Small Planetary Bodies Research Articles

Lunar energetic neutral atom (ENA) spectra measured by the interstellar boundary explorer (IBEX)

The solar wind continuously flows out from the Sun, filling interplanetary space and directly interacting with the surfaces of small planetary bodies and other objects throughout the solar system. A significant fraction of these ions backscatter from the surface as energetic neutral atoms (ENAs). The first observations of these ENA emissions from the Moon were recently reported from the Interstellar Boundary Explorer (IBEX). These observations yielded a lunar ENA albedo of ∼10% and showed that the Moon reflects ∼150 metric tons of neutral hydrogen per year. More recently, a survey of the first 2.5 years of IBEX observations of lunar ENAs was conducted for times when the Moon was in the solar wind. Here, we present the first IBEX ENA observations when the Moon is inside the terrestrial magnetosheath and compare them with observations when the Moon is in the solar wind. Our analysis shows that: (1) the ENA intensities are on average higher when the Moon is in the magnetosheath, (2) the energy spectra are similar above ~0.6* solar wind energy but below there are large differences of the order of a factor of 10, (3) the energy spectra resemble a power law with a “hump” at ∼0.6 * solar wind energy, and (4) this “hump” is broader when the Moon is in the magnetosheath. We explore potential scenarios to explain the differences, namely the effects of the topography of the lunar surface and the consequences of a very different Mach number in the solar wind versus in the magnetosheath.

Differentiated Planetesimals and the Parent Bodies of Chondrites

Meteorites are samples of dozens of small planetary bodies that formed in the early Solar System. They exhibit great petrologic diversity, ranging from primordial accretional aggregates (chondrites), to partially melted residues (primitive achondrites), to once fully molten magmas (achondrites). It has long been thought that no single parent body could be the source of more than one of these three meteorite lithologies. This view is now being challenged by a variety of new measurements and theoretical models, including the discovery of primitive achondrites, paleomagnetic analyses of chondrites, thermal modeling of planetesimals, the discoveries of new metamorphosed chondrites and achondrites with affinities to some chondrite groups, and the possible identification of extant partially differentiated asteroids. These developments collectively suggest that some chondrites could in fact be samples of the outer, unmelted crusts of otherwise differentiated planetesimals with silicate mantles and metallic cores. This may have major implications for the origin of meteorite groups, the meaning of meteorite paleomagnetism, the rates and onset times of accretion, and the interior structures and histories of asteroids.

Si isotope fractionation between Si-poor metal and silicate melt at pressure–temperature conditions relevant to metal segregation in small planetary bodies

Experimental investigations of Si isotope fractionation between Si-bearing metal alloy and silicate phases have to date been limited to high pressure (1–7GPa) and high temperature (1800–2200°C) conditions at highly reducing conditions, to optimize applicability of results to early core formation processes in the Earth. Here, we assess the extent and mechanism of Si isotopic fractionation at conditions relevant to metal segregation in small (km-scale) planetary bodies, using samples obtained from an industrial-scale blast furnace of Tata Steel (IJmuiden, the Netherlands).During the low-pressure, high-temperature process of steelmaking inhomogeneous blast furnace burden consisting of pre- and untreated iron ore, iron silicates and coke is reduced to oxygen fugacities near the C–CO buffer, resulting in the segregation of a metal phase containing only ∼0.3wt% Si. Seven sample sets, each comprising a metal alloy and a silicate slag, were taken during tapping of the blast furnace at tapping temperatures between 1400°C and 1600°C. We find large isotopic mass fractionation between metal and silicate, with Δ30Sisilicate–metal varying between 0.7‰ and 1.6‰, values that are as high as previously obtained in high-pressure, highly reduced experiments. A model for metal–silicate Si isotope fractionation in blast furnaces can explain both the sense and magnitude of fractionation, if the presence of SiO-bearing vapour is explicitly taken into account. Our data indicate that significant Si isotope fractionation can occur between metal and silicate at low-pressure, high-temperature and only mildly reducing conditions for which Si solubility in molten Fe-rich metal is low. This suggests an important role for SiO at low confining pressures. Our data can be applied to models of aubrite meteorite formation through high-temperature differentiation of an enstatite chondrite parent body. Our calculations suggest a far larger degree of rehomogenisation during differentiation than previously thought on the basis of metal–silicate Si isotopic fractionation measured in natural meteorites that re-equilibrated at low temperature.

