Our Moon is in the news these days, with the successful first soft landing earlier this month of a spacecraft on its far side, China’s Chang’e 4 lander. It’s an important landmark because the far side of the Moon still remains relatively little understood, despite it being on our nearest neighbour in space. Thats' because the far side is always turned away from us - we can never observe it directly from Earth, and the Moon’s mass itself blocks any spacecraft there from any direct radio communication with Earth.
So why is it that we only ever see one side of the Moon? The gravitational pull between the Earth and the Moon is the cause – Earth’s gravity, being stronger, has basically braked the Moon’s original rotation speed so that it now rotates once for every orbit it makes around the Earth. This means that the same side is always facing the Earth. This effect is known as tidal locking, and in fact almost all the moons in the Solar System are tidally locked to their planet.
However, the Moon is a special case because Earth has no other satellites. When there are multiple satellites around a planet, for example those of Jupiter and Saturn, tidal effects can get a lot more complicated because of the gravitational pull of the satellites on each other, as well as that of the planet.
For example, 3 of Jupiter’s largest moons, Io, Europa and Ganymede, orbit relatively close to each other and to Jupiter itself. The resulting gravitational tug in several directions has caused these three moons to align their orbits such that Europa orbits twice as fast as Ganymede and Io orbits twice as fast as Europa, a situation known as ‘resonance’. But the power of the tidal effects doesn’t stop there.
Volcanoes and cracked ice
Io, the innermost of these 3 moons, gets so stretched, deformed and heated by these competing forces that it is covered in constantly erupting volcanoes. Europa, being further away from massive Jupiter, experiences less extreme tidal stress, but still enough to crack its frozen icy surface in all directions. It’s believed that beneath this ice there is a liquid ocean, and evidence has been found for plumes of water vapour issuing from the cracks, though no space mission has had the opportunity yet to image them close up.
Plumes of water vapour
And at Jupiter’s neighbour, Saturn, the fascinating small moon Enceladus is in resonance with its larger neighbour, Dione. The effects of the resulting tidal stretching are perhaps the most spectacular in our solar system: the famous ‘tiger stripe’ fissures in the ice around Enceladus’ south pole, and the massive plumes of water vapour streaming from them.
https://solarsystem.nasa.gov/moons/earths-moon/in-depth/
https://solarsystem.nasa.gov/moons/jupiter-moons/europa/in-depth/
https://solarsystem.nasa.gov/moons/jupiter-moons/io/in-depth/
https://solarsystem.nasa.gov/moons/saturn-moons/enceladus/in-depth/
Image credits:
1. NASA/JPL/DLR https://solarsystem.nasa.gov/resources/1039/galileo-sees-io-erupt/?category=moons/jupiter-moons_io
2. NASA/JPL-Caltech/SETI Institute https://www.nasa.gov/sites/default/files/thumbnails/image/pia19048.jpg
3. NASA/JPL-Caltech https://www.jpl.nasa.gov/news/news.php?feature=4755