Kōbe-shi, Japan
Kōbe-shi, Japan

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Wada K.,Chiba Institute of Technology | Tanaka H.,Hokkaido University | Suyama T.,Nagano City Museum | Kimura H.,Center for Planetary Science | And 2 more authors.
Astrophysical Journal | Year: 2011

Collisional growth of dust aggregates is a plausible root of planetesimals forming in protoplanetary disks. However, a rebound of colliding dust aggregates prevents dust from growing into planetesimals. In fact, rebounding aggregates are observed in laboratory experiments but not in previous numerical simulations. Therefore, the condition of rebound between dust aggregates should be clarified to better understand the processes of dust growth and planetesimal formation. We have carried out numerical simulations of aggregate collisions for various types of aggregates and succeeded in reproducing a rebound of colliding aggregates under specific conditions. Our finding is that in the rebound process, the key factor of the aggregate structure is the coordination number, namely, the number of particles in contact with a particle. A rebound is governed by the energy dissipation along with restructuring of the aggregates and a large coordination number inhibits the restructuring at collisions. Results of our numerical simulation for various aggregates indicate that they stick to each other when the mean coordination number is less than 6, regardless of their materials and structures, as long as their collision velocity is less than the critical velocity for fragmentation. This criterion of the coordination number would correspond to a filling factor of 0.3, which is somewhat larger than that reported in laboratory experiments. In protoplanetary disks, dust aggregates are expected to have low bulk densities (<0.1gcm-3) during their growth, which would prevent dust aggregates from rebounding. This result supports the formation of planetesimals with direct dust growth in protoplanetary disks. © 2011. The American Astronomical Society. All rights reserved..

Crawford I.A.,Birkbeck College | Crawford I.A.,Center for Planetary science | Anand M.,Open University Milton Keynes | Anand M.,Natural History Museum in London | And 7 more authors.
Planetary and Space Science | Year: 2012

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. © 2012 Elsevier Ltd. All rights reserved.

Kimura H.,Kobe University | Kimura H.,Center for Planetary Science
Monthly Notices of the Royal Astronomical Society | Year: 2016

Photoelectron emission is crucial to electric charging of dust particles around main-sequence stars and gas heating in various dusty environments. An estimate of the photoelectric processes contains an ill-defined parameter called the photoelectric quantum yield, which is the total number of electrons ejected from a dust particle per absorbed photon. Here we revisit the so-called small particle effect of photoelectron emission and provide an analytical model to estimate photoelectric quantum yields of small dust particles in sizes down to nanometers. We show that the small particle effect elevates the photoelectric quantum yields of nanoparticles up to by a factor of 103 for carbon, water ice, and organics, and a factor of 102 for silicate, silicon carbide, and iron. We conclude the surface curvature of the particles is a quantity of great importance to the small particle effect, unless the particles are submicrometers in radius or larger. © 2016 The Author Published by Oxford University Press on behalf of the Royal Astronomical Society.

Nakajima K.,Kyushu University | Yamada Y.,Hokkaido University | Takahashi Y.O.,Center for Planetary Science | Takahashi Y.O.,Kobe University | And 3 more authors.
Journal of the Meteorological Society of Japan | Year: 2013

In this paper, we examine the steady state responses of models participating in the Aqua-Planet Experiment Project (APE) to the zonal asymmetry of equatorial sea surface temperature (SST) anomalies (SSTAs). Experiments were performed using three different SSTA distributions, i.e., two localized SSTAs with a common shape but different intensities, and an SSTA that varied with zonal wavenumber one. The structure of the responses obtained differs significantly among the models; however, some common features are also present. The principal features of the responses to the localized SSTAs are a positive precipitation anomaly over the warm SSTA, a widespread negative precipitation anomaly along the intertropical convergence zone, a pair of Rossby wavetrains along the equatorward flanks of mid-latitude westerly jets originating from a pair of upper tropospheric anticyclones that develop to the east of the warm SSTAs, and zonally wavelike precipitation and geopotential anomalies along the baroclinic zones. The structure of the tropical responses differs considerably from the Matsuno-Gill pattern, and the magnitude of the responses is almost proportional to the intensity of the localized SSTA in each of the models. The responses to the zonal wavenumber one SSTA are dominated by zonal wavenumber one structures. Around the longitudes of the warm (cold) SSTA, tropical precipitation increases (decreases). At longitudes east of the positive precipitation anomaly, the region of nearly zero absolute vorticity near the equator in the upper troposphere expands polewards, and the midlatitude westerly jets become narrower and stronger. To the west of the positive precipitation anomaly, the upper tropospheric region of nearly zero absolute vorticity shrinks, and the mid-latitude jets become weaker but broader, so that the regions of westerly winds extends to the equator, which results in the development of a zonal mean westerly wind anomaly around the equator. The longitudinal shift of the upper tropospheric westerly zonal wind anomaly relative to the precipitation anomaly is in marked contrast to that associated with the Walker circulation and the convection center around the Maritime Continent. © 2013, Meteorological Society of Japan.