Highly siderophile element and osmium isotope evidence for postcore formation magmatic and impact processes on the aubrite parent body

Abstract– Aubrites exhibit a wide range of highly siderophile element (HSE—Re, Os, Ir, Ru, Rh, Pt, Pd, Au) concentrations and 187Os/188Os compositions. Their HSE concentrations are one to three orders of magnitude less than chondrites, with the exception of the Shallowater and Mt. Egerton samples. While most aubrites show chondritic HSE abundance ratios, significant enrichments of Pd and Re relative to Os, Ir, and Ru are observed in 12 of 16 samples. Present‐day 187Os/188Os ratios range from subchondritic values of 0.1174 to superchondritic values of up to 0.2263. Half of the samples have 187Os/188Os ratios of 0.127 to 0.130, which is in the range of enstatite chondrites. Along with the brecciated nature of aubrites, the HSE and Re‐Os isotope systematics support a history of extensive postaccretion processing, including core formation, late addition of chondritic material and/or core material and potential breakup and reassembly. Highly siderophile element signatures for some aubrites are consistent with a mixing of HSE‐rich chondritic fragments with a HSE‐free aubrite matrix. The enrichments in incompatible HSE such as Pd and Re observed in some aubrites, reminiscent of terrestrial basalts, suggest an extensive magmatic and impact history, which is supported by both the 187Re‐187Os isotope system and silicate‐hosted isotope systems (Rb‐Sr, K‐Ar) yielding young formation ages of 1.3–3.9 Ga for a subset of samples. Compared with other differentiated achondrites derived from small planetary bodies, aubrites show a wide range in HSE concentrations and 187Os/188Os, most similar to angrites. While similarities exist between the diverse groups of achondrites formed early in solar system history, the aubrite parent body(ies) clearly underwent a distinct evolution, different from angrites, brachinites, ureilites, howardites, eucrites, and diogenites.

A new spectral calculation scheme for long‐term deformation of Maxwellian planetary bodies

Viscoelasticity plays an important role in long‐term deformation of large‐scale topographies on small planetary bodies, such as the Moon. Although we should consider increases in viscosity due to secular cooling during long‐term deformation, previous spectral calculation schemes for a Maxwellian viscoelastic body have problems in calculating deformation with temporal and radial variation in viscosity. To resolve these problems, we propose a new formulation of the constitutive equation of a Maxwell body. Compared with conventional time domain spectral schemes, our scheme, with its second‐order approximation in time, demonstrates significant improvement in both accuracy and stability for long time steps. As a result, the computational costs for time‐consuming calculations, such as the long‐term evolution of surface topography, are greatly reduced. Using this scheme, we investigate the effect of planetary cooling on the deformation of an initially hot but monotonically cooling Moon. Our calculation results show that the effective elastic thickness depends on the wavelength of deformation for loads emplaced at very early times (<20 Myr). The results of our calculations also show that without additional heat sources other than initial accretion heat, hardening due to secular cooling prevents substantial isostatic compensation and crustal lateral flow underneath large impact basins. This result indicates that small planetary bodies, such as the Moon, do not retain the memory of their initial state during the accretion phase for very long and that their current viscoelastic states reflect sustained heat production and/or transport from their deep interior.

