Since August of 2012, NASA’s Curiosity Rover has been exploring Gale Crater on Mars. The site was picked because evidence suggests that the crater once was a lake filled with liquid water. We were hoping evidence for organic chemistry on the Martian surface might be found there. So far, the analysis of the surface from previous rovers have shown there are not any organic compounds, but we have kept looking.
To begin with, an organic compound is a molecule that is often associated with life (but certainly not exclusively. Both geology and biology can be responsible for the creation of organic compounds). An organic molecule has carbon bonded to hydrogen within its structure. For example, methane is an organic compound and its chemical formula is CH4. The C stands for carbon and the H for hydrogen. All four of the hydrogens are chemically bonded to the carbon atom, thus classifying it as an organic compound. On the other hand, carbon dioxide—the main component of Mars’ atmosphere—is not an organic compound. Its chemical formula is CO2 (the O stands for oxygen), and as you can see, there is no hydrogen present. Carbon dioxide does not make the cut to be considered organic under this system of classification.
Before looking at the surface, we wanted to look at the atmosphere to see if we could find anything we might have missed in the past. It’s been known that organics have existed in the atmosphere before Curiosity landed, but in December of 2014, it was discovered that organics have also been detected on the surface. We also found that the organics in the Martian atmosphere are behaving in a very interesting way.
We’ve known methane has been in Mars’ atmosphere for quite some time. That is something we can observe from here on Earth. But, the measurements curiosity took showed something we did not expect. During the twenty months of observing the methane levels in the atmosphere above Gale Crater, the usual average levels averaged at .7 parts per billion (that’s .7 nanogram of methane per liter of atmospheric volume), but for a two month period, those levels averaged at 7.2 parts per billion. That means that the levels of methane present in the atmosphere became ten times higher for a small period of time, and then became ten times lower again. We don’t know why.
This new evidence suggested that Mars might be more organically active than we had thought in the past. Our excitement grew. The improved technology on Curiosity had shown that we had missed some things with our older rovers. What might we find on the ground?
After identifying several minerals taken from a sample collected off the surface, the material was then heated up so that Curiosity’s on-board spectrometers could identify what gases were given off (a spectrometer looks at the light a material gives off, and from there the molecules and elements in the sample can be identified). The results were quite surprising.
Curiosity identified eight different organic compounds and water from a sample that was taken from only a 2.6 inch hole in the mudstone the sample was collected from. The surface of Mars is very hostile to the preservation of organics—this is due to Mars’ thin atmosphere—and we were not all that optimistic about being able to detect any respectable amounts less than two meters below the surface. If they found this much exciting data only a couple inches into a martian rock, what might we find if they dug even deeper?
The findings from the surface sample also show that Gale Crater was once a lake. In fact, the calculations from many of these findings show that twenty percent of Mars used to be covered in liquid water. As the planet’s magnetic field diminished (for reasons we aren’t sure of), the atmosphere of the planet was no longer protected from the Sun. Mars’ atmosphere then began to thin and this caused the planet to heat up. The heat caused 87% of the surface water to evaporate and escape into space (Mars has a weaker gravitational attraction than the Earth).
The thing that is most interesting is why the methane levels in the atmosphere are changing. The abundance of methane disappeared all together for three months sometime after these findings. Then, the methane came back again. From the evidence, we can conclude that the surface and the atmosphere are interacting. Why are the methane levels fluctuating so intensely, and where did it come from in the first place?
There are three hypotheses we have so far to answer this question. When ultraviolet radiation from the Sun interacts with the organics on the surface of Mars, methane could be generated (and destroyed). Another possibility is that olivine interacting with water on Mars might be producing the methane. One of the minerals identified in the surface sample Curiosity took was indeed olivine. We know that when olivine and water interact under pressure within the presence of carbon dioxide (the main component of Mars’ air), methane can be produced.
The third possibility is the most exciting, especially if you are an astrobiologist. If there are microbes living in liquid water on Mars—we know there was once liquid water and that there is currently frozen water—then their metabolic processes could be generating the methane. Methane is a fingerprint of life. Its presence doesn’t make the presence of life on Mars conclusive, but it sure does make it seem more likely.