----- Original Message ----- From: "Constantine Thomas" <[EMAIL PROTECTED]> To: <[EMAIL PROTECTED]> Sent: Wednesday, June 19, 2002 4:34 AM Subject: [jupiter_list] Europan Chaos > I had an odd thought the other day - why haven't we ever seen 'half-formed' > Chaos terrain on Europa? I ask because if Chaos were formed by ice diapirs, > then it'd presumably take some time for them to form so you'd be more > likely to see them in various stages of completion, right? But if they were > formed by water melting through, it'd be formed very quickly and you'd > mostly see fully-formed chaos. > > Is it just that we haven't seen enough of the surface to see examples of > 'half-formed' Chaos? Or is it that we wouldn't really recognise a > "half-formed Chaos" because we don't have enough topography data? (I'd > imagine the diapir would cause a big topographic bulge on top of it, then > maybe a small broken up area would form at the centre of the dome, which > would gradually increase in size to make a 'full Chaos'). Or do people > think we have seen half-formed Chaos and I just don't know about it? ;-) > > If it's none of the above, and we genuinely don't see half-formed chaos > terrain, would that mean it *is* more likely that they form rapidly, as a > result of water melting through the surface? > Consty ____________ The answer to this puzzle lies in the fact that Pappalardo & Company DO think that chaotic terrain is the result of a melt-through -- but that it's a local, shallow melting event caused when a rising diapir contacts a patch of particularly briny ice, whose melting point is low enough that the somewhat warmer ice of the diapir melts it into liquid. There's good reason to think that such local concentrations of brine may exist in patches and layers all over Europa, as they do in terrestrial polar ice. When rising diapirs contact such briny patches close to the surface, they produce the surface melt-through events that create chaotic terrain (in which the "matrix" in which the big broken icebergs float may be slushy, partially melted ice, rather than flat-out liquid, before it quickly refreezes). In many other places, such near-surface brine doesn't exist, and so -- though diapirs may create very many subsurface pockets of liquid brine (which in turn may sprout other diapirs) -- the diapirs that actually reach the surface just produce bulges in the solid surface ice (the "lenticulae"). Moreover, the limited analyses of gradual surface topography made from Galileo suggest that all the patches of chaotic terrain actually lie on top of substantially elevated bulges of terrain -- which makes sense if the melted areas were relatively shallow and lying on top of a subsurface upward bulge of ice, but doesn't make sense if a thin Europan ice crust melted through completely and the underlying ocean just gushed out. I really think the overall evidence for the "thick crust" model of Europa's ice is overwhelming. One other important point: Pappalardo's theory says that diapirs considerably enlarge themselves through positive feedback -- for, as their warmth softens the surrounding ice, it flexes more in response to Jupiter's changing tidal forces, thus generating more frictional heat that further softens the nearby ice... All diapirs may start as very small initial events. == You are subscribed to the Europa Icepick mailing list: [EMAIL PROTECTED] Project information and list (un)subscribe info: http://klx.com/europa/
