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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

The Moon's time-variable exosphere

Earth's Moon does not have a conventional gaseous atmosphere, but instead an “exosphere” of particles ejected from the surface. Colaprete et al. have used NASA's LADEE orbiter to investigate how the exosphere varies over time, by using the glow from sodium and potassium atoms as a probe (see the Perspective by Dukes and Hurley). The exosphere composition varies by a factor of 2 to 3 over the course of a month, as different parts of the Moon are exposed to sunlight. There are also increases shortly after the Moon passes through streams of meteoroids.

Science, this issue p. 249; see also p. 230

Abstract

Despite being trace constituents of the lunar exosphere, sodium and potassium are the most readily observed species due to their bright line emission. Measurements of these species by the Ultraviolet and Visible Spectrometer (UVS) on the Lunar Atmosphere and Dust Environment Explorer (LADEE) have revealed unambiguous temporal and spatial variations indicative of a strong role for meteoroid bombardment and surface composition in determining the composition and local time dependence of the Moon’s exosphere. Observations show distinct lunar day (monthly) cycles for both species as well as an annual cycle for sodium. The first continuous measurements for potassium show a more repeatable variation across lunations and an enhancement over KREEP (Potassium Rare Earth Elements and Phosphorus) surface regions, revealing a strong dependence on surface composition.

The Lunar Atmosphere and Dust Environment Explorer (LADEE) mission, which operated in lunar orbit between 6 October 2013 and 18 April 2014, had the goal to determine the composition of the lunar atmosphere and investigate the processes that control its distribution and variability, including sources, sinks, and surface interactions (1). The Ultraviolet and Visible Spectrometer (UVS) (2) was designed to make observations of the lunar exosphere and search for dust. The LADEE orbit was retrograde and equatorial; thus, observations were restricted to between about ±20° latitude. However, this orbit did provide synoptic observations of lunar exospheric species with temporal cadence of typically less than 12 hours. These observations provide new constraints on the processes, sources, and sinks that govern concentrations of species in the lunar exosphere. UVS observed resonant scattering from sodium (Na) and potassium (K). Although Na and K are minor constituents in the lunar exosphere, the brightness of their emission lines makes them readily observable, and thus they serve as excellent proxies to understanding the processes that govern the composition of the lunar exosphere (36).

Potential sources of Na and K in the exosphere include photon-stimulated desorption (PSD), sputtering, and impact vaporization (Fig. 1). The role of PSD and sputtering have been estimated by evaluating the change in Na abundances as the surface is shielded from solar wind sputtering as the Moon passed through Earth’s magnetotail (7). From these previous Earth-based observations and modeling, PSD was generally considered to be the dominant source, by a factor of ~10 to 100, over sputtering or impact vaporization (6, 7). However, the question of whether photons simply re-excite material that has been previously vaporized by other processes or whether they act as a primary source process of atoms from glasses and minerals has not been previously answered. The only other space-based observations of Na were made by the Telescope for Visible Light (TVIS) instrument on board the Kaguya spacecraft (8). Kaguya TVIS observations were limited to the lunar nightside, looking in the antisolar direction toward the extended Na tail; thus, they preferentially observed the “hottest” portion of the Na population and were not continuous across the entire lunation (9). These observations showed a continuous decrease of the inferred Na surface density from first to third quarter, including phases when the Moon passes through Earth's magnetosphere, with a variation of about 50% across a lunation.

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).

Observations of K from Earth are more difficult than for Na because a strong telluric O2 band overwhelms the stronger of the two K lines, whereas the smaller scale height for K results in relatively small concentrations at altitudes where Earth-based observations can be made. Besides its initial discovery (3), only two other K observations have been published (10, 11), and these were limited to observations on single days. There are therefore no observations of K over the course of a lunation.

Earth-based observations have detected increases in the overall concentration of Na in the lunar exosphere associated with some meteoroid streams (1216). However, these observations were intermittent and limited in time, often focusing on only a few measurements about an individual stream; thus, Na variability associated with meteoroid streams has been inconclusive (17). Annual variability of lunar-sodium tail brightness has been found, with a peak in the Na tail brightness in February, but no explanation for the annual variations has been provided (9, 17). The UVS observations allow for continuous monitoring of the exosphere, both during and outside intervals of increased stream activity, through the course of more than five lunations, which allows for a much less ambiguous measure of immediate Na activity associated with meteoroid streams. In addition to these Na observations, these observations are the first to assess the response of K to meteoroid stream activity.

