Planetary protection issues for Europa, like those for other solar system bodies, have two components — forward contamination and backward contamination.1,2 The former relates to the potential for inadvertently transporting terrestrial organisms to Europa and either causing potential harm to extant europan life or contaminating experiments to determine whether life exists. The latter relates to the return to Earth of samples from Europa that have the potential to contain living organisms, with the possibility that inadvertent release of such organisms into the terrestrial environment may cause harm. These issues are dealt with separately below.
Prevention of the forward contamination of Europa and other planetary bodies is motivated by two different, but complementary, objectives. 3,4 The first is the desire to minimize the introduction of any material that can interfere with or confound measurements of the europan environment, especially as they pertain to the efforts to understand prebiotic or biotic activity. The second is the very unlikely, but non-zero, probability of introducing a terrestrial microorganism that might conceivably take hold in some ecological niche in the europan environment and, as a result, become directly or indirectly detrimental to the indigenous biosphere.
Differences Between Mars and Europa
To date, considerations of forward contamination have focused principally on Mars, the most plausible abode of past or present life to which spacecraft have been dispatched. Several characteristics of the martian environment have, however, led to the drafting of planetary protection requirements that emphasize the preservation of science rather that the protection of biospheres.5 These characteristics include the following:
Indications based on the Viking mission and subsequent studies that the martian surface is not, in general, conducive to the proliferation of life. The lack of liquid water and resources to support metabolism, an average temperature well below freezing, and the presence in the near-surface regolith of oxidizing materials that can destroy organic molecules or organisms all make life extremely unlikely.
The low current rate of geologic activity on the martian surface. Spacecraft hardware on the martian surface thus may not be substantially disturbed for millions, perhaps billions, of years.
The likelihood that plausible martian environmental niches conducive to life are localized rather than globally connected. Thus, even if a single location were to become contaminated by terrestrial organisms or organic molecules, it is unlikely that the contamination would spread either regionally or globally.
Europa's environment, however, is sufficiently different from that of Mars that more attention may have to be paid to the protection of any indigenous europan biota. 6 Factors arguing for a possible shift toward protection of potential biospheres over preservation of the science include the following:
Europa has been geologically active at global scales in the geologically recent past. It has been resurfaced relatively recently, and the average age of the surface is probably no more than 10 million to 100 million years. Radiogenic heating in Europa's rocky core and the dissipation of tidal energy in its icy shell drive the extrusion of water or ice onto the surface, resulting in the erasure of existing surface features and, presumably, the entrainment and subduction of old surface materials.
The global character of any ocean enhances the hazard of contamination. If terrestrial organisms were to make their way into a europan ocean and were able to grow and multiply using resources there, they would quickly be distributed globally.
Thus, spacecraft hardware and other contaminants emplaced onto Europa's surface would be incorporated into the ice, and, if it exists, a sub-ice ocean on a time scale of 10 million to 100 million years. Moreover, it must be assumed that contamination that did make its way into the ocean has the potential to contaminate the entire ocean and not just a localized part of it.
Europa's harsh radiation environment would only partly mitigate this effect. Although radiation can break the chemical bonds in organic molecules like those found in terrestrial organisms, thus effectively killing any introduced organisms, the radiation would penetrate only to very shallow depths (meters) into the subsurface. Thus, a spacecraft that made an uncontrolled descent onto Europa's surface and buried debris into the subsurface, or a successful lander that deployed a subsurface penetrator, could emplace material at depths where terrestrial organisms might survive for extremely long periods of time.
Protecting Science Versus Protecting Biospheres
Although it is premature to assume that Europa has either an ocean or indigenous biota, prudence dictates the adoption of controls on forward contamination that assume both are present. Similarly, although it is extremely unlikely that any known terrestrial organism could survive the long journey to, and intense radiation around, Europa, nevertheless the survival of terrestrial organisms in a variety of extreme environmental conditions, including low temperatures and high radiation, is well documented.7,8 Moreover, a variety of terrestrial microorganisms could possibly grow in a europan ocean, including methanogens, sulfate reducers, anaerobic methane oxidizers, oligotrophic anaerobic heterotrophs capable of growing (albeit slowly) on low concentrations of organic compounds, and acetogens (John Baross, University of Washington, private communication, 1999).
Thus, prudence also dictates that measures be taken to ensure that terrestrial organisms are not inadvertently transferred to Europa. The scope of the provisions will depend on the degree to which emphasis is placed on preserving the scientific integrity of future observations or on protecting any indigenous europan organisms.
