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Register a new userCloud clearing in the wake of Saturns Great Storm of 2010 - 2011 and suggested new constraints on Saturns He/H2 ratio
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Saturns Great Storm of 2010 - 2011 produced a planet-encircling wake that slowly transitioned from a region that was mainly dark at 5 microns in February 2011 to a region that was almost entirely bright and remarkably uniform by December of 2012. The uniformity and high emission levels suggested that the entire wake region had been cleared not only of the ammonia clouds that the storm had generated and exposed, but also of any other aerosols that might provide significant blocking of the thermal emission from Saturns deeper and warmer atmospheric layers. Our analysis of VIMS wake spectra from December 2012 provides no evidence of ammonia ice absorption, but shows that at least one significant cloud layer remained behind: a non-absorbing layer of 3 - 4 optical depths (at 2 microns) extending from 150 to ~400 mbar. A second layer of absorbing and scattering particles, with less than 1 optical depth and located near 1 bar, is also suggested, but its existence as a model requirement depends on what value of the He/H2 ratio is assumed. The observations can be fit well with just a single (upper) cloud layer for a He/H2 ratio of 0.064 in combination with a PH3 deep volume mixing ratio of 5 ppm. At lower He/H2 ratios, the observed spectra can be modeled without particles in this region. At higher ratios, in order to fit the brightest wake spectrum, models must include either significant cloud opacity in this region, or significantly increased absorption by PH3, NH3, and AsH3. As the exceptional horizontal uniformity in the late wake is most easily understood as a complete removal of a deep cloud layer, and after considering independent constraints on trace gas mixing ratios, we conclude that the existence of this remarkable wake uniformity is most consistent with a He/H2 mixing ratio of 0.055 (+0.010, -0.015), which is on the low side of the 0.038 - 0.135 range of previous estimates.
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Observations of planets throughout our Solar System have revealed that the Earth is not alone in possessing natural, inter-annual atmospheric cycles. The equatorial middle atmospheres of the Earth, Jupiter and Saturn all exhibit a remarkably similar phenomenon - a vertical, cyclic pattern of alternating temperatures and zonal (east-west) wind regimes that propagate slowly downwards with a well-defined multi-Earth-year period. Earths Quasi-Biennial Oscillation (QBO, observed in the lower stratospheres with an average period of 28 months) is one of the most regular, repeatable cycles exhibited by our climate system, and yet recent work has shown that this regularity can be disrupted by events occurring far away from the equatorial region, an example of a phenomenon known as atmospheric teleconnection. Here we reveal that Saturns equatorial Quasi-Periodic Oscillation (QPO, with a ~15-year period) can also be dramatically perturbed. An intense springtime storm erupted at Saturns northern mid-latitudes in December 2010, spawning a gigantic hot vortex in the stratosphere at $40^circ$N that persisted for 3 years. Far from the storm, the Cassini temperature measurements showed a dramatic $sim10$-K cooling in the 0.5-5 mbar range across the entire equatorial region, disrupting the regular QPO pattern and significantly altering the middle-atmospheric wind structure, suggesting an injection of westward momentum into the equatorial wind system from waves generated by the northern storm. Hence, as on Earth, meteorological activity at mid-latitudes can have a profound effect on the regular atmospheric cycles in the tropics, demonstrating that waves can provide horizontal teleconnections between the phenomena shaping the middle atmospheres of giant planets.
Using astrometric observations spanning more than a century and including a large set of Cassini data, we determine Saturns tidal parameters through their current effects on the orbits of the eight main and four coorbital moons. We have used the latter to make the first determination of Saturns Love number, $k_2=0.390 pm 0.024$, a value larger than the commonly used theoretical value of 0.341 (Gavrilov & Zharkov, 1977), but compatible with more recent models (Helled & Guillot, 2013) for which $k_2$ ranges from 0.355 to 0.382. Depending on the assumed spin for Saturns interior, the new constraint can lead to a reduction of up to 80% in the number of potential models, offering great opportunities to probe the planets interior. In addition, significant tidal dissipation within Saturn is confirmed (Lainey et al., 2012) corresponding to a high present-day tidal ratio $k_2/Q=(1.59 pm 0.74) times 10^{-4}$ and implying fast orbital expansions of the moons. This high dissipation, with no obvious variations for tidal frequencies corresponding to those of Enceladus and Dione, may be explained by viscous friction in a solid core, implying a core viscosity typically ranging between $10^{14}$ and $10^{16}$ Pa.s (Remus et al., 2012). However, a dissipation increase by one order of magnitude at Rheas frequency could suggest the existence of an additional, frequency-dependent, dissipation process, possibly from turbulent friction acting on tidal waves in the fluid envelope of Saturn (Ogilvie & Li, 2004). Alternatively, a few of Saturns moons might themselves experience large tidal dissipation.
