Comparative study of methods for finding extraterrestrial life based on solar system exploration and the role of microscopy in it

We present a comparative study of methods with a table summarizing them in searching extraterrestrial life. This work represents an update and improvement of our previous publication. In these works, microorganisms are supposed to be the object of extraterrestrial life exploration. We selected several characteristics and molecules which can be detected and analyzed in searching for extraterrestrial life. We also evaluated the search technique which targets those characteristics and molecules from several points of view. In this revision, "Generality for terrestrial microorganism" and "Source of false positives" were resolved into sub-issues and described in detail. We also introduced new issues, i.e., "Possible meaning of negative detection" and "Science objectives other than life search." We classified life in space into three types: Earth life, Earth-independent life (life that emerged from non-life independently of Earth life), and Earth-kin life (extraterrestrial life sharing a common ancestor life with Earth life). All the methods which are effective in detecting Earth life are potentially useful for the detection of Earth-kin life. In contrast, their applicability is not guaranteed for Earth-independent life. The three major conclusions of the previous article were re-evaluated and confirmed to remain unchanged: (i) there is no realistic single detection method which can conclude on the discovery of life independent of Earth, (ii) there is no single best method which is superior to the other methods in all respects, and (iii) there is no single method which can distinguish between Earth life and life independent of Earth. Subsequently, we discussed the combination of the detection methods and dividing extraterrestrial microorganism search into multiple steps: the survey-and-detection step and the analysis-and-conclusion step. We also emphasized the importance of imaging; even with not enough resolution for observing the detailed morphology of a microbial cell, it is highly worthwhile to distinguish whether the detected molecules are concentrated at micron size or diffused thinly. Of course, high-resolution imaging provides information with regard to the morphology of microorganisms. Accordingly, imaging is necessary for both the survey-and-detection and the analysis-and-conclusion steps. To realize a space exploration mission or instrument for extraterrestrial life search, not only in scientific but programmatic sense, it is important to design it so that scientific results can be obtained even if no life is found. The studies included in this article primarily supposed Mars as a target; however, they are valid for a wide variety of planets of the solar system, as well as their moons and small objects.