Recently, Curiosity ran some atmospheric experiments on the Martian surface only to discover some surprising results. The rover did not find any traces of methane on the Martian surface. This result is puzzling because previous surveys of the planet by satellites revealed the presence of methane, only to be contradicted by our on-the-ground experiments. This adds more fuel to the heated debate regarding the history of life on the Red Planet.
To date, the only mission that looked for life directly was the Viking program. The results from the lander's soil analysis looked promising at first, but further tests and other issues have cast serious doubt on the validity of those results. Currently, whether or not the Viking lander found evidence of extraterrestrial life is in dispute, though most scientists agree the Viking experiment needs more evidence to support it before accepting the conclusion. Other missions to Mars have looked for signs of life (such as water and methane) without testing for life specifically.
One of the deepest Martial mysteries revolves around the methane problem. Our probes have discovered what appear to be plums of methane around the Martian surface, helping to revitalize the "life on Mars" argument. With these plums, many scientists suspected that microbial life once existed on Mars or currently resides below the Martian surface. We came to this conclusion because we find most of the methane on Earth is released as a byproduct of biological activity.
On September 19, NASA unveiled Curiosity's results and they came up rather methane-free. This result is disappointing and disturbing. Disappointing because we currently regard methane as a planetary signature that could mean life exists somewhere on its surface. Disturbing because every test we've conducted from Earth and from space tell us that Curiosity should have found at least a trace of the odorless chemical.
Our "current" estimates say methane should exist in concentrations of about 1.3 parts per billion. Curiosity's Tunable Laser Spectrometer is specifically designed for hunting down methane in the Martian atmosphere. In the beginning, the results from Curiosity's tests looked promising because they initially found higher levels of methane than originally expected, leading some scientists to speculate concentrations exceeding several parts per billion. But, NASA determined the initial results came as a result of "Florida air" that was still in the spectrometers chamber when Curiosity landed.
As the contamination was cleared, Curiosity continued to look for signs of methane with no luck. Curiosity's experiment was also designed to compare carbon isotope rations of methane in an attempt to determine whether or not the gas was produced by biological activities. But, with an absence of methane, Curiosity can't run than experiment.
So, where is all of the methane that's suppose to be on Mars? In the words of Ricky Ricardo, "Methane, you've got some 'splainin' to do!"
It's possible that the methane exists in the upper Martian atmosphere and isn't found near its surface. It's also possible whatever is causing the methane plums are more isolated. As an example, on Earth when we find such plums, the methane tends to stay in the same general area without contaminating its surroundings. Given some of these possibly, Curiosity could simply have been in the wrong place at the wrong time to make any substantial discovery.
The next step is for Curiosity to perform something known as a "methane enrichment experiment" which will increase the Sample Analysis at Mars suite's sensitivity up to 10 times. This could potentially show us that methane does exist on the surface, just at lower levels. In addition, the ExoMars spacecraft, currently scheduled to launch in 2016, will conduct a more thorough study of the Martian atmosphere and hopefully help us to understand this conundrum.
In the end, these experiments are vital for our understanding and the formulation of our opinion on whether or not Mars had life at some point in its history. Methane found near the ground could be the result of biological activity, but if the methane only exists in the upper atmosphere, that could indicate some bizarre chemical process taking place up there.
Mars seems to house one of the most intricate riddles in the solar system. Our measurements continue to reveal unexpected results. The jury on past-Martian life is still out, but at the moment, these results don't bode well.