New Frontiers in Solar System Exploration (2003)

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Europa and Large Satellites he outer solar system is dominat- life or perhaps sterile today, these T ed by the giant ice and gas plan- ets, whose enormous gravitational fields sweep up much of the debris moons seem certain to contain the basic ingredients for life. Knowing whether or not life exists on them that passes through their orbits. As today is equally compelling. the solar system evolved, this debris Europa holds the most promise for formed a host of satellites around understanding the biological potential Jupiter, Saturn, Uranus, and Neptune, of icy satellites. There is convincing and very large satellites formed evidence that the incessant pushing around all but Uranus. These major and pulling on Europa by Jupiter’s grav- moons, all larger than Pluto and two itational field generates sufficient tidal larger than Mercury, are interesting heating to sustain a global ocean of liq- worlds in their own right. uid water just a few tens of kilometers Why are these worlds worthy of beneath the satellite’s icy surface. national and international exploration Moreover, there is geological evidence and research? The most compelling 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- Icy Europa as seen by Voyager 2 (top) and Galileo (above). The view from Galileo of ronments, a Europa mission would Europa’s Conamara region shows objects as small as 60 m across. The discontinu- help us to understand how tidal heat- ities in the linear features create the strong impression that pieces of the surface have fractured and drifted before refreezing into new positions. ing can affect the evolution of worlds. 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. Jupiter’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 Europa’s internal structure may include a water layer some 100 km thick. 6 New Frontiers in Solar System Exploration 

Europa Geophysical Explorer Guiding Themes Addressed Important Planetary Science Questions Addressed Profile The Origin What planetary processes are responsible for generating Europa Geophysical Explorer and sustaining habitable worlds? and Evolution Where are the habitable zones in the solar system? Mission Type: Orbiter of Habitable How has the suspected ocean varied throughout Europa’s Worlds history? Cost Class: Large What is keeping the ocean from freezing? Priority Measurements: • Obtain high-resolution images of Europa’s surface. Processes How do the processes that shape the contemporary character of planetary bodies operate and interact? How Planetary • Characterize its internal heat What is the chemical composition of Europa’s suspected Systems Work ocean? 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 1990s, 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 In addition to mapping the satellite’s topography, the Europa Geophysical Explorer Geophysical Explorer. will use radar to probe the structure of Europa’s icy surface layer. 7