Interest in Jupiter’s moon Europa has intensified with exciting new findings in the last few years from NASA’s Galileo mission, currently in orbit around Jupiter, which has made numerous close passes by Europa. This summary of our understanding of Europa, the proposed strategy for its exploration, and the possibility for indigenous life there follows closely the 1999 NRC report A Science Strategy for the Exploration of Europa.1 The task group quotes and paraphrases liberally from that report in the next sections, and the reader is directed to it for greater detail.
Perhaps the most exciting facet of Europa is that an ocean of liquid water may lie beneath its surface covering of ice. Although there is currently no direct evidence for such an ocean, intriguing indirect evidence has been seen from various spacecraft. Europa’s reflectance characteristics indicate that its surface is composed of water ice. Local- and global-scale ice tectonics dominates the geology, with a very large number of cracks crisscrossing its surface. Galileo ’s gravity measurements show that Europa has a differentiated interior, with the outermost 80- to 170-km layer predominantly water and/or water ice. Europa’s magnetic signature suggests the presence of an electrically conducting layer near the surface. The likely explanation is that the water layer is liquid, with dissolved salts providing the ions for electrical conduction. Europa also has an extremely thin atmosphere composed of gases derived from its surface.
High-resolution images of the surface show features that are best or most easily explained as resulting from the presence of at least partially melted material at shallow depths.2 These features include plates of ice that appear to have rafted to new locations and then frozen in place (Figure 2.1)3, 4 and cycloidal cracks that can be interpreted as being due to the effects of diurnal tidal stresses on a surface ice shell decoupled from Europa’s interior (Figure 2.2).5
Because of the likely existence of liquid water, at least as a transient or intermittent species, life could exist within or below Europa ’s icy shell. The other requirements for life—access to biogenic elements and access to a source of energy—may be available at the water-rock boundary at the bottom of the water layer.6 Some researchers have argued against this idea on the grounds that Europa’s ocean is a closed system and, therefore, its water would rapidly become chemically reduced as the result of interactions with hot rocks in hydrothermal systems,7 which would mean that a europan ocean would not be an energetically favorable environment for life. This contrary view has, however, been challenged on the grounds that even if the ocean is reducing, abundant redox chemistry could still take place, providing an energy source for metabolism.8
Another possibility is that life exists not in the deep oceanic interior of Europa but near the surface. If this is the case, then the ultimate power source for a europan biosphere may be chemical species created by interactions between the surface ice and energetic particles in the jovian radiation environment.9 While no evidence for life exists, it is the potential for life that makes Europa an exciting target for further exploration.
The most important questions about Europa include whether liquid water exists and whether it has lubricated the motion of surface blocks seen in the Galileo images. Researchers also seek to learn the composition of the deep interior and the non-icy parts of the surface. The nature of the tectonic processes and the abundance of geochemical energy sources are important as well for learning about the potential for—or the history of—life on Europa.
EUROPA EXPLORATION STRATEGY
The important questions about Europa that are outlined above can be addressed through a series of spacecraft missions that carry out the following key measurements, extracted from the 1999 NRC report by the Committee on Planetary and Lunar Exploration (COMPLEX):10
Measuring Europa’s global topography and gravity, and determining how Europa’s shape changes as it orbits Jupiter;
Characterizing Europa’s geology and surface composition on a global scale;
Mapping the thickness of Europa’s ice shell and determining the interior structure;
Distinguishing between any intrinsic europan magnetic field and induction and/or plasma effects; and
Sampling the geochemical environment of Europa’s surface and possible ocean.
COMPLEX concluded that Europa is an exciting object for further study, with the potential for major new discoveries in planetary geology and geophysics, and the potential for studies of extraterrestrial life. In addition, COMPLEX concluded that the results obtained by Galileo (revealing geologically recent or ongoing geologic activity, regarding the possible presence of liquid water, and indicating the potential for present or past biological activity) make Europa a high-priority target for further exploration.
The two highest-priority overall science goals identified in the 1999 report by COMPLEX for Europa exploration reflect the emphasis on the potential for life as a major driver in Europa’s exploration. They are the following:
Determining whether liquid water has existed in substantial amounts subsequent to the period of planetary formation and differentiation, whether it exists now, and whether any liquid water that is present is globally or locally distributed; and
Understanding the chemical evolution that has occurred in the liquid-water environment and the potential for an origin and the possible continued existence of life on Europa.
