A Report of the Task Group on the Forward Contamination of Europa
Planetary protection is an essential consideration for exploration of planets or satellites that may have experienced prebiotic chemical evolution or that may have developed life. Recent observations of Jupiter's satellite Europa indicate that it has been geologically active in the relatively recent past and that liquid water might exist beneath a surface ice shell some 10 to 170 km thick. Moreover, water might exist closer to the surface on an intermittent basis if the ice shell is cracked or otherwise punctured owing to the action of internal and external forces.
We know that life arose rapidly on Earth, perhaps in ancient hydrothermal systems. In these systems, cold ocean water is taken up and circulated through a geothermally heated zone, where it interacts chemically with minerals, and is then released back into the ocean. Its high temperature and dissolved mineral content result in a state of physical and chemical disequilibrium when it mixes again with the cold water. On Earth, the subsequent reactions to reestablish equilibrium were able to provide energy to support metabolism. Europa may also have such geothermal zones if a global ocean of liquid water exists below the surface.
Terrestrial microorganisms provide the only available reference point for evaluating whether life might already be present on Europa or whether it could be introduced by a contaminated spacecraft. On Earth, life is found in some of the most extreme environments. These include extreme heat, cold, pressure, salinity, acidity, dryness, and radiation. Microorganisms are remarkably resilient and have survived exposure to the space environment for more than 5 years aboard the Long Duration Exposure Facility and for millions of years in permafrost regions on Earth's surface. Moreover, in some circumstances, the ability to survive one form of environmental stress may confer the ability to survive in another stressful environment. Many common bacteria are, for example, desiccation resistant, and there is evidence suggesting that the mechanisms that evolved to permit survival in very dry regions also confer resistance to irradiation. Organisms capable of surviving a particular set of extreme conditions cannot, therefore, be assumed to be necessarily confined to environments possessing those conditions.
Even though current information is not sufficient to conclude whether Europa has an ocean, native life, or environments compatible with terrestrial life, it is also insufficient to dismiss these possibilities at this time. Thus, future spacecraft missions to Europa must be subject to procedures designed to prevent its contamination by terrestrial organisms. This is necessary to safeguard the scientific integrity of future studies of Europa's biological potential and to protect against potential harm to europan organisms, if they exist, and is mandated by obligations under the United Nations' Treaty on Principles Governing the Activities of States in the Exploration and Use of Outer Space, Including the Moon and Other Celestial Bodies (U.N. Document No. 6347 January 1967).
Current NASA requirements for the protection of other planetary environments are based on categorizing the mission as to type and the target object as to its likelihood of harboring life. The current procedures for planetary protection use protocols derived from those originally developed for the Viking missions to Mars in the 1970s. Determining whether or not this methodology is applicable to Europa missions was the central facet of the task group's deliberations.
The Task Group on the Forward Contamination of Europa concluded that current cleaning and sterilization techniques are satisfactory to meet the needs of future space missions to Europa. These techniques include Viking-derived procedures such as cleaning surfaces with isopropyl alcohol and/or sporicides and sterilization by dry heating, as well as more modern processes such as sterilization by hydrogen peroxide, assuming that final sterilization is accomplished via exposure of the spacecraft to Europa's radiation environment. The technological drawbacks of current prelaunch sterilization techniques are such that the use of such techniques is likely to increase the complexity and, hence, the cost of a mission.
The task group also concluded that the current spore-based culturing techniques used to determine the bioload on a spacecraft should be supplemented by screening tests for specific types of extremophiles, such as radiation-
NOTE: "Executive Summary" reprinted from Preventing the Forward Contamination of Europa, National Academy Press,Washington, D.C., 2000, pp. 1-2.