Sodium atoms in the lunar exotail: Observed velocity and spatial distributions

The lunar sodium tail extends long distances due to radiation pressure on sodium atoms in the lunar exosphere. Our earlier observations measured the average radial velocity of sodium atoms moving down the lunar tail beyond Earth (i.e., near the anti-lunar point) to be ～12.5 km/s. Here we use the Wisconsin H-alpha Mapper to obtain the first kinematically resolved maps of the intensity and velocity distribution of this emission over a 15° × 15 ° region on the sky near the anti-lunar point. We present both spatially and spectrally resolved observations obtained over four nights bracketing new Moon in October 2007. The spatial distribution of the sodium atoms is elongated along the ecliptic with the location of the peak intensity drifting 3° east along the ecliptic per night. Preliminary modeling results suggest the spatial and velocity distributions in the sodium exotail are sensitive to the near surface lunar sodium velocity distribution. Future observations of this sort along with detailed modeling offer new opportunities to describe the time history of lunar surface sputtering over several days.     

Orbital mapping of the lunar magnetic field

Magnetometer data obtained during the first four lunations after the deployment of the Apollo 15 subsatellite have been used to construct contour maps of the lunar magnetic field referred to 100 km altitude. These contour maps cover a relatively small band on the lunar surface. Within the region covered there is a marked near side-far side asymmetry. The near-side field is generally weaker and less structured than the far-side field. The strongest intrinsic lunar magnetic field detected is between the craters Van de Graaff and Aitken, centered at 20°S and 172°E. The variation in field strength with altitude for this feature suggests that its scale size is on the order of 80 km. A magnetization contrast between this region and its surroundings of the order of 6 × 10^–5 emu-cm^–3 is obtained assuming a 10-km thick slab. Preliminary Apollo 16 magnetometer data at extremely low altitude (0 to 10 km) show a very structured magnetic field with field strengths up to 56. Large compressions in the magnetic field magnitude, just above the lunar limb regions, are occasionally detected when the Moon is in the solar wind. The occurrence of limb compressions is strongly dependent on the selenographic coordinates of the lunar region on the solar wind terminator beneath the orbit of the sub-satellite. The discovery of remanent magnetization of varying strength over much of the lunar surface and its correlation with limb compression source regions supports the hypothesis that limb compressions are due to the deflection of the solar wind by regions of strong magnetization at the lunar limbs. If this hypothesis is correct, then the map of lunar regions associated with compressions indicates that the northerly equatorial region on the far side is less strongly magnetized than the southerly equatorial region on the far side.Paper dedicated to Professor Harold C. Urey on the occasion of his 80th birthday on 29 April, 1973.     

Dry separation of respirable lunar dust: Providing samples for the lunar airborne dust toxicity advisory group

In order to study the toxicity of lunar dust, the respirable size fraction of lunar soil needed to be separated with an apparatus that possesses the following capabilities: no use of liquid; fully recoverable sample; and use of only small sample quantities (<1 gm). We report the design of a simple apparatus that meets these requirements and implements an inertial-impaction mechanism established in aerosol science. Lunar soil was agitated at a frequency of 100 Hz using a vibration table with a containment chamber under a constant air flow. The air flow carried the lofted lunar dust particles past four impactors (four T-junction connectors), upon which a fraction of large particles were captured during the impaction. The fine particles in the air flow were then collected by an end-of-the line membrane filter. Detailed examination of particles on the filter showed that the majority (∼80-90 wt%) are <3 μm (geometric diameter), suggesting a high level of effectiveness for the apparatus.     

Precession of the lunar core

Goldreich (Goldreich, P. [1967]. J. Geophys. Res. 72, 3135) showed that a lunar core of low viscosity would not precess with the mantle. We show that this is also the case for much of lunar history. But when the Moon was close to the Earth, the Moon’s core was forced to follow closely the precessing mantle, in that the rotation axis of the core remained nearly aligned with the symmetry axis of the mantle. The transition from locked to unlocked core precession occurred between 26.0 and 29.0 Earth radii, thus it is likely that the lunar core did not follow the mantle during the Cassini transition. Dwyer and Stevenson (Dwyer, C.A., Stevenson, D.J. [2005]. An Early Nutation-Driven Lunar Dynamo. AGU Fall Meeting Abstracts GP42A-06) suggested that the lunar dynamo needs mechanical stirring to power it. The stirring is caused by the lack of locked precession of the lunar core. So, we do not expect a lunar dynamo powered by mechanical stirring when the Moon was closer to the Earth than 26.0-29.0 Earth radii. A lunar dynamo powered by mechanical stirring might have been strongest near the Cassini transition.     

