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he outer solar system is dominat- ed by the giant ice and gas plan- ets, whose enormous gravitational fields sweep up much of the debris that passes through their orbits. As the solar system evolved, this debris formed a host of satellites around Jupiter, Saturn, Uranus, and Neptune, and very large satellites formed around all but Uranus. These major moons, all larger than Pluto and two larger than Mercury, are interesting worlds in their own right. Why are these worlds worthy of national and international exploration and research? The most compelling Icy Europa as seen by Voyager 2 (top] and Galileo (above]. The view from Galileo of Europa's Conamara region shows objects as small as 60 m across. The discontinu- ities in the linear features create the strong impression that pieces of the surface have fractured and drifted before refreezing into new positions. motivation relates to understanding the origin and survival of life and the limits on where life can be found in a planetary system. Water is essential to life as we know it, and the large icy satellites may contain the largest reser- voirs of liquid water in the solar sys- tem. fupiter's moon Europa may be the best place in the solar system beyond Earth to search for signs of life. Saturn's moon Titan provides a natural laboratory for the study of organic chemistry over spans of time and distance unattainable in terrestrial laboratories. The origin and evolution of the satellite systems of the giant planets also provide analogs for under- standing planetary systems around other stars, some of which may be abodes for life. Perhaps teeming with ~ Fuzz {~: ~~ ~~f~ ~;f5~:ffff~ ~f/~'f:~ life or perhaps sterile today, these moons seem certain to contain the basic ingredients for life. Knowing whether or not life exists on them today is equally compelling. Europa holds the most promise for understanding the biological potential of icy satellites. There is convincing evidence that the incessant pushing and pulling on Europa by fupiter's grav- itational field generates sufficient tidal heating to sustain a global ocean of liq- uid water just a few tens of kilometers beneath the satellite's icy surface. Moreover, there is geological evidence of recent transfer of material between the surface and the water layer. If a cold europan ocean is in direct contact with Europa's warmer rocky mantle, the result could be an environment in which complex chemical processes can occur. Such an environment could lead to the beginnings of life. A Europa mission with the goal of confirming the presence of an interior ocean is the first step in understanding icy satellites' potential as abodes for life. Characterizing Europa's ice shell and understanding its geological histo- ry are also vital components of such a mission. By exploring the extent to which organic chemistry progresses toward life in extreme planetary envi- ronments, a Europa mission would help us to understand how tidal heat- ing can affect the evolution of worlds. Europa's internal structure may include a water layer some 100 km thick.

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Europa Geophysical Explorer C. ~ ~ ~ ~ r r ~ ~ r,~,~,~,~,~,r,r,r,r,r,.; i ~ ~ ~ ~ a a a a ~ Profile Europa Geophysical Explorer Mission Type: Orbiter Cost Class: Large Priority Measurements: Obtain high-resolution images of Europa's surface. Characterize its internal heat sources. Determine its surface composition. Sound the ice shell to determine its thickness and structure. Search for temporal variations in its magnetic properties. These concepts are key to understand- ing the origin and evolution of water- rich environments in icy satellites. The Europa Geophysical Explorer mission proposed by the SSE Survey is an extension of orbiter concepts stud- ied by NASA in the l990s, and would pave the way for more ambitious Europa missions in the future, such as a lander or even a submarine to explore the ocean (if it exists). The Europa Geophysical Explorer would be tasked with confirming the presence of an ocean, identifying areas of recent surface activity, finding areas with possible biotic or prebiotic com- pounds, and characterizing the europan environment in preparation for further study. The large cost (in excess of $1 bil- lion) of this mission would place it in the same class as Galileo and Cassini/ Huygens. Such large missions have been a traditional focus for interna- tional cooperation in which NASA and other national space agencies can leverage their resources to accomplish what might otherwise be too costly to achieve. Galileo and Cassini/Huygens provide perfect examples of partner- ships that have proved highly success- ful. NASA should engage prospective international partners in the planning and implementation of the Europa Geophysical Explorer. Guiding Themes Addressed Important Planetary Science Questions Addressed In addition to mapping the satellite's topography, the Europa Geophysical Explorer will use radar to probe the structure of Europa's icy surface layer.