Free Astronomy Magazine May-June 2015

23 MAY-JUNE 2015 PLANETOLOGY U ntil a few decades ago, it was hard to imagine that in the solar system there may have been liquid water on plan- etary bodies other than Earth. It was known that there is some in comets in the form of ice, it was suspected that in that same state there could have been some on Mars, and only the most optimistic astrono- mers hoped one day to be able to find it on some large moons. Computer models of the ‘70s indicated that on Ganymede, the largest natural satellite of Jupiter (and of the entire solar system) and whose diameter exceeds that of the planet Mercury, there could have been a global ocean hidden be- neath a very thick sur- face layer composed of rocks and ice. As easily guessed, the times were pre- mature in or- der for as- t ronome r s to have the necessary