A Proposed Discovery Mission
Photo: NASA/JPL [Larger 99K GIF]
Photo: NASA/JPL [Larger 99K GIF] | NOTE: The following information was provided to the Astrobiology Web by the Prinicpal Investigator on this proposed mission with their permission to distribute it freely as we deemed appropriate. This is not an official NASA or JPL website and, as such, the information presented here is in no way endorsed or validated by NASA or JPL. Neither the Astrobiology Web or Reston Communications has any contractual or bidding relationship with NASA, JPL, or any of the proposing parties. We just think these are exciting ideas and wanted to share what we have learned with our readers. |
D. SCIENCE
D.1.1 Background
Europa is one of the most interesting objects in the solar system because underneath its surface of ice may be an ocean of liquid water. The existence of an ocean and the possibility that it may harbor life is a fundamental motivation in the exploration of Europa. It has been assumed that the outer solar system is beyond the reach of the Discovery Program and hence that any investigations that seeks to test for an Europan ocean would have to await more expensive and elaborate missions. However, we propose here a Discovery class mission that can reach Europa and return data that will provide an abundance of new information as to the nature of its surface and possible evidence for the presence of an ocean.
Europa, one of the four large satellites of Jupiter, has been known since Galileo first observed it in 1610. Its orbit is only 9.5 times the radius of Jupiter, making it, after Io, the second Galilean satellite from Jupiter. One of the most remarkable features of Europa is the smoothness of its surface. Voyager observations of features near the terminator on Europa indicated that relief is not more than a few hundred meters (Lucchitta and Soderblom 1982). The surface can broadly be divided into two basic units (Buratti and Golombek 1988): relatively light colored ice makes up the plains units while darker, redder material makes up the dark spots and bands that characterize the mottled terrain units. The clean ice on the surface of Europa has exceptionally high reflectivity --- as high as freshly fallen snow.
If Europa were in a perfectly circular, synchronous orbit it would experience negligible tidal heating from Jupiter. However, Europa, Io, and Ganymede are in a Laplace Resonance that forces non-zero eccentricities (Cassen et al. 1982). Physically, the energy dissipated by tidal forces in Europa comes from the orbital energy of the satellite and the rotational energy of Jupiter. This tidal heating together with the decay of radioactive elements and the release of energy from earlier periods represents the heat sources that are warming Europa's interior (eg. Squyres et al. 1983).
Europa's low density leads to the suggestion that there might be a significant layer of ice near its surface. Earth's moon, which is similar in size to Europa, and Io both have densities of about 3.5 g cm- 3 compared to Europa's 3.0 g cm- 3. If Europa has differentiated into a rocky core of silicates with Io-like densities then this would imply a surface layer of water ice of over 100 km. If the heat flow is large enough, the lower layers of the ice would melt forming an ocean. If instead, Europa's interior is composed of hydrated silicates --- the water is distributed within the body --- then the surface shell may be relatively thin and completely frozen.
The notion of a subice ocean on Europa has been extensively discussed in the scientific literature for over 25 years (Lewis 1971, Fanale et al. 1977, Smith et al. 1979, Cassen et al. 1979, Squyres et al. 1983, Ross and Shubert 1987, Ojakangas and Stevenson 1989, McDonald et al. 1997) and in the popular literature as well (Clarke 1982). In addition, some models for the origins of life on Earth suggest a submarine origin at hydrothermal vents (Corliss et al. 1979) and others have invoked impact melting of a frozen ocean (Bada et al. 1994). Within this framework there have been speculations about life in an ocean on Europa (Reynolds et al. 1983). Unfortunately, the discussion of oceans on Europa, while rich in theory, has been devoid of any clear experimental tests. The question therefore arises: Are there direct tests that can be achieved in near-term low-cost missions that can indicate the presence of an ocean on Europa? We suggest that the surface of Europa may hold chemical and isotopic evidence of an ocean and that the Europa Ice Clipper is the near-term low cost mission to return this data.
The surface of Europa may hold clues to the presence of an ocean in several ways, all of which are related to the linear surface features. Squyres et al (1983) suggested that the mechanism responsible for the smooth surface of Europa was resurfacing by water ejected from an ocean through cracks in the ice (see also Crawford and Stevenson 1988). Lucchitta and Soderblom (1982) present several theories of how these lineations may have formed and the likelihood that these may be fissures through the ice. Transport of water to the surface of Europa through its lineations leads to speculation of the chromophores responsible for the darkening of the lineations. It is not unreasonable to assume that a soil-based silicate (Lucchitta and Soderblom 1982), or sulfoxide polymers (Hapke 1989) might be constituents, due to the rocky core of Europa and sulfur implantation from the Io plasma torus. Perhaps the most intriguing theories are that the chromophores are actually organic molecules (Cassen et al 1979; Squyres et al. 1983). A recent analysis (McDonald et al. 1997) concludes that simple molecular species such as CH4 and NH3 would not be stable on the surface, while hydrocarbons with at least four carbons, and possibly simple nitriles and aldehydes, would be stable as condensates. They conclude that if there is organic synthesis in a sub-surface ocean and the dark lineations on Europa do represent seepage vents of water from this ocean, then the lineations will retain a record of that organic chemistry. The likely species to be detected are the more complex organic macromolecules that would form in the interior ocean, such as polymerized hydrocarbons, aldehydes, and poly-HCN, and propagate to the surface through the fissures. Thus the surface materials may hold materials which are the direct result of ocean-based processes and have been carried to the surface through cracks. We propose to analyse and collect these materials.
The dark surface materials, possibly of ocean origin, and the "probable cause" warrants sending an investigative spacecraft to Europa. Our methods will investigate these materials during the encounter and transmit that data to Earth. This could then provide the basis for a more detailed, and expensive mission to characterize the ocean. We will also return samples to the Earth for analysis. Our approach here is predicated on the ability of terrestrial laboratories to analyse extremely small amounts of material --- a capability that will undoubtedly improve over the time of this project. Thus, our sample return strategy returns material amounts measured in the nanograms --- levels that are ample for measurement. The science conducted on the returned sample (isotope and elemental analysis at precisions adequate for cosmochemistry) can only be done with samples returned to sophisticated terrestrial laboratories and it is unlikely that even an expensive in situ missions can achieve these measurements at the desired precision.
The Europa Ice Clipper is a flyby mission. To obtain samples of the surface of Europa we will use an impact sampling method. As we approach Europa a 10 kg hollow copper sphere is released on a impact trajectory. The spacecraft then diverts to fly through the plume of surface material that is created by the impact. The ability to create a plume, predict its properties, and sample the particles in the plume while protecting the spacecraft are the basis for this mission design.
The current target site is at -75S, 66E in an area not imaged by Voyager and just recently seen by Galileo (see image on the Fact Sheet). The results of the main Galileo Europa encounter will arrive at Earth during the feasibility study of the Ice Clipper. We will tailor our target area based on these results.