Meteoritic Influence on Sodium and Potassium Abundance in the Lunar

Geophysical Research Letters

RESEARCH LETTER Meteoritic inﬂuence on sodium and potassium abundance 10.1002/2016GL069541 in the lunar exosphere measured by LADEE

Key Points: 1 2,3 4 5 • Lunar exospheric potassium Jamey R. Szalay , Mihály Horányi , Anthony Colaprete , and Menelaos Sarantos abundance is significantly enhanced during the Geminids, lasting 4–8 days 1Southwest Research Institute, San Antonio, Texas, USA, 2Laboratory for Atmospheric and Space Physics, University of after peak meteoroid flux Colorado Boulder, Boulder, Colorado, USA, 3Department of Physics, University of Colorado Boulder, Boulder, Colorado, • Sodium and potassium undergo 4 5 different evolutions in the lunar USA, Division of Space Sciences and Astrobiology, NASA Ames Research Center, Moffett Field, California, USA, NASA exosphere Goddard Space Flight Center, Greenbelt, Maryland, USA • Weak correlation is found between sporadic meteoritic input and exospheric neutral abundance Abstract The Lunar Atmosphere and Dust Environment Explorer (LADEE) orbited the Moon for approximately 6 months, taking data with the Lunar Dust Experiment (LDEX), Ultraviolet-Visible Correspondence to: Spectrometer (UVS), and Neutral Mass Spectrometer (NMS). Here we compare coincident LDEX J. R. Szalay, measurements of meteoritic influx to exospheric column densities of Na and K derived by UVS. We report a [email protected] strong correlation of exospheric potassium and meteoroid ejecta during the Geminids meteoroid shower, exhibiting a much stronger response than sodium. With the exception of the Geminids, we find a weak Citation: correlation between the sporadic meteoroid influx as measured by LDEX and exospheric density as Szalay, J. R., M. Horányi, A. Colaprete, measured by UVS. and M. Sarantos (2016), Meteoritic influence on sodium and potassium abundance in the lunar exosphere measured by LADEE, Geophys. Res. Lett., 43, doi:10.1002/2016GL069541. 1. Introduction The Moon has a tenuous exosphere with a host of constituents, including neutral atoms, charged particles, and Received 10 MAY 2016 dust ejecta [Stern, 1999]. The discovery of sodium and potassium in the lunar exosphere [Potter and Morgan, Accepted 6 JUN 2016 1988; Tyler et al., 1988] has motivated a wide range of studies using these neutrals as efficient probes for the Accepted article online 13 JUN 2016 behavior of the lunar exosphere [Stern, 1999]. Lunar exospheric neutrals are believed to come from a variety of sources: photodesorption, ion sputtering, impact vaporization, and thermal desorption. The relative strengths of these source mechanisms are still actively debated in the literature. Once in the lunar exosphere, these neutrals can be lost via photoionization, charge exchange, jeans escape, and radiation pressure [Morgan and Killen, 1997; Stern, 1999]. Recent modeling efforts have been undertaken to understand the dynamics of neutrals in the lunar exosphere, suggesting the need for better in situ measurements to constrain model parameters [Sarantos et al., 2010, 2012]. Ground-based measurements of the lunar sodium tail found photodesorption to be a primary source of exospheric sodium and observed a minimum Na density near full Moon [Potter et al., 2000]. More recent measurements of the near-tail taken from lunar orbit by the Kaguya spacecraft indicated a similar source temperature [Tenishev et al., 2013]. A synodic trend for surface density was found, in which lunar sodium decreased by approximately 20% from first to last quarter of the lunar cycle [Kagitani et al., 2010]. A small change in the actual exospheric content from full Moon to third quarter can go unappreciated from an Earth observer, as the brightness seen from Earth increases for geometrical reasons when we look at the densest portions of the subsolar exosphere at third quarter [Sarantos et al., 2010], consistent with a spatial variation of density as cosine squared of the solar zenith angle [Potter and Morgan, 1994]. Thus, the difference between measured behaviors of the exosphere by Kaguya and ground-based observations is most likely due to their different observing geometries. The response of the lunar exosphere to meteoroid bombardment has been previously investigated, finding ©2016. The Authors. This is an open access article under the an exospheric increase of sodium during the Leonids and Taurids meteoroid showers [Verani et al., 1998; Smith terms of the Creative Commons et al., 1999]. However, ground-based measurements of the lunar exosphere are not able to determine the Attribution-NonCommercial-NoDerivs sodium and potassium density profile throughout the entire lunar cycle. License, which permits use and distribution in any medium, provided The Lunar Atmosphere and Dust Environment Explorer (LADEE) mission was uniquely equipped to investigate the original work is properly cited, the use is non-commercial and no the effect of meteoroid bombardment on the lunar exosphere [Elphicetal., 2014]. Onboard, it carried the Lunar modifications or adaptations are made. Dust Experiment (LDEX) [Horányi et al., 2014], Ultraviolet-Visible Spectrometer (UVS) [Colaprete et al., 2014],

