Thermal evolution of the core with a high thermal conductivity

Monday, 15 December 2014: 12:05 PM
Stephane Labrosse, Ecole Normale Supérieure Lyon, Lyon, France
The rate at which heat is extracted across the core mantle boundary
(CMB) is constrained by the requirement of driving dynamo action in
the core. This constraint can be computed explicitly using the
entropy balance of the core and depends on the thermal conductivity,
whose value has been revised upwardly. A high order model for the
core thermal structure is derived and the implications for the core
cooling rate and thermal evolution obtained, using the recent values
of the thermal conductivity. Because of the depth increase of the
thermal conductivity, a CMB heat flow equal to the isentropic value
(13.25TW at present) leads to 700 km thick stably stratified layer
at the top of the core. Considering a CMB heat flow larger than the
isentropic value leads to an inner core less than 700 Myr old and
the thermal evolution of the core is largely constrained by the
conditions for dynamo action without an inner core. Analytical
calculations for that period show that a CMB temperature larger than
7000 K must have prevailed 4.5 Gyr ago if the geodynamo has been
driven by thermal convection for that whole time. This emphasizes
the questions of the onset of the geodynamo and of the duration of a
basal magma ocean.