SH51B-0271 0800h
Measuring Pickup Ions to Characterize the Lunar Surface and Exosphere
It has been known for some time that measurement of the ion components, born from neutral exospheres imbedded in the solar wind, can be used to determine the composition and structure of the parent neutral exospheres (Hartle et al., 1973, Hartle and Thomas, 1974, Luhmann, 1996). The ion pickup process has been observed and verified for more than two decades, including pickup ions born from cometary comas, exospheres of Venus, Mars and Titan, and interstellar gases. Several observations (Mall, et al., 1998 and Hilchenbach et al., 1992) of lunar pickup ions have been reported from passing spacecraft including observations of metallic elements that were presumably sputtered from the lunar surface. The ions so formed, primarily by photoionization, electron impact and charge exchange, are picked up and accelerated by the motional electric field $E = -V \times B$, where $V$ is the plasma bulk velocity and $B$ the magnetic field. The unique orbital characteristics of pickup ions make it possible to infer important details about their sources. For a given ion mass, energy, and incoming direction, the ion trajectory can be mapped back to a point where the velocity vanishes at the cusp of a cycloid. When the gyroradius is much greater than the neutral source scale height (most cases), this cusp point is, it can be safely assumed, the source point. This also requires that the source velocity is much less than pickup acceleration integrated from the source point to spacecraft (again, true in most cases). This makes it possible to derive the neutral exosphere density at that point, assuming the ionization rate is known. When this measurement scheme is carried out on numerous orbits of a mission, it will be possible to derive neutral exosphere densities of all those species whose pickup ions can be measured. With the exception of H$+$, ion gyroradii are much greater than their source gas scale heights for typical solar wind conditions. Then, for a given ion mass, a spectrometer in lunar orbit will measure ions produced in the exosphere at increasing distances from the spacecraft as it sweeps from low to high energies, until the surface source is reached. Then, sweeping to even higher energies, the spectrometer will measure exospheric ions from greater distances until the ion flux becomes undetectable as the source point passes through more than one neutral scale height. As will be shown, there are several advantages to this approach, including the ability to determine the neutral exosphere below the spacecraft orbit (from the surface to the orbit) instead of just along the spacecraft track and the ability to map surface mineralogy and composition through sputtered ions.
SH51B-0272 0800h
Apollo 12 Glass Spherule Ages and the Meteoroid Bombardment History of the Moon
With 5 g of soil collected by the Apollo 12 astronauts, we are continuing our study of the meteoroid bombardment history of the Moon and the inner solar system. The Moon, by virtue of its lack of water and air, preserves evidence of ancient meteoroid impacts that would quickly be eroded away or otherwise obscured on Earth. We use the $^{40}$Ar/$^{39}$Ar isochron technique to determine the ages of individual glass spherules which are produced in meteoroid impacts on the Moon. The spherules in our study are $\sim$200 microns in diameter, and typically contain $\sim$100 fmol of radiogenic $^{40}$Ar. In Culler et al. (Science 287, p. 1785, 2000), we reported the ages of spherules from an Apollo 14 soil sample. The distribution of spherule ages from a well-mixed sample of the lunar regolith is a proxy for the impactor flux to the Moon since the end of mare basalt volcanism, over 3 billion years ago. The surprising abundance of spherules with young ages that we found in the Apollo 14 soil suggests that the meteoroid bombardment rate in the inner solar system increased suddenly, by a factor of approximately 4, about 400 million years ago. We are testing this hypothesis by examining spherules from a second lunar location; our Apollo 12 samples were collected 180 km away from the Apollo 14 spherules studied by Culler et al. (2000). We have so far analyzed over 100 spherules from Apollo 12 sample 12023. Geochemical analyses of each spherule indicate that the vast majority were formed in impacts, rather than in volcanic eruptions. Preliminary indications are that we are confirming an overabundance of spherules with ages younger than 400 million years. We will present final, calibrated geochronological results for over 150 spherules at the conference. Observing an apparent increase in spherule production at two widely spaced locations on the Moon favors the hypothesis of increased meteoroid bombardment, and disfavors the possibility that special geological circumstances at particular lunar locations led to an increase in spherule production or preservation 400 million years ago. Understanding this meteoroid flux increase will be an interesting future direction in planetary science.
