Radioactivity
Mars is made of note-quite the same stuff as earth (different uncompressed bulk density, for a start) but clearly has radiogenic isotopes in its makeup. These generate internal heat which, although not enough to run constant volcanism as on Earth does cause episopic volcanism (mars ain't volcanically dead, just dormant). That also causes internal warming. Many other Mars researchers will froth at the mouth at how LOW I put the heat production, but they're wrong, says he, humbly...
Either way, there's internal heat. Just a question of how deep you go. I say 20 km, some say 2 km. I think they're badly wrong, but we can all live with 20 km, and so can Mars' microbes...