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Figure 1. The LDEX impact rates and UVS-derived column densities for Na and K. Raw and smoothed (over 4 days) data are indicated by the green and black lines, respectively. For the LDEX measurements, the green line shows the 1 day moving average. Gray contours indicate 1–4𝜎 error bars for the diﬀerence between raw and smoothed values. Meteoroid showers for which LDEX observed enhanced dust densities are shown by the vertical lines. The vertical orange bars mark the periods when the Moon was in the Earth’s magnetotail (±30∘ from full Moon).

and Neutral Mass Spectrometer (NMS) [Mahaffy et al., 2014]. LDEX discovered a permanently present, asymmetric dust cloud which is continually sustained by meteoroid bombardment [Horányi et al., 2015]. UVS recorded exospheric densities at a significantly higher cadence than previous studies, elucidating the exospheric trends on short and long timescales (monthly to semiannual). It observed the noontime content of the Na exosphere as seen from the same vantage point on the dayside to peak at ∼30∘ past full Moon [Colaprete et al., 2016]. Here we use the in situ LDEX data as a proxy for meteoritic influx and compare it to the Na and K column densities measured by UVS. In section 2, we describe the measurements taken by these two instruments. We determine the lifetime of exospheric potassium by measuring its response to the strong Geminids meteoroid shower in section 3. In section 4, we investigate the response of Na and K to long-term trends in the sporadic meteoroid background and additional strong meteoroid showers, followed by the conclusions in section 5.

2. LDEX and UVS Measurements LDEX is an impact ionization dust detector, capable of individually detecting impacts from particles with radii greater than approximately 0.3 μm. In addition to mapping the density distribution of dust in the lunar exosphere [Horányietal., 2015], the Moon’s dust cloud was observed to wax and wane with lunar phase, exhibiting a synodic dependence [Szalay and Horányi, 2015]. UVS is a point spectrograph which took spectra in the range of 230–810 nm typically at a cadence of approximately two measurements per day. It was able to successfully make measurements of the sodium and potassium brightness, from which abundances were derived. UVS found both sodium and potassium exhibit synodic trends; however, the two species behaved differently ([see Colaprete et al., 2016, Supplementary Material] for a full description of these measurements). Figure 1 shows the LDEX impact rates along with the sodium and potassium column densities derived by UVS over lunar noon for the low-altitude phase of the mission. Due to the variation in dust impact detections as a function of altitude and lunar phase, we average the LDEX-measured impact rates taken every minute over 1 day (4 days) shown in green (black) to determine an average meteoritic influx. LDEX observed a small number of “unusual” periods of enhanced activity, with all but one corresponding to a known meteoroid shower [Szalay and Horányi, 2016]. Of those periods, the Geminids was identified to be the strongest producer of impact ejecta.

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Figure 2. The Na and K abundances as a function of lunar phase, averaged over the period from January to April 2014. (top) Abundances both in color (scaled from minimum to maximum value) and radial extent to illustrate the phase dependence, in a frame with the Sun on the x axis and the direction of orbital motion about the Sun on the y axis. Orange areas indicate when the Moon is in the magnetotail, approximately ±30∘.

UVS observed synodic trends in both Na and K column densities. In Figure 1, the UVS measurements are shown at their nominal cadence (green) and smoothed overa4daysliding window (black) to illuminate synodic trends. By comparing UVS and LDEX data, we find that UVS recorded only one significant enhancement temporally coincident with a known meteoroid shower, the Geminids. This shower most clearly influenced the instantaneous potassium content of the exosphere.

