A review of earth-based radar mapping of the moon

Lunar radar mappings carried out in the late 1960's and 1970's have provided several valuable insights into lunar surface processes. These radar mappings used the delay-Doppler technique developed by Gordon Pettengill and his colleagues. These radar mappings also needed the narrow antenna beams, now available with large radio telescopes such as those at the Arecibo and Haystack Observatories. Two-element radar interferometers have provided resolution of the delay-Doppler ambiguity at meter wavelengths and provided topographic information at centimeter wavelengths. These techniques have provided high-resolution lunar radar-maps at 3.8-cm, 70-cm, and 7.5-m wavelengths, a set of wavelengths which span the window available for Earth-based radar mapping of the Moon.These radar maps have been used along with other Earth-based and Apollo orbital measurements to define surface units. The radar maps and these other data can describe physical properties such as small-scale (centimeter sized) blockiness and surface chemistry (titanium and iron) content. These estimates of lunar surface properties rely heavily upon extrapolation of surface sampling results.Presented at the IAU-COSPAR Julian Schmidt Symposium on 100 Years of Lunar Mapping held at Lagonissi, Greece, 25–27 May, 1978.     

A review of volatiles in the Martian interior

Multiple observations from missions to Mars have revealed compelling evidence for a volatile‐rich Martian crust. A leading theory contends that eruption of basaltic magmas was the ultimate mechanism of transfer of volatiles from the mantle toward the surface after an initial outgassing related to the crystallization of a magma ocean. However, the concentrations of volatile species in ascending magmas and in their mantle source regions are highly uncertain. This work and this special issue of Meteoritics & Planetary Science summarize the key findings of the workshop on Volatiles in the Martian Interior (Nov. 3–4, 2014), the primary open questions related to volatiles in Martian magmas and their source regions, and the suggestions of the community at the workshop to address these open questions.     

A note on tidal deceleration of the moon

The discrepancy between the observed apparent acceleration of the Moon in longitude (1) and the actual lunar laser ranging data (3), (4) is of the order of ~ 9 × 10^–23 rad s^–2. It cannot be explained by the rms errors in (1) and (3), (4); processes connected with the internal Earth's dynamics and accelerating the Earth in its rotation might be responsible for the phenomenon, leading to the decreasing of the principal moment of the Earth's inertia ~ – 3.2 × 10²⁹m² kg cy^–1.     

Solar wind and other gases in the regoliths of the Pesyanoe parent object and the moon

We report new data from Pesyanoe‐90,1 (dark lithology) on the isotopic signature of solar wind (SW) Xe as recorded in this enstatite achondrite which represents a soil‐breccia of an asteroidal regolith. The low temperature (≤800°C) steps define the Pesyanoe‐S xenon component, which is isotopically consistent with SW Xe reported for the lunar regolith. This implies that the SW Xe isotopic signature was the same at two distinct solar system locations and, importantly, also at different times of solar irradiation. Further, we compare the calculated average solar wind “SW‐Xe” signature to Chass‐S Xe, the indigenous Xe observed in SNC (Mars) meteorites. Again, a close agreement between these compositions is observed, which implies that a mass‐dependent differential fractionation of Xe between SW‐Xe and Chass‐S Xe is >1.5%_o per amu. We also observe fractionated (Pesyanoe‐F) Xe and Ar components in higher temperature steps and we document a fission component due to extinct ²⁴⁴Pu. Interestingly, the Pesyanoe‐F Xe component is revealed only at the highest temperatures (>1200°C). The Pesyanoe‐F gas reveals Xe isotopic signatures that are consistent with lunar solar energetic particles (SEP) data and may indicate a distinct solar energetic particle radiation as was inferred for the moon. However, we cannot rule out fractionation processes due to parent body processes. We note that ratios ³⁶Ar/³⁸Ar≤5 are also consistent with SEP data. Calculated abundances of the fission component correlate well with radiogenic ⁴⁰Ar concentrations, revealing rather constant ²⁴⁴Pu/K ratios in Pesyanoe, and separates thereof, and indicate that both components were retained. We identify a nitrogen component (δ¹⁵N = 44%_o) of non‐solar origin with an isotopic signature distinct from indigenous N (δ¹⁵N = ?33%^o). While large excesses at ¹²⁸Xe and ¹²⁹Xe are observed in the lunar regolith samples, these excesses in Pesyanoe are small. On the other hand, significant ¹²⁶Xe isotopic excesses, comparable to relative excesses observed in lunar soils and breccias, are prominent in the intermediate temperature steps of Pesyanoe‐90,1.     

