Origins and Implications of the Apollo 16 Breccia Noble Gas Suite

<p>The Apollo 16 landing site is dominated by regolith breccias; consolidated regolith palaeo-soils [5,7,8]. Each regolith soil (and, by extension, each regolith breccia) is composed of fragments of rock sourced from different impacts and lithological units [e.g. 2,3]. Because of this, these samples probe the impact history of the lunar surface across a wide range of time. McKay et al. (1986) reported the trapped argon isotope ratios (i.e., <sup>40</sup>Ar/<sup>36</sup>Ar<sub>Tr</sub>) values of regolith breccias and used these values to semi-quantitatively model breccia formation ages [see also 4]. Two groups of regolith breccias were identified at the Apollo 16 landing site: (i) the ‘ancient’ group, lithified immature regolith (i.e., <30 I<sub>s</sub>/FeO), and (ii) a ‘younger’ group that generally have higher levels of maturity. Joy et al. (2011) used the <sup>40</sup>Ar/<sup>36</sup>Ar<sub>Tr</sub> ratios to model that: (i) the ancient samples closed from soils to breccias between ~3.8 and 3.4 Ga, consistent with regolith developed and consolidated after the Imbrium basin-forming event, and during a time of declining basin-forming impacts, and (ii) that the young breccias were assembled in the Eratosthenian period between ~2.5 and 1.7 Ga, providing insight into post-basin bombardment impact processes.</p><p>A third set of regolith breccias identified by Jerde et al. (1987, 1990), (the soil-like breccias), have no reported noble gas or exposure age information. Joy et al. (2011) inferred that these samples were likely consolidated into breccias in the last 2 Ga (based on their I<sub>s</sub>/FeO maturity being similar to the Apollo 16 soils). They, therefore, may extend the current archive of impact and regolith processes into the Eratosthenian and Copernican periods.</p><p>Whole-rock samples were laser step heated and the extracted gases were measured using a Thermo Scientific Helix-MC noble gas magnetic sector mass spectrometer. Preliminary analysis of our data shows these breccias are dominated by a solar wind composition component, with minor spallation and radiogenic contributions. The concentrations of evolved gases suggest the samples are more similar in terms of noble gas budget to the present day Apollo 16 soil samples (based on analysis using data collated by Curran et al. 2020), than the ancient gas-poor Apollo 16 regolith breccias (McKay et al. 1986). Thus, these noble gas data are consistent with the petrological characterisation and Is/FeO classification [5,6] of these breccias being comparable to present day Apollo 16 soil samples. Solar wind composition gas concentrations comparable to present day soil samples suggest these new breccias represent consolidated regolith of comparable maturity, perhaps suggesting these soil-like breccias were formed around the same time period as the ‘younger’ group.</p><p>References: [1] Curran, N.M., et al., 2020, PSS, 182, 104823. [2] Donohue, P.H., et al., 2013, 44<sup>th</sup> LPSC, A#2897.; [3] Fagan, A.L., et al., 2013, GCA, 106, 429-445.; [4] Fagan, A.L., et al., 2014, Earth Moon Planets, 112, 59–71.; [5] Jerde, E.A., et al., 1987, J. Geophys. Res., 92(B4), E526– E536.; [6] Jerde, E.A., et al., 1990, EPSL, 98(1), 90-108.; [7] Joy, K.H., et al., 2011, GCA, 75(22), 7208-7225.; [8] McKay, D.S., et al., (1986), J. Geophys. Res., 91(B4), 277– 303.</p>

Directly comparing coronal and solar wind elemental fractionation

Context. As the solar wind propagates through the heliosphere, dynamical processes irreversibly erase the signatures of the near–Sun heating and acceleration processes. The elemental fractionation of the solar wind should not change during transit, however, making it an ideal tracer of these processes. Aims. We aim to verify directly if the solar wind elemental fractionation is reflective of the coronal source region fractionation, both within and across different solar wind source regions. Methods. A backmapping scheme was used to predict where solar wind measured by the Advanced Composition Explorer (ACE) originated in the corona. The coronal composition measured by the Hinode Extreme ultraviolet Imaging Spectrometer (EIS) at the source regions was then compared with the in situ solar wind composition. Results. On hourly timescales, there is no apparent correlation between coronal and solar wind composition. In contrast, the distribution of fractionation values within individual source regions is similar in both the corona and solar wind, but distributions between different sources have a significant overlap. Conclusions. The matching distributions directly verify that elemental composition is conserved as the plasma travels from the corona to the solar wind, further validating it as a tracer of heating and acceleration processes. The overlap of fractionation values between sources means it is not possible to identify solar wind source regions solely by comparing solar wind and coronal composition measurements, but a comparison can be used to verify consistency with predicted spacecraft-corona connections.