In the search for life in our Solar system, several candidate worlds stand out for various reasons: Mars, Saturn’s moons Enceladus and Titan, and Jupiter’s moon Europa. A press conference and Nature paper announced today present a new model that may help provide a better understanding of the dynamics on Europa, tantalizing hints about the possibility of dynamic processes below the ice.
Europa is the second of Jupiter’s large moons (also known as Galilean moons, for their discoverer Galileo). As such, it is closer than the other two icy moons Ganymede and Callisto, but not as close in as volcanically-active Io. Europa consists of a relatively smooth surface of water ice, with no atmosphere to speak of. Over several decades of observation, observers have found ever-stronger evidence for a global ocean underneath Europa’s ice; since water needs some kind of protective layer to stay liquid, the ice plays somewhat the same role that Earth’s atmosphere does. Despite how common it is in our daily experience, water is strange: while most substances are more dense in their solid form, ice is less dense than liquid water, so it floats on top of oceans or in a drink glass. In this way, an ice layer can actually insulate liquid water beneath, keeping it warm; this is how arctic marine species can continue to forage throughout the winter when open water may not be available.
Obviously, the analogy breaks down after a certain point: Earth’s surface is kept warm by the combination of a warm interior and infrared sunlight, some of which is kept from reradiating into space by greenhouse gases (water vapor, carbon dioxide, methane, and so forth). Without some sort of greenhouse effect, Earth’s water would freeze at night and a higher proportion of the surface would freeze over every winter. Too much is a bad thing as well, which of course is the problem we face with global climate change. Europa is much farther from the Sun than we are, and its subsurface water is kept liquid by a warm interior and insulated by the thick ice layer above.
The ice on Europa is fairly smooth and free of cratering (in contrast to the Moon or Mercury), which doesn’t mean it hasn’t experienced meteorite impacts, but rather that its surface has been smoothed over. Much as Earth’s weather and water erode craters away, some process or processes erase the craters on Europa, leaving it with a surface only about 60 million years old, as compared to the parts of the Moon’s surface, which are as much as 4.9 billion years old. Evidently, Europa is a dynamic place, despite being tidally-locked to Jupiter like our Moon is, keeping one face toward its host planet.
Europa is still not absolutely smooth, however: its surface is covered in fissures, grooves, and domes known as chaos terrains. These domes, which can be as high as 200 meters, are the focus of the new model proposed by B. E. Schmidt, D. D. Blankenship, G. W. Patterson, and P. M. Schenk. The researchers began by comparing certain features on Europa’s surface to ice layers in places such as Greenland and Antarctica on Earth. Below the ice in Greenland and the like, hydrothermal vents melt the ice from below, but surrounding ice prevents the liquid water from flowing away. In this way, a lake begins to grow, with ice on all sides and above. However, the difference in density stresses the ice, fracturing it into blocks that float on top of the water at a higher level than the surrounding ice. Since the cracks allow heat energy to escape, the overall temperature of the region cools off, refreezing the water. The formerly-floating blocks now make a dome – precisely how the chaos terrains appear in topographic studies of Europa’s surface. On Earth, no matter how big the ice cap, eventually the ice must come to an end in land or open ocean, but on Europa with solid ice extending all the way around the globe, the surface must crack and refreeze continually over the millennia.
For this model to be successful, the surface ice on Europa must be very thick: the lakes of water formed by the process described must are about 3 kilometers deep, with the formerly-floating blocks also about that thickness. The surface ice layer itself therefore must be thicker than 10 kilometers, a figure within the realms of possibility from data collected by the Galileo probe and other observations. (Other models suggest a thinner ice layer, but while these can describe the surface cracking and reforming, they currently have difficulty in handling the chaos terrains.) In addition, chemical impurities in Europa’s ice are consistent with the hydrothermal vent proposal.
For life as we know it on Earth, it isn’t sufficient to have liquid water: other processes must exist to carry nutrients (and possibly organisms themselves) through the environment. On Earth, the job is done by tides from the Moon and Sun, along with convection due to differential warming; one possible model for the origin of life places it near hydrothermal vents on the ocean floor, with organisms subsequently distributed by currents of air and water. Could Europa, with its global ocean churning by vents below and Jupiter’s gravity above, sustain life also below the ice?