Barophiles are microorganisms that thrive under conditions of high hydrostatic pressure, and all known examples inhabit marine environments. Europa’s ice shell may be 10 to 170 km thick, and thus the pressure in the upper layers of a hypothetical europan ocean would have to be 13 to 210 MPa (130 to 2,100 bars), assuming that the ice has the same density as water. Studies indicate that most organisms cannot grow when the pressure exceeds 60 MPa, and many are indeed killed at that pressure. Known terrestrial organisms could withstand the pressure near the top of Europa’s putative ocean, especially if the ice shell is relatively thin. A combination of high pressure and low temperature would, however, decrease the molecular volume of a microbe’s macromolecules and would probably bring about its death. For example, Bacillus subtilis, one of the indicator organisms currently used to determine the bioload on a spacecraft (see Chapter 5), can survive pressures of 30 MPa provided that the temperature is greater than 20 °C.
Radiation-resistant organisms are of particular relevance to any discussion of the forward contamination of Europa. The bacterium Deinococcus radiodurans, for example, can grow continuously, without mutation or any effect on its growth rate, in the presence of 6,000 rad/hr (a dose rate found 1 mm beneath Europa’s surface ice).4 This organism can also survive acute exposures to ionizing radiation of 3 Mrad (at -70 °C) without lethality—a dose that induces about 130 double-strand breaks (DSBs) per chromosome. Furthermore, viable cells are readily recovered from cultures even after exposure to 8 Mrad (at -70 °C).5 This ability is extraordinary since most cells cannot survive irradiation at more than 500 to 100,000 rad6or 1 to 3 DSBs per haploid chromosome.7Recent advances have led to insights into this bacterium’s exceedingly efficient DNA repair capabilities,8,9which have been shown to be partly responsible for its resistance to radiation.10,11
Because there are no known radioactive environments that can explain the evolution of D. radiodurans’s resistance to radiation, there is general agreement that this organism ’s resistance to radiation is a secondary characteristic developed in response to some other environmental stress. The consensus view is that the mechanisms that evolved to permit survival in very dry environments also confer resistance to radiation.12
It is possible that other desiccation-resistant microorganisms, not yet described as radiation-resistant, could pose a threat to the europan biosphere. Such organisms can only pose a threat if they can survive a multi-year journey to Europa. Despite the discrediting of the oft-repeated claim that live bacteria were recovered from Surveyor 3’s camera after surviving on the Moon’s surface from April 20, 1967, to November 20, 1969, experiments conducted aboard a variety of spacecraft including the European Retrievable Carrier and the Long Duration Exposure Facility indicate that a variety of common terrestrial bacteria are able to withstand the space environment for periods as long as 6 years.13,14Since the radiation-resistance characteristics of many common organisms (and most extremophiles) are unknown, it is conceivable that many bacteria classified as desiccation-and/or radiation-resistant will survive in extraterrestrial environments.
On Europa, life-sustaining, near-surface environments may exist within or under regions of water ice, since ice will provide microbes with some degree of radiation protection. In addition to requiring water in the liquid state, genetic repair would certainly also be dependent on the presence of a source of carbon and energy. There is currently little or no evidence for any organic matter on Europa’s surface due to the lack of observable spectral features of CH bonds in Galileo ’s infrared spectra of Europa. Nevertheless, the presence of carbon in material recycled from the interior via geologic processes or in cometary and meteoritic debris cannot be discounted.15
The ability (or inability) of terrestrial organisms to adapt to, and survive and multiply in, extreme terrestrial environments reveals much about the resilience of life in stressful circumstances. Given that these organisms have had millions of years to come to terms with their particular physical and chemical environments, their ability to cope provides some insight into the problems facing terrestrial organisms suddenly introduced into extraterrestrial environments. Earth’s polar regions present two particularly telling examples, the cryptoendolithic environments found in the polar deserts and permafrost.
Antarctic cryptoendolithic environments exist where communities of microorganisms have colonized the surface layers of porous rocks to depths of a few millimeters. Photosynthetic members of the community utilize sunlight that penetrates the translucent rock crust. The ambient air in the Antarctic desert is rarely above 0 °C, but the near-surface regions of rocks exposed to the Sun are warmed by solar radiation. The cryptoendolithic colonies obtain water from snow, which melts when it falls on warm and dry rock surfaces.