P43C-01
Lunar Laser Ranging Science: Recent Progress and Future Plans
Since it's initiation by the Apollo 11 astronauts in 1969, lunar laser ranging (LLR) has strongly contributed to our understanding of the Moon's internal structure and the dynamics of the Earth-Moon system. LLR science results include tests of gravitational physics and ephemeris information from the orbit, lunar science from rotation and solid-body tides, and Earth science. \underline{Science from the orbit}: LLR data provide for high-accurate research in gravitational physics including tests of the equivalence principle (EP), search for the time variation of the gravitational constant G, and geodetic precession. The EP test is used for an accurate determination of the Eddington parameter beta. The analysis is sensitive to astronomical parameters such as orbit, masses, and obliquity. The dissipation-caused acceleration in orbital longitude is -25.9 {"/cent}2, dominated by tides on Earth with a 1% lunar contribution. Lunar ephemerides are a product of the LLR analysis used by current and future spacecraft missions. \underline{Lunar science}: variations in the lunar orientation and rotation plus tidal deformation provide unique information on lunar interior structure, physical properties, and energy dissipation. Analyses of the LLR data determine the second-degree lunar tidal Love numbers k2 (potential) and h2 (vertical displacement), tidal dissipation related Q, and two interactions at the fluid-core/solid-mantle boundary. The determined Love numbers are k2=0.0199±0.0025 and h2=0.043±0.008, consistent with elastic models of the interior. Tidal dissipation is strong, Q has a weak dependence on tidal frequency and is about 29 at a one month period and 36 at one year. The core/mantle interactions are dissipation, indicating a fluid core of about 20% the Moon's radius, and an oblateness interaction. \underline{Earth science}: Station positions and motions, Earth rotation variations, and precession are determined from analyses. At the same time, the current distribution of the retroreflectors is not optimal and signals are weak. We discuss a recently proposed next-generation of LLR experiment. In particular, we suggest that upcoming missions to the Moon should carry either new sets of laser retroreflector arrays or laser transponders pointed at Earth or both of these instruments. A wider geographic distribution of new instruments on the lunar surface than the current distribution would improve the accuracy of the lunar science parameters several times. A bright transponder source on the Moon would open LLR to dozens of satellite laser ranging stations which cannot detect the current weak signals from the Moon. Properties of the lunar interior, including liquid core and solid inner core, can be determined from lunar rotation, orientation, and tidal response. Anticipated improvements in Earth geophysics and geodesy would include the positions and rates for the Earth stations, Earth rotation, precession rate, nutation, and tidal influences on the orbit. Improvements are also expected in several tests of general relativity. Science investigations with optical transponders on the Moon can also be used as a prototype demonstration for later laser ranging to Mars; a lunar installation would provide valuable early feedback on their operational characteristics. The work described here was carried out at the Jet Propulsion Laboratory, California Institute of Technology under a contract with the National Aeronautics and Space Administration.
P43C-02 INVITED
The heat flow of the Moon: What do we know, and how do we measure it?
With the exception of the Earth, the Moon is the only terrestrial body for which the heat flow has been measured in situ. During the Apollo 15 and 17 missions, two probes at each landing site were inserted into the lunar regolith, and between 1971 and 1977 data concerning the temperature profile and thermal conductivity were collected. One of the more intriguing results of this experiment was that the derived heat flow at the Apollo 15 site appeared to be higher than that at the Apollo 17 site (21 vs. 16 mW m-2). In retrospect, the interpretation of the Apollo heat flow experiment data have turned out to be more complicated than originally acknowledged. Thermal conductivity estimates derived from two different techniques (a heating experiment and the measurement of the annual thermal wave) were found to be discordant by a factor of two. The Apollo-era studies neglected to consider the 18.6-year precession of the lunar orbit plane that acts to modulate the annual thermal wave by a non-negligible factor. The average temperature at a given depth was found to increase slowly with time, for which no good explanation currently exists. Finally, it is now known that heat producing elements in the lunar crust are distributed in a highly asymmetric manner, and if one would like to obtain the average heat flow of the Moon, more than two measurements would certainly be required. Our next big leap forward will certainly come with the acquisition of new data. While astronauts could emplace heat flow probes on the Moon, such stations would be limited to the number of human landing sites. A promising robotic alternative is the use of an electro-mechanical "mole" such as the HP3 that is being developed for ESA's ExoMars mission. After being deployed on the surface, such a device could hammer its way several meters below the surface, most likely deeper than any rotary drill could achieve. Thermophysical properties would be measured in the mole itself, as well as in a trailing payload compartment that could include a densitometer, thermal conductivity experiment, and electrical permittivity probe. Sensors embedded in the tether that connect the mole to the surface electronics box would perform long-term monitoring of the temperature profile.
