Philae, amino acids, and the origins of life

Space: the final frontier
Last week, the Philae probe successfully landed on comet 67P, an icy lump of rock which is over 3 miles long and nearly 2 miles wide – and over 300 million miles away.

Comet_67P_on_22_August_2014_NavCam

Comet 67P, or Churyumov–Gerasimenko. ESA/Rosetta/NAVCAM

One of the reasons that comets are of such interest to scientists that the European Space Agency would spend 1.4 billion euros sending the Rosetta spacecraft 4 billion miles on a mission that would take ten years to come to fruition, is that comets could hold clues to the origin of life on Earth.

Three basic theories of the origin of organic molecules on Earth have been proposed:

  • The “primeval soup” theory; that they were formed of inorganic compounds in the atmosphere.
  • The “iron–sulfur world theory” suggested a hydrothermal origin of life, where organic molecules were synthesized at hydrothermal vents (“black smokers”) on the ocean floor.
  • The “panspermia” theory is that they rained down on earth from outer space, from celestial bodies like comets (which are also thought to be the source of the large amounts of water found on this planet).

Comets are debris left over from the formation of the Solar System, and are thought to have deposited both water and amino acids, the building-blocks of life, on the Earth, contributing to our planet’s development and perhaps ‘seeding’ the first life on earth – and maybe on other planets, too.

Scratch and sniff
Despite being only the size of a washing machine, the Philae lander was equipped with state-of-the-art technology for investigating the comet 67P. The high-quality photographs coming back from the probe have been visually stunning, but the images are only part of the work. The scientific goals of the mission are also to examine the “elemental, isotopic, molecular and mineralogical composition” of the comet. To that end, they have (among other instruments) gas chromatography and mass spectrometry equipment to identify samples taken from below and above the comet’s surface.

Analysis of the chemical composition of comets and meteorites has, up to now, largely been limited to those that have crashed to Earth, so it will be exciting for scientists to discover if the variety of amino acids found on earth-based objects from outer space are mirrored on an object still in space.

Life on Mars?
Amino acids were detected in minute quantities in samples taken from the Apollo moon landings (after analysis of the samples on Earth), though the findings were tentative and contamination or spontaneous generation (in situ synthesis) during testing couldn’t be ruled out.

Although one might expect organic molecules to be destroyed by the extreme heat of entry into Earth’s atmosphere, there’s evidence that the interior of meteorites remains comparatively cool. The inside of the ALH84001 meteorite (from Mars), for example, stayed around 40° C during its descent through the earth’s atmosphere – cool enough for live bacteria to survive, had any been present.

Some important precursors to organic compounds vital to life (as we know it) have been detected in space by astronomers, including a simple ‘sugar’ molecule, glycolaldehyde, and cyanomethanimine and ethanamine, precursors of adenine and alanine (respectively). In 2009 NASA scientists discovered the amino acid glycine in gas and dust material ejected from comet Wild 2 using the Stardust probe.

A number of amino acids are believed to have been detected in the Murchison, Orgueil, and Ivuna meteorites, including the six ‘essential’ amino acids, and several others including the alpha- and beta- isomers of aminoisobutyric acid.

Beta-aminoisobutyric acid, also known as BAIBA, is a fascinating amino acid that has been written about on this blog already (See: What is BAIBA?).

Shock and awe
One recent study demonstrated that it was possible to create amino acids in the laboratory using ‘shock synthesis’. Mimicking the impact of an icy comet onto a rocky planet, they detected D- and L-alanine, as well as α-aminoisobutyric acid and isovaline in the resulting material. So one theory is that amino acids aren’t present in large quantities on a comet until a violent impact – like crashing to Earth – generates them.

Comet_67P_on_19_September_2014_NavCam_mosaic

Comet the hour, comet the man. ESA/Rosetta/NAVCAM

Despite its relatively short life-span, its batteries having run dry due to an unfortunate sheltered final landing spot, Philae managed to collect science data from its instruments (including ROLIS, COSAC, Ptolemy, SD2 and CONSERT), which will be analysed over the coming months and years, along with ongoing observations of the comet from the Rosetta spacecraft.

Will 67P divulge clues to the the origin of life on Earth? Will Philae and the ESA find the amino acids they’re looking for? Only time will tell.

Selected references and further reading:
Plaxco KW, Gross M. Astrobiology: A Brief Introduction. Baltimore: Johns Hopkins University Press; 2011. 352 p.
Ehrenfreund P, Glavin DP, Botta O, Cooper G, Bada JL. Extraterrestrial amino acids in Orgueil and Ivuna: Tracing the parent body of CI type carbonaceous chondrites. PNAS. 2001 Feb 27;98(5):2138–41.
NASA Researchers Make First Discovery of Life’s Building Block in Comet.
Martins Z, Price MC, Goldman N, Sephton MA, Burchell MJ. Shock synthesis of amino acids from impacting cometary and icy planet surface analogues. Nature Geosci. 2013 Dec;6(12):1045–9.

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