Sasaki Takanori Online

T. Sasaki, G. R. Stewart & S. Ida, ApJ (2010)

Takanori Sasaki — Sun, 09 May 2010 15:00:52 +0000

Takanori Sasaki, Glen R. Stewart & Shigeru Ida, Origin of the Different Architectures of the Jovian and Saturnian Satellite Systems, ApJ, 174, 1052-1064 (2010).

Abstract

The Jovian regular satellite system mainly consists of four Galilean satellites that have similar masses and are trapped in mutual mean-motion resonances except for the outer satellite, Callisto. On the other hand, the Saturnian regular satellite system has only one big icy body, Titan, and a population of much smaller icy moons. We have investigated the origin of these major differences between the Jovian and Saturnian satellite systems by semi-analytically simulating the growth and orbital migration of proto-satellites in an accreting proto- satellite disk. We set up two different disk evolution/structure models that correspond to Jovian and Saturnian systems, by building upon previously developed models of an actively supplied proto-satellite disk, the formation of gas giants, and observations of young stars. Our simulations extend previous models by including the (1) different termination timescales of gas infall onto the proto-satellite disk and (2) different evolution of a cavity in the disk, between the Jovian and Saturnian systems. We have performed Monte Carlo simulations and have shown that in the case of the Jovian systems, four to five similar-mass satellites are likely to remain trapped in mean-motion resonances. This orbital configuration is formed by type I migration, temporal stopping of the migration near the disk inner edge, and quick truncation of gas infall caused by Jupiter opening a gap in the solar nebula. The Saturnian systems tend to end up with one dominant body in the outer regions caused by the slower decay of gas infall associated with global depletion of the solar nebula. The total mass and compositional zoning of the predicted Jovian and Saturnian satellite systems are consistent with the observed satellite systems.

Read the full article

Numerical Experiments on Variation of Species Diversity

Takanori Sasaki — Thu, 03 Apr 2008 07:35:26 +0000

In this study, on the basis of the assumption of “random-way” and “size-dependent-chosen” evolution, we reconsider an evolving food web model and discuss the mechanism of diversification, stabilization, and extinction of species. By adopting the size-dependent evolutionary system, we derived a particular evolutionary pattern of the diversity that maintained equilibrated state over the long term. And we confirmed that size-dependent evolutionary system is stable whereas size-independent evolutionary system raises the fluctuation and extinction of species diversity. The results implied that considering the size-dependent system would be essential in establishing food web numerical model.

Imperfect Equilibration of Hf-W System by Giant Impacts

Takanori Sasaki — Thu, 03 Apr 2008 07:34:30 +0000

Hf-W chronometry provides constraints on the timing of planetary accretion and differentiation, as segregation of a metal core from silicates should induce strong fractionation of Hf from W. In previous studies, it was assumed that a giant impact raise up perfect resetting on Hf-W chronometer. In this study, we showed the difficulty of achieving perfect equilibration of Hf-W system by giant impacts.

To achieve perfect equilibration, iron must be split into small droplets less than meter-scale. However, since the sedimentation velocities of small droplets are slow, the Rayleigh-Taylor instability between the upper metal-containing and the lower metal-free layers results in quick overturn of the layers. Thus, the lower metal-free layers cannot be equilibrated.

Sketch of the Rayleigh-Taylor instability

1st sketch: After a giant impact, the ejecta and broken impactor are mixed each other and accrete to the protoearth as a mixture. Accreted metal in this mixture split into metal droplets, then these droplets sink in mixture layer.
2nd sketch: If these droplets is small enough to achieve perfect equilibration, the mixture layer that holds samll metal droplets, upper layer in first sketch, is equilibrated of Hf-W system. Then next, the Rayleigh-Taylor instability between the mixture layer with metal droplets and silicate melt layer of metal free occurs immediately because the growth of the Rayleigh-Taylor instability is by far faster than the Stokes sedimentation of metal droplets.
3rd sketch: Once the Rayleigh-Taylor instability has grown, the mixture layer with metal droplets sink into the core as a cluster, thus the equilibrated layer and non-equilibrated layer counterchange immediately.
4th sketch: On this occasion, metal droplets in mixture layer cannot interact with silicate outside the cluster. Thus, there exists two regions in the mantle such as equilibrated region (last sketch, lower layer) and non-equilibrated region (last sketch, upper layer).