P43C-03
New Lunar Paleointensity Measurements, Ancient Lunar Dynamo or Lunar Dud?
We analyze published and new paleointensity data from Apollo samples to reexamine the hypothesis of an early (3.9 to 3.6 Ga) lunar dynamo. Our new paleointensity experiments on four Apollo samples use modern absolute and relative measurement techniques. Our samples (60015, 76535, 72215, 62235) have ages ranging from 3.3 to 4.2 Ga, bracketing the putative period of a lunar dynamo. Samples 60015 (anorthosite) and 76535 (troctolite) failed during absolute paleointensity experiments, using the IZZI-modified Thellier-Thellier method. Samples 72215 and 62235 recorded a complicated, multi-component magnetic history that includes a low temperature (< 500°C) component with a high intensity (~90 μT), and a high temperature (> 500°C) component with a low intensity (~2 μT). These two samples were also subjected to a relative paleointensity experiment (sIRM), from which neither provided unambiguous evidence for a thermal origin of the recorded remanent magnetization. We found similar multi-component behavior in several published experiments on lunar samples. We test and present several magnetization scenarios in an attempt to explain the complex magnetization recorded in lunar samples. Specifically, an overprint from exposure to a small magnetic field (i.e. IRM) results in multi-component behavior (similar to lunar sample results), from which we could not recover the correct magnitude of the original TRM. The non-unique interpretation of these multi-component results combined with IRM (isothermal remanent magnetization) contamination during Apollo sample return ( Strangway et al., 1973), indicates that techniques incapable of distinguishing between single- and multi-component records (e.g., sIRM), cannot be reliably used to infer magnetic conditions of the early Moon. In light of these new experiments and a thorough reevaluation of existing paleointensity measurements, we conclude that there is a paucity of lunar samples that demonstrate a primary thermal remanent magnetization. As relative paleointensity measurements for lunar samples are calibrated using absolute paleointensities, the lack of acceptable absolute paleointensity measurements renders the interpretation of relative paleointensity measurements unreliable. Consequently, current lunar paleointensity measurements are inadequate to determine the existence and strength of an early lunar magnetic field. Surface magnetometry measurements and the return of magnetically uncontaminated samples from future missions are much needed for further progress in understanding the characteristics and origin of lunar crustal remanent magnetization.
P43C-04 INVITED
The Apollo Seismic Data, Recent Re-analyses, and What's Next
The seismic data that were acquired with a network of four stations established during the Apollo lunar landing missions and operated for eight years from 1969 to 1977 provided us with wealth of information, much of it quite unexpected, concerning the internal structure and dynamics of the Moon. Major findings included the existence of weak but numerous deep moonquakes, fewer but strong shallow moonquakes, and a lunar deep interior differentiated into a crust and upper mantle constituting a very thick lithosphere, underlain by a lower mantle similar to the Earth's asthenosphere. The acquired data were fully digital, and thus are allowing more sophisticated analyses using the high computer power that has become available only recently. New analyses have identified many times more deep moonquakes and their source regions than previously known, and their occurrence times are now better defined relative to tidal stress variations within the Moon. This is helping us to understand their source mechanisms and their relationship to the internal structure of the Moon. New analyses are also providing more reliable estimates of seismic velocity distribution and the possible extra-solar-system origin of shallow moonquakes. With all these old and new findings, are we ready to formulate a set of hypotheses regarding the origin and evolution of the Moon that are to be tested with future experiments on the Moon? My answer to this question is: No, we are not. The data acquired with the Apollo seismic network had many limitations: (a) the network covered only the front center of the Moon; (b) the instruments did not have broad enough frequency response to record very-long-period seismic waves often useful in inferring the internal structure; and (c) eight years of observation was simply too short to record many seismic events that occur only at longer intervals. A global coverage with many more seismic stations than what we had with the Apollo network is sorely needed to answer questions most geochemists are asking, including more reliable seismic velocity variations with depth and their regional variations. Thus, with our future experiments on the Moon, we must concentrate on both quality and quantity of the data to be acquired, surpassing what we did 30+ years ago.
P43C-05 INVITED
Moon geophysics and Lunar environemental monitoring: Apollo data reprocessing and perspectives with the MoonTwin project.
