It could be hydrocarbons. We just found oil on on Mars! Black gold, Tharsis tea! Now if only we had an atmosphere to burn it in.
tbc I don't believe there's oil on mars nor do I think it is remotely likely.
“Methane (CH4) is abundant on the giant planets—Jupiter, Saturn, Uranus and Neptune—where it was the product of chemical processing of primordial solar nebula material. On Earth, though, methane is special. Of the 1,750 parts per billion by volume (ppbv) of methane in Earth’s atmosphere, 90 to 95 percent is biological in origin.”[1]
[1] https://www.scientificamerican.com/article/methane-on-mars-t...