n the early 2020’s the European Space Agency intends to launch a special space probe to Jupiter, the fifth planet out from the Sun and the largest in the Solar System. The probe is to be called the Jupiter Icy Moons Explorer. It is being referred to using the acronym “JUICE”, which again underlines the huge investment that seems to go into making interesting and novelty acronyms for major science projects. Among other things, this mission should confirm what we suspect is the case for large satellites of major planets, that inside, under the ice there may be oceans of liquid water.
We live on a lovely, wet world. About two thirds of our planet’s surface is covered by water, and if we smoothed the Earth into an absolutely smooth sphere, the entire surface would be covered by a deep ocean. Using our planet as the typical ocean world, it was very easy for us to jump to astronomical conclusions. Venus, the next planet toward the Sun is far too hot for liquid water oceans, and Mars, the next planet out from ours is too cold. Therefore, any water on an object orbiting the Sun further out than Mars would be present only as ice, which would never be warm enough to melt. Not for the first or probably last time in our history, we were led to a logical but wrong conclusion because we did not take everything into account.
The main clue lies in our own oceans. Along the mid-ocean ridges, where new seabed is emerging at plate boundaries, there are hydrothermal vents, where very hot, mineral-laden water emerges, supporting colonies of exotic animals that owe nothing to the Sun for their existence. If our Sun suddenly switched off, our atmosphere would liquefy; the oceans would be covered by ice, but down along the mid-ocean ridges, life would go on.
A strong clue to other sources of warmth is Io, Jupiter’s closest large moon. Instead of being frozen solid, it is hot and covered with volcanoes. It is the most geologically active place in the Solar System. Io is being pulled slightly into an egg shape by Jupiter’s gravity, but since the moon is also rotating on its axis, it is constantly flexing and changing shape, which produces a huge amount of heat – enough to melt Io’s interior.
The heating of Io is too intense for it to be a good place to look for oceans or life. However the next moon out, Europa, is far enough for this tidal heating to be less severe, and many scientists believe there could be a deep ocean underneath the ice, with hydrothermal vents and possibly life. Now there is growing evidence that ice-covered oceans are not out of question for the two outermost large moons, Ganymede and Callisto, under very thick layers of insulating ice.
The JUICE spacecraft will be able to probe down through the ice and detect any oceans underneath. It will even tell us how deep they are. This space mission will almost certainly change the way we think about the objects orbiting the cold, dark outer reaches of the Solar System, and also our ideas about extraterrestrial life.
Mercury is very low in the west after sunset. Mars and Saturn still dominate the southwestern sky. Look for Venus and Jupiter close together in the eastern sky before dawn. The Moon will reach last quarter on the 10th and be new on the 18th.
Ken Tapping is an astronomer with the National Research Council’s Dominion Radio A