Seiji Sugita

Farside Gravity Field of the Moon from Four-Way Doppler Measurements of SELENE (Kaguya)

The farside gravity field of the Moon is improved from the tracking data of the Selenological and Engineering Explorer (SELENE) via a relay subsatellite. The new gravity field model reveals that the farside has negative anomaly rings unlike positive anomalies on the nearside. Several basins have large central gravity highs, likely due to super-isostatic, dynamic uplift of the mantle. Other basins with highs are associated with mare fill, implying basalt eruption facilitated by developed faults. Basin topography and mantle uplift on the farside are supported by a rigid lithosphere, whereas basins on the nearside deformed substantially with eruption. Variable styles of compensation on the near- and farsides suggest that reheating and weakening of the lithosphere on the nearside was more extensive than previously considered.

Spectroscopic measurements of vapor clouds due to oblique impacts

Vapor clouds generated by oblique impacts were observed spectroscopically. The observations here concentrate on the earliest downrange-moving component, among multiple components of vapor clouds generated by hypervelocity impacts of quartz projectiles into dolomite targets. The spectrum of impact vapor simultaneously exhibits blackbody radiation, molecular band emission, and atomic line emission, but their relative ratios change with time and space. The spectrum of the earliest component is dominated by the line/band emissions. The strong band/line emissions demonstrate the presence of a gas phase. The ratio of line/band emission to blackbody radiation is higher in near-vertical impacts than shallower angle impacts. Ratios of normalized intensities of emission lines indicate that a Boltzmann distribution with an equilibrium temperature ranging from 4000 K to 6000 K approximates well the distribution of calcium atoms in energy levels in the impact vapor. Furthermore, the temperature of impact vapor appears to be controlled by the vertical component of the impact velocity. The degree of self-absorption of a calcium emission line, which comes from vaporized dolomite target, indicates that only a very small mass of impact vapor is involved in the observed atomic radiation process. This observation, as well as the short lifetime and extremely high temperature, suggests that jetting may be the dominant source of the line emissions in the earliest stage vapor component at impact velocities less than 6 km/s. The experimental techniques and analysis methods presented in this study are applicable to other components of impact-generated vapor and will provide new information on vaporization mechanisms in hypervelocity impacts.

Felsic highland crust on Venus suggested by Galileo Near‐Infrared Mapping Spectrometer data

Received 2 March 2008; revised 29 July 2008; accepted 18 September 2008; published 31 December 2008. [1] We evaluated the spatial variation of Venusian surface emissivity at 1.18 mm wavelength and that of near-surface atmospheric temperature using multispectral images obtained by the Near-Infrared Mapping Spectrometer (NIMS) on board the Galileo spacecraft. The Galileo NIMS observed the nightside thermal emission from the surface and the deep atmosphere of Venus, which is attenuated by scattering from the overlying clouds. To analyze the NIMS data, we used a radiative transfer model based on the adding method. Although there is still an uncertainty in the results owing to the not well known parameters of the atmosphere, our analysis revealed that the horizontal temperature variation in the near-surface atmosphere is no more than ±2 K on the Venusian nightside and also suggests that the majority of lowlands likely has higher emissivity compared to the majority of highlands. One interpretation for the latter result is that highland materials are generally composed of felsic rocks. Since formation of a large body of granitic magmas requires water, the presence of granitic terrains would imply that Venus may have had an ocean and a mechanism to recycle water into the mantle in the past.

Spectroscopic characterization of hypervelocity jetting : Comparison with a standard theory

Symmetric collision between two identical plates has yielded successful theoretical models for the jetting process. Consequently, assessment of impact jetting at planetary scales has been largely based on the theories developed for such specific types of collisions. Little experimental work has been done, however, to measure both temperature and target-to-projectile mass ratio of jetting created by spherical projectiles impacting planar targets, which typify planetary impacts. The goal of this study is to examine the validity of applying planar-impact theories to jetting due to impacts of spherical projectiles into planar targets. Using a newly developed spectroscopic approach, we observe jetting created by copper spheres impacting planar dolomite targets at hypervelocities. In contrast with previous experiments using quartz projectiles, the observed mean temperatures of jets due to copper projectiles does not correlate well with the vertical component of impact velocity. Instead, the observed temperatures of jets show much better correlation with impact velocity than the vertical component of impact velocity and impact angle. The experiments also reveal that the target-to-projectile mass ratio within a jet increases with impact angle (measured from the horizontal). In order to understand the significance of these experimental results, they were then compared with a jetting model for asymmetric collisions based on standard theories. Such a comparison indicates qualitative consistencies, such as complete vaporization of the carbonate target (as opposed to mere degassing of carbon dioxide due to incomplete vaporization of carbonate) and higher target-to-projectile mass ratio in a jet at higher impact angles. Quantitative comparison, however, also reveals significant inconsistencies between theory and experiments, such as an impact-angle effect on jet temperature and a correlation in jet temperatures between projectile and target components. In order to resolve these inconsistencies, new factors such as viscous shear heating and the nonsteady state nature of the jetting processes may need to be considered.