Considerations on a suction drill for lunar surface drilling and sampling: II. experimental tests in vacuum conditions

Drilling devices with the ability to create narrow, but deep, boreholes in a planetary surface are important tools for the exploration of the structure and properties of planetary surface layers. Therefore, they are usually proposed for planetary landers for the Moon, Mars, Venus or small planetary bodies. A method based on the use of a cold gas flow for ejecting debris particles from the borehole has recently been suggested and investigated theoretically (Komle et al. in Acta Geotech 3:201–214, 2008a). The current paper reports on laboratory experiments designed to validate this method under vacuum or low pressure conditions. Two different sample materials were used: (1) glass beads in the sub-millimeter size range, and (2) the certified lunar analog material JSC-1A, a finely crushed basaltic rock. For both materials, the suction process for removing the particles from a simulated borehole worked well and with a moderate gas consumption.

An Internal Heating Model to Elucidate the Shape of a Small Planetary Body

Small planetary bodies usually have irregular shapes. If they are large enough to be heated to a partial melting status, the deforming force of gravity could overcome the internal forces and make the shape transfigure from potato-like to spherical. We have developed a model to calculate the thermal history of a planetoid and apply the model to asteroids, since ample evidence has shown that many asteroids could have undergone differentiation. After revealing the relation between the shape and the ratio of the melt part, we also examine the surface roughness of these asteroids and suggest that 280 km would be a critical radius for an asteroid to develop a virtually globular contour.

Discrete element modeling of Martian pit crater formation in response to extensional fracturing and dilational normal faulting

[1] Pit craters, circular to elliptical depressions that lack a raised rim or ejecta deposits, are common on the surface of Mars. Similar structures are also found on Earth, Venus, the Moon, and smaller planetary bodies, including some asteroids. While it is generally accepted that these pits form in response to material drainage into a subsurface void space, the primary mechanism(s) responsible for creating the void is a subject of debate. Previously proposed mechanisms include collapse into lave tubes, dike injection, extensional fracturing, and dilational normal faulting. In this study, we employ two-dimensional discrete element models to assess both extensional fracturing and dilational normal faulting as mechanisms for forming pit craters. We also examine the effect of mechanical stratigraphy (alternating strong and weak layers) and variation in regolith thickness on pit morphology. Our simulations indicate that both extensional fracturing and dilational normal faulting are viable mechanisms. Both mechanisms lead to generally convex (steepening downward) slope profiles; extensional fracturing results in generally symmetric pits, whereas dilational normal faulting produces strongly asymmetric geometries. Pit width is established early, whereas pit depth increases later in the deformation history. Inclusion of mechanical stratigraphy results in wider and deeper pits, particularly for the dilational normal faulting, and the presence of strong near-surface layers leads to pits with distinct edges as observed on Mars. The modeling results suggest that a thicker regolith leads to wider but shallower pits that are less distinct and may be more difficult to detect in areas of thick regolith.

Accurate Localization for Space Rovers and its Computational Reduction

AbstractTo localize a rover on small planetary body, a method using round-trip propagation delay of radio waves is most promising. In order to improve the estimation accuracy, it is necessary to estimate rotational motion of the small planetary body. A method of localization has been expanded to estimate also the rotational parameters of the small planetary body. The expanded problem includes complex nonlinear and dynamical issues that it cannot be solved analytically. So, the estimation problem has been formulated as an optimization problem to minimize the loss function defined based on the estimation errors derived in comparison with actual measurement data. A solution for the optimization problem has been proposed, which uses Powell's conjugate direction method for local searches. Although this solution does not require calculation of derivatives, it requires a large amount of computation since several forward calculations of the state are required for each minimum search. In this paper, a method to select measurement data is described, which provides as accurate estimation as the original results with reducing the computational amount. The main idea of selection is to conserve the sensitivity of the measurement data. The proposed method to select data is compared with the results using the decimated data of equal interval. Simulation results and experimental results are shown to evaluate computational reduction and estimation accuracy.