Echo simulation of lunar penetrating radar: based on a model of inhomogeneous multilayer lunar regolith structure

Lunar Penetrating Radar(LPR) based on the time domain Ultra-Wideband(UWB) technique onboard China's Chang'e-3(CE-3) rover, has the goal of investigating the lunar subsurface structure and detecting the depth of lunar regolith. An inhomogeneous multi-layer microwave transfer inverse-model is established. The dielectric constant of the lunar regolith, the velocity of propagation, the reflection, refraction and transmission at interfaces, and the resolution are discussed. The model is further used to numerically simulate and analyze temporal variations in the echo obtained from the LPR attached on CE-3's rover, to reveal the location and structure of lunar regolith. The thickness of the lunar regolith is calculated by a comparison between the simulated radar B-scan images based on the model and the detected result taken from the CE-3 lunar mission. The potential scientific return from LPR echoes taken from the landing region is also discussed.     

Seismoacoustic lunar fields and lunar electromagnetic (nonthermal) emission

For better insight into lunar radio emissions, observations of the Moon were made during the maximal Geminids meteor shower and during the lunar eclipse without external effects. Statistical processing of the obtained data was carried out. It was found that the lunar endogenous and exogenous processes are displayed in both the seismic-emission fields and lunar nonthermal electromagnetic emissions. Both types of signals demonstrate good correlation. The seismic and electromagnetic emission processes have common periodicities, some of which determine the internal structure of the Moon. Similar regularities are expected for other bodies of the Solar System.     

Central peaks in lunar craters

Central peak features in 580 craters on the lunar near side were systematically studied, and detailed studies were made of several features of interest, in an attempt to define the mechanism of central peak formation. The peaks tend to be elongated along the preferred directions of the lunar grid. A weak correlation exists between peak size and crater size. Some peaks appear to have been formed or modified by volcanic processes, though strictly mechanical processes are known to produce central peaks on Earth. A more detailed knowledge of impact mechanics is required to account for the observations of the lunar central peaks.     

The lunar disturbance effect

Apollo photographs indicate that the lunar disturbance effect is due to changes in the photometric function of the lunar soil caused by rearrangement of the soil particles, rather than to any physical or chemical differences between the uppermost layer and the underlying materials.Paper presented to the NATO Advanced Study Institute on Lunar Studies, Patras, Greece, September 1971.     

Lunar dust and lunar simulant activation and monitoring

Abstract— NASA plans to resume human exploration of the Moon in the next decade. One of the pressing concerns is the effect that lunar dust (the fraction of the lunar regolith <20 μm in diameter) will have on systems, both human and mechanical, due to the fact that various problems were caused by dust during the Apollo missions. The loss of vacuum integrity in the lunar sample containers during the Apollo era ensured that the present lunar samples are not in the same condition as they were on the Moon; they have been passivated by oxygen and water vapor. To mitigate the harmful effects of lunar dust on humans, methods of “reactivating” the dust must be developed for experimentation, and, ideally, it should be possible to monitor the level of activity to determine methods of deactivating the dust in future lunar habitats. Here we present results demonstrating that simple grinding, as a simple analog to micrometeorite crushing, is apable of substantially activating lunar dust and lunar simulant, and it is possible to determine the level of chemical activity by monitoring the ability of the dust to produce hydroxyl radicals in aqueous solution. Comparisons between ground samples of lunar dust, lunar simulant, and quartz reveal that ground lunar dust is capable of producing over three times the amount of hydroxyl radicals as lunar simulant and an order of magnitude more than ground quartz.     

Rotation of the moon and lunar coordinate systems

The need for precise definition of lunar reference systems is stressed and the principles on which systems of lunar coordinates could be based are established. Differences between coordinate systems defined by the dynamical properties of the lunar configuration and the rotational motion of the lunar globe about its centre of gravity are outlined, and rigorous mathematical formulae relating those systems have been developed. The principles of reduction of measurements are outlined and in the Appendix the absolute coordinates obtained for 700 lunar features are presented.Paper presented to the NATO Advanced Study Institute on Lunar Studies, Patras, Greece, September 1971.     

