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  1. Through Thick or Thin: Exploring Europa's Outer Layer of Ice

    When NASA’s Galileo spacecraft sent back images and data of the Jovian moon Europa, scientists began thinking seriously that life just might exist on this enigmatic, frozen world.

    Europa appears to have all the conditions necessary for the emergence of life: liquid water, organic chemicals, and energy. A layer of ice covers Europa, but there is strong evidence – the most convincing comes from Galileo’s magnetometer – that a salty ocean may lie underneath. Organic chemicals are prevalent throughout the universe and could have been deposited on Europa by comets and meteors. Tidal forces exerted by Jupiter could provide enough energy for life, and if Europa is anything like its close neighbor Io, subsurface volcanism may provide yet another source of energy.

    While scientists are eager to explore Europa’s putative ocean for signs of life, the ice layer covering the moon is also the object of a great deal of interest. For one thing, before any attempt can be made to access the ocean, the thickness of the ice layer needs to be determined. In addition, the ice layer itself may harbor life.

    “Our work has been on understanding the geology and geological systems of Europa, and what we have come to understand is that everything on the surface has a linkage to the ocean,” says Richard Greenberg of the University of Arizona. “In the chaotic terrain – the areas where the ice appears to have melted down into the warmer ocean before refreezing – there are a number of niches where organisms could survive and prosper in the ice.”

    Organisms couldn’t live right on the surface of the ice because the temperature is a frigid -160 C (-260 F), too cold for even a microbe to survive other than in a state of complete hibernation. But brief exposure to the outer ice layer may provide the organisms in Europa’s ocean with a steady source of nutrition. The food – oxidants produced by radiation bombarding the outer ice layer – would drain downward into the ocean at the areas of ice melt-through. The mixing of ice crust and subsurface ocean may provide microorganisms with another source of food as well. It is possible that when cracks opened in the ice layer, organisms living in the ocean would be exposed to the faint rays of the sun.

    “There’s a whole zone within meters of the surface where organisms could photosynthesize, as long as they had access to sunlight,” says Greenberg.

    Greenberg thinks the cracks in the ice could provide an ideal habitat for organisms on Europa. The cracks are caused by the tidal pull of the gas giant Jupiter, which squeezes Europa like a tennis ball. Greenberg believes that Europa’s ice is only a few kilometers thick, and that the cracks reach down to the ocean below. If true, the tidal motion of Europa’s ocean periodically would push water up and down in the cracks. In this way, the cracks would act as passageways where organisms can continually reach the surface.

    Robert Pappalardo of Brown University has very different ideas of how organisms might access the oxidants and sunlight on Europa’s surface. Pappalardo disagrees with Greenberg about the thickness of Europa’s ice; based on geophysical modeling, he believes Europa’s crust is about 20 kilometers thick. Still, Pappalardo says that a thicker ice crust would not prevent interaction between the icy surface and the liquid ocean. He suggests that diapirs – columns of warmer ice that slowly plough their way upwards through the top ice layer – could provide the needed interchange.

    “On Earth, salt rises as diapirs through overlying higher-density sediments – notably in Iran and off the Texas Gulf coast,” says Pappalardo. “We can best visualize the process by picturing a Lava Lamp. On Europa, warm ice forms the rising blobs, as these are buoyant relative to the colder and denser overlying ice. The blobs could carry ocean material and any potential organisms up toward the surface.”

    The diapirs would act as a sort of elevator, bringing up the ocean-dwelling inhabitants and bringing down the oxidants formed on the surface of the moon.

    “As they impinge on the cold near-surface ice, diapirs can melt out pockets of the surface ice, allowing it and entrained oxidants to drain downward into the deep portions of Europa’s ice shell and presumably back into the ocean. In this way, diapiric stirring can help move microbes toward the surface, and can move nutrients down into the ocean.”

    Although the two scientists agree that interaction between the ice crust and ocean might support life, they strongly disagree about the thickness of that crust. Their disagreement is based on the estimated amount of energy available on Europa. Greenberg, for instance, believes Europa’s constant tidal flexing generates enough heat to maintain both a liquid ocean and a thin ice crust.