An episode of dynamical instability is thought to have sculpted the orbital structure of the outer solar system. When modeling this instability, a key constraint comes from Jupiters fifth eccentric mode (quantified by its amplitude M55), which is an important driver of the solar systems secular evolution. Starting from commonly-assumed near-circular orbits, the present-day giant planets architecture lies at the limit of numerically generated systems, and M55 is rarely excited to its true value. Here we perform a dynamical analysis of a large batch of artificially triggered instabilities, and test a variety of configurations for the giant planets primordial orbits. In addition to more standard setups, and motivated by the results of modern hydrodynamical simulations of the giant planets evolution within the primordial gaseous disk, we consider the possibility that Jupiter and Saturn emerged from the nebular gas locked in 2:1 resonance with non-zero eccentricities. We show that, in such a scenario, the modern Jupiter-Saturn system represents a typical simulation outcome, and M55 is commonly matched. Furthermore, we show that Uranus and Neptunes final orbits are determined by a combination of the mass in the primordial Kuiper belt and that of an ejected ice giant.
The abundance of deuterium in giant planet atmospheres provides constraints on the reservoirs of ices incorporated into these worlds during their formation and evolution. Motivated by discrepancies in the measured deuterium-hydrogen ratio (D/H) on Jupiter and Saturn, we present a new measurement of the D/H ratio in methane for Saturn from ground-based measurements. We analysed a spectral cube (covering 1151-1160 cm$^{-1}$ from 6 February 2013) from the Texas Echelon Cross Echelle Spectrograph (TEXES) on NASAs Infrared Telescope Facility (IRTF) where emission lines from both methane and deuterated methane are well resolved. Our estimate of the D/H ratio in stratospheric methane, $1.65_{-0.21}^{+0.27} times 10^{-5}$ is in agreement with results derived from Cassini CIRS and ISO/SWS observations, confirming the unexpectedly low CH$_{3}$D abundance. Assuming a fractionation factor of $1.34 pm 0.19$ we derive a hydrogen D/H of $1.23_{-0.23}^{+0.27} times 10^{-5}$. This value remains lower than previous tropospheric hydrogen D/H measurements of (i) Saturn $2.10 (pm 0.13) times 10^{-5}$, (ii) Jupiter $2.6 (pm 0.7) times 10^{-5}$ and (iii) the proto-solar hydrogen D/H of $2.1 (pm 0.5) times 10^{-5}$, suggesting that the fractionation factor may not be appropriate for stratospheric methane, or that the D/H ratio in Saturns stratosphere is not representative of the bulk of the planet.
Saturns mid-sized moons (satellites) have a puzzling orbital configuration with trapping in mean-motion resonances with every other pairs (Mimas-Tethys 4:2 and Enceladus-Dione 2:1). To reproduce their current orbital configuration on the basis of Crida & Charnozs model of satellite formation from a hypothetical ancient massive rings, adjacent pairs must pass 1st-order mean-motion resonances without being trapped. The trapping could be avoided by fast orbital migration and/or excitation of the satellites eccentricity caused by gravitational interactions between the satellites and the rings (the disk), which are still unknown. In our research, we investigate the satellite orbital evolution due to interactions with the disk through full N-body simulations. We performed global high-resolution N-body simulations of a self-gravitating particle disk interacting with a single satellite. We used $N sim 10^5$ particles for the disk. Gravitational forces of all the particles and their inelastic collisions are taken into account. As a result, dense short-wavelength wake structure is created by the disk self-gravity and global spiral arms with $m sim$ a few is induced by the satellite. The self-gravity wakes regulate the orbital evolution of the satellite, which has been considered as a disk spreading mechanism but not as a driver for the orbital evolution. The self-gravity wake torque to the satellite is so effective that the satellite migration is much faster than that was predicted with the spiral arms torque. It provides a possible model to avoid the resonance capture of adjacent satellite pairs and establish the current orbital configuration of Saturns mid-sized satellites.
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