Before there was life as we know it, there were molecules. And after many seemingly unlikely steps these molecules underwent a magnificent transition: they became complex systems with the capability to reproduce, pass along information and drive chemical reactions.
But the host of steps leading up to this transition has remained one of science's beloved mysteries — until now.
Image: Artist's conception of an exoplanet with an atmosphere, via NASA/JPL.
New research suggests that the building blocks of life — prebiotic molecules — may form in the atmospheres of planets, where the dust provides a safe platform to form on and various reactions with the surrounding plasma provide enough energy necessary to create life.
"If the formation of life is like a jigsaw puzzle — a very big and complicated jigsaw puzzle — I like to imagine prebiotic molecules as some of the individual puzzle pieces," said St. Andrews professor Dr. Craig Stark. "Putting the pieces together you form more complicated biological structures making a clearer, more recognizable picture. And when all the pieces are in place the resulting picture is life."
We currently think prebiotic molecules form on the tiny ice grains in interstellar space. While this may seem to contradict the readily accepted belief that life in space is impossible, the surface of the grain actually provides a nice hospitable environment for life to form as it protects molecules from harmful space radiation.
"Molecules are formed on the dust surface from the adsorption of atoms and molecules from the surrounding gas," Stark told Universe Today. "If the appropriate ingredients to make a particular molecular compound are available, and the conditions are right, you're in business."
By "conditions," Stark is hinting at the second ingredient necessary: energy. The simple molecules that populate the galaxy are relatively stable; without an incredible amount of energy they won't form new bonds. It has been thought that life could form in lightning strikes and volcanic eruptions for this very reason.
So Stark and his colleagues turned their eyes to the atmospheres of exoplanets, where dust is immersed in a plasma full of positive ions and negative electrons. Here the electrostatic interactions of dust particles with plasma may provide the high energy necessary to form prebiotic compounds.
In a plasma the dust grain will soak up the free electrons quickly, becoming negatively charged. This is because electrons are lighter, and therefore quicker, than positive ions. Once the dust grain is negatively charged it will attract a flux of positive ions, which will accelerate toward the dust particle and collide with more energy than they would in a neutral environment.
In order to test this, the authors studied an example atmosphere, which allowed them to examine the various processes that may turn the ionized gas into a plasma as well as determine if the plasma would lead to energetic enough reactions.
"As a proof of principle we looked at the sequence of chemical reactions that lead to the formation of the simplest amino acid glycine," Stark said. Amino acids are great examples of prebiotic molecules because they are required for the formation of proteins, peptides and enzymes.
Their models showed that "the plasma ions can indeed be accelerated to sufficient energies that exceed the activation energies for the formation of formaldehyde, ammonia, hydrogen cyanide and ultimately the amino acid glycine," Stark told Universe Today. "This may not have been possible if the plasma was absent."
The authors demonstrated that with modest plasma temperatures, there is enough energy to form the prebiotic molecule glycine. Higher temperatures may also enable more complex reactions and therefore more intricate prebiotic molecules.
Stark and his colleagues demonstrated a viable pathway to the formation of a prebiotic molecule, and therefore life, in seemingly common conditions. While the origin of life may remain one of science's beloved mysteries, we continue to gain a better understanding, one puzzle piece at a time.
The paper has been accepted for publication in the journal Astrobiology and is available for download here.