84 research outputs found
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
The Ganymede Laser Altimeter (GALA) for the Jupiter Icy Moons Explorer (JUICE): Mission, science, and instrumentation of its receiver modules
The Jupiter Icy Moons Explorer (JUICE) is a science mission led by the European Space Agency, being developed for launch in 2023. The Ganymede Laser Altimeter (GALA) is an instrument onboard JUICE, whose main scientific goals are to understand ice tectonics based on topographic data, the subsurface structure by measuring tidal response, and small-scale roughness and albedo of the surface. In addition, from the perspective of astrobiology, it is imperative to study the subsurface ocean scientifically. The development of GALA has proceeded through an international collaboration between Germany (the lead), Japan, Switzerland, and Spain. Within this framework, the Japanese team (GALA-J) is responsible for developing three receiver modules: the Backend Optics (BEO), the Focal Plane Assembly (FPA), and the Analog Electronics Module (AEM). Like the German team, GALA-J also developed software to simulate the performance of the entire GALA system (performance model). In July 2020, the Proto-Flight Models of BEO, FPA, and AEM were delivered from Japan to Germany. This paper presents an overview of JUICE/GALA and its scientific objectives and describes the instrumentation, mainly focusing on Japan’s contribution
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