COMPLEX has made no attempt to determine the relative weight that should be given to these imperatives. Indeed, such a determination would require a study in itself. Such a detailed consideration of imperatives, particularly as they apply to the Europa Orbiter mission, is currently being undertaken by the Space Studies Board's Task Group on the Forward Contamination of Europa. Rather than attempt to prejudge the outcome of the task group's study, COMPLEX outlines two possible positions, one that favors preserving the integrity of studies of Europa's biotic and prebiotic conditions and another favoring the protection of possible europan organisms.
Preserving Scientific Integrity. A great deal of attention has been paid to the procedures that need to be taken to prevent contaminating planetary bodies to such an extent that future scientific studies are compromised.9 Most of the attention has been paid to Mars, for which current procedures include careful cleaning of spacecraft, ensuring that orbiters do not crash within 50 years of launch, and determination of the geographic locations of landing and crash sites.10 The detailed requirements for Europa remain to be determined, but they would probably be similar to those for Mars. That is, they would be designed to protect Europa for a finite period of time during which biological exploration could proceed unencumbered by terrestrial contamination.
One issue of particular concern for Europa is the potential for inclusion of organic material in spacecraft arriving at Europa. This concern stems from the great interest in finding out whether or not any kinds of primitive biochemical building blocks — including amino acids and sugars, fatty acids, and also nitrogen bases and phosphate esters — are available on Europa. The search for biochemicals would likely begin on Europa's surface. Indeed, it is possible that the coloring agent in the "dirty" ice may contain interesting organic material. Analysis of ice cores would likely be undertaken with technology that has already been developed for ice analyses on Earth; subsequent analysis of material from beneath the ice might be even more revealing. Precautions would have to be taken to ensure against contamination during the operation of coring and other types of subsurface sampling devices.
If biochemical signatures were ever detected on Europa, immediate consideration would have to be given to the possibility that they were in fact contaminants from Earth. Mass-isotopic analysis of any such material might be an easy way to confirm the nonterrestrial origin of the material. Beyond that, the presence of biochemicals ought not to be taken as prima facie evidence of past or existing life. Further analysis, including measurements of optical activity, hydrolyzable polymers, and many others would have to be performed. In every case, the caveat would be to ensure that the measuring device did not contaminate or interfere with any of the measurements being undertaken. (For example, nylon and other hydrolyzable materials should not be included as a part of any of the instrumentation without proper analysis and precautions.)
Preserving Indigenous Organisms. Accepting the goal of not harming possible europan organisms requires that planetary protection proceed not just for a brief period of biological exploration. Rather, it must continue indefinitely or until it can be demonstrated that no ocean or no organisms are present.
This requirement means that any spacecraft reaching Europa's surface must have undergone some form of bioload reduction, perhaps analogous to that achieved following the dry heating of the Viking landers. Moreover, this requirement would probably apply equally to orbiters that would eventually impact Europa's surface and to landers. A reduced level of protection might apply to spacecraft whose design or mission profile was such that the most heavily shielded portion of its interior would receive a sterilizing dose of radiation prior to contacting Europa's surface.
Sample-return missions from Europa are still a long way off, and the opportunity to learn from planned sample-return missions to Mars is obvious. Indeed, all of the discussion about planetary protection for upcoming Mars missions is directly applicable to Europa.11 The gist of those discussions is that the threat of returning material from space that is potentially hazardous or deleterious to the terrestrial environment is exceedingly small but cannot be set at zero; as a result, appropriate precautions must be taken.12 Indeed, a recent NRC study concluded that samples from Europa "should be contained and handled similarly to samples returned from Mars."13 Included among these precautions is that attention must be paid to the important matter of public relations and education. Given the high level of public interest in the exploration of Europa and the search for life elsewhere in the solar system, it is important to ensure that any rumors or misinformation about potential hazards do not become exaggerated. 14
Nonetheless, as the dates for potential sample-return missions draw nearer, it would be prudent to appoint a program oversight committee that can reconsider these issues in the light of new knowledge and technology.
Studies of Europa can benefit in several ways from cooperative interactions between NASA and other federal agencies. For example, the National Science Foundation (NSF), through its Office of Polar Programs, funds a broad variety of research activities in the Antarctic and other icy terrestrial regions; much of the work relating to ice cores and glaciers has clear importance for planning studies of Europa. Studies of Lake Vostok, mentioned in Chapter 4, are an example of research in which close cooperation between NASA and NSF is critical. Scientific activity in the Antarctic is governed under the terms of the Antarctic Treaty, which guarantees cooperation and unrestricted scientific access that in turn requires coordination through various national agencies and with international bodies (e.g., the Scientific Committee for Antarctic Research). Because NSF is tasked with coordinating all U.S. Antarctic research activities, access by NASA must be through interagency cooperation. In turn, NASA can provide technologies useful to NSF-funded scientists, so mutually beneficial cooperative activities can occur.
