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تسجيل مستخدم جديدObliquity evolution of the minor satellites of Pluto and Charon
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New Horizons mission observations show that the small satellites Styx, Nix, Kerberos and Hydra, of the Pluto-Charon system, have not tidally spun-down to near synchronous spin states and have high obliquities with respect to their orbit about the Pluto-Charon binary (Weaver et al. 2016). We use a damped mass-spring model within an N-body simulation to study spin and obliquity evolution for single spinning non-round bodies in circumbinary orbit. Simulations with tidal dissipation alone do not show strong obliquity variations from tidally induced spin-orbit resonance crossing and this we attribute to the high satellite spin rates and low orbital eccentricities. However, a tidally evolving Styx exhibits intermittent obliquity variations and episodes of tumbling. During a previous epoch where Charon migrated away from Pluto, the minor satellites could have been trapped in orbital mean motion inclination resonances. An outward migrating Charon induces large variations in Nix and Styxs obliquities. The cause is a commensurability between the mean motion resonance frequency and the spin precession rate of the spinning body. As the minor satellites are near mean motion resonances, this mechanism could have lifted the obliquities of all four minor satellites. The high obliquities need not be primordial if the minor satellites were at one time captured into mean motion resonances.
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Pluto and its five known satellites form a complex dynamic system. Here we explore where additional satellites could exist exterior to Charon (the innermost moon) but interior of Hydra (the outermost). We also provide dynamical constraints for the ma
sses of the known satellites. We show that there are significant stable regions interior of Styx and between Nix and Kerberos. In addition, we show that coorbitals of the known small satellites are stable, even at high inclinations, and discuss mass constraints on undiscovered satellites in such orbits.
We consider the formation of satellites around the Pluto-Charon binary. An early collision between the two partners likely produced the binary and a narrow ring of debris, out of which arose the moons Styx, Nix, Kerberos and Hydra. How the satellites
emerged from the compact ring is uncertain. Here we show that a particle ring spreads from physical collisions and collective gravitational scattering, similar to migration. Around a binary, these processes take place in the reference frames of most circular orbits, akin to circular ones in a Keplerian potential. Ring particles damp to these orbits and avoid destructive collisions. Damping and diffusion also help particles survive dynamical instabilities driven by resonances with the binary. In some situations, particles become trapped near resonances that sweep outward with the tidal evolution of the Pluto-Charon binary. With simple models and numerical experiments, we show how the Pluto-Charon impact ring may have expanded into a broad disk, out of which grew the circumbinary moons. In some scenarios, the ring can spread well beyond the orbit of Hydra, the most distant moon, to form a handful of smaller satellites. If these small moons exist, New Horizons will find them.
The New Horizons spacecraft mapped colors and infrared spectra across the encounter hemispheres of Pluto and Charon. The volatile ices CH$_4$, CO, and N$_2$, that dominate Plutos surface, have complicated spatial distributions resulting from sublimat
ion, condensation, and glacial flow acting over seasonal and geological timescales. Plutos H$_2$O ice bedrock is also mapped, with isolated outcrops occurring in a variety of settings. Plutos surface exhibits complex regional color diversity associated with its distinct provinces. Charons color pattern is simpler, dominated by neutral low latitudes and a reddish northern polar region. Charon near infrared spectra reveal highly localized areas with strong NH$_3$ absorption tied to small craters with relatively fresh-appearing impact ejecta.
The Pluto-Charon binary system is the best-studied representative of the binary Kuiper-belt population. Its origins are vital to understanding the formation of other Kupier-belt objects (KBO) and binaries, and the evolution of the outer solar-system.
The Pluto-Charon system is believed to form following a giant impact between two massive KBOs at relatively low velocities. However, the likelihood of a random direct collision between two of the most massive KBOs is low, and is further constrained by the requirement of a low-velocity collision, making this a potentially fine-tuned scenario. Here we expand our previous studies and suggest that the proto-Pluto-Charon system was formed as a highly inclined wide-binary, which was then driven through secular/quasi-secular evolution into a direct impact. Since wide-binaries are ubiquitous in the Kuiper-belt with many expected to be highly inclined, our scenario is expected to be robust. We use analytic tools and few-body simulations of the triple Sun-(proto-)Pluto-Charon system to show that a large parameter-space of initial conditions leads to such collisions. The velocity of such an impact is the escape velocity of a bound system, which naturally explains the low-velocity impact. The dynamical evolution and the origins of the Pluto-Charon system could therefore be traced to similar secular origins as those of other binaries and contact-binaries (e.g. Arrokoth), and suggest they play a key role in the evolution of KBOs.
NASAs New Horizons spacecraft has revealed the complex geology of Pluto and Charon. Plutos encounter hemisphere shows ongoing surface geological activity centered on a vast basin containing a thick layer of volatile ices that appears to be involved i
n convection and advection, with a crater retention age no greater than $approx$10 Ma. Surrounding terrains show active glacial flow, apparent transport and rotation of large buoyant water-ice crustal blocks, and pitting, likely by sublimation erosion and/or collapse. More enigmatic features include tall mounds with central depressions that are conceivably cryovolcanic, and ridges with complex bladed textures. Pluto also has ancient cratered terrains up to ~4 Ga old that are extensionally fractured and extensively mantled and perhaps eroded by glacial or other processes. Charon does not appear to be currently active, but experienced major extensional tectonism and resurfacing (probably cryovolcanic) nearly 4 billion years ago. Impact crater populations on Pluto and Charon are not consistent with the steepest proposed impactor size-frequency distributions proposed for the Kuiper belt.
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