Geophysical Research Letters

First direct detection of ions originating from the Moon by MAP-PACE IMA onboard SELENE (KAGUYA)

Authors


Abstract

[1] The Moon has no global intrinsic magnetic field and only has a very thin atmosphere. Ion measurements made from lunar orbit provide us with information regarding interactions between the solar wind and planetary surface, the surface composition through secondary ion mass spectrometry and the source and loss mechanisms of planetary tenuous atmosphere. An ion energy mass spectrometer MAP-PACE IMA onboard a lunar orbiter SELENE (KAGUYA) has detected low-energy ions at 100-km altitude. The MAP-PACE measurements have elucidated that the ions originate from the lunar surface and exosphere and that the ions are at least composed of He+, C+, O+, Na+ and K+. Following the discovery of the lunar Na and K exospheres by the ground-based observation, MAP-PACE IMA have found the He, C and O exospheres around the Moon.

1. Introduction

[2] The Moon, Mercury, and some of the planetary satellites and asteroids maintain thin atmospheres. The Moon and its atmosphere have been the most extensively investigated, and the findings are thought to be the cornerstone for the studies of the other celestial bodies. The lunar atmosphere is called ‘surface-bounded exosphere’ because it is thin enough to be regarded as an exosphere and it bounds on the solid surface. Because of rare collisions between lunar atmospheric particles, the atmosphere presumably preserves information regarding the lunar surface [Smyth and Marconi, 1995]. The atmospheric particles sputtered by the solar wind (SW) potentially give us surface composition information through secondary ion mass spectrometry (SIMS) [Managadze and Sagdeev, 1988; Elphic et al., 1991].

[3] Ground-based observations elucidated the existence of the lunar Na and K exospheres [Potter and Morgan, 1988; Tyler et al., 1988]. Photoionized lunar exospheric particles are transported by the SW in a cycloidal motion if the SW electric and magnetic fields are steady [Cladis et al., 1994]. Thus in-situ measurements of the ions by a lunar orbiter provide continuous monitoring of the lunar exosphere and allow us to investigate its source and loss mechanisms.

[4] In contrast to many ground-based observations, there have been only limited in-situ ion measurements around the Moon [e.g., Hodges et al., 1972; Hodges, 1975]. The ion analyzers on board AMPTE/IRM and WIND detected ions which were supposed to originate from the Moon, while the satellites were distant from the Moon [Hilchenbach et al., 1993; Mall et al., 1998]. MAP-PACE (MAgnetic field and Plasma experiment-Plasma energy Angle and Composition Experiment) onboard SELENE (SELenological and ENgineering Explorer) (KAGUYA) has detected ions originating from the Moon at an altitude of 100 km. In this paper, we show the results of the first in-situ measurements of the mass-identified low-energy ions around the Moon.

2. Instrumentation

[5] SELENE is a Japanese lunar orbiter which was launched on 14 September 2007 from Tanegashima Space Center in Japan. SELENE is a polar orbiter, the orbit altitude of which is 100 km. The orbit plane moves west by ∼1.1 degrees per orbit in longitude. MAP is one of the scientific instruments onboard SELENE. MAP consists of LMAG (Lunar MAGnetometer) and PACE. LMAG is a triaxial flux gate magnetometer that is equipped at the top plate of a 12-m long mast in order to reduce an offset of the interference magnetic fields caused by the spacecraft [Shimizu et al., 2008]. LMAG measures the vector magnetic field with a sampling frequency of 32 Hz and a resolution of 0.1 nT. PACE consists of four sensors: ESA (Electron Spectrum Analyzer)-S1, ESA-S2, IMA (Ion Mass Analyzer) and IEA (Ion Energy Analyzer) [Saito et al., 2008a; Yokota et al., 2005]. ESA-S1 and S2 measure the distribution function of low-energy electrons below 15 keV, while IMA and IEA measure the distribution function of low-energy ions below 28 keV/q.

[6] SELENE is a three-axis stabilized satellite. ESA-S1 and IMA continuously face to the Moon and measure anti-moonward electrons and ions, while ESA-S2 and IEA are mounted on the opposite side of the satellite and measure moonward electrons and ions. Each sensor has a hemispherical field of view. Three-dimensional distribution functions of electrons and ions are measured by a pair of electron sensors (ESA-S1 and S2) and a pair of ion sensors (IMA and IEA), respectively. As shown in Figure 1, SW ions are measured by IEA on the dayside when the Moon is exposed to the SW. IMA measures the SW only around the day-night terminator. One of the main roles of IMA is to measure ions coming from the Moon and to identify the ion species by mass analysis.

Figure 1.

Schematic view of the ion measurement configuration of SELENE.

