Wednesday, March 25, 2015

Astrobiology : The basics of life.

Life as we know it is composed of three basic types of chemical compounds: nucleic acids (DNA & RNA), proteins, and lipids. One aspect of the study of abiogenesis (the origin of life) has been to determine how the basic compounds could have been formed from simple chemical precursors that we already know can be formed from simple physical and chemical processes.

One of the first experiments in to examine how the basic biomolecules could have been formed was performed in 1952 and is known as the Miller-Urey experiment. In this experiment, a selection of simple reducing gases was repeatedly condensed and boiled while being exposed to electric sparks. The experiment was setup to mimic the conditions of the early atmosphere here on Earth. The reducing gases are those that would have been generated from geologic processes before living things started adding oxygen to the mix. The sparks represented lightening, which was thought of as a plausible energy source that could drive the reactions to generate interesting chemistry.

The experiment was able to produce hydrogen cyanide (HCN), formaldehyde (CH2O), and other simple molecules. These molecules then reacted to form over 40 different amino acids, in sufficient amounts to color the condensed liquid pink after a day. Several variations of the experiment have been done that produce different mixes of small biomolecules, including all the purines and pyrimidines used in the bases of RNA/DNA as well as the ribose sugars used to construct nucleotides.

Most of these experiments required the presence of ammonia (NH4) and methane (CH3) to produce interesting biomolecules. This is a problem, because these gases are now thought to be relatively rare in the early atmosphere because of their absence in the volcanic gas emissions that would have contributed heavily to the early atmosphere.

A new set of conditions has been identified (Patel, Nature 2015) that is able to generate the precursors to amino acids, nucleic acids (sugars and purines/pyrimidines), and lipids. The key components were hydrogen cyanide (HCN) and hydrogen sulfide (HS) from the atmosphere and ultraviolet light from the sun interacting in a water (H2O) bath. The process was sped up by the presence of dissolved copper ions. Interestingly, the proposed reactions generate all the basic compounds used in our type of life, but also don't generate many other simple compounds that our type of life doesn't use.

I consider this topic to be part of astrobiology because the same basic physical processes which are studied in research into abiogenesis here on Earth have also been playing out at sites throughout the universe. The same basic compounds of life have been generated on wet worlds wherever they are found. This not only suggests that life may be ubiquitous through the universe, but that when life is found on planets like ours that it may also be composed of similar basic types of molecules.

Lots of other simple chemicals have been identified in space using spectroscopy. These have also been generated through common physical and chemical processes and may represent the basic chemicals found in other types of life that started in environments distinct from our own.

  1. Miller-Urey experiment:
  2. Recent analysis of Miller-Urey:
  3. Patel, Nature 2015:
  4. Molecules in space: