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?
To find out, Curiosity is armed with the most sophisticated laboratory for detecting organic molecules yet flown in space. The Sample Analysis at Mars
(SAM) instrument suite can sniff gases directly from the Martian air or vaporize, in a controlled fashion, samples drilled from the soil. Mission scientists follow careful protocols to ensure that the material sampled is from Mars rather than carried from Earth. The result is exquisite sensitivity for detecting organic molecules, far better than what the first such experiment on the Viking landers of the 1970's could do.
As a result, SAM was able not only to discover organic molecules in the Martian soil, but also to determine they are rich in the element sulfur. And this provides a clue to their preservation. If the ancient lake was rich in sulfur, as suggested by detection of sulfate compounds in several places on Mars, the sulfur would have become attached to the original organics. And sulfur-containing organics tend to be better preserved in the soil than other organic molecules. Hence, over billions of years, these are the molecules that would have survived. But that survival comes at a cost, which is that much of the evidence for what formed the organics is lost to the depths of geologic time. That is, we do not know whether these organics were formed by biology or geology.
Nonetheless, with organics confirmed to be present in the Martian soil, the search is on to understand their origin, as well as that of gaseous methane -- the simplest organic molecule -- that Curiosity has been sniffing repeatedly in the atmosphere, coming from an as yet unidentified source. Curiosity will continue to sample the Martian soil for as long as it is able. Future landed and orbital missions will carry even more sensitive instruments to try to determine whether Martian organics come from biology, or not. Curiosity's discoveries may not have told us whether life once formed on Mars, but they have in a single stroke made the Red Planet a much more interesting target in the search for life.