Shock metamorphism on the moon

Shock metamorphism of the lunar samples is discussed. All types of lunar glasses formed by various-size collision-type impact are found as impact glass, ropy glass and agglutinates. The agglutinates bonded by crystal and glassy materials contain hydrogen and helium from the solar wind components. Lunar shocked minerals of plagioclase and silica show anomalous compositions and densities. There are typical two formation processes on planetary materials formed by shock events; that is (1) shocked quartz formed by silica-rich target rocks (esp. on evolved planets of the Earth and Mars), and (2) shocked silica with minor Al contents formed from plagioclase-rich primordial crusts of the Moon. The both shocked silica grows to coarse-grain normal crystals after high-temperature metamorphism which cannot distinguish the original main formation event of impact process.     

The evolution of the moon: A finite element approach

Thermal convection has considerable influence on the thermal evolution of terrestrial planets. Previous numerical models of planetary convection have solved the system of partial differential equations by finite difference methods, or have approximated it by parametrized methods. We have evaluated the applicability of a finite element solution of these equations. Our model analyses the thermal history of a self-gravitating spherical planetary body; it includes the effects of viscous dissipation, internal melting, adiabatic gradient, core formation, variable viscosity, decay of radioactive nucleides, and a depth dependent initial temperature profile. Reflecting current interest, physical parameters corresponding to the Moon were selected for the model.Although no initial basalt ocean is assumed for the Moon, partial melting is observed very early in its history; this is presumably related to the formation of the basalt maria. The convection pattern appears to be dominated by an L-2 mode. The present-day lithospheric thickness in the model is 600 km, with core-mantle temperatures close to 1600 K. Surface heat flux is 25.3 mW m^–2, higher than the steady state-value by about 16%.The finite element method is clearly applicable to the problem of planetary evolution, but much faster solution algorithms will be necessary if a sufficient number of models are to be examined by this method.Notation coefficient of thermal expansion - _ij Kronecker delta - absolute or dynamic viscosity - perturbation in temperature - thermal diffusivity - kinematic viscosity - density - stress tensor - B.P. before present - c specific heat at constant pressure or volume (Boussinesq approximation) - d depth of convection - E ^* activation energy for creep - g gravity - Ga billions of years - H(t) heat generation per unit mass per unit time at timet - k Boltzmann's constant - K mean thermal conductivity - Ma millions of years - p pressure - q heat flux - q _ss steady-state heat flux - Ra Rayleigh number - S volumetric heat sources, includes radioactivity and viscous dissipation - t time - T temperature - u verocity vector - V ^* activation volume for creep     

A magnetic dynamo in the moon?

The existence of fossil lunar magnetism has caused speculation that the Moon had, at one time, an internally produced dynamo magnetic field. Quantitative analysis of this idea, constrained by the largest iron lunar core compatible with observations, implies that the Moon would have had to rotate faster than its breakup angular velocity in order to support a dynamo magnetic field.A paper presented at the Lunar Science Institute Conference on Geophysical and Geochemical Exploration of the Moon and Planets, January 10–12, 1973.     

Meteoritic material on the moon

Three types of meteoritic material are found on the Moon: micrometeorites, ancien planetesimal debris from the ‘early intense bombardment’, and debris of recent, crater-forming projectiles. Their amounts and compositions have been determined from trace element studies. The micrometeorite component is uniformly distributed over the entire lunar surface, but is seen most clearly in mare soils. It has a primitive, C1-chondrite-like composition, and comprises 1-1.5% of mature soils. Apparently it represents cometary debris. The mean annual influx rate is 2.4 × 10^?9 g cm^?2 yr^?1. It shows no detectable time variation or dependence on selenographic position. The ancient component is seen in highland breccias and soils more than 3.9 AE old. It has a fractionated composition, with volatiles depleted relative to siderophiles. The abundance pattern does not match that of any known meteorite class. At least two varieties exist (LN and DN, with Ir/Au, Re/Au 0.25-0.5 and > 0.5 the C1 value). Both seem to represent the debris of planetesimals that produced the mare basins and highland craters during the first 700 Myr of the Moon's history. It appears that the LN and DN objects impacted at less then 10 km s^?1, had diameters less than 100 km, contained more than 15% Fe, and were not internally differentiated. Both were depleted in volatiles; the LN objects also in refractories (Ir, Re). This makes it unlikely that the LN bodies served as important building blocks of the Moon. The crater-forming component has remained elusive. Only a possible hint of this component has been seen, in ejecta from Dune Crater and Apollo 12 KREEP glasses of Copernican (?) origin.     

