ISSN 1062-8738, Bulletin of the Russian Academy of Sciences: Physics, 2007, Vol. 71, No. 7, pp. 953–956. © Allerton Press, Inc., 2007. Original Russian Text © G.K. Ustinova, 2007, published in Izvestiya Rossiiskoi Akademii Nauk. Seriya Fizicheskaya, 2007, Vol. 71, No. 7, pp. 986–989.

Charge States of Inert Gas Ions in Lunar Ilmenites G. K. Ustinova Vernadsky Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences, Moscow, 119991 Russia e-mail: [email protected] Abstract—Fractionation of particles of solar corpuscular radiation significantly changes from flare to flare; therefore, the average regularities derived from the contents of inert gases in lunar samples of different cosmicray exposure age are of particular importance. Simulation of the charge states of inert gas ions (Ne, Ar, Kr, and Xe) in lunar ilmenites suggests possibility of higher solar flare activity and higher hardness of the solar proton spectrum ~1 billion years ago. DOI: 10.3103/S1062873807070180

INTRODUCTION Inert gases in lunar ilmenites [1, 2] have been analyzed from the point of view of charge states of ions in two components of solar corpuscular radiation: solar wind (SW), with an energy E < 1 MeV/nucleon, and solar energetic particles (SEPs), E ~ 1– 50 MeV/nucleon, which are associated with solar flares. Ionization of atoms, depending on the first ionization potential, occurs in either the chromosphere or the bottom layers of the corona; the charge states of ions are formed according to the local temperature and electron density and remain invariable in subsequent processes [3]. The SW and SEP components have different isotopic and elemental compositions. It is believed that SEPs reach such high energies due to the shock-wave acceleration upon reconnection of magnetic fields during flares prior to the injection from the corona and/or due to the acceleration in the heliospheric magnetic fields. In this case, fractionation of the SEP component inevitably occurs, proportionally to A/Z or (A/Z)2 (and A/Q or (A/Q)2, where Q the ion charge, if the ionization is incomplete) [4]. For isotopes

i and j of the same element, fractionation in the shock wave is proportional to Ai/Aj or (Ai/Aj )2, i.e., is a conventional mass fractionation, which arises, for example, in diffusion due to the different volatilities of light and heavy isotopes of inert gases. This circumstance hinders understanding of the observed fractionation effects in samples of extraterrestrial materials. INERT GASES IN LUNAR ILMENITES The contents of inert gases He, Ne, Ar, Kr, and Xe in ilmenites of lunar soil 71501 (I71) and regolith breccia 79035 (I79), with cosmic-ray exposure ages of ~100 million years and ~1 billion years, respectively, were measured using closed-system step etching [1, 2]. Two components of captured inert gases were revealed: nonfractioned SW gases in near-surface fractions (1–3) and SEP gases (enriched in heavy isotopes) in fairly deep fractions (13–16) (see rows 3 and 6 in Tables 1 and 2, respectively). In Fig. 1, approximating polynomials clearly demonstrate that the mass numbers of neon, argon, krypton, and xenon increase with an increase in

Table 1. Simulation of the ratios of inert gas ions in the SW and SEP components of solar corpuscular radiation in lunar ili

j

menites I71 (the parameter KI 71 = (Ai/ Q I71 )2/(Aj/ Q I71 )2, where i and j are the isotopes in the ratios under consideration, A is the isotope mass number, and Q is the ion charge) 20

20

36

Composition

Ne ---------22 Ne

36

1 2 3

Solar system [7] SW: I 71(1) [1, 2]

13.68 13.81

5.31 5.46

37.65 13.91

3307 2043

4

SWI 71 = I 71(1) D 0 / D 0

13.81

5.46

40.3

5 6

SEP: SWI 71KI 71 SEP: I 71(13) [1, 2]

11.41 11.21

4.90 4.68

38.09 38.64

i

j

Ar --------38 Ar

Ne ---------36 Ar

* At QI71 = 18–19 for 82Kr and QI71 = 19 for 84Kr.

