Comets: Interstellar Johnny Appleseeds?

A QUANTUM OF SCIENCE

Are comets the disseminators of the seeds of life?

It was January 2, 2004. The NASA space probe known as Stardust passed through the tail of the Wild-2 comet, five years after the craft was launched on its mission to collect interstellar dust and particle samples from the tail, or coma, of the comet – particles that might well come from beyond Earth’s solar system. A specially-designed collector called an extra low density aerogel was used to capture the particles, and the probe’s camera took high-resolution images of the comet’s nucleus.

On January 15, 2006, the Stardust probe returned to Earth. A sonic boom and a fireball heralded its return over Utah’s Great Salt Lake desert. It was travelling at almost 29,000 miles per hour – the fastest re-entry speed into Earth's atmosphere ever achieved by a man-made object.

Since then, scientists in the Stardust Mission of NASA’s Jet Propulsion Laboratory have been hard at work analyzing the microscopic dust and particles it collected on its seven-year, three billion mile journey. And the results of that analysis are breathtaking.

Some things were expected. Silicate crystals had been predicted based on spectroscopic observations, but their presence confirmed not only those predictions but also the belief that the comet contained matter from outside the solar system.

Even more exciting, organic materials were detected. Astrobiologists have long been aware of aliphatic hydrocarbons (long chains of carbon and hydrogen) diffused throughout space, but the hydrocarbons found in the coma of Wild-2 were much longer than standard interstellar chains, indicating greater complexity. Methylamine and ethylamine, while simple molecules, were an exciting find as well because the nitrogen they contain is essential for life.

But on August 16, 2009, Dr. Jamie Elsila of NASA's Goddard Space Flight Center in Greenbelt, Maryland announced something astonishing. Scientists analyzing the Stardust samples had detected the presence of glycine, the simplest amino acid – and one of the critical building blocks of almost all life on Earth.

Rigorous testing was required in order to confirm this result. Every effort was made to ensure that "earth grime" did not contaminate the samples. The Johnson Space Center in Webster, Texas maintained the comet particles (it is also the home of most of the moon rocks recovered by the Apollo missions) and over 150 scientists from some of the most prestigious laboratories in the world helped with the analysis.

Among the tests they performed as confirmation of the results was an isotopic analysis. Isotopes of an element contain different numbers of neutrons than the most common version of that element, and the prevalence of different isotopes of a given element are well characterized. Using that information, scientists were able to confirm that the isotopic prevalence found in the glycine detected in the comet particles was not terrestrial contamination.

Where did the glycine come from, then? Theories abound, but the one with the greatest antiquity is the theory of panspermia (also known as exogenesis). First proposed in ancient Greece, many respected scientists since the Renaissance have expressed support for the idea that life came to Earth from outer space. This is one of the core areas of study in the field known as astrobiology, a multidisciplinary science that has existed formally since NASA established the first astrobiology program in 1960. Combining physics, astronomy, chemistry, biology and even more specialized sciences, astrobiology concerns itself with the study of the origin, evolution, distribution, and future of life in the universe.

The presence of even the simplest amino acid in the tail of a comet is a profound piece of evidence supporting the idea of panspermia. If this theory is correct, comets might well be the Johnny Appleseeds of the universe, slowly sowing their seeds of complex pre-biotic molecules throughout the galaxy – and suggesting that life on other worlds may be far more common than scientists once thought.

For more information:

NASA article on the discovery
http://www.nasa.gov/mission_pages/stardust/news/stardust_amino_acid.html

SCIENCE Magazine article on Wild-2 analysis (2006)
http://xray.physics.sunysb.edu/research/pdf_papers/2006/sandford_science_2006.pdf

More on the Wild-2 comet
http://en.wikipedia.org/wiki/Comet_Wild_2

More on Panspermia
http://en.wikipedia.org/wiki/Panspermia

More on Astrobiology:
http://en.wikipedia.org/wiki/Astrobiology

© AQOS / P. Smalley (2009)
Reproduction with attribution is appreciation