We calculated the isotopic evolution of Hf-W system in consideration of partial resetting of this chronometer. Our study provided three implications: (1) collision conditions and the number of the giant impact events affect the age estimation of the core formation, (2) the Earth’s W isotope ratio indicates that more than two-tenth of the volume of protoearth’s mantle must be equilibrated at each giant impact, and (3) Mars should have experienced a late extensive equilibration event that involve metal-silicate more than three-tenth volume of Mars’ mantle, which is potentially a single giant impact.

The age of the last giant impact for the Earth

The age of the last giant impact as a function of the resetting ratio of each giant impact for fitting the observational W data of Earth’s samples. The number of giant impacts is 2 to 10 from left to right.

Subaru Infrared Spectroscopy of the Pluto

Takanori Sasaki — Thu, 03 Apr 2008 07:30:59 +0000

Several previous observations of Pluto in the J, H, and K bands have derived its surface composition of mixture ice of N2, CH4, and CO. Hydrocarbons generally have a fundamental C-H stretching mode in the L band (2.8 – 4.0 um), the absorption of which is much stronger than the overtones in the J, H, and K bands. However, it is difficult to make precise spectroscopic measurements in the L band owing to the high background and variable telluric extinctions. So the spectral resolution of previous data of Pluto in the L band was insufficient to examine the absorption features of hydrocarbons of Pluto’s surface.

We have conducted an infrared spectroscopy of the Pluto-Charon system in the L band with the adaptive optics system on the Subaru telescope on 2002 May 28. Thanks to the adaptive optics system, we were able to derive the high resolution data of Pluto. The spectra is dominated by the strong and broad absorptions of methane, but include some additional features between 3.05 and 3.9 um possibly due to hydrocarbon molecules other than methane. Comparing the observed spectra and simple model calculations by Hapke’s bidirectional model, we considered the effects of some hydrocarbon molecules to the shape of the spectrum of Pluto. We discussed the evolution of components on Pluto through some processes (non-equilibrium condensation, photochemical reaction, cosmic-ray irradiation, hydrodynamic escape, and external reservoir’s addition), and predicted the second major components (HCN, C2H2, C2H6), which was consistent with our observation.

Observational spectrum compared with two model spectra.

Solid curve: the observation smoothed by running average of 31 pixels.
Dotted curve: synthetic spectrum of N2-CH4-CO ices, those CH4 ices are diluted in N2 ices.
Dashed curve: synthetic specctrum of N2-CH4-CO ices and C2H2:CH4 = 1:30, C2H6:CH4 = 1:10 with diluted CH4 ices.

Subaru Infrared Spectroscopy of the Asteroid Karin

Takanori Sasaki — Thu, 03 Apr 2008 05:31:33 +0000

“Space weathering” is the term applied to darkening albedo, reddening spectral slope, and obscuring absorption bands of planetary surface materials with time. The mismatch between reflectance spectra of most common asteroids (S-type asteroids) and most common meteorites (ordinary chondrites) might be caused by the space weathering. Recent laboratory experiments simulating micrometeorite impact heating confirmed Hapke’s old hypothesis that the spectral darkening / reddening are caused by formation of nanophase iron particles. In the meantime, a recent study of celestial mechanics discovered a new-born group of asteroids, “Karin cluster group”, which is thought to be remnants of a collisional breakup only 5.8 million years ago.

We have conducted a near-infrared (J, H, and K bands) spectroscopy of the brightest asteroid 832 Karin among the Karin cluster group. The spectroscopic observation was performed by the Subaru telescope with the Cooled Infrared Spectrograph and Camera for OHS on 2003 September 14. For different rotational phases of Karin, we derived different spectra such as reddened spectrum like that of S-type asteroid and un-reddened spectrum like that of ordinary chondrite. Karin could be an impact fragment preserving an old surface and is probably one of the cone-shaped fragments at low-velocity impact that formed the Karin cluster group. Our result supports the idea that S-type asteroids are parent bodies of ordinary chondrites.

Observational phase of 832 Karin

Light-curve of 832 Karin is based on data obtained by the 1.8-m VATT at the Vatican observatory at Mt. Graham, Arizona, by the 1-m Schmidt telescope at the Kiso ovservatory, Japan, and by the 40-cm telescope in Fukuoka University of Education, Japan. Our observation corresponds to the phase of rotations of red, green, and blue regions. This figure was adopted and modified from Yoshida et al. (2004).

