Kiyoshi Kuramoto

Partitioning of H and C between the mantle and core during the core formation in the Earth: Its implications for the atmospheric evolution and redox state of early mantle

Partitioning of H and C among fluid, silicate melt, and molten metallic iron within a growing Earth at temperatures 2000–2500 K and pressures 0.2–5 GPa is estimated by using a thermodynamic model based on the recent knowledge on gas solubility into silicate melts and molten metallic iron. The repulsive interactions among H, C, and S dissolved in molten metallic iron are taken into account. It is shown that partition coefficient of H between molten metallic iron and silicate melt increases with pressure and temperature. Under the presence of Fe-rich metal, CO2 content in silicate melt is suggested to be very low because of low oxygen fugacity under such condition. Assuming a homogeneous accretion of planetesimals with the composition given by the two-component model slightly modified from Ringwood [1977] and Waanke [1981], it is shown that C is preferentially partitioned to molten metallic iron and quite less to silicate melt, whereas a substantial proportion of H is partitioned to silicate melt as H2O and also to molten metallic iron as interstitial atoms. For such concentrations of H, C, and S, the effect of thermodynamic interaction among them in molten metallic iron is not strong enough to cause the oversaturation of graphite. H and C partitioned to molten metallic iron may account for a significant portion of the density deficit in Earths core. The estimated amount of H2O partitioned to silicate melt is possibly large enough to explain the sources for (1) H2O in the hydrosphere and mantle, (2) oxygen which partially oxidizes ferrous iron to ferric iron in the mantle, and (3) oxidant for metals which may fail to segregate to the core and act as the source of the highly siderophile elements in the mantle of the present Earth. The higher H/C and the lower C/36Ar ratios in the silicate Earth including the hydrosphere compared to various classes of meteorites are possibly explained if these elements are derived from the early mantle material after the H and C partitioning to molten metallic iron and core segregation. Accretion of the late veneer material such as the highly oxidized, CI chondrite-like material seems difficult to explain such elemental abundance pattern without invoking unknown large volatile reservoir in the mantle.

Formation of a hot proto-atmosphere on the accreting giant icy satellite: Implications for the origin and evolution of Titan, Ganymede, and Callisto

Judging from accretion energy and accretion time for the giant icy satellites it is suggested that a proto-atmosphere is formed by the evaporation of icy materials during accretion of these bodies around the proto-gaseous giant planets. We study the blanketing effect of proto-atmosphere during accretion of these satellites in the gas-free environment. We use a gray atmosphere model in which the condensation of H2O in a convective atmosphere is taken into account. The numerical results strongly suggest that the accretion energy flux is large enough to increase the surface temperature higher than ∼500 K during accretion due to the blanketing effect of proto-atmosphere as long as the accretion time is shorter than 105 years. Such a high surface temperature causes the formation of a deep water-rich ocean due to the melting of icy materials. Also a rocky core should be eventually formed by sinking of rocky materials through the water-rich ocean during accretion. Therefore, the apparent difference in the surface geologic features between Ganymede and Callisto can hardly be explained by whether or not these bodies have experienced the formation of rocky core. Stability of hydrostatic structure of the proto-atmosphere is also studied. Vigorous escape of the proto-atmosphere is likely to occur under high surface temperature. A large portion of accretional energy is possibly consumed by the vigorous escape during accretion. Thus, the giant icy satellites may lose a significant amount of icy materials during their accretions. This can explain the ice-depleted composition of Titan inferred from the observed mean densities of Saturnian satellites, if the accretion occurs within 104–105 years. Such a significant loss of icy materials is also expected for Ganymede and Callisto. The escape and catalytic reaction in the hot proto-atmosphere may play an important role in formation of the present N2-abundant and CO-depleted atmosphere of Titan.

