Planning ahead is something that sets astronomy and space exploration apart. Decadal surveys and years of technical effort for missions give the field a much longer time horizon than many others. In the near future, scientists know that there will be many ways to search for biosignatures anywhere from nearby ocean worlds (i.e. Titan) to distant potentially habitable exoplanets. However, it is not clear what these biosignatures would look like. After all, there is only Earth’s biosphere to study at the moment, and it would be unfortunate to miss any clues about another just because it didn’t look like those found on Earth. Now, a team led by researchers from the Santa Fe Institute (SFI) has developed a framework that could help scientists look for biosignatures that may be completely different from those on Earth.
This framework is based on stoichiometry. Stoichiometry is a common characteristic of high school chemistry classes and the study of chemical relationships. There are some obvious stoichiometric ratios on earth that are clearly formed by life as we know it. The generalization of these relationships to be applicable everywhere was the focus of the SFI paper. There were three main principles that together make up the new framework.
UT video about biosignatures.
The first principle is that stoichiometric values change with the cell size of individual cells. For example, as bacteria get larger, the concentration of RNA increases while the concentration of individual proteins decreases. When these cells die, their size would help determine what concentration of molecules is being released into the environment.
The environmental distribution is also influenced by the second principle – that the number of cells in a neighborhood follows a power law distribution with respect to their size. For example, according to the simplest power law distribution curve, there are most likely many more small cells than large ones. This size ratio, together with the stoichiometries associated with these different sizes, then led to the third principle.
Example of a power law distribution. Lower values on the x-axis (in this case particle sizes) lead to large quantities (y-axis).
Credit – Hay Cranes / PD / Wikipedia
Further application of this stoichiometric principle leads to a result that can be applied more generally to biospheres. In this case, the size of a particular particle is a determining factor in its relationship to the liquid that surrounds it.
Let’s keep using RNA and proteins as an example. RNA is an order of magnitude larger than a protein. According to the first principle, it is also found more frequently in larger cells. However, according to the second principle, larger cells are less widespread in the environment. Therefore, in a biologically active system, proteins that are smaller are more likely to have a higher concentration in a surrounding fluid than RNA, which is larger. Hence the third principle that its size determines the concentration of a particle in a surrounding liquid.
UT video about the possibility of life on Titan.
The immediate application of this framework is the study of ocean worlds like Titan or Enceladus, which are likely to contain liquid bodies that could contain concentrations of biological molecules. Unfortunately, there are currently no systems that can accurately measure the particle size and that could launch missions into these worlds. But that doesn’t mean there won’t be any in the future. The potential for using this framework now requires a little more technical expertise to develop such a system. And it’s already clear how good the astronomy and space research community is at this.
SFI – Origins of Life researchers develop a new ecological biosignature
Journal of Mathematical Biology – Generalized Stoichiometry and Biogeochemistry for Astrobiological Applications
Astrobiology – Exoplanet Biosignatures: A Framework for Their Evaluation
UT – What does it take to find life? Search the universe for biosignatures
Artistic idea of life on another planet
Photo credit: NASA