Low temperature spectroscopy and irradiation effects on Solar System ices – Kuiper belt objects as a case study

Since their discovery in 1992, Kuiper Belt Objects (KBOs)—small planetary bodies beyond the orbit of Neptune and ‘precursors’ to short periodic comets— have received considerable attention from the planetary sciences, astronomy, and physical chemistry community. At the low temperatures of KBOs of 30–50 K, a chemical modification of their surface ices such as water, nitrogen, carbon monoxide, carbon dioxide, methanol, and possibly ammonia is predominantly induced via non-equilibrium chemistry through ionizing radiation from solar wind particles, solar wind photons, and galactic cosmic ray (GCR) particles. Laboratory experiments simulating the interactions of ionizing radiation with icy surfaces at ultralow temperatures offer a remarkable opportunity for understanding the chemical processing of primitive bodies and allow for an establishment of a ‘chemical’ time-line dating back to the origin of our Solar System. Since prebiotic molecules like amino acids and their precursors can be synthesized in these ices as well, those studies also explore scenarios where in our Solar System astrobiologically important molecules might have been synthesized since the formation of the Solar System 4.6 billion years ago. The symposium ‘Kuiper Belt Objects – Laboratory Studies, Models, and Observations’ at the Pacifichem 2010 in Honolulu, Hawaii, focussed on the interplay between laboratory spectroscopy, theoretical investigations, and fundamental laboratory studies on specific molecular processes, which lead to a chemical modification of KBO surfaces with the overall goal to extract generalized concepts on the chemical processing of KBO surfaces and related bodies in the outer Solar System. Four ‘radiation’ sources are considered ranging from high energy electrons and MeV nuclei (simulating GCR particles) via solar photons to thermal chemistry of open shell reactants like hydroxyl radicals and hydrogen atoms at low temperatures. In detail, Zheng et al. (DOI: 10.1039/c1cp20528e) demonstrated that an investigation of the chemistry of (suprathermal) oxygen atoms, formed in the decomposition of water molecules upon interaction with energetic electrons in the track of galactic cosmic ray particles (GCRs) yields vital laboratory data on the interacting of non-equilibrium species with waterrich KBO ices. Kim et al. (DOI: 10.1039/ c1cp20658c) proposed that non-equilibrium effects of triplet carbon monoxide in nitrogen-rich ices lead to the formation of the diazirinone molecule (N2CO), which was recently characterized for the very first time spectroscopically. These results were amplified by Pilling et al.’s study (DOI: 10.1039/c1cp20592g) on the processing of water–carbon dioxide ices by heavy MeV particles of the galactic cosmic radiation field. Non-equilibrium effects are also induced via the photolysis of water-bearing ices as outlined by Kinugawa et al. (DOI: 10.1039/c1cp20595a) and Andersson et al. (DOI: 10.1039/ c1cp21138b) utilizing sophisticated time-offlight spectroscopy of the photofragments produced at 157 nm. Oba et al. (DOI: 10. 1039/c1cp20596j), Hidaka et al. (DOI: 10.1039/c1cp20645a), and He et al. (DOI: 10.1039/c1cp21601e) showed nicely that reactions of hydroxyl radicals and hydrogen atoms can also result in a rich chemistry on KBO surfaces. Finally, Kayi et al. (DOI: 10.1039/c1cp20656g) demonstrated via electronic structure calculations that the astrobiologically relevant glycine amino acid is thermodynamically less stable by 31–37 kJ mol 1 compared to its methylcarbamic acid isomer thus raising questions of a possible photon-induced isomerization of both structures. We hope that the timeliness of this topic in light of the currently operating New Horizon Mission (NASA) on its way to (ex-planet) Pluto and the Rosetta Mission (ESA) launched in 2004, which is currently on its route to comet 67P/ChuryumovGerasimenko, triggers further laboratory, spectroscopy, and theoretical investigations on the stability and chemical processing of ices of Kuiper belt objects in the next decade. Department of Chemistry, University of Hawaii at Manoa, 2545 McCarthy Mall, Honolulu, HI 96822-2275, USA. E-mail: ralfk@hawaii.edu b State Key Laboratory of Molecular Reaction Dynamics, Institute of Chemistry, Chinese Academy of Sciences, No. 2, North 1st Street, ZhongGuanCun, Beijing 100190, P. R. China Kanagawa Institute of Technology, Atsugi 243-0292, Japan Department of Chemistry, National Dong Hua University, Hualien, 974, Taiwan PCCP Dynamic Article Links

