Lunar Backscatter and Neutralization of the Solar Wind: First Observations of Neutral Atoms from the Moon D

GEOPHYSICAL RESEARCH LETTERS, VOL. 36, L12104, doi:10.1029/2009GL038794, 2009 Click Here for Full Article

Lunar backscatter and neutralization of the solar wind: First observations of neutral atoms from the Moon D. J. McComas,1 F. Allegrini,1 P. Bochsler,2 P. Frisch,3 H. O. Funsten,4 M. Gruntman,5 P. H. Janzen,6 H. Kucharek,2 E. Mo¨bius,2 D. B. Reisenfeld,6 and N. A. Schwadron7 Received 20 April 2009; accepted 22 May 2009; published 18 June 2009.

[1] The solar wind continuously flows out from the Sun, with the geocorona [Schmitt et al., 1991; Wargelin et al., filling interplanetary space and impinging directly on the 2004]. The Moon emits gamma rays and neutrons when lunar regolith. While most solar wind ions are implanted galactic GeV cosmic ray protons and alphas interact with into the lunar dust, a significant fraction is expected to scatter the surface [Thompson et al., 1997; Feldman et al., 1998]. back and be emitted as energetic neutral atoms (ENAs). The Moon is also brighter than the Sun at gamma-ray However, this population has never been observed, let alone energies of 100–300 MeV,with fluxes ranging from 1–5Â characterized. Here we show the first observations of 10À7 cmÀ2sÀ1 [Orlando and Strong, 2008]. backscattered neutral atoms from the Moon and determine [3] Solar wind ions, which are 96% H and 4% He with that the efficiency for this process, the lunar ENA albedo, is only traces of heavy species, have a typical speed of 10%. This indicates that the Moon emits 150 metric 400 km sÀ1 (1 keV/nucleon) and impinge continuously tons of hydrogen per year. Our observations are important on the sunward side of the Moon. These ions weather the for understanding the universal processes of backscattering lunar surface, which is also ‘‘gardened’’ by micrometeorites. and neutralization from complex surfaces, which occur In the geomagnetic tail, plasma sheet ions are incident on the wherever space plasmas interact with dust and other small Moon isotropically at mean energies of 4keV. The solar bodies throughout our solar system as well as in wind sputters neutral atoms out of the regolith with energies exoplanetary systems throughout the galaxy and beyond. of several eV. This sputtering erodes surface grains at a rate Citation: McComas, D. J., et al. (2009), Lunar backscatter and of 0.01–0.04 nm/yr [e.g., Wehner et al., 1963; Johnson and neutralization of the solar wind: First observations of neutral Baragiola, 1991], however, the porous nature of the rego- atoms from the Moon, Geophys. Res. Lett., 36, L12104, lith may trap >90% of forward-directed sputtered species doi:10.1029/2009GL038794. [Hapke, 1986]. Ion sensors onboard the SELENE spacecraft in lunar orbit recently detected backscattered solar wind 1. Introduction particles and found that 0.1–1% of the solar wind protons bombarding the lunar surface backscatter from the Moon as [2] The Moon emits a broad array of scattered or sec- ions [Saito et al., 2008]. During the backscatter process, ondary radiation and particles. The geometric albedo (GA) however, incident solar wind ions are mostly neutralized, of the Moon depends on the surface composition and ranges and the vast majority should exit as ENAs with a broad from 11.3% for visible light to 2–8% in the interval distribution of energies up to the solar wind energy. These 60–170 nm. (The geometric albedo is the ratio of the ENAs then travel along ballistic trajectories, unimpeded by surface brightness at zero phase angle (i.e., reflected back magnetic fields from the lunar surface, Earth’s magnetotail, toward the incident radiation source) to an idealized fully or interplanetary space, making them observable at signif- reflective (and diffusely scattering) surface of the same icant distances from the Moon. Finally, solar wind passing geometrical cross-section.) The latter contribution is pro- downstream along the terminators of the Moon can be duced by the fluorescence of surface minerals [Flynn et al., neutralized by passing through the lunar exosphere 1998]. The total lunar GA drops to 0.014% at 5 nm in the [Futaana et al., 2008] or very small levitated dust grains soft X-ray region [Flynn et al., 1998]. Images of the Moon [Wimmer-Schweingruber and Bochsler, 2003]. Pickup ions by ROSAT and Chandra showed that the illuminated side from this lunar exosphere were observed with the AMPTE glows in X-rays from 0.5–7 keV, while the dark side spectra SULEICA sensor in the mass ranges around O and Si show charge exchange products from solar wind interactions [Hilchenbach et al., 1993]. However, to date, no neutral atoms from any of these lunar sources have been directly 1Southwest Research Institute, San Antonio, Texas, USA. observed. 2Space Science Center, University of New Hampshire, Durham, New Hampshire, USA. 3Department of Astronomy and Astrophysics, University of Chicago, 2. Observations Chicago, Illinois, USA. 4Los Alamos National Laboratory, Los Alamos, New Mexico, USA. [4] NASA’s Interstellar Boundary Explorer (IBEX) mis- 5Astronautics and Space Technology Division, University of Southern sion was launched on October 19, 2008 and is currently in a California, Los Angeles, California, USA. 5 6 very high altitude (3 Â 10 km apogee) Earth orbit, Department of Physics and Astronomy, University of Montana, making the first all-sky maps of ENAs produced by the Missoula, Montana, USA. 7Department of Astronomy, Boston University, Boston, Massachusetts, interaction of the heliosphere with the local interstellar USA. medium at heliocentric distances of 100 AU [McComas et al., 2004, 2009]. On December 3, 2008, early in the first Copyright 2009 by the American Geophysical Union. orbit after IBEX’s higher energy sensor (IBEX-Hi) [Funsten 0094-8276/09/2009GL038794$05.00