The formation of the Moon is probably results from a large impact between a Mars-sized planet and the Earth. The size of the Moon's core, the thickness of the crust and the structure of the lunar mantle are among the few parameters able to constrain this impact, along with the depth and vigor of the magma ocean that appeared on the young moon, after re-accretion around Earth's orbit. These parameters are therefore crucial to understand how our planet was affected by the impact, from both the energetic and volatile budget point of view, and how a body like the moon evolves. The reprocessing of the data recorded by the 4 ALSEP stations (Apollo 12, 14, 15 and 16), which were the first and, to date, the only successful geophysical stations in Planetary sciences, have shed new light on the interior of the Moon and in the determination of the parameters listed above. Very large uncertainties however remain. A first example is in the crustal thickness. The seismic crustal thickness estimates vary from 58 km to 30±5 km near the Apollo 12 landing site. When the lateral variations are taken into account, a mean crustal thickness beneath the Apollo stations of 34±5 km is found. Comparable uncertainties are found in the deep structure of the Moon, which is not directly constrained by seismology. Interior structure models obtained from joint inversion of the density, moment of inertia, Love number (k2) and using the seismic data apriori for the upper mantle and middle mantle show that a wide range of acceptable core models with 1%-2% lunar mass fit the data.These two extreme examples of lunar interior structure show that large uncertainties remain. Most are related to the lack of goo geophysical data. The Apollo seismometers had limited performance, especially in terms of frequency bandwidth and limited coverage of th network. Only two heat flow measurements were made by Apollo and all geodetic beacons are close to the equator; Other are related to the large lateral variations, already detected in the crustal thickness, and probably also existing in the lunar mantle. Consequently,most of the geophysical methods developed during the last two decades (e.g. long period body waves inversions, free oscillations inversions, receiver function analysis, etc) cannot be used on the Moo data. The deployment of a new network of geophysical stations on the Moon is therefore the aim of several projects in USA and Europe. We focus here on the MoonTwin project. The goal of the MoonTwin is to deploy 2 landers on the Moon, including one near the south pole, and is proposed as the NEXT mission of the ESA AURORA program. These landers will first perform severa technology demonstrations necessary to future MSR missions including a precision soft landing. After landing, science of the Moon and from the Moon will be performed.In addition to the geophysical objectives described above, which can be accomplished by seismometry, geodetic, heat flow measurements and magnetometry, other objectives more related to exploration and Science on the Moon will be covered: the first one will be to better understand and monitor the potential hazard lunar seismic events pose to a permanent habitat on the Moon, the rate of micrometeoroides impacts and the level of radiation. The second one will be to perform a first pilot experiment of radio-astronomy on the Moon, by using the benefit of the polar station, which will be regularly in occultation from the Earth radio-astronomical noise.
P43C-06
How Tides Control Some Individual Deep Moonquake Nests
Seismic stations emplaced during the Apollo missions operated from 1969 to 1977, and the most numerous signals identified in these data were deep moonquakes (DMQ). These occurred mostly at depths of 700-1200 km within a few hundred discrete clusters or nests, and within individual nests DMQ occurrence times correlated strongly with the gravitational tide generated by the Earth. However, the relationship with tides differs markedly at different nests, and we still don't understand either the mechanical origin of DMQ or why the tides exercise such strong control on their occurrence. We here investigate what controls episodes of DMQ occurrence in about a dozen DMQ nests where the temporal pattern is distinct and highly repeatable. In these nests, plots of occurrence times vs anomalistic phase exhibit a very regular drift pattern influenced by both the anomalistic and draconic periods, and often modulated by a roughly sinusoidal component having a period of about seven months. We show that extrema of the normal component of the tidal stress often exhibit a nearly identical pattern. In several of these nests there is a linear relationship between the normal and shear components of the tidal stress when DMQ occur. If we use a Mohr circle analysis to interpret these observations they suggest the static friction coefficient is extraordinarily low, about 0.1 or less. This is lower than ordinary geological materials, and may indicate that trapped fluids or other exotic phases play a role in the faulting process.
P43C-07
Lunar Tidal Deformation and the Internal Structure of the Moon
Monitoring tidally-induced deformations of the Moon from orbiting and landed spacecraft would provide important clues on the constitution of the lunar interior, thereby augmenting the Apollo seismic data record. A major advantage is that the tidal forcing function for a synchronously rotating satellite like the Moon is accurately known and precise tidal potential catalogues can be produced. A higher temporal resolution and precision than with space techniques alone can be achieved by additionally emplacing one or several landers on the lunar surface. However, tidal measurements may be significantly impeded by instrumental drift, instrument coupling to the surface, and local sources of noise. We will present relations between key tidal parameters that can be retrieved from an instrument suite monitoring tidally-induced changes of local gravity, tilt, latitude and strain at the surface and the constitution of the deep interior of the Moon. A most promising approach would involve time-varying gravitational field observations from an orbiting spacecraft combined with long-term monitoring of tidally-induced gravity changes at the lunar surface.