Possible lunar ring dikes

Terrestrial ring dike structures are features consisting of one or more series of concentric fracture systems along which the central block often subsided and up through which lavas intruded and extruded and other volcanic features formed. Before the lunar probe satellites, a search for lunar features that showed characteristics of terrestrial ring dikes was conducted using the LAC charts andKuiper Atlas photographs. More recently the search was extended on the nearside features and to the farside features using the Lunar Orbiter series of photographs resulting in a catalog of 559 nearside candidates and 82 farside. Features exhibiting one or more of the following four criteria were included as lunar analogs to terrestrial ring dikes: (1) inner ridge(s) approximately concentric with the crater wall, (2) inner rill(s) approximately concentric with the crater wall, (3) outer ridge(s) and/or rill(s) approximately concentric with the crater wall, and (4) interior and exterior slopes of the crater wall approximately equal (implying extrusion of lava along a ring fracture). Equal slopes are in contradistinction to a central source eruptive feature or an impact feature both of which usually produce craters with walls whose inner slopes are about twice as steep as their outer flanks, which characterize the vast majority of lunar craters. Features exhibiting each of the four criteria were found and some had combinations of two or more including rills merging into ridges, e.g., in Taruntius and Posidonius. Gambart is an example of equal inner and outer slopes, while Hesiodus A and Marth are two of the best examples of complete inner rings concentric with the outer rings. Ten percent of the candidates were probable impact craters but had subsequent volvanic activity of a ring dike nature. The initial search showed a distribution of the possible lunar ring dikes that was non-random and strongly associated with the margins of the maria, further implying that they are volcanic features. This relation was upheld when extended by the recent survey. The anticipated dearth of farside ring dikes was corroborated in our study and their distribution is restricted to those few mare-like areas on the farside, further supporting the volcanic nature of these features     

The mapping of lunar radar scattering characteristics

This is the first of four articles describing a comprehensive series of radar maps of the entire visible lunar hemisphere carried out at wavelengths of 3.8 and 70 cm and analyzing the echoes in both orthogonal senses of circular polarization. In this paper, the basic techniques of delay-Doppler mapping by radar are developed, and the particular steps employed in mapping the Moon are outlined. Succeeding articles present the results obtained and discuss the way in which these results relate to other, nonradar measurements as well as to the actual lunar surface properties.     

The lunar seismogram

The unexpected and unusual characteristics of the lunar seismogram have given rise to various speculations regarding their origin: secondary ejecta, diffusive wave propagation and wave propagation effects in a self-compacted powder layer with a linearly increasing velocity with depth. Many of the characteristics can be explained, qualitatively, by the simple theory of a self-compacting, dry powder layer for which the velocity varies as the sixth root of the depth. This gives a very low seismic velocity at the lunar surface which, in turn, allows the signal to have a long duration, a lack of correlation between horizontal and vertical displacements, a signal envelope that changes with source to receiver separation and a varying spectrum over the duration of the signal. To explain the long duration of the seismic signal quantitatively, it is necessary to include scattering of the normally incident rays at the surface by shallow surface undulations. The sixth root velocity-depth dependence is consistent with the measured variation, with pressure, of the compressibility and velocity of lunar samples.     

Regolith history of lunar meteorites

Abstract— The regolith evolution of the lunar meteorites Dhofar (Dho) 081, Northwest Africa (NWA) 032, NWA 482, NWA 773, Sayh al Uhaymir (SaU) 169, and Yamato (Y‐) 981031 was investigated by measuring the light noble gases He, Ne, and Ar. The presence of trapped solar neon in Dho 081, NWA 773, and Y‐981031 indicates an exposure at the lunar surface. A neon three‐isotope diagram for lunar meteorites yields an average solar ²⁰Ne/²²Ne ratio of 12.48 ± 0.07 representing a mixture of solar energetic particles neon at a ratio of 11.2 and solar wind neon at a ratio of 13.8. Based on the production rate ratio of ²¹Ne and ³⁸Ar, the shielding depth in the lunar regolith of NWA 032, NWA 482, SaU 169, and Y‐981031 was obtained. The shielding depth of these samples was between 10.5 g/cm² and >500 g/cm². Based on spallogenic Kr and Xe, the shielding depth of Dho 081 was estimated to be most likely between 120 and 180 g/cm². Assuming a mean density of the lunar regolith of 1.8 g/cm³, 10.5 g/cm² corresponds to a depth of 5.8 cm and 500 g/cm² to 280 cm below the lunar surface. The range of regolith residence time observed in this study is 100 Ma up to 2070 Ma.     

