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Mary MacDonaldAST330: The MoonYokota, S., Saito, Y., Asamura, K., Tanaka, T., Nishino, M. N., Tsunakawa, H., Shibuya, H., Matsushima, M., Shimizu, H., Takahashi, F., Fujimoto, M., Mukai, T., Terasawa, T. 2009, First direct detection of ions originating from the Moon by MAP-PACE IMA onboard SELENE (KAYUGA), Geophys. Res. Lett., 36, L11201.IntroductionMost people do not think of the Moon as having an atmosphere. However, studies have shown that the moon maintains a thin atmosphere which has been the subject of extensive study. Ground-based observations have detected the presence of K and Na in the tenuous atmosphere (Potter and Morgan, 1988). This paper presents the first in situ detection of ions in the Moon’s atmosphere thought to have originated from the surface of the Moon. The detections were made with the MAP-PACE Ion Mass Analyzer (IMA) onboard the SELENE lunar orbiter. The paper is brief, succinct discovery paper, written with a lot of emphasis on the instrumentation, which is ground-breaking. The authors do not delve very deeply into the complicated dynamics of the lunar atmosphere, but provide a useful result for those who want to.Previous Study of the Lunar AtmosphereExtensive study of the lunar atmosphere began after pressure gauges placed by the Apollo astronauts measured surface pressures that indicated an atmosphere of about 106 atoms cm-1 in density (Potter and Morgan 1988). Ground-based observations revealed the presence of K and Na in the atmosphere (Potter and Morgan 1988). However, given the likely dynamic nature of the lunar atmosphere, longer observation periods than ground-based observations can provide are needed. Because the lunar atmosphere is so sparse, interactions between ions and molecules are rare and each species can be considered a separate, independent exosphere. Stern (1999) points out that ionosphere (ion component of the lunar atmosphere) is highly directional, and is constrained by the directions of the solar wind and nearby magnetic fields. This directional factor presents another reason that in situ measurements are necessary.InstrumentsThe MAP-PACE (MAgnetic field and Plasma experiment – Plasma energy Angle and Composition Experiment) instrument aboard SELENE (KAGUYA) consists of four sensors: ESA (Electron Spectrum Analyzer) –S1 and –S2, which measure electrons, IEA (Ion Energy Analyzer), and IMA (Ion Mass Analyzer), both of which measure ions. One of the most important aspects of this instrument is its orientation. SELENE is a polar orbiter. IMA and IEA are located on opposite sides of the spacecraft such that IMA is facing the Moon at all times and IEA is facing away from the Moon at all times. The orientation of the orbit is such IMA only detects solar wind ions coming directly from the Sun around the day-night terminator. At all other points in the orbit, IMA detects only ions coming from the Moon. ObservationsYokota et al. report that on the daytime side of the Moon, IEA, which is facing away Moon, detects ions of about 2 keV (from the solar wind), and detects no flux on the nighttime side. IMA, which is facing the surface of the Moon, detects solar wind ions of ~2 keV around the terminator, and detects a smear of ions of other energies at other points in its orbit. Themajority of the other detections made by IMA are ions of < 1 keV detected during the first half of the dayside orbits. The Mass SpectrometerThe ions of detected by IMA were also measured with IMA’s mass spectrometer, which is a time-of-flight (TOF) mass spectrometer. According to Yokota et al. (2005), the TOF method used in this instrument involves inducing an electric field in the cylindrically-shaped IMA instrument which accelerates the particles. Consequently, the equation of motion for a charged particle in the cylinder is given by:zqCzqEzm0)(−==which is the equation of motion for a simple harmonic oscillator. Therefore the motion of the particle can be modeled by:.)sin(02mqCtAz=+=ωϕωFrom this expression for frequency, an expression for the period of motion (the time-of-flight) that depends only on mass can be derived:qCmT0π=.In the case of this mass spectrometer, the period is equal to twice the time of flight (Yokota et al. 2005). Therefore the time of flight is directly proportional to the mass of the particle, and the TOF profile of a detection corresponds to its mass profile. The majority of detections are high-energy H and He ions, due to the solar wind. The authors choose to present their findings in a sort of raw-count histogram which includes these SW ions. This is a poor choice of representation because the counts for the lower energy (non-solar wind) ions are not properly displayed. The use of graphical rather than quantitative representations throughout this paper leaves the reader with a lack of context in which to consider the results. Thankfully, for the most important results of the paper, the TOF profiles, only the low-energy (non-solar wind) detections are presented. Along with previously-known ions of Na+ and K+, this paper reports on detections of He+, C+, and O+. The He+ is of a low energy and therefore most likely not a solar wind contribution.A Lunar Surface OriginThe authors proceed to present evidence that the low-energy ion detections are of lunar surface origin. The ions at the surface and atmosphere are most likely a few eV, and would need to be accelerated through an electric field of a given magnitude in order to obtain the energies of several hundred eV detected by IMA. Conveniently, the SELENE spacecraft is equipped with the instruments necessary to measure both the magnetic field of the Moon and the velocity of the solar wind. These two quantities allow for the calculation of the vector electric field between the Moon and the spacecraft. The calculated magnitude of the electric field is within the range of a few millivolts/m that would cause surface and atmospheric ions to be accelerated from a few eV to several hundred eV. Furthermore, the z-component of the electric field is positive, non-zero.Because of the orientation of the spacecraft, this means that the electric field is always directed northward. This explains the clustering of detections in the first-half of the day-side orbits (which covers the northern hemisphere). The electric field provides a mechanism by which ions can be transported from the exosphere to the detectors aboard the spacecraft. However, the


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Mt Holyoke AST 330 - The Moon

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