Accretion, core formation, H and C evolution of the Earth and Mars

Abstract Recent understandings of planetary accretion have suggested that accumulation of a small number of large planetesimals dominates intermediate to final growth stages of the terrestrial planets, with impact velocity high enough to induce extensive melting of the planetesimal and target materials, resulting in formation of a large molten region in which gravitational segregation of silicate and metal, that is, core formation proceeds. In case of homogeneous accretion, volatiles contained in each planetesimal are likely subjected to partitioning among gas, silicate melt, and molten metallic iron at significantly high temperatures and pressures in such a massive molten region. Each phase would subsequently form the proto-atmosphere, -mantle, or-core, respectively. Such chemical reprocessing of H and C associated with core formation, which is followed by both degassing from mantle and atmospheric escape, may result in a diverse range of H 2 O CO 2 in planetary surface environments, which mainly depends on the H and C content relative to metallic iron in planetary building stones. This may explain inferred difference in volatile distribution between the Earths (relatively H 2 O-rich, CO 2 -poor) and the martian (H 2 O-poor, CO 2 -rich) surface environments. Such volatile redistribution may be systematically described by using the retentivity of H 2 O, ξ, defined as follows: ξ = 1 − ([ CO ] 0 + 2[ CH 4 ] 0 + 2[ C(gr) ] 0 ) [ H 2 O] 0 , where [ i ] 0 represents mol number of species i partitioned into non-metallic phases, that is, gas and silicate melt in impact-induced molten region. When ξ > 0.5, relatively H 2 O-rich and CO 2 -poor surface environment may eventually evolve, although a small portion of H 2 O partitioned into the non=metallic phases are possibly consumed by subsequent chemical reactions with reduced C-species with producing CO 2 and H 2 . When ξ 2 O consumption by the above reactions and selective loss of H 2 to space may result in relative H 2 O-depleted and CO 2 -rich surface environment. Given the building stone composition by the two-component model by Ringwood (1977) and Wanke (1981), ξ is found to decrease with increasing the mixing fraction of the volatile-rich component: ξ > 0.5 for the mixing fraction smaller than about 15–20% and ξ H C ratio in the building stone. The estimated mixing fraction of the volatile-rich component, about 10% for the Earth and 35% for Mars, is consistent with the observed difference in volatile distribution between the surfaces of both planets.

Tidal deformation of Ganymede: Sensitivity of Love numbers on the interior structure

Tidal deformation of icy satellites provides crucial information on their subsurface structures. In this study, we investigate the parameter dependence of the tidal displacement and potential Love numbers (i.e., h2 and k2, respectively) of Ganymede. Our results indicate that Love numbers for Ganymede models without a subsurface ocean are not necessarily smaller than those with a subsurface ocean. The phase lag, however, depends primarily on the presence/absence of a subsurface ocean. Thus, the determination of the phase lag would be of importance to infer whether Ganymede possesses a subsurface ocean or not based only on geodetic measurements. Our results also indicate that the major control on Love numbers is the thickness of the ice shell if Ganymede possesses a subsurface ocean. This result, however, does not necessarily indicate that measurement of either of h2 or k2 alone is sufficient to estimate the shell thickness; while a thin shell leads to large h2 and k2 independent of parameters, a thick shell does not necessarily lead to small h2 and k2. We found that to reduce the uncertainty in the shell thickness, constraining k2 in addition to h2 is necessary, highlighting the importance of collaborative analyses of topography and gravity field data.

