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.     

Mercury and Moon He exospheres: Analysis and modeling

Helium is one of the first elements clearly identified in the lunar exosphere (Hoffman, J.H., Hodges, R.R., Johnson, F.S., Evans, D.E. [1973]. Proc. Lunar Sci. Conf. 3, 2865–2875). Apollo 17 measured the He density at the surface during four lunations. It confirmed the expected day to night asymmetry of the He exosphere with a maximum density near the dawn terminator on the nightside. Few years later, the first detection of Mercury’s He exosphere was successfully obtained by Mariner 10 (Broadfoot, A.L., Shemansky, D.E., Kumar, S. [1976]. Geophys. Res. Lett. 3, 577–580). These observations highlighted similar global distribution of the He exosphere at Mercury and at the Moon, but also significant differences that have never been convincingly explained.In this paper, we model the He exosphere at the Moon and Mercury with the same approach. The energy accommodation of the exospheric He particles interacting with the surface can be roughly constrained using Apollo 17 and Mariner 10 measurements. Neither a low energy accommodation, as suggested by Shemansky and Broadfoot (Shemansky, D.E., Broadfoot, A.L. [1977]. Rev. Geophys. 15, 491–499), nor a full energy accommodation, as suggested by Hodges (Hodges Jr., R.R. [1975]. The Moon, 14, 139–157), can fit all the observations. These observations and their modeling suggest a diurnal variation of the energy distribution of the He ejected from the surface that cannot be explained satisfactorily by any of the present theories on the gas–surface interaction in surface-bounded exospheres.     

The lunar exosphere: The sputtering contribution

We have extended our Monte Carlo model of exospheres [Wurz, P., Lammer, H., 2003. Icarus 164 (1), 1-13] by treating the ion-induced sputtering process from a known surface in a self-consistent way. The comparison of the calculated exospheric densities with experimental data, which are mostly upper limits, shows that all of our calculated densities are within the measurement limits. The total calculated exospheric density at the lunar surface of about 1×¹⁰⁷ m⁻³ as result of solar wind sputtering we find is much less than the experimental total exospheric density of about ¹⁰¹² m⁻³. We conclude that sputtering contributes only a small fraction of the total exosphere, at least close to the surface. Because of the considerably larger scale height of atoms released via sputtering into the exosphere, sputtered atoms start to dominate the exosphere at altitudes exceeding a few 1000 km, with the exception of some light and abundant species released thermally, e.g. H₂, He, CH₄, and OH. Furthermore, for more refractory species such as calcium, our model indicates that sputtering may well be the dominant mechanism responsible for the lunar atmospheric inventory, but observational data does not yet allow firm conclusions to be drawn.     

Constraints on Mercury’s Na exosphere: Combined MESSENGER and ground-based data

We have used observations of sodium emission obtained with the McMath-Pierce solar telescope and MESSENGER’s Mercury Atmospheric and Surface Composition Spectrometer (MASCS) to constrain models of Mercury’s sodium exosphere. The distribution of sodium in Mercury’s exosphere during the period January 12-15, 2008, was mapped using the McMath-Pierce solar telescope with the 5″ × 5″ image slicer to observe the D-line emission. On January 14, 2008, the Ultraviolet and Visible Spectrometer (UVVS) channel on MASCS sampled the sodium in Mercury’s anti-sunward tail region. We find that the bound exosphere has an equivalent temperature of 900-1200 K, and that this temperature can be achieved if the sodium is ejected either by photon-stimulated desorption (PSD) with a 1200 K Maxwellian velocity distribution, or by thermal accommodation of a hotter source. We were not able to discriminate between the two assumed velocity distributions of the ejected particles for the PSD, but the velocity distributions require different values of the thermal accommodation coefficient and result in different upper limits on impact vaporization. We were able to place a strong constraint on the impact vaporization rate that results in the release of neutral Na atoms with an upper limit of 2.1 × 10⁶ cm⁻² s⁻¹. The variability of the week-long ground-based observations can be explained by variations in the sources, including both PSD and ion-enhanced PSD, as well as possible temporal enhancements in meteoroid vaporization. Knowledge of both dayside and anti-sunward tail morphologies and radiances are necessary to correctly deduce the exospheric source rates, processes, velocity distribution, and surface interaction.     