Optimizing Virtual Superimpositions: User–centered Design for a UAR Supported Smart Home System

To localize a rover on small planetary body, a method using round-trip propagation delay of radio waves is most promising. In order to improve the estimation accuracy, it is necessary to estimate rotational motion of the small planetary body. A method of localization has been expanded to estimate also the rotational parameters of the small planetary body. The expanded problem includes complex nonlinear and dynamical issues that it cannot be solved analytically. So, the estimation problem has been formulated as an optimization problem to minimize the loss function defined based on the estimation errors derived in comparison with actual measurement data. A solution for the optimization problem has been proposed, which uses Powell's conjugate direction method for local searches. Although this solution does not require calculation of derivatives, it requires a large amount of computation since several forward calculations of the state are required for each minimum search. In this paper, a method to select measurement data is described, which provides as accurate estimation as the original results with reducing the computational amount. The main idea of selection is to conserve the sensitivity of the measurement data. The proposed method to select data is compared with the results using the decimated data of equal interval. Simulation results and experimental results are shown to evaluate computational reduction and estimation accuracy.

Accurate Localization for Space Rovers and its Computational Reduction

To localize a rover on small planetary body, a method using round-trip propagation delay of radio waves is most promising. In order to improve the estimation accuracy, it is necessary to estimate rotational motion of the small planetary body. A method of localization has been expanded to estimate also the rotational parameters of the small planetary body. The expanded problem includes complex nonlinear and dynamical issues that it cannot be solved analytically. So, the estimation problem has been formulated as an optimization problem to minimize the loss function defined based on the estimation errors derived in comparison with actual measurement data. A solution for the optimization problem has been proposed, which uses Powell's conjugate direction method for local searches. Although this solution does not require calculation of derivatives, it requires a large amount of computation since several forward calculations of the state are required for each minimum search. In this paper, a method to select measurement data is described, which provides as accurate estimation as the original results with reducing the computational amount. The main idea of selection is to conserve the sensitivity of the measurement data. The proposed method to select data is compared with the results using the decimated data of equal interval. Simulation results and experimental results are shown to evaluate computational reduction and estimation accuracy.

Estimation Method of Rotation Parameters of a Small Planetary Body and Position of a Rover Based on Measurements of Round-trip Propagation Delay

Estimation Method of Rotation Parameters of a Small Planetary Body and Position of a Rover Based on Measurements of Round-trip Propagation Delay

1A2-B18 電波を用いた往復伝播時間測定による位置同定法の実験的研究

A method for accurate localization of a space rover on a surface of a small planetary body has been developed for guiding it to a destination point. The method uses round-trip propagation delay of radio waves, range, between a mother spacecraft and the rover. Numerical simulations proved that the developed method is able to estimate the position of the rover with reasonable accuracy assuming that the measurement noise is Gaussian. Laboratory experiments have been conducted via range measurement tool to validate the convergence of the proposed method of estimation, whose results are reported in this paper. These results proved that the estimation converges even the measurement data includes real noise existed in the range measurement tool, which supports the feasibility of the proposed method of estimation.

Evidence for high-pressure core-mantle differentiation from the metal–silicate partitioning of lithophile and weakly-siderophile elements

