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.1.2 Science Goals and Objectives
Science Goals and Objectives
The overall science goal for the Europa Ice Clipper is to sample the surface of Europa to understand the processes that shape it and to look for evidence of a subsurface ocean. In addition we will address questions at to the formation of Europa and the source of water for the Galilean satillites. We are proposing the following specific science objectives.
To explore for the presence of an ocean we will:
-- determine of the structural properties of the surface ice.
-- determine the presence of light organics (eg., CO2, and C2, C3, C4 compounds), volatile inorganics (eg. NH3), and soluble salts in the surface ice.
-- analyze organic and/or mineral phases in the surface ice.
To understand the formation of Europa and the source of water we will:
-- measure the D/H and the oxygen isotopes of the surface ice in returned samples.
-- return to Earth samples of the refractory components of the surface ice.
Structure: The structural properties of the surface ice on Europa could provide clues to the presence of an ocean. A fluffy snow-like surface could be the result of precipitation from vapor clouds ejected through cracks in the ice shell. Alternatively the direct condensation of water vapor (essentially vacuum condensation) onto the surface could result in a compact hard surface. The nature of the plume generated by the impactor will depend on the strength and cohesiveness of the surface. For a nominal 10 kg impactor mass and an impact velocity of 10 km/s the amount of ejected mass is a sensitive function of the strength of the surface. For compact ice the ejected mass is estimated to be about 4800 kg while for loose material in which gravity is the determinate of ejection the mass could be as high as 2.5x105 kg for material with the consistency of sand. Even more extreme, light fluffy surfaces like the ice of C/SL9 would result in the ejection of about 107 kg. Crater sizes and depths vary with the radius of the impactor; for our impactor, we expect a crater radius of order 10 meters. Thus, a very sensitive measure of Europa's crustal strength (to depths of 10 m or so) will be available from imaging the impact - generated plume. An estimate of the total ejecta mass will be possible from images of the impact plume.
Organics: A potential key signature of an ocean is the presence of complex organics in the surface ice. These organics may be the result of biological or abiological processes in the ocean. Abiotic processes that could produce organics include: synthesis in deep hydrothermal vents, cosmic ray synthesis and even photochemistry in the water. Irradiation on the ice surface may also produce and alter organics and this factor must be considered in the analysis. Nonetheless, as we discussed above, if organics are produced in an ocean then the signature of this process could be recognizable in the distribution of organics seen in the ice. The organics in the plume particles will be measured with a time-of-flight mass spectrometer after particles impact on a metal foil.
Volatile Species: The presence of light organics and volatile inorganics in the surface ice of Europa could be compelling evidence of an active source of these materials related to a subice ocean. McDonald et al. (1996) argue that there are no mechanisms for the direct production of light organics and over geologically short timescales they would migrate out of the surface ice and be lost to space. Even clathrate formations --- potential sources of CH4 and CO2 -- - would be unstable at the surface due to the low pressures. We note however that the impact itself may cause the production of light organics from refractory organics on the surface. If the impact-induced source can be corrected, then detection of these volatile organics could be a strong indication of an ocean and very recent activity.
Soluble salts: Progressive freezing of liquid water exposed to the surface will concentrate soluble salts. These would be incorporated into the ice surface as discrete particles that could be recognized as such in the particle analyzer by their relative concentrations and types of cations and anions. The ratios of these elements could be indicative of water solubility and hence provide further evidence of an ocean, and one that deposits material on the surface.
Isotopes: The deuterium to hydrogen ratio and the oxygen isotopes will be measured in water molecules returned to Earth. These measurements will address questions related to the major sources of volatiles in the solar system. From studies of the D/H ratio in the outer solar system and comets it is clear that there are variations in the D/H in major solar system reservoirs. The protosolar value is about 8x10- 4, the Galileo result form Jupiter is about 6x10- 4. In general the large planets in the outer solar system have D/H results that are consistent (within error bars) with the protosolar value. By contrast, standard mean ocean water on Earth is about 10 times the protosolar level and the comet Hally results gives a D/H that is about twice the terrestrial value. Comet Semarkona also give a D/H that is larger than terrestrial. For Europa the key question is the source of its H ; does it come from the Jovian subnebula and would therefore reflect the Jovian value -- close to the presolar value? Or does the water on Europa reflect accretion from a cometary source and the associated much higher value of D/H? To address these question we will measure the D/H on Europa to within 20% of it's expected range (between 10- 4 and 10- 5). The isotopes of oxygen will also provide information on the source processes and formation of Europa. Here the central comparison is with the meteorites. To differentiate a Europa isotopic ratio from values characterizing, for example the Eucrites, requires a precision of 1 per mil in the 17O and 18O determinations.
Refractory Species: Samples returned from Europa will include particles composing the refractory component of the surface material. It is likely that this will include both silicates and kerogen-like organics. Laboratory analysis of the solid organics could allow for discrimination between sources in an ocean interior to Europa and production by irradiation on the ice surface. Elemental and mineralogical analysis of the silicate particles should provide considerable information on the nature of Europa's interior and possibly about the planetary nebula form which the galilean satellites formed. Meteoritic infall and interplanetary dust particles (IDP) provide an additional source of surface material. It concentration with respect to ice and to material from within Europa provides information on the relative timescale of the various processes that deposit material on the surface. Meteoritic and IDPs can be distinuished from Europa-derived particles by the elemental composition.