Jupiter’s Europa, Saturn’s Enceladus and Titan have the strongest evidence.
The Asteroid Ceres has joined Mars and several satellites of the giant planets in the list of locations in the solar system that may harbor life. The dwarf planet has water. Now, infrared spectra taken from the orbiting Dawn spacecraft reveal organic-rich regions in and around its ~50-kilometer Ernutet crater. Ceres definitely has about 25% water ice, but there is a reasonable likelihood that there is liquid water as well.
Mars also has a lot of water ice for sure but some of it could be liquid.
Recent spacecraft studies of Comet 67P/Churyumov-Gerasimenko with Rosetta and of Ceres with the Dawn space probe provide evidence that complex organic molecules and even amino acids are ubiquitous on small bodies in the solar system and that water ice is abundant in the asteroid belt.
Europa’s ocean may have an Earthlike chemical balance. Europa’s is closest to the surface (less than 10 km and possibly less than 1 km in places), and hence potentially best suited for eventual direct exploration. Enceladus’ ocean is deeper—5–40 km below its surface—but fractures beneath the south pole of this moon allow ice and gas from the ocean to escape to space where it has been sampled by mass spectrometers aboard the Cassini Saturn Orbiter. Titan’s ocean is the deepest—perhaps 50–100 km—and no evidence for plumes or ice volcanism exist on the surface. In terms of the search for evidence of life within these oceans, the plume of ice and gas emanating from Enceladus makes this the moon of choice for a fast-track program to search for life. If plumes exist on Europa—yet to be confirmed—or places can be located where ocean water is extruded onto the surface, then the search for life on this lunar-sized body can also be accomplished quickly by the standards of outer solar system exploration.
Both Ganymede and Callisto have surfaces dominated by impact craters, and even the so-called “grooved terrain” on Ganymede indicative of geologic activity seems very ancient. To support the ancient craters requires thick, cold crusts, so that their oceans are likely deep below the surface—perhaps 100 km or more. Because both objects are more massive than Europa and possess as much ice as rock, some of the ice must be in the form of the high pressure phases that are denser than liquid water. Thus their oceans are likely sandwiched between layers of ice above and below, making contact with the underlying silicate (thought to be important for life) unlikely.
According to astronomers with the Royal Observatory of Belgium, Dione’s icy surface is floating on an ocean lying 62 miles beneath the surface. Data suggest the ocean is a few dozen miles deep.
Scientists believe Dione’s ocean has likely been around for the entirety of the moon’s existence, offering plenty of time for the development of microbial life. The ocean is too deep to access, but scientists are hopeful that the moon ejects ocean spray through polar geysers like Saturn’s other liquid-logged moons, Titan and Enceladus.