The Emergence of Multiple Robust Zonal Jets from Freely Evolving, Three-Dimensional Stratified Geostrophic Turbulence with Applications to Jupiter

Three-dimensional numerical simulations of freely evolving stratified geostrophic turbulence on the β plane are presented as a simplified model of zonal jet formation on Jupiter. This study samples the parameter space that covers the low, middle, and high latitudes of Jupiter by varying the central latitude of the β plane. The results show that robust zonal jets can emerge from initial small-scale random turbulence through the upscale redistribution of the kinetic energy in the spectral space. The resulting flow’s sensitivities to the flow’s deformation radius LD and the two-dimensional Rhines length Lβ = (U/β)^½ (U is the characteristic turbulence velocity and β is the meridional gradient of the planetary vorticity) are tested, revealing that whether the outcome of the upscale energy transfer becomes dominated by jets or vortices depends on the relative values of LD and Lβ. The values of Lβ and LD are varied by tuning the β-plane parameters, and it is found that the flow transitions from a jet-dominated regime in Lβ ≲ LD to a vortical flow in Lβ ≳ LD. A height-to-width ratio equal to f/N, the Coriolis parameter divided by the Brunt–Väisälä frequency, has previously been established for stable vortices, and this paper shows that this aspect ratio also applies to the zonal jets that emerge in these simulations

Saturn’s Northern Hemisphere Ribbon: Simulations and Comparison with the Meandering Gulf Stream

Voyager observations of Saturn in 1980–81 discovered a wavy feature engirdling the planet at 47°N planetographic latitude. Its latitude coincides with that of an eastward jet stream, which is the second fastest on Saturn after the equatorial jet. The 47°N jet’s wavy morphology is unique among the known atmospheric jets on the gas giant planets. Since the Voyagers, it has been seen in every high-resolution image of this latitude for over 25 years and has been termed the Ribbon. The Ribbon has been interpreted as a dynamic instability in the jet stream. This study tests this interpretation and uses forward modeling to explore the observed zonal wind profile’s stability properties. Unforced, initial-value numerical experiments are performed to examine the nonlinear evolution of the jet stream. Parameter variations show that an instability occurs when the 47°N jet causes reversals in the potential vorticity (PV) gradient, which constitutes a violation of the Charney–Stern stability criterion. After the initial instability development, the simulations demonstrate that the instability’s amplitude nonlinearly saturates to a constant when the eddy generation by the instability is balanced by the destruction of the eddies. When the instability saturates, the zonal wind profile approaches neutral stability according to Arnol’d’s second criterion, and the jet’s path meanders in a Ribbon-like manner. It is demonstrated that the meandering of the 47°N jet occurs over a range of tropospheric static stability and background wind speed. The results here show that a nonlinearly saturated shear instability in the 47°N jet is a viable mechanism to produce the Ribbon morphology. Observations do not yet have the temporal coverage to confirm the creation and destruction of eddies, but these simulations predict that this is actively occurring in the Ribbon region. Similarities exist between the behaviors found in this model and the dynamics of PV fronts studied in the context of meandering western boundary currents in Earth’s oceans. In addition, the simulations capture the nonlinear aspects of a new feature discovered by the Cassini Visual and Infrared Mapping Spectrometer (VIMS), the String of Pearls, which resides in the equatorward tip of the 47°N jet. The Explicit Planetary Isentropic Coordinate (EPIC) model is used herein

A complex storm system in Saturn’s north polar atmosphere in 2018

Producción CientíficaSaturn’s convective storms usually fall in two categories. One consists of mid-sized storms ∼2,000 km wide, appearing as irregular bright cloud systems that evolve rapidly, on scales of a few days. The other includes the Great White Spots, planetary-scale giant storms ten times larger than the mid-sized ones, which disturb a full latitude band, enduring several months, and have been observed only seven times since 1876. Here we report a new intermediate type, observed in 2018 in the north polar region. Four large storms with east–west lengths ∼4,000–8,000 km (the first one lasting longer than 200 days) formed sequentially in close latitudes, experiencing mutual encounters and leading to zonal disturbances affecting a full latitude band ∼8,000 km wide, during at least eight months. Dynamical simulations indicate that each storm required energies around ten times larger than mid-sized storms but ∼100 times smaller than those necessary for a Great White Spot. This event occurred at about the same latitude and season as the Great White Spot in 1960, in close correspondence with the cycle of approximately 60 years hypothesized for equatorial Great White Spots.Ministerio de Economía, Industria y Competitividad - Fondo Europeo de Desarrollo Regional (project AYA2015-65041-P)Gobierno Vasco (project IT-366-19