Modern slope processes on the moon

Slope movements of material in lunar craters are investigated based on remote spectral studies carried out on board the Clementine spacecraft, and data obtained during the large-scale survey on board the LRO (Lunar Reconnaissance Orbit) spacecraft. The morphological analysis of crater forms based on large-scale images and spectral and spectropolarized assessments of the exposition age (or maturity) of the slope material has led to the conclusion that the formation process of observed outcrops probably is a modern feature. The lower age limit of these structures is estimated at 40–80 years. Thus, slope movements of surface materials can continue at the present time, regardless of the age of the crater studied. Slope movements of crushed granular material lead to fresh outcrops of subsurface layers of marine or continental landscapes and, therefore, extend our capabilities to research the deep material of the Moon. To analyze this phenomenon, craters of 16 and 30 km have been selected. The length of fresh outcrops, while depending strongly on the dimensions of the craters, can be up to several kilometers. In connection with this, the prospect appears of remote analysis of rocks that came to the surface from depths of at least several hundred meters. In this case, there are openings for the contact analysis of subsurface material without the use of labor-intensive operations associated with the delivery of equipment for deep drilling to the lunar surface.     

A comparison of the magnetic field anomalies for the moon and earth

A comparative analysis of the anomalous magnetic field of the Moon, information about which was obtained by the Apollo 15 subsatellite, and the anomalous magnetic field of the Earth, involving data provided from surveys at various altitudes (up to 500 km) is given. As a result of spectral analysis of these fields it is shown that the main difference of the spectra is in the lower intensity of long period lunar anomalies and the increased rate of their damping with height, which is probably connected with the absence of any kind of magnetization by induction.     

Laboratory studies on seismic and electrical properties of the moon

Laboratory measurements of seismic wave velocities and electrical properties of Apollo lunar samples and similar material of terrestrial origin are discussed in this paper. Measurements of the electrical properties show that in the frequency range above a few hundred Hz the outer region of the Moon may be considered as a low loss dielectric. This observation supports a longstanding speculation that dry, powdered rocks in which the dielectric loss tangent is frequency-independent over a wide range of frequency are present in the uppermost lunar surface layers. The surface layers of the Moon are likely to have an extremely low electrical conductivity. Thus future electromagnetic probing of the Moon to a few hundred kilometer depth is possible in the few kHz frequency range. Based on ultrasonic experiments with pressure as a variable, we next present the elastic constants and equations of state of lunar materials and characteristic dispersion of seismic wave velocities of the Moon. We find thatP andS wave velocities increase sharply within the first 30 km depth and then level off gradually. Combining this observation with lunar seismic and geophone data, we believe that the first 30 km of the Moon may be interpreted as a scattering region. If H₂O exists on the Moon, H₂O may occur at some shallow depth beneath the outermost surface layer in solid ice interlocking cracks and pores and mineral grains. The rocks in this permafrost state have relatively low seismic velocity and highQ. If permafrost does exist, we would expect a wide range of electrical conductivity and dielectric constant. Future electromagnetic probing of the Moon should yield very usefull information on the physical state of the lunar interior; when this electrical information is combined with the seismic information, we should learn much more about the internal constitution and the state of the Moon than is known today.     

Thermal history and evolution of the moon

Possible models for the thermal evolution of the Moon are constrained by a wide assortment of lunar data. In this work, theoretical lunar temperature models are computed taking into account different initial conditions to represent possible accretion models and various abundances of heat sources to correspond to different compositions. Differentiation and convection are simulated in the numerical computational scheme.Models of the thermal evolution of the Moon that fit the chronology of igneous activity on the lunar surface, the stress history of the lunar lithosphere implied by the presence of mascons, and the surface concentrations of radioactive elements, involve extensive differentiation early in lunar history. This differentiation may be the result of rapid accretion and large-scale melting or of primary chemical layering during accretion. Differences in present-day temperatures for these two possibilities are significant only in the inner 1000 km of the Moon and are not resolvable with presently available data.If the Apollo 15 heat flow is a representative value, the average uranium concentration in the moon is 65±15 ppb. This is consistent with achondritic bulk composition (between howardites and eucrites) for the Moon.Paper dedicated to Professor Harold C. Urey on the occasion of his 80th birthday on 29 April 1973.     

