https://phys.org/news/2018-11-colonizing-mars-contaminating-native-life.html

Life, scientists suggest, has some basic requirements. It could exist anywhere in the universe that has liquid water, a source of heat and energy, and copious amounts of a few essential elements, such as carbon, hydrogen, oxygen, nitrogen and potassium.

Mars qualifies, as do at least two other places in our solar system. Both Europa, one of Jupiter's large moons, and Enceladus, one of Saturn's large moons, appear to possess these prerequisites for hosting native biology.