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Register a new userOn the Instability of Saturns Hypothetical Retrograde Co-orbitals
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We find an interesting fact that fictitious retrograde co-orbitals of Saturn, or small bodies inside the retrograde 1:1 resonance with Saturn, are highly unstable in our numerical simulations. It is shown that in the presence of Jupiter, the retrograde co-orbitals will get ejected from Saturns co-orbital space within a timescale of 10 Myr. This scenario reminds us of the instability of Saturn Trojans caused by both the Great Inequality and the secular resonances. Therefore, we carry out in-depth inspections on both mechanisms and prove that the retrograde resonance overlap, raised by Great Inequality, cannot serve as an explanation for the instability of retrograde co-orbitals, due to the weakness of the retrograde 2:5 resonance with Jupiter at a low eccentricity. However, we discover that both $ u_5$ and $ u_6$ secular resonances contribute to the slow growth of the eccentricity, therefore, are possibly the primary causes of the instability inside Saturns retrograde co-orbital space.
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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.
Aims. 2015 BZ509 is the first asteroid confirmed to be in retrograde co-orbit resonance (or 1/-1 resonance) with the giant planets in the solar system. While Saturn is the only giant planet whose Trojans are not discovered until now, we identify some small bodies among Centaurs and Damocloids that are potentially in 1/-1 resonance with Saturn in the present study. Methods. We integrate numerically the motion of the 1000 clones (include the nominal orbit) of each Centaur whose orbit has a semi-major axis between 9.3 au and 9.8 au and an inclination i > 90 deg. To confirm and evaluate the 1/-1 resonant configurations mentioned above, we introduce a useful one-degree integrable approximation for planar 1/-1 resonance. Results. We identify four candidates potentially in 1/-1 resonance with Saturn. The capture in this particular resonant state during the 40000 yr integration timespan is very common for 2006 RJ2 (906/1000 clones), 2006 BZ8 (878/1000 clones), and 2017 SV13 (998/1000 clones), and it is less likely for 2012 YE8 (426/1000 clones). According to our statistical results, 2006 RJ2 is the best candidate to be currently in a 1/-1 mean motion resonance with Saturn, and 2017 SV13 is another important potential candidate. Moreover, 2012 YE8 and 2006 BZ8 are also Centaurs of interest but their current and long-term 1/-1 resonant state with Saturn is less likely. The proportions of the clones captured in the relative long-term stable co-orbit resonance (over 10000 yr) are also given. Conclusions. Small bodies in retrograde co-orbit resonance with giant planets may be more common than previously expected. Identification of these potential mysterious minor bodies encourages the search for such objects on a larger scale in our solar system. The findings of this paper are also useful for understanding the origin and dynamical evolution of the Centaurs and Damocloids on retrograde orbits.
We demonstrate dynamical pathways from main-belt asteroid and Centaur orbits to those in co-orbital motion with Jupiter, including the retrograde (inclination $i>90^o$) state. We estimate that at any given time, there should be $sim1$ kilometer-scale or larger escaped asteroid in a transient direct (prograde) orbit with semimajor axis near that of Jupiters ($asimeq a_J$), with proportionally more smaller objects as determined by their size distribution. Most of these objects would be in the horseshoe dynamical state, which are hard to detect due to their moderate eccentricities (spending most of their time beyond 5 AU) and longitudes relative to Jupiter being spread nearly all over the sky. We also show that $approx$1% of the transient asteroid co-orbital population is on retrograde orbits with Jupiter. This population, like the recently identified asteroid (514107) 2015 BZ$_{509}$, can spend millions of years with $asimeq a_J$ including tens or hundreds of thousands of years formally in the retrograde 1:-1 co-orbital resonance. Escaping near-Earth asteroids (NEAs) are thus likely the precursors to the handful of known high-inclination objects with $asimeq a_J$. We compare the production of jovian co-orbitals from escaping NEAs with those from incoming Centaurs. We find that temporary direct co-orbitals are likely dominated by Centaur capture, but we only find production of (temporary) retrograde jovian co-orbitals (including very long-lived ones) from the NEA source. We postulate that the primordial elimination of the inner Solar Systems planetesimal population could provide a supply route for a metastable outer Solar System reservoir for the high-inclination Centaurs.
The D68 ringlet is the innermost feature in Saturns rings. Four clumps that appeared in D68 around 2014 remained evenly spaced about 30 degrees apart and moved very slowly relative to each other from 2014 up until the last measurements were taken in 2017. D68s narrowness and the distribution of clumps could either indicate that we have a collection of source bodies in a co-orbital configuration or imply that an outside force confines the observed dust and any source bodies. In this paper we explore the possibility that these four clumps arose from four source bodies in a co-orbital configuration. We find that there are no solutions with four masses that produce the observed spacings. We therefore consider whether an unseen fifth co-orbital object could account for the discrepancies in the angular separations and approach a stable stationary configuration. We find a range of solutions for five co-orbital objects where their mass ratios depend on the assumed location of the fifth mass. Numerical simulations of five co-orbitals are highly sensitive to initial conditions, especially for the range of masses we would expect the D68 clumps to have. The fragility of our D68 co-orbital system model implies that there is probably some outside force confining the material in this ringlet.
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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