SCIENCE HIGHLIGHTS – September 2005


Hurford Estimates Elastic Lithosphere Thickness on Europa

Recent Paper Accepted for Publication in Icarus

The unknown thickness of Europa's icy crust (overlying a presumed watery ocean) is "the critical" issue driving research on this moon of Juputer.  Images of the surface, taken by the Galileo spacecraft, reveal terrains which appear to have been formed by melting of the icy crust, suggesting that the crust might be quite thin, < 10km.  Other features on the surface suggest that ice is thick enough for convection to occur, ~20 km.  
          Small scale deformations can probe the outermost portion of Europa's icy crust.  Europa's surface is covered with ridges, which many believe form along cracks in the crust.  The material comprising these ridges load the surface and cause flexure of the crust along the crack, producing a secondary uplift or fore-bulge.  The distance from the crack to the fore-bulge can be used to estimate the elastic lithosphere thickness (the thickness of the portion of the crust that behaves elastically).  This is the subject of a recent paper by Terry Hurford of  the Planetary Geodynamics Lab, now in press in Icarus.

Terry Hurford, a recent PhD from the University of Arizona, recently joined Code 698, the Planetary Geodynamics Lab. His PhD "Tides and Tidal Stress: Applications to Europa" (right) involves modeling deformation in the icy satellite. 

Solid line is a topographic profile for solid line in inset. The forebulge region (left of the topographic peak) is well represented by numerical models treating the ridge as a line load (dotted) or a distribured load (dashed), and suggest an elastic lithopshere of 350+/-50 m.

In "Flexure of Europa's lithosphere due to ridge-loading" Hurford and co-authors R.A. Beyer, B. Schmidt, B. Preblich, A.R. Sarid and R. Greenberg, report a systematic survey of fore-bulges at 145 locations across Europa's surface. Topographic profiles were determined using 2D photoclinometry. An example is shown in the Figure. The inset shows an image of the crust of Europa. The dark line near the very obvious crack at the center is the location of the topographic profile shown by the solid line below. Numerical models were used to estimate the thickness of the elastic lithosphere (upper portion of the icy crust).
          They find that certain terrains are associated with thicker elastic lithospheres. They also compared estimates of the elastic lithosphere with the relative age of the ridge which is loading the surface, finding younger ridge loads associated with thicker lithospheric estimates.

Contact:  Terry Hurford, Code 698, hurfordt@core2gsfc.nasa.gov