102 research outputs found
Was Martian mantle wet? A possible consequence of rapid core formation
Degassing of H2O in the planetary interior possibly plays an important role in the evolution of surface environment as well as geologic activity on the terrestrial planets. Mars may be such a planet that well preserves the materials and the geologic features directly related to early evolution of H2O. H2O content in the interior of proto-Mars during accretion and also core formation were investigated. Geodetic data shows that Mars has a dense core. The existence of iron-rich core on Mars may be also supported by the depletion of siderophile elements in SNC meteorites assuming that these samples came from Mars. Isotope systematics of these meteorites indicate that the core formation occurred very early, probably concurrently with Mars formation. Considering the kinetics of metal segregation from silicate, the melting of silicate is likely to precede the core formation. Once the core formation occurs, substantial amount of gravitational energy is released and thus the planetary interior is heated. This energy may be large enough to keep the silicate material in partially molten state along with the accretional heating. Under such circumstances, the silicate melt probably migrates to the surface. Early crustal formation, therefore, is also likely to be associated with the core formation
Significantly high polarization degree of the very low-albedo asteroid (152679) 1998 KU
We present a unique and significant polarimetric result regarding the
near-Earth asteroid (152679) 1998 KU , which has a very low
geometric albedo. From our observations, we find that the linear polarization
degrees of 1998 KU are 44.6 0.5\% in the R band
and 44.0 0.6\% in the V band at a solar phase angle of 81.0\degr. These
values are the highest of any known airless body in the solar system (i.e.,
high-polarization comets, asteroids, and planetary satellites) at similar phase
angles. This polarimetric observation is not only the first for primitive
asteroids at large phase angles, but also for low-albedo (< 0.1) airless
bodies.
Based on spectroscopic similarities and polarimetric measurements of
materials that have been sorted by size in previous studies, we conjecture that
1998 KU has a highly microporous regolith structure comprising
nano-sized carbon grains on the surface.Comment: 9 pages, 5 figures, and 3 tables, accepted for publication in A&
Pluto’s ocean is capped by gas hydrates
Many icy solar system bodies possess subsurface oceans. At Pluto, Sputnik Planitia’s location near the equator suggests the presence of a subsurface ocean and a locally thinned ice shell. To maintain an ocean, Pluto needs to retain heat inside. On the other hand, to maintain large variations in ice shell thickness, Pluto’s ice shell needs to be cold. Achieving such an interior structure is problematic. Here we show that the presence of a thin layer of clathrate hydrates (gas hydrates) at the base of the ice shell can explain both the long-term survival of the ocean and the maintenance of shell thickness contrasts. Clathrate hydrates act as a thermal insulator, preventing the ocean from complete freezing while keeping the ice shell cold and immobile. The most likely clathrate guest gas is methane either contained in precursor bodies and/or produced by cracking of organic materials in the hot rocky core. Nitrogen molecules initially contained and/or produced later in the core would likely not be trapped as clathrate hydrates, instead supplying the nitrogen-rich surface and atmosphere. The formation of a thin clathrate hydrate layer capping a subsurface ocean may be an important generic mechanism maintaining long-lived subsurface oceans in relatively large but minimally-heated icy satellites and Kuiper Belt Objects
Obliquity of an Earth-like planet from frequency modulation of its direct imaged lightcurve: mock analysis from general circulation model simulation
Direct-imaging techniques of exoplanets have made significant progress
recently, and will eventually enable to monitor photometric and spectroscopic
signals of earth-like habitable planets in the future. The presence of clouds,
however, would remain as one of the most uncertain components in deciphering
such direct-imaged signals of planets. We attempt to examine how the planetary
obliquity produce different cloud patterns by performing a series of GCM
(General Circulation Model) simulation runs using a set of parameters relevant
for our Earth. Then we use the simulated photometric lightcurves to compute
their frequency modulation due to the planetary spin-orbit coupling over an
entire orbital period, and attempt to see to what extent one can estimate the
obliquity of an Earth-twin. We find that it is possible to estimate the
obliquity of an Earth-twin within the uncertainty of several degrees with a
dedicated 4 m space telescope at 10 pc away from the system if the stellar flux
is completely blocked. While our conclusion is based on several idealized
assumptions, a frequency modulation of a directly-imaged earth-like planet
offers a unique methodology to determine its obliquity.Comment: 29 pages, 18 figures, accepted for publication in Ap
Science Objectives of the Ganymede Laser Altimeter (GALA) for the JUICE Mission
Laser altimetry is a powerful tool for addressing the major objectives of
planetary physics and geodesy, and have been applied in planetary explorations
of the Moon, Mars, Mercury, and the asteroids Eros, and Itokawa. The JUpiter
Icy Moons Explorer (JUICE), led by European Space Agency (ESA), has started
development to explore the emergence of habitable worlds around gas giants. The
Ganymede Laser Altimeter (GALA) will be the first laser altimeter for icy
bodies, and will measure the shape and topography of the large icy moons of
Jupiter, (globally for Ganymede, and using flyby ground-tracks for Europa and
Callisto). Such information is crucial for understanding the formation of
surface features and can tremendously improve our understanding of the icy
tectonics. In addition, the GALA will infer the presence or absence of a
subsurface ocean by measuring the tidal and rotational responses. Furthermore,
it also improves the accuracy of gravity field measurements reflecting the
interior structure, collaborating with the radio science experiment. In
addition to range measurements, the signal strength and the waveform of the
laser pulses reflected from the moon's surface contain information about
surface reflectance at the laser wavelength and small scale roughness.
Therefore we can infer the degrees of chemical and physical alterations, e.g.,
erosion, space weathering, compaction and deposition of exogenous materials,
through GALA measurements without being affected by illumination conditions.
JUICE spacecraft carries ten science payloads including GALA. They work closely
together in a synergistic way with GALA being one of the key instruments for
understanding the evolution of the icy satellites Ganymede, Europa, and
Callisto.Comment: 10 pages, 6 figures, accepted for publication in Trans. JSASS
Aerospace Tech. Japa
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