Sources of sodium in the lunar exosphere: Modeling using ground-based observations of sodium emission and spacecraft data of the plasma

Observations of the equatorial lunar sodium emission are examined to quantify the effect of precipitating ions on source rates for the Moon’s exospheric volatile species. Using a model of exospheric sodium transport under lunar gravity forces, the measured emission intensity is normalized to a constant lunar phase angle to minimize the effect of different viewing geometries. Daily averages of the solar Lyman α flux and ion flux are used as the input variables for photon-stimulated desorption (PSD) and ion sputtering, respectively, while impact vaporization due to the micrometeoritic influx is assumed constant. Additionally, a proxy term proportional to both the Lyman α and to the ion flux is introduced to assess the importance of ion-enhanced diffusion and/or chemical sputtering. The combination of particle transport and constrained regression models demonstrates that, assuming sputtering yields that are typical of protons incident on lunar soils, the primary effect of ion impact on the surface of the Moon is not direct sputtering but rather an enhancement of the PSD efficiency. It is inferred that the ion-induced effects must double the PSD efficiency for flux typical of the solar wind at 1 AU. The enhancement in relative efficiency of PSD due to the bombardment of the lunar surface by the plasma sheet ions during passages through the Earth’s magnetotail is shown to be approximately two times higher than when it is due to solar wind ions. This leads to the conclusion that the priming of the surface is more efficiently carried out by the energetic plasma sheet ions.     

A primary analysis of microwave brightness temperature of lunar surface from Chang-E 1 multi-channel radiometer observation and inversion of regolith layer thickness

In China’s first lunar exploration project, Chang-E 1 (CE-1), a multi-channel microwave radiometer was aboard the satellite, with the purpose of measuring microwave brightness temperature (Tb) from lunar surface and surveying the global distribution of lunar regolith layer thickness. In this paper, the primary 621 tracks of swath data measured by CE-1 microwave radiometer from November 2007 to February 2008 are collected and analyzed. Using the nearest neighbor interpolation to collect the Tb data under the same Sun illumination, global distributions of microwave brightness temperature from lunar surface at lunar daytime and nighttime are constructed. Based on the three-layer media modeling (the top dust-soil, regolith and underlying rock media) for microwave thermal emission of lunar surface, the CE-1 measured Tb and its dependence upon latitude, frequency and FeO + TiO₂ content, etc. are discussed. The CE-1 Tb data at Apollo landing sites are especially chosen for validation and calibration on the basis of available ground measurements. Using the empirical dependence of physical temperature upon the latitude verified by the CE-1 multi-channel Tb data at Apollo landing sites, the global distribution of regolith layer thickness is further inverted from the CE-1 brightness temperature data at 3 GHz channel. Those inversions at Apollo landing sites and the characteristics of regolith layer thickness for lunar maria are well compared with the Apollo in situ measurements and the regolith thickness derived from the Earth-based radar data. Finally, the statistical distribution of regolith thickness is analyzed and discussed.     

Observations of metallic species in Mercury’s exosphere

From observations of the metallic species sodium (Na), potassium (K), and magnesium (Mg) in Mercury’s exosphere, we derive implications for source and loss processes. All metallic species observed exhibit a distribution and/or line width characteristic of high to extreme temperature - tens of thousands of degrees K. The temperatures of refractory species, including magnesium and calcium, indicate that the source process for the atoms observed in the tail and near-planet exosphere are consistent with ion sputtering and/or impact vaporization of a molecule with subsequent dissociation into the atomic form. The extended Mg tail is consistent with a surface abundance of 5-8% Mg by number, if 30% of impact-vaporized Mg remains as MgO and half of the impact vapor condenses. Globally, ion sputtering is not a major source of Mg, but locally the sputtered source can be larger than the impact vapor source. We conclude that the Na and K in Mercury’s exosphere can be derived from a regolith composition similar to that of Luna 16 soil (or Apollo 17 orange glass), in which the abundance by number is 0.0027 (0.0028) for Na and 0.0006 (0.0045) for K.     

Chang’E-1 gamma ray spectrometer and preliminary radioactive results on the lunar surface

The Chang’E-1(CE-1) spacecraft took a gamma-ray spectrometer (hereafter, CGRS) to detect the element distributions on the lunar surface in a circular, 200 km altitude, polar orbit with approximately 2 h periodicity. CGRS consists of two large CsI(Tl) crystals as the main and anticoincidence detectors. The large CsI crystal of CGRS has a higher detector effective area than other lunar gamma ray spectrometers. For its 1-year mission, gamma ray spectra including many peaks of major elements and trace elements on the lunar surface have been measured by CGRS. Global measurement within 0.55-0.75 MeV is given here to describe the distribution of radioactive composition (e.g., uranium and thorium) on the lunar surface. Although CGRS has a lower energy resolution that cannot separate the uranium peak from others in this energy region, 609 keV uranium gamma ray line dominates the shape of the spectrum in this energy region. Therefore, the radioactive map can indirectly describe the uranium distribution on the lunar surface. The radioactive map shows that the higher radiation is concentrated in the Procellarum KREEP Terrene (PKT) on the nearside with an oval shape. The secondary high-radiation is located in South Pole-Aitken (SPA) basin. Lunar highlands have lower concentration. The relationship between radiation and topography displays different linear correlations for lunar highlands and SPA basin, which imply the different processes for these two regions.     

