Solar flare ions have energies capable of inducing nuclear reactions; consequently, there has been much discussion in the astrophysical literature of the cumulative effect of solar flareinduced nuclear reactions on the isotopic composition of the solar surface (e.g. Lingenfelter and Hudson, 1980; Ramaty, 1986). It has proved difficult to evaluate this question theoretically. A strong case for the importance of surface nuclear reactions is based on studies of variations in 15N/14N in lunar soils (Kerridge, 1980). The most likely source of the N is the solar wind. Models attempting to explain this phenomenon by mixing solar wind nitrogen with another hypothetical source fall short of matching all of the aspects of N abundance and isotopic systematics (e.g., Kerridge et al., 1992). Interestingly, the N isotopic variations appear to be correlated with the epoch or antiquity of solar wind exposure. The sense of the variations is such that the solar wind 15N/14N ratio is required to increase with time, as would be expected if surface nuclear reactions were important. This interpretation has been challenged recently with the suggestion that isotopically light N was implanted with higher energy than solar wind, presumably by solar flares (Kerridge, 1992). Kerridge's new interpretation still leaves a number of questions unanswered, so the jury is still out. However, this view makes the present-day solar wind 15N/14N ratio more ambiguous than previously (also cf. Kim et al., 1992). Direct measurement of 15N/14N in the solar wind would be an important constraint on these interpretations. In addition, two different laboratories have reported the presence of 14C on the surfaces of lunar materials, apparently of solar wind origin (see e.g Fireman, 1980).
Neither the 15N or 14C observations are easily reconciled with present estimates of solar flare activity. Further, the required particle fluences to produce the 15N variations should have produced a photospheric B abundance much larger than is observed (Kerridge et al., 1977). There is no obvious lunar explanation for the N observations, although the solar problems created have caused some authors (e.g. Geiss and Bochsler, 1982) to conclude that there must be lunar processes involved. Measurement of key isotopic abundances, e.g. D, 6Li, 10B/11B, 15N, 19F, 50V, 138La, etc. in a returned solar wind sample could resolve these issues. All these nuclei have low natural abundances, but are relatively highyield reaction products.
14C and 10Be in the Solar Wind
This section provides documentation for science objective 18, from
Table 1-2 of the proposal. Begemann et al.
(1972) and Fireman et al. (1977) measured excess 14C on the
surfaces of lunar rocks which they both ascribed to the solar wind,
with 14C/H~10-12. More
recent estimates by Jull et al., ~10-13(1994)
and </= 3.5 x 10-14(1995) are lower due to higher
estimates of in situ production by solar cosmic rays. For a 2 year
exposure the flux estimates correspond to 1800 (1994) and </= 600
14C/cm2 (1995). Modern
accelerator mass spectrometry (AMS) can routinely measure
106/cm2,
requiring about 600 and 1700 cm2 for a
measurement based on the most recent estimates. In the lid of the sample
reentry capsule (SRC) about 5000 cm2 is set aside for
collection of radioactive nuclei.
No information is available on the solar wind 10Be flux but solar surface production rates are likely of the same order of magnitude as 14C, and AMS sensitivities for 10Be are 3 times better than for 14C. It is reasonable to attempt a 10Be measurement on about the same amount of collector material as for 14C.
Micrometeorite background is a potential problem as it may not be possible to completely protect materials in the SRC lid from secondary micrometeorite impact ejecta unlike the collector arrays. The actual amount of SRC lid area available will be better defined as the SRC design matures. The potential effects of secondary ejecta can also be evaluated. It is likely that sufficient area with low solid angle for ejecta can be identified.
In Phase B, suitable lid radioactive collector materials will be identified. Allowance must be made for production by galactic cosmic ray (GCR) and solar flare (SCR) nuclear reactions. There are several possibilities: (a) use a thin foil of high Z material such as Au for which GCR production rates are low and SCR rates 0. For a 10 mg/cm2 Au foil (about 1/4 mil) the apparent fluences due to GCR reactions would be about 10/cm2, negligible. A thick Au foil supporting the thinner one would provide a check on GCR background. There are disadvantages to Au in that it would probably reach a temperature of about 250oC which may be too hot for any spacecraft component in the SRC lid and that solar wind implantation is only about 40-50% efficient in Au. (b) use Si metal for which SCR background is negligible, accept higher GCR background, but exploit the ability to do controlled etching and only analyze the outer 200 nm for radioactive nuclei. Si has the advantage of lower temperatures and high implantation efficiency.
AMS measurements are anticipated to be made at
Lawrence Livermore National Laboratory.