Chains of magnetic crystals like magnetite (Fe3O4) or greigite (Fe3S4) are studied as potential biosignatures in the search for extraterrestrial life. On Earth, certain bacteria produce magnetite or greigite with uniform size, shape, and purity, forming chains that help them navigate magnetic fields. Similar crystals found in Martian meteorite ALH84001 sparked debate about their possible biological origin. However, non-living processes can also produce comparable magentic crystal forms, complicating interpretation. Scientists now assess multiple traits and contextual evidence to distinguish between biological and abiotic origins. Understanding both pathways is crucial to avoid false positives and guide life detection missions to Mars and other planetary environments.
Environmental prevalence: Magnetotactic bacteria (MTB) are commonly found in a diversity of low-oxygen, aquatic environments on Earth, including marine and freshwater systems. Similar aquatic environments were likely common on Mars and are found today in some icy moons. If life exists, or ever existed, in those environments, it may have produced similar magnetic crystal structures. [Congruence]
EnvironmentsConvergent evolution: The prevalence of MTB across multiple bacterial lineages (e.g., α-Proteobacteria, δ-Proteobacteria, Nitrospira) suggests convergent evolution and widespread biological utilization of magnetite biomineralization. This suggests that the development of intracelular magnetic crystals for magnetotaxis could be a common evolutionary adaptation in habitable aquatic environments. [General]
EnvironmentsUniversality of magnetosome chains in MTB: In the majority of MTB, magnetosomes are aligned in one or more chains parallel to the long axis of the cell, which is the most magnetically efficient arrangement. If MTB ever evolved on other planets, chains of magnetosomes would be the most likely structural arrangement. [General]
EnvironmentsLate evolutionary innovation: Representatives of the magnetotactic prokaryotes are phylogenetically associated with five major lineages within the domain Bacteria. No magnetotactic Archaes has yet been discovered. This implies that magnetotaxis on Earth is likely a late evolutionary innovation (after the bacteria-archaea split) and therefore chains of magnetic particles may not be a prevalent feature in life elsewhere. [General]
EnvironmentsMultiple biogenic characteristics: Chains of magnetite produced by MTB are characterized by a set of strict morphological criteria (e.g., narrow size distribution in the single-domain range, elongation along [111] axis, low defect density, specific truncated hexa-octahedral geometry). These features are optimized for magnetic dipole alignment and are not reproduced by abiotic systems. [Production, General]
EnvironmentsStructural instability: Nano-crystal alignment into chains is difficult to achieve abiotically given its energetic unfavourability. Chains of magnetic crystals are unlikely to form in abioitc environments [General]
EnvironmentsThe magnetite crystals in ALH84001 are not aligned in chains, and therefore are likely formed abiotically. [Emmy Hughes] [General]
EnvironmentsThe alignment of nanocrystalline magnetite in chains would indicate production by magnetosomes; however, most if not all of the crystals in ALH84001, after a series of analyses, are demonstrated to not be aligned in chains at all.
Magnetite chains are likely misinterpreted abiotic semi-periodic structures. [Emmy Hughes]
[General] EnvironmentsAbiotic serrated grain edges, meaning irregular edges of grains which can suggest periodic or semi-periodic structures, can be mistaken for magnetite chains.