Preventing the Forward Contamination of Europa (2000)

The task group considered and discussed the history of planetary protection, the particular characteristics of Europa, and the variety of life on Earth in order to develop a set of requirements for the planetary protection of Europa. Unfortunately, the task group was unable to reach a complete consensus on these requirements. Although a majority of the members did agree on a specific requirement, a rationale, and a proposed procedure for meeting that requirement, two significant independent minority views were also expressed.

This chapter outlines the task group’s recommended planetary protection strategy for Europa and goes on to explain the areas of general agreement as well as the points on which the views of the two minority subsets diverged from that of the majority. The consequences of these viewpoints are described along with the arguments pro and con.

The task group is in complete agreement that planetary protection is an important goal for all space missions. Limiting the forward contamination of Europa is necessary to preserve the scientific integrity of future biological studies and to protect any indigenous life forms. NASA has a scientific, moral, and legal responsibility to take this task seriously, even if living up to these responsibilities is costly. The task group is also in complete agreement that the current evidence for the existence of a global ocean is persuasive but not definitive. And, of course, even if there were definitive evidence for an ocean, it would still be premature to assume the presence of indigenous biota on Europa.

Given the uncertainties in our knowledge of the diversity of life on Earth and the recent discoveries of organisms living in extreme environments, the majority of the task group believes that a conservative approach must be taken to protecting the europan environment. Furthermore, since Europa, unlike Mars, may have a global ocean, a viable organism could colonize the entire subsurface via the ocean connection.

Although it is premature to conclude that either an ocean or biota exist on Europa, it is prudent to implement planetary protection procedures that assume the existence of both. In this case, we are obliged to protect the europan environment for an open-ended period, until it can be demonstrated that no oceans or organisms are present. This viewpoint mandates that the bioload on any spacecraft sent to Europa must be reduced to such a level that the probability of inadvertent contamination of a europan ocean by viable organisms is very low, either in the next 100 years or at any time in the future.

The task group therefore recommends the following standard: for every mission to Europa, 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. This standard calls for explicit calculation of the probability of contamination posed by each particular mission. It allows spacecraft designers to take advantage of the bioload reduction that occurs from radiation in the jovian environment (see Chapter 2). The value of 10^-4 was chosen because of its historical precedents in the planetary protection resolutions issued by COSPAR.¹

NASA must devise a method for carrying out this calculation. An example of how such a calculation might be done is given in Appendix A. The task group’s suggested methodology subdivides the bioload into common microorganisms, spores, radiation-resistant spores, and highly radiation-resistant nonspore microorganisms (e.g., Deinococcus radiodurans; see Chapter 3). Assays of the spacecraft and its assembly environment would determine the abundance of these organisms. Multiplying the various survival factors by the probability that an organism will reach the global ocean and grow provides an overall probability that must be less than 10^-4 in order for a mission to meet this standard.

The task group emphasizes that the sample calculation is intended purely to illustrate a methodology NASA could use to certify that a particular mission meets the 10^-4 standard. The task group lacks the time, the resources, and the expertise to establish definitive values for each and every parameter considered in the calculation. Instead, the task group has chosen plausible but conservative values for each parameter considered in the calculation. Similarly, it makes no attempt to include either the uncertainties associated with the parameters

entering into the calculation or the possibility that some parameters may be correlated. Indeed, the task group explicitly assumes that all factors are independent.

NASA may decide that more detailed calculations and considerations are necessary or that the calculations for a particular mission show that the probability threshold of 10^-4 is exceeded at some high level of confidence, given the error bars estimated for the various factors. Future studies such as those recommended in Chapter 7 will naturally change the numerical values in the required calculation.

The task group was unable to reach complete consensus on a number of issues relevant to determining the appropriate planetary protection requirements for Europa. Two independent minority viewpoints were expressed by two subsets of the task group. Recognizing that reasonable people may disagree on the interpretation of complex scientific issues, the task group presents here the majority viewpoint and both minority viewpoints so that they may be discussed and retained for the historical record.

The first point of disagreement concerned the applicability to Europa of the current approach to planetary protection as recommended in NRC reports, as adopted by NASA, and as ratified by the international scientific community, embodied by COSPAR. According to this approach, the planetary protection measures applicable to a particular spacecraft depend on the type of mission envisioned and the degree to which its destination is of interest to studies of the processes of chemical evolution and/or the origin of life (see Chapter 1). Application of this methodology requires some detailed knowledge about the object to which the spacecraft is being sent.

