P51D-01 INVITED
The Lunar Plasma and Dust Environment
The lunar surface is exposed to a variety of plasma conditions as a function of local time, solar activity, and orbital position. The wind, UV radiation, magnetospheric plasmas, and meteoroid impacts result in a complex, time-dependent environment, which creates a natural dusty plasma laboratory. The charging, possible subsequent mobilization, and transport of fine lunar dust have remained controversial subjects, and have been suggested to lead to the formation of a 'dusty exosphere', extending tens to hundreds of kilometers above the surface. The outstanding questions include: What are the parameters and variability of the near- surface plasma environment? What is the charge and electric field distribution on and above the surface? What is the microphysics of electrostatic dust mobilization? What is the role of micrometeoroid bombardment? It is crucial for future human/robotic exploration and for science utilization of the Moon to answer these questions. The dusty plasma group at the University of Colorado has over a decade of experience in theoretical and experimental studies including dust charging experiments, laboratory simulation of electrostatic dust transport, dust levitation, and theoretical modeling. The experimental facility is now being updated to include a strong UV source to generate a photoelectron sheath above a surface of lunar regolith analog material. The near-surface plasma is characterized using plasma-, electric field- and emissive probes. Multiple populations of electrons in the lunar plasma (photoelectrons emitted from the surface, solar wind electrons, etc.) require the modification of existing probe data analysis methods. Also under development are advanced instruments to measure the characteristics of lofted dust particles. We present here a summary of recent results and planned experiments in our dusty plasma program. [Plasma probes for the lunar surface, X. Wang, M. Horanyi and S. Robertson, J. Geophys. Res., 113, A08108, doi:10.1029/2008JA013187, 2008; A laboratory model of the lunar surface potential near boundaries between sunlit and shadowed regions, X. Wang, M. Horányi, Z. Sternovsky, S. Robertson, and G. E. Morfill, Geophys. Res. Lett., 34, L16104, doi:10.1029/2007GL030766, 2007; Variability of the lunar photoelectron sheath and dust mobility due to solar activity, Z. Sternovsky, P. Chamberlin, M. Horanyi, S. Robertson and X. Wang, , J. Geophys. Res. in print]
P51D-02 INVITED
Lunar plasma measurement by MAP-PACE onboard KAGUYA(SELENE)
Low energy charged particles around the Moon were observed by Moon orbiting satellites and plasma instrumentation placed on the lunar surface in 1960s and 1970s. Though there were some satellites that explored the Moon afterwards, most of them were dedicated to the global mapping of the lunar surface. There has been almost no new information about the low energy charged particles around the Moon except the low energy electron measurement by Lunar Prospector, the lunar wake plasma data obtained by WIND during its Moon fly-by, and reports on remote detection of the lunar ions, lunar electrons and ULF waves generated by electron beams around the lunar wake. MAP (MAgnetic field and Plasma experiment) was developed for the comprehensive measurement of the magnetic field and three-dimensional plasma around the Moon. MAP consists of MAP-LMAG (Lunar MAGnetometer) and MAP-PACE (Plasma energy Angle and Composition Experiment). MAP-PACE consists of 4 sensors: ESA (Electron Spectrum Analyzer)-S1, ESA-S2, IMA (Ion Mass Analyzer), and IEA (Ion Energy Analyzer). PACE ion sensors discovered new features of low energy ions around the Moon since MAP started continuous observation last December. The in-situ measurement of low energy ions around the Moon is realized almost three decades after the Apollo period. In addition, nobody has ever measured mass identified low energy ions around the Moon at 100km altitude. PACE-IMA has succeeded in the in-situ measurements of the lunar tenuous ionized atmosphere and has discovered the existence of alkali ions that are originated from the lunar surface or lunar atmosphere. PACE- IMA has also discovered low energy ions that might be related to a meteor shower. PACE ion sensors have found solar wind reflection on the lunar surface. Instead of being absorbed by the lunar surface, quite a large amount of solar wind ions are reflected back from the Moon. The reflected ions are accelerated above solar wind energy picked up by the solar wind motional electric field. Besides these observations, PACE ion sensors have observed the ion acceleration above a magnetic anomaly and stray ions from the solar wind detected deepest inside the wake at 100km above the midnight-equator. These are the newly discovered phenomena in which the proton reflection at the lunar surface is playing crucial roles. PACE electron sensors and LMAG are used as an electron reflectometer that detects magnetic anomalies on the lunar surface. Though the coverage of the magnetic anomaly detection is still limited, magnetic anomalies has been detected with high special resolution.
