RheoMan: a five-year, ERC-funded (Advanced Grant), project to model the rheology of the Earth's mantle

Sep 18, 2014 Dislocations from extra solar systems Results

RheoMan disseminates and explores other worlds. In his PhD thesis Baptiste Journaux (ENS Lyon) has modelled with us the structure of dislocations in high-pressure ice X which is thought to constitute the mantle of ocean planets recently detected in remote extra solar systems. The results just appeared:

B. Journaux, R. Caracas, P. Carrez, K. Gouriet, P. Cordier, I. Daniel (2014) Elasticity and dislocations in ice X under pressure, Physics of the Earth and Planetary Interiors, 236, 10-15, doi: 10.1016/j.pepi.2014.08.002

 

The relevance of ice X: Ocean Planets:

Some massive ice-rich planets are considered to be in between rocky terrestrial planets and gaseous giant ones. Depending on their distance to the star and properties of their atmospheres, some of them may form a surface water ocean. They are called “Ocean-Planets”.

Ocean Planets

From Zeng & Sasselov (2013)

 

Possible candidates for belonging to this category of Ocean-Planets would be: Kepler-11b, Kepler-18b, Kepler-20b, Kepler-20c, Kepler-20d, Kepler-20f, Kepler-22b, Kepler-68b or 55Cnc-e.

Those planets would be composed of a large icy mantle essentially formed of high pressure polymorphs of ice VII and X. The transition toward high-pressure ice would occur below a thick ocean, about 100km thick. See for instance the model proposed by Fu et al. (2010)

Interieor Ocean Planets

H2O region of the 5 MEarth, 50% H2O planet

 

Depending on the thermal structure of these bodies, one can expect convection in the thick icy layers. Hence a better understanding of the rheology of high-pressure ices is needed.

 

Water ices

H2O exhibits a rich spectra of possible crystal structure depending on the pressure – temperature conditions

 Phase diagram

The phase diagram of water

 

Among those, ice X is the structure stable beyond the megabar and roughly up to 4 Mbar.

The structure of ice X can be seen as an intergrowth between a body-centered arrangement of an oxygen sublattice and a face-centered cubic arrangement of a hydrogen sublattice, shifted by ¼ along each of the three directions of the primitive cubic unit cell (a = 2.78 Å at 62 GPa, 300 K). The resulting space group is Pn3m, with O and H in the respectively 2a(000) and 4b(¼ ¼ ¼) Wyckoff positions.

 

 Ice X