Rhea, the grey rock in the foreground, is a cratered, icy world about 950 miles wide. By contrast, Titan, the hazy yellow behemoth in the background, is Saturn’s largest moon at 3,200 miles across.
Despite their differences in size and composition, both have the potential to host complex chemistry, and even the precursors for life. Rhea has a tenuous, oxygen-rich atmosphere created by charged particles from Saturn’s magnetic field striking and splitting apart water molecules. Titan has lakes and rivers of liquid methane and ethane, a thick hydrocarbon atmosphere, and may have a subsurface ocean, making it one of the best hopes for extraterrestrial life in the solar system.
The vast majority of planets discovered around other stars have been gas giants like Saturn and Jupiter that probably aren’t well suited to life. But if those planets have moons that can support their own ecosystems, the galaxy might be full of potential for life. Recent research suggests that current surveys should be able to detect potentially habitable moons by their gravitational tugs on their host planets. Even if Rhea and Titan turn out to be lifeless, studying them can give astronomers an idea of what to expect on exo-moons.
Image: NASA/JPL/Space Science Institute. Color composite by Jason Major.