In-situ production of organic molecules at the poles of the Moon
P13D-1730
S.T. Crites1*, P.G. Lucey1, and D.J. Lawrence2
1. Hawaii Institute of Geophysics and Planetology,
University of Hawaii at Manoa, Honolulu, HI
2. Space Department, JHU/APL, Laurel, MD
*[email protected]
I. Introduction
V. Proton-stimulated organic synthesis: previous work
VIII. Results of proton flux modeling
Samples returned by the Apollo missions showed no trace of organic materials.
However, the poles of the Moon are utterly unlike the equatorial regions, and the
LCROSS impactor detected a range of organic compounds including C2H4,
CH3OH, and CH4 (Colaprete et al., 2010). These compounds may have been
delivered directly by comets, or they may have developed in situ from simple ices
or even solar wind gas. If these compounds developed in situ the poles of the
Moon provide an opportunity to test models of inorganic synthesis that can be
applied to many surfaces in the solar system and interstellar clouds.
In order to estimate organic production due to proton irradiation, we need an
estimate of a production rate for this process, and a calculation of proton flux in
the regolith. The production rate due to protons can be obtained from previous
lab experiments irradiating ices and measuring resulting organics. Moore and
Hudson (1998) performed experiments exploring organic synthesis in mixtures of
H2O ices mixed with simple C, H, O, and N-bearing ices. They irradiated thinlydeposited ice layers at temperatures below 20 K with a beam of 1 MeV protons.
Although most galactic cosmic rays have higher energy, Moore and Hudson
chose 1 MeV because the stopping power for protons by H2O ice is maximum at
this energy, and thus these protons could be expected to contribute the most to
the radiation dose (Hudson and Moore 1995).
We calculated the proton flux with depth in the regolith as a result of GCR protons. Figure 10 (a) shows the flux
of protons at all energies with depth in the regolith. In order to compare our results directly to Moore and
Hudson’s experiments using 1 MeV protons, we report the flux with depth of protons of energies between 0.1 and
10 MeV. Protons decline with depth but there is a small peak of 0.009 p/cm2-s at a depth of 8 cm below the
surface for protons in this energy band (Figure 10 (b)). Figure 11 shows the energy spectrum of protons, with the
band around 1 MeV highlighted in pink. The protons in this band are only a small portion of the protons affecting
the regolith, with the ratio of 1 MeV protons to the total proton flux of all energies around 100.
Production of organics in situ requires the presence of the relevant elements
(combinations of C, H, O, and N), sufficient mobility of elements to react with one
another, and an energy source to drive reactions. This project is to assess the
galactic cosmic ray protons flux in the lunar regolith as a potential energy source
and contribute to an assessment of the plausibility of production of organics on the
Moon from in-situ inorganic material.
Figure 1: From Colaprete et al. (2010). A variety of
organic compounds were detected in the plume from
the LCROSS mission.
We also performed simulations for FAN soil with 1, 3, 10, and 30 wt% H2O. A comparison of proton flux with and
without water show that the water content of the regolith does not affect the proton flux significantly (Figure 12).
Moore and Hudson found that complex organic molecules began to appear after
a dose of about 10 eV/molecule, and production rates leveled off after a dose of
about 17 eV/molecule, or a beam fluence of 1.5 x 1015 p/cm2 (Figure 8).
II. Presence of C, H, O, N
The solar wind, comets, and asteroids all deliver volatiles to the Moon,
as shown in Figure 2. However, high daytime temperatures prevent Comets
these volatiles from being retained over much of the Moon’s surface in
significant quantitites. The unique nature of the poles provide an
exception. Because of the low obliquity of the Moon, regions in
topographic lows at the poles are permanently shaded from sunlight Asteroids
and measurements from the Diviner Lunar Radiometer have
confirmed the extremely cold nature of some of these regions (Paige
et al., 2010). The temperatures measured by Diviner (Figure 3) are
low enough to trap even very volatile ices such as CO2. Figure 4
Interplanetary Dust Particles
shows vacuum evaporation rates from Zhang and Paige (2009),
demonstrating that a wide range of compounds are stable at the
Figure 2: From Lucey (2000).
temperatures of polar cold traps.
Solar Wind
Figure 10 (a): Flux of protons of all energies with depth in the regolith. (b): Flux of protons between 0.1 and 10 MeV with depth in the regolith.
Both plots are for a dry FAN soil composition.
Figure 8: From Moore and
Hudson (1998). Changes in
column density of synthesized
molecules from a mixture of
H2O + CH4.
The Moon
Giant Molecular Clouds
Possible sources of lunar polar volatiles.
VI. Modeling proton dose in the regolith: Methods
Using the particle transport code MCNPX (Pelowitz, 2008), we can estimate the
proton flux in the regolith to be used with the production rates of Moore and
Hudson (1998). The GCR flux varies with solar activity; we picked a solar
modulation parameter of Phi=300 for this simulation. We simplified the GCR
flux to protons only and modeled the Moon as a sphere with radius 1738 km and
FAN composition. We ran simulations for dry FAN soil and FAN soil with 1, 3,
10, and 30 wt% H2O.