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.     

‘Direct’ evidence for water (H2O) in the sunlit lunar ambience from CHACE on MIP of Chandrayaan I

Direct detection of water in its vapour phase in the tenuous lunar environment through in situ measurements carried out by the Chandra’s Altitudinal Composition Explorer (CHACE) payload, onboard the Moon Impact Probe (MIP) of Chandrayaan I mission vindicates the presence of water on the surface of the moon in form of ice at higher lunar latitudes inferred from IR absorption spectroscopy, (especially that of OH), by the Moon Mineralogy Mapper (M³) of Chandrayaan I. The quadrupole mass spectrometer based payload, CHACE, sampled the lunar neutral atmosphere every 4 s with a broad latitudinal (∼40°N to 90°S, with a resolution of ∼0.1°) and altitudinal (from 98 km up to impact on the lunar surface with a resolution of ∼0.25 km) coverage in the sunlit side of the moon for the first time. These two (CHACE and M³) complementary experiments are shown to collectively provide unambiguous signatures for the distribution of water in solid and gaseous phases in Earth’s moon.     

Light scattering by complex particles in the Moon's exosphere: Toward a taxonomy of models for the realistic simulation of the scattering behavior of lunar dust

It is suspected that the lunar exosphere has a dusty component dispersed above the surface by various physical mechanisms. Most of the evidence for this phenomenon comes from observations of “lunar horizon glow” (LHG), which is thought to be produced by the scattering of sunlight by this exospheric dust. The characterization of exospheric dust populations at the Moon is key to furthering our understanding of fundamental surface processes, as well as a necessary requirement for the planning of future robotic and human exploration.We present a model to simulate the scattering of sunlight by complex lunar dust grains (i.e. grains that are non-spherical and can be inhomogeneous in composition) to be used in the interpretation of remote sensing data from current and future lunar missions. We numerically model lunar dust grains with several different morphologies and compositions and compute their individual scattering signatures using the Discrete Dipole Approximation (DDA). These scattering properties are then used in a radiative transfer code to simulate the light scattering due to a dust size distribution, as would likely be observed in the lunar exosphere at high altitudes 10's of km. We demonstrate the usefulness and relevance of our model by examining mode: irregular grains, aggregate of spherical monomers and spherical grains with nano-phase iron inclusions. We subsequently simulate the scattering by two grain size distributions (0.1 and radius), and show the results normalized per-grain. A similar methodology can also be applied to the analysis of the LHG observations, which are believed to be produced by scattering from larger dust grains within about a meter of the surface.As expected, significant differences in scattering properties are shown between the analyses employing the widely used Mie theory and our more realistic grain geometries. These differences include large variations in intensity as well as a positive polarization of scattered sunlight caused by non-spherical grains. Positive polarization occurs even when the grain size is small compared to the wavelength of incident sunlight, thus confirming that the interpretation of LHG based on Mie theory could lead to large errors in estimating the distribution and abundances of exospheric dust.     

Regarding the possible generation of a lunar nightside exo-ionosphere

The non-condensing neutral helium exosphere is at its most concentrated levels on the cold lunar nightside. We show herein that these He atoms are susceptible to impact ionization from primary and secondary electrons flowing in the vicinity of the negatively-charged nightside lunar surface. The secondary electron beams are a relatively recent discovery and are found to be emitted from the nightside surface at energies consistent with the negative surface potential. The effect is to create an electron impact-created ionosphere in nightside regions, possibly especially potent within polar craters.     

