If the sum of all NXs is found to be much less than one, it is known by the Poisson statistic that this sum is equal to the probability of any organism being successful. Hence,
Pc = Sum (NXs) in the limit of a small value (e.g., 10-4).
To begin the calculation, it is important to recognize that different classes of organisms are critical to these calculations, and methods must be devised to estimate their abundances. It is assumed that the spacecraft will be cleaned and/or treated to reduce its bioburden of organisms. As a starting point, the current procedures for cleaning and validating a Mars lander that does not carry life-detection experiments will be assumed. Under these procedures, the lander spacecraft is certified to be carrying a total available bioload of not more than 300,000 culturable “spores,” where the “spores” are defined to be heat-shock-resistant organisms. However, some portions of a Mars spacecraft are solid materials, encapsulated components, and occluded surfaces, and they are not included in the above levels of bioload because they are not “available” for release to the martian environment. For the sake of argument, however, the task group considered a worst-case scenario, in which the long-term corrosive action of ocean water ultimately liberates all organisms, wherever they reside. To adjust the estimate of bioload, typical values for surface and buried microbial density on items manufactured inside and outside of controlled environments (e.g., clean rooms) must be taken into account. These values are given in current planetary protection guidelines.1
From a series of many thousands of samplings of the Viking landers and subsequent culture studies, information is available on the types of organisms present under clean room assembly conditions. Chemolithoautotrophic organisms were not specifically tested for because at the time, it was widely expected that organic compounds would be present in the martian soil.
The Viking planetary protection studies characterized only the aerobic, mesophilic organisms that grow on trypticase soy agar (TSA) plates. The ratio of total culturable cells to “spores” was found to be quite variable but typically ranged from 3:1 to 60:1.2 Under present protocols the spacecraft assay is for heat-shock-resistant organisms, presumed to be spores. To be safe, the task group assumed a value of 50 (see the next paragraph) as the factor by which to multiply the number of heat-shock-resistant organisms measured for a given spacecraft to estimate the actual number of organisms that could grow in culture. This value is significantly higher than the average observed. More than 55 percent of the organisms were gram-positive cocci (Staphylococcus and Micrococcus), characteristic of organisms found in the human body. Bacillus organisms accounted for about 24 percent, the Corynebacterium-Brevibacterium group accounted for about 15 percent, and Actinomycetes and yeasts constituted the remaining few percent.3
In studies done on the Surveyor spacecraft before they were sent to the Moon, aerobes outnumbered anaerobes by 5 to 1, both overall and for just the “spore” fraction.4 Even though autotrophy was not assayed, a search was made for psychrophilic microorganisms, but none were detected on spacecraft surfaces.5 The factor-of-50 multiplier (see preceding paragraph) takes into account these additional anaerobic microorganisms, which were not assayed in the Viking studies.
In summary, the number of chemolithotrophic organisms on spacecraft is small compared to the number of more common heterotrophs. Since organic compounds may be present in the putative europan ocean, the task group accepts the nominal spacecraft assays that use TSA as the growth medium and assumes them to be indicative of the overall population of organisms that pose a contamination threat.
Type A organisms are all those organisms that are culturable using the standard TSA plating technique. Type B are those known as “spores ” in the standard protocol, as determined by their resistance to heat shock. Type C are a subset of Type B and are resistant to higher radiation doses. Type D are a subset of Type A and are also resistant to high radiation doses. To avoid unnecessarily elaborate testing and analysis procedures, the task group suggests that Types C and D be determined by a simple screening test using exposure to 60Co or by some other well-established procedure for dosing with ionizing radiation. The classification criteria suggested at this time are as follows:
Type C—Organisms with 10 percent or greater survival above 0.8 Mrad; and
Type D—Organisms with 10 percent or greater survival above 4.0 Mrad.
For this example, the task group took a level of 10 times the Mars available “spore” bioburden, or 3 × 106 culturable heat-shock-resistant organisms for the total spacecraft bioload, based on typical spacecraft sizes and