A NASA-funded research team has confirmed the origin of organic matter found in lunar samples collected during the Apollo missions. It has long been known that the soil samples collected in the late 60s and early 70s contained amino acids, but the technology to determine where they came from has not been available until now.
Amino acids are the building blocks of proteins, which are central to creating organic structures like skin and hair. When the organic matter was found in soil samples brought back from the moon during the Apollo missions, there was a big question mark over how exactly it had ended up there, as the environment on the Lunar surface is completely inhospitable to any known forms of life.
It's generally accepted that there are four possibilities. Firstly, it's possible that the solar wind – a weak charged particles from the sun – contained the constituent elements of amino acids, and landed them on the lunar surface, with them later forming together to form the organic matter.
The second possibility is similar to the first, but with the precursor molecules arriving on the Lunar surface from rocket exhaust, while the third postulates that they were delivered to the Moon via asteroids, which are thought to habor chemical reactions that create amino acids.
The final possibility has perhaps always been the most likely – that the organic compounds are simply the product of terrestrial contamination, from equipment brought to the moon during the Apollo missions, or introduced during handling when they arrived back on Earth.
When they were first analyzed, scientists were able to determine that the samples contained organic material, but technological limitations meant that they couldn't ascertain their origin with any degree of certainty. With all the benefits of modern technology, the new team of researchers attacked the mystery once more.
Taking seven samples for analysis, the team first found that there were very low concentrations of amino acids present – between 105 to 1,910 parts per billion. Utilizing the high sensitivity of equipment in NASA's Goddard Astrobiology Analytical Laboratory, the researchers were then able to determine the isotopic composition of the samples, which provided answers to the decades old mystery.
Analysis revealed that the samples contained more Carbon-12 isotopes than larger, less reactive Carbon-13 – which correlates with what you would expect to find in terrestrial life. This essentially confirms that the organic matter originates from terrestrial contamination, rather than from asteroids, solar wind or rocket exhaust.
Furthermore, the volume of amino acids found in the samples relative to their position on the lunar surface simply doesn't tally with any of the other theories. For example, if solar winds were responsible for the matter, then the sample taken from closest to the surface would contain a higher number of amino acids. This wasn't found to be the case.
Similarly, if rocket exhaust was responsible for the amino acids, then they would have been found in a higher concentration directly beneath the lunar module. Again, this wasn't found to be the case.
The more the researchers studied the samples, the more the evidence mounted up. There are two versions of amino acid molecules – left and right – which are mirror images of one another. Terrestrial life favors left-handed amino acids, so you'd expect to find more of that type if the samples were contaminated by equipment or handling on Earth. When the researchers studied the soil in the lab, they found just that, further suggesting that terrestrial contamination is to blame for the presence of the organic matter.
However, while most of the evidence points to the amino acids originating back on Earth, it's likely that a small percentage of the matter came from asteroids, with the researchers finding some amino acids that are very rare in terrestrial biology, but commonly found in meteorites.
"People knew amino acids were in the lunar samples, but they didn't know where they came from," said Jamie Elsila of NASA's Goddard Space Flight Center in Greenbelt, Maryland. "The scientists in the 1970s knew the right questions to ask and they tried pretty hard to answer them, but they were limited by the analytical capabilities of the time. We have the technology now, and we've determined that most of the amino acids came from terrestrial contamination, with perhaps a small contribution from meteorite impacts."
The research is also a reminder of the importance of contamination control for future missions.
"This work highlights the fact that even with thoughtful and careful contamination control efforts, trace organics in extraterrestrial samples can be overwhelmed by terrestrial sources," said Elsila. "Future missions emphasizing organic analysis must consider not only contamination control but also include 'witness samples' that record the environment and potential contamination as the mission is built and launched to understand the unavoidable contamination."