Hydrocarbons like methane and petroleum are common not only on the Earth, but also on most other planetary bodies in our Solar System, as well as interplanetary dust and meteorites. They appear to have been a common constituent of the materials that formed these bodies, and under heat and pressure hydrocarbon fluids would make their way towards the surface. Trace elements in the rocks, including the inert gas helium, would be swept up by such streams, and this would provide the only known explanation for the clear, strong association of hydrocarbons with helium. But all petroleum on Earth also possesses molecules of unquestionably biological origin. If the oil did not derive from biological materials, then only a massive sub-surface microbial life that pervades all oil-bearing regions could account for this. The sub-surface conditions on many other planetary bodies will be quite similar to those on Earth. Therefore I suggested that such life could be widespread in the Solar System, and that one should look for evidence in the first place in the carbonaceous Martian meteorites. The primary food source would be the upwelling hydrocarbons, together with oxygen available from iron and sulfur oxides in the rocks. Carbon dioxide and water will be produced, and the solids that remain will be iron and sulfur in lower oxidation states. The famous Martian meteorite contains indeed low oxidation iron particles and iron sulfide, together with hydrocarbons, a combination characteristic of oil-bearing regions on Earth.