Ice, Ice, Baby! Rifting on the Icy Satellites of the Outer Solar System

The Jovian moons of Europa and Ganymede, and the Saturnian moon Enceladus have all been shown to have prominent extensional features analogous to rifts on Earth.  For my senior thesis, I am developing numerical models in order to better understand the geodynamics of how these rifts form, and in particular, how the emplacement of magmatic dikes can affect their evolution.

“Icy satellites” is the term we use to describe most of the collected moons of the large, gaseous Jovian planets of the outer solar system (Jupiter, Saturn, Uranus and Neptune).  As the name suggests, these moons are composed primarily of ices (water ice, as well as hydrocarbon ices like ethane or methane), but are cold enough that these ices behave more like rocks here on Earth.  The mean surface temperature on Enceladus is only about 75 Kelvin, or -325 degrees Fahrenheit.  These moons range in size from bigger than planets (Ganymede is larger than Mercury) to smaller than countries (Enceladus is a mere 500 km across - smaller than Colorado).  It has also been hypothesized that many of these moons have subsurface oceans beneath their icy crusts, which could potentially be refuges for extra-terrestrial life.

Three icy satellites, along with the Earth and Moon, to scale.

Despite these geologic differences, a number of features on these worlds are identified as similar to terrestrial rifts (such as the East African Rift, or the Basin and Range in Western North America).  These rifts have been categorized based upon observed morphology as either narrow rifts with localized extension or wide rifts with extension over a larger region.  Although past geodynamic models have been able describe the formation of these rifts, they require the use of unusual initial conditions (surface temperature, ice crust thickness, etc.) that conflict with satellite observations and other geodynamic models.  Our hypothesis is that by including previously ignored effects of magmatic dikes into these geodynamic models, we will be able to rectify this conflict between model predictions and observable parameters.

Examples of the different types of rifts on the different icy satellites, along with hypothetical geologic cross sections across the rifts, displaying the different observed morphology, and hypothesized structure.

Dikes are vertical, sheet-like magmatic intrusions.  Dikes and other volcanic processes on these moons are driven by cold liquid water and hydrocarbons (as opposed to molten rock, here on Earth), in a process called “cryovolcanism.”  Dikes have been inferred to exist at least on Enceladus due to the observation of large water-ice plumes originating from rifts on the moon’s southern pole.

Cassini-Huygens image of water geysers erupting from the rifts on Enceladus.

This research can potentially provide further insight into the general processes of extension and rifting, in addition to further understanding of these strange alien worlds.

For more information about the geodynamics involved in my model, and general information on rifting on both Earth and these icy satellites, see the following two papers:

• Buck, W.R. (1991), Modes of continental lithospheric extension, J. Geophys. Res., 96 (B12), 20, 161-20, 178. (direct link)
• Nimmo, F. (2004), Dynamics of rifting and modes of extension on icy satellites, J. Geophys. Res., 109, E01003, doi: 10.1029/2003JE002168. (direct link)

-James T. Keane, 3/14/2011