Tikalon Blog by Dev Gualtieri

Lunar Organics

December 21, 2015

While the Earth is thought by most to be the only known reservoir of organic chemicals, the universe has been found to contain many organics. In 1937, Pol Swings and Léon Rosenfeld used spectroscopy to detect the diatomic radical, methylidyne (C-H), in interstellar gas clouds.[1]

Eventually, more complex organic molecules, including polycyclic aromatic hydrocarbons, were detected in nebulae, and polycyclic aromatic hydrocarbons have been detected also on Saturn's moon, Titan.[2] Fullerene (C₆₀) was detected in a planetary nebula by NASA's Spitzer space telescope in 2010.[3] In just a few decades, we've gone from discovery of a few simple cosmic organic molecules, like methane, discovered in 1991, to compounds as complex as those found in petroleum and coal.

Cosmic cocktails, anyone?

Ethanol, also known as grain alcohol, is one of the organic molecules discovered in space.

(Via Wikimedia Commons.)

The abundance of organic compounds in space might have been seed material for the genesis of life on Earth. Formation of the complex constituents of life would have been easier if such basic ingredients were abundant. In fact, the sugar molecule, glycolaldehyde, was detected around the protostellar binary IRAS 16293-2422, an object quite near the Earth, by radio astronomers at the Atacama Large Millimeter/submillimeter Array.[4-5] Glycolaldehyde is a precursor to the formation of ribonucleic acid, RNA, and its presence in an emergent star system indicates that such chemicals would have condensed onto planets as they formed.

This conjecture was buttressed by the detection of glycine (NH₂CH₂COOH), one of the twenty amino acid building blocks of life, in the comet, Wild 2. Glycine was found in a return sample from this comet by NASA's Stardust spacecraft.[6-7] I wrote about the Stardust mission in a previous article (Stardust, August 20, 2014).

The nucleus of comet, Wild 2, as seen by the Stardust spacecraft.

(NASA Image.)

On January 2, 2004, the Stardust spacecraft collected debris surrounding Wild 2 in a collection grid filled with aerogel. The aerogel, which is 99% empty space, gently captured samples of the comet's dust trail, and the collection module was returned to Earth, where it landed by parachute on January 15, 2006. Jamie Elsila, principal investigator of the glycine study and an astrochemist at NASA's Goddard Space Flight Center (Greenbelt, Maryland), remarked at the time of this discovery,

"Glycine is an amino acid used by living organisms to make proteins, and this is the first time an amino acid has been found in a comet... Our discovery supports the theory that some of life's ingredients formed in space and were delivered to Earth long ago by meteorite and comet impacts."[7]

Lunar regolith (soil) specimens collected during the Apollo missions were found to contain up to 70 parts-per-billion (ppb) of amino acids, the most abundant being glycine.[8-9] A problem of interpretation arose because of several possible sources for these small concentrations. These amino acids could have been contamination from terrestrial sources through two routes; namely, material brought to the moon by the Apollo spacecraft, or contamination during handling on Earth.[9]

Another possibility was that the compounds were synthesized from precursors in lunar module rocket exhaust.[9] Likewise, a similar thing might have happened with material transported from the Sun to the Moon via the solar wind.[9] Finally, the amino acids could have come from an infall of asteroid and cometary material.[9] As published in a recent paper in Geochimica et Cosmochimica Acta, it appears that the lunar sample amino acids were from terrestrial contamination.

Apollo 12 astronaut, Alan L. Bean, collecting lunar regolith.

Bean was the pilot for the Lunar Module that landed in the Oceanus Procellarum (Ocean of Storms). Photographer Charles "Pete" Conrad, Jr., is mirrored in Bean's visor.

(NASA photo.)

As lead author of the Geochimica et Cosmochimica Acta paper, Jamie Elsila, explains,

"People knew amino acids were in the lunar samples, but they didn't know where they came from... The scientists in the 1970s... 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."[9]

The research team, composed of scientists from the Goddard Space Flight Center, the Catholic University of America (Washington, DC), and the NASA Johnson Space Center (Houston, Texas) analyzed return samples from the Apollo 16 and Apollo 17 missions using time-of-flight mass spectrometry, ultrahigh-performance liquid chromatography with fluorescence detection, and compound-specific carbon isotopic ratios.[8] All measurements were consistent with terrestrial sources, and it was concluded that terrestrial biological contamination was a primary source of the amino acids.[8]

They also looked for a correlation between distance from the Apollo 17 lunar module rocket exhaust and amino acid concentration, and none was found.[8] As for a solar wind source, the carbon-13 isotope concentration was higher than that of the solar wind. Furthermore, surface regolith that should have had more amino acids from solar wind exposure than deeper soil specimens were found to have less.[8-9]

The conclusion is that greater care should be taken in future sample return missions to prevent terrestrial contamination.[9] This research was supported by NASA's Lunar Advanced Science and Exploration Research (LASER) Program, the NASA Astrobiology Institute, and the Goddard Center for Astrobiology.

Lunar Organics

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