The NSF also supports studies of life in extreme environments, most recently through its Life in Extreme Environments (LExEn) program. These studies extend and complement work in exobiology and astrobiology funded by NASA. Cooperation between the two agencies may enable more rapid progress and reduce overlap in exobiological research.
Another important area of potential interagency cooperation is the development and testing of electronic and optical components hardened to survive the intense europan radiation environment. Although NASA and DOD requirements once drove innovations in electronics, commercial users have recently become dominant. As a result, radiation-resistant hardware is not being developed as extensively as it once was, and the development of radiation-hardened electronic components has become prohibitively expensive. Thus locating off-the-sheet electronic and optical components suitable for spacecraft use requires extensive searching, and extensive testing must be done to prove their capabilities.
Finding such components, however, does not necessarily result in identifying a steady source of suitable components, because variations across manufacturing lots may lead to significant differences in the radiation tolerance of a given component, or subcomponents may come from different suppliers. Moreover, most companies are not forthcoming about uniformity in manufacturing and do not trace from which lot a particular set of units came, or whether procedures or suppliers have changed. Thus, testing of the actual flight components is often necessary. Further, miniaturization of electronic components has led to a higher degree of catastrophic failures, as opposed to the mere inconvenience of transient losses of information or operations.
Agencies besides NASA that would benefit from access to a long-term, stable supply of radiation-hardened components include the Department of Defense and the Department of Energy, which need flight components that can survive exposure to high levels of radiation. Similarly, the National Oceanic and Atmospheric Administration will need to pay greater attention to radiation hardening as its increasing role in the monitoring of space weather (i.e., disturbances in the solar-terrestrial environment) will require the deployment of operational monitoring satellites beyond geosynchronous orbits. Moreover, civilian users, such as the communications-satellite industry, may require radiation-hardened components for communications satellites that may be placed at altitudes within Earth's radiation belts. A cooperative program to provide better sources for radiation-hardened electrical and optical components could offer broad benefits. The December 1998 announcement that NASA and other government agencies will have access to a radiation-hardened version of the Pentium processor, thanks to an agreement between Intel Corporation and the Department of Energy, is a useful beginning.
1. L.B. Hall, "Foundations of Planetary Quarantine," Planetary Quarantine: Principles, Methods, and Problems, L.B. Hall, ed., Gordon and Breach Science Publishers, London, 1971, page 5.
2. G.B. Phillips, "Back Contamination," Planetary Quarantine: Principles, Methods, and Problems, L.B. Hall, ed., Gordon and Breach Science Publishers, London, 1971, page 121.
3. Space Studies Board, National Research Council, Biological Contamination of Mars: Issues and Recommendations, National Academy Press, Washington, D.C., 1992, page 14.
4. A.G. Haley, Space Law and Government, Appleton-Century-Crofts, New York, 1963, page 282.
5. Space Studies Board, National Research Council, Preventing the Forward Contamination of Europa, National Academy Press, Washington, D.C., 2000, in preparation.
6. Space Studies Board, National Research Council, Preventing the Forward Contamination of Europa, National Academy Press, Washington, D.C., 2000, in preparation.
7. See, for example, K. Horikoshi and W.D. Grant, eds., Extremophiles: Microbial Life in Extreme Environments, John Wiley and Sons, New York, 1998.
8. M.J. Daly and K.W. Minton, "Resistance to Radiation," Science 270: 1318, 1995.
9. See, for example, Space Studies Board, National Research Council, Biological Contamination of Mars: Issues and Recommendations, National Academy Press, Washington, D.C., 1992.
10. D.L. De Vincenzi, P. Stabekis, and J. Barengoltz, "Refinement of Planetary Protection Policy for Mars Missions," Advances in Space Research 18: 311, 1996.
11. See, for example, Mars Sample Handling and Requirements Panel, Final Report, Office of Space Science, National Aeronautics and Space Administration, Washington, D.C., 1999.
12. Space Studies Board, National Research Council, Mars Sample Return: Issues and Recommendations, National Academy Press, Washington, D.C., 1997.
13. Space Studies Board, National Research Council, Evaluating the Biological Potential in Samples Returned from Planetary Satellites and Small Solar System Bodies: Framework for Decision Making, National Academy Press, Washington, D.C., 1998, page 81.
14. M.S. Race, "Mars Sample Handling and Planetary Protection in a Public Context, Advances in Space Research 22: 391, 1998.