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How cold are the icy satellites' surfaces? It's pretty darned cold. Temperature of course depends on distance from the Sun, but also on albedo (surface reflectivity), as bright surfaces are cooler and dark surfaces are warmer. On the satellites of Jupiter, temperatures range from about 140 K (about -210 °F) in dark material at the equators of Ganymede and Callisto, to about 50 K (-370 °F) in bright icy patches of Europa's poles. What is "tidal heating"? If a moon travels in an elliptical orbit around its parent planet, then it is repeated squished and squeezed with every orbit as it moves closer to and farther from the parent planet, producing heat. (An analogy is the heat produced by repeatedly bending a paper clip back and forth.) Satellite orbits will tend to circularize after a (geologically) short period of time, unless they are "forced" to have a large eccentricity due to gravitational interaction with other satellites. (For example, this is the case with Io, Europa, and Ganymede today.) Tidal heating falls off markedly with distance from the parent planet (which helps explain why Io is so volcanically active today, but Europa is cooler, and Ganymede is probably comparatively cold inside.) Warm ice is much more easily squished than cold ice, so tidal heating concentrates in ice the is already warm; thus, there can be a feedback in which tidal heating produces warm ice, which in turn produces even more tidal heating. What are "convection" and "diapirs"? Convection is the transfer of heat by vertical circulation. In icy satellite interiors, ice can be heated through radiogenic or tidal heating, whereas the near-surface ice is generally very cold. The warm ice is less dense than the cold ice, so it warm ice tends to rise, and the denser cold ice tends to sink, if the viscosity (consistency) of the ice in question is not too stiff. Warmer and thicker ice layers are more likely to convect. Lava Lamps and miso soup provide nice illustrations of convection. Earth's mantle convects, with warm rock masses moving upward through colder and denser ones. A rising or sinking blob of material, e.g. a rising blob of warm ice, is called a "diapir" when it pierces and moves through another material. Diapirism can also be triggered when a lighter material underlies a denser material of different composition. On Earth, salt commonly rises as diapirs through overlying denser sedimentary rocks, as in Iraq and Iran. Is the weird surface of the Uranian moon Miranda really due to its having broken apart and reassembled? Probably not, but this is not to say that the moon hasn't been shattered to bits in its past, or that its geology isn't fascinating. Theoretical models indicate that a moon in the location and with the size of tiny Miranda (only about 500 miles in diameter) should have been shattered by large impacts several times in the very early days of the Solar System. The moon we know as Miranda is the last incarnation of such moon-breaking events, and its probably shows no sign of these past calamities. Its surface does show three giant ovoidal "coronae" with geological characteristics that suggest they formed due to upwelling of the surface from below. This can be explained by diapirism during a past episode of tidal heating of the tiny moon. How old is Europa's surface, and how do we know? Galileo imaging shows that Europa's surface is sparsely cratered, meaning that the surface is probably young. Simulations of comet and asteroid orbits indicate that it is primarily comets which slam into Europa and the other Galilean satellites. From the modeled and observed numbers of comets in the vicinity of Jupiter today, estimates can be made for the surface age of Europa based on the number and sizes of the moon's impact craters. By this method, Kevin Zahnle and coworkers estimate that Europa's surface has an average age of about 50 million years (plus or minus a factor of five uncertainty). This is a blink of an eye by geological standards. Does Europa have a subsurface ocean, and how do we know? Europa's geological features (including bands, ridges, chaos, and multi-ringed impact structures) are indicative of warm, mobile, glacial ice at relatively shallow depths, and sometimes reaching the surface. Beneath this warm "slushy" ice could be a liquid water ocean, but it is difficult to be certain with the data that are in hand. Tidal heating is greatly aided by the presence of an ocean, so a warm near-surface is suggestive of a subsurface ocean. Also, the surface of Europa seems to have "slipped" relative to its interior in a process called "nonsynchronous rotation," judging by the pattern of the satellite's large-scale fractures--a subsurface ocean would greatly facilitate this. Recent work by Greg Hoppa and Randy Tufts indicates that Europa's bizarre "cycloidal" shaped features probably owe their origin to cracking in response to tidal flexing of Europa's icy shell on the very rapid time scale of its 3.55 day orbit; this requires large tides, best facilitated by liquid water. The magnetometer aboard Galileo has indicated an induced magnetic field at Europa, which is clear evidence for a conductive substance inside--this is strong evidence for at least some liquid water at depth, but a salty partial melt cannot be excluded. But the Galileo spacecraft was not designed specifically to test the hypothesis of a subsurface ocean on Europa. To know for sure whether Europa has an ocean, a Europa Orbiter mission is needed, which would measure the daily tidal fluctuations of Europa's surface. How thick are Europa's icy outer shell and its possible ocean? The sum of theoretical and observational data indicate that Europa's the icy shell is ~15 to 25 km thick, overlying an ocean approximately 80 km deep. A comprehensive theoretical estimate of the ice shell thickness is given by Ojakangas and Stevenson (1989), who look at the influences of tidal and radiogenic heating. They find that the ice shell should be an average thickness of ~20 to 30 km. Unintuitively, they find that the radiogenic heat from the rock-rich interior of Europa would not make the ice shell much thinner, because there is a feedback: more heat from the interior thins the ice shell, and so there is less total tidal heating of the ice shell, so the ice shell does not thin significantly overall. More recently, Bill McKinnon has investigated this further: in modeling significant tidal