NASA’s Curiosity rover has discovered organic compounds — molecules containing carbon and hydrogen, along with other elements — on Mars, and this is good news in the search for life. Organic molecules are the stuff of life: they are the building blocks of the living cell and at the center of all its processes. Life on Earth synthesizes organic compounds from water and carbon dioxide, using the power of the sun or tapping energy from hydrothermal vents at the floor of the ocean to build these molecules.
But the discovery in Gale Crater on Mars isn’t evidence of life itself. To understand why, we have to remember that while the solar system is full of organic molecules, we haven’t found life anywhere but on Earth so far. Organic molecules can be made by natural processes completely divorced from life itself. Such processes are easily replicated in the laboratory, and organic molecules collected from meteorites — which include amino acids — show no evidence in their chemical makeup of having been derived from life.
So what is the significance of the discovery reported by NASA on Thursday? It is that organic molecules have survived in the soil of Mars. This is remarkable because they survived only inches below Mars’ surface, a surface where the radiation that penetrates the thin Martian atmosphere easily destroys organic molecules. Other searches for organic molecules on Mars have been unsuccessful, and these failures have in part been blamed on the harsh environment for preserving them.
Gale Crater, which the Curiosity rover has been exploring for more than five years, appears to have once held a large lake. Perhaps sediments accumulated in this lake that could have trapped, concentrated, and protected organic molecules. But the lake dried up several billion years ago. Could such sediments have protected organic molecules for such a vast expanse of time?