Liquid Fe metal–liquid silicate partition coefficients for the lithophile and weakly-siderophile elements Ta, Nb, V, Cr, Si, Mn, Ga, In and Zn have been measured in multianvil experiments performed from 2 to 24 GPa, 2023–2873 K and at oxygen fugacities of −1.3 to −4.2 log units relative to the iron-wüstite buffer. Compositional effects of light elements dissolved in the metal liquid (S, C) have been examined and experiments were performed in both graphite and MgO capsules, specifically to address the effect of C solubility in Fe-metal on siderophile element partitioning. The results were used to examine whether there is categorical evidence that a significant portion of metal–silicate equilibration occurred under very high pressures during core-mantle fractionation on Earth. Although the depletion of V from the mantle due to core formation is significantly greater than that of Nb, our results indicate that both elements have similar siderophile tendencies under reducing conditions at low pressures. With increasing pressure, however, Nb becomes less siderophile than V, implying that average metal–silicate equilibration pressures of at least 10–40 GPa are required to explain the Nb/V ratio of the mantle. Similarly the moderately-siderophile, volatile element ratios Ga/Mn and In/Zn are chondritic in the mantle but both volatility and core-mantle equilibration at low pressure would render these ratios strongly sub-chondritic. Our results indicate that pressures of metal–silicate partitioning exceeding 30–60 GPa would be required to render these element ratios chondritic in the mantle. These observations strongly indicate that metal–silicate equilibration must have occurred at high pressures, and therefore support core-formation models that involve deep magma oceans. Moreover, our results allow us to exclude models that envisage primarily low-pressure (<1 GPa) equilibration in relatively small planetary bodies. We also argue that the core cannot contain significant U as this would require metal–silicate equilibration at oxygen fugacities low enough for significant amounts of Ta to have also been extracted from the mantle. Likewise, as In is more siderophile than Pb but similarly volatile and also quite chalcophile it would have been difficult for Pb to enter the core without reversing the relative depletions of these elements in the mantle unless metal–silicate equilibration occurred at high pressures >20 GPa.

High-pressure melting relations in Fe–C–S systems: Implications for formation, evolution, and structure of metallic cores in planetary bodies

We present new high-pressure temperature experiments on melting phase relations of Fe–C–S systems with applications to metallic core formation in planetary interiors. Experiments were performed on Fe–5 wt% C–5 wt% S and Fe–5 wt% C–15 wt% S at 2–6 GPa and 1050–2000 °C in MgO capsules and on Fe–13 wt% S, Fe–5 wt% S, and Fe–1.4 wt% S at 2 GPa and 1600 °C in graphite capsules. Our experiments show that: (a) At a given P– T, the solubility of carbon in iron-rich metallic melt decreases modestly with increasing sulfur content and at sufficiently high concentration, the interaction between carbon and sulfur can cause formation of two immiscible melts, one rich in Fe-carbide and the other rich in Fe-sulfide. (b) The mutual solubility of carbon and sulfur increases with increasing pressure and no super-liquidus immiscibility in Fe-rich compositions is likely expected at pressures greater than 5–6 GPa even for bulk compositions that are volatile-rich. (c) The liquidus temperature in the Fe–C–S ternary is significantly different compared to the binary liquidus in the Fe–C and Fe–S systems. At 6 GPa, the liquidus of Fe–5 wt% C–5 wt% S is 150–200 °C lower than the Fe–5 wt% S. (d) For Fe–C–S bulk compositions with modest concentration of carbon, the sole liquidus phase is iron carbide, Fe 3C at 2 GPa and Fe 7C 3 at 6 GPa and metallic iron crystallizes only with further cooling as sulfur is concentrated in the late crystallizing liquid. Our results suggest that for carbon and sulfur-rich core compositions, immiscibility induced core stratification can be expected for planets with core pressure less than ∼6 GPa. Thus planetary bodies in the outer solar system such as Ganymede, Europa, and Io with present day core–mantle boundary (CMB) pressures of ∼8, ∼5, and 7 GPa, respectively, if sufficiently volatile-rich, may either have a stratified core or may have experienced core stratification owing to liquid immiscibility at some stage of their accretion. A similar argument can be made for terrestrial planetary bodies such as Mercury and Earth’s Moon, but no such stratification is predicted for cores of terrestrial planets such as Earth, Venus, and Mars with the present day core pressure in the order ⩾136 GPa, ⩾100 GPa, and ⩾23 GPa. (e) Owing to different expected densities of Fe-rich (and carbon-bearing) and sulfur-rich metallic melts, their settling velocities are likely different; thus core formation in terrestrial planets may involve rain of more than one metallic melt through silicate magma ocean. (f) For small planetary bodies that have core pressures <6 GPa and have a molten core or outer core, settling of denser carbide-rich liquid or flotation of lighter, sulfide-rich melt may contribute to an early, short-lived geodynamo.

