Dissociation of CH4 at high pressures and temperatures: diamond formation in giant planet interiors?
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Experiments using laser-heated diamond anvil cells show that methane (CH4) breaks down to form diamond at pressures between 10 and 50 gigapascals and temperatures of about 2000 to 3000 kelvin. Infrared absorption and Raman spectroscopy, along with x-ray diffraction, indicate the presence of polymeric hydrocarbons in addition to the diamond, which is in agreement with theoretical predictions. Dissociation of CH4 at high pressures and temperatures can influence the energy budgets of planets containing substantial amounts of CH4, water, and ammonia, such as Uranus and Neptune. (+info)
Grain size-sensitive creep in ice II.
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Rheological experiments on fine-grained water ice II at low strain rates reveal a creep mechanism that dominates at conditions of low stress. Using cryogenic scanning electron microscopy, we observed that a change in stress exponent from 5 to 2.5 correlates strongly with a decrease in grain size from about 40 to 6 micrometers. The grain size-sensitive creep of ice II demonstrated here plausibly dominates plastic strain at the low-stress conditions in the interior of medium- to large-sized icy moons of the outer solar system. (+info)
New dust belts of Uranus: one ring, two ring, red ring, blue ring.
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We compared near-infrared observations of the recently discovered outer rings of Uranus with Hubble Space Telescope results. We find that the inner ring, R/2003 U 2, is red, whereas the outer ring, R/2003 U 1, is very blue. Blue is an unusual color for rings; Saturn's enigmatic E ring is the only other known example. By analogy to the E ring, R/2003 U 1 is probably produced by impacts into the embedded moon Mab, which apparently orbits at a location where nongravitational perturbations favor the survival and spreading of submicron-sized dust. R/2003 U 2 more closely resembles Saturn's G ring, which is red, a typical color for dusty rings. (+info)
Dissociation of methane into hydrocarbons at extreme (planetary) pressure and temperature.
(4/6)
Constant-pressure, first-principles molecular dynamic simulations were used to investigate the behavior of methane at high pressure and temperature. Contrary to the current interpretation of shock-wave experiments, the simulations suggest that, below 100 gigapascals, methane dissociates into a mixture of hydrocarbons, and it separates into hydrogen and carbon only above 300 gigapascals. The simulation conditions (100 to 300 gigapascals; 4000 to 5000 kelvin) were chosen to follow the isentrope in the middle ice layers of Neptune and Uranus. Implications on the physics of these planets are discussed. (+info)
Clouds of high contrast on Uranus.
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Near-infrared images of Uranus taken with the Hubble Space Telescope in July and October 1997 revealed discrete clouds with contrasts exceeding 10 times the highest contrast observed before with other techniques. At visible wavelengths, these 10 clouds had lower contrasts than clouds seen by Voyager 2 in 1986. Uranus' rotational rates for southern latitudes were identical in 1986 and 1997. Clouds in northern latitudes rotate slightly more slowly than clouds in opposite southern latitudes. (+info)
Superionic and metallic states of water and ammonia at giant planet conditions.
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The phase diagrams of water and ammonia were determined by constant pressure ab initio molecular dynamic simulations at pressures (30 to 300 gigapascal) and temperatures (300 to 7000 kelvin) of relevance for the middle ice layers of the giant planets Neptune and Uranus. Along the planetary isentrope water and ammonia behave as fully dissociated ionic, electronically insulating fluid phases, which turn metallic at temperatures exceeding 7000 kelvin for water and 5500 kelvin for ammonia. At lower temperatures, the phase diagrams of water and ammonia exhibit a superionic solid phase between the solid and the ionic liquid. These simulations improve our understanding of the properties of the middle ice layers of Neptune and Uranus. (+info)