The emission line strengths for both Na and K were derived from UVS limb observations at around local noon. These noon limb-viewing activities typically covered spacecraft solar longitudes between –25° and +55° with a telescope viewing altitude of around 40 km (2, 18).

The derived tangent column densities of Na and K show variations by a factor of 2 to 3 over the course of a lunation (Fig. 2). The minimum-to-maximum ratio of Na column density changes continuously during the five lunations measured by LADEE, but the factor of 2 for Na seen on the last month is significantly different from the ~50% seen in Kaguya observations (9). A factor of two change in the lunar Na content was inferred from modeling ground-based observations taken before UVS observations (7) and attributed to the solar wind increasing the PSD rate via ion-enhanced diffusion inside grains (18). However, the model in (18) predicted that the exosphere rose continuously between exit and subsequent reentry to the magnetosphere. The Kaguya TVIS data showed a continuous decline in Na through the magnetotail, whereas LADEE UVS data show Na peaks near full Moon. These differences are perhaps a difference in vantage points as suggested to reconcile the Kaguya results with the analysis of ground-based observations (9).

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).

As the Moon passes into Earth’s magnetotail, the total Na is seen by UVS first to decrease and then increase through full Moon to a maximum about 30° of lunar phase, after which it begins to decrease again. Although detailed modeling is required to understand the cause of these trends, some of this variation could be the result of adsorbed species being released by solar wind sputtering as particles spend more time on the surface than on ballistic trajectories. The absence of sputtering in the magnetotail should allow for the Na surface reservoir to increase, perhaps explaining the increase in exospheric concentration several days after the full Moon; then, as the Moon comes out of the magnetotail, sputtering begins to release adsorbed particles again and exospheric Na decreases as it is lost to space. This paradigm suggests then that most of the Na particles do not get lost to the surface on their first bounce, unlike hypothesized in recent models based on ground-based observations (7, 15). It was suggested that some of the daily variation observed by Kaguya could be explained if there were surficial enhancements of Na in selenographic longitudes around 90°± 90°, resulting in differences in PSD rates between near-side and far-side regolith (9). Consistent with this idea of a surface dependency, the new LADEE UVS data are suggestive of a different dependence in PSD between mare versus highlands soils (Fig. 3).

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)].

Potassium shows a much more regular trend over time, with intensity variations over a lunation of approximately a factor of 2 (Fig. 2). This more systematic monthly trend for K (compared to Na) is reminiscent of the extreme variation of K surface abundance due to the enrichment found in KREEP (Potassium Rare Earth Elements and Phosphorus) soils (19), which can be as much as a factor of 10. Any variation introduced by the solar wind appears to be masked by the strong variation of surface K. In addition to the much larger variation in K over the course of a lunation, the K dependence on surface composition is more clearly shown by the location of the peak tangent column densities (Fig. 3). Given the anticipated influence of the magnetotail on sputtering (7, 18), a minimum in column density for both Na and K could be expected to be centered at around a longitude of 0°, with maxima to either side. However, the K peak occurs to the west, centered on the Oceanus Procellarum and the Mare Imbrium regions, areas of maximum surface K as measured by the Lunar Prospector mission (20). Sodium is more symmetric about 0° longitude, with a local minimum at 0°. There is some correlation of Na with surface albedo (Fig. 3), which suggests a possible dependence on composition (e.g., Mare or Highlands composition); however, albedo is also a function of surface maturity and roughness, and these factors are expected to influence how adsorbates bind on grains. Additionally, the structure around 0° longitude could also be a reaction to being inside the magnetotail.

