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Register a new userDynamics of cloud features on Uranus
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Near-infrared adaptive optics imaging of Uranus by the Keck 2 telescope during 2003 and 2004 has revealed numerous discrete cloud features, 70 of which were used to extend the zonal wind profile of Uranus up to 60deg N. We confirmed the presence of a north-south asymmetry in the circulation (Karkoschka, Science 111, 570-572, 1998), and improved its characterization. We found no clear indication of long term change in wind speed between 1986 and 2004, although results of Hammel et al. (2001, Icarus 153, 229-235) based on 2001 HST and Keck observations average ~10 m/s less westward than earlier and later results, and 2003 observations by Hammel et al. (2005, Icarus 175, 534-545) show increased wind speeds near 45deg N, which we dont see in our 2003-2004 observations. We observed a wide range of lifetimes for discrete cloud features: some features evolve within ~1 hour, many have persisted at least one month, and one feature near 34deg S (termed S34) seems to have persisted for nearly two decades, a conclusion derived with the help of Voyager 2 and HST observations. S34 oscillates in latitude between 32deg S and 36.5deg S, with a period of $sim$1000 days, which may be a result of a non-barotropic Rossby wave. It also varied its longitudinal drift rate between -20deg /day and -31deg /day in approximate accord with the latitudinal gradient in the zonal wind profile, exhibiting behavior similar to that of the DS2 feature observed on Neptune (Sromovsky et al., Icarus 105, 110-141, 1993). S34 also exhibits a superimposed rapid oscillation with an amplitude of 0.57deg in latitude and period of 0.7 days, which is approximately consistent with an inertial oscillation.
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We aim to locate the stability region for Uranus Trojans (UT hereafter) and find out the dynamical mechanisms responsible for the structures in the phase space. Using the spectral number as the stability indicator, we construct the dynamical maps on the (a0, i0) plane. The proper frequencies of UTs are determined precisely so that we can depict the resonance web via a semi-analytical method. Two main stability regions are found, one each for the low-inclination (0-14deg) and high-inclination regime (32-59deg). There is also an instability strip in each of them, at 9deg and 51deg respectively. All stability regions are in the tadpole regime and no stable horseshoe orbits exist for UTs. The lack of moderate-inclined UTs is caused by the nu5 and nu7 secular resonances. The fine structures in the dynamical maps are shaped by high-degree secular resonances and secondary resonances. During the planetary migration, about 36.3% and 0.4% of the pre-formed orbits survive the fast and slow migrations (with migrating time scales of 1 and 10Myr) respectively, most of which are in high inclination. Since the low-inclined UTs are more likely to survive the age of the solar system, they make up 77% of all such long-life orbits by the end of the migration, making a total fraction up to 4.06E-3 and 9.07E-5 of the original population for the fast and slow migrations, respectively. About 3.81% UTs are able to survive the age of the solar system, among which 95.5% are on low-inclined orbits with i0<7.5deg. However, the depletion of the planetary migration seems to prevent a large fraction of such orbits, especially for the slow migration model.
Uranus provides a unique laboratory to test our understanding of planetary atmospheres under extreme conditions. Multi-spectral observations from Voyager, ground-based observatories, and space telescopes have revealed a delicately banded atmosphere punctuated by storms, waves, and dark vortices, evolving slowly under the seasonal influence of Uranus extreme axial tilt. Condensables like methane and hydrogen sulphide play a crucial role in shaping circulation, clouds, and storm phenomena via latent heat release through condensation, strong equator-to-pole gradients suggestive of equatorial upwelling and polar subsidence, and through forming stabilising layers that may decouple different circulation and convective regimes as a function of depth. Weak vertical mixing and low atmospheric temperatures associated with Uranus negligible internal heat means that stratospheric methane photochemistry occurs in a unique high-pressure regime, decoupled from the influx of external oxygen. The low homopause also allows for the formation of an extensive ionosphere. Finally, the atmosphere provides a window on the bulk composition of Uranus - the ice-to-rock ratio, supersolar elemental and isotopic enrichments inferred from remote sensing and future textit{in situ} measurements - providing key insights into its formation and subsequent migration. This review reveals the state of our knowledge of the time-variable circulation, composition, meteorology, chemistry, and clouds on this enigmatic `Ice Giant, summarising insights from more than three decades of observations, and highlighting key questions for the next generation of planetary missions. As a hydrogen-dominated, intermediate-sized, and chemically-enriched world, Uranus could be our closest and best example of atmospheric processes on a class of worlds that may dominate the census of planets beyond our own Solar System.