Near-infrared Brightness of the Galilean Satellites Eclipsed in Jovian Shadow: A New Technique to Investigate Jovian Upper Atmosphere

Based on observations from the Hubble Space Telescope and the Subaru Telescope, we have discovered that Europa, Ganymede, and Callisto are bright around 1.5 μm even when not directly lit by sunlight. The observations were conducted with non-sidereal tracking on Jupiter outside of the field of view to reduce the stray light subtraction uncertainty due to the close proximity of Jupiter. Their eclipsed luminosity was 10^(–6)-10^(–7) of their uneclipsed brightness, which is low enough that this phenomenon has been undiscovered until now. In addition, Europa in eclipse was <1/10 of the others at 1.5 μm, a potential clue to the origin of the source of luminosity. Likewise, Ganymede observations were attempted at 3.6 μm by the Spitzer Space Telescope, but it was not detected, suggesting a significant wavelength dependence. It is still unknown why they are luminous even when in the Jovian shadow, but forward-scattered sunlight by hazes in the Jovian upper atmosphere is proposed as the most plausible candidate. If this is the case, observations of these Galilean satellites while eclipsed by the Jovian shadow provide us with a new technique to investigate the Jovian atmospheric composition. Investigating the transmission spectrum of Jupiter by this method is important for investigating the atmosphere of extrasolar giant planets by transit spectroscopy.

Breaking the Habit: The Peculiar 2016 Eruption of the Unique Recurrent Nova M31N 2008-12a

Since its discovery in 2008, the Andromeda galaxy nova M31N 2008-12a has been observed in eruption every single year. This unprecedented frequency indicates an extreme object, with a massive white dwarf and a high accretion rate, which is the most promising candidate for the single-degenerate progenitor of a type-Ia supernova known to date. The previous three eruptions of M31N 2008-12a have displayed remarkably homogeneous multi-wavelength properties: (i) From a faint peak, the optical light curve declined rapidly by two magnitudes in less than two days; (ii) Early spectra showed initial high velocities that slowed down significantly within days and displayed clear He/N lines throughout; (iii) The supersoft X-ray source (SSS) phase of the nova began extremely early, six days after eruption, and only lasted for about two weeks. In contrast, the peculiar 2016 eruption was clearly different. Here we report (i) the considerable delay in the 2016 eruption date, (ii) the significantly shorter SSS phase, and (iii) the brighter optical peak magnitude (with a hitherto unobserved cusp shape). Early theoretical models suggest that these three different effects can be consistently understood as caused by a lower quiescence mass-accretion rate. The corresponding higher ignition mass caused a brighter peak in the free-free emission model. The less-massive accretion disk experienced greater disruption, consequently delaying re-establishment of effective accretion. Without the early refueling, the SSS phase was shortened. Observing the next few eruptions will determine whether the properties of the 2016 outburst make it a genuine outlier in the evolution of M31N 2008-12a.

Extremely strong polarization of an active asteroid (3200) Phaethon

The near-Earth asteroid (3200) Phaethon is the parent body of the Geminid meteor stream. Phaethon is also an active asteroid with a very blue spectrum. We conducted polarimetric observations of this asteroid over a wide range of solar phase angles α during its close approach to the Earth in autumn 2016. Our observation revealed that Phaethon exhibits extremely large linear polarization: P = 50.0 ± 1.1% at α = 106.5°, and its maximum is even larger. The strong polarization implies that Phaethon’s geometric albedo is lower than the current estimate obtained through radiometric observation. This possibility stems from the potential uncertainty in Phaethon’s absolute magnitude. An alternative possibility is that relatively large grains (~300 μm in diameter, presumably due to extensive heating near its perihelion) dominate this asteroid’s surface. In addition, the asteroid’s surface porosity, if it is substantially large, can also be an effective cause of this polarization.(3200) Phaethon is a near-Earth asteroid discovered in 1983 that has large inclination and eccentricity. Here, the authors perform polarimetric observation of Phaethon over a wide range of solar phase angle and report that the asteroid exhibits a very strong linear polarization.

Polarimetric Study of Near-Earth Asteroid (1566) Icarus

We conducted a polarimetric observation of the fast-rotating near-Earth asteroid (1566) Icarus at large phase (Sun-asteroid-observers) angles

Significantly high polarization degree of the very low-albedo asteroid (152679) 1998 KU2

\alpha

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= 57 deg--141deg around the 2015 summer solstice. We found that the maximum values of the linear polarization degree are