Lunar Soils: A Long-Term Archive for the Galactic
Environment of the Heliosphere?
Robert F. Wimmer-Schweingruber* and Peter Bochsler*
*Physikalisches Institut, Universitdt Bern, Sidlerstrasse 5, CH-3012 Bern, Switzerland
Abstract.
Solar wind implanted in surface layers (~ 0.03/^m) of lunar soil grains has often been analyzed to infer the
history of the solar wind. In somewhat deeper layers, and thus presumably at higher implantation energies, a
mysterious population, dubbed "SEP" for "solar energetic particle", accounts for the majority of the implanted
gas - several orders of magnitude more than expected from the present-day flux of solar energetic particles.
In addition, its elemental and isotopic composition is distinct from that of the solar system. While the heavy
Ne isotopes are enriched relative to 20Ne, 15N is depleted relative to 14N - a behavior that is hard to explain
with acceleration of solar material. N is overabundant with respect to the noble gases (especially Ar). Here we
show that interstellar pick-up ions (PUIs) which are ionized and accelerated in the heliosphere and subsequently
implanted in lunar regolith grains can account for the properties of the "SEP" population. This implies that lunar
soils preserve samples of the galactic environment of the solar system and may eventually be used as an archive
for solar system "climate".
INTRODUCTION
14
Lunar soils contain a record of implanted noble gases
since the formation of the lunar regolith (soil) more than
3 • 109 years ago. Figure 1 shows the 20Ne/22Ne isotope
ratio vs. the cumulative release of 20Ne for the Apollo
17 lunar soil sample 71501. The dominant contribution
in the outermost layers of the grains which outgas first
is of solar wind composition, hence this population is
termed solar wind (SW). In later extraction steps, a population is found with a different composition than that of
the the solar wind. It appears that this component has
been implanted at energies far exceeding typical solar
wind energies, but still below 100 keV/nuc [1], hence
this population has been called "SEP" (for Solar Energetic Particles) [2]. Despite its apparent similarity with
the composition of actual SEPs previously determined by
in-situ observations with various spacecraft-borne instruments [3], this population cannot originate from SEPs. It
exceeds by several orders of magnitude the amount expected from present-day fluxes of solar energetic particles. The "SEP" to solar wind abundance ratio measured
in lunar soils (10% - 40%) disagrees with present-day
expectations, even when allowing for the fact that solar wind neon may be trapped with an efficiency of only
about 10%. Enhanced solar activity in the past is probably unable to explain the amount of the "SEP" component because the energy for acceleration needs to come
SW
• CSSE
oSH
13
0)
C
12-
11
0.0
Gas release curve
for 71501 pyroxene
0.2
0.4
"SEP"-?"
0.6
1.0
FIGURE 1. 20Ne/22Ne isotopic ratio versus the cumulative
fraction of released neon for lunar soil 71501 pyroxene. Two
different extraction techniques were applied, closed system
stepped etching (CSSE, full symbols), and stepped heating
(SH, open symbols). SW denotes solar wind isotopic composition, "SEP" that of the "SEP" component. Data are from Wieler
etal. [2].
from the solar magnetic field which has not varied that
much in the past [4, 5]. Here, we argue that this controversial "SEP" component is not solar, but originates
CP598, Solar and Galactic Composition, edited by R. F. Wimmer-Schweingruber
© 2001 American Institute of Physics 0-7354-0042-3/017$ 18.00
399
0.8
fraction Ne released
predominantly in interstellar pick-up ions (PUIs). We
propose that this gas component should be more adequately termed "HEP" for "Heliospheric Energetic Particles". We will show that this new interpretation explains
the compositional signatures of the HEP component, the
amount of implanted HEP gas, as well as the implantation energy range.