The lunar dust environment

Each year the Moon is bombarded by about 10⁶ kg of interplanetary micrometeoroids of cometary and asteroidal origin. Most of these projectiles range from 10 nm to about 1 mm in size and impact the Moon at 10–72 km/s speed. They excavate lunar soil about 1000 times their own mass. These impacts leave a crater record on the surface from which the micrometeoroid size distribution has been deciphered. Much of the excavated mass returns to the lunar surface and blankets the lunar crust with a highly pulverized and “impact gardened” regolith of about 10 m thickness. Micron and sub-micron sized secondary particles that are ejected at speeds up to the escape speed of 2300 m/s form a perpetual dust cloud around the Moon and, upon re-impact, leave a record in the microcrater distribution. Such tenuous clouds have been observed by the Galileo spacecraft around all lunar-sized Galilean satellites at Jupiter. The highly sensitive Lunar Dust Experiment (LDEX) onboard the LADEE mission will shed new light on the lunar dust environment. LADEE is expected to be launched in early 2013.Another dust related phenomenon is the possible electrostatic mobilization of lunar dust. Images taken by the television cameras on Surveyors 5, 6, and 7 showed a distinct glow just above the lunar horizon referred to as horizon glow (HG). This light was interpreted to be forward-scattered sunlight from a cloud of dust particles above the surface near the terminator. A photometer onboard the Lunokhod-2 rover also reported excess brightness, most likely due to HG. From the lunar orbit during sunrise the Apollo astronauts reported bright streamers high above the lunar surface, which were interpreted as dust phenomena. The Lunar Ejecta and Meteorites (LEAM) Experiment was deployed on the lunar surface by the Apollo 17 astronauts in order to characterize the lunar dust environment. Instead of the expected low impact rate from interplanetary and interstellar dust, LEAM registered hundreds of signals associated with the passage of the terminator, which swamped any signature of primary impactors of interplanetary origin. It was suggested that the LEAM events are consistent with the sunrise/sunset-triggered levitation and transport of charged lunar dust particles. Currently no theoretical model explains the formation of a dust cloud above the lunar surface but recent laboratory experiments indicate that the interaction of dust on the lunar surface with solar UV and plasma is more complex than previously thought.     

Star-Calibrated lunar photography

Accuracy in the determination of the absolute coordinates of lunar features or the physical libration of the Moon can be greatly improved by a technique providing lunar photographs on starcalibrated plates. An outline of the photographic technique and the computer programme for the reduction of the plates is given. In the Appendix the technical details of the double slide plate carrier which, adjusted to the Data Corporation Camera, secures high quality stellar images, are presented.Work supported in part by Contract F61052-68-C-0002 between Air Force Cambridge Research Laboratories (OAR), U.S. Air Force and the Department of Astronomy, University of Manchester.     

Characterization of lunar dust and a synopsis of available lunar simulants

Understanding the formation and evolution of the soil and dust of the Moon addresses the fundamental question of the interactions of space with the surface of an airless body. The physical and chemical properties of the lunar dust, the <20 μm portion of lunar soil, are key properties necessary for studies of the toxicity and the electrostatic charging of the dust. These properties have been largely overlooked until recent years. Although chemical and physical studies of the <20 μm portion of lunar soil have been the topic of several studies, there is still need for further studies, primarily of the <1 μm particles. This paper presents a review of the studies of lunar dust that have been conducted to date. As many preparations for future exploration or science activities on the Moon require testing using lunar soil/dust simulants, we also include a brief review of past and current simulants.     

Detectability of lunar X-rays

We have examined several theories that imply the generation of X-rays by the Moon. The X-ray fluxes to be expected at the top of the Earth's atmosphere are estimated and compared. For example, we find that an X-ray flux is to be expected when the Moon is full and K_p high, as a consequence of the configuration of the auroral electrons in space deduced from the long tail model of the magnetosphere. The X-ray photons are caused by energetic electrons in the tail that bombard the lunar surface. Alternatively, Gold has suggested that lunar X-rays are produced by the bombardment by solar-wind electrons; this results in a lunar phase dependence that is different from the long tail model. The background is discussed and we conclude that the lunar X-ray flux may be detectable. Experiments of this kind may provide useful tools for investigating the models.     

