| 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 ejecting into
space.
The size of the satellite directly after accretion (this body is referred here as
proto-Enceladus) is unknown. It can be estimated in two ways. First, if the average mass
outflow is equal to the present rate then the satellite’s original mass was ~30% bigger than
today. Second, we assume here that density of proto-Enceladus was equal to the present
density of Mimas because they were formed in the same part of the nebula. Mimas is dead, so
it preserves original composition. Both approaches give similar initial Enceladus’ radius
(~296 km) and its surface area (~1.1x106 km2). The present values are: 252 km and
7.99x105 km2.
The loss of matter 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.
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
hot region to fill the void in statu nascendi. The motion includes:
Subsidence of the lithosphere of SPT.
Flow of matter in the mantle.
Motion of lithospheric 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 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.
Note also that we do not know the present age of the satellite surface. Age assessment
depends on the assumed model of the flux of meteorites. For the lunar-like flux, cratered
plains of Enceladus are 4.2 Gyr old, and only 1.7 Gyr old, if cometary impact rates are used
(1,2). If ‘cometary’ chronology is correct then we have no data concerning 2/3 of Enceladus
history. During that time there could be a number of activity cycles, and the total decrease of
the surface area could be 300,000 km2.
If our hypothesis is confirmed, then Enceladus will be an exceptional body,
possibly representing a new class of celestial bodies: bodies decreasing as a result
of endogenic activity. Are other bodies similar to Enceladus? Dione seems be a
good candidate. Its activity is predicted and observed(3,4). Its gravity is too low to
retain gases. Its present high density could be a result of partial loss of volatiles
in the past. Moreover, it is in orbit-orbit resonance, so substantial tidal heating is
possible.
Acknowledgments
This work was partially supported by the National Science Centre (grant
2011/01/B/ST10/06653).
References and Notes:
1. J.R., Spencer, et al. Enceladus: An Active Cryovolcanic Satellite, in: M.K.
Dougherty et al. (eds.), Saturn from Cassini-Huygens, Springer Science, (2009), p.
683.
2. K. Zahnle et al., Cratering rates in the outer Solar System. Icarus 163, 263
(2003).
3. JPL press release. Cassini finds hints of activity at Saturn moon Dione,
http://saturn.jpl.nasa.gov/news/cassinifeatures/feature20130529/ May, 29 (2013)
4. L. Czechowski, Parameterized model of convection driven by tidal and radiogenic
heating. Adv. Space Res. 38, 788 (2006). |