Extensive observations of Europa by NASA's Galileo spacecraft (see chapter 5) indicate that this large moon of Jupiter has been geologically active in the relatively recent past and that liquid water could exist beneath its surface shell of water ice—a moon-encircling shell that appears to be between 6 miles (10 km) and 106 miles (170 km) thick.
Even though scientists do not have sufficient information at this time to state with confidence that Europa has a subsurface, liquid-water ocean, native (extraterrestrial) life, or supports environments that might prove compatible with terrestrial life (at the microbial level), such intriguing possibilities cannot be quickly dismissed. Therefore, consistent with international planetary-body-protection agreements, all future robot spacecraft missions to Europa must be subjected to rigorous procedures to prevent forward contamination by hitchhiking terrestrial microorganisms. Unless steps are taken to prevent Europa's contamination by terrestrial microorganisms, the future scientific integrity of the search for alien life on this Jovian moon will be severely compromised.
The discovery of extremophiles—terrestrial organisms living in extreme environments—has caused scientists to recognize the uncertainties in their knowledge of the diversity of life on their own home planet. So care must be taken to avoid introducing terrestrial microorganisms to the surface or subsurface of Europa. If this moon has an extensive, perhaps global, subsurface ocean, then the accidental introduction of a viable microorganism from Earth could result in a rapid spread of forward contamination throughout the entire aquatic domain. Any native Europan life-forms could be overwhelmed quickly by the microbial invaders from Earth and soon could become extinct, long before scientists had a chance (through surrogate robot spacecraft missions) to study them. Consequently, national and international planetary protection specialists are currently recommending that for every future mission to Europa, the probability of the robot spacecraft contaminating a Europan ocean with a viable terrestrial organism at any time in the future (that is, to the next 100 years or so) should be less than 10-4 per mission.
As part of its suggested planetary protection guidelines for Europa, COSPAR allows aerospace engineers and spacecraft designers to take advantage of the intense ionizing radiation environment encountered in the Jovian environment to help reduce the bioload on any robot spacecraft visiting Europa. One of NASA/JPL's radiation dose models for Europa indicates that at a 3.9-inch (10-cm) depth in Europan ice, the natural ionizing radiation dose is approximately 5,000 rad (50 gray) per month. By way of comparison, the natural ionizing radiation environment at the surface of Earth is on the order of 0.1 rad (0.001 gray) per year. Similarly, the ionizing dose limits (wholebody) for astronauts is 25 rads (0.25 gray) per month and 50 rad (0.5 gray) per year, not to exceed a career total of between 100 rad (1 gray) to 400 rad (4 gray), depending on age and sex. Even a brief visit to the surface of Europa would prove fatal to an astronaut, due to acute radiation syndrome. So in all likelihood, only radiation-hardened, well-sterilized robot exploring machines will search for life in any (suspected) Europan ocean later this century.
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