PLANETARY MUSINGS

A thin shell dynamo at Mercury?

Stanley, S, J Bloxham, WE Hutchison, and MT Zuber, Thin shell dynamo models consistent with Mercury's weak observed magnetic field, EPSL, 234, 27-38, (2005), doi:10.1016/j.epsl.2005.02.040.

Ever since Mercury's magnetic field was discovered by Mariner 10 back in 1974-1975 its origin has been somewhat of enigma. The basic observation was that the rate of increase in strength of the magnetic field as the spacecraft got closer to the planet was consistent with an internal field with a dipolar character. The strength of the field, however, is pretty weak compared to the Earth's. One possibility is that the magnetic field is generated by a dynamo, just one that doesn't behave exactly like the ours. Stanley et al [2005] focus on the idea that the character of the dynamo and magnetic field may be a function of the relative thickness of the liquid outer core. The Earth's inner core is only about 35% of the radius of the whole core, but that isn't necessarily true for other planets.

Stanley et al., argue that based on magnetostrophic balance assumtpions that the ratio of the strength of the dipolar component of Mercury's magnetic field to the inferred toroidal component is a factor of 10-1000 less than the Earth's if a dynamo is involved. They set out to assess whether a dynamo in a thin shell (inner core is 70% or more of radius of core) could explain this phenomena. The basic result was that yes it could if the modified Rayleigh number (a modified scaling of the ratio of buoyancy to Coriolis forces in this case) was modest.

One of the conclusions that they had that isn't clear to me is how "differential rotation" (apparently the mechanism for creating the toroidal field) can drive a dynamo. Where is the differential rotation, between the inner core and the mantle? They also suggested that it may be possible for the character of the magnetic field to vary with position inside and outside the tangent cylinder (imagine a cylinder placed around the inner core along the axis of rotation). If true that might be observable by the MESSENGER spacecraft when it gets to Mercury in 2011. Though I wonder what exactly we might look for in those data...