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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