953

Ar --------84 Kr

82

Kr --------84 Kr

130

84

Xe -----------132 Xe

Kr -----------132 Xe

0.2004 0.2037

0.1653 0.1659

20.73 12.46

3984

0.2037

0.1659

20.77

1348 1308

0.2052* 0.2079

0.1609 0.1586

7.55 7.97

954

USTINOVA

Table 2. The same as in Table 1 but for I79 20

20

36

Composition

Ne ---------22 Ne

36

1 2 3

Solar system [7] SW: I79(3–4) [1, 2]

13.68 13.47

5.31 5.43

37.65 10.67

3307 2160

4

SWI79 = I79(3–4) D 0 / D 0

13.47

5.43

30.91

5 6

SEP: SWI79KI79 SEP: I79(16–17) [1, 2]

11.13 11.12

4.87 4.72

38.16 37.51

i

j

Ar --------38 Ar

Ne ---------36 Ar

Ar --------84 Kr

82

Kr --------84 Kr

130

84

Xe -----------132 Xe

Kr -----------132 Xe

0.2004 0.2119

0.1653 0.1761

20.73 4.64

4212

0.2119

0.1761

7.74

1598 1547

0.2315* 0.2306

0.1708 0.1635

3.13 3.53

* At QI79 = 21–22 for 82Kr and QI79 = 23 for 84Kr.

the etch depth. A similar regularity is observed for helium isotopes [1]. At the same time, for the He, Ne, and Ar gases, higher diffusion losses of lighter gases are observed in the element ratios of near-surface fractions. It can be seen in Fig. 2 that the 20Ne/36Ar ratio increases with the etch depth. The same regularity was observed for the 4He/36Ar ratios. Indeed, introduction of corrections to the self-diffusion coefficients D0 listed in Table 3 [5] equalizes the composition of SWI71 with the composition of inert gases in the solar wind [6] and Solar system [7] (Table 1, rows 2, 4). Such equalization does not occur in the regolith breccia I79 (see rows 2 and 4 in 20Ne/22Ne

84Kr/86Kr

14

3.35 3.30

13

3.25 3.20

12

3.15 11

0

4

8

12 16 20

36Ar/38Ar

3.10

0

4

8

12 16 20

8

12 16 20 Etch steps

132Xe/134Xe

6

2.76

5

2.72 2.68

4 2.64 3 2

2.60 0

4

8

12 16 20

2.56

0

4

Fig. 1. Dependences of the isotopic ratios of inert gases on the step etch depth for samples of lunar ilmenites: (crosses) lunar soil I 71 and (diamonds) regolith breccia I79 (according to the data of [1, 2]) and approximating polynomials for (solid lines) I71 and (dotted lines) I79.

Table 2 and Fig. 2, respectively); this problem will be considered in more detail at the end of the paper. However, the effect of near-surface diffusion is not clearly observed in the case of heavy gases in both ilmenites I71 and I79 (Fig. 2). With allowance for only mass fractionation, a paradoxal conclusion was drawn in [1, 2]: the ratios of light gases (4He/36Ar and 20Ne/36Ar) increase with depth, whereas the ratio 84Kr/132Xe remains constant. SIMULATION OF FRACTIONATION EFFECTS It can easily be seen that the above-mentioned paradox can easily be explained within the mechanism of fractionation of solar gases upon shock-wave acceleration. Indeed, for isotopes of each pair of light inert gases, A/Z = 2; hence, their relative content did not change during acceleration, remaining the same as in the SW component. In the case of heavy gases, A/Z = 2.33 and 2.44 for 84Kr and 132Xe, respectively; therefore, the 84Kr/132Xe ratio was smaller in the SEP component during shock-wave acceleration in comparison with the SW component. Thus, this ratio should decrease with increasing etch depth. However, in experiments, this effect is obscured by large diffusion losses of lighter 84Kr near the surface, as a result of which a constant 84Kr/132Xe ratio is observed. Nevertheless, it is most likely that ionization in the chromosphere was incomplete. For example, in the current solar wind, the average charge states QSW of Ne, Ar, Kr, and Xe ions are, respectively, 8, 9, 12, and 14; they correspond to the range of ionization potentials P ≈ 200–400 eV [8]. The observed ratios of isotopes and elements in the SEP components of the I71(13) and I79(16–17) samples, extracted from the depth, can be obtained by converting their ratios in the SW components I71(1) and I79(3–4), corrected to near-surface diffusion (Tables 1, 2, rows 4), taking into account the fractionation of the SEP component in the shock wave proportionally to (A/Q)2. Simulation shows (Tables 1–3, rows 5, 6) that, for the lunar soil ilmenite I71 with a cosmic-ray exposure age of ~100 million years, the average charge states QI71 of Ne, Ar, Kr, and Xe ions are,