P51D-03
Lunar Potential Determination Using Apollo-Ear Data and Modern Measurements and Models
Since the Apollo era the electric potential of the Moon has been a subject of great interest and debate. Deployed on the surface by three Apollo missions (Apollo 12, 14 and 15), the Suprathermal Ion Detector Experiments (SIDE) were used to infer a sunlit surface potential of about +10 V from the energy spectra of lunar ionospheric thermal ions accelerated toward the Moon. More recently, electron distributions measured from orbit by the Lunar Prospector Electron Reflectometer (LP ER) have been used to infer negative lunar surface potentials, primarily in shadow. We will present initial results from a study to combine lunar surface potential measurements from both SIDE and LP ER to calibrate an advanced lunar surface charging model that includes electric currents from the plasma environment, photoelectron and secondary electron emission from the surface, as well as the effects of the wake formed downstream by the solar wind-lunar interaction.
P51D-04
Studying the Lunar Ionosphere with SELENE Radio Science Experiment
Lunar ionosphere is generally thought to be as thin as 1 cm-3; the process that will prevent the accumulation of newly produced ions near the lunar surface is the impingement of the solar wind magnetic field on the lunar surface, which induces an electric field that sweeps away ions. In harmony with this prediction, most of the radio occultation experiments performed with radio stars failed to detect the lunar ionosphere. Radio occultation experiments conducted with the Soviet Luna 19 and 22 spacecraft, on the other hand, detected large electron densities near the dayside lunar surface. Vyshlov (1974) obtained peak electron densities of 500--1000 cm-3 at heights of 5--10 km, with a gradual decrease at higher altitudes with a scale height of 10--30 km. The measured densities are difficult to explain theoretically, and thus the generation mechanism of the lunar ionosphere is a major issue, with even the validity of the previous observations still under debate. If a thick lunar ionosphere exists, possible mechanisms to maintain the ionized layer are the effect of the remnant magnetic field which stands off the solar wind magnetic field, certain processes that enhance the neutral gas concentration, or charged dust grains that are lifted up by the near-surface electric field. The electron density profiles above the lunar surface are being observed by radio occultation during the SELENE (KAGUYA) mission using sub-satellites. The systematic measurements will establish the morphology of the lunar ionosphere and reveal its dependence on various conditions, thereby providing clues to the generation mechanism. The S-band (2.2GHz) and X-band (8.5GHz) signals transmitted by the Vstar sub- satellite is received at the Usuda Deep Space Center in Japan. The most serious error source is the temporal variation in the terrestrial ionosphere during measurements. In the region where the contribution of the lunar ionosphere is virtually absent, i.e. at altitudes above ~100 km, a gradual variation caused by the terrestrial ionosphere is observed. This variation is extrapolated into the near-moon portion and subtracted from the observed one, thereby eliminating the influence of the terrestrial ionosphere to some extent. In addition to this method, we also use the Rstar sub- satellite, which transmits coherent two signals in S-band, to measure the terrestrial ionosphere during the lunar occultation of Vstar; the subtraction of the Rstar's measurement from the Vstar's measurement gives the lunar ionosphere. The opportunities of the latter method are rather limited, however. More than 100 measurements using Vstar and more than 10 measurements using Rstar and Vstar have been conducted during the first half of the mission. Although the error due to the fluctuation of the terrestrial ionosphere is rather significant, there seems to be a tendency that the electron density increases on the morning side of the moon.
