The first paper considers the effect of the solid-liquid phase change on convection in the solid and proposes scaling relationships for the temperature difference across the layer and the velocity at the upper boundary as function of the Rayleigh number. Applications to Titan or Ganymede show that convection in these layers is quite efficient and that the ice is expected to melt, at least partially, in contact with the rocky core.
The second paper adds the effects of salts entering the ice layer from the underlying rocky core. The ice-salt mixture is expected to be denser than pure water ice which could impede convection. This effect is quantified using a buoyancy number which compares the negative buoyancy of salts to the positive buoyancy contribution from temperature variations. For large values of the buoyancy number, a stable layer of salt-rich ice develops at the bottom of the ice layer but convection proceeds largely unaffected in the upper salts-poor layer, with a limited but significant entrainement of salts from the lower layer to the overlying ocean.
Full citations:
Lebec, L., Labrosse, S., Morison, A., and Tackley, P. J. Scaling of convection in high-pressure ice layers of large icy moons and implications for habitability. Icarus, 396:115494, 2023.
]]>Lebec, L., Labrosse, S., Morison, A., Bolrão, D. P., and Tackley, P. J. Effects of salts on the exchanges through high-pressure ice layers of large ocean worlds. Icarus, 412:115966, 2024.
Full citation:
Hernandez, J.-A., Caracas, R., & Labrosse, S. (2022). Stability of high-temperature salty ice suggests electrolyte permeability in water-rich exoplanet icy mantles. Nature Communications, 13(1), 3303.
Available on the journals website
]]>The American space probe New Horizon made history when it performed the first (and to this date, only) fly-by of the dwarf planet Pluto on 14th July, 2015. The collected data was enough to change in drastic ways our understanding of this remote world. In particular, it showed that Pluto is still geologically active despite being far away from the Sun and having limited internal energy sources.
Perhaps the most striking feature on Pluto’s surface is Sputnik Planitia. This bright plain, slightly larger than France, is an impact crater filled with nitrogen ice. Pressure and temperature conditions at the surface of Pluto allow the gaseous nitrogen in its atmosphere to coexist with solid nitrogen. Climate models of Pluto predict exchange of nitrogen between the atmosphere of Pluto and Sputnik Planitia. Moreover, the surface of the ice exhibits remarkable polygonal features (see Figure). This is a manifestation of thermal convection in the nitrogen ice, constantly organizing and renewing the surface of the ice. The buoyancy source for such convection remained enigmatic, as the heat coming from Pluto’s interior would produce a surface topography in discrepancy with the observations.
In a study published on December 16th in Nature, Adrien Morison, Gaël Choblet and myself show that sublimation of the nitrogen ice powers convection in the ice layer of Sputnik Planitia by cooling down its surface. Manifestions of that sublimation are visible at the surface of the ice, in particular the uneven structure at small scale visible on the picture. Studying the growth of those structure from the centers of the polygons to their boundaries even allowed quantification of the convection in the ice layer: surface velocities of the ice are estimated to be similar to that of tectonic plates on Earth. In the study at hand, numerical simulations show that the cooling from sublimation is able to power convection in a way that is consistent with numerous data coming from New Horizons and following studies: size of polygons, amplitude of topography and surface velocities. It is also consistent with the timescale at which climate models predict sublimation of Sputnik Planitia, around 1 or 2 million years.
The dynamics of this nitrogen ice layer is in a way more similar to the dynamics of Earth’s oceans rather than that of Jupiter’s and Saturn’s icy moons: it is powered by the climate. Such climate-powered dynamics of a solid layer could also occur at the surface of other planetary bodies, such as Triton (one of Neptune’s moons), or Eris and Makemake (from Kuiper’s Belt).
Check out the paper on the journal’s webpage.
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