One minority subgroup expressed the view that the current strategy of protection via categorization is broad enough to be applicable to Europa. Indeed, this approach has already been applied to recommendations for the prevention of back contamination when europan sample are returned to Earth.2 The implication of this minority view is that the first missions to Europa should be subject to a somewhat augmented version of the protocols currently applied to Mars missions. Thus, orbiters and simple landers would be subject to Viking-level cleaning, while landers with life-detection experiments and/or deep penetrators would be subject to a stricter Viking-level sterilization procedure. Suggested augmentations to the existing cleaning protocols for Mars missions would include assaying for radiation-resistant microbes in addition to spores and the use of molecular-based, cell-detection methods in addition to conventional culturing techniques.

The majority viewpoint is more conservative and argues that Europa must be treated as a special case. The basis for this viewpoint is the current relative ignorance of Europa’s possible biology, its possible subsurface ocean—which could allow life to be globally connected —and its possible geologic activity, which may recycle surface material into the ocean on a time scale comparable to the age of the surface, and may also provide a source of chemical energy in the form of organic debris and inorganic substrates entrained from the surface. The majority viewpoint is also based on the possibility that an impacting spacecraft could implant debris sufficiently deep within the ice that it would be protected from radiation.

The second point of disagreement within the task group concerned the likelihood of the survival and proliferation of a terrestrial organism that might reach a europan ocean (see F₇in the sample calculation contained in Appendix A).

This minority subset of the task group argued that while it is just conceivable that a terrestrial organism might survive in an oceanic environment on Europa, experience from studies of extreme terrestrial environments suggests that such an organism’s ability to grow and multiply—the real danger to future scientific studies and, potentially, to the survival of indigenous organisms, if they exist—is indistinguishable from zero.

This second subset asserted that no known terrestrial organisms could survive the successive assaults of cold, aridity, and radiation likely to be experienced in transit from Earth to Europa and then finally proliferate in a saline, oceanic environment under extreme hydrostatic pressure. They believe that the combination of physical and

chemical extremes on Europa has no counterpart on Earth, so that no terrestrial organism could have adapted simultaneously to all of them.

Although less than 1 percent of all living species have been characterized to date, both the physiological ecology and the behavior of microbial communities, as well as the environments to which terrestrial microorganisms can adapt, are reasonably well studied. The minority maintained that the known facts are sufficient to form scientifically valid conclusions about the survival and proliferation of terrestrial organisms on Europa.

It argued further that even if organisms that had simultaneously adapted to all the extreme environmental parameters on Europa did exist on Earth, the probability that a spacecraft would be contaminated with significant numbers of these organisms is infinitesimally small. This minority subset would nonetheless be willing to subject future Europa missions of all types to the Viking-level cleaning procedures, so as to significantly reduce their initial bioload.

A majority of the members of the task group did not accept these views. They recommended a more conservative approach and set the probability of proliferation at the relatively small, but finite, value of 10^-6(see F₇ in the Appendix A). They argued that prudence is necessary given the variety of life seen in extreme environments on Earth, our ignorance of the extremes of life’s adaptability, and our lack of knowledge of the europan ocean. As we learn more, F₇, like the other probability factors discussed in Appendix A, may be updated.

The majority viewpoint is that a common standard should be set according to which, for every mission to Europa, the probability of contaminating a europan ocean with a viable terrestrial organism at any time in the future should be less than 10^-4per mission. NASA would establish the assays and calculations for confirming this figure. The two independent minority viewpoints would both allow future missions to Europa to be governed by the (possibly updated) standards for planetary protection of Mars.

¹ See, for example, COSPAR, “Resolution 26 COSPAR Position with regard to the Florence Report of its Consultative Group on Potentially Harmful Effects of Space Experiments,” Article 5, COSPAR Information Bulletin No. 20, 1964, page 26.

² Space Studies Board, National Research Council, Evaluating the Biological Potential in Samples Returned from Planetary Satellites and Small Solar System Bodies, National Academy Press, Washington, D.C., 1999.