Even if there turns out to be no life or no sophisticated prebiotic chemistry, these goals remain legitimate drivers for a better understanding of Europa’s geologic history.
The particular scientific goals of the first mission are expected to be determination of whether a global ocean of liquid water exists beneath the icy surface, determining, if possible, the spatial and geographical extent of liquid water, determining the bulk composition of the surface material, and characterizing the global geologic history and the nature of any ongoing surface and atmospheric processes. These science objectives can best be met by one or more near-polar-orbiting spacecraft.
EUROPA’S RADIATION ENVIRONMENT
The intense radiation near the surface of Europa is a key factor governing the viability of organisms that might be carried to Europa on spacecraft. Ionizing radiation causes biological effects, such as genetic damage, that result in significant morbidity or death once sufficient damage accumulates. The intensity of the radiation environment in the Jupiter system has been measured by several spacecraft, and its variation with depth below the surface of the ice can be predicted. Accordingly, the rate at which microorganisms with a specified radiation tolerance would succumb in the vicinity of Europa can be determined.
Europa lies deep within the magnetosphere of Jupiter, which is the volume of space above Jupiter’s atmosphere that is affected by Jupiter ’s magnetic field. This magnetosphere extends up to 10 million km from Jupiter (i.e., it encompasses a volume 1,000 times that of the Sun) and is filled with ionizing, magnetically trapped particle radiation. The mechanism of magnetic trapping of radiation at Jupiter is the same as that which operates in Earth’s van Allen belts. Jupiter’s magnetosphere is, aside from the Sun, the dominant source of energetic charged particles and radio emissions in the solar system.
First discovered as a radio source, the magnetosphere of Jupiter interacts strongly with the innermost Galilean satellite Io, as evidenced by the modulation of decametric radio emissions at the orbital period of Io. The study of Jupiter’s decimetric radio emissions led to the first determinations of the approximate strength and direction of the jovian magnetic moment. The first spacecraft visits to Jupiter, in 1973 and 1974 by Pioneer 10 and Pioneer 11, respectively, confirmed the existence of the magnetosphere and revealed its disklike configuration, which rotates approximately with the planet itself. The later visits by Voyager 1 and Voyager 2 in 1979 revealed the importance of Io ’s plasma torus, a region of sulfur- and oxygen-dominated plasmas maintained by the escape of SO2 and other S- and O-bearing molecules from Io. The plasma torus mediates the interaction of Io with the jovian
magnetosphere. The Ulysses spacecraft also encountered Jupiter in 1992 and explored the high-latitude, dusk-side magnetosphere whose configuration appears to reflect its interaction with the solar wind. The latest spacecraft to explore Jupiter is Galileo, which began its orbital tour in 1995 and has provided detailed measurements of the interactions of the magnetosphere with Jupiter’s satellites as well as synoptic views of global dynamics.
Europa orbits Jupiter at a distance of 671,000 km and is continually bombarded by magnetically trapped, ionizing radiation. This magnetospheric particle flux is the dominant component of the radiation environment at Europa. Galactic cosmic radiation and solar particle radiation cannot access Europa because of Jupiter’s magnetic field except at energies exceeding ~90 GeV, where fluxes are negligible.
The radiation flux in the vicinity of Europa varies on many time scales. There are fluctuations that can exceed an order of magnitude with magnetospheric activity over times of minutes. Similarly, smaller fluctuations, typically by a factor of less than two, occur with the 11.2-hour synodic Jupiter rotation period as seen by Europa, because of the variations of trapped particle intensities with magnetic latitude (Jupiter’s rotational and magnetic axes are misaligned by 10 degrees). Moreover, variations of up to roughly an order of magnitude have been observed over the 25-year time span between the Pioneer spacecraft encounters with Jupiter and the Galileo mission.
Galileo magnetic field and charged-particle data also imply that the radiation environment varies across the surface of Europa, being only one-fifth as high over the leading hemisphere as over the trailing hemisphere. This happens because the ice surface of Europa absorbs trapped particles as magnetic flux tubes drift across the moon from trailing side to leading side (owing to magnetospheric rotation), depleting the particle population on the leading side.
