When the Galileo spacecraft flew near Europa -- one of Jupiter's moons -- in the late 1990s, it observed ridges and other features on the surface of the moon's icy outer crust. Now two researchers have developed a model to explain how these features are created. Eric Gaidos from the Jet Propulsion Laboratory in the US and Francis Nimmo from Cambridge University in the UK say that friction from tidal forces caused by nearby Jupiter melts some of the ice and creates short-lived pockets of water near the moon's surface (Nature 405 637). Previously it was thought that the tidal forces fractured the icy crust, allowing liquid water from the ocean that exists underneath the ice to rise to the surface.
Gaidos and Nimmo say that the tidal forces from Jupiter can cause regions of ice near a fault or defect to move relative to one another. This relative motion causes frictional heating that increases the local temperature of the ice and makes it less viscous. This “warm ice”, which is estimated to have a temperature of about 273 K, then flows upwards by a few tens of centimetres over the course of one tidal cycle. “We suggest that such motion over the course of many cycles could be responsible for the formation of structures such as ridge pairs,” say the researchers. Their model predicts pockets of liquid water near the moon’s surface, which would, however, refreeze within tens of years.