3. Exospheric Response to the Geminids To quantify the exospheric response to a strong meteoroid input, we first calculate the average behavior of each species as a function of lunar phase, averaged during the quiescent period of January to April 2014 (Figure 2). The synodic dependence discussed in Colaprete et al. [2016] is evident in this diagram. We then subtract this average abundance from the Na and K signals to quantify the magnitude of anomalous enhancements. Figure 3 shows these residual abundances after subtraction. Unlike the UVS data, which are taken at the same local time for each measurement, the LDEX data shown are the impact rate averaged over a sliding window of 24 h. While these data are sufficient to characterize large changes in exospheric dust influx, to accurately determine the smaller-scale synodic modulation over long timescales involves comparing the densities as a function of local time and altitude [Szalay and Horányi, 2015], which would decrease our temporal resolution. Since the synodic variation in the LDEX impact rates is consid- erably smaller than the UVS data, and due to the complexities in accurately representing this modulation, to calculate the LDEX residual, we simply subtract off the average impact rate of 1.2 min−1 from the LDEX data. As shown in Figure 3, the dust and potassium abundance is strongly enhanced during the Geminds, while a sodium response is less pronounced. Given the strong response measured during the Geminids, we focus on this period to understand the response of the lunar exosphere to meteoroid bombardment. We employ a simple model for exospheric response,

ṅ = 𝛼M − n∕𝜏 (1)

where n is the exospheric neutral density, 𝜏 is the characteristic lifetime time of the atom considered, M is meteoritic influx, and 𝛼 is a scaling constant for the meteoritic influx to gauge its efficiency to produce

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Figure 3. (first and third rows) The column abundances measured by UVS for Na and K, with the average synodic behavior in green. (second and fourth rows) The residual abundances, after the synodic trend has been removed. (fifth row) The LDEX impact rates, with a constant value of 1.2 min−1 subtracted off. Gray bars indicate 2𝜎,4𝜎,and6𝜎.

neutral atoms. We note that 𝜏 is an all encompassing “general” lifetime, such that all loss processes are con- tained within this one time constant in an average sense. This is a simplification, as there are a variety of loss processes for each exospheric species; however, it gives us a quantitative and data-driven estimate for the total exospheric lifetime in this scenario. We numerically integrate equation (1) to find n(t), which gives an estimate of the meteoritically generated component of the exospheric cloud density. The integration is performed by assuming n(0)=0 at the begin- ning of the LDEX data set, such that the solution during the Geminids is independent of n(0). We use the LDEX impact rates as a proxy for meteoritic input, M. These impact rates are the residual LDEX instantaneous impact rates taken every minute, averaged over a sliding window of 24 h as shown in Figure 3. To derive 𝜏 in this ini- tial study, we assume that dust influx is solely responsible for sustaining the anomalous neutral exospheric density enhancement during the Geminids. Significantly more complex and comprehensive models exist to study the behavior and evolution of the lunar exosphere, which have a variety of additional sources and sinks [Sarantos et al., 2010, 2012; Hurley et al., 2015]. Figure 4 shows the normalized residual potassium density along with model-predicted densities for decay timescales of 2 to 10 days. Using this simple model, we find that the potassium density enhancement decays on a timescale of 𝜏 ≈ 4–8 days, following the decay in meteoroid bombardment. We did not derive a timescale for Na during the Geminids as it is not clear if the minor enhancement observed during this time period was actually due to meteoroid bombardment.

4. Response to the Sporadic Meteoroids and Other Showers During the period of October to the end of December 2013, the Moon experienced a multitude of meteoroid showers [McBeath, 2015; Szalay and Horányi, 2015]. To investigate the response of the exosphere to sporadic meteoroids, we therefore focus on the data taken during January to April 2014. As estimated by directly comparing average dust impact rates to exospheric column densities, neither species exhibits a strong correlation with sporadic meteoritic influx, with both potassium and sodium having correlation coef- ficients of ∼0.5 with dust impact rates. While both the LDEX and UVS data have long-term, periodic character, repeating on the lunar synodic period, their peaks occur at different lunar phases. The Na and K densities gen- erally peak around 30∘ past full Moon, while the LDEX-measured densities peak later during the Moon’s last

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Figure 4. LDEX (gray) and UVS potassium (black) residual measurements (from Figure 3, second and ﬁfth rows) for ±5 days around the Geminids peak ﬂux to the Moon. Potassium decay rates of 2 to 10 days are shown by colored lines.

