After more than four decades of searching for organic molecules on the surface of Mars, scientists have conclusively found them in mudstones on the lower slopes of Mount Sharp. A variety of organic compounds were discovered by NASA's Curiosity rover, which heated the Martian rocks to 500° Celsius to release the chemicals.

The finding is significant—for life to have ever existed on Mars there would almost certainly need to be organic molecules to get it started; they're the basic building blocks of life as we know it. And if life did get started, it would have left organic molecules behind. However the confirmation of organics on Mars raises more questions than it answers. Based upon the information scientists have gleaned so far, they cannot determine whether these organics were produced by life, delivered to the surface of Mars by meteorites, or are the byproduct of geological processes on Mars.

The Viking landers reached the surface of Mars during the summer of 1976 amid some expectation that they might find evidence of past life, if not life itself. However, when Viking landers sampled the Martian soil they found no past life, nor did their gas chromatograph mass spectrometers find any organic molecules. Nada.

Scientists still aren't entirely sure why they found no organics on Mars at the time, but it's probably because the Viking spacecraft touched down in a place where the surface materials they scooped up were essentially little bits and pieces of volcanic rock that had been blowing around on the surface for who knows how long. Although scientists expected to at least find bits of organics delivered to the surface by meteors, the harsh radiation on the Martian surface would have destroyed anything but the most recent arrivals.

Not what we expected

When it came time to plan where Curiosity would land in 2012, scientists were guided by the objective of searching for organic molecules and figured the best place to look for them would be around an old lake bed. Eventually, NASA chose a site called Gale Crater, because there were many signs that water was present over its history. "Gale Crater ended up having a fabulous record of lake deposits," said Jennifer Eigenbrode, a biogeochemist and geologist at NASA's Goddard Space Flight Center.

After landing, Curiosity drove through some of the lowest depressions in Gale crater and sampled rocks. In 2014, scientists reported the first detection of chlorinated hydrocarbons. While this was interesting, it was a pretty bland set of organic molecules, and the researchers were unable to rule out contamination left over from the rover's time on Earth.

"While it was fascinating to find this, it was not exactly what we expected," Eigenbrode said. "Assuming a geological, meteoritic or biological source, you would expect a diversity of sources. It inspired us to find a better set of rocks for more organics."

And so Curiosity drove on to the base of Mount Sharp and sampled some of the mudstones (a rock that formed from muds and clays) found there. Surely, if organic molecules existed on Mars, the scientists thought, they would be here. Everything else told them that the lake environment in which the mudstones formed 3 billion years ago would have been a likely place for life to exist.

The right sample

To obtain a sample, Curiosity's rover drills into a rock and delivers a fine powder to the Sample Analysis at Mars (SAM) suite of instruments. The powder essentially falls into a cup akin to a small thimble, where it is placed into a small oven and heated to temperatures of 500ºC, and above. As the sample heats up, gases come off and are channeled into a tube where they are sniffed by a mass spectrometer. The rest of this gas is sent into a long, skinny tube that separates each molecule.

The result of this careful analysis is that scientists have conclusively found thiophenic, aromatic, and aliphatic organic compounds on Mars. These results are reported Thursday in a new paper in Science, of which Eigenbrode is the lead author. She was especially intrigued by the discovery of sulfur within the organic molecules, which probably helps explain how the organic molecules were preserved on the surface of Mars for a long period of time despite the harsh radiation.

With this analysis, scientists searching for indications of past life on Mars are really pushing the suite of instruments on Curiosity pretty far. But this won't be the last opportunity to search for organic molecules on Mars. In July 2020, the European Space Agency and Russia's Roscosmos will launch the ExoMars rover to Mars. That summer, NASA also plans to launch the Mars 2020 rover, which should be able to perform additional observations.

The ExoMars rover is particularly tantalizing to scientists because it will have both an instrument to detect organics like the SAM on Curiosity, as well as the capacity to go significantly deeper below the surface of Mars to get samples, perhaps as far as two meters. Although scientists are not sure, they suspect the organic molecules at this depth, if they were indeed produced by biological processes long ago, would stand a much better chance of being protected from radiation on the surface.

And what if microorganisms did, in fact, exist on Mars billions of years ago? Confirming it would involve a pretty in-depth analysis of rocks on Mars, which is probably beyond the capability of any of these rovers. One option is to return samples, which may happen in the 2020s. The other option is to send humans.

Science, 2018. DOI: 10.1126/science.aas9185 (About DOIs).