Hiroki Senshu

Thermal evolution of a growing Mars

[1]xa0We developed a new numerical model which can simulate the thermal history and metal-silicate separation of a growing Mars. In this model the thermal disturbance caused by planetesimal impacts is calculated for each impact event by taking into account the effects of shock heating, crater excavation, and isostatic rebound. A metallic blob is assumed to form at the base of a magma pond if an impact site is heated above the melting temperature. Sinking of the metal blobs is traced assuming Stokes velocity. Their coalescence during sinking is treated by a Monte Carlo approach. A series of simulations is carried out assuming that Mars is formed by the runaway growth from a swarm of planetesimals as was suggested by recent numerical simulations of the planetary accretion process. Our numerical results show that (1) no global magma ocean is formed during accretion, (2) metal-silicate separation takes place without global scale melting, and (3) instead of a metallic core, a metal-rich layer is formed at the late stage of accretion.

THERMAL EVOLUTION AND LIFETIME OF INTRINSIC MAGNETIC FIELDS OF SUPER-EARTHS IN HABITABLE ZONES

We have numerically studied the thermal evolution of different-mass terrestrial planets in habitable zones, focusing on the duration of dynamo activity to generate their intrinsic magnetic fields, which may be one of the key factors in habitability of the planets. In particular, we are concerned with super-Earths, observations of which are rapidly developing. We calculated the evolution of temperature distributions in the planetary interior using Vinet equations of state, the Arrhenius-type formula for mantle viscosity, and the astrophysical mixing-length theory for convective heat transfer modified for mantle convection. After calibrating the model with terrestrial planets in the solar system, we apply it for 0.1-10 M{sub +} rocky planets with a surface temperature of 300 K (in habitable zones) and Earth-like compositions. With the criterion of heat flux at the core-mantle boundary (CMB), the lifetime of the magnetic fields is evaluated from the calculated thermal evolution. We found that the lifetime slowly increases with planetary mass (M{sub p} ), independent of the initial temperature gap at the CMB ({Delta}T{sub CMB}), but beyond the critical value M{sub c,p} ({approx}O(1) M{sub +}) it abruptly declines from the mantle viscosity enhancement due to the pressure effect. We derived M{sub c,p} as a function ofmorexa0» {Delta}T{sub CMB} and a rheological parameter (activation volume, V*). Thus, the magnetic field lifetime of super-Earths with M{sub p} >M{sub p,c} sensitively depends on {Delta}T{sub CMB}, which reflects planetary accretion, and V*, which has uncertainty at very high pressure. More advanced high-pressure experiments and first-principle simulation, as well as planetary accretion simulation, are needed to discuss the habitability of super-Earths.«xa0less

COHESION OF AMORPHOUS SILICA SPHERES: TOWARD A BETTER UNDERSTANDING OF THE COAGULATION GROWTH OF SILICATE DUST AGGREGATES

Adhesion forces between submicrometer-sized silicate grains play a crucial role in the formation of silicate dust agglomerates, rocky planetesimals, and terrestrial planets. The surface energy of silicate dust particles is the key to their adhesion and rolling forces in a theoretical model based on the contact mechanics. Here we revisit the cohesion of amorphous silica spheres by compiling available data on the surface energy for hydrophilic amorphous silica in various circumstances. It turned out that the surface energy for hydrophilic amorphous silica in a vacuum is a factor of 10 higher than previously assumed. Therefore, the previous theoretical models underestimated the critical velocity for the sticking of amorphous silica spheres, as well as the rolling friction forces between them. With the most plausible value of the surface energy for amorphous silica spheres, theoretical models based on the contact mechanics are in harmony with laboratory experiments. Consequently, we conclude that silicate grains with a radius of

Laser link experiment with the Hayabusa2 laser altimeter for in-flight alignment measurement

0.1~mu

LED minilidar for Mars rover

m could grow to planetesimals via coagulation in a protoplanetary disk. We argue that the coagulation growth of silicate grains in a molecular cloud is advanced either by organic mantles rather than icy mantles or, if there are no mantles, by nanometer-sized grain radius.

Ubiquitous Interactive Visualization of 3-D Mantle Convection through Web Applications Using Java

We report results of a laser link experiment between a laser altimeter called light detection and ranging (LIDAR) aboard Hayabusa2 and ground-based satellite laser ranging stations conducted when the spacecraft was near the Earth before and after the gravity assist operation. Uplink laser pulses from a ground station were successfully detected at a distance of 6.6xa0million km, and the field of view direction of the receiving telescope of the LIDAR was determined in the spacecraft frame. The intensities of the received signals were measured, and the link budget from the ground to the LIDAR was confirmed. By detecting two successive pulses, the pulse intervals from the ground-based station were transferred to the LIDAR, and the clock frequency offset was thus successfully calibrated based on the pulse intervals. The laser link experiment, which includes alignment measurement of the telescopes, has proven to be an excellent method to confirm the performance of laser altimeters before they arrive at their target bodies, especially for deep space missions.Graphical abstract.