quarter or the waning gibbous phase [Szalay and Horányi, 2015]. Sputtering was suggested to be a sink for exospheric neutrals [Colaprete et al., 2016], which becomes active as the Moon exits the magnetotail, possibly explaining the lag between dust and exospheric neutrals were sporadics to play a dominant role. However, more complex modeling for the neutral exosphere and dust ejecta is required to further understand the sporadics’ role in exospheric neutral production. In addition to the Geminids, the Leonids and Quadrantids are comparably strong meteoroid showers which also coincided with LADEE’s operational period. Due to operational constraints, LADEE was unable to make sufficient measurements to characterize the lunar response during the Leonids. Figure 5 shows the impact angle contours for both the Geminids and Quadrantids during their respective peak flux times. Both meteoroid showers are observed to have similar zenith hourly rates at Earth [McBeath, 2015] yet are registered in different intensities by both LDEX and UVS. Since the Quadrantids hit at a high selenographic latitude, their contribution to dust production in the equatorial plane where LDEX measured is reduced. This is reflected in the smaller impact rate enhancement registered by LDEX during the Quadrantids compared to the Geminids. However, since the transport of exospheric neutrals is a more global process than the impact ejecta detected

Figure 5. (top) The surface potassium abundance (gray scale) from Lunar Prospector measurements [Prettyman et al., 2006] with the UVS potassium column densities overplotted (colored dots) as a function of selenographic latitude and longitude. Impact angle contours are shown as solid (dotted) lines for the peak Geminids (Quantrantids) times. (bottom) The UVS column densities, both raw (green) and smoothed over a 20∘ sliding window (black) along with the average surface potassium abundance (blue). Each has been normalized by its standard deviation and centered about the origin. Data points marked with crosses indicate ±3 days around the Geminids peak time.

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by LDEX, and both showers bombard comparable amounts of lunar potassium, the Quadrantids would be expected to generate a similarly large exospheric enhancement. Given that impact vapor production is a power law in velocity [Morgan and Killen, 1997; Sarantos et al., 2012; Collette et al., 2014] and that both showers impact at relatively large velocities (>30 km/s), it is unlikely that the potassium content between the two impactor populations plays a signiﬁcant role as the amount of liberated material is 10–100 times larger than that of the impactor [Cintala, 1992]. If the Quadrantids produce a comparable amount of potassium as the Geminids, the data would suggest that potassium mobility is decreased at higher latitudes.

5. Discussion and Conclusions Photoionization and loss to the surface are two mechanisms to lose neutrals from the lunar exosphere. Na and K have photoionization lifetimes of 0.6 and 0.4 days, respectively [Stern, 1999]. The derived characteristic lifetime of potassium in this work of 4–8 days indicates that K atoms spend a significant amount of their lifetime (≃ 𝜏 − 0.4 days) on the surface and shielded from UV [Sprague et al., 1992]. Since the Geminids, from which 𝜏 is derived, impact the Moon at approximately 2a local time, much of the potassium liberated by this shower may not reach the sunlit side of the Moon in a single ballistic bounce. The difference between 𝜏 and the photoionization lifetime gives an indication of the residence time for potassium on the surface. Compared to potassium, sodium does not exhibit a similarly large response to the Geminids. Since their photoionization lifetimes are essentially the same, both the sources and sinks for these two species could be significantly different. With the exception of the Geminids, we did not find strong correlations between the observed impact rates and exospheric densities on short timescales. There is a weak but present correlation on the scale of a lunar phase between the two; however, the exospheric column densities precede the observed dust density enhancements. The offset could be due to sinks by sputtering if sporadic meteoroids play a dominant role or it could arise from a variety of synodic effects not related to dust. One such effect is the variation in radiation pressure throughout the mission. Although perhaps contributing to a portion of the observed brightness variation, changes in radiation pressure have been accounted for in calculating the UVS column densities [Colaprete et al., 2016, Supplementary Material]. UVS measurements showed a marked correlation between the surface abundance and column density of potassium (Figure 5), indicating that the exospheric density depends not only on the production mechanism but also on the K source regions [Colaprete et al., 2016]. This correlation gives increased confidence that the potassium lifetime is relatively short (significantly less than a full lunation). Were potassium to have a much longer lifetime, the exospheric density would be more smeared and not track with the surface so closely. We find that meteoroid bombardment can be responsible for liberating significant quantities of lunar potassium during intense shower periods, specifically during the Geminids meteoroid shower. We suggest that while meteoroid bombardment is not dominantly responsible for exospheric neutral generation, it appears to play a comparatively more significant role for potassium than sodium. Unraveling why these two species respond so differently to intense meteoritic bombardment, and only to selected meteoroid showers, may require more complex modeling to identify the differences in their sources and sinks on the lunar surface.

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