Astro2020 Science White Paper: Triggered High-Priority Observations of Dynamic Solar System Phenomena

Unexpected dynamic phenomena have surprised solar system observers in the past and have led to important discoveries about solar system workings. Observations at the initial stages of these events provide crucial information on the physical processes at work. We advocate for long-term/permanent programs on ground-based and space-based telescopes of all sizes - including Extremely Large Telescopes (ELTs) - to conduct observations of high-priority dynamic phenomena, based on a predefined set of triggering conditions. These programs will ensure that the best initial dataset of the triggering event are taken; separate additional observing programs will be required to study the temporal evolution of these phenomena. While not a comprehensive list, the following are notional examples of phenomena that are rare, that cannot be anticipated, and that provide high-impact advances to our understandings of planetary processes. Examples include: new cryovolcanic eruptions or plumes on ocean worlds; impacts on Jupiter, Saturn, Uranus, or Neptune; extreme eruptions on Io; convective superstorms on Saturn, Uranus, or Neptune; collisions within the asteroid belt or other small-body populations; discovery of an interstellar object passing through our solar system (e.g. 'Oumuamua); and responses of planetary atmospheres to major solar flares or coronal mass ejections.Comment: Astro2020 white pape

Mission to the Trojan Asteroids: lessons learned during a JPL Planetary Science Summer School mission design exercise

The 2013 Planetary Science Decadal Survey identified a detailed investigation of the Trojan asteroids occupying Jupiter's L4 and L5 Lagrange points as a priority for future NASA missions. Observing these asteroids and measuring their physical characteristics and composition would aid in identification of their source and provide answers about their likely impact history and evolution, thus yielding information about the makeup and dynamics of the early Solar System. We present a conceptual design for a mission to the Jovian Trojan asteroids: the Trojan ASteroid Tour, Exploration, and Rendezvous (TASTER) mission, that is consistent with the NASA New Frontiers candidate mission recommended by the Decadal Survey and the final result of the 2011 NASA-JPL Planetary Science Summer School. Our proposed mission includes visits to two Trojans in the L4 population: a 500 km altitude fly-by of 1999 XS143, followed by a rendezvous with and detailed observations of 911 Agamemnon at orbital altitudes of 1000 - 100 km over a 12 month nominal science data capture period. Our proposed instrument payload - wide- and narrow-angle cameras, a visual and infrared mapping spectrometer, and a neutron/gamma ray spectrometer - would provide unprecedented high-resolution, regional-to-global datasets for the target bodies, yielding fundamental information about the early history and evolution of the Solar System. Although our mission design was completed as part of an academic exercise, this study serves as a useful starting point for future Trojan mission design studies. In particular, we identify and discuss key issues that can make large differences in the complex trade-offs required when designing a mission to the Trojan asteroids.Comment: 38 pages, 8 tables, 4 figures. Accepted for publication in Planetary and Space Scienc

A possible influence of the Great White Spot on Saturn kilometric radiation periodicity

The periodicity of Saturn kilometric radiation (SKR) varies with time, and its two periods during the first 5 years of the Cassini mission have been attributed to SKR from the northern and southern hemisphere. After Saturn equinox in August 2009, there were long intervals of time (March 2010 to February 2011 and September 2011 to June 2012) with similar northern and southern SKR periods and locked SKR phases. However, from March to August 2011 the SKR periods were split up again, and the phases were unlocked. In this time interval, the southern SKR period slowed down by ~ 0.5% on average, and there was a large jump back to a faster period in August 2011. The northern SKR period speeded up and coalesced again with the southern period in September 2011. We argue that this unusual behavior could be related to the so-called Great White Spot (GWS), a giant thunderstorm that raged in Saturn's atmosphere around that time. For several months in 2011, the visible head of the GWS had the same period of ~ 10.69 h as the main southern SKR modulation signal. The GWS was most likely a source of intense gravity waves that may have caused a global change in Saturn's thermospheric winds via energy and momentum deposition. This would support the theory that Saturn's magnetospheric periodicities are driven by the upper atmosphere. Since the GWS with simultaneous SKR periodicity measurements have only been made once, it is difficult to prove a physical connection between these two phenomena, but we provide plausible mechanisms by which the GWS might modify the SKR periods