66,285 research outputs found
Voyager Encounters Saturn: Scientific Highlights
Observations generated by Voyager 1's encounter with Saturn are disclosed. Atmospheric conditions, the rings, new moons and the five inner moons are described. Titan, Hyperion and Iapetus are discussed in detail, as is Saturn's magnetosphere
Half moons are pinch points with dispersion
"Pinch points," singular features observed in (quasi-)elastic neutron
scattering, are a widely discussed hallmark of spin liquids with an emergent
gauge symmetry. Much less attention has been paid to "half moons," distinctive
crescent patterns at finite energy, which have been observed in experiments on
a number of pyrochlore magnets, and in a wide range of model calculations. Here
we unify these two phenomena within a single framework, paying particular
attention to the case of ordered, or field-saturated states, where pinch points
and half moons can be found in bands of excitations above a gap. We find that
half moons are nothing other than pinch points inscribed on a dispersing band.
Molecular dynamics simulations of the kagome lattice antiferromagnet are used
to explore how these bands evolve into the ground state and excitations of a
classical spin liquid. We explicitly demonstrate that this theory can reproduce
the pinch points and half moons observed in NdZrO.Comment: 6 pages, 4 figures. Supplementary material: 10 pages, 3 figure
Resonant Removal of Exomoons During Planetary Migration
Jupiter and Saturn play host to an impressive array of satellites, making it
reasonable to suspect that similar systems of moons might exist around giant
extrasolar planets. Furthermore, a significant population of such planets is
known to reside at distances of several Astronomical Units (AU), leading to
speculation that some moons thereof might support liquid water on their
surfaces. However, giant planets are thought to undergo inward migration within
their natal protoplanetary disks, suggesting that gas giants currently
occupying their host star's habitable zone formed further out. Here we show
that when a moon-hosting planet undergoes inward migration, dynamical
interactions may naturally destroy the moon through capture into a so-called
"evection resonance." Within this resonance, the lunar orbit's eccentricity
grows until the moon eventually collides with the planet. Our work suggests
that moons orbiting within about 10 planetary radii are susceptible to this
mechanism, with the exact number dependent upon the planetary mass, oblateness
and physical size. Whether moons survive or not is critically related to where
the planet began its inward migration as well as the character of inter-lunar
perturbations. For example, a Jupiter-like planet currently residing at 1AU
could lose moons if it formed beyond 5AU. Cumulatively, we suggest that an
observational census of exomoons could potentially inform us on the extent of
inward planetary migration, for which no reliable observational proxy currently
exists.Comment: 6 Figures, Accepted for Publication in The Astrophysical Journa
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