Elsevier

Icarus

Volume 273, 15 July 2016, Pages 36-44
Icarus

Lunar exospheric helium observations of LRO/LAMP coordinated with ARTEMIS

https://doi.org/10.1016/j.icarus.2015.10.033Get rights and content

Highlights

  • LRO/LAMP UV spectrograph detected fluorescence of HeI 584 Å in the lunar exosphere.

  • LAMP-derived He source rate is directly related to the solar wind α-particle flux.

  • LAMP-derived He surface density is consistent with LACE measurements in 1973.

  • These observations offer insight on He density on both latitude & local solar time.

  • These observations will help constraining models of lunar volatiles transport.

Abstract

We present results from Lunar Reconnaissance Orbiter’s (LRO) UV spectrograph LAMP (Lyman-Alpha Mapping Project) campaign to study the lunar atmosphere. Several off-nadir maneuvers (lateral rolls) were performed to search for resonantly scattering species, increasing the illuminated line-of-sight (and hence the signal from atoms resonantly scattering the solar photons) compared to previously reported LAMP’s “twilight observations” (Cook, J.C., Stern, S.A. [2014]. Icarus 236, 48–55). Helium was the only element distinguishable on a daily basis, and we present latitudinal profiles of its line-of-sight column density in December 2013. We compared the helium line-of-sight column densities with solar wind alpha particle fluxes measured from the ARTEMIS (Acceleration, Reconnection, Turbulence, & Electrodynamics of Moon’s Interaction with the Sun) twin spacecraft. Our data show a correlation with the solar wind alpha particle flux, confirming that the solar wind is the main source of the lunar helium. We also support the finding by Benna et al. (Benna, M. et al. [2015]. Geophys. Res. Lett. 42, 3723–3729) and Hurley et al. (Hurley, D.M. et al. [2015]. Icarus, this issue), that a non-zero contribution from endogenic helium, coming from radioactive decay of 232Th and 238U, is present. Moreover, our results suggest that not all of the incident alpha particles are converted to thermalized helium, allowing for a non-negligible fraction to escape as suprathermal helium or simply backscattered from the lunar surface. We compare LAMP-derived helium surface density with the one recorded by the mass spectrometer LACE (Lunar Atmospheric Composition Experiment) deployed on the lunar surface during the Apollo 17 mission, finding good agreement between the two measurements. The LRO/LAMP roll observations presented here are in agreement with the most recent lunar exospheric helium model (Hurley, D.M. et al. [2015]. Icarus, this issue) around mid- to high-latitudes (50–70°) regardless of the local solar time, while there is an underestimation of the model around the low- to mid-latitudes (10–30°), especially around the dawn terminator. The LRO/LAMP roll observations presented here provide unique coverage of local solar time and latitude of the lunar exospheric helium, filling a gap in the knowledge of the structure of the lunar exosphere as a whole. These observations will inform future models of transport of volatiles, since at the terminator the analytic expressions for the surface temperature, essential to determine the energy distribution, the residence time, and the hop length of the particles, is least accurate.

Introduction

Helium was one of the first species identified in the lunar exosphere by the Apollo 17 surface-based mass spectrometer LACE (Lunar Atmospheric and Composition Experiment; Hoffman et al., 1973). The maximum surface density of helium was recorded by LACE at the night side, as expected from a non-condensable gas whose density follows the T−5/2 dependence on surface temperature (Hodges and Johnson, 1968). The strong correlation between the helium abundance recorded by LACE and the Kp index of geomagnetic activity (Hodges and Hoffman, 1974), along with the longitudinal dependence of helium density (Hodges, 1973) made clear that the main source (albeit not unique) of lunar exospheric helium is neutralization of solar wind alpha particles (He++) that impinge the lunar surface at energies of the order of 4 keV (Hodges, 1978). In fact, the alpha particle flux incident on the lunar surface, and thus the implantation of the solar wind at the surface, falls off as a function of solar zenith angle (Tanaka et al., 2009, Leblanc and Chaufray, 2011, Crider and Vondrak, 2000). The main loss process for helium is Jeans (or thermal) escape, given its low mass, followed by photo-ionization. However, LACE measured only 70% of the density of helium expected from the solar wind alpha particle flux, suggesting that a non-negligible fraction of solar wind He++ is lost from the Moon as ions or suprathermal neutrals.

