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The shape and internal structure of an asteroid is a result of its violent collisional history, and Asteroid 25143 Itokawa is an important target for investigating the internal structure and formation processes associated with rubble pile asteroids. The surface of Itokawa contains numerous boulders and is very rough, but 20% of the surface is covered with centimeter-sized gravels that are called ”smooth terrain”. These flat areas are associated with low areas of the gravitational potential and are considered to be formed by down-slope migration and accumulation of pebbles. We propose a method to constrain the interior density distribution of an asteroid by modeling the gravity field and fitting the smooth terrains to equi-potential surfaces. Density models that are composed of two distinct lobes best explain the data if the ”head” and ”body” have different densities of 2,450 kg/m3 and 1,930 kg/m3, respectively. Alternatively, the two lobes could have comparable densities if there was a compressed ”neck” of higher density between the two. Three layer models that treat the densities of the head, neck and body separately are largely unconstrained. However, when using an independent estimate of the center-of-mass/center-of-figure offset as determined from modeling the YORP spin-up of Itokawa, our models require the density of the head to be larger than the global average.

A fundamental linear relationship exists between the efficiency of thermoremanent magnetization measured at room temperature and the magnitude of the ambient field at the time of acquisition. The magnetic efficiency (the ratio of thermoremanent magnetization to saturation remanent magnetization) multiplied by the saturation magnetization is proportional to the magnetizing field, where the proportionality constant is independent of magnetic mineralogy and domain state. The empirical constant for this equation was determined using existing experimental data of single domain through pseudosingle domain to multidomain states of iron, meteoritic iron, magnetite, maghemite, pyrrhotite, and hematite. We show that the acquired magnetic efficiency is closely related to two types of demagnetizing fields that act as barriers against domain wall pinning during magnetic acquisition. The first relates to the saturation magnetization that is derived from the distribution of Bohr magnetons within the crystal lattice, and the second originates from grain shape. Theoretical considerations imply a factor of two difference in the magnetic efficiency acquired during laboratory and geologic timescales. This equation reveals that troilite may be a potentially important magnetic carrier for extraterrestrial magnetism. Using magnetic scanning techniques, our relationship allows for estimating the paleointensity from samples that contain more than one magnetic species.

International audience; Ages of geological units of planetary bodies are determined from impact crater counts on their surface. These ages are model-dependent, and several models largely used in the community assume a constant production function and a constant cratering rate over the last 3 Ga. We have mapped the population of small impact craters (>200 m in diameter) formed over a population of large impact craters (>5 km in diameter) with layered ejecta on Acidalia Planitia, Mars. We have deduced the age of each large impact crater under the assumption of a constant impact rate and constant production function. The impact rate inferred from this set of ages is, however, not constant and show a significant increasing during the last~1 Ga compared to chronology models commonly used. We interpret this inconsistency as an evidence for temporal variations in the size-frequency distribution (SFD) of impactors in the main belt, consistent with recent studies argued for a late increasing of the large impactor flux on Earth and the Moon.

The lunar geological record has much to tell us about the earliest history of the Solar System, the origin and evolution of the Earth-Moon system, the geological evolution of rocky planets, and the near-Earth cosmic environment throughout Solar System history. In addition, the lunar surface offers outstanding opportunities for research in astronomy, astrobiology, fundamental physics, life sciences and human physiology and medicine. This paper provides an interdisciplinary review of outstanding lunar science objectives in all of these different areas. It is concluded that addressing them satisfactorily will require an end to the 40-year hiatus of lunar surface exploration, and the placing of new scientific instruments on, and the return of additional samples from, the surface of the Moon. Some of these objectives can be achieved robotically (e.g. through targeted sample return, the deployment of geophysical networks, and the placing of antennas on the lunar surface to form radio telescopes). However, in the longer term, most of these scientific objectives would benefit significantly from renewed human operations on the lunar surface. For these reasons it is highly desirable that current plans for renewed robotic surface exploration of the Moon are developed in the context of a future human lunar exploration programme, such as that proposed by the recently formulated Global Exploration Roadmap., Accepted for publication in a forthcoming Special Issue of Planetary and Space Science on "Scientific Preparations for Lunar Exploration"

Article history: We estimate the impact flux and cratering rate as a function of latitude on the terrestrial planets using a model distribution of planet crossing asteroids and comets (Bottke, W.F., Morbidelli, A., Jedicke, R., Petit, J.-M., Levison, H.F., Michel, P., Metcalfe, T.S., 2002. Icarus 156, 399-433). After determining the planetary impact probabilities as a function of the relative encounter velocity and encounter inclination, the impact positions are calculated analytically, assuming the projectiles follow hyperbolic paths during the encounter phase. As the source of projectiles is not isotropic, latitudinal variations of the impact flux are predicted: the calculated ratio between the pole and equator is 1.05 for Mercury, 1.00 for Venus, 0.96 for the Earth, 0.90 for the Moon, and 1.14 for Mars over its long-term obliquity variation history. By taking into account the latitudinal dependence of the impact velocity and impact angle, and by using a crater scaling law that depends on the vertical component of the impact velocity, the latitudinal variations of the cratering rate (the number of craters with a given size formed per unit time and unit area) is in general enhanced. With respect to the equator, the polar cratering rate is about 30% larger on Mars and 10% on Mercury, whereas it is 10% less on the Earth and 20% less on the Moon. The cratering rate is found to be uniform on Venus. The relative global impact fluxes on Mercury, Venus, the Earth and Mars are calculated with respect to the Moon, and we find values of 1.9, 1.8, 1.6, and 2.8, respectively. Our results show that the relative shape of the crater size-frequency distribution does not noticeably depend upon latitude for any of the terrestrial bodies in this study. Nevertheless, by neglecting the expected latitudinal variations of the cratering rate, systematic errors of 20-30% in the age of planetary surfaces could exist between equatorial and polar regions when using the crater chronology method.

Seismic waves generated by meteoroid impacts have been detected by the Apollo lunar seismic network. These waves are sensitive to the crustal structure beneath both the seismic stations and the impact sites. We use a Markov chain Monte-Carlo algorithm in order to invert for lateral variations of crustal thickness on the Moon. The inversion uses travel times of seismic waves originating from multiple meteoroid impact locations. Previous seismic investigations constrained the crustal thickness solely for Apollo stations 12, 14 and 16, whereas our approach enables to estimate the crustal thickness at locations far from the Apollo network, and to build a first crustal thickness map based on seismic data. Here we compare our crustal thickness estimates based on seismic travel times to those based on inversions of the topography and gravity field. Both methods turn out to be coherent in the sense that highland sites possess a thicker crust than mare sites.

This special issue of Planetary and Space Science on “Scientific Preparations for Lunar Exploration” is comprised of papers presented at a workshop of the same name, which was hosted at ESA ESTEC in Noordwijk, the Netherlands on the 6th and 7th February 2012. This workshop was attended by around 180 scientists from a large spectrum of scientific disciplines, including space and planetary sciences, astronomy, physical sciences, life sciences, medicine and human physiology. The goal of this workshop was to identify and highlight the major scientific unknowns surrounding future exploration activities on the Moon and to identify the scientific work that needs to be performed to enable the exploration activities of the future.