Stair-Step Faulting

This week’s image shows stair-step scarps on Ganymede, one of the Jovian moons. This image was captured by the Galileo satellite. NASA’s archive page for this image describes its dimensions thus: “North is to the right of the picture and the Sun illuminates the surface from the west (top). The image is centered at -14 degrees latitude and 320 degrees longitude, and covers an area approximately 16 by 15 kilometers (10 by 9 miles).” So these cliffs are on quite a large scale! The fault has pulled the surface apart, dividing the crust into several blocks, which have become rotated producing these steep scarps. 

This suggests that tectonic processes have played an important role in shaping the surface of Ganymede, which shouldn’t be surprising given the forces exerted on the Jovian moons by the gravity of Jupiter and the resonances between the various Galilean Satellites. As we saw on Io a few weeks back these effects can be quite dramatic, but unlike its neighbours Ganymede is not undergoing tidal heating. Ganymede is not being resurfaced to the same extent as Io, but rifting is still occurring across the moon, leading to the formation of younger, terrain. Grooved terrains like this are common across the young, bright regions of Ganymede. 

Ganymede is a very interesting body. It is not only the largest moon of Jupiter, but the largest moon in all of the solar system. It might look small in comparison to the gas giant it orbits, but Ganymede is larger than Mercury, and if it orbited the sun directly would be a substantial planet in its own right. Its large size means that is has a spherical shape and a differentiated interior. Its internal structure is divided into a core, likely composed of metallic materials, a silicate mantle much like that of the earth, and a crust. The crust of Ganymede is believed to mainly consist of water, and there appears to be a liquid ocean present beneath the surface. Under the cold conditions in this part of the solar system the icy crust will behave much more like terrestrial rock than the ices we are familiar with on Earth. This means that we can look to geology to interpret features on the moon’s surface.

The striking landscape shown in this week’s image formed as a result of faulting of the crust. The fault has pulled the surface apart, dividing the crust into several blocks, which have become rotated producing these steep scarps. This sort of landscape shows that tectonic processes have played an important role in shaping the surface of Ganymede. As we saw on Io a few weeks back the Galilean satellites are under a lot of stresses, both from the gravity of Jupiter and the resonances between the moons.  However, unlike its neighbours Ganymede is not undergoing tidal heating. Consequently, Ganymede is not being resurfaced to the same extent as Io, but rifting is still occurring across the moon, leading to the formation of younger terrain.

Extensional faulting is a tectonic process which occurs across the solar system whenever sections of a planets crust move apart, allowing new material to rise from below. On Earth the formation of extensional faults is largely due to the pulling apart of tectonic plates. However this is quite an unusual arrangement. On most other planets there are no true plates and faulting occurs due to other sources of tectonic stress, such as the bulging or contraction of the crust. Compared to most solar system bodies Ganymede and Europa actually behave a lot like Earth. Their icy surfaces fracture into plates, which a likely floating on the liquid oceans below. Clearly the materials are very different, but under these conditions they behave a lot like tectonic plates producing faults like those seen in this image. 

Image Credit: NASA/JPL/Brown University https://photojournal.jpl.nasa.gov/catalog/PIA02582