The $mu$ and $ u$ rings of Uranus form a secondary ring-moon system with the satellites Puck, Mab,Portia, and Rosalind. These rings are tenuous and dominated by micrometric particles, which can be strongly disturbed by the solar radiation pressure. We performed a numerical analysis of the orbital evolution of a sample of particles under the influence of the solar radiation force and the planetary oblateness, combined with the gravitational interaction with the close satellites. The most likely result is a collisions and the deposition of particles onto the surface of these satellites. Since this mechanism tends to cause a depletion of material of the rings, we investigate additional sources for these dust particles. Adopting a rough estimative of the flux of interplanetary meteoroids, we found that the ejections from Mab could generate a ring with optical depth comparable with the observations. A similar analysis was carried out for the F-ring dust band. The damping due to the Saturns oblateness prevents the overstated changes of the eccentricity and increases in the lifetime of the particles. Therewithal photometric study of the F-ring using Cassini images revealed that substantial secular increase in the brightness of Saturns F ring has occurred in the last 25 years. The shapes of the phase curves from Cassini and Voyager are similar, suggesting that although the number of dust particles has increased, the overall distribution of sizes is unchanged. The dust bands that permeate the rings of Uranus were observed late in 2007 during the equinox, when the Sun crossed the ring plane. Images taken with the VLT were processed and then combined to result in long-exposure frames. For each frame, the north and south radial profiles were extracted. They will be used to develop a photometric model.
The core accretion theory of planet formation has at least two fundamental problems explaining the origins of Uranus and Neptune: (1) dynamical times in the trans-Saturnian solar nebula are so long that core growth can take > 15 Myr, and (2) the onset of runaway gas accretion that begins when cores reach 10 Earth masses necessitates a sudden gas accretion cutoff just as the ice giant cores reach critical mass. Both problems may be resolved by allowing the ice giants to migrate outward after their formation in solid-rich feeding zones with planetesimal surface densities well above the minimum-mass solar nebula. We present new simulations of the formation of Uranus and Neptune in the solid-rich disk of Dodson-Robinson et al. (2009) using the initial semimajor axis distribution of the Nice model (Gomes et al. 2005; Morbidelli et al. 2005; Tsiganis et al. 2005), with one ice giant forming at 12 AU and the other at 15 AU. The innermost ice giant reaches its present mass after 3.8-4.0 Myr and the outermost after 5.3-6 Myr, a considerable time decrease from previous one-dimensional simulations (e.g. Pollack et al. 1996). The core masses stay subcritical, eliminating the need for a sudden gas accretion cutoff. Our calculated carbon mass fractions of 22% are in excellent agreement with the ice giant interior models of Podolak et al. (1995) and Marley et al. (1995). Based on the requirement that the ice giant-forming planetesimals contain >10% mass fractions of methane ice, we can reject any solar system formation model that initially places Uranus and Neptune inside the orbit of Saturn. We also demonstrate that a large population of planetesimals must be present in both ice giant feeding zones throughout the lifetime of the gaseous nebula.
We present optical (g, R_c, and I_c) to near-infrared (J) simultaneous photometric observations for a primary transit of GJ3470b, a Uranus-mass transiting planet around a nearby M dwarf, by using the 50-cm MITSuME telescope and the 188-cm telescope, both at Okayama Astrophysical Observatory. From these data, we derive the planetary mass, radius, and density as 14.1 pm 1.3 M_earth, 4.32^{+0.21}_{-0.10} R_earth, and 0.94 pm 0.12 g cm^{-3}, respectively, thus confirming the low density that was reported by Demory et al. based on the Spitzer/IRAC 4.5-micron photometry (0.72^{+0.13}_{-0.12} g cm^{-3}). Although the planetary radius is about 10% smaller than that reported by Demory et al., this difference does not alter their conclusion that the planet possesses a hydrogen-rich envelope whose mass is approximately 10% of the planetary total mass. On the other hand, we find that the planet-to-star radius ratio (R_p/R_s) in the J band (0.07577^{+0.00072}_{-0.00075}) is smaller than that in the I_c (0.0802 pm 0.0013) and 4.5-micron (0.07806^{+0.00052}_{-0.00054}) bands by 5.9% pm 2.0% and 3.0% pm 1.2%, respectively. A plausible explanation for the differences is that the planetary atmospheric opacity varies with wavelength due to absorption and/or scattering by atmospheric molecules. Although the significance of the observed R_p/R_s variations is low, if confirmed, this fact would suggest that GJ3470b does not have a thick cloud layer in the atmosphere. This property would offer a wealth of opportunity for future transmission-spectroscopic observations of this planet to search for certain molecular features, such as H2O, CH4, and CO, without being prevented by clouds.
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