A Proposed Geobiology-Driven Nomenclature for Astrobiological In Situ Observations and Sample Analyses

Abstract

As the exploration of Mars and other worlds for signs of life has increased, the need for a common nomenclature and consensus has become significantly important for proper identification of nonterrestrial/non-Earth biology, biogenic structures, and chemical processes generated from biological processes. The fact that Earth is our single data point for all life, diversity, and evolution means that there is an inherent bias toward life as we know it through our own planet's history. The search for life "as we don't know it" then brings this bias forward to decision-making regarding mission instruments and payloads. Understandably, this leads to several top-level scientific, theoretical, and philosophical questions regarding the definition of life and what it means for future life detection missions. How can we decide on how and where to detect known and unknown signs of life with a single biased data point? What features could act as universal biosignatures that support Darwinian evolution in the geological context of nonterrestrial time lines? The purpose of this article is to generate an improved nomenclature for terrestrial features that have mineral/microbial interactions within structures and to confirm which features can only exist from life (biotic), features that are modified by biological processes (biogenic), features that life does not affect (abiotic), and properties that can exist or not regardless of the presence of biology (abiogenic). These four categories are critical in understanding and deciphering future returned samples from Mars, signs of potential extinct/ancient and extant life on Mars, and in situ analyses from ocean worlds to distinguish and separate what physical structures and chemical patterns are due to life and which are not. Moreover, we discuss hypothetical detection and preservation environments for extant and extinct life, respectively. These proposed environments will take into account independent active and ancient in situ detection prospects by using previous planetary exploration studies and discuss the geobiological implications within an astrobiological context.

Keywords: 954–967; Biogenicity; Evaporites. Astrobiology 21; Geobiology; Mars Sample Return; Nomenclature.

FIG. 1.

Comparisons of the origins of life on Earth and history of sedimentary features on Mars. Relationship between habitable periods on Earth and Mars. During the periods when the first life evolved on Earth, the martian surface was warmer, wetter, and habitable. The age of sedimentary rocks from the Opportunity rover landing site is ∼3.5 Gyr (late Noachian/early Hesperian). It is also in this time period where the surface of Mars likely had water stable on the surface (McLennan et al., ; Squyres et al., 2005) in liquid form versus recent discoveries of surface brine fluids on present-day slopes (Ohja et al., 2015).

FIG. 2.

Conceptual probabilities of validation of extinct and extant life. Astrobiological mission strategies need to move toward life validation alongside life detection. For extinct (ancient) life, the probability of detection of former biological components or other true biological features are at the mercy of the preservation medium and its robustness over geologic time. For extant (active) life, the availability of “positive” detections is only limited by the instrumentation and physical proximity to the microbial communities.

FIG. 3.

Biogenic preservation and biotic information over modern and geologic time. High-level investigation space using terrestrial life (“as we know it”) and the loss of biotic information within biogenic preserved settings with respect to time.

FIG. 4.

“Pendulum” diagram showing examples and definitions of the proposed astrobiology nomenclature for an NaCl hopper crystal. The example above is for a single pigmented halite hopper crystal (biogenic) and then brought from left-to-right showing how features lose their biological validity due to the definitions proposed in this aricle (Original credit: Frank A. Corsetti).

FIG. 5.

A proposed update to the classical perspective of habitability for planetary systems. The classic Venn diagram of energy sources, solvents, climate conditions, and C, H, O, N, P, and S has been used for discussions into martian habitability and taking global and local measurements from orbiters, rovers, and landers to determine how “habitable” a location was. That assessment did not take into account the need for the four environmental and aqueous features to overlap in time. It would only be the time frame of the combined overlap of all four products that life, as we know it, would have the highest probability for survival after a separate origin and last universal common ancestor should microbial evolution took place outside of Earth. Box 2.2 of the National Academy of Sciences Dynamic Habitability chapter in An Astrobiology Strategy for the Search for Life in the Universe discusses these features in further detail.