heating of the rocky interior, he finds that the ice shell can become no less than ~10 km thick. Additional support for a ~15 to 25 km ice shell comes from observations of pits, domes, and spots on Europa's surface. The size and spacing of the features suggests that they are due to convection of the ice shell. Modeling suggests that convection can initiate in Europa's ice shell only if the shell is >10 to 25 km thick, consistent with the thicknesses derived from thermal modeling. Data from radio tracking of the Galileo spacecraft as it has passed Europa tell of the internal distribution of its mass and suggest an outer layer of water and/or ice that is about 100 km thick, allowing for an ocean about 80 km deep. Do "icebergs" really exist on Europa? In close-up pictures of Europa's "chaos" region, it can be seen that there are "blocks" or "plates" which have moved about relative to one another. These have informally been referred to as "icebergs," but this term is improperly suggestive, invoking visions of ice chunks floating in terrestrial polar seas. On Europa it would be extremely difficult, if not impossible, to melt from a subsurface ocean all the way to the surface. One reason is that a prohibitively large amount of heat energy would be needed. Another reason (demonstrated by David Stevenson) is that ice should flow faster than it can melt, meaning that even if a sufficient heat source were available, a hole in the base of Europa's ice shell would fill in with warm, glacially flowing ice before it could melt. Instead, the chaos areas may represent areas in which Europa's ice shell has partially melted above a large diapir or assemblage of diapirs. Who first found that Europa's surface is ice-rich? According to David Morrison in the introduction to the book "Satellites of Jupiter," both Americans and Russians can lay some claim to the discovery. He writes: "Near-infrared photometry provided the first clue to surface composition; in a landmark abstract Kuiper (1957) suggested from limited data, never published, that Europa and Ganymede had water ice surfaces, and Moroz (1961) later came to the same conclusion from similar broadband infrared observations." When will the Europa Orbiter get there, and how will it determine whether there is an ocean? Launch of the Europa Orbiter mission has been delayed until perhaps 2006 or 2007. Travel time to Jupiter is about 3 years, then it takes another 2.5 years or so to drop into Europa orbit. This means arrival in 2013 or 2014. The mission itself would last only a month or two after that, but enough to make critical measurements. Europa's tidal deformation will be determined using Doppler measurement of radio signals (to determine the fluctuating gravity field), and an altimeter will measure the rising and falling of the surface tides. If Europa is frozen solid, the daily tidal fluctuation of its icy surface would be only about 1 m, but if there is a subsurface ocean (whether shallow or deep) the tidal fluctuation would be about 30 m. The orbiter will probably have a radar sounder instrument capable of detecting subsurface pockets of water and perhaps an ocean. The orbiter would also have a camera or cameras to image most of the surface at better than 300 m/pixel resolution and spots at about 10 m/pixel. Perhaps it would also carry a spectrometer, magnetometer, thermal instrument, and/or mass spectrometer. Will a lander mission be sent to Europa, and what instruments will it carry? If a Europa Orbiter mission finds strong evidence for a subsurface ocean on Europa, then there will be strong scientific justification to send a follow-on lander mission to Europa. A seismometer could detect for certain whether there is liquid water at depth and how deep that water is (assuming the satellite is seismically active). An electromagnetic technique called "magnetotellurics" could measure the current generated in subsurface water in response to the ambient magnetic field. Certainly it would be important to sample the chemistry of the reddish non-ice material on Europa's surface to test for its compatibility with life and for the presence of any organic materials. The idea of actually melting all the way through Europa's ice shell to an ocean (if technologically possible) is probably a long time off. How has Galileo enhanced what scientists got back from the two Voyager spacecraft flybys of Europa in the late 1979? The best imaging resolution from the Voyager spacecraft was about 2 km per pixel. Now we see down to 6 meters per pixel! We had little clue before of even the range of surface features on Europa. Now we know the detailed morphologies of Europan features and their geographical distributions. Plus other Galileo instruments have provided critical information as to the composition, magnetic field, and thermal properties of Europa. From Voyager, there was a hint of the range of features on the surface and how they formed, that the surface might be young (few large craters), and that there may be a subsurface ocean. With the detail provided by Galileo, all of these things seem more certain today. Why is the recent finding of folds on Europa significant? It's impossible to fully understand the geology of Europa (or any icy satellite) until we understand the nature of its extension and compression. If there are only extensional features, then it could mean the whole satellite has expanded. If so, this would have important ramifications for the satellite interior. Or if the satellite has subduction zones, then this would be a way for the satellite to cycle surface material into the interior. Understanding the nature of compressional features tells us (in part) how the icy crust is recycled--we know that new material comes up to the surface to form the plentiful dark bands (extensional features), and now we believe that some (though not all) of the extension is taken up by making folds, which can cycle some shallow subsurface material back into the deeper subsurface. Some other icy satellites show large ridges, and some have been suggested to be folds (e.g. on Enceladus and Dione at Saturn), but no unique and convincing evidence of folds have yet been found on any other icy satellite. When the Cassini spacecraft arrives at Saturn in 2004, it will be very interesting to see whether these satellites show evidence for long-wavelength folds based on high resolution imaging of their surfaces.
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