| Summary
We suggest that the mass loss from South Polar Terrain (SPT) is the main driving force of
the following tectonic processes on Enceladus: subsidence of SPT, flow in the mantle and
motion of plates.
1. Introduction
Enceladus, a satellite of Saturn, is the smallest celestial body in the Solar System where
volcanic activity is observed. Every second, the mass of ~200 kg is ejected into space from
the South Polar Terrain (SPT) – [1, 2, 3].
The loss of matter from the body’s interior should lead to global compression of the crust.
Typical effects of compression are: thrust faults, folding and subduction. However, such
forms are not dominant on Enceladus. We propose here special tectonic model that could
explain this paradox.
2. Subsidence of SPT and tectonics
The volatiles escape from the hot region through the fractures forming plumes in the
space. The loss of the volatiles results in a void, an instability, and motion of solid matter into
the hot region to fill the void. The motion includes :
Subsidence of the ‘lithosphere’ of SPT.
Flow of the matter in the mantle.
Motion of plates adjacent to SPT towards the active region.
If emerging void is being filled by the subsidence of SPT only, then the velocity of subsidence is
~0.05 mm/
yr-1. However, all three types of motion are probably important, so the
subsidence is slower but mantle flow and plates’ motion also play a role in filling the
void.
Note that in our model the reduction of the crust area is not a result of compression but it
is a result of the plate sinking. Therefore the compressional surface features do not have to be
dominant.
3. Models of subsidence
The numerical model of suggested process of subsidence is developed. It is based on the
typical set of equation: Navier-Stokes equation for incompressible viscous liquid, equation of
continuity and equation of heat conduction. The Newtonian and non-Newtonian rheologies
are used. The preliminary results of the model indicate that the subsidence rate of ~0.05
mm/
yr-1 is possible if we assume Newtonian rheology of the ice. For non-Newtonian
rheology more probable value is ~0.01 mm/
yr-1. In this case the velocity of motion of the
‘mantle’ material is higher.
4. Experimental model
The map of the STP shows the low polygonal region surrounded by the characteristic
‘arcs’. In the laboratory model we observed the regular pentagonal plate (model of STP)
sinking in viscoelastic material. Its rheology corresponds to assumption that icy plates are
warm enough to creep like glaciers. In the laboratory model we have found ‘kinks’ formed
above vertices of the plate. Contrary to expectations (the viscoelastic material behaves like
the fluid for the considered time scale) these ‘kinks’ appear to be stable features. The ‘kinks’
are similar to the ‘arcs’ in the STP. This fact suggests that ‘plates’ adjacent to the SPT could
behave like glaciers.
5. Conclusions
Our hypothesis is a natural consequence of observed mass loss. In our opinion this mass
loss is a main factor driving tectonic motions. Of course, it does not exclude some
form of solid state convection in the icy mantle, but in fact this convection is not
needed.
The time of operation of this form of tectonics is not known. There are some
observations suggesting that the activity in the STP is now decreasing. The periodic
changes of activity are possible. Other observations suggest that in the past there were
other centers of activity similar to the present STP [1]. The indication of the future
activity centers is less certain. However, the ovoid-shaped depression up to 2 km
deep, of size 200x140 km with the center at 200E, 15S is a good candidate – [7,
8].
Acknowledgements
The research is partly supported by National Science Centre (grant 2011/ 01/ B/
ST10/06653).
References
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683.
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(2006).
[3] Langenberg, H., Planetary science:ÂJets of mystery Nature GeoscienceÂ1, 816
(2008)Â
doi:10.1038/ngeo373.
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[6] Czechowski, L., Tectonics of Enceladus (2013) submitted.
[7] Czechowski, L., Enceladus - the vanishing satellite presenting in EGU 2014,
Vienna.
[8] Czechowski, L., Some remarks on the early evolution of Enceladus. Planet. Sp. Sc.
104, 185-199 (2014). |