The long residence time
of the lead-210 precursor to polonium-210 allows the
mapping of gas vents which have been active over the
last approximately 60 years. Because radon and
polonium are daughter products of the decay of
uranium, the background level of alpha particle
activity is a function of the lunar crustal uranium
distribution.
Using radioactive radon and polonium as tracers,
the Apollo 15 and 16 Command Module orbital alpha
particle experiments obtained evidence for the release
of gases at several sites beneath the orbit tracks,
especially over the Aristarchus Plateau and Mare
Fecunditatis [1]. Aristarchus crater had previously
been identified by ground-based observers as the site
of transient optical events [2]. The Apollo 17 surface
mass spectrometer showed that argon-40 is released
from the lunar interior every few months, apparently in
concert with some of the shallow moonquakes that are
believed to be of tectonic origin [3]. The latter tectonic
events could be associated with very young scarps
identified in the lunar highlands [4] and are believed to
indicate continued global contraction. Such quakes
could open fissures leading to the release of gases that
are trapped below the surface. A primary goal of the
APS was to map gas-release events, thus allowing both
an appraisal of the current level of tectonic activity on
the Moon and providing a probe of subsurface uranium
concentrations.
Analysis: The APS consisted of five pairs of
silicon ion-implanted detectors which were collimated
to a 45° half-angle aperture. The detectors were
positioned on the five outward-pointing faces of a cube
which was mounted at the tip of one of the spacecraft
booms. The five analog signals sent to the electronics
box were digitized using a common 8bit ADC,
spanning the nominal energy range between 4.5 MeV
and 6.6 MeV (corresponding to 0.02 MeV per
channel). The detector resolution was approximately
0.1 MeV. During data acquisition, LP spun at 12 RPM
with its spin axis approximately perpendicular to the
ecliptic. Thus, three of the APS faces swept through
nadir when the spacecraft was near the equator, while
the other two faces were either nadir- or zenithpointing when the spacecraft was near the poles.
The primary lunar alpha-particle signal is from
reflected interplanetary alpha particles. Therefore, we
have examined only APS data acquired during periods
of low interplanetary alpha particle flux. To minimize
statistical uncertainties, spectra were summed over all
LP mapping cycles when the instrument was turned on
(approximately 229 days over 16 months).
Discussion: Figure 1 shows two sample spectra
summed over 15° × 10° regions where radon-222 (red
line; centered just west of Aristarchus at 22.5°N,
55°W) and polonium-210 (blue line; centered at
37.5°N, 35°E) peaks were detected. Characteristic 1σ
systematic uncertainties are shown. Polonium-218 is
expected to be found in concert with radon-222, but
with a peak magnitude which is at most half that of
radon. The radon-detection spectrum in Figure 1 may
have a polonium-218 peak at 6.0 MeV, but it is only
slightly above the detection threshold. This spectrum
does not show a polonium-210 peak, indicating a new
source of radon outgassing in this area of the lunar
surface. The polonium-210-detection spectrum shows
neither a radon-222 or polonium-218 peak, indicating
radon outgassing events in this area sometime in the
past 60 years, but not in the past several days.
Figure 2 shows the spectrum obtained over the
Aristarchus plateau (black line; centered at 22.5°N,
45°W), the only location on the lunar surface where we
detect both radon-222 and polonium-210 peaks. For
comparison we also show a typical spectrum where no
peaks were found (green line; centered at 22.5°N,
105°E). The magnitude of the polonium-210 peak
relative to that of the radon-222 peak on the
Aristarchus spectrum indicates that the radon
outgassing activity in this area has been much greater
in the past than it is at present.
Lunar and Planetary Science XXXIII (2002)
1835.pdf
RADON-222 AND POLONIUM-210 MAPS: S. L. Lawson, et al.
To yield lunar alpha particle maps, we summed
over a 0.2 MeV energy range centered on each of the
three alpha particle energies noted above. Figure 3
shows our polonium-210 map of the lunar surface,
while figure 4 is our radon-222 map. There is a wide
spatial distribution of polonium-210 detections at the
2σ threshold, with more on the nearside and few in the
maria. Elevated radon count rates can be seen over the
Aristarchus plateau and Kepler crater (8°N, 37°W).
Conclusions: The LP APS data were studied to
map sites of radon release on the lunar surface. We
found only a faint indication of alpha particles
resulting from the decay of polonium-218. However,
our radon-222 alpha particle map indicates that radon
gas is presently emanating from the vicinity of craters
Aristarchus and Kepler. The LP gamma-ray
spectrometer, which effectively has significantly
higher spatial resolution than the APS, identified
thorium enrichments at these two craters [5]. Thorium
and uranium are both incompatible elements whose
lunar surface abundances are highly correlated; thus, it
is likely that the radon-222 alpha particles measured
using the LP APS originate from Kepler and
Aristarchus. Our detection of radon over Aristarchus is
consistent with one of the results of the Apollo 15
APS. The polonium-210 distribution mapped by the
APS indicates a variability in time and space of lunar
gas release events.
Figure 1. Sample LP APS spectra.
Figure 2. APS spectrum over Aristarchus compared
with a spectrum lacking alpha particle detections.
Figure 3. LP APS polonium-210 map (sum over
5.2–5.4 MeV energy range).
F i g u r e 4 . LP APS radon-222 map (sum over
5.4–5.6 MeV energy range).
References: [1] Gorenstein P. (1993) in Remote
Geochemical Analysis, C.M. Pieters and P.A.J.
Englert, eds., 235. [2] Middlehurst B. (1967) Rev.
Geophys., 5, 173. [3] Hodges R. and J. Hofman (1975)
Proc. Lunar Sci. Conf. 6th, 3039. [4] Schultz P. (1976)
Moon Morphology, Univ. of Texas, Austin.
[5] Lawrence D.J. et al. (2000) JGR, 105, 20307.