Kolokolova L.,University of Maryland College Park | Kimura H.,Center for Planetary Science
Astronomy and Astrophysics | Year: 2010

Context. We study how the electromagnetic interaction between the monomers in aggregates affects the polarization of cosmic dust. Aims. We aim to show that the electromagnetic interaction depends on the porosity and composition of the aggregates and contributes significantly to the spectral gradient of polarization (polarimetric color). The results may explain the observations of some comets that demonstrated atypical negative polarimetric color in the visible and also a reverse of the positive polarimetric color to the negative one in the near-infrared. Methods. We performed computer simulations of the light scattering by aggregates consisting of spheres made of a variety of materials: transparent, absorptive, and the material similar to that of the dust in comet Halley. We studied how the number of monomers covered by the electromagnetic wave at a single period (on the light path equal to one wavelength) affects their interaction by considering linear clusters of 2 and 10 monomers of radius of 0.1 μm. Results. Electromagnetic interaction between the monomers in aggregates depolarizes the light. The interaction becomes stronger if more monomers are covered by the electromagnetic wave at a single period. Thus, the porosity of aggregates influences their polarization. The electromagnetic interaction also depends on composition and is stronger for transparent materials. Conclusions. Electromagnetic interaction between the monomers in aggregates may explain why the polarimetric color of comet dust decreases as observations move from the visible to the near-infrared since a longer wavelength covers more monomers. It may also explain why some comets exhibit negative polarimetric color even in the visible; these comets may have more compact dust. Strong electromagnetic interaction resulted either from compactness or transparency of the material can explain the negative polarimetric color of interplanetary dust and debris disks and contribute to the polarization of asteroids. In general, the spectral dependence of polarization is a promising tool for studying the properties of cosmic dust particles, particularly their porosity. © ESO, 2010.

Rymer A.M.,Johns Hopkins Applied Physics Laboratory | Mitchell D.G.,Johns Hopkins Applied Physics Laboratory | Hill T.W.,Rice University | Kronberg E.A.,Max Planck Institute for Solar System Research | And 3 more authors.
Geophysical Research Letters | Year: 2013

A 2-3 day periodicity observed in Jupiter's magnetosphere (superposed on the giant planet's 9.5 h rotation rate) has been associated with a characteristic mass-loading/unloading period at Jupiter. We follow a method derived by Kronberg et al. () and find, consistent with their results, that this period is most likely to fall between 1.5 and 3.9 days. Assuming the same process operates at Saturn, we argue, based on equivalent scales at the two planets, that its period should be 4 to 6 times faster at Saturn and therefore display a period of 8 to 18 h. Applying the method of Kronberg et al. for the mass-loading source rates estimated by Smith et al. () based on data from the third and fifth Cassini-Enceladus encounters, we estimate that the expected magnetospheric refresh rate varies from 8 to 31 h, a range that includes Saturn's rotation rate of ∼10.8 h. The magnetospheric period we describe is proportional to the total mass-loading rate in the system. The period is, therefore, faster (1) for increased outgassing from Enceladus, (2) near Saturn solstice (when the highest proportion of the rings is illuminated), and (3) near solar maximum when ionization by solar photons maximizes. We do not claim to explain the few percent jitter in period derived from Saturn Kilometric Radiation with this model, nor do we address the observed difference in period observed in the north and south hemispheres. ©2013. American Geophysical Union. All Rights Reserved.