INTERSTELLAR MATTER IN THE
HELIOSPHERE
Interstellar neutral gas entering the inner heliosphere undergoes ionization mainly by charge exchange with solar
wind ions or by ionization with solar ultraviolet. Once a
particle is ionized, it experiences Lorentz forces due to
the relative motion of the interplanetary magnetic field
which is carried outwards from the sun with the solar wind plasma. The newly created ion is "picked up"
and, together with the solar wind particles, it is gradually swept to the boundaries of the heliosphere. The
pick-up process results in particle distributions which are
extremely non-thermal with speeds ranging from about
—v sw to +vsw in the solar wind reference frame. These
velocity distributions are unstable to the generation of
hydromagnetic waves [6, 7] which decay by generating turbulence in the ambient solar wind plasma. In regions of high turbulence, PUIs are much more efficiently
accelerated than solar wind particles, because of their
wide initial energy distribution. Strong power-law tails
develop at speeds above vsw [8] which contain up to
10% - 20% of the total PUI flux. Depending on the ionization properties of a given atom, it can penetrate more
or less deeply into the heliosphere. Helium has particularly low ionization rates and, as a consequence, interstellar pick-up (HEP-) helium is similarly abundant
as (true) SEP helium at the relevant suprathermal energies - even at 1 AU - resulting in characteristic differences between the He/O abundance ratios in CIRassociated SEPs (He/O ~ 157 [9]) and the solar wind
(He/O ~ 75 [10]). Similarly, interstellar neon penetrates
more deeply into the heliosphere than most other species
(except helium) because of its relatively low ionization
rates. Neon and helium are mainly ionized by solar ultraviolet [11], and the present-day flux of interstellar
neon PUIs reaches its maximum near the orbit of the
Earth. There it amounts to about 2.5 x 10~5 of the solar wind neon flux, depending somewhat on solar activity. Neutral hydrogen penetrates about as far as the orbit of Jupiter. Then it is partly ionized by ionization, but
mainly by resonant charge exchange with solar wind protons [11]. The charge-exchange cross section of nitrogen
is currently not known [11]. Nitrogen is probably ionized around 2-3 AU. Interstellar H, He, N, O, and Ne
400
PUIs have been observed [12, 13, 14] at the orbit of the
Earth or along the trajectory of the Ulysses spacecraft.
The isotopic composition of interstellar pick-up helium,
4
He/3He - 4400 [13], or - 5800 [15], differs significantly from the solar wind 4He/3He isotopic abundance
ratio which amounts to typically 2300 [16, 17].
From in-situ investigations we know that interstellar He pick-up ions account for a large fraction of the
suprathermal particle population [8, 18], ranging from
50% during solar quiet times to ~ 10% during disturbed
times [19]. Using the best currently known ionization
rates for He and Ne [11] and the formalism of Vasyliunas and Siscoe [20] one can easily compute that the average ratio of interstellar Ne/He is depleted by about a
factor ~ 10 relative to its solar value. Hence we may expect that interstellar Ne accounts for between 1 - 5 % of
the suprathermal flux of Ne. Indeed, Mobius et al.'[21]
report that 8% of the suprathermal Ne associated with
CIRs at 1 AU (and hence connected to a shock location
beyond 1 AU) is singly charged.
CONTEMPORARY LOCAL
INTERSTELLAR MEDIUM [LISM]
The density of the galactic interstellar medium varies
over many orders of magnitude, from ~ 0.01cm"3 to
more than 106cm~3 in dense molecular clouds with a
galactic average of about % gal ~ 1 cm"3 [22]. Presently,
the Sun and the heliosphere are located near the edge [23]
of the so-called local interstellar cloud which has a density of 0.05 < nH < 0.25cm~3 [24, 25, 26, 27, 28], a
comparatively tenuous environment [29]. For this work
we will adopt a value of HH = O.lcm"3. The elemental composition of the local interstellar cloud is roughly
comparable to the solar composition [30] although we
would expect the "metals" to be enriched due to the
chemical evolution of the ISM. The isotopic composition of the ISM is inferred from radio observations of
interstellar molecules. The solar 14N/15N isotopic abundance ratio (200 ± 55) [31] is apparently somewhat lower
than the terrestrial value (~ 272). The corresponding ratio in the ISM at the galactocentric distance of the Sun is
450 ± 22 [32]. It is generally assumed that the LISM has
been subject to further nucleosynthetic processing since
the solar system decoupled from galactic matter at the
time of its formation. Thus, it appears that the abundance
of 14N increases with time with respect to 15N. 14N is
the more 'secondary' isotope of the two (i. e. the species
which is produced in stars and recycled by stellar winds
and stellar explosions into the ISM after some 'primary'
nuclei such as 12C, 15N, or 16O have been produced in
earlier generations of stars).
