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-resistant organisms. In addition, 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.
The task group recommends a number of studies that would improve knowledge of Europa and that would better define the issues related to minimization of forward contamination. These include studies on the following topics:
Ecology of clean room and spacecraft-assembly areas, with emphasis on extremophiles such as radiation-resistant microbes;
Detailed comparisons of bioload assay methods;
Desiccation- and radiation-resistant microbes that may contaminate spacecraft during assembly;
Autotroph detection techniques; and
Europa’s surface environment and its hydrologic and tectonic cycles.
The task group was unable to reach complete agreement on the central issue of the planetary protection standards that must be met by future missions to Europa. The majority if its members believe that Europa ’s potential importance to studies of chemical evolution and the origin of life is great but that detailed understanding of the europan environment and the survival of terrestrial organisms in extreme conditions is so limited that the current planetary protection methodology is not readily applicable to Europa missions. Uncertainties demand conservatism, and, thus, the very first mission to Europa must meet the highest reasonable level of safeguard.
In practice, this means that the bioload of each Europa-bound spacecraft must be reduced to a sufficiently low level at launch that delivery of a viable organism to a subsurface ocean is precluded at a high level of probability. This approach allows mission planners to take advantage of the bioload reduction likely to occur en route, particularly while in Jupiter’s radiation environment. One consequence of this view is that Europa must be protected from contamination for an open-ended period, until it can be demonstrated that no ocean exists or that no organisms are present. Thus, we need to be concerned that over a time scale on the order of 10 million to 100 million years (an approximate age for the surface of Europa), any contaminating material is likely to be carried into the deep ice crust or into the underlying ocean.
Thus, the task group’s majority concluded that spacecraft sent to Europa must have their bioload at launch reduced to such a level that, taking into account the natural additional reduction that occurs after launch, the probability of contaminating a europan ocean with a viable terrestrial organism at any time in the future should be less than 10-4 per mission. How this standard might be implemented by a combination of Viking-level cleaning and sterilization, accompanied by bioload reduction in the europan radiation environment, is illustrated by a probabilistic calculation offered by the task group (Appendix A).
In addition to the majority view, this report presents two independent minority viewpoints that argue for less stringent planetary protection requirements.*
The minority viewpoints supporting less-stringent planetary protection procedures than those advocated by the majority are based on two independent arguments. One subset of the task group argued that the planetary protection provisions for Europa should be broadly consistent with the current policies, practices, and protocols. The other subset argued that studies of the organisms found in extreme terrestrial environments suggest that no known terrestrial organism has a significant probability of surviving and multiplying in a europan ocean. The practical consequences of both of these views is that Europa missions should be subject to essentially the same planetary protection requirements that are currently applied to Mars missions. That is, spacecraft (including orbiters) without biological experiments should be subject to at least Viking-level cleaning, but sterilization is not necessary.