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1.4 keV) energy channels as the solar wind protons reached energies of 1 keV. Thus, the lunar interaction produces backscattered neutrals with a broad energy spectrum from the incident solar wind energy downward throughout the range of energies measurable by IBEX. [6] We simulated the interaction of solar wind protons with the lunar regolith utilizing the Stopping and Range of Ions in Matter (SRIM) code [Ziegler et al., 2008], which has been used extensively for modeling ion-solid interactions and is ideally suited to simulate amorphous-rimmed grains characteristic of the regolith [Keller and McKay, 1993]. We assumed a regolith composition 80% by mass, typical for highlands, and 20% for maria based on the IBEX viewing geometry. Similar calculations assuming significant compositional variations, including the addition of 10% (by number) implanted H, provided similar results. The SRIM simulations indicate that, with the exception of near grazing collisions, the backscatter probability for ENAs within the IBEX-Hi energy range decreases nearly linearly with increasing energy up to the solar wind energy. [7] Using this result, we convolved the IBEX-Hi angular and energy response functions with a modeled ENA energy spectrum, assuming for each measurement an ENA spectrum had a cut off at the incident solar wind energy and increased linearly with decreasing energy. We also convolved the detailed shapes of the IBEX-Hi energy pass bands, which Figure 1. (top) First detection of lunar ENAs and (bottom) included, for example, integrating the lowest energy channel geometry for the IBEX observations (Earth, Moon, and down to its 1% response level at 238 eV. In this process, the spacecraft not to scale). IBEX spins at roughly 4 rpm with slopes of the backscattered distributions as a function of its 7° FWHM field of view sweeping over the Moon each spin for 10 hours on December 3, 2008. The ENAs are summed in 6° bins with the lunar direction indicated by the white arrow. The light blue horizontal line shows the lunar ENAs. The wider range of pixels at 1600 is an instru- mental effect that we did not remove from this raw data plot. The color bar indicates triple coincidence counts from the lowest two ESA steps (0.38–0.95 keV FWHM) summed over 96 spins. IBEX detects ENAs produced by backscattering and neutralization of the incident solar wind protons at an angle of 98° from the initial solar wind velocity vector. et al., 2009] was turned on, the Moon crossed through its field of view (FOV), as shown in Figure 1. The pointing of IBEX’s spin axis results in observations of solar wind ions backscattered as ENAs into a narrow angular range around 98° relative to the solar wind velocity vector. Even though the Moon was 2 Â 105 km away, IBEX’s exceptional sensitivity allowed this first detection of lunar ENAs. Figure 2. ENAs detected by IBEX-Hi and associated solar [5] Figure 2 shows count rates of lunar ENAs from wind conditions during the passage of the Moon across the IBEX-Hi; the requirement of ‘‘triple coincidence’’ reliably IBEX-Hi field of view. (top) The ENAs are triple coinci- suppresses noise [Funsten et al., 2009]. The color coded dence measurements with a linearly interpolated background curves represent the four lowest energy pass bands of the subtraction and a running average smoothing over each eight instrument, spanning 0.38–2.5 keV FWHM. For the first consecutive data points. (middle) The solar wind proton third of the 10 hours of lunar viewing, the solar wind flux and (bottom) speed (equivalently proton energy on speed was low and the flux decreased significantly. During right scale) are from the ACE spacecraft 1.2 Â 106 km this interval most of the ENAs were detected in the lowest, upstream from the Moon. Using the measured solar wind and least sensitive, energy pass band (0.38–0.59 keV). Over speed, the ACE-Moon distance, the Moon-IBEX distance the remainder of the lunar viewing, the solar wind speed and a typical ENA energy of 500 eV gives the 84 minute (energy of incident protons) increased, producing enhanced and 63 minute delays for the start and end of the event, fluxes in the second (0.52–0.95 keV) and then third (0.84– respectively, as shown.

