Recent imagery from the Hubble Space Telescope adds to the evidence that Jupiter's largest moon, Ganymede, harbors a salty ocean beneath its icy surface comprised of more water than found on Earth. With this discovery, Ganymede joins the growing number of tantalizing worlds in our solar system in which liquid water is prevalent. On Earth, where there is liquid water, there is life. As such, understanding our solar system's water worlds has potentially profound ramifications for the emergence of life on Earth, and the existence of life elsewhere in our solar system and the universe (helping us identify these same characteristics in Earth-like worlds around other stars). In addition to Earth, our present list of water worlds includes Jupiter's moons Europa, Ganymede and Callisto, Saturn's moons Enceladus and Titan, and Neptune's moon Triton. Including the vast hydrocarbon seas on Titan's surface, a compelling scientific quest could be defined focused on the diversity and distribution of liquid resources across the solar system. Enceladus and Europa may be the two worlds in our solar system best suited to search for life as we know it; Titan is likely the best place to search for life as we don't know it.
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