http://jlevine.lbl.gov
SH51B-0273 0800h
Low Energetic Neutral Atom (LENA) imaging of the Moon
Since the Moon possesses neither global magnetic field nor atmosphere, the solar wind can directly precipitate on its surface causing atomic sputtering. The sputtered low energetic neutral atoms(LENAs) can be used to image the precipitation area and characterize the sputtering process remotely. Imaging the sputtered LENAs with sufficiently high mass resolution reveals (1) the moon surface elemental composition, (2) the solar wind surface interaction pattern, and (3) one of the most significant sources of the Moon's exosphere. The relative yield of different elements reflects the surface elemental composition, then the LENA images can be converted to the surface composition maps. The Lunar surface magnetic anomalies could stand off the solar wind locally. The solar wind flux cannot reach the surface in these areas and hence the LENA flux disappears. We have calculated the LENA flux from sunlit hemisphere, which reveals that the anomalous field can be clearly seen on LENA images. The strength of the magnetic anomaly can also be estimated from the shape of the ENA flux voids on the image by assuming the pressure balance condition. Since the solar wind is a supersonic flow and has a finite thermal velocity, it can also reach areas not accessible by solar photons such as permanently shadowed craters and areas beyond the limb. We have calculated expected sputtered LENA flux from the dark side of the moon in two different models. We first treated the solar wind flow into the shadowed region parallel to the magnetic field considering the ambipolar diffusion. We then considered the finite-gyroradius effect of individual solar wind protons by using the particle concept. The calculation revealed that the sputtered ENAs can be imaged and will provide valuable information on the unlit region.
SH51B-0274 0800h
Structure of the Moon's Wake-Tail under different IMF conditions: Hybrid simulations
We have studied the structure and properties of the Moon's wake by means of a two dimensional (global) hybrid simulation (particle ions, fluid electrons). Three different interplanetary magnetic field (IMF) configurations have been examined when the angle between the direction of the solar wind flow and the IMF is 0, 45, and 90 degrees. The Moon acts as a diamagnetic obstacle removing plasma from the solar wind flow. The tail refilling process on the moon's nightside can be described by plasma expansion into a vacuum driven by the thermal motion of particles along the IMF magnetic field lines. We examine properties of the wake-tail formed behind the obstacle embedded in the solar wind flow up to 50 Moon radii using up to 200 million particles in each simulation run with a realistic size for the Moon's body. Results of our study suggest that kinetic processes drive the physical processes in the Moon's wake-tail and their understanding is beyond the ideal MHD description. The orientation of the IMF field influences the structure of the Moon's wake. Counterstreaming plasma in Moon's downstream tail represent an unstable plasma configuration which excites different wave modes whose nature depends on the structure of the tail (i.e. on the orientation of the IMF). We shall provide analysis of these waves for the three cases studied thus far. A comparison of the simulation results with Wind satellite Moon flyby data and Lunar Prospector orbiter data will be carried out wherever possible.
SH51B-0275 0800h
Impact of Lunar Dust on the Exploration Initiative
From the Apollo era it is known that dust grains on the Moon can cause serious problems for exploration activities. Such problems include adhering to clothing and equipment, reducing external visibility on landings, and causing difficulty to breathing and vision within the spacecraft. An important step in dealing with dust-related problems is to understand how dust grains behave in the lunar environment. The electrostatic charging of dust grains and the lunar surface by interaction with the local plasma environment and the photoemission of electrons due to solar UV and X-rays is a likely cause of some of these problems. It can also act as a mechanism for transporting dust away from the surface, as the like-charged surface and dust act to repel each other. There is much evidence to suggest that sunlight scattered at the terminators, as observed by surface landers and astronauts in orbit, is caused by "dust clouds" composed of $\sim$0.1 \mum grains at altitudes of $\sim$1-100 km. In order to explain these observations, we propose a dynamic "fountain" model in which charged dust grains follow ballistic trajectories, subsequent to being accelerated upwards through a narrow sheath region by the surface electric field. These dust grains could affect the optical quality of the lunar environment for astronomical observations and interfere with exploration activities.