Energetics of asteroid dynamos and the role of compositional convection

The conditions under which a dynamo can operate in the core of a small planetary body or asteroid are examined. Compositional convection driven by inner core growth is thermodynamically much more efficient than thermal convection at driving a dynamo, but whether asteroid cores crystallize in this fashion is currently uncertain. Inner core solidification will drive dynamo activity in cores larger than ≈50–150 km in radius. Dynamo activity requires core cooling rates exceeding ∼0.001–0.1 K/My if compositional convection occurs. In the absence of an inner core, cooling rates of ∼1–100 K/My or heating by 60Fe within 10–20 Myr of solar system formation are required to drive a dynamo. If inner core growth is important (as for the IVB iron meteorite parent body) then a dynamo should develop with magnetic paleointensities that depend on the core sulphur content. If 60Fe decay is dominant, the frequency of asteroid dynamo occurrence is predicted to decay with distance from the Sun.

電波の伝搬遅延時間測定による小天体探査ローバの位置同定法

Investigation of small planetary bodies is now attracted and some missions on asteroids and comets have been realized. Such bodies are full of geographical features that next demand would be direct surface investigation by a rover. In order to navigate the rover to the specific points, the localization of the rover is required. However, conventional localization method is not adequate on small bodies whose size is less than 1[km]. One of the conventional methods, localization by camera images obtained on the rover needs a map made by a mother spacecraft. The resolution of the map is not enough for the rover's local view. Another method, localization by observing the movements of stars and the sun has little accuracy. Global positioning system (GPS) is not practical for small planetary bodies because it needs several orbiters. In this paper, we propose a reasonable method which localizes the rover on a small body with reasonable accuracy, with reasonable time and with reasonable resource. This method measures two way range between a rover and a mother spacecraft repeatedly. Even this method uses only one orbiter, the localization accuracy is estimated about 1[m] assuming ITOKAWA-size asteroidwhoseradiusisatmost600[m].

Percolative core formation in planetesimals

The percolation of liquid iron alloy through crystalline silicates potentially played an important role during core formation in small bodies of the early solar system, such as asteroids and planetesimals. This is because heat production by radioactive decay of 26Al and 60Fe, which is believed to be the main heat source in early-formed small planetary bodies, will initially cause Fe–S melts to form, well before the silicates start to melt. In order to test the feasibility of percolation, the effect of pressure on the dihedral angle between Fe–O–S liquid and olivine has been investigated from 1.5 to 5.0 GPa, a pressure range that is relevant for the interiors of large asteroids. Texturally-equilibrated dihedral angles increase from 54° to 98° over this pressure range. The dihedral angle reaches the critical value of 60° at 2–3 GPa depending on the olivine composition (Fe#). This change in dihedral angle is related to the oxygen content of Fe–O–S phase, which decreases with increasing pressure, because oxygen dissolved in the melt reduces the Fe–S melt/olivine interfacial energy. These results show that Fe–O–S liquid can form an interconnected network and percolate through silicate aggregates under conditions of high oxygen fugacity and low pressure, even when the melt fraction is small. Therefore, percolation is likely to have been the dominant core formation mechanism in small relatively-oxidised planetary bodies with a radius less than about 1300 km.

New ideas on the early solar system

John Bridges, Jamie Gilmour and Ian Sanders consider developments on understanding the role of planetesimal formation in the early solar system, highlighting the work of the late Robert Hutchison. Abstract At the 12 October 2007 RAS Specialist Discussion Meeting, the UK meteorites and planetary science community discussed a range of new and existing ideas about how the earliest planetesimals formed. The meeting was held in honour of the late Dr Robert Hutchison, a meteorite specialist and Gold Medal winner of the RAS. Many of the models that have been used over the past 40 years are based on what we have learnt about asteroids from primitive meteorites. These are believed to be fragments of what were originally small planetary bodies (planetesimals) that failed to aggregate into a large planet between the orbits of Mars and Jupiter.