Triton’s Southern Latitudes are Covered by Non-Volatile Ices

On the surface composition of Triton’s southern latitudes

Authors:

Holler et al

Abstract:

We present the results of an investigation to determine the longitudinal (zonal) distributions and temporal evolution of ices on the surface of Triton. Between 2002 and 2014, we obtained 63 nights of near-infrared (0.67–2.55 μμm) spectra using the SpeX instrument at NASA’s Infrared Telescope Facility (IRTF). Triton has spectral features in this wavelength region from N2, CO, CH4, CO2, and H2O. Absorption features of ethane (C2H6) and 13CO are coincident at 2.405 μμm, a feature that we detect in our spectra. We calculated the integrated band area (or fractional band depth in the case of H2O) in each nightly average spectrum, constructed longitudinal distributions, and quantified temporal evolution for each of the chosen absorption bands. The volatile ices (N2, CO, CH4) show significant variability over one Triton rotation and have well-constrained longitudes of peak absorption. The non-volatile ices (CO2, H2O) show poorly-constrained peak longitudes and little variability. The longitudinal distribution of the 2.405 μμm band shows little variability over one Triton rotation and is 97 ±± 44°° and 92 ±± 44°° out of phase with the 1.58 μμm and 2.35 μμm CO bands, respectively. This evidence indicates that the 2.405 μμm band is due to absorption from non-volatile ethane. CH4 absorption increased over the period of the observations while absorption from all other ices showed no statistically significant change. We conclude from these results that the southern latitudes of Triton are currently dominated by non-volatile ices and as the sub-solar latitude migrates northwards, a larger quantity of volatile ice is coming into view.

Uranian Moon Ariel is Warmer Than it Should be

Elastic thickness and heat flux estimates for the uranian satellite Ariel

Authors:

Peterson et al

Abstract:

The surface of Ariel, an icy satellite orbiting Uranus, shows extensional tectonic features suggesting an episode of endogenic heating in the satellite’s past. Using topography derived from stereo-photoclinometry, we identified flexural uplift at a rift zone suggesting elastic thickness values in the range 3.8–4.4 km. We estimate the temperature at the base of the lithosphere to be in the range 99–146 K, depending on the strain rate assumed, with corresponding heat fluxes of 28–92 mW/m2. Neither tidal heating, assuming Ariel’s current eccentricity, nor radiogenic heat production from the silicate core are enough to cause the inferred heat fluxes. None of three proposed ancient mean-motion resonances produce equilibrium tidal heating values in excess of 4.3 mW/m2. Thus, the origin of the inferred high heat fluxes is currently mysterious.

Was Uranus' Moon Miranda Resurfaced by Convection?

Global resurfacing of Uranus's moon Miranda by convection

Authors:

Hammond et al

Abstract:

Miranda, an icy moon of Uranus, is one of the most visually striking and enigmatic bodies in the solar system. Three polygonal-shaped regions of intense deformation, dubbed "coronae," dominate the surface of Miranda. Here we use numerical methods to show that sluggish-lid convection in Miranda's ice shell, powered by tidal heating, can simultaneously match the global distribution of coronae, the concentric deformation pattern, and the estimated heat flow during formation. The expected rheological conditions in Miranda's ice shell lead to the development of low-order convection that produces surface deformation patterns similar to those observed. We find that satellite core size strongly controls convection geometry and that low-order convection patterns are much more stable for core radii less than half the satellite radius.

Uranus' Cupid is Doomed

Cupid is Doomed: An Analysis of the Stability of the Inner Uranian Satellites

Authors:

French et al

Abstract:

We have explored the stability of the inner Uranian satellites using simulations based on the most recent observational data. We find that, across a wide range of mass assumptions, the system is unstable, resulting in the eventual crossing of orbits and probable subsequent collision of moons. Cupid and Belinda are usually the first satellites to cross orbits, and they do so on a time scale of 10^3-10^7 years. Cressida and Desdemona are generally the next pair to cross, on a time scale of 10^5-10^7 years. We show that the crossing times are highly sensitive to initial conditions and that Cupid's instability is related to its resonant interactions with Belinda. We also show that a previously discovered power law, which relates orbit crossing time to satellite mass, is valid across a wide range of masses. We generalize the power law to handle two unstable orbital pairs with overlapping lifetimes and show that it can be used to extend the time span of studies of orbital stability in a computationally efficient manner. Our results suggest that the current Uranian satellite system is in transition and that the moons will continue to collide and reaccrete for the foreseeable future.

How Grooved Terrain on Icy Moons Formed

A Common Origin for Ridge-and-Trough Terrain on Icy Satellites by Sluggish Lid Convection

Authors:

Barr et al

Abstract:

Ridge and trough terrain is a common landform on icy satellites of the outer solar system. Examples include the grooved terrain on Ganymede, gray bands on Europa, coronae on Uranus's moon Miranda, and ridges and troughs in the northern plains of Saturn's small, but active, moon Enceladus. Regardless of setting, the heat flow and strain rates associated with the formation of each of these terrains are similar: heat flows of order tens to a hundred milliwatts per meter squared, and deformation rates of order 10−16 to 10−12 s−1. Barr (2008) and Hammond & Barr (2014a) have previously shown that the conditions associated with the formation of ridge and trough terrain on Ganymede and the south polar terrain on Enceladus are consistent with solid-state ice shell convection in a shell with a weak surface. Here, we show that sluggish lid convection can simultaneously create the heat flow and deformation appropriate for the formation of ridge and trough terrains on a number of satellites. This conclusion holds regardless of the thickness of the satellites' ice shells. For convection to deform their surfaces, the ice shells must have yield stresses similar in magnitude to the daily tidal stresses exerted by the gravitational pull from their parent planets. This suggests that tidal and convective stresses must act together to deform the surface, and that the spatial pattern of tidal cracking on the surfaces of the moons controls the locations of ridge and trough terrain.