Moonquakes and lunar tectonism

With the succesful installation of a geophysical station at Hadley Rille, on July 31, 1971, on the Apollo 15 mission, and the continued operation of stations 12 and 14 approximately 1100 km SW, the Apollo program for the first time achieved a network of seismic stations on the lunar surface. A network of at least three stations is essential for the location of natural events on the Moon. Thus, the establishment of this network was one of the most important milestones in the geophysical exploration of the Moon. The major discoveries that have resulted to date from the analysis of seismic data from this network can be summarized as follows:

1. Lunar seismic signals differ greatly from typical terrestrial seismic signals. It now appears that this can be explained almost entirely by the presence of a thin dry, heterogeneous layer which blankets the Moon to a probable depth of few km with a maximum possible depth of about 20 km. Seismic waves are highly scattered in this zone. Seismic wave propagation within the lunar interior, below the scattering zone, is highly efficient. As a result, it is probable that meteoroid impact signals are being received from the entire lunar surface.
2. The Moon possesses a crust and a mantle, at least in the region of the Apollo 12 and 14 stations. The thickness of the crust is between 55 and 70 km and may consist of two layers. The contrast in elastic properties of the rocks which comprise these major structural units is at least as great as that which exists between the crust and mantle of the earth. (See Toks?zet al., p. 490, for further discussion of seismic evidence of a lunar crust.)
3. Natural lunar events detected by the Apollo seismic network are moonquakes and meteoroid impacts. The average rate of release of seismic energy from moonquakes is far below that of the Earth. Although present data do not permit a completely unambiguous interpretation, the best solution obtainable places the most active moonquake focus at a depth of 800 km; slightly deeper than any known earthquake. These moonquakes occur in monthly cycles; triggered by lunar tides. There are at least 10 zones within which the repeating moonquakes originate.
4. In addition to the repeating moonquakes, moonquake ‘swarms’ have been discovered. During periods of swarm activity, events may occur as frequently as one event every two hours over intervals lasting several days. The source of these swarms is unknown at present. The occurrence of moonquake swarms also appears to be related to lunar tides; although, it is too soon to be certain of this point.

These findings have been discussed in eight previous papers (Lathamet al., 1969, 1970, 1971) The instrument has been described by Lathamet al. (1969) and Sutton and Latham (1964). The locations of the seismic stations are shown in Figure 1.     

Comparison and analysis on lunar rotation with lunar gravity field models

Understanding the structure of and dynamic processes in the deep interior of planets is crucial for understanding their origin and evolution. An effective way to constrain them is through observation of rotation and subsequent simulation. In this paper, a numerical model of the Moon’s rotation and orbital motion is developed based on previous studies and implemented independently. The Moon is modeled as an anelastic body with a liquid core. The equations of the rotation were nonlinear and the Euler angles are cross coupled. We solve them numerically via the Runge-Kutta-Fehlberg (RKF) and multi-steps Adams-Bashforth-Moulton (ABM) predictor-corrector numerical integration. We have found that adequate accuracy is maintained by taking twelve steps per day using eleventh differences in the integrating polynomial. The lunar orbital and rotational equations are strongly coupled, so we integrated the rotation and motion simultaneously. We refer to other planetary informations from the newest planetary and lunar ephemeris INPOP17a, which is reported had fitted the longest LLR (Lunar Laser Ranging) observation data. Using the model GL660B from GRAIL (Gravity Recovery and Interior Laboratory) mission, we firstly compare our numerical results with the INPOP17a to prove the reasonability of our model. After that we apply the lunar gravity model CEGM02 determined from Chang’E-1 mission and SGM100h from SELENE mission to our model, the difference between results from CEGM02 and GL660B are less than \(-0.20 \sim0.15\) arc-second, and \(-0.25 \sim0.20\) arc-second for GL660B and SGM100h. Compared to SGM100h, the results show that the low degree and order coefficients (less than 6 from this paper) of lunar gravity field were improved in CEGM02 as expected. It is the first time to demonstrate that these models can be applied to lunar rotation model. These results manifest that a development of the gravity field measure will help us to know the rotation motion more precisely.