3. Observation

[7] Figure 2 shows a typical measurement of ions originating from the Moon. During the period of the measurement, the Moon was in the SW, located at (51, −22, 5)RE in the Geocentric Solar Ecliptic (GSE) coordinate system where RE indicates the radius of the Earth. Figure 2 (top) shows energy-time spectrograms from IEA and IMA. The signatures repeated at two-hour intervals in the spectrograms are due to the orbital period of two hours. Figure 2 (middle) shows the satellite position in the Selenocentric Solar Ecliptic (SSE) coordinate system. The point at latitude 0° and longitude 0° is the subsolar point. The longitude from −90° to 90° corresponds to the dayside. Figure 2 (bottom) shows the magnitude and directions of the magnetic field B in the SSE coordinate system measured by MAP-LMAG. IEA measures SW ions with energies of ∼2.0 keV on the dayside orbits moving from the north-pole to the south-pole, while IEA has no ion detection in the lunar wake. SW ions are also measured by IMA near the terminator at latitude close to ±90°. As reported by Saito et al. [2008b], IMA detects reflected/scattered SW ions with energies of ∼1.0 keV when IEA directly detects SW ions. What we concentrate on here is that ions with energies of a few hundreds of electronvolts coming from the Moon have been detected by IMA mainly in the first half intervals of the dayside orbits. We propose that the origin of the low-energy ions is the lunar surface and exosphere.

Figure 2.

(top) Energy-time spectrograms of ions measured by IEA and IMA of MAP-PACE between 1:00 and 12:00 UT on 2 June 2008. (middle) The satellite position in the Selenocentric Solar Ecliptic (SSE) coordinate system is plotted. (bottom) Magnitude and directions of the magnetic field B in the SSE coordinate system measured by MAP-LMAG.

[8] IMA is a time-of-flight (TOF)-type mass spectrometer that identifies the ion species by measuring the flight time of ions inside the sensor [Yokota et al., 2005]. For the measurement period between 1:00 and 12:00 UT on 2 June 2008, the IMA data were obtained in an operational mode for mass analysis. The mass profiles obtained by IMA are shown in Figure 3. The TOF profile corresponds to the mass profile. Figure 3 (top) shows the energy/charge-TOF profile of all the ions measured by IMA for the measurement period. H+ of ∼2 keV/q and He++ of ∼4 keV/q are the two major components of the SW. The low-energy (<2 keV) H+ is due to the SW H+ reflected/scattered at the lunar surface. The ions originating from the lunar surface and exosphere are distributed at energies from 10 to 800 eV and at flight time from 95 to 600 ns (white rectangle) detached from the SW component. Figure 3 (middle) shows the mass profile of the ions inside the white rectangle in Figure 3 (top). The mass profile demonstrates that the ions at least include He+, C+, O+, Na+ and K+. The energy of He+ is nearly the same as that of the other species and is smaller than that of the reflected/scattered SW H+. Therefore, the origin of He+ is probably the Moon interior or the SW He++ absorbed in the lunar surface. Compared with the mass profile obtained around the Mercury [Zurbuchen et al., 2008], there is a possibility that the peaks of the water group and silicon are also included in the IMA mass profile. Figure 3 (bottom) show where the ions are detected by the SELENE satellite. Almost all the detection points (rectangles) are located within the northern half of the dayside orbits.

Figure 3.

(top) Energy-time-of-flight (TOF) profile of the ions measured by IMA of MAP-PACE between 1:00 and 12:00 UT on 2 June 2008. (middle) TOF profile of the low-energy and heavy ions (white rectangle of Figure 3, top). Thin curves indicate the calibration data. (bottom) Detection points (rectangle) of the low-energy and heavy ions (white rectangle of Figure 3, top) in the SSE coordinate system.

[9] Here we show evidence that the low-energy heavy ions measured by IMA originate from the Moon. We know that the energy of the ions is a few hundreds electronvolts and that almost all the detection points are during the half intervals of the dayside orbits. The features themselves are evidence for the detection of ions originating from the lunar surface and exosphere at 100-km altitude [Yokota and Saito, 2005]. When the Moon is in the SW, ions emitted from the lunar surface and exosphere are transported by the SW [Cladis et al., 1994]. The energy of almost all the ions desorbed from the surface by the solar photons or SW ions are a few electronvolts and below [Smyth and Marconi, 1995; Madey et al., 1998]. The energy of photoionized lunar exospheric particles is also below a few electronvolts. The Larmor radius of the ions is large compared with the satellite altitude of 100 km. Thus, the ions originating from the Moon move straight along to the ambient electric field E from the originating points to the detection points at 100-km altitude. If ∣E∣ is a few millivolts per meter, the ions are accelerated to a few hundreds electronvolts at the satellite altitude. Moreover, the northward electric field (Ez > 0) causes the situation that the SELENE satellite detects the ions originating from the Moon only in the north half intervals of the dayside orbits because Ez > 0 prevent ions generated at the south hemisphere from reaching the satellite altitude.