Influence of majorite on hot plumes

A mini-lidar to observe the activity of Martian atmosphere is developed. The 10cm-cube LED mini-lidar was designed to be onboard a Mars rover. The light source of the mini-lidar is a high powered LED of 385nm. LED was adopted as light source because of its toughness against circumference change and physical shock for launch. The pulsed power and the pulse repetition frequency of LED beam were designed as 0.75W (=7.5nJ/10ns) and 500kHz, respectively. Lidar echoes were caught by the specially designed Cassegrain telescope, which has the shorter telescope tube than the usual to meet the 10cm-cube size limit. The high-speed photon counter was developed to pursue to the pulse repetition frequency of the LED light. The measurement range is no shorter than 30m depending back-ground condition. Its spatial resolution was improved as 0.15m (=1ns) by this photon counter. The demonstrative experiment was conducted at large wind tunnel facility of Japan Meteorological Agency. The measurement target was smoke of glycerin particles. The smoke was flowed in the wind tunnel with wind speed of 0 – 5m. Smoke diffusion and its propagation due to the wind flow were observed by the LED mini-lidar. This result suggests that the developed lidar can pursue the structure and the motion of dust devil of >2m.

Feasibility of retrieving dust properties and total column water vapor from solar spectra measured using a lander camera on Mars

We have designed a new system for real-time interactive visualization of results taken directly from large-scale simulations of 3-D mantle convection and other large-scale simulations. This approach allows for intense visualization sessions for a couple of hours as opposed to storing massive amounts of data in a storage system. Our data sets consist of 3-D data for volume rendering with sets ranging as high as over 10 million unknowns at each timestep. Large scale visualization on a display wall holding around 13 million pixels has already been accomplished with extension to hand-held devices, such as the OQO and Nokia N800. We are developing web-based software in Java to extend the use of this system across long distances. The software is aimed at creating an interactive and functional application capable of running on multiple browsers by taking advantage of two AJAX-enabled web frameworks: Echo2 and Google Web Toolkit.

LED-powered mini-lidar for martian atmospheric dust studies

The influence of MgSiO3 majorite on mantle convection has been investigated via 2-D numerical simulations that incorporate the stability field of majorite. According to a recent first principles study, wadsleyite decomposes into an assemblage of majorite plus periclase with a large negative Clapeyron slope. Since the stability field of majorite is limited to be greater than ~2200u2009K in a depth range of 500–660u2009km for Mg2SiO4, very hot upwelling plumes are expected to be strongly influenced by the phase transitions related to majorite. These hot upwellings are occasionally observed in simulations, even though the average temperature of hot plumes is far less than the stability field of majorite. The dynamics of these upwellings are controlled by the release and the absorption of latent heat induced by majorites phase transitions as well as by the interruption of currents due to the large negative Clapeyron slope related to majorite.

Dynamics of hypervelocity jetting during oblique impacts of spherical projectiles investigated via ultrafast imaging: ULTRAFAST IMAGING OF IMPACT JETTING

Dust and water vapor are important constituents in the Martian atmosphere, exerting significant influence on the heat balance of the atmosphere and surface. We have developed a method to retrieve optical and physical properties of Martian dust from spectral intensities of direct and scattered solar radiation to be measured using a multi-wavelength environmental camera onboard a Mars lander. Martian dust is assumed to be composed of silicate-like substrate and hematite-like inclusion, having spheroidal shape with a monomodal gamma size distribution. Error analysis based on simulated data reveals that appropriate combinations of three bands centered at 450, 550, and 675xa0nm wavelengths and 4 scattering angles of 3°, 10°, 50°, and 120° lead to good retrieval of four dust parameters, namely, aerosol optical depth, effective radius and variance of size distribution, and volume mixing ratio of hematite. Retrieval error increases when some of the observational parameters such as color ratio or aureole are omitted from the retrieval. Also, the capability of retrieving total column water vapor is examined through observations of direct and scattered solar radiation intensities at 925, 935, and 972xa0nm. The simulation and error analysis presented here will be useful for designing an environmental camera that can elucidate the dust and water vapor properties in a future Mars lander mission.