Methane on Mars

In 2003, studies of the Martian atmosphere revealed a surprising increase in methane. The discovery is of considerable interest, as it hints at the presence of life. Most methane on Earth results from biological activity. The Martian methane eventually dissipated but there was a similar occurrence a decade later.
Before the fall of 2013, the Curiosity rover used its Tunable Laser Spectrometer to test the atmosphere. It determined that methane levels were extremely low--just 1.3 parts per billion, or a tiny fraction of what was detected a decade before.
In November 2013, however, Curiosity detected a spike. Methane levels shot up from 0.7 parts per billion to about 5 parts per billion. A week later, there was a further increase, to 7 ppb. Three weeks after that, it went up again, to 9 ppb. Six weeks later, the methane had vanished.
Why do methane levels suddenly spike at long intervals and what is the source of the gas? The fact that releases of methane occur several years apart and, in 2013, during the local fall and winter, argue against a biological origin. Subsurface life, however, may explain the facts.
Mars has many caves. Some may be very deep and extensive. In the distant past, vast quantities of liquid water existed underground. Huge outflow channels attest to the storage capacity of the Martian interior, and internal heat sources which kept water in liquid form. Bodies of underground water may persist in some areas. Any life they harbor relies on planetary heat not sunlight. Methane may be a byproduct of biological activity, but even if it is produced at a steady rate it does not enter the atmosphere continually, but in bursts. This is because underground lakes have upper ice covers. Most of the time, they prevent methane from entering the atmosphere.
After it is produced, methane (a light gas) rises to the top of the lake habitats and accumulates under the ice. As more accumulates pressure builds up. Finally pressure reaches critical mass; methane forces its way up through the ice. The sudden release of several years of accumulated gas causes the spikes. After the pressure falls the ice settles back down, resealing holes or cracks, and the cycle starts over again.
Someday robots may explore Martian caves to test this hypothesis.