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solar wind ions on various solids [e.g., Bu¨hler et al., 1966], which are typically 80 to 90% [e.g., Grimberg et al., 2009]. We stress that these experimental results represent efficiencies for He at solar wind energies interacting with solids. Hydrogen is lighter, and presumably, has a somewhat lower trapping efficiency because it interacts more strongly with heavier ions in the regolith. Thus, an ENA albedo of 10% or even higher should not be surprising. Furthermore, our SRIM simulations over all the surface orientations relative to the solar wind indicate a total backscatter yield of 30%. Comparing this with the total ENA albedo of 10% yields the escape probability of a backscattered ENA from the lunar surface of 30%, which is consistent with simulation results of the escape of sputtered particles from a regolith [Cassidy and Johnson, 2005].

3. Conclusion Figure 3. Observed ENA flux as a function of solar wind [10] In conclusion, we report here the first observations flux. The measurements show ENA observations from of ENAs from the Moon. We show that their energy Figure 2 where the solar wind was propagated individually spectrum and total flux follow the variations in the incident for each sample. Error bars represent statistical counting solar wind energy and flux. We also derive an ENA neutral uncertainty only; additional errors associated with IBEX-Hi’s hydrogen albedo for the Moon of 10%, for the first time. angular and energy responses are largest for the lowest solar This measured ENA albedo results in a continuous emission of hydrogen atoms from the Moon at a rate of 3 Â 1024 sÀ1 wind energies and times when the Moon was at the edge of 5 À1 the FOV, which correspond to the right side of this figure (1.5 Â 10 kg yr ). The implications of these observations where the solar wind flux >3.5 Â 108 cmÀ2 sÀ1. The solar are far reaching. In addition to quantifying the production of wind ions produce lunar ENAs through backscatter and neutrals from the interaction of the solar wind with a solid neutralization on the lunar surface. Multiplying the measured body such as the Moon, these results can be applied to the slope of 0.005 srÀ1 by 2p and accounting for ENAs below solar wind interaction with other objects throughout the solar IBEX-Hi’s energy threshold leads to a total lunar ENA system from tiny dust grains to asteroids, Kuiper belt objects, albedo of 10%. the moons of Mars, and when the solar wind pressure is high enough, even Mercury, which appears to have a surface similar to the lunar highlands [Taylor et al., 2006]. These energy derived from SRIM dropped out of the calculation neutrals augment the neutral hydrogen geocorona and, even- making these results highly model independent. tually, will again become ionized and swept out of the [8] The measured ENA flux observed at IBEX (scaled heliosphere with the solar wind. We have thus identified back to the lunar surface with the distance squared) is another source of heliospheric pickup ions. Plasma-surface shown in Figure 3 as a function of incident solar wind flux. interactions are also important in a broad range of astrophys- Clearly, the measured flux is well correlated with the ical contexts. About 16% of cool stars have debris disks that incident solar wind, as expected for the solar wind being signal the presence of exoplanet systems [Trilling et al., the source population for the ENAs observed by IBEX. 2008], while up to 28% of known giant gas exoplanets may The slope of this curve is the ENA albedo as viewed by have moons, with some of them located in regions of intense IBEX at its unique backscatter angle of 98°. This albedo stellar wind [Scharf, 2006]. The formation of lunar ENAs is accounts for the probabilities of proton backscatter, neutral- directly relevant to plasma-surface interactions on these solid ization, and escape from the rough lunar surface, integrated surfaces as well as in interactions with the gaseous/dusty over the approximately quarter of the Moon that is simulta- proto-stellar nebulae that surround accreting stars during neously hit by the solar wind and viewed by IBEX. We find a their T-Tauri phases. The results from this study thus provide best fit slope of 0.005 srÀ1. Assuming that this value is important clues to the evolution of dust and rocky moons in similar at other viewing angles, which seems reasonable exoplanet systems throughout and beyond our Galaxy. given the lunar surface roughness on various scales, the ENA albedo within the IBEX-Hi energy range emitted into [11] Acknowledgments. We gratefully acknowledge all of the con- 2p sr is 3.8%. SRIM simulations indicate that ENA fluxes tributions made by the outstanding men and women of the IBEX team who have been and are making this mission a tremendous success. This work below IBEX-Hi’s energy range may be one to two times the was funded by NASA as a part of the Explorer Program under contract number observed, leading to an estimate of the full ENA NNG05EC85C; parts of IBEX were also funded by the Swiss Prodex albedo of 10%. program. [9] This ENA albedo is consistent with the recent measurements of 0.1–1% backscatter efficiency of solar wind References protons [Saito et al., 2008], since the fraction that comes off Bu¨hler, F., J. Geiss, J. Meister, P. Eberhardt, J. C. Huneke, and P. Signer (1966), Trapping of the solar wind in solids: Part I. Trapping probability ionized is only expected to be a few percent of the back- of low energy He, Ne and Ar ions, Earth Planet. Sci. Lett., 1, 249–255. scattered neutral fraction. Our estimate is also consistent with Cassidy, T. A., and R. E. Johnson (2005), Monte Carlo model of sputtering experimentally determined trapping efficiencies for light and other ejection processes within a regolith, Icarus, 176, 499–507.

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