The deep nitrogen-covered basin on Pluto, informally named Sputnik Planitia, is located very close to the longitude of Pluto's tidal axis and may be an impact feature, by analogy with other large ...basins in the Solar System. Reorientation of Sputnik Planitia arising from tidal and rotational torques can explain the basin's present-day location, but requires the feature to be a positive gravity anomaly, despite its negative topography. Here we argue that if Sputnik Planitia did indeed form as a result of an impact and if Pluto possesses a subsurface ocean, the required positive gravity anomaly would naturally result because of shell thinning and ocean uplift, followed by later modest nitrogen deposition. Without a subsurface ocean, a positive gravity anomaly requires an implausibly thick nitrogen layer (exceeding 40 kilometres). To prolong the lifetime of such a subsurface ocean to the present day and to maintain ocean uplift, a rigid, conductive water-ice shell is required. Because nitrogen deposition is latitude-dependent, nitrogen loading and reorientation may have exhibited complex feedbacks.
Context. The Rossby wave instability in astrophysical disks is a potentially important mechanism for driving angular momentum transport in disks. Aims. We attempt to more clearly understand this ...instability in an approximate three-dimensional disk model environment which we assume to be a single homentropic annular layer we analyze using disk shallow-water theory. Methods. We consider the normal mode stability analysis of two kinds of radial profiles of the mean potential vorticity: the first type is a single step and the second kind is a symmetrical step of finite width describing either a localized depression or peak of the mean potential vorticity. Results. For single potential vorticity steps we find there is no instability. There is no instability when the symmetric step is a localized peak. However, the Rossby wave instability occurs when the symmetrical step profile is a depression, which, in turn, corresponds to localized peaks in the mean enthalpy profile. This is in qualitative agreement with previous two-dimensional investigations of the instability. For all potential vorticity depressions, instability occurs for regions narrower than some maximum radial length scale. We interpret the instability as resulting from the interaction of at least two Rossby edgewaves. Conclusions. We identify the Rossby wave instability in the restricted three-dimensional framework of disk shallow water theory. Additional examinations of generalized barotropic flows are needed. Viewing disk vortical instabilities from the conceptual perspective of interacting edgewaves can be useful.
The linear normal mode stratorotational instability (SRI) is analytically re-examined in the inviscid limit where the length-scales of horizontal disturbances are large compared their vertical and ...radial counterparts. Boundary conditions different from channel walls are also considered. This quasi-hydrostatic, semigeostrophic (QHSG) approximation allows one to examine the effect of a vertically varying Brunt-Väisäilä frequency, N2. It is found that the normal mode instability persists when N2 increases quadratically with respect to the disc vertical coordinate. However, we also find that the SRI seems to exist in this inviscid QHSG extreme only for channel wall conditions: when one or both of the reflecting walls are removed, there is no instability in the asymptotic limit explored here. It is also found that only exponential-type SRI modes (as defined by Dubrulle et al.) exist under these conditions. These equations also admit non-normal mode behaviour. Fixed Lagrangian pressure conditions on both radial boundaries predict there to be no normal mode behaviour in the QHSG limit. The mathematical relationship between the results obtained here and that of the classic Eady problem for baroclinic instability is drawn. We conjecture as to the mathematical/physical nature of the SRI. The general linear problem, analysed without approximation in the context of the Boussinesq equations, admits a potential vorticity-like quantity that is advectively conserved by the shear. Its existence means that a continuous spectrum is a generic feature of this system. It also implies that in places where the Brunt-Väisäilä frequency becomes dominant the linearized flow may two-dimensionalize by advectively conserving its vertical vorticity.
These observations suggest that CH 4 on Pluto has a complex history, involving reservoirs of different composition, thickness and stability controlled by volatile processes occurring on different ...timescales. In order to interpret these observations, we use a Pluto volatile transport model able to simulate the cycles of N 2 and CH 4 ices over millions of years. By assuming fixed solid mixing ratios, we explore how changes in surface albedos, emissivities and thermal inertias impact volatile transport. This work is therefore a direct and natural continuation of the work by Bertrand et al. (2018), which only explored the N 2 cycles. Results show that bright CH 4 deposits can create cold traps for N 2 ice outside Sputnik Planitia, leading to a strong coupling between the N 2 and CH 4 cycles. Depending on the assumed albedo for CH 4 ice, the model predicts CH 4 ice accumulation (1) at the same equatorial latitudes where the Bladed Terrain Deposits are observed, supporting the idea that these CH 4-rich deposits are massive and perennial, or (2) at mid-latitudes (25°− 70°), forming a thick mantle which is consistent with New Horizons observations. In our simulations, both CH 4 ice reservoirs are not in an equilibrium state and either one can dominate the other over long timescales, depending on the assumptions made for the CH 4 albedo. This suggests that long-term volatile transport exists between the observed reservoirs. The model also reproduces the formation of N 2 deposits at mid-latitudes and in the equatorial depressions surrounding the Bladed Terrain Deposits, as observed by New Horizons. At the poles, only seasonal CH 4 and N 2 deposits are obtained in Pluto's current orbital configuration. Finally, we show that Pluto's atmosphere always contained, over the last astronomical cycles, enough gaseous CH 4 to absorb most of the incoming Lyman-α flux.