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respectively, 8, 14, 18–19, and 18; these values lie in the range of P ≈ 700–800 eV. The best correspondence for Kr is observed at different charge states of the isotopes, specifically: QI71 = 18–19 (in equal proportions) for 82Kr and 19 for 84Kr. For the regolith breccia I79 with a cosmic-ray exposure age of ~1 billion years, the average charge states QI79 of Ne, Ar, Kr, and Xe ions correspond to the higher range of P (≈900–1000 eV); they are 8, 16, 21–23, and 23 respectively. The best correspondence for Kr is observed at QI79 = 21–22 (in equal proportions) for 82Kr and 23 for 84Kr. RESULTS AND DISCUSSION The charge states of ions of most elements in the current SEP component of corpuscular radiation from gradually developing solar flares correspond to ionization under conditions of thermal equilibrium at the temperature T ≈ 2 × 106 K, a value typical of the solar corona (for example, for iron ions, the charge QFe ≈ 10) [9]. However, in rigid pulse flares, which are significantly enriched in 3He and heavy ions, much higher charge states of ions were observed (QFe ≈ 20), which correspond to T ≈ 10 × 106 K [10], i.e., to much hotter layers of the corona. The higher average charge states of the inert gas ions in the SEP components for ~1 billion years in comparison with those for the last ~100 million years and the current period (Table 3) indicate high flare (pulse) activity of the Sun and, on average, its higher luminosity in past on the given time scale. It can be seen well in Fig. 2 that the ratios for heavy isotopes in ilmenites I79 are smaller than those for ilmenites I71 by a factor of almost 2. This difference may be due to insufficient correction of the cosmogenic component contribution in [1, 2]. For all isotopes except Xe, the correction was performed separately for each etch step in samples I71 and I79 on the basis of the contents of cosmogenic gases measured at deeper etch steps. In the case of xenon, due to the strong contamination by atmospheric Xe, the same current phenome20Ne/36Ar

84Kr/132Xe

50

10

955

Table 3. Self-diffusion coefficients D0 of inert gases [5] and charge states Q of their ions: QSW in the current solar wind [8] and QI71 and QI79 in the solar wind averaged over ~100 million years and ~1 billion years respectively (this study) Parameter cm2 s–1

D0, Z QSW (200–400 eV)* QI71 (700–800 eV)* QI79 (900–1000 eV)*

Ne

Ar

Kr

Xe

0.452 10 8 8 8

0.156 18 9 14 16

0.08 36 12 18–19 21–23

0.048 54 14 18 23

* The range of ionization potentials is indicated in parentheses [5].

nological composition of cosmogenic Xe was used to take into account the cosmogenic components in I71 and I79. However, the results considered above suggest that the energy spectrum of solar flare protons was harder ~1 billion years ago, a circumstance that should significantly change the ratios of cosmogenic isotopes. It was shown in [11] that the yield of all cosmogenic isotopes increases with increasing hardness of the spectrum of initiating particles; however, the increase rate is different for different isotopes, as a result of which their ratios change. As an example, Fig. 3 shows these regularities the Xe and Kr isotopes under consideration. It can be seen that the cosmogenic ratio 84Kr/132Xe depends strongly on the spectral hardness, decreasing by a factor of almost 2 with a change in γ only from 3 (the current average spectrum of solar protons) to 2.5 (the spectrum of galactic cosmic rays). Specifically Isotope content, atoms/106 Si; Relative isotope content 84Kr/132Xe 105 103

84Kr/86Kr

101

132Xe/134Xe

10 –1 10 –3

84Kr

10 –5

86Kr

40 30

1

20

0

2

3

4

6

10 4

8

12

16

4

0

4

8

12 16 Etch steps

Fig. 2. Dependences of the elemental ratios of inert gases on the step etch depth for samples of lunar ilmenites (designations are the same as in Fig. 1).

