Cycloids

AMONG the weirdest things revealed about Europa by Voyager in 1979 were the cycloid-shaped ridges, especially those crossing over Astypalaea and points east, where fortuitous lighting showed them well. We already saw these Voyager images in Figs. 6.3 and 9.1a, as well as a close-up of the same cycloids in Fig. 11.5. The International Astronomical Union, the same organization that recently decreed that Pluto cannot be called a planet, in its collective wisdom decided to call these features “flexi”, and gave official names to some of them. But everyone calls them cycloids, in reference to their geometric shape—chains of arcs connected at pointy cusps. IAU nomenclature is random and inexplicable, the inevitable result of a huge amount of discussion over long dinners, but in this case it was harmless. (In a later chapter I’ll discuss how some IAU - sanctioned misnomers actually promoted misunderstanding about Europa.)

Galileo images showed cycloidal cracks everywhere on Europa. Like most cracks, they usually show up as double ridges, but can also develop into strike-slip fault (like Astypalaea Linea) or dilation bands (like Fig. 8.3 or most of the bands in the Wedges region). They may also remain as hard-to-spot simple cracks. The beautiful cycloids that formed across Astypalaea (Fig. 6.3) are typical in form and size: Each is ~100 km long, and chains often comprise a dozen or more connected arcs running across a thousand kilometers or more.

Cycloids are ubiquitous on Europa. You could scan almost any of the Galileo are ubiquitous on Europa. You could scan almost any of the Galileo images and find beautiful examples. Some of my favorites run roughly north-south in the northern leading hemisphere (Fig. 12.1). Along much of the length of these cracks, double ridges have formed. Elsewhere the cracks have opened, forming dilational bands.