One Big Question: How might we find the building blocks of extraterrestrial life in the lab?
When scientists look for extraterrestrial life, they typically turn their focus upwards, to planets like Mars, moons like Enceladus and exoplanets far beyond our own solar system. But for Laura Rowe, an assistant professor of chemistry at Valparaiso University in Indiana, the search begins in the lab.
In April, a member of Rowe's lab presented work she and her team did in this area of research during the Experimental Biology 2017 meeting in Chicago. Because we couldn't be there to listen, we instead asked Rowe for insight into this fascinating field of inquiry. As part of our One Big Question series we asked: How might we find the building blocks of extraterrestrial life in the lab?
Here's what she had to say:
NASA broadly defines Life as: "A self-sustaining chemical system capable of Darwinian evolution."
In the field of astrobiology the more conservative stance on extraterrestrial life is that it will be carbon based and will have required liquid water to evolve. Living organisms on Earth use a handful of elements to construct larger macromolecules that are fundamental to life as we know it: DNA, RNA, and proteins. All three of these macromolecules are polymeric in nature. This means that they all link together smaller, repeating units (monomers) with different chemical bonds to make large and diverse macromolecules. For DNA and RNA these smaller monomer units are called nucleotides, whereas for proteins these smaller monomeric units are called amino acids.
Therefore, when looking for the building blocks of life in outer space, we would likely be looking for either DNA, RNA, proteins or the smaller building blocks of these large molecules: nucleotides and amino acids.
The "One Big Question" is how might we discover the building blocks of extraterrestrial life in the lab?
If we initially assume (which may or may not be a correct assumption) that extraterrestrial life will have some form of DNA, RNA, or proteins, and thus some form of nucleotides and/or amino acids present, then a first effort in the lab would be to determine if the nucleotides and amino acids that are used for life on our planet would be stable enough to exist in the environmental conditions present on planets and moons that are known to have, or have had, liquid water.
The surface of Mars, for example, is exposed to very high levels of radiation due to its thin atmosphere, while its soil is known to have a considerable amount of perchlorate present. The high energy of the radiation, and the strong oxidizing nature of perchlorate, has a tendency to break chemical bonds that are found in nucleotides and amino acids. Since the evolution of even simple microbial life is believed to have taken hundreds of millions of years, nucleotides and amino acids must be able to resist rapid degradation in the environment for life to have evolved.
There are a limited number of nucleotides (5) and amino acids (20 or 22) that living organisms on Earth use to build DNA, RNA, and proteins. However, there are many more nucleotides and amino acids in existence. Exposing nucleotides and amino acids to the extreme conditions they may be exposed to extraterrestrially and observing their stability, is a good first step to identifying possible extraterrestrial life building blocks in the lab. If an unusual amino acid, or nucleotide, that is not used in life on Earth is found to be much more stable than the "natural" or "proteinogenic" amino acid or nucleotide under these conditions, then it may be a molecule to be on the look-out for during future space exploration.
Expeditions to Mars have given us many specifics of the surface conditions that we can attempt to mimic in the lab to do this; whereas the two other extraterrestrial life hot spots in our Solar System, Europa and Enceladus, have mainly speculative conditions in their subsurface oceans. It is exciting, however, that the Europa Clipper mission has been announced recently, so that we should have much more information about Europa in the next decade.
In order to possibly discover the building blocks of extraterrestrial life in the lab on Earth, we must have some idea of what environmental conditions exist on extraterrestrial bodies with liquid water. If the nucleotides and amino acids that life on Earth use are sufficiently stable when exposed to these conditions, then it is reasonable to assume they may be present in extraterrestrial organisms. If, however, they degrade quickly, then we should look at alternative nucleotide and amino acid structures that may impart a unique stability to the building blocks, or open up the scope of our definition of potential building blocks for extraterrestrial life entirely.
On Earth, life has evolved in surprisingly extreme conditions; extremophile organisms thrive in Antarctic ice, sulfuric hot springs, and even radioactive waste sites. Thus, it is reasonable to assume that if liquid water is present on an extraterrestrial body that contains basic elements, then living organisms could have evolved even under very hostile environmental conditions. However, the exact building blocks and structure of that life could be very different then the building blocks and structure of life on Earth.