Mercury's sodium exosphere

Mercury's neutral sodium exosphere is simulated using a comprehensive 3D Monte Carlo model following sodium atoms ejected from Mercury's surface by thermal desorption, photon stimulated desorption, micro-meteoroid vaporization and solar wind sputtering. The evolution of the sodium surface density with respect to Mercury's rotation and its motion around the Sun is taken into account by considering enrichment processes due to surface trapping of neutrals and ions and depletion of the sodium available for ejection from the surfaces of grains. The change in the sodium exosphere is calculated during one Mercury year taking into account the variations in the solar radiation pressure, the photo-ionization frequency, the solar wind density, the photon and meteoroid flux intensities, and the surface temperature. Line-of-sight column densities at different phase angles, the supply rate of new sodium, average neutral and ion losses over a Mercury year, surface density distribution and the importance of the different processes of ejection are discussed in this paper. The sodium surface density distribution is found to become significantly nonuniform from day to night sides, from low to high latitudes and from morning to afternoon because of rapid depletion of sodium atoms in the surfaces of grains mainly driven by thermal depletion. The shape of the exosphere, as it would be seen from the Earth, changes drastically with respect to Mercury's heliocentric position. High latitude column density maxima are related to maxima in the sodium surface concentration at high latitudes in Mercury's surface and are not necessarily due to solar wind sputtering. The ratio between the sodium column density on the morning side of Mercury's exosphere and the sodium column density on the afternoon side is consistent with the conclusions of Sprague et al. (1997, Icarus 129, 506-527). The model, which has no fitting parameters, shows surprisingly good agreement with recent observations of Potter et al. (2002, Meteor. Planet. Sci. 8, 3357-3374) successfully explaining their velocity and column density profiles vs. heliocentric distance. Comparison with this data allows us to constrain the supply rate of new sodium atoms to the surface. We also discuss the possible origins of the strong high latitude emissions (Potter and Morgan, 1990, Science 248, 835-838; 1997a, Adv. Space Res. 19, 1571-1576; 1997b, Planet. Space Sci. 45, 95-100; Sprague et al., 1998, Icarus 135, 60-68) and the strong variations of the total content of the sodium exosphere on short (Potter et al., 1999, Planet. Space Sci. 47, 1441-1449) and long time scales (Sprague et al., 1997, Icarus 129, 506-527).     

A reanalysis of the Apollo light scattering observations, and implications for lunar exospheric dust

Conspicuous excess brightness, exceeding that expected from coronal and zodiacal light (CZL), was observed above the lunar horizon in the Apollo 15 coronal photographic sequence acquired immediately after orbital sunset (surface sunrise). This excess brightness systematically faded as the Command Module moved farther into shadow, eventually becoming indistinguishable from the CZL background. These observations have previously been attributed to scattering by ultrafine dust grains (radius ∼0.1 microns) in the lunar exosphere, and used to obtain coarse estimates of dust concentration at several altitudes and an order-of-magnitude estimate of ∼10⁻⁹ g cm⁻² for the column mass of dust near the terminator, collectively referred to as model “0”.We have reanalyzed the Apollo 15 orbital sunset sequence by incorporating the known sightline geometries in a Mie-scattering simulation code, and then inverting the measured intensities to retrieve exospheric dust concentration as a function of altitude and distance from the terminator. Results are presented in terms of monodisperse (single grain size) dust distributions. For a grain radius of 0.10 microns, our retrieved dust concentration near the terminator (∼0.010 cm⁻³) is in agreement with model “0” at z=10 km, as is the dust column mass (∼3–6×10⁻¹⁰ g cm⁻²), but the present results indicate generally larger dust scale heights, and much lower concentrations near 1 km (<0.08 cm⁻³ vs. a few times 0.1 cm⁻³ for model “0"). The concentration of dust at high altitudes (z>50 km) is virtually unconstrained by the measurements. The dust exosphere extends into shadow a distance somewhere between 100 and 200 km from the terminator, depending on the uncertain contribution of CZL to the total brightness. These refined estimates of the distribution and concentration of exospheric dust above the lunar sunrise terminator should place new and more rigorous constraints on exospheric dust transport models, as well as provide valuable support for upcoming missions such as the Lunar Atmosphere and Dust Environment Explorer (LADEE).     

Investigation of local time dependence of Mercury's sodium exosphere based on a numerical simulation

Mercury has a surface-bounded exosphere (SBE) similar to that of the Moon. One of the atmospheric species, sodium, was found by ground-based observations to be the most prominent component. Mercury's sodium SBE is known to be non-uniform with respect to local time (LT) in low-latitude regions: the sodium column density in the dawn-side region is larger than that in the dusk-side region, and the sodium abundance is the largest in the morning-noon region. To reveal the production processes for the exosphere near Mercury's surface, the LT dependence of the exosphere was investigated through a numerical simulation. Three data sets of sodium column densities observed for the dawn-side hemisphere, observed by Sprague et al. [1997. Distribution and abundance of sodium in Mercury's atmosphere, 1985-1988. Icarus 12, 506-527], were compared with results simulated by a 3D Monte Carlo method, and the source rates and density of sodium of the planetary surface were estimated. In the simulation, the photon-stimulated desorption (PSD) and thermal desorption (TD) processes were assumed as the release mechanisms. The sodium source rates for the three data sets, at respective heliocentric distances of about 0.33, 0.42, and 0.44 AU, were estimated as 1-4×10⁸ Na/cm²/s with weak LT dependence. In contrast, the expected sodium surface density showed clear dependence on LT and the heliocentric distance. The sodium surface density decreases from early morning to noon by a few orders, and, particularly for large heliocentric distances, the surface is in a condition of sodium excess and depletion with respect to the surface sodium density assumed by Killen et al. [2004. Source rates and ion recycling rates for Na and K in Mercury's atmosphere. Icarus 171, 1-19] in the early morning and morning-noon regions, respectively. This study implies that the decrease in sodium surface density from the early morning to noon regions might produce the characteristic LT dependence in the low-latitude dawn-side region.     

