Martian life signs: Meteorite amino acids
- Published: Apr 16, 2013
- Author: Steve Down
- Channels: HPLC
Terrestrial vs. planetary origins
Image: Katsuhiro Mouri & Shuji Kobayashi (Nagoya City Science Museum/Planetarium)
Before scientists planted labs on Mars, they were examining Martian meteorites for clues to the possibility of past life on the planet. These missiles were expelled from Mars by the impact of other space matter and some found their way to Earth where they have been collected and scrutinised closely. The discovery of the building materials for life in the meteorites, such as amino acids and nucleobases, would be a strong indication that life once existed on the red planet.
As it happens, several meteorites from Mars have been captured and analysed over the past 20 years or so and amino acids have been found. However, they are thought to be contaminants from the surrounding terrestrial environment, for a number of reasons. The ratios of the D- and L-isomers of the meteoritic amino acids are close to those of amino acids found at the landing site. In addition, the amino acid distributions from two particular meteors are very different to each other, but similar to the local terrestrial distributions.
In 2012, this train of thought was rudely interrupted by new studies which pointed to an extraterrestrial origin of organic compounds in Martian meteorites. One of the studies found extraterrestrial amino acids in meteorites from Antarctica that had been thermally altered either on Mars or during ejection from the planet.
In an effort to try and clarify things, scientists from the US have now carried out detailed studies on another meteorite recovered from Antarctica. Roberts Massif 04642, as it is known, is about 225 million years old but landed on Earth about 700,000 years ago. It was recovered as a single piece.
Michael Callahan from NASA Goddard Space Flight Center and The Goddard Center for Astrobiology, with colleagues from the NASA Postdoctoral Program Administered by Oak Ridge Associated Universities, Bullis School, Potomac, and Pennsylvania State University analysed the meteorite for amino acids as well as nucleobases, which are the building blocks of RNA and DNA. This is the first time that nucleobases have been targeted in a Martian meteorite and the first time that amino acids have been analysed in this particular meteorite.
Nucleobases and amino acids
The meteorites were powdered and treated to extract the organic components. Any amino acids present were reacted with o-phthaldialdehyde/N-acetylcysteine to produce fluorescent derivatives for analysis by HPLC with fluorescence and mass spectrometer detectors.
Two sequential HPLC columns, packed with C18 and phenylhexyl material, respectively, were used to separate the derivatives. The mass spectrometer was operated in electrospray ionisation mode and extracted ion chromatograms were produced from the data, corresponding to the ions expected for the amino acids. The detection limits were 10-15 to 10-16 mol for both detectors.
The search for nucleobases and their analogues also used a phenylhexyl column and LC/MS with positive-ion electrospray ionisation and tandem mass spectrometry. The detection limits for nucleobases like cytosine, thymine, uracil, adenine and guanine and their analogues like purine, xanthine and isoguanine were 1 ppb.
Amino acid clues
The nucleobases were absent from the meteorite, which was not unexpected. These compounds are known to decompose at the high temperatures which the rocks would have experienced.
On the other hand, several amino acids were found. They included those that are not incorporated into proteins and those which are. The absence of common proteinogenic amino acids such as serine, valine, glutamic acid and aspartic acid combined with the detection of low levels of glycine and L-alanine suggest that meteorite contamination from the terrestrial environment is unlikely.
Several n-ω-amino acids such as δ-aminovaleric acid were also detected and their distribution in this meteorite was similar to that reported for other carbonaceous meteorites. These compounds are formed by thermal reactions and might originate via reactions of the Fischer-Tropsch type on Mars during igneous processing, although this is just conjecture at this stage.
Their Martian origin is supported by the fact that, although some of the n-ω-amino acids have been found previously in Antarctic ice, the levels were far lower than those found in this meteorite.
The team also measured the carbon isotope ratio abundances of some of the amino acids in the meteorite. The values are consistent with those for organic compounds previously measured in Martian meteorites, adding further support to their extraterrestrial origins.
The confirmation of amino acids in the meteorites can be used to direct experiments on the surface of Mars by the Sample Analysis at Mars module on the Curiosity rover. The results of those experiments will help to decide whether a new Martian mission to recover rocks for return to Earth is necessary in the search for past or present Martian life.
Meteoritics and Planetary Science 2013 (Article in Press): "A search for amino acids and nucleobases in the Martian meteorite Roberts Massif 04262 using liquid chromatography-mass spectrometry"
Article by Steve Down
The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.