Sulfur chemistry in laser-simulated impact vapor clouds: implications for the K/T impact event

Abstract One of the most promising mechanisms for the mass extinction at the K/T boundary event is blockage of sunlight by sulfuric acid aerosol, which is induced by impact vaporization of sulfate in evaporite deposits around the K/T impact site. One of the advantages of this hypotheses is that it may cause an impact winter much longer than that by silicate dust and soot due to a global wildfire. However, the residence time of sulfuric acid aerosol in the stratosphere depends strongly on the ratio of SO 2 /SO 3 in the K/T impact vapor. If SO 3 was dominant, the blockage of sunlight by the sulfuric acid aerosol would not last longer than that by silicate dust and soot. The chemical reaction of sulfur oxides in an impact vapor cloud has not been studied extensively before. This study carries out chemical equilibrium calculations, kinetic model calculations, and laser irradiation experiments with a quadrupole mass spectrometer to estimate the SO 2 /SO 3 ratio in the K/T impact vapor cloud. The results strongly suggest that most of sulfur oxides in the K/T impact vapor cloud may have been SO 3 , not SO 2 . The sulfuric acid aerosol may not have been able to block the sunlight for a long time.

Nonstop Mars Sample Return System Using Aerocapture Technologies

In this study, preliminary assessment of a Martian nonstop sample return system is made as a part of Mars Exploration with Landers and Orbiters (MELOS) mission, which is currently under investigation in Japan Aerospace Exploration Agency. In a mission scenario, an atmospheric entry vehicle of aero-maneuver ability is flown into the Martian atmosphere, collects the Martian dust particles and atmospheric gases during the hypersonic atmospheric flight, exits the Martian atmosphere, and is inserted into a parking orbit from which a return system departs for the earth. In order to accomplish controlled flight and successful orbit insertion, aerocapture technologies are introduced into the vehicle guidance and control system. A conceptual design is obtained as a result of the preliminary system analysis.

Response of Piezoelectric Lead–Zirconate–Titanate to Hypervelocity Silver Particles

A lead–zirconate–titanate (PZT) element was studied by bombarding silver particles in the mass range from 1 to 100 pg, and the velocity from 2 to 6 km/s. Output signals were uniquely identified on impact and characterized by Fourier analysis. It was found that incident energies above 100 nJ were uniquely determined by a single PZT element. We discussed its potential as a real-time detector for space dust and debris.

Rotational-Temperature Measurements of Chemically Reacting CN Using Band-Tail Spectra

The vibrational state of chemically reacting CN radicals does not necessarily have a Boltzmann distribution because it may be influenced by the chemical reaction leading to the formation of the CN radicals. Here, we develop a new method to measure the rotational temperature of chemically reacting nonequilibrium cyanide radicals using the band tails of their emission spectra. Because of the very short relaxation time scales, both the translational and rotational states reach a thermal equilibrium even when the vibrational state does not have a Boltzmann distribution. The method proposed in this study has two advantages. First, it is not sensitively affected by self-absorption. Second, it does not require as high a wavelength resolution as other methods because it uses the overall shape of the tail part of the CN emission bands. Thus, our method is more suitable for high-speed temperature measurements, where a high-wavelength-resolution measurement is difficult to obtain. To investigate the validity of our method, we carried out laser-ablation experiments within an N 2 - H 2 O - CO 2 - Ar gas mixture using graphite targets and measured the rotational temperature of laser-induced CN and C 2 radicals using the proposed method. The rotational temperatures exhibit reasonable trends as functions of time and beam cross sections, strongly suggesting that our method is useful for translational-rotational-temperature estimation of chemically reacting nonequilibrium CN radicals.

Bayesian spectral deconvolution with the exchange Monte Carlo method

An analytical method to deconvolute spectral data into a number of simple bands is extremely important in the analysis of the chemical properties of matter. However, there are two fundamental problems with such deconvolution methods. One is how to determine the number of bands without resorting to heuristics. The other is difficulty in avoiding the parameter solution trapped into local minima due to the hierarchy and the nonlinearity of the system. In this study, we propose a novel method of spectral deconvolution based on Bayesian estimation with the exchange Monte Carlo method, which is an application of the integral approximation of stochastic complexity and the exchange Monte Carlo method. We also experimentally show its effectiveness on synthetic data and on reflectance spectral data of olivine, one of the most common minerals of terrestrial planets.

Shock compression response of forsterite above 250 GPa

Shocked forsterite above 250 GPa indicates incongruent crystallization of MgO, its phase transition, and remelting. Forsterite (Mg2SiO4) is one of the major planetary materials, and its behavior under extreme conditions is important to understand the interior structure of large planets, such as super-Earths, and large-scale planetary impact events. Previous shock compression measurements of forsterite indicate that it may melt below 200 GPa, but these measurements did not go beyond 200 GPa. We report the shock response of forsterite above ~250 GPa, obtained using the laser shock wave technique. We simultaneously measured the Hugoniot and temperature of shocked forsterite and interpreted the results to suggest the following: (i) incongruent crystallization of MgO at 271 to 285 GPa, (ii) phase transition of MgO at 285 to 344 GPa, and (iii) remelting above ~470 to 500 GPa. These exothermic and endothermic reactions are seen to occur under extreme conditions of pressure and temperature. They indicate complex structural and chemical changes in the system MgO-SiO2 at extreme pressures and temperatures and will affect the way we understand the interior processes of large rocky planets as well as material transformation by impacts in the formation of planetary systems.