observations of distant objects in an effort to assess whether NASA needs to treat samples returned from small solar system bodies differently from samples returned from Mars. To identify the requirements for the origin and survival of living organisms, the task group examined contemporary views on the range of conditions under which life can originate, the conditions required for the preservation of metabolically active organisms in terrestrial environments, and the somewhat different conditions needed to preserve living organisms in a dormant form. Based on this analysis, the task group identified six parameters (liquid water, energy sources, organic compounds, temperature, radiation intensity, and natural influx to Earth) as relevant to its assessment and formulated the following six questions to help determine how returned samples should be handled.

  1. Does the preponderance of scientific evidence indicate that there was never liquid water in or on the target body?

  2. Does the preponderance of scientific evidence indicate that metabolically useful energy sources were never present?

  3. Does the preponderance of scientific evidence indicate that there was never sufficient organic matter (or CO2 or carbonates and an appropriate source of reducing equivalents)1 in or on the target body to support life?

  4. Does the preponderance of scientific evidence indicate that subsequent to the disappearance of liquid water, the target body has been subjected to extreme temperatures (i.e., >160 °C)?

  5. Does the preponderance of scientific evidence indicate that there is or was sufficient radiation for biological sterilization of terrestrial life forms?

  6. Does the preponderance of scientific evidence indicate that there has been a natural influx to Earth, e.g., via meteorites, of material equivalent to a sample returned from the target body?

For the purposes of this report, the term ''preponderance of scientific evidence" is not used in a legal sense but rather is intended to connote a nonquantitative level of evidence compelling enough to research scientists in the field to support an informed judgment. In applying the questions, the task group drew on existing data on the origin, composition, and environmental conditions (past and present) of each small body or planetary satellite examined and then determined whether the quality and weight of the evidence were convincing enough to allow making judgments and deriving findings. The answers to the questions, taken together, were used by the task group to reach a considered conclusion that the potential for a living entity to be in on a returned sample was either "negligible" or "not negligible." Because of the incomplete current state of knowledge about small solar system bodies, there are no definitive answers to the questions, and so all judgments regarding biological potential are qualitative (not quantitative).

The questions allow for a conservative, case-by-case approach to assessing whether or not special physical and biological isolation and handling of returned samples (containment) would be warranted, taking into account information about the different small bodies, natural influx to Earth of material from small bodies, and the possible nature of putative extraterrestrial life. An answer of "yes" to any question argues against the need for special containment beyond what is needed for scientific purposes. (Sample-handling requirements to support scientific investigations are currently under study by NASA.) For containment procedures to be necessary, an answer of "no" needs to be returned to all six questions. For such samples, strict containment and handling as outlined in Chapter 7 are required.

The task group chose to consider only two possible alternatives for containment and handling of samples returned from small solar system bodies: either (1) strict containment and handling of returned samples as outlined in the Mars report (NRC, 1997) or (2) no special containment beyond what is needed for scientific purposes. The task group ruled out intermediate or compromise procedures involving partial containment. In certain cases (e.g., P- and D-type asteroids) the limitations of the available data led the task group to be less certain, and therefore more conservative, in its assessment of the need for containment.

The following section summarizes the task group's findings with regard to the potential for a living entity to be present in samples returned from select planetary satellites and small solar system bodies. The selection was

1  

 For the purposes of this report, CO2 or carbonates and an appropriate source of reducing equivalents is equivalent to "organic matter" to accommodate chemolithoautotrophs.



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