When the Apollo missions brought back lunar samples, scientists were surprised to find traces of organic matter in the form of amino acids. The origin of these molecules was shrouded in mystery, and no explanation had enough evidence to back it up. Over 40 years later, a team of NASA-funded scientists has identified that most of those amino acids were due to contamination from Earth.
Amino acids are the building blocks of proteins. They are made of hydrogen, carbon, oxygen and nitrogen – elements that are relatively common in the universe. They are mostly formed by living creatures, but can also form through non-biological processes.
In the lunar samples, scientists found amino acids at very low concentrations (105 to 1,910 parts-per-billion). Nothing can survive on the Moon, so a team of researchers came up with four different possible explanations for how they got to the Moon.
The first possibility is that solar wind might have formed them on the surface of the Moon. The solar wind is a stream of plasma blown from the surface of the Sun that contains all the components of amino acids.
The second cause could have been asteroids. Amino acids can form inside asteroids and the lunar surface has been bombarded enough to leave a significant amount of these molecules.
The third scenario considered contamination from Earth as the cause. Amino acids could have been present in the instruments used on the Moon, or they could have made their way into samples back on Earth during analysis.
The last explanation blames the presence on the Apollo rocket exhaust, which contains precursor molecules that could potentially have combined to form amino acids.
Lab technology has made enormous leaps forward since the samples were collected, allowing scientists to look at the origin of the atoms in these amino acids. Elements exist in slightly different versions called isotopes. Carbon, for example, can be carbon-12, which is the most common version, or the rarer carbon-14, which is used for carbon dating. Isotopes are formed in very specific ways, so by identifying the isotopes they knew where the amino acids were formed.
The amino acids found were mostly made of carbon-12, which is the preferred form for life on Earth, rather than carbon-13 which is found in the solar wind and in asteroids. Another characteristic typical of Earth's amino acids is that they have a particular orientation: organisms make amino acids that are "left-handed," while non-biological processes on asteroids can make both left- and right-handed molecules. Most amino acids found were left-handed. Fuel contamination was excluded, too, because the sample near the landing had a similar contamination to the one far away.
"People knew amino acids were in the lunar samples, but they didn't know where they came from," Jamie Elsila, lead author of the study, said in a statement. "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 discovery is important for future missions that aim to recover organic molecules from celestial bodies. The possible contamination should be taken into account before any analysis is conducted.
The research was published by Geochimica et Cosmochimica Acta.