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How surface composition and meteoroid impacts mediate sodium and potassium in the lunar exosphere

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Science  15 Jan 2016:
Vol. 351, Issue 6270, pp. 249-252
DOI: 10.1126/science.aad2380
  • Fig. 1 The primary sources and sinks of the lunar Na and K exosphere.

    The yellow region represents the sodium exosphere having a greater extent on the dayside due to higher sodium temperatures. The sources and sinks include photon-stimulated desorption (from solar ultraviolet), sputtering (from solar wind protons) and meteoroid impact vaporization (e.g., the Geminid meteoroid stream). These processes are affected as the Moon passes through Earth’s magnetosphere (green region) and sheath (red region).

  • Fig. 2 The total line-of-site column densities for sodium and potassium.

    (A and B) Sodium. (C) Potassium. The column densities were derived from data taken while the telescope grazing point was between a spacecraft solar longitude of 165° to 180° (near local noon). In (A), the last three observed lunations are shown with green-shaded regions indicating when the Moon was in Earth’s magnetotail. (B) and (C) show the Na and K column densities, respectively, for the entire mission period. In (B), a fit to the minima in column concentration (solid blue curve) is shown to highlight the long-term Na trend. Also shown in (B) and (C) are the approximate beginning and end dates (gray-shaded regions) for three meteoroid streams (Leo, Gem, and Qua) and the observed peak (blue dashed lines) and the approximate dates of the full Moon (red arrows). Average absolute and relative (point-to-point) uncertainties are shown by the red points toward the upper right corner of (B) and (C).

  • Fig. 3 Column densities for sodium and potassium as a function of selenographic longitude.

    (A) Sodium. (B) Potassium. The approximate entry into (blue squares) and exit out of (red points) Earth’s magnetotail are shown. Data was acquired at about solar noon. Data acquired during the Geminids stream are indicated with an X or a +. Also shown is the scaled surface albedo averaged between ­–15° and –22° latitude [green line in (A)] and in the relative concentration of K at the latitude of –20° in the lunar soil as a function of selenographic longitude as derived from Lunar Prospector observations [green line in (B)].

  • Fig. 4 The lifetimes of sodium released by meteoroid impacts, as estimated with a Monte Carlo method.

    In these simulations, the effect of a meteoroid stream is modeled by a sudden release of Na test particles, which are tracked until loss by photoionization, sputtering, or deposition into permanently shadowed regions (18). The fraction of atoms lost on the first bounce much exceeds gravitational escape, reflecting losses to sputtering after particles recycle for the first time (A). Prolonged residence of released Na on surficial grains between bounces would continue to affect the exosphere well after the meteoroid stream encounter, with an exponential time decay as long as 90 days (B).

Supplementary Materials

  • How surface composition and meteoroid impacts mediate sodium and potassium in the lunar exosphere

    A. Colaprete, M. Sarantos, D. H. Wooden, T. J. Stubbs, A. M. Cook, M. Shirley

    Materials/Methods, Supplementary Text, Tables, Figures, and/or References

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    • Materials and Methods
    • Supplementary Text
    • Figs. S1 to S5
    • References

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