Meteoritic Influence on Sodium and Potassium Abundance in the Lunar
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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 model- ing 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 tem- perature [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], SZALAY ET AL. METEORITIC INFLUENCE ON LUNAR EXOSPHERE 1 Geophysical Research Letters 10.1002/2016GL069541 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 1–4 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 exo- sphere [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 approx- imately 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. SZALAY ET AL. METEORITIC INFLUENCE ON LUNAR EXOSPHERE 2 Geophysical Research Letters 10.1002/2016GL069541 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 syn- odic 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 behav- ior of each species as a function of lunar phase, averaged during the quiescent period