Helium resonant scattering emission (584 Å) was detected for the first time remotely by LAMP (Stern et al., 2012), opening an avenue for the remote study of the lunar exosphere. LAMP observations also showed a decrease in helium density by a factor of 2 when the Moon was in the Earth’s magnetotail, where the solar wind source is suppressed (Feldman et al., 2012). Episodic bursts (or “flares”) of helium density were observed in detailed LAMP time-series analysis (Cook and Stern, 2014). These bursts are apparently uncorrelated with solar activity or meteor showers, implying that an endogenic source is likely involved, i.e. radioactive decay of 232Th and 238U into lead in the mantle followed by release triggered by moonquakes. Hodges et al. (1973) suggested that this radiogenic process should create helium at rates comparable to or higher than the rate from the solar wind, although most of such helium (90%) would remain trapped within the interior (assuming that the same venting rates acting on argon apply to helium).

There are still lingering questions concerning the sources and distribution of lunar exospheric helium, such as: how important is the contribution of the endogenic lunar helium to the exosphere? What is the distribution of exospheric helium as a function of the local solar time and latitude? How exactly is helium thermally accommodated to the lunar surface? In order to address these questions, the UV spectrograph LAMP on board of the Lunar Reconnaissance Orbiter (Chin et al., 2007) performed a campaign to search for helium atmospheric emissions at the same time of the science phase of LADEE (Lunar Atmosphere and Dust Environment Explorer; Elphic et al., 2014), i.e. from October 2013 to April 2014, which was studying the Moon in an equatorial, retrograde orbit. We will focus on the lateral rolls observations of December 2013 and describe them in Section 2, show the results in Section 3 and discuss them in Section 4. Section 5 summarizes the results and points to future work.

Section snippets

LAMP UV spectrograph

LAMP is a photon-counting imaging spectrograph that covers a bandpass of 575–1965 Å. Its detector is a double-delay line microchannel plate. Data are collected within 2D arrays where in the horizontal direction is stored the information on wavelength (1024 columns) and in the vertical direction (32 rows) is stored the spatial information. The instrument collects data as pixel-list events within 4 ms intervals, so we can readily integrate signals over longer timescales and regions of interest.

The model

The helium exospheric model we use is a Monte Carlo model of the lunar exosphere that follows test particles launched with a spatial and velocity distribution representative of thermalized solar wind alpha particles from the point of release until the eventual loss from the system (Hurley et al., this issue). It follows the particles along their trajectories by solving the equation of motion under lunar gravity using a 4th order Runge–Kutta algorithm. Thus the initial positions are on the

Discussion

One of the greatest advantages of using these off-nadir maneuvers of LRO is to obtain a line-of-sight column density along different local solar times at once. In fact, LAMP “twilight observations”, like those of Feldman et al. (2012) and Cook and Stern (2014), are obtained in the nominal nadir-looking mode, and the local solar time sampled is the same as the spacecraft. Moreover, these observations must be taken close to the terminators to maximize the length of the illuminated column beneath

Conclusions and future work

The UV spectrograph LAMP on board of the Lunar Reconnaissance Orbiter (LRO), performed a campaign to search for atmospheric emissions of helium fluorescence line (584 Å) at the same time of the science phase of LADEE (October 2013–April 2014), which was studying the Moon in an equatorial, retrograde orbit. LRO was tilted toward the direction of motion (pitches) both backwards and forward, and laterally (rolls, here discussed).

We discussed here the lateral rolls performed in December 2013, and we

Acknowledgments

We thank the Lunar Reconnaissance Orbiter project and project team at NASA’s Goddard Space Flight Center for conducting the LAMP atmospheric observations. LAMP is funded by NASA under contract NNG05EC87C, whose financial support we gratefully acknowledge.

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