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Summary

Preventing the Forward Contamination of Europa. Task Group on the Forward Contamination of Europa
Task Group on the Forward Contamination of Europa. National Academies Press, Washington, DC (2000)

Available online: http://www.nap.edu/catalog/9895.html

NASA’s Planetary Protection Office requested that the Space Studies Board (SSB) undertake a study to evaluate the planetary protection (PP) requirements and methods used to prevent contamination of Europa by terrestrial organisms in future orbiter and lander missions and that it recommend any necessary changes. The SSB was asked to:

  1. assess levels of cleanliness and sterilization required to prevent the forward contamination of Europa by future spacecraft missions;
  2. review methods used to achieve appropriate cleanliness and sterilization for spacecraft and recommend alternatives based on new science and technology; and
  3. identify scientific investigations that should be done to reduce uncertainty in the above assessments.

In its work, the task group considered many dimensions of the questions, including spacecraft cleaning techniques, resilience of terrestrial organisms, the space environment at Jupiter, and cost implications of alternative PP approaches. In the end, the task group reached consensus on some, but not all aspects of their charge.

Conclusions and Recommendations

  1. To meet planetary protection requirements and obligations, each Europa bound spacecraft must be cleaned, sterilized, and/or subjected to sufficient radiation prior to contact with Europa’s surface so that the probability of contaminating a possible europan ocean with a viable terrestrial organism at any time in the future is less than 10 -4.
    NOTE: two minority views were also expressed by two members of the task group.
    • One argued that PP provisions for Europa should be broadly consistent with the current practice of spacecraft categorization by type of mission (flyby, orbiter or lander) and the degree to which the target body is of interest to studies related to chemical evolution.
    • The other view argued for PP requirements similar to Mars missions (subject to Viking level cleaning but not sterilization) based on studies that suggest no known terrestrial organism has a significant probability of surviving and multiplying in a Europan ocean.

The practical consequence of both minority views is that Europa missions should be subjected to the same PP requirements that are currently applied to Mars missions (spacecraft without biological experiments should be subject to at least Viking-level cleaning, but sterilization is not necessary).

  1. Current cleaning and sterilization techniques are satisfactory to meet the needs of future space missions to Europa. These techniques include Viking-derived techniques (e.g., washing surfaces with isopropyl alcohol and/or sporicides, and dry heat sterilization), as well as modern processes such as hydrogen peroxide sterilization, assuming that final sterilization is accomplished via exposure of the spacecraft to Europa’s radiation environment.
  2. The current culture-based method used to determine the bioload on a spacecraft should be supplemented by screening tests for specific types of extremophiles, such as radiation-resistant organisms.
  3. Modern molecular methods, such as those based on the polymerase chain reaction (PCR), may prove to be quicker and more sensitive for detecting and identifying biological contamination than NASA’s existing culturing protocols for planetary protection.
  4. The task group recommends that a series of scientific and technical investigations be conducted to reduce uncertainty in calculating the probability of contaminating Europa as a result of spacecraft missions. These investigations include targeted research in the following areas:
    • Ecology of clean room and spacecraft-assembly areas, with emphasis on extremophiles such as radiation-resistant microbes to allow targeting bioload-reduction techniques to the specific organisms present in these artificial environments.
    • Detailed comparisons of bioload assay methods (i.e., strengths and weaknesses of the various molecular-based techniques; which methods can best extend current culturing techniques? Can quick PCR methods replace culture-based assays? Can improved detection techniques be developed to readily distinguish between living and dead organisms?).
    • Desiccation- and radiation-resistant organisms (abundance, survivability, capacity for growth in possible europan environments).
    • Autotroph detection techniques (sensitivity and reliable estimates of abundances of chemoautotrophs).
    • Europa’s surface environment, and its hydrologic and tectonic cycles (Europa’s surface chemical, thermal, and radiation characteristics; transport mechanisms and time scales. Does recycling occur? On what temporal and spatial scales? Does Europa have a groundwater or hydrothermal system?).