VII. Neutron flux at the surface: Sanity check
Figure 3: Based on figure from Paige et al. (2010)
(UCLA/JPL/GSFC/NASA). Surface temperatures at the lunar south
pole measured by the Diviner Lunar Radiometer when temperatures
were near the annual maximum.
Figure 4: From Zhang and Paige (2009). Calculated evaporation rates for
a variety of organic and inorganic compounds. The coldest temperatures
detected by Diviner Lunar Radiometer, represented by the blue line, are
sufficient to trap even extremely volatile compounds.
In order to check our assumptions, we calculated neutron flux induced by the
GCR protons in our simulation. The neutron flux is well studied and understood
because of its importance to planetary neutron and gamma ray spectroscopy.
We compared our neutron calculations to those found in the literature (Figure 9),
and they are in good agreement. This gives us confidence in the validity of our
proton calculations.
This work
III. Mobility of elements
Temperatures cold enough to trap volatiles also prevent movement of the
elements. However, indirect illumination by reflected light off topographic
highs can heat some permanently shadowed regions. Models suggest
substantial temperature variation in the shallow subsurface (Figure 5,
Vasavada et al. (1999)), and meteorite impacts provide heating to deeper
layers.
IV. Energy source
Experiments have shown that both Lyman α UV radiation and energetic
protons can stimulate organic synthesis (Moore and Hudson 1998). The
lunar surface is illuminated by scattered interstellar Lyman α UV radiation
and galactic cosmic ray protons. However, Lyman α is confined to the
optical surface and erodes surface ice (Morgan and Shemansky, 1991), so
we investigate the deeper penetrating protons in the upper few centimeters
where ices are better protected from loss.
Figure 5: From Vasavada et
al. (1999). Modeled diurnal
minimum,
mean,
and
maxiumum temperatures as a
function of depth on the
Moon.
McKinney et al. (2006)
Figure 9: Comparison of neutron
albedo flux in thermal and epithermal
energy ranges as calculated by our
simulations and McKinney et al.
(2006).
Figure 11: Energy spectrum of cosmic ray protons in the
regolith for our simulation. The 1 MeV band is highlighted in
pink.
Figure 12: Ratio of proton flux in FAN soil with 30 wt% H2O to dry
FAN soil. The ratio of fluxes is close to one, indicating that the
amount of water present in the regolith is not an important
regulator of proton flux.
X. Implications and conclusions for organic synthesis
To compare directly to Moore and Hudson’s experiment, we calculate the time required to accumulate the fluence of
1.5 x 1015 p/cm2 reported by Moore and Hudson as being approximately equal to a dose of 17 eV/molecule. Using
our maximum flux of 0.009 p/cm2-s for 1 MeV protons only, we calculate that 5 By are required to accumulate this
fluence. This time is an upper limit for time required to accumulate a dose capable of initiating organic synthesis,
since there are 100x more protons available, and most are of higher energy. Hudson and Moore (1995) report that
there is a peak in efficiency for stopping power of protons at 1 MeV, so these may be the most effective. However,
the abundant higher energy protons also contribute to the radiation dose accumulated by ices in the regolith. Simple
calculations using the maximum flux of 2.15 p/cm2-s with no scaling for variations in proton effectiveness with
energy result in 200 My to accumulate a fluence of 1.5 x 1015 p/cm2 . The timescale for accumulation of the required
dose is in reality probably somewhere between our upper and lower limits of 5 By and 200 My.
Although energetic protons provide the energy source for initiation of reactions, the transfer of electrons actually
induces chemical reactions to occur. We calculated the electron flux with depth in the regolith as well as the energy
spectrum of electrons produced by GCR proton interactions with the regolith (not shown). A calculation of electron
flux produced by the Moore experiment would provide a means for comparison to these simulations.
By comparing our MCNPX particle flux simulations with experimental data from Moore and Hudson (1998), we find
that proton flux from GCR provides ample exposure to plausibly drive organic synthesis over timescales close to, or
probably much less than the age of the surface and the longevity of the cold traps. In addition, since many organics
are more refractory than ices, the organics produced may remain even as the extent of cold traps changes with time.
Future work includes assessing the contribution from higher-energy GCR protons on the radiation dose; explicitly
considering the electrons produced in the Moore experiment for comparison to our simulations; and potentially
considering the effects of regolith composition variation.
Figure 6: From Wilson et al.
(1997).
GCR particle type
abundances for nucleons of
energy 200 MeV.
The
majority of GCR particles are
protons.
Galactic cosmic rays, or energetic particles originating outside the solar
system, provide a constant source of proton irradiation to potentially
stimulate reactions on the Moon’s surface. Because the Moon does not
have a magnetic field to deflect them, its surface is constantly bombarded
by these particles. The GCR flux is composed of ~89% protons., ~9%
alpha particles, and <1% heavy nuclei, as shown in figure 6. Figure 7
shows the energy spectrum for the GCR flux.
Figure 7: From McKinney et al. (2006). Proton and
alpha particle fluxes for different values of the solar
modulation parameter (Phi).
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