Reflectance specctra of 832 Karin (2 sets)

Red points: first set of 832 Karin
Blue points: last set of 832 Karin
Violet squares: S(IV)-type asteroid 584 Semiramis (from SBN Data Set 52 Color)
Green diamonds: L6 ordinary chondrite Paranaiba (from RELAB Public Spectroscopy Database)
These data are normalized to the unity at 1.0 um. The data points corresponding to the mean values for every 10 pixels are plotted with their error-bars of standard deviation. The data are removed in the wavelength range where the telluric absorptions were strong and the error-bars are large.

In the same season, Yoshida et al. (2004) performed a multispectral (B, V, R, and I bands) Karin observation, and their visible observations of Karin are consistent with our observation. Thus, the spectral changes according to the rotational phase due to space weathering were detected in both the visible and the infrared observations.

T. Sasaki & Y. Abe, EPS (2007)

Takanori Sasaki — Thu, 27 Sep 2007 15:00:11 +0000

Takanori Sasaki & Yutaka Abe, Rayleigh-Taylor Instability after Giant Impacts: Imperfect equilibration of Hf-W system and its effect on the core formation age, EPS, 59, 1035-1045 (2007).

Abstract

Hf-W chronometry provides constraints on the timing of planetary accretion and differentiation, as the segre- gation of a metal core from silicates should induce strong fractionation of Hf from W. In most previous studies, it was assumed that a giant impact would perfectly reset the Hf-W chronometer. Here, we show the difficulty of achieving perfect equilibration of the Hf-W system. Perfect equilibration requires iron to split into small droplets. However, since the sedimentation velocities of small droplets are low, the Rayleigh-Taylor instability between the upper metal-containing and lower metal-free layers results in quick overturning of the layers, unless iron droplets were uniformly distributed in the entire mantle. Therefore, the lower metal-free layers cannot be equilibrated. We calculated the isotopic evolution of the Hf-W system, taking into account the partial resetting of this chronometer. Our study led to three conclusions: (1) collision conditions and the number of giant impact events affect the age estimation of core formation, (2) the Earth’s W isotope ratio indicates that more than two-tenths of the volume of the protoearth’s mantle must have been equilibrated at each giant impact, and (3) Mars should have experienced a late, extensive equilibration event; it could have been a single giant impact.

Read the full article

T. Sasaki et al., ApJ (2005)

Takanori Sasaki — Fri, 31 Dec 2004 15:00:06 +0000

Takanori Sasaki, Ai Kanno, Masateru Ishiguro, Daisuke Kinoshita & Ryosuke Nakamura, Search for nonmethane hydrocarbons on Pluto, ApJ, 618, L57-L60 (2005).

Abstract

We have carried out infrared high-resolution spectroscopy of the Pluto-Charon system in the L band with the adaptive optics system on the Subaru telescope. The spectrum is dominated by the strong and broad absorption features of methane but includes some additional features. Comparing the spectrum with model calculations, we suggest that absorption features around 3.1, 3.2, and 3.35 μm could be an indication of nonmethane hydrocarbons on Pluto’s uppermost surface. Implications of the estimated mass ratio between hydrocarbons for the formation and evolution of Pluto are discussed.

Read the full article

T. Sasaki et al., ApJ (2004)

Takanori Sasaki — Tue, 09 Nov 2004 15:00:34 +0000

Takanori Sasaki, Sho Sasaki, Jun-ichi Watanabe, Tomohiko Sekiguchi, Fumi Yoshida, Hideyo Kawakita, Tetsuharu Fuse, Naruhisa Takato, Budi Dermawan & Takashi Ito, Mature and fresh surfaces on the new-born asteroid Karin, ApJ, 615, L161-L164 (2004).

Abstract

Here we report a near-infrared (J, H, and K bands) spectroscopy of 832 Karin, the brightest asteroid among the Karin cluster group, which is thought to be the remnants of a collisional breakup only 5.8 million years ago. The spectroscopic observation was performed by the Subaru telescope with the Cooled Infrared Spectrograph and Camera for OHS on 2003 September 14. For different rotational phases of Karin, we derived different spectra such as a reddened spectrum like that of an S-type asteroid and an unreddened spectrum like that of ordinary chondrite. Karin could be an impact fragment preserving an old surface and is probably one of the cone-shaped fragments at the low-velocity impact that formed the Karin cluster group. Our result supports the idea that S- type asteroids are parent bodies of ordinary chondrites.

Read the full article