O

Bands are generally linear, curvilinear, or cycloidal features, with sharp parallel to subparallel margins. The interior of smooth bands consists of either very subdued ridges and troughs or material with little or no structure. It is common that the material forming the bands has a relatively lower albedo than the surrounding plains. Smooth bands are interpreted as regions of crustal extension in which the low relief and lack of internal structure can be due to small-scale fracturing or the emplacement of infilling material [373] (Fig. 6.20).

'Cycloids' are chains of scalloped lines linked arc-to-arc at their cusps, extending over the icy plains for hundreds of kilometers. Evidently, the cuspate form is the result of pressure on the underside of the thin ice shell induced by diurnal tides in the enclosed ocean. This caused Europa's ice shell to flex. Jupiter's immense gravity caused the ice to bulge, cyclically, over the period of an orbit. Once the tidal force exceeded the tensile strength of the ice, it began to crack. The crack propagated relatively slowly across the ever-changing stress field, following a curved path. As soon as the stress subsided, the fracture halted. Later, when the stress built up again, the crack restarted, but it did so along a new curve, and so on. This tidal cycle occurs about every 85 h. The distinctive scalloped appearance resulted from the fact that successive curved fractures shared cusps. The creation of these cycloids could

Fig. 6.20. This mosaic of Galileo images shows chaos and gray bands crossing Europa. The icy crust has been broken apart revealing a darker underlying material. The smooth gray band is an area where the crust has been fractured, separated, and filled in with material from the interior. Obviously, the moon has been subjected to intense geological deformation. (Credit: NASA/JPL/ Caltech)

Fig. 6.20. This mosaic of Galileo images shows chaos and gray bands crossing Europa. The icy crust has been broken apart revealing a darker underlying material. The smooth gray band is an area where the crust has been fractured, separated, and filled in with material from the interior. Obviously, the moon has been subjected to intense geological deformation. (Credit: NASA/JPL/ Caltech)

only happen if the tidal bulge could slide freely over the interior, implying there is a deep ocean separating the thin icy shell from the underlying silicate lithosphere. There is apparently no evidence that these cycloids are still being formed. The absence of new formation does not mean there is no longer a liquid ocean. It may simply mean that the icy crust has finally become thick enough that the stresses cannot overcome its tensile strength to fracture and create the cycloids. However, some scientists believe we should still be seeing current activity if a liquid ocean still exists. Obviously, if a new cycloid had been observed in the process of formation, it would be evidence that the ocean tides were still active [374]. According to Lee et al., cycloidal cracks are probably the most important among Europa's surface features for proving a subsurface liquid ocean, since the tidal stress that produces them can only approach the ice failure strength if an ocean is present [375].

'Chaos' features can be described as materials that form irregular areas containing blocks or polygons of preexisting crustal material, with other intervening material that appears to lie at the same or a lower level than the surrounding plains [376] (Fig. 6.21). Chaos regions seem to be most simply explained by melt-through events. A natural explanation is that the Chaos regions represent areas where deep, warm ocean waters have come into contact with the overlying ice. Partial melt-through events, which should be more common, may also allow disruption of the surface ice and formation of Chaos [377]. According to Greenberg et al., 18% of the surface of Europa is fresh appearing chaos while another 4% appears as slightly modified chaos. There is much more older chaotic terrain that is overprinted by tectonic structures. This chaos suggests that the effects of having liquid water under a very thin ice shell have dominated Europan geology. Chaos regions are widespread on Europa, and widespread chaos regions suggest that there has been occasional zero shell thickness [378]. Like craters, chaos areas may have been formed continuously over at least the entire geological age of the surface, leaving only the most recent

Telescopes Mastery

Telescopes Mastery

Through this ebook, you are going to learn what you will need to know all about the telescopes that can provide a fun and rewarding hobby for you and your family!

Get My Free Ebook


Post a comment