Using scientific instruments in novel ways is a common practice, but still results in significant new discoveries. But sometimes, it doesn’t happen so much as a “that’s funny” moment as an intentional new use of old equipment. A new paper from researchers that Imperial College London (ICL), PhD student Solomon Hirsch and his advisor Mark Sephton, shows how the gas chromatograph-mass spectrometers that have been a mainstay of Martian probes since the Viking era could be used to find extant life there.
The formation of their idea is based around the presence of Intact polar lipids (IPLs), a type of organic biomarker that makes up the structural components of many cells, especially in single-celled lifeforms like bacteria and archaea. Typically, detection of these IPLs is done using solvent extraction of liquid chromatography methods, which are impractical for in-situ analysis on another planet, but the new paper shows how it can be done with pyrolysis-gas chromatography-mass spectrometry.
Before they could do any testing, though, the researchers had to decide what IPL signature they would look for in the data. IPLs degrade very quickly, which marks their presence as a distinct “viable life signature” since they only exist for a few hours or days after death, and degrade into other biomarkers post-mortem. Archaea and bacteria also use different IPLs in their structures, and as such would have different signatures both for their living ones and for their dead ones.
Fraser discusses the story of the search for life on Mars.For living bacteria, the researchers chose 1,2-Dioleoyl-sn-glycero-3-phosphocoline, which in the lovely language of organic chemistry, is shortened to DOPC, and which degrades to oleic acid after the cell’s death. For archaea, the even longer organic chemistry name can be shortened to DPPE for the living IPL, with the post-mortem expected result called archaeol.
To test their theory, the researchers pyrolized (i.e. burned) samples of each of the four expected chemicals. The DOPC found in bacterial IPLs produced a distinct additional biomarker known as glyceryl monooleate, which wasn’t present when burning the degraded form of oleic acid. They also found they could use a reagent called hexamethyldisilazane to improve sensitivity of their finding, though its unclear how viable of an option that is for in-situ measurements.
Unfortunately, the distinction between living IPLs and their degradation product was much more difficult for archaea. While the test might prove useful in hinting at the presence of once extant life, it wouldn’t be as useful in determining whether it was actually still living or not.
Another fundamental assumption of the ICL research is that cells on Mars would use the same organic molecules as part of their strucutre. Fraser discusses how that might happen.There are some additional challenges to overcome before this research can truly be implemented on other planets. Inorganic molecules commonly found on other planets, such as iron oxides and perchlorates (which are especially common on Mars) can obscure the signatures of these organic compounds. Additionally, there are more types of organic chemicals that make up IPLs here on Earth than were studied in this first research paper - so not finding evidence of the specific type of IPLs found in these certain types of bacteria does not necessarily mean there weren’t other types present.
But ultimately what the study authors suggest is to use this tool as a first screener in situ for a preliminary peek at whether or not life might be present in a collected sample. Those samples could be collected anywhere from the sub-surface of Mars to the geysers on Enceladus, but they would still have the same signature. Given that the work can be done with a tried and true flight tested system which has been in use for decades, there’s really no harm in looking for these signatures as part of the standard suite of tests on future missions - and if researchers happen to find a tell-tale sign of organic molecules or not, that’s one other data point in the long story of our search for life in the universe.
Learn More:
Simon Levey, ICL/Phys.org - Signs of recent life on Mars could be detected using new simple test
Solomon Hirsch & Mark A. Sephton - Intact polar lipids as organic biomarkers of viable extraterrestrial life
UT - A Simple Instrument Could Find Martian DNA - If It Exists
