Astrobiologists have provided new insight into how radiation exposure can destroy the amino acid glycine, even when it’s trapped in ice.
Glycine is a basic building block of life, and studying glycine can help us determine how and if this tiny molecule was involved in the construction of life’s first cells on Earth, as well as its potential role in the origins of life on other worlds.
Compound of Life
Glycine is one of the 20 amino acids that are commonly used by living organisms on Earth to build proteins. In fact, it’s the smallest and simplest protein-building amino acid used by life. But this simple molecule could have something big to teach us about life’s potential in the Universe.
“Simple, yes. Boring, no,” says Perry Gerakines of the Cosmic Ice Lab at NASA’s Goddard Space Flight Center. “Glycine is the smallest amino acid, and therefore the simplest, but that also makes it the most straightforward to study.”
The relative ease of working with glycine means that it is a good starting-point for studying how molecules are affected by various environmental conditions.
“All of the sample measurements are infrared spectra and these give a sort of fingerprint of each molecule that identifies the different types of structural groups within it,” said Gerakines. “The more structural groups a molecule has, the more difficult it is to identify and quantify its infrared fingerprint.”
As the simplest amino acid, astrobiologists have long been interested in glycine’s potential role in life’s origin and evolution. But glycine is also important because scientists have actually spotted the molecule beyond our planet.
In 2003, Astrobiologists supported by the Exobiology and Evolutionary Biology (Exo/Evo) element of NASA’s Astrobiology program first reported the potential detection of glycine beyond Earth in the hot molecular cores of star-forming regions (1). In 2009, glycine was also found in samples that were collected from the comet Wild 2 and returned to Earth by NASA’s Stardust spacecraft. There are also numerous studies that show glycine and other amino acids are present in meteorites that have fallen to Earth. One can safely assume that meteorites have delivered glycine to the surface of other planets in the Solar System as well.
The fact that there are sources of glycine beyond our planet is important. Glycine’s presence as a natural product in space means that it could have been one of the first molecules freely available to prebiotic cells after the Earth formed.
When the Earth first solidified from the spiralling mess of rock and dust circling the young Sun, glycine could have been delivered to the planet by asteroids or interplanetary dust. This non-biological molecule would have been part of the list of ingredients from which the very first cells on Earth might have been built.
And Earth isn’t the only world that glycine could have rained down upon. The molecule’s presence in space means that it could be important to life’s origins on any planet or celestial body in our solar system and beyond.
Because of glycine’s likely role in life’s origin on Earth, Gerakines and his colleagues wanted to know how the molecule behaves in conditions present on other worlds. If meteorites delivered glycine to any (or every) planet in the early Solar System, it’s important to know if and how it could have survived on those worlds.
In addition, because glycine is found in every living organism on Earth, it’s interesting to know if traces of the molecule might be left behind on the surface of other planets that once supported life. And if glycine could be left behind after cells die, what does this mean for larger, more complex molecules that could act as biosignatures?
With support from the Goddard Center for Astrobiology, the team encased samples of the molecule in ice and blasted them with a beam of MeV protons. The ice offered some protection from the radiation-induced destruction of glycine that the researchers were going for.
Six mixtures of water ice and glycine were tested, and the rate at which the molecules were destroyed was studied at temperatures ranging from 15 to 280 K (roughly -260°C to 7°C).
The results are particularly useful in understanding how glycine might survive in ice on the surface of a planet like Mars. Radiation levels at the martian surface are much higher than on Earth, and if left unprotected at the surface of Mars, glycine might not last long. But how might glycine fare if it’s sheltered in subsurface or polar ice on the red planet?
“The answer to that question depends upon the level of radiation to which the ice is exposed,” said Gerakines. “For example, under the surface of Mars, the radiation dose drops with depth. So, at depths of a few centimeters glycine could survive for a few hundred million years in ice, but at a depth of about 2 meters it could survive for billions of years.”
Even though we’ve been sending robotic explorers to Mars since the 1960s, we still don’t know much about how molecules evolve over time in martian conditions. It’s not a stretch to think that glycine and other carbonaceous molecules might have been delivered to Mars by impacts, and that they are now resting near the planet’s frozen surface.
If ancient life ever existed on Mars, there is also the possibility that biomolecules were left behind, and that these bio-signatures are just sitting there waiting to be studied. But before we invest time and money in missions to search for these molecules, we first have to understand if they could survive and what they might look like after being exposed to radiation on Mars for millions (or billions) of years.
“If glycine’s radiation destruction rate is typical of other biological molecules, then these results imply that ancient bio-signatures would most likely be found at depths below 1 to 2 meters,” said Gerakines. “Extrapolating these results to other molecules is not simple, but building a larger database with different classes of molecules (amino acids, nucleobases, polycyclic aromatics, sugars, alcohols, etc.) would be the best way to determine the best bio-signature to survive in the martian subsurface radiation environment.”
The survival of ice-bound molecules is not only relevant to the search for bio-signatures on other planets. It’s also important to understand how molecules from Earth survive beyond our planet. This relates to planetary protection, and making sure we don’t contaminate planets like Mars in our exploration efforts.
In terms of contaminating Mars, the results of the study indicate that we most likely haven’t caused any issues thus far.
“To date, we have not left the top-most surface of Mars, and the radiation environment there (as recently determined by Curiosity) is so high that any biological organisms would not survive without protection,” said Gerakines.
They study also has applications in developing future missions to Mars. Many researchers are hoping for a sample-return mission to the red planet, which would bring back rocks from the martian surface for them to study. What is the likelihood of molecules originating on Mars being found in such samples?
“In that case,” said Gerakines, “I would not expect any surface rocks (within a few centimeters of the surface of Mars) to contain any organic signatures if they have been exposed to the martian radiation environment for more than a few hundred million years.”
So far, robotic missions have only scraped the surface of the red planet. As Gerakines and his colleagues have shown, this region is not only inhospitable for living organisms, it’s also a dangerous place for the molecules of life themselves. If we’re going to search for bio-signatures on Mars, we need to go deeper. This means developing technologies and methods for collecting samples from well below the planet’s icy topsoil.