The tantalizing possibility that life may exist on Mars has inspired both scientific exploration and popular culture for a long time. The question of whether we are alone in the universe has recently been reignited by some intriguing data from NASA’s Curiosity Rover, which has been trundling around Mars since 2012. Before we review that data, let’s start with some statistics on the Red Planet:

• Mars is approximately half the size of the Earth.
• It is about 140,000,000 miles away from Earth and can be reached by space craft in six months.
• Earth and Mars were both formed about 4.5 billion years ago.
• Long, long ago Mars had a significant amount of surface water, including an ocean the size of the Arctic Ocean, and continues to have water-containing ice caps on its poles, as well as subsurface water.
• Mars looks red due to the presence of iron oxide (rust) on its surface.
• The atmosphere on Mars is only 1/16th as dense as Earth’s, comprised of 96% carbon dioxide, 1.9% argon, 1.9% nitrogen and trace amounts of oxygen, water vapor, and methane. .

Based on data from infrared telescopes, we have been reasonably certain for many years that the atmosphere of Mars contains small amounts of methane. The recent excitement about the possibility of life on Mars is driven by theories about the source of that methane. Further, since methane in the Martian atmosphere is converted to carbon dioxide over time, the presence of methane in the atmosphere indicates that there is ongoing methane production on or within the planet.

There are two plausible sources of methane on Mars. The first is a process known as serpentinization, in which water and carbon dioxide react with certain types of rock to produce methane. This process is almost certainly happening to some degree on Mars, and thus accounts for at least some of the methane observed in the atmosphere.

The second and vastly more exciting possible source of Mars’ methane is methogenic bacteria. In oxygen-free environments, methogenic bacteria convert carbon dioxide and hydrogen into methane and water. These bacteria are extremely common on Earth and are responsible for the production of methane in swamps and in the digestive tracts of humans and animals.

When NASA’s Curiosity Rover arrived on Mars, part of its mission was to study methane in the atmosphere. The early results were confusing. Curiosity initially detected only 0.7 parts per billion (ppb) of methane, a value so low that it could have been a measurement error. This left open the possibility that the earlier data from the infrared telescopes was incorrect. And then something fascinating happened. The methane concentration in the atmosphere shot up to 7-9 ppb before declining over a period of months as it was converted to carbon dioxide. As this cycle repeated several times, it became clear that plumes of methane were being periodically released from the ground.

Unfortunately, the big question of whether this methane was generated by Martian bacteria remains open. But the good news is that we know how to answer this question. Just as is the case with all atoms, carbon atoms come in several configurations, each with different atomic weights known as isotopes. (The extra weight comes from extra neutrons.) Living organisms tend to use the lightest and most common isotope, carbon-12. Therefore methane, CH4, which is produced by biological sources such as bacteria, would contain mostly this lighter isotope. In contrast, methane produced from non-biological processes such as serpentinization would contain higher concentrations of heavier carbon isotopes such as carbon-13.

Scientists at NASA are currently trying to determine if the instruments on the Curiosity Rover may be able to measure the relative concentrations of the different carbon isotopes in the methane on Mars. Even if Curiosity is unsuccessful, in 2016 the European Space Agency will be launching the Mars Trace Gas Mission which is specifically designed to address this question. Therefore, in the near future we can expect to get a strong indication of whether or not the methane on Mars was generated by bacteria.

Because the discovery of life in the universe somewhere other than Earth would be perhaps the most significant discovery ever, I think it is unlikely to become generally accepted that life exists on Mars based solely on a mixture of carbon isotopes in atmospheric methane. Therefore, if future measurements do determine that methane in Martian air is primarily made of carbon-12, I expect the next step would be to send a probe to Mars which can drill further into the ground. Drilling into the Martian soil would allow us to search for either living bacteria or traces of dead bacteria from long ago.

Although it will be many years until we have a chance to take a soil sample from deep below the Martian surface, I predict that living bacteria will be found. Everything in the universe is comprised of the same atoms that all follow the same natural laws. Therefore, if life can take hold here on Earth, it seems equally likely that it could have arisen other places as well. Time will tell.

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