132Xe

134Xe

10 –7

8

5

6 γ

Fig. 3. Dependences of the contents and ratios of cosmogenic isotopes 84Kr, 86Kr, 132Xe, and 134Xe, produced from the neighboring target nuclei, on the exponent γ of the power energy proton spectrum F(E) ~ E–γ (the excitation functions of krypton isotopes at Rb, Sr, Y, and Zr nuclei and the excitation functions of xenon isotopes at Cs, Ba, La, and Ce nuclei are taken from [12] and [13, 14], respectively; the data on the cosmic abundance of elements are taken from [7]).

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overestimation of the cosmogenic contribution to the 84Kr/132Xe ratio due to the use of the same isotopic composition of cosmogenic Xe in I71 and I79 in the estimation of this ratio in [2] should lead to low ratios of the captured components of these gases in ilmenites I79 in Fig. 2. Note that no diffusion mechanism can be responsible for the observed difference in the elemental ratios of heavy gases in lunar ilmenites of different age, because such an effect should manifest itself even to greater extent in the case of light gases. However, the decrease in the 84Kr/132Xe ratios in the old breccia can be caused by additive Xe impurities, product of fission of transuranium elements (primarily 238U and 244Pu). They are present in different amounts mainly in crystalline lunar rocks and absent in the lunar soil, which was formed after decay of these elements; they are also characteristic of lunar breccias, into which Xe could be introduced as a result of collision from a reservoir in lunar rocks with previously accumulated fission Xe [15]. The possible presence of fission Xe impurity in breccia ilmenites I79 leads to a decrease in the calculated charge states of ions; hence, the values of QI79 listed in Table 3 should be considered only as upper limits. REFERENCES 1. Benkert, J.-P., Baur, H., Signer, P., and Wieler, R., J. Geophys. Res. E, 1993, vol. 98, no. 7, p. 13147.

2. Wieler, R. and Baur, H., Meteoritics, 1994, vol. 29, no. 5, p. 570. 3. Meyer, J.-P., Astrophys J. Suppl. Ser., 1985, vol. 57, no. 1, pp. 151, 173. 4. Eichler, D. and Hainebach, K., Phys. Rev. Lett., 1981, vol. 47, no. 21, p. 1560. 5. Spravochnik “Fizicheskie velichiny” (Handbook on Physical Quantities), Grigor’ev, I.S. and Meilikov, E.Z., Eds., Moscow: Energoatomizdat, 1991, pp. 375; 414. 6. Geiss, J. and Bochsler, P., Rapports Isotopiques Dans Le Systeme Solaire, Paris: CNES, 1985, p. 213. 7. Anders, E. and Grevesse, N., Geochim. Cosmochim. Acta, 1989, vol. 53, no. 14, p. 197. 8. Geiss, J., Abstracts of Papers, ESA Worksh. Sol. Heliosph. Sp. Plasm. Phys., ESA SP-235, 1985, p. 37. 9. Arnaud, M. and Rothenflug, R., Astron. Astrophys. Suppl. Ser., 1985, vol. 60, no. 3, p. 425. 10. Labrador, A.W., Leske, R.A., Mewaldt, R.A., et al., Abstracts of Papers, 27th ICRC, Hamburg, 2001, p. 3149. 11. Ustinova, G.K., Izv. Akad. Nauk, Ser. Fiz., 2003, vol. 67, no. 4, p. 443. 12. Regnier, S., Lavielle, B., Simonoff, M., and Simonoff, G.N., Phys. Rev. C: Nucl. Phys., 1982, vol. 26, no. 3, p. 931. 13. Kaiser, W.A., Philos. Trans. R. Soc. London, Ser. A, 1977, vol. 285, no. 1327, p. 337. 14. Ustinova, G.K., Lunar and Planetary Science 36th, Houston: LPI, 2005, p. 36. CD#1021. 15. Pepin, R.O., Becker, R.H., and Rider, P.E., Geochim. Cosmochim. Acta, 1995, vol. 59, no. 23, p. 4997.

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