The Saturn system - Introduction to Planetary Geomorphology - page 186

that Enceladus experiences actively erupting geysers. The
eruptions spew water vapor, other gasses, and dust above the
surface at a rate of ~100 kg/s ( Fig. 9.16 ), some of which
blankets the surrounding terrain. This material is also
thought to inject ions into Saturn ' s magnetosphere and to
“

Geophysical models now suggest that the interior of
Enceladus ( Fig. 9.17 ) could be differentiated into a core
containing silicates, overlain by a global or regional liquid
water zone and an outer ice shell. Internal heat to keep the
water zone liquid and to “ drive ” the geysers is probably
generated at least partly by tidal heating associated with
the orbit of neighboring Dione, as discussed by Francis
Nimmo et al.( 2007 ).
The global map of Enceladus ( Fig. 9.18 ) shows the key
terrains and named features. As reviewed by John Spencer
et al.( 2009 ), the principal terrains are (1) cratered plains,
(2) western (leading) hemisphere fractured plains, (3) east-
ern (trailing) hemisphere fractured plains, and (4) the
south polar regions, as well as various smooth plains.
Impact crater morphologies vary with size and apparent
age. Craters of diameter <1 km are typical bowl-shaped
features, while craters 1 - 40 km in diameter show a wide
range in stages of degradation, including very shallow
features that result from viscous relaxation. Degradation
increases with crater size, by geographic location toward
the south (especially toward the south polar terrain), and
with inferred terrain age. Many of the larger craters have
central peaks, upwarped central domes ( Fig. 9.19 ), and
lobate ejecta deposits. Moreover, many craters in the
cratered plains are tectonically modi ed by fine fractures.
Tectonic features include scarps, troughs, ridges, and
grooves ( Fig. 9.20 ), giving some parts of Enceladus a
super cial resemblance to Europa. However, unlike on
Europa, high-standing ridges and ridge-complexes are
mostly lacking. Tectonic features clearly truncate heavily
cratered terrain, and the existence of the fine fractures
cutting many of the impact craters on Enceladus would

feed ” the E Ring.

Figure 9.16. A Cassini image of Enceladus showing active geysers of
mostly water and water-ice jetting from the terrain in

the south polar area. Despite the small size of this satellite (504 km),
it experiences substantial geologic activity, apparently driven by
tidal stresses in the interior (NASA PIA 12733).

“ tiger-stripe ”

Figure 9.17. A diagram showing the possible interior of Enceladus
(surface topography exaggerated) and the potential convection
with the generation of geysers (from Spencer et al., 2009 ; with
permission from Springer Science+Business Media B. V.: Dordrecht:
Springer, Saturn from Cassini 724, Enceladus:

An Active Cryovolcanic Satellite, M. Dougherty, L. Esposito, and
S. Krimigis (eds.), Fig. #3).

- Huygens, 2009 , 683 -