SH51B-0276 0800h
Low Energy Charged Particle Measurement by Japanese Lunar Orbiter SELENE
SELENE (SELenological and Engineering satellite) is a Japanese lunar orbiter that will be launched in 2006. The main purpose of this satellite is to study the origin and evolution of the moon by means of global mapping of element abundances, mineralogical composition, and surface geographical mapping from 100km altitude. PACE (Plasma energy Angle and Composition Experiment) is one of the scientific instruments onboard the SELENE satellite. PACE consists of 4 sensors: ESA (Electron Spectrum Analyzer)-S1, ESA-S2, IMA (Ion Mass Analyzer), and IEA (Ion Energy Analyzer). ESA-S1 and S2 measure three-dimensional distribution function of low energy electrons below 17keV. ESA basically employs a method of a top hat electrostatic analyzer with angular scanning deflectors at the entrance and toroidal electrodes inside. IMA and IEA measure the three-dimensional distribution function of low energy ions below 28keV/q. IMA has an ability to discriminate the ion mass with high mass resolution. IMA consists of an energy analyzer that is basically the same as ESA and an LEF (Linear Electric Field) TOF (Time Of Flight) ion mass analyzer. IEA consists of only an energy analyzer that is the same as the energy analyzer of IMA. Each sensor has hemi-spherical field of view (FOV). With two pairs of sensors ESA-S1 & IMA, and ESA-S2 & IEA, which are installed on the +Z and -Z surface of the spacecraft, three-dimensional distribution function of low energy electrons and ions are observed. The scientific objectives of PACE are 1) to measure the ions sputtered from the lunar surface and the lunar atmosphere, 2) to measure the magnetic anomaly on the lunar surface using two ESAs and a magnetometer onboard SELENE simultaneously as an electron reflectometer, 3) to resolve the moon - solar wind interaction, 4) to resolve the moon - Earth's magnetosphere interaction, and 5) to observe the Earth's magnetotail. Sputtered ions from the lunar surface will be measured for the first time. Recently, ground-based observations have revealed the existence of tenuous alkali-atmosphere around the moon. The rarefied atmosphere is thought to be produced mainly by solar photons and the solar wind. Sputtering by the solar wind, that is one of the source mechanisms of the tenuous atmosphere, presumably produces the secondary ions reflecting the composition of the lunar surface. In-situ measurements of low energy ions around the moon will provide us with fruitful information on the lunar surface and atmosphere.
SH51B-0277 0800h
Large Negative Lunar Surface Potentials
Previous observations by Lunar Prospector (LP) revealed the presence of monoenergetic field-aligned upward-going electron beams of moderate energies ($<$100 eV) coming from the lunar night side, both in the solar wind wake and in the magnetotail. Simultaneous observations of energy-dependent loss cones established that these beams likely consisted of secondary electrons accelerated from the lunar surface by an electrostatic potential. Due to the moderate potentials implied and the rather large Debye lengths (and therefore electric field scale heights) of $\sim$100-1000 m in these low-density regions, these potentials were not considered hazardous. A recent re-examination of LP data, however, has shown that, in more energetic plasma environments like the magnetotail plasmasheet, upward-going electron beams sometimes reach energies as high as 1-2 keV. These more energetic beams are commonly seen above the lunar night side and (though less frequently) sometimes even forward of the terminator, implying that plasmasheet currents are sometimes high enough to charge the surface negative despite photoemission. Higher plasma densities in the plasmasheet mean Debye lengths and therefore scale heights as small as $\sim$1-20 m, which along with the large potentials implied, could lead to electric fields as high as $\sim$100-1000 V/m. These fields are large enough to present a serious hazard to astronauts and equipment on the lunar surface. Though we are unfortunately limited by the low time and therefore spatial resolution of the LP measurements, I present here a first-cut characterization of the global distribution of large negative lunar surface potentials.