Lunar exospheric helium observations of LRO/LAMP coordinated with ARTEMIS
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.
References (38)
Planetary coronae and atmospheric evaporation
Planet. Space Sci.
(1963)- et al.
Sporadic increases in lunar atmospheric helium detected by LAMP
Icarus
(2014) New upper limits on numerous atmospheric species in the native lunar atmosphere
Icarus
(2013)Temporal variability of lunar exospheric helium during January 2012 from LRO/LAMP
Icarus
(2012)An analytic function of lunar surface temperature for exospheric modeling
Icarus
(2015)- et al.
Mercury and Moon He exospheres: Analysis and modeling
Icarus
(2011) Imaging the South Pole–Aitken basin in backscattered neutral hydrogen atoms
Planet. Space Sci.
(2015)The ARTEMIS mission
Space Sci. Rev.
(2011)Variability of helium, neon, and argon in the lunar exosphere as observed by the LADEE NMS instrument
Geophys. Res. Lett.
(2015)- et al.
Theory of planetary atmospheres
Lunar Reconnaissance Orbiter overview: The instrument suite and mission
Space Sci. Rev.
The solar wind as a possible source of lunar polar hydrogen deposits
J. Geophys. Res.: Planets (1991–2012)
The Lunar Atmosphere and Dust Environment Explorer mission
Space Sci. Rev.
LAMP: The Lyman Alpha Mapping Project on NASA’s Lunar Reconnaissance Orbiter mission
Space Sci. Rev.
LRO–LAMP observations of the LCROSS impact plume
Science
Far‐ultraviolet reflectance properties of the Moon’s permanently shadowed regions
J. Geophys. Res.: Planets (1991–2012)
First results from ARTEMIS, a new two-spacecraft lunar mission: Counter-streaming plasma populations in the lunar wake
Space Sci. Rev.
Mercury’s helium exosphere
J. Geophys. Res.
Helium and hydrogen in the lunar atmosphere
J. Geophys. Res.
Cited by (13)
Gas storage and transport in porous media: From shale gas to helium-3
2021, Planetary and Space ScienceSELMA mission: How do airless bodies interact with space environment? The Moon as an accessible laboratory
2018, Planetary and Space ScienceCitation Excerpt :The composition of noble gases in the lunar exosphere, measured by the Apollo Lunar Atmospheric Composition Experiment (LACE) experiment and additionally inferred from studies of gas trapped in lunar regolith samples brought to Earth indicated that species such as helium (He) are dominated by a solar wind source, but with additional contributions probably from the interior of the Moon (Hodges Jr. and Hoffman, 1975; Wieler et al., 1996). Both LADEE and LAMP observations confirm the solar wind source; they are halted when the Moon is in Earth's magnetotail (Feldman et al., 2012; Cook and Stern, 2014; Hurley et al., 2016; Grava et al., 2016). Because the solar wind impinges on the lunar surface with energies of about 1 keV/nuc H, He and other solar wind species are absorbed in the surface material (in the regolith grains and rocks) and are trapped.
Contributions of solar-wind induced potential sputtering to the lunar surface erosion rate and it's exosphere
2018, Nuclear Instruments and Methods in Physics Research, Section B: Beam Interactions with Materials and AtomsCitation Excerpt :Finally in Section 6 we calculate the density of the sputtered atoms that will contribute in developing the lunar exosphere. The lunar exosphere composition has been studied and identified by different instruments [12–20]. The sources of elements in the lunar exosphere are mainly atomic sputtering by solar-winds, vaporization by micrometeorites impact and photon stimulated desorption [21,22].
Upper limit of helium-4 in the sunlit lunar exosphere during magnetotail passage under low solar wind condition: Result from CHACE aboard MIP in Chandrayaan-1
2017, IcarusCitation Excerpt :Due to very less number of positive detections and poor SNR, in this paper we restrict ourselves to estimating the upper limit of He based on the detection threshold of the instrument. Lunar He is known to have a uniform latitudinal distribution, as reported by LAMP in the terminator region (Feldman et al., 2012; Grava et al., 2016). This is attributed to the multiple hops undergone by the ballistic (Moon-bound) He atoms on the lunar surface (Shemansky and Broadfoot, 1977), resulting into uniform latitudinal distribution.
Space plasma physics science opportunities for the lunar orbital platform - Gateway
2023, Frontiers in Astronomy and Space Sciences