Amino Acids: In Hot Water
Well over 100 different amino acids have been detected in meteorites, and it’s probable that they originally formed on the asteroids that spawned these space rocks. Amino acids are the building blocks of living organisms, and quite a bit of research has gone into discovering how they may have formed.
Chief among the possibilities is a “hydrothermal process” – chemical reactions stimulated by heat in the presence of water, a process that occurs on Earth in areas like deep-sea hydrothermal vents. Because asteroids may be heated by radioactive decay, hydrothermal formation may also have occurred on these rocky, sub-planetary bodies.
Less effort has gone into discovering how the amino acids survived on the parent bodies after formation, and then on the meteorites that brought them to Earth.
An ongoing project funded by the NASA Exobiology program is attempting to remedy that shortcoming by looking at how three simple amino acids interact with minerals that are present on many asteroids and meteorites.
“People have done decomposition experiments with amino acids for 50 years or more, at high temperatures, but they never put minerals in there,” says Tom McCollom, a research associate in the Center for Astrobiology and the Laboratory for Atmospheric and Space Physics at the University of Colorado. “In a hydrothermal environment on an asteroid or in a meteorite, there are certainly going to be some minerals around. Do they have anything to do with catalyzing and accelerating the decomposition, or are there some cases where they retard it? The potential role of minerals is a big gap in the research.”
More broadly, the study helps to explore the “prebiotic chemistry” that life built upon as it got started. “We also want to understand the quantitative aspects of the chemistry that would have gone on before any kind of life, that set the stage for biological organisms to come along,” McCollom says.
Prebiotic chemistry, he points out, “formed the stuff that the first organisms were made of, and perhaps provided them with nutrients as well.” Getting a better understanding of the decay of amino acids is as relevant to the origin of life on Earth as it is to the search for life beyond Earth, McCollom says.
Heat may stimulate reactions that form amino acids, but it also can accelerate their degradation, and thus while some scientists believe life on Earth may have gotten started at deep-sea hydrothermal vents, a question arises: Why didn’t the heat decompose the amino acids once they formed?
The same “heat is a double-edged sword” situation applies to asteroids. The presence of clay minerals on meteorites indicates that asteroids had hydrothermal circulation, probably driven by magma warmed by radioactive decay.
“A lot of meteorites show evidence of hydrothermal alteration, and yet we see amino acids on them,” McCollom notes. “Quite an assortment of different kinds has been found, including all 23 amino acids that organisms use, and a whole host – perhaps 100 or more — of amino acids that biology does not use. Amino acids are pretty abundant in meteorites. So why did they not decompose? Or were the amino acids being made as fast or faster than they were decomposing?”
In previous experiments, McCollom looked at norvaline, a non-biological amino acid that is found in meteorites and is also produced synthetically and marketed, for some reason, as a nutritional supplement to body-builders. He is also looking at two simple, biological amino acids, alanine and aspartic acid.
For minerals, McCollum chose pyrite, pyrrhotite, magnetite (which are all found on meteorites) and hematite, a form of iron oxide that is common in the crust and some hydrothermal systems on Earth. All of these minerals affect the levels of dissolved sulfur compounds and hydrogen in the solution, McCollom says, and thus could have a major impact on reaction rates.
McCollom placed the minerals and three amino acids in an apparatus that simulates conditions at deep-sea vents, with a pressure of about 350 atmospheres and a temperature of 155 to 175 degrees Celsius (about 300 to 350 degrees Fahrenheit). In the first experiments, norvaline decomposed much faster when minerals were present, McCollom says, showing that “minerals can accelerate the reaction rates by several orders of magnitude.”
Decomposition was measured for hundreds or thousands of hours by quantifying breakdown products from the amino acids.The rapid decay rates do not support the long-term survival of amino acids in meteorites, McCollom concedes.
By accelerating the breakdown, “minerals make it harder to explain why amino acids are in meteorites, but it gives you some quantitative constraint on reaction rates. Perhaps the meteorites were exposed to a lower temperature, or there was another reaction making these compounds at the same rate as they were being decomposed, or faster. You could have a tension between the two reactions, and maybe you end up getting a certain amount as a steady-state concentration."
There are other possibilities. Perhaps the destructive process stalled on the meteorites, or the amino acids reached the meteorites after they broke off from the asteroids, and were not subject to hydrothermal conditions.
The results may help illuminate the difficult issue of how life began, McCollom adds. “We want to have a good concrete assessment of the suite of pre-biotic molecules that were available when life originated and began evolving on the early Earth. For different sorts of environments, we want to place a control on what was available for fledgling organisms to incorporate into their biological and biochemical pathways.”