[10] To obtain E for the measurement period, E = −V × B is calculated as shown in Figure 4, where V indicates the SW velocity measured by MAP-PACE IEA and B is the magnetic field measured by MAP-LMAG (see Figure 2). The bold horizontal lines below Figure 4 (bottom) indicate the intervals of the dayside orbits. Because IEA measures the SW ions only on the dayside, E is plotted only for the intervals of the dayside orbits. Figure 4 (top) shows ∣E∣ = 0 ∼ 3 mV/m and Ez > 0 almost all the intervals of the dayside orbits. Figure 4 (bottom) compares the energy-time spectrogram of the low-energy heavy ions (inside the white rectangle in Figure 3) measured by IMA and the potential difference ∣E∣ × L, where L is the length between the satellite position and the foot point of E on the lunar surface. Note that the energy-time spectrogram in Figure 2 includes all the count data obtained by IMA. ∣E∣ × L is plotted only when E is directed from the lunar surface to the satellite. The agreement between the energy/charge of the low-energy heavy ions and ∣E∣ × L strongly supports that the origin of the ions measured by IMA is the lunar surface and exosphere. Note that the energy-time spectrogram reflects the origination point of the ions. The ions with energies close to ∣E∣ × L are emitted from the lunar surface, while the origin of the ions with energies below ∣E∣ × L is the lunar exosphere. The ions with energies higher than ∣E∣ × L are probably due to the high-energy component of the sputtered ions.

Figure 4.

(top) Electric field derived from E = −V × B in the SSE coordinate system, where V indicates the SW velocity from IEA of MAP-PACE. (bottom) The potential differences (black dots) estimated by ∣E∣ × L are superposed over the energy-time spectrogram of the low-energy and heavy ions (white rectangle in Figure 3, top), where L is the length between the satellite position and the foot point of E on the lunar surface. The bold horizontal lines below Figure 4 (bottom) indicate the dayside intervals of the satellite orbit.

4. Summary and Discussion

[11] MAP-PACE IMA on board SELENE has detected ions originating from the Moon. The detected ions include at least He+, C+, O+, Na+ and K+. In addition to the lunar Na and K exospheres which the ground-based observations found, the MAP-PACE IMA observation demonstrates the presence of the He, C and O exospheres. The TOF profile in Figure 3 suggests that IMA has also detected some ions in a mass range between Na+ and K+. Al+ that was possibly from the Moon was detected by WIND [Mall et al., 1998]. There is a possibility that the peak of K+ in the TOF profiles includes Ar+. The existence of Ar on the Moon was reported by Hoffman et al. [1973].

[12] Although the secondary ions sputtered by the SW reflect the lunar surface composition, the TOF profile measured by MAP-PACE IMA disagrees with the mass profile estimated by Elphic et al. [1991]. That is probably because the peaks of He+, C+, O+, Na+ and K+ in the TOF profile are not due to the SW sputtering. Potter and Morgan [1994] proposed that the solar photon play a dominant role for the lunar Na and K exospheres. However, the SW sputtering produces the secondary ions of 103∼104 cm−2 s−1 [Elphic et al., 1991] that is detectable by MAP-PACE IMA. Because Al+ is emitted not by the solar photon but by the SW from the lunar surface [Elphic et al., 1991; Madey et al., 1998], the potential presence of Al+ in the TOF profile suggests the SW sputtering effect. We believe that the long-term data of MAP-PACE IMA enable the remote composition analysis of the lunar surface proposed by Managadze and Sagdeev [1988] and Elphic et al. [1991].

[13] The lunar Na and K exospheres have been studied by the ground-based observation. The disadvantage of the ground-based observation is that the observation time is limited and depends on the atmosphere condition. The ion measurement of MAP-PACE IMA onboard SELENE gives us a tool for continuous monitoring of the lunar exospheres including Na, K and other species and also enable us to investigate the source and loss mechanisms.

Acknowledgments

[14] The authors wish to express their sincere thanks to all the team members of MAP-PACE and MAP-LMAG for their great support in processing and analyzing the MAP data. The authors also wish to express their grateful thanks to all the system members of the SELENE project. The authors are also greatly indebted to Yasuo Arai of High Energy Accelerator Research Organization for providing TDC chips that were indispensable for the TOF measurement of IMA. SELENE-MAP-PACE sensors were manufactured by Mitaka Kohki Co. Ltd., Meisei Elec. Co., Hamamatsu Photonics K.K., and Kyocera Co.

Ancillary