The isotopic composition of interstellar neon is
presently unknown. Since 20Ne is certainly a primary
nucleus, it is generally believed that the ISM neon has
been decreasing its 20Ne/22Ne isotopic abundance ratio
since the formation of the Sun. However, presentlyavailable information from anomalous cosmic rays
originating in accelerated PUIs that have been further
accelerated at the heliospheric termination shock [33],
gives no evidence for such a decrease within the present
experimental uncertainties [34] .
We have summarized the isotopic and elemental signatures for other elements expected from galactic chemical evolution and the corresponding values measured
in lunar soils in Table 1. The inferred HEP abundances
are generally consistent with the behavior expected from
galactic nucleosynthetic evolution. To our knowledge,
no non-terrestrial D has been identified in lunar soils so
far due to contamination issues. Because H and D PUIs
reach their maximum production rate at several AU, well
beyond the orbit of the Earth and the Moon, the HEP
component should account for a considerably smaller
fraction of implanted gas in lunar soils than in the case
of He and Ne. Possibly, some asteroidal regoliths might
have conserved a signature of interstellar HEP D. Note
that in HEP N the heavier isotope is depleted with respect
to the lighter one - a behavior that is hard to explain as
due to accelerated solar wind or to to deeper implantation
of the heavier isotope in lunar soil grains.
PAST AND FUTURE OF THE
HELIOSPHERE
We now proceed to show that the amount and implantation energy of the HEP component follow naturally from
the interpretation presented in this work. In its history,
the solar system has orbited the galactic center about 18
times, assuming present-day orbital elements. Along its
galactic orbit, the Sun and the heliosphere must have encountered interstellar clouds of varying density. In fact,
the observation that the velocity dispersion of similar
types of stars increases with their age is generally explained as the result of episodic encounters with dense
molecular clouds [41], as is the possible outward galactic migration of the solar system to its present galactocentric distance suggested by Wielen et al. [42]. The observed size and density distributions of such clouds follow power laws, with the denser clouds being smaller
than their more tenuous counterparts [e. g. 43].
Obviously, in a first approximation, the flux of interstellar PUIs will be proportional to the neutral density in the local environment, as long as their progenitor particles are atomic. With increasing density, molecular species will grow in importance, and, consequently,
the radial distribution of the PUI flux will be altered be-
401
cause of the different ionization properties of molecules.
This may explain why nitrogen, which forms very stable molecules and bounds strongly to interstellar dust
particles, is too abundant in implanted gases when compared with e. g. Ar which, as a noble gas, does not form
molecules.
ACCELERATION TO SUPRATHERMAL
ENERGIES
Finding an estimate for the flux, O, of suprathermal, accelerated PUIs which are of prime importance for our interpretation of the HEP population of implanted gases in
lunar soils, is somewhat more involved. In the following,
we assume that it scales as a power law, O ~ nl^~s. The
case where S = 0 describes the situation in which the flux
is strictly proportional to the interstellar neutral density
and in which the ISM density has no influence on the acceleration process. The severe mass loading of the solar
wind however, will lead to strongly enhanced turbulence,
as is observed in the vicinity of comets [44, 45, 46, 47].
In-situ observations of accelerated PUIs strongly support the view that these particles are efficiently accelerated in turbulent regions of the heliosphere by a mechanism such as transit-time damping [48] of magnetic field
fluctuations [8,49] or possibly by other means. This type
of acceleration can be viewed as a diffusion process in
momentum space. The associated diffusion tensor, Dpp,
increases with the square of the normalized power in the
fluctuations of the magnetic field, ($B/B)2, It is to be
expected that the flux of accelerated PUIs, O, is proportional to the product of the number of PUIs available for
acceleration and the efficiency of the injection or acceleration process. The number of PUIs available for acceleration is obviously proportional to the neutral density at heliocentric distance r, nn(r). This in turn is proportional to the neutral number density in the interstellar
medium, #H. The efficiency of the injection and acceleration mechanism is inversely proportional to the acceleration time. This, in turn, is inversely proportional to Dpp.