New views of the lunar plasma environment

A rich set of new measurements has greatly expanded our understanding of the Moon–plasma interaction over the last sixteen years, and helped demonstrate the fundamentally kinetic nature of many aspects thereof. Photon and charged particle impacts act to charge the lunar surface, forming thin Debye-scale plasma sheaths above both sunlit and shadowed hemispheres. These impacts also produce photoelectrons and secondary electrons from the surface, as well as ions from the surface and exosphere, all of which in turn feed back into the plasma environment. The solar wind interacts with sub-ion-inertial-scale crustal magnetic fields to form what may be the smallest magnetospheres in the solar system. Proton gyro-motion, solar wind pickup of protons scattered from the dayside surface, and plasma expansion into vacuum each affect the dynamics and structure of different portions of the lunar plasma wake. The Moon provides us with a basic plasma physics laboratory for the study of fundamental processes, some of which we cannot easily observe elsewhere. At the same time, the Moon provides us with a test bed for the study of processes that also operate at many other solar system bodies. We have learned much about the Moon–plasma interaction, with implications for other space and planetary environments. However, many fundamental problems remain unsolved, including the details of the coupling between various parts of the plasma environment, as well as between plasma and the surface, neutral exosphere, and dust. In this paper, we describe our current understanding of the lunar plasma environment, including illustrative new results from Lunar Prospector and Kaguya, and outstanding unsolved problems.     

Self-consistent modelling of Mercury’s exosphere by sputtering, micro-meteorite impact and photon-stimulated desorption

A Monte-Carlo model of exospheres (Wurz and Lammer, 2003) was extended by treating the ion-induced sputtering process, photon-stimulated desorption, and micro-meteorite impact vaporisation quantitatively in a self-consistent way starting with the actual release of particles from the mineral surface of Mercury. Based on available literature data we established a global model for the surface mineralogy of Mercury and from that derived the average elemental composition of the surface. This model serves as a tool to estimate densities of species in the exosphere depending on the release mechanism and the associated physical parameters quantitatively describing the particle release from the surface.Our calculation shows that the total contribution to the exospheric density at the Hermean surface by solar wind sputtering is about 4×10⁷ m^-3, which is much less than the experimental upper limit of the exospheric density of 10¹² m^-3. The total calculated exospheric density from micro-meteorite impact vaporisation is about 1.6×10⁸ m^-3, also much less than the observed value. We conclude that solar wind sputtering and micro-meteorite impact vaporisation contribute only a small fraction of Mercury’s exosphere, at least close to the surface. Because of the considerably larger scale height of atoms released via sputtering into the exosphere, sputtered atoms start to dominate the exosphere at altitudes exceeding around 1000 km, with the exception of some light and abundant species released thermally, e.g. H₂ and He. Because of Mercury’s strong gravitational field not all particles released by sputtering and micro-meteorite impact escape. Over extended time scales this will lead to an alteration of the surface composition.     

Bearing strength of lunar soil

Bearing load vs penetration curves have been measured on a 1.3 g sample of lunar soil from the scoop of the Surveyor 3 soil mechanics surface sampler, using a circular indentor 2 mm in diameter. Measurements were made in an Earth laboratory, in air. This sample provided a unique opportunity to evaluate earlier, remotely controlled, in-situ measurements of lunar surface bearing properties. Bearing capacity, measured at a penetration equal to the indentor diameter, varied from 0.02–0.04 N cm^–2 at bulk densities of 1.15 g cm^–3 to 30-100 N cm^–2 at 1.9 g cm^–3. Deformation was by compression directly below the indentor at bulk densities below 1.61 g cm^–3, by outward displacement at bulk densities over 1.62 g cm^–3. Preliminary comparison of in-situ remote measurements with those on returned material indicates good agreement if the lunar regolith at Surveyor 3 has a bulk density of 1.6 g cm^–3 at 2.5 cm. depth; definitive comparison awaits both better data on bulk density of the undisturbed lunar soil and additional mechanical-property measurements on returned material.