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A hydrocarbon is any compound with molecules each consisting of just hydrogen and carbon with no other element. Hydrocarbons are found in crude oil and are the primary constituent of refined fuels. On Earth, they are thought to have been generally formed from material of biological origin, though abiotic hydrocarbons are present in the output of deep ocean vents and they are also present on other bodies in the solar system which are presumed to be of non-biological origin. Thus, to some extent a hydrocarbon is a biosignature, but certain ratios between various hydrocarbons as well as ratios between various hydrocarbons and other non-hydrocarbon material can be better biosignatures.
Carbon atoms tend to bond to four other atoms, each of which can be carbon or hydrogen (or some other element, in which case the compound is not a hydrocarbon), and hydrocarbons can be so-constructed in various ways, including long chains, and branches. Some classifications:
The simplest alkane is methane with one carbon and four hydrogen atoms. Next is ethane, with two bonded carbon atoms, each also bonded with three hydrogen atoms.