P51D-05 INVITED
The dynamic lunar exosphere: clues from sodium
Of the known species of the lunar exosphere, sodium is the most extensively studied due to its favorable spectroscopic properties and it could be suggestive of the evolution of lunar volatiles. The distribution of sodium about the Moon is highly non-thermal, implying contributions from the energetic sources: photon- stimulated desorption, impact vaporization, and ion sputtering. We review recent findings about the interaction of the sodium surface reservoir with the variable solar UV flux, meteoritic influx and the ion flux of solar wind and magnetospheric origin. In particular, we detail the role of solar wind and terrestrial plasma sheet ions in controlling the content and morphology of exospheric sodium, including the effects of ion- enhanced grain diffusion. We correlate unpublished ground-based observations of the lunar sodium emission with simultaneous observations from plasma instruments aboard Lunar Prospector indicating the importance of interplanetary and magnetospheric conditions. Given the existing similarities between the lunar and Hermean exospheres, we discuss the implications of results from MESSENGER's first Mercury flyby leading to constraints on the sources of sodium in the lunar environment.
P51D-06
Lunar Horizon Glow: A Quantitative Indicator of Exospheric Dust
During the Apollo missions, horizon glow (HG) was observed by astronauts in lunar orbit just prior to orbital sunrise. These observations were further supported by excess brightness which appeared along the horizon in coronal photographs from Apollo 15 and 17. Horizon glow may also be present in star tracker measurements acquired during the Clementine mission, though it would be heavily masked by coronal and zodiacal light (CZL). The most likely cause of HG is thought to be forward scattering of sunlight by submicron dust grains in the lunar exosphere above the terminator, extending to 10's of km or higher in altitude. Such a dust population is thought to arise from charged lunar dust that has been electrostatically lofted from the surface, since strong surface electric fields are believed to exist at the terminator. Additional contributions to exospheric dust will arise from meteoritic ejecta. With many missions now returning to the Moon, it is important to be able to distinguish and quantify the observable sources of UV-VIS optical emission, specifically HG from lunar exospheric dust, CZL, and line emission from exospheric gases. We have developed a code which simulates 3D (2D spatial plus spectral) intensities of horizon glow arising from lunar exospheric dust, as it would be viewed from an orbiter in lunar shadow. The dust vertical profile used is the semi-empirical model proposed by Murphy and Vondrak. Dust scattering properties as a function of grain size are computed using Mie Theory. The code also incorporates CZL intensities as formulated by Hahn et al., as well as Na D-line emission as observed by Potter and Morgan, in order to contrast these three emission sources near the limb via their distinct spatial distributions, spectral intensities and dependence on solar elongation angle. We include a simulation of lunar HG, as it might be observed by the UV/Vis spectrometer aboard the Lunar Atmosphere and Dust Environment Explorer (LADEE).
