by Bruce Moomaw
Cameron Park - April 17, 2000 - Sixteen years ago they predicted that if Europa's outer ice shell is sitting on a layer of slush or liquid -- rather than being directly attached to Europa's main rock body -- Jupiter's tidal tuggings on that shell would very slightly accelerate the rate at which the outer shell rotated relative to Europa's main body, so that it would very slowly slide around the rest of the moon, gliding on the soft middle layer.
The resultant very slow changes in the direction of the tidal stresses on the ice shell -- and the fact that the shell would also have to "stretch" over the two small tidal bulges in Europa's rocky main body -- would produce a pattern of cracks in it that closely matched the pattern seen in the surface ice on the anti-Jupiter side of the moon.
Hoppa and Greenberg said that Europa's already-known surface marks matched that pattern perfectly, and predicted that Europa's Jupiter-facing side would show a similar pattern of marks. Now Galileo's first good photos of that side have matched their predictions beautifully.
It should be emphasized that this additional "non-synchronous rotation" of Europa's outer surface (if it exists) is extremely slow. Comparison of Galileo's photos with the photos of Europa taken 20 years earlier by the Voyager spacecraft show no detectable difference, proving that Europa's surface must slide around its main rock body once every 12,000 years if not much longer.
But this is hardly surprising, since Hoppa and Greenberg's calculations had predicted that such a complete slew occurs only every 10 million years or so.
As it is, the confirmation of their predictions provides still another piece of very suggestive evidence for the existence of a subsurface Europan ocean either of very soft subsurface ice, ice-water slush, or flat-out liquid water.
As P.H. Figeredo reported, "the lineaments [seen on Europa's surface] show that [Europa's ice surface] completed [at least] a full rotation relative to the tidally deformed interior."
But how thick is the overlying solid-ice shell? Much of the evidence seen by Galileo suggests that it's very thin: only a few kilometers thick, and perhaps even less than one kilometer in some areas.
One of the strongest pieces of evidence for this has been the widely publicized "chaotic terrain": big areas in which Europa's original ice crust seems to have shattered, and the pieces ("icebergs") have then become jaggedly mounted in a new layer of refreezing liquid.
The obvious implication is that these are areas where the outer ice has melted through until an underlying ocean of water or slush gushed to the surface.
P.J. Thomas stated: "If the chaos regions are the remnants of catastrophic melt-through events, reasonable estimates for basal and shell heating predict that the ice crust will be only a few km thick for 10% of [Europa's] observed surface age" -- and that, based on his calculations, "We anticipate that a more realistic model... should make the ice thinner still."
David J. Stevenson stated at the Conference that "variations in ice thickness are likely to be very small -- one percent or less -- even if there are two or more orders of magnitude in the spatial variation of tidal heating. The reason is that the lowermost ice can flow laterally, more quickly than the ice can thin by melting, and [thus filling] in the hollows."
Another theory is that the chaotic areas have been disrupted by "diapirs" -- plumes of warmer but still solid tidally heated ice that very slowly rise up through the solid ice layer -- but G.C. Collins and James W. Head argued that this doesn't explain why the shattered surface "icebergs" seem to have sunk into some liquid or slushy material.
They believe that chaotic areas are caused by small local pockets of melted water or brine in the ice layer near the surface. And Stevenson and H. Wang, in another paper, calculated that local concentrations of tidal stress can melt such near-surface pockets, so that "the existence of liquid water does not necessarily mean the existence of an ocean."
The evidence that the upper layer of super-cold and brittle ice on Europa is very thin now seems certain -- but there may well be a thicker layer of warmer and softer ice between it and any ocean, and if an ocean does exist it may be an ocean of ice slush rather than liquid water, which would be a more hostile environment for any Europan life.
The possibility of Europan life, however, is very much alive. Chris Chyba and E. Pierazzo argued that crashing comets could deliver large amounts of the organic molecules that serve as precursors of life to Europa, as they may also have done to Earth.
And while one increasingly popular interpretation of "Galileo's" infrared spectra of Europa's surface is that it may contain astonishingly high concentrations of sulfuric acid -- more concentrated than a car battery -- some biologists have pointed out that some bacteria thrive in cave environments with comparable concentrations of sulfuric acid; and D. Fernandez-Remolar discussed a river in Spain which is equally acid from natural sources, but nevertheless filled with "a very diverse sulfur and iron [consuming bacterial] community".
However, Europa's ocean floor may not have enough volcanic hydrothermal vents to provide such bacteria with an adequate 'food' source -- indeed, N.A. Spaun and James Head suggested at this Conference that the geological evolution of Europa has underlain any ocean with a floor of magnesium sulfate (Epsom salt) dozens of kilometers thick, which would probably cover any such vents -- but Chyba has suggested elsewhere that Jupiter's radiation belts might help Europa manufacture large amounts of the chemicals such bacteria would need in Europa's surface ice, and that they could diffuse downwards from there into regions of liquid water that could support microbes.
In short, Europan life, like Martian life, is still very much an open question scientifically, and we'll have to wait longer for more information on it.
The next stage will be the Europa Orbiter, which NASA hopes to have arrive at the moon some time between 2008 and 2010 to try to probe through the ice layer with long-wavelength radar and look for an underlying liquid water layer -- and even if the ice turns out to be too thick for this, the Orbiter can use a laser altimeter to measure the height of the tidal bulges in the ice, which (as I noted above) will be far greater if there is an underlying layer of water or slush.
The laser altimeter by itself should establish once and for all whether the Europan ocean exists -- and if the radar can locate local thin patches in the surface ice, they will make excellent landing sites for future spacecraft equipped to analyze the ice and look for signs of frozen microbial life.
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