    “The tides on Europa distort the shape of the body once every 85 hours,” says Greenberg, “so Europa gets enough tidal heating from Jupiter to maintain both a liquid ocean and a thin crust. There’s been a lot of evidence that this is so. The convection of water in the ocean is enough to heat the ice and cause melt-throughs. You just need to do the math, calculating heat rates and tidal rates for Europa, to understand how the planet is warm enough for a liquid ocean and a thin ice crust.

    “Rather than give any particular number for the thickness of Europa’s crust,” Greenberg continues, “the main line of argument is that the crust is thin enough for the ocean to be a factor, and thin enough for the cracks on the surface to reach down to the ocean below. With a crust 20-km-thick, that is very difficult.”

    But Pappalardo cites models of Europa’s tidal heat that predict a crust thickness of somewhere between 20 to 30 kilometers. Based on this data, and on speculation about other possible energy sources on Europa, he suggests it is highly improbable that Europa could generate the heat necessary to sustain a thinner ice shell.

    “One could imagine turning up the amount of interior heat flow from Europa’s rocky core to such a great extent that the ice shell becomes only 6 or so kilometers thick,” says Pappalardo. “This is an extreme possibility, one that has not been supported by geophysical modeling.

    “How would the heating be turned up so high?” Pappalardo asks. “To generate such heat flow, Europa would have to possess a rocky interior that is partially molten, and this interior would have to have been molten over the entire history of the solar system. If it were ever colder, it would not then be able to become molten. But so far, no geophysical model predicts such a hot Europa interior today.”

    “Moreover, this scenario raises a potential conundrum,” Pappalardo continues. “If Europa is tidally heated to such an extent that it has a molten rock interior, then this large amount of tidal dissipation would act to damp out Europa’s high [orbital] eccentricity,” reducing the tidal forces, “and the interior in turn would cool. In other words, it has not been demonstrated that Europa’s high eccentricity is compatible with a rocky interior. There are many issues which such a model must address before it can be considered credible, let alone compelling.”

    Ronald Greeley, professor of geology at Arizona State University, heads the NASA Astrobiology Institute’s Europa Focus Group. In his opinion, what matters more than the question of thick or thin ice is the question of the age of Europa’s surface. No-one knows whether the cracks in Europa’s ice were made recently or are millions of years old.

    “The age, or timing, of formation is extremely important,” says Greeley. “With time, an originally thin crust might thicken as cooling progresses. Many investigators fail to say if they are referring to a given thickness for today’s conditions, or if they are restricting comments to the time when the features they are investigating were first formed. Disregarding differences in opinion on how certain surface features might form, the ice crust thickness could be of different thicknesses in different places and at different times.”

    Knowing the age of Europa’s surface would tell us whether the moon has an active geology. Scientists try to determine the age of a moon or planet by counting the number of impact craters caused by asteroids. The theory is that the fewer the craters, the younger the surface. An active surface – like that of our own planet – tends to erase evidence of old impacts, while on a non-active surface – like our moon – the evidence can remain for billions of years.

    “Although,” says Greeley, “Europa is lightly cratered from impacts, suggesting relative youth in the geologic context, the surface could still be millions of years old and not necessarily reflect today’s crustal properties.”

    Another problem with crater-counting is that impact rates differ according to location in the solar system. For example, Jupiter has such immense gravity that it attracts a lot of asteroids, which could increase the cratering rate on its moons relative to better-understood standards like our own moon. Unfortunately, only crude estimates are available for the rate of impacts in the present Jupiter environment.

    For Greeley, uncertainty about the age of Europa’s surface leads to uncertainty about many of the moon’s other proposed features. For instance, he says the only strong evidence that Europa’s ocean is currently liquid is the magnetometer data sent by the Galileo probe. But that data is open to other interpretations, and is not absolute proof of a liquid ocean.

    “The real bottom line is that we do not currently have the right kind of information to say if liquid water exists today beneath the surface, nor can we say how thick an ice crust might be even if there is liquid water at depth. For definitive statements, these data must come from a future mission, such as the Europa Orbiter.”

    What Next?

    The Europa Orbiter is tentatively scheduled to arrive at Jupiter in 2010, and settle into a 200-kilometer orbit about Europa in 2011 or 2012. The proposed Orbiter will use radar to measure the thickness of the ice crust and to determine whether liquid water exists below the ice. Along with mapping the surface and measuring the topography of the moon, the Orbiter will also try to detect any signs of recent geological activity.