The Pluto system was recently explored by NASA's New Horizons spacecraft, making closest approach on 14 July 2015. Pluto's surface displays diverse landforms, terrain ages, albedos, colors, and ...composition gradients. Evidence is found for a water-ice crust, geologically young surface units, surface ice convection, wind streaks, volatile transport, and glacial flow. Pluto's atmosphere is highly extended, with trace hydrocarbons, a global haze layer, and a surface pressure near 10 microbars. Pluto's diverse surface geology and long-term activity raise fundamental questions about how small planets remain active many billions of years after formation. Pluto's large moon Charon displays tectonics and evidence for a heterogeneous crustal composition; its north pole displays puzzling dark terrain. Small satellites Hydra and Nix have higher albedos than expected.
Pluto's surface is covered in numerous CH4 ice deposits, that vary in texture and brightness, as revealed by the New Horizons spacecraft as it flew by Pluto in July 2015. These observations suggest ...that CH4 on Pluto has a complex history, involving reservoirs of different composition, thickness and stability controlled by volatile processes occurring on different timescales. In order to interpret these observations, we use a Pluto volatile transport model able to simulate the cycles of N2 and CH4 ices over millions of years. By assuming fixed solid mixing ratios, we explore how changes in surface albedos, emissivities and thermal inertias impact volatile transport. This work is therefore a direct and natural continuation of the work by Bertrand et al. (2018), which only explored the N2 cycles. Results show that bright CH4 deposits can create cold traps for N2 ice outside Sputnik Planitia, leading to a strong coupling between the N2 and CH4 cycles. Depending on the assumed albedo for CH4 ice, the model predicts CH4 ice accumulation (1) at the same equatorial latitudes where the Bladed Terrain Deposits are observed, supporting the idea that these CH4-rich deposits are massive and perennial, or (2) at mid-latitudes (25°− 70°), forming a thick mantle which is consistent with New Horizons observations. In our simulations, both CH4 ice reservoirs are not in an equilibrium state and either one can dominate the other over long timescales, depending on the assumptions made for the CH4 albedo. This suggests that long-term volatile transport exists between the observed reservoirs. The model also reproduces the formation of N2 deposits at mid-latitudes and in the equatorial depressions surrounding the Bladed Terrain Deposits, as observed by New Horizons. At the poles, only seasonal CH4 and N2 deposits are obtained in Pluto's current orbital configuration. Finally, we show that Pluto's atmosphere always contained, over the last astronomical cycles, enough gaseous CH4 to absorb most of the incoming Lyman-α flux.
•We simulate N2 and CH4 volatile transport on Pluto over millions of years.•Net accumulation of CH4 ice is obtained in the equatorial regions, suggesting that the Bladed Terrain are thick CH4 deposits.•Results also suggest perennial CH4 ice deposits in the mid latitudes, which explains the thick mantle of volatile ice observed•Polar deposits and mid-latitude N2 ice deposits are seasonal in the model and should disappear in summer.•Pluto's atmosphere always contained enough gaseous CH4 to block most of the Lyman-α flux over the last astronomical cycles.
The Kuiper Belt is a broad, torus-shaped region in the outer Solar System beyond Neptune’s orbit. It contains primordial planetary building blocks and dwarf planets. NASA’s New Horizons spacecraft ...conducted a flyby of Pluto and its system of moons on 14 July 2015. New Horizons then continued farther into the Kuiper Belt, adjusting its trajectory to fly close to the small Kuiper Belt object (486958) 2014 MU69 (henceforth MU69; also informally known as Ultima Thule). Stellar occultation observations in 2017 showed that MU69 was ~25 to 35 km in diameter, and therefore smaller than the diameter of Pluto (2375 km) by a factor of ~100 and less massive than Pluto by a factor of ~106. MU69 is located about 1.6 billion kilometers farther from the Sun than Pluto was at the time of the New Horizons flyby. MU69’s orbit indicates that it is a “cold classical” Kuiper Belt object, thought to be the least dynamically evolved population in the Solar System. A major goal of flying past this target is to investigate accretion processes in the outer Solar System and how those processes led to the formation of the planets. Because no small Kuiper Belt object had previously been explored by spacecraft, we also sought to provide a close-up look at such a body’s geology and composition, and to search for satellites, rings, and evidence of present or past atmosphere. We report initial scientific results and interpretations from that flyby.
The short time evolution of three-dimensional small perturbations is studied. Exhibiting spectral asymptotic stability, thin discs are none the less shown to host intensive hydrodynamical activity in ...the shape of non-modal growth of initial small perturbations. Two mechanisms that lead to such behaviour are identified and studied, namely, non-resonant excitation of vertically confined sound waves by stable planar inertia-coriolis modes that results in linear growth with time, as well as resonant coupling of those two modes that leads to a quadratic growth of the initial perturbations. It is further speculated that the non-modal growth can give rise to secondary stratorotational instabilities and thus lead to a new route to turbulence generation in thin discs.
Observations made during the New Horizons flyby provide a detailed snapshot of the current state of Pluto's atmosphere. Whereas the lower atmosphere (at altitudes of less than 200 kilometers) is ...consistent with ground-based stellar occultations, the upper atmosphere is much colder and more compact than indicated by pre-encounter models. Molecular nitrogen (N2) dominates the atmosphere (at altitudes of less than 1800 kilometers or so), whereas methane (CH4), acetylene (C2H2), ethylene (C2H4), and ethane (C2H6) are abundant minor species and likely feed the production of an extensive haze that encompasses Pluto. The cold upper atmosphere shuts off the anticipated enhanced-Jeans, hydrodynamic-like escape of Pluto's atmosphere to space. It is unclear whether the current state of Pluto's atmosphere is representative of its average state--over seasonal or geologic time scales.
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