On Earth, according to conventional theory, the largest, by mass and volume, identifiable trace of past life is subsurface oil and natural gas deposits. Nearly all coal and oil on Earth and most sedimentary source rocks associated with coal, oil, and natural gas contain molecules of biological origin and is proof of past life. If Mars possessed an Earth-like biosphere in the past, Mars may contain subsurface deposits of oil and natural gas indicating past life. Life might still exist in these deposits. Subsurface oil and natural gas on Mars would probably cause seepage of hydrocarbon gases such as methane at favorable locations on the Martian surface. Further, if Mars contains substantial subsurface* life, the most detectable signature of this life on the Martian surface would be gases generated by the life percolating up to the surface and venting into the Martian atmosphere. In this paper, systems that can detect evidence of subsurface oil and gas, including ground penetrating radar and infrared gas sensors are explored. The limitations and future prospects of infrared gas detection and imaging technologies are explored. The power, mass, and volume requirements for infrared instruments able to detect venting gases, especially methane, from an aerobot is estimated. The maximum range from the infrared sensor to the gas vent and the minimum detectable gas density or fraction of the Martian atmosphere -- as appropriate for the instrument type -- is estimated. The bit rate and bit error rate requirements for transmitting the data back to Earth are also estimated.