Critical review of chirality indicators of extraterrestrial life

Abstract

The major revolution in modern astronomy recognizing the universe as teeming with exoplanets, the discovery of liquid water in solar moons, and the continuing focus on Mars exploration, all accelerate the re-evaluation of potential biomarkers for extraterrestrial life. Based on life on planet Earth which relies heavily on chiral molecules and especially on homochiral families, the detection of molecules with these structural properties appears in all road-maps as prime indicators of extraterrestrial life. This review analyzes the strengths, bounds and potential weaknesses of relying on chirality and on homochirality as biomarkers, along with recommendations of how to practically use it. Some of the main issues presented, discussed and answered include: what is the extent to which chirality can be expected to be a universal feature of life; is detection of chirality enough or do we need also to detect homochirality; how justified is it to view life on Earth as purely homochiral; what are the weaknesses of the need to invent an arbitrary label of handedness (needed to define homochirality) and what are the pitfalls that emerge from these weaknesses; what stands behind a detected specific value of enantiomeric excess and what affects its values as we consider old, extinct life, just emerging embryonic life, or extant but rare life; how can one quantify the degree of homochirality; and, what are relevant experimental approached for detecting chirality on-ground and from distance? Finally, a summary with a concise list of recommendations is provided, along with a brief outlook.