Data on the radiation environment of Europa have been compiled from information gathered by the Pioneer 10 and 11, Voyager 1 and 2, and Galileo missions. Available measurements include electron intensities in the energy range 30 keV to > 10 MeV and ion intensities from 30 keV to > 100 MeV. Data on ion composition— separation of protons from helium and from ions with atomic number Z > 6—are available above about 500 keV/nucleon. These data are input to standard radiation-dose models that calculate the rate of energy deposition versus depth below the target surface.
The results are summarized in Figure 2.3, which shows the radiation dose accumulated at various depths in the europan ice as a function of exposure time. Contributions from jovian electrons, electron bremsstrahlung, and ions are shown. The electron-bremsstrahlung component consists of X rays generated by the high-energy electrons at depth as they are stopped by the target material. The contribution from ions and electrons with energies below 30 keV may safely be ignored even though their energy flux may be significant. This is because particles with such very low energies penetrate Europa’s surface to a depth of only about 10 µm for the electrons and even less for the ions.
The units of radiation dose are the rad or the gray (1 rad is 100 erg/g and 1 gray is 1 joule/kg) of energy deposition in the target material. No corrections have been made for the relative biological effectiveness (RBE) of different forms of ionizing radiation, based on the rates of linear energy transfer (LET). In higher organisms, such as humans, it is well established that high-LET radiation (e.g., stopping protons, heavy ions) has a greater biological effect than does low-LET radiation (e.g., electrons and X rays), as reflected by RBE values as high as 20 depending on the specific biological end point. However, no experimentally established RBE values for microbes are available.
Figure 2.3 shows that the radiation environment at 10-cm depth in europan ice is ~5 krad per month. The radiation dose is dominated by electrons and bremsstrahlung over depth values down to approximately 1 meter, below which protons dominate. At greater depths, the radiation environment continues to decrease, reaching values similar to those in Earth ’s biosphere below depths of 20 to 40 m (not shown). Hence, once microorganisms are transported below a shallow depth at Europa—at most a few tens of meters below the surface—radiation is no longer a significant environmental factor.
For comparison, the natural radiation environment at the surface of Earth gives an average dose of about 0.1 rad per year. The ionizing radiation exposure limits for astronauts are 25 rad per month and 50 rad per year, not to exceed 100 to 400 rad total in a career, depending on age and sex (these are whole-body doses). Many microorganisms tolerate far more radiation; D. radiodurans, for instance can grow and reproduce in a 6-krad/hour environment.
1 Space Studies Board, National Research Council, A Science Strategy for the Exploration of Europa, National Academy Press, Washington, D.C., 1999.
2 G.C.Collins et al., “Evaluation of Models for the Formation of Chaotic Terrain on Europa, ” Journal of Geophysical Research 105: 1709, 2000.
3 M.H. Carr et al., “Evidence for a Subsurface Ocean on Europa,” Nature 391: 363, 1998.
4 N.A. Spaun, et al., “Conamara Chaos Region, Europa: Reconstruction of Mobile Polygonal Ice Blocks, Geophysical Research Letters 25: 4277, 1998.
5 G.V. Hoppa et al., “Formation of Cycloidal Features on Europa,” Science 285: 1899, 1999.
6 B.M. Jakosky and E.L. Shock, “The Biological Potential of Mars, the Early Earth, and Europa,” Journal of Geophysical Research 103: 19359, 1998.
7 E.J. Gaidos, K.H. Nealson, and J.L. Kirschvink, “Life in Ice-Covered Oceans,” Science 284: 1631, 1999.
8 T.M. McCollom, “Methanogenesis as a Potential Source of Chemical Energy for Primary Biomass Production by Autotrophic Organisms in Hydrothermal Systems on Europa,” Journal of Geophysical Research 104: 30,729, 1999.
9 C.F. Chyba, “Energy for Microbial Life on Europa,” Nature 403: 381, 2000.
10 Space Studies Board, National Research Council, A Science Strategy for the Exploration of Europa, National Academy Press, Washington, D.C., 1999.