As we have already mentioned, Dpp is proportional to the
amount of turbulence, ($B/B)2, and this has been shown
to be proportional to nn(r) and hence to /ZH [7]. Admittedly, the acceleration process of pick-up ions in turbulent regions in the heliosphere is not fully understood.
Nevertheless, a power-law dependence of the acceleration efficiency on density is not without merits. Density
inhomogeneities in the interstellar medium would certainly also contribute to a compression and subsequent
enhancement of turbulence, for example. Moreover, in
a dense interstellar environment, the fast magnetosonic
speed in the heliosphere is enhanced due to the large
pressure contribution of interstellar pick-up ions, result-
TABLE 1. Summary of solar and expected and observed behavior of the HEP component in lunar soils. * best
estimate and/or expected temporal evolution, n.d.: not detectable, dec.: decreasing, inc.: increasing ^ assumed
terrestrial * J.M. Weygand, pers. comm., 2000 nrse: neutron rich species enriched
Solar
Signature
D/H
He/4He
12
C/13C
14
N/15N
20
Ne/22Ne
36
Ar/38Ar
3
84Kr/86Kr
Xe isotopes
Reference
no deuterium
- 4.3 x 10~4
-891"
200 ±55
13.7±0.3
5.6 ±0.6*
3.296 ±0.013
solar wind
_
[16]
_
[31]
[16]
[39]
[40]
HEP*
5
-2-3xlO" (dec.)
< 4 x 10~4 (inc.)
< 89 (dec.)
> 200 (inc.)
< 13.7 (dec.)
< 5.6 (dec.)
< 3.3 (dec.)
heavier (nrse)
ing in stronger interplanetary shocks and formation of
corotating shocks nearer to one AU. Thus we assume
that the flux of accelerated, suprathermal PUIs is proportional to a power of the neutral particle density, O ~ n^~s,
where we believe that S w 1.
Therefore, during times when the heliosphere encounters dense clouds, the flux of suprathermal PUIs is considerably higher than today. Simplistically inserting the
average neutral hydrogen density in the galaxy, % gai ~
lcm~3, corresponding to the passage through a moderately dense cloud complex, we arrive at a flux 100 times
greater than today.
In order to find the average flux to which lunar soils
have been exposed during the history of the solar system,
we need to weight the expected flux for a given density
of the LIC with the occurrence rate for clouds of that
density. In a previous paper [50], we considered a large
volume, LQ, with LO ~ 103pc which contains one cloud
of that dimension and a density no, two clouds with
twice the density, and half its linear dimension, etc. down
to a scale LI ~ O.lpc (typically considered to be the
size of the smallest and densest clouds). This reflects
the mass-conserving, stationary state of fragmenting and
collapsing clouds. The resulting mass distribution of the
clouds, nm ~ w~ 2 , lies close to observed values, nm ~
w~L8[51]. Assuming that the average density of this
model cloud conglomerate reflects % gai ~ 1 cm"3, one
can derive no w 0.05 cm~3. Next, we need to account for
the fact that only a fraction, r|max, of the available pick-up
ions can be accelerated. rjmax can be expressed as a size,
LTJ below which clouds contribute at most rjmax of their
density to suprathermal particles. Following our previous
approach [50, 4] this can be evaluated and yields
=
(3-D)-
<Ptodlay
Observed HEP
Reference
n.d.