P51D-07
LCROSS Impact Simulations and Predictions
The primary objective of the Lunar Crater Observation and Sensing Satellite (LCROSS) is to confirm the presence or absence of water ice that might have trapped out over time from the lunar exosphere into permanently shadowed inter-crater regolith near the lunar poles. It will provide a critical ground-truth for Lunar Prospector and LRO neutron and radar maps, making it possible to assess the total lunar water inventory and to provide significant insight into the processes that delivered hydrogen to the polar regions. Non-detection of water could lead to significant changes in the architecture of lunar operations and settlement. Ong and Asphaug (LPSC 2008) study the fraction of volatile material that remains bound to the Moon and Mars during comet and asteroid impacts and calculate a mass of water retained over the past 2 Ga of order 1E10-1E11 tons, a few times the water ice inferred by Feldman et al. (Science 1998) on the basis of Lunar Prospector neutron detection. This flux includes small contemporary events but is dominated by major discrete contributions in the past. Whatever the mechanism for the delivery and possible retention of lunar water, interest in the possible presence of water ice has both scientific and operational foundations. If water is present in the upper meters to the few percent level, LCROSS will find it by using a 2000 kg kinetic impactor -- the empty Atlas V Centaur upper stage -- to excavate more than 250 metric tons of regolith. The thermal and spectral signature of the impact flash and the crater ejecta that gets launched into sunlight will be studied in detail, and the results transmitted to Earth before the 700 kg shepherding spacecraft also impacts the Moon. These two impact experiments and their aftermaths will also be observed from a number of Lunar-orbital and Earth-based assets. For the purpose of mission planning, asset security, and scientific prediction, we have conducted a variety of calculations based upon several models covering different aspects of the event. Crater scaling laws are used to obtain fundamental estimates of crater diameter and ejecta mass. We also apply the RAGE adaptive mesh hydrocode to model the short-timescale (0.1 s) thermal plume that is expected to occur immediately after the impact. We also conduct a number of large scale (millions particle) smooth-particle hydrodynamics (SPH) calculations, which take into account impactor geometry and realistic terrain (e.g. hills or troughs). We also apply the ZEUSMP hydrocode to model crater formation and ejecta mass-velocity distribution. We have also developed two semi-analytic models, the first being a Monte Carlo model of the distribution of expected ejecta, based on scaling models using a plausible range of crater and ejecta parameters, and the second being a simple model of observational predictions for the shepherding spacecraft that will impact the lunar surface 4 minutes later. Results of these calculations will be presented. For the initial thermal plume we predict an initial expansion velocity of 7 km/s and a maximum temperature of 1200 K. Scaling laws for crater formation and the SPH calculation predict a crater with a diameter of 15 m, a total ejecta mass of 1000 tons, with 10 tons reaching an altitude of 2 km above the target. There is no risk of LCROSS ejecta impacting Moon-orbiting assets. Both the SPH and ZEUSMP calculations predict a maximum ejecta velocity of 1 km/s. The semi-analytic Monte Carlo calculations produce more conservative estimates (by a factor of 5) for ejecta reaching 2 km.
P51D-08 INVITED
The MOON micro-seismic noise : first estimates from meteorites flux simulations
The Moon is considered to be a seismically quiet planet and most of the time, the Apollo seismograms were flat when not quakes was occuring. We show in this paper that this might not be the case if more sensitive data are recorded by future instruments and that a permanent micro-seismic noise is existing due to the continuous impacts of meteorites. We perform a modeling of this noise by using, as calibrated seismic data, those generated by the impacts of the Apollo S4B or LEM, by taking care on the scaling law, necessary to express the seismic force with respect to the mass and velocity of the impactors. We also parametrize the dependence of the amplitude of the seismic coda, associated to the maximum amplitude of the seismograms, with respect to the epicentral distance and to the source geometry. This enabling us to use the seismic data of the S4B impacts as empirical waveforms for the modeling of the natural impacts. The frequency/size law of meteoroids impacting the Moon and the associated probability of NEO impacts are however not known precisely. Uncertainties as large as a factor of 3-5 remain, especially for the moderate-sized impacts which are not observed on the Earth, due to the shielding by the atmosphere. We therefore use several meteoroid mass/frequency laws from the literature to generate, with a random simulator, a history of impacts on the Moon during a given period. The seismic signals generated by succession of seismic sources and estimate the frequency/amplitude relationship of such seismic signals. Our results finally provide an estimate for the meteoritic seismic background on the Moon. This background noise was not recorded by the Apollo seismic experiment due insufficient resolution. Such an estimate can be used in designing a new generation of lunar seismometers, for estimating the probability of detecting proposed impacts due to nuggets of strange quark matter , and to inform future lunar based experiments, which require very stable ground, such as optical interferometry moon-based telescopes or gravity waves detectors.