• 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.

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