Some individual meteoroid streams led to a significant increase in these exospheric species. Three major streams that occurred during the LADEE mission are indicated in Fig. 2. In the Na data set, although there are peaks in the Na column abundance, it is not obvious that these are the result of the streams (as opposed to whatever is causing the monthly variation). Indeed the noon-time immediate response to streams of an exosphere populated by previously adsorbed atoms is expected to be rather small, because most streams have radiants on the lunar nightside (21, 22), with larger variations in response to the streams expected at around dawn. However, in the K noontime data there is a clear increase at around the time of the Geminid (Gem) meteoroid stream (approximately between 4 and 16 December) and also smaller signatures at around the time of the Leonid (Leo) (6 to 30 November) and Quadrantid (Qua) (1 to 10 January) streams. In addition to these enhancements at or around individual streams, the overall rise in Na from October 2013 to a peak in December 2013, and then a gradual decline until the end of the mission, can be attributed to a cumulative response to meteoroid streams. A similar trend was apparent in Kaguya data (9), but, as with earlier studies (17), it could not be correlated to any particular process. The UVS data suggest a strong link between these streams and the exosphere, lasting far beyond the initial encounter. The meteoroid impacts expose fresh Na and K, but also supply these species directly to the exosphere at increased rates.

The large instantaneous responses of the dayside to meteor showers are indirect evidence of the role of adsorbed particles. If all particles were lost to the surface in one bounce, it would take more than an order of magnitude enhancement of impact vaporization during showers over sporadic impacts for this source to reach the rate of 2 × 106 Na atoms cm−2 s−1 required to explain the Na atmosphere (7). Hence, vapor introduced by meteoroid impacts can reside for considerable periods on the surface, adding to the overall adsorbed Na and K surface reservoir. We performed Monte Carlo simulations to estimate the total residence time for Na and K in the exosphere and soil. In the simulations, a one-second injection of Na or K was assumed to occur from the nightside hemisphere, with the duration between hops and loss rates for sputtering and photoionization taken from published values (see the supplementary materials). The simulations show that a release of ejecta from a single injection will persist in the exosphere-surface system for much longer than the ionization lifetime would suggest (Fig. 4). This is the result of each particle residing in the soil for approximately an ionization lifetime (i.e., several days) between bounces, combined with the many bounces that it has to take before being lost from the exosphere. Residence times in the lunar environment of 45 to 90 days (mainly on the lunar surface) can be expected before escape to the solar wind, which would explain the long-term smooth increase and decrease in the Na column density observed as the result of meteoroid streams (Fig. 2). Shorter times are predicted if some fraction (but smaller than 50%) of recycled particles are trapped permanently between bounces.

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).

These observations provide constraints on the sources and sinks of Na and K in the lunar exosphere. The nearly continuous, synoptic nature of the observations illustrates the contribution of impacts and surface composition. The independent confirmation by Kaguya and LADEE of the lunar Na trend between November and April provides the strongest evidence yet for an annual variation of the Na exosphere. This trend is likely the cumulative response of Na to meteoroid streams, whose annual activity peaks from November through January and then subsides until the summer. The substantial residence time for Na at the surface suggested by this interpretation inevitably leads to the conclusion that Na migrates toward the poles like other volatiles (e.g., water) in these cycles of adsorption and redesorption. The K measurements show a strong but, contrary to Na, short-lived response to the Geminids meteoroid shower. Outside of the meteoroid streams, K shows a regular variation across a lunation that correlates strongly with the abundance of potassium in the lunar bulk soil. Combined, these results and recent studies of the Mercurian exosphere (23, 24) indicate a pronounced role for meteoroid impact vaporization and surface exchange in determining the composition of surface-bounded exospheres. However, the details of how the exosphere depends on surface composition and responds to meteoroid streams are not yet understood.

Supplementary Materials

www.sciencemag.org/content/351/6270/249/suppl/DC1

Materials and Methods

Supplementary Text

Figs. S1 to S5

References (2534)

References and Notes

  1. Materials and methods are available as supplementary materials on Science Online
Acknowledgments: We thank R. Killen for constructive discussions and the three reviewers who helped to greatly improve this paper. LADEE UVS was supported through the NASA Lunar Quest Program. Additional funding for M.S. was through NASA grants NNX14AG14A, NNX13AP94G, and NNX13AO74G. All LADEE UVS data are available online at the NASA Planetary Data System (PDS), including all raw and calibrated spectra and derived sodium and potassium line strengths.
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