(2.17±0.05)xKT 4
_
-300
11.3±0.3
-4.9
3. 151 ±0.015
heavier (nrse)
[35, 36]
[37]
_
[38]
[2]
[37]
[40]
[40]
'H
where D is the power-law index of the distribution of the
number of clouds in a given size interval, R the powerlaw index for the density distribution in a given size interval. We have plotted the resulting enhancement factor of the long-term average compared to the presentday suprathermal pick-up flux (O)/(r|o9today) for various values of D in Figure 2. The solid curve in Fig-
FIGURE 2. Enhancement factor <$/(r|o(ptoday) vs. 5 for D =
1.5, 1.75, 2.00, 2.25, and 2.50. The solid curve is for D = 2.00,
the dashed and dotted curves for other values of D are arranged
symmetrically and the values of D are indicated. For all curves,
LI = 10"1, Tjmax = 0.1, Tjo = 10~3, R=l, ptoday = 0.1 Cm~ 3 .
ure 2 is for D = 2.00, the dashed and dotted curves for
other values of D are arranged symmetrically and the
values of D are indicated. We do not cover the entire
range of values that D might have. Probable values for
D are 1.2 < D < 2.7 with the more extreme ones being
less probable. For the curves in Figure 2, LO/LI = 104,
Tlmax = 0.1, R = 1, fttoday = O.lcm"3 and todays ac-
Po
•no
3-D-R(l+S)
402
celeration efficiency r|o = 10 3. The enhancement factor (O) /(t|o9today) amounts to about 300 for 5 = 1 and
D = 2. The long-term average of the flux of accelerated,
i. e. suprathermal, pick-up ions is between 100 and 1000
times higher than today. The curves begin to flatten at
S ~ 1 because then only the more tenuous clouds contribute in the non-linear fashion envisioned in our previous paper [50]. The denser clouds can only contribute a
fixed maximum fraction (t|max) of their density. Therefore, the growth with S slows down.
varying depths. Encounters with dense interstellar clouds
are only of short duration (~ 104 — 105 y) and may possibly serve as markers for lunar soil dating, much as volcanic ashes in terrestrial sediments aid in understanding
climate archives. Obviously, improvements in the dating of lunar soils are important. As we believe we have
shown in this work, the lunar regolith may not only serve
to understand the ancient solar wind (and thus processes
active in the early history of the Sun), but also as an
archive for heliospheric climate, and as a "travel diary"
for the voyage of the solar system through the galaxy.
CONCLUSIONS
ACKNOWLEDGMENTS
We have shown that the non-solar composition of the
HEP component measured in lunar soils may be explained by its interstellar origin. The accelerated PUIs
have energies which lie in the appropriate range to explain the implantation depth of the HEP component. As
we have already mentioned, it accounts for 10% - 40%
of the total amount of implanted gas in lunar soils.
Mewaldt et al. [52] have simulated the isotopic composition of Ne implanted in lunar soils (assumed to be
pure SiO2) by applying the TRIM package [53] to the
measured interplanetary spectrum of particles [54]. Assuming a solar value of 13.7 for the impinging solar and
suprathermal 20Ne/22Ne isotope abundance ratio they
find that beyond a depth of several 1000 A this ratio lies
at 11.3 in the implanted gas, essentially confirming work
by Wieler et al. (mentioned in [1]), and Bochsler and
Wimmer[55] that was not formally published. The reason
for this depth-dependent variation in composition lies in
the different ranges of the different isotopes in matter.
The Ne isotopic ratio of the deeply implanted particles
can be achieved with different combinations of energy
spectral index and isotope ratio in the suprathermal particles [52]. The amount of the HEP component remains
unexplained in their scenario, they find that an increase
by a factor ~ 100 in the suprathermal tails is needed.
Considering that interstellar neon contributes several
percent to the flux of suprathermal particles, our enhancement factor of ~ 300 shows that accelerated interstellar pick-up ions contributed a substantial fraction to
the total flux of suprathermal particles in the past.
This interpretation has implications beyond the mere
explanation of a badly understood gas component in
the lunar soil. Since their formation more than 3 • 109
years ago, lunar soils have been preserving samples
of the LISM through which the solar system has migrated. Deciphering this > 3 • 109 year archive will
not be easy. The moon is constantly being bombarded
by micro-meteorites and occasionally by larger meteorites. This results in the process known as "lunar gardening", a constant tilling of the lunar soil to greatly
403
We wish to thank Rainer Wieler, Otto Eugster, Andre
Maeder, Fritz Biihler, and Kurt Marti for helpful discussions. This work was supported by the Schweizerischer
Nationalfonds.
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