Density and stress distribution in the moon

A model is presented for the lateral variations of density within the Moon. The model gives rise to a gravitational potential which is equal to the observed potential at the lunar surface, moreover, it minimizes the total shear-strain energy of the Moon. The model exhibits lateral variations of about ±0.25 g cc^–1 within 50 km depth. The variations, however, reduce to ±0.06 and ±0.008 g cc^–1 within layers at 50–135 and 135–235 km respectively, and they become negligible below this region. The associated stress differences are found to be about 50 bar within 600 km depth, having their maximum values of about 90 bars at a depth of about 250 km. On the basis of these stress differences a strength of about 100 bar is concluded for the upper 400 km of the lunar interior for the last 3.3 b.y.Paper dedicated to Professor Harold C. Urey on the occasion of his 80th birthday on 29 April, 1973.The Lunar Science Institute is operated by the Universities Space Research Association under Contract No. NSR 09-051-001 with the National Aeronautics and Space Administration. This paper is Lunar Science Institute Contribution No. 117.     

Depletion of sulfur on the surface of asteroids and the moon

Abstract— Data from the X‐ray and γ‐ray spectrometers onboard the Near Earth Asteroid Rendezvous (NEAR) spacecraft were used to constrain the chemical and mineralogical composition of asteroid 433 Eros (McCoy et al. 2001). The bulk composition appears to be consistent with that of L to H chondrites (Nittler et al. 2001). However, there appeared to be a marked depletion relative to ordinary chondritic composition in the S/Si ratio (0.014 ± 0.017). We investigate space weathering mechanisms to determine the extent to which sulfur can be preferentially lost from the surface regolith. The two processes considered are impact vaporization by the interplanetary meteoroid population and ion sputtering by the solar wind. Using impact data for Al projectiles onto enstatite, we find that the vaporization rate for troilite (FeS) is nine times as fast as that for the bulk of the regolith. If 20% of the iron is in the form of troilite, then the net vaporization rate, normalized to bulk composition, is 2.8 times faster for sulfur than for iron. Sputtering is equally efficient at removing sulfur as impact vaporization.     

On the accretion of the earth and moon

The bodies which produced the premare impact craters on the moon contained a much higher proportion of smaller bodies in the earliest observable times than subsequently. This suggests that the earth and moon accreted from small objects with only an occasional large planetoid.If the earliest observable lunar craters are 4.3 × 10⁹ yr old, the half-life of the primitive planetesimals which produced the giant lunar craters larger than 161 km in diameter, was 143 × 10⁶ yr, while the half-life of the primitive planetesimals which produced lunar craters larger than 1 km in diameter was only 88 × 10⁶ yr. The half-life of the bodies which produced 1 km craters was still shorter, about 75 × 10⁶ yr.     

Seismic effects from major basin formations on the moon and mercury

Grooved and hilly terrains occur at the antipode of major basins on the Moon (Imbrium, Orientale) and Mercury (Caloris). Such terrains may represent extensive landslides and surface disruption produced by impact-generatedP-waves and antipodal convergence of surface waves. Order-of-magnitude calculations for an Imbrium-size impact (10³⁴ erg) on the Moon indicateP-wave-induced surface displacements of 10 m at the basin antipode that would arrive prior to secondary ejecta. Comparable surface waves would arrive subsequent to secondary ejecta impacts beyond 10³ km and would increase in magnitude as they converge at the antipode. Other seismically induced surface features include: subdued, furrowed crater walls produced by landslides and concomitant secondary impacts; emplacement and leveling of light plains units owing to seismically induced ‘fluidization’ of slide material; knobby, pitted terrain around old basins from enhancement of seismic waves in ancient ejecta blankets; and perhaps the production and enhancement of deep-seated fractures that led to the concentration of farside lunar maria in the Apollo-Ingenii region.