Tanaka K.K.,Hokkaido University | Yamamoto T.,Hokkaido University | Kimura H.,Center for Planetary Science
Astrophysical Journal | Year: 2010

We construct a theoretical model for low-temperature crystallization of amorphous silicate grains induced by exothermic chemical reactions. As a first step, the model is applied to the annealing experiments, in which the samples are (1) amorphous silicate grains and (2) amorphous silicate grains covered with an amorphous carbon layer. We derive the activation energies of crystallization for amorphous silicate and amorphous carbon from the analysis of the experiments. Furthermore, we apply the model to the experiment of low-temperature crystallization of an amorphous silicate core covered with an amorphous carbon layer containing reactive molecules. We clarify the conditions of low-temperature crystallization due to exothermic chemical reactions. Next, we formulate the crystallization conditions so as to be applicable to astrophysical environments. We show that the present crystallization mechanism is characterized by two quantities: the stored energy density Q in a grain and the duration of the chemical reactions τ. The crystallization conditions are given by Q>Q min and τ < τcool regardless of details of the reactions and grain structure, where τcool is the cooling timescale of the grains heated by exothermic reactions, and Q min is minimum stored energy density determined by the activation energy of crystallization. Our results suggest that silicate crystallization occurs in wider astrophysical conditions than hitherto considered. © 2010. The American Astronomical Society. All rights reserved..

Suzuki A.I.,Center for Planetary Science | Nakamura A.M.,Kobe University | Kadono T.,University of Occupational and Environmental Health Japan | Wada K.,Chiba Institute of Technology | And 2 more authors.
Icarus | Year: 2013

Ejecta patterns are experimentally examined around craters formed in a layer of glass beads by vertical impacts at low velocities. The diameters of the constituent glass beads of three different targets range 53-63μm, 90-106μm, and 355-500μm. The impact velocities and ambient pressures range from a few to 240ms-1 and from 500Pa to the atmospheric pressure, respectively. Various ejecta patterns are observed around craters and are classified into two major classes based on whether they have concentric ridges or not. We propose a possible formation model for the ridges in which the wake created by a projectile as it passes through the atmosphere causes the crater rim to collapse: The model can explain the observation that the degree of collapse of the resultant crater rim depends on the impact velocity and ambient pressure. Using the ratio between the hydrodynamic drag of the airflow induced by the wake and the gravitational force of the degraded part of the rim, we calculate the critical conditions of the impact velocity and ambient pressure necessary for the wake to erode the rim. The conditions turn out to be roughly consistent with the boundary between the two morphological classes. As a result, it is possible that the projectile wake triggers the collapse of the crater rim, leading to a ground-hugging flow that settles to form the distal ridge observed in this study. This mechanism may play a role in producing ejecta morphologies on planetary bodies with atmosphere. © 2013 Elsevier Inc.

Masters A.,University College London | Masters A.,Center for Planetary science | Thomsen M.F.,Los Alamos National Laboratory | Badman S.V.,Japan Aerospace Exploration Agency | And 6 more authors.
Geophysical Research Letters | Year: 2011

Detecting plasma dynamics in Saturn's magnetosphere is essential for understanding energy flow through the system. It has been proposed that both the Dungey and Vasyliunas cycles operate at Saturn, and the competition between these cycles has been debated. We examine data taken by the Cassini spacecraft in Saturn's post-dawn magnetosphere, ∼17.5 Saturn radii from the planet, and identify an example of return flow from magnetotail reconnection. The flow included water group ions and had elevated ion temperatures (of order 1 keV), consistent with Vasyliunas cycle return flow. The flow was also supercorotating (∼200 km s-1, ∼120% of corotation), which is highly atypical of Saturn's outer magnetosphere. Our results suggest that return flows are time-variable, and our results concerning Dungey cycle return flows are inconclusive. We propose that supercorotating flows in Saturn's dawn magnetosphere strongly influence the current system that is responsible for the planet's main auroral emission. Copyright 2011 by the American Geophysical Union.

Kuramoto K.,Hokkaido University | Kuramoto K.,Center for Planetary science | Umemoto T.,Hokkaido University | Ishiwatari M.,Hokkaido University | Ishiwatari M.,Center for Planetary science
Earth and Planetary Science Letters | Year: 2013

Hydrodynamic escape of hydrogen driven by solar extreme ultraviolet (EUV) radiation heating is numerically simulated by using the constrained interpolation profile scheme, a high-accuracy scheme for solving the one-dimensional advection equation. For a wide range of hydrogen number densities at the lower boundary and solar EUV fluxes, more than half of EUV heating energy is converted to mechanical energy of the escaping hydrogen. Less energy is lost by downward thermal conduction even giving low temperature for the atmospheric base. This result differs from a previous numerical simulation study that yielded much lower escape rates by employing another scheme in which relatively strong numerical diffusion is implemented. Because the solar EUV heating effectively induces hydrogen escape, the hydrogen mixing ratio was likely to have remained lower than 1. vol% in the anoxic Earth atmosphere during the Archean era. © 2013 Elsevier B.V.

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