Moons are Small Gods in the Darkness
There are over three hundred of them in our solar system. Some are bigger than planets, and no two are the same. Moons are the constant companions of our planets, and without these strange and wonderful worlds, our solar system would be a very different, chaotic, and inhospitable place. Our moons are a host of guardian angels watching over us. They are our small gods.
Last night, the moon rose over the UK under lunar eclipse. Between around 9pm and 10:30pm it slowly emerged from behind the Earth’s shadow, and we (at least those who weren’t completely shrouded in cloud) were treated to one of the finest astronomical spectacles in a decade. But far from being just a beauty in the night sky, our moon, and the moons of other planets, are fundamental features of our solar system, and without them, we would be in a lot of trouble.
Moons are guardian angels
Moons orbiting around their planets act to stabilise the orbits and rotations of those host planets. Large moons (those with a small planet-to-moon ratio) can stop the axis of a planet’s rotation from changing too much. Earth’s large moon limits the variation of tilt of the Earth’s rotational axis- its obliquity, to within 2 degrees. Mars’s moon Phobos is too small to have this effect, so Mars’s obliquity varies by up to 50 degrees over thousands of years, making the planet hostile to life due to extreme seasonal changes. If we lost our own moon, then over a long period of time, our obliquity may oscillate by the same amount or more than Mars, over geologic time. Such changes would mean that our seasonal biomes would be seriously disrupted or destroyed, and at certain time, the combined rotation and orbit around the sun may make the Earth completely inhospitable to life.
The gravitational pull between moons and their host planets creates tides on those planets, and on themselves. Such interactions can be seem in water (as seen on Earth due to our own moon) and in the flexible rock itself. This flexing of the rock can create heat within the body of the moon and planet. In the icy far reaches of space, this heat can be enough to drive volcanism (as on Io around Jupiter) or cryovolcanism (As on Enceladus around Saturn) or to melt ice to create subsurface oceans of water which may be habitable for alien life (as on Europa around Jupiter). Without tides on Earth, fertile plains relied on by humans, and by a whole range of organisms, would not exist. Many plants and creatures that have developed to live in a periodically wet and dry environment, would never have appeared, and species diversity would be measurably poorer.
Moons are the answers to our prayers*
*by prayers I mean scientific questions
Study of the moons around us can offer valuable and often unexpected answers to questions of the formation of our solar system, and of the processes still operating on a planetary scale.
Looking at the sheer variety of moons has given us a number of ways to explain how all the bits of our solar system came together. The classic model for the assembly of planets also holds for some moons. This ‘accretion model’ predicts that after the sun formed, the remaining giant cloud of gas and dust clumped together due to gravity into grains, rocks, boulders and, eventually planets that orbited the sun. At the same time, if smaller bodies fell into orbit around a planet, instead of the sun, they became moons. We can see examples of this kind of moon formation in the landscapes of the Jovian moons Ganymede and Callisto. Ganymede formed very close to Jupiter from a lot of debris, in a quick and very energy-rich process. This energy led to the rapid segregation of ice and rock, which can be seen on its disrupted surface. In contrast, Callisto formed much more slowly, further away from Jupiter, and there was less energy involved in its formation. As a result, the rock and ice didn’t differentiate and its surface is smooth. Thus, both the amount and type of material contributing to the accretion process, as well as the energy of formation, is critical in creating the variety of many of our moons.
But accretion is not the only way that a planet can get itself a moon. Neptune’s largest moon Triton is a bit of anomaly, as it is one of the only natural satellites that orbits its planet the wrong way. Most moons orbit their planets in the same direction as the planet rotates, a legacy of the original rotation of the gas cloud. But Triton is in retrograde – it travels clockwise, whileNeptunerotates anticlockwise. This piece of information alone is enough to tell us that the moons formed separately. Leading speculation is that it was an object ejected from the nearby Kuiper belt of asteroids, which subsequently became locked in orbit aroundNeptune. So moons can be stolen, as well as home grown.
Our own moon, Luna, is an example of yet another way you can get a moon – planetary destruction. Studies of the composition of lunar and terrestrial rocks has shown that they have very similar compositions, minus a bit of iron in the former. The prevailing theory for such a similar composition is that our moon was in fact derived from Earth. But how do you get a bit of Earth 300,000km out? By hitting it. Really hard. So the theory goes that around 4 billion years ago, a planet the size of Mars collided with the proto-Earth, melting the outer layers of both, and spewing vast amounts of molten rock into space, which soon reformed into our own moon. This is known, somewhat unsurprisingly, as the Giant Impact Hypothesis. They even have a funky computer model. The hypothesis has recently been thrown into doubt after new analysis of lunar rocks from Apollo 17 found significant quantities of water within them, which wouldn’t be expected if Luna formed by a high energy, rock melting, water vaporising impact. Time will tell, and maybe we will find yet another way of getting ourselves a moon.
Observing the moons of other planets can not only tell us about how they got there, but also what they are doing now, and how they are responding to large scale forces in the solar system. In particular, they show us the large scale effects of the tidal gravitational pull on rock, rather than water. When rock of a moon is put under tensional and compressional stresses during an orbit around its planet, it generates heat, and if the forces are strong enough, the heat may be great enough to generate some truly magnificent volcanic and cryovolcanic features.
The surface of Io is smooth and, probe images have shown, constantly changing. Episodic impacts of its surface would be expected to leave a mark in the form of craters, and the complete absence of these can only mean that the surface is new. Near constant, large scale volcanism is responsible for resurfacing the entire moon on a very short timescale, and it is the heat generated by the tidal friction of Io with Jupiter and other nearby moons, that that drives the eruptions.
For moons which are composed of more ice than rock, which is the case for most Saturnian and Jovian satellites, the tidal friction heating melts the lower layers of ice in contact with the rocky core. The heat driven expansion of this liquid causes a spectacular phenomenon of cryovolcanism, or ice volcanism. Perhaps the finest example of this in all the solar system are the plumes of water ice spewing more than 300 km above the surface of the small Saturnian moon Enceladus. These plumes are fed by isolated pickets of warmer water, and play a big part in feeding the rings of Saturn amidst which the moon sits.
By observing the surfaces of moons, we are also able to assess changes in the structure and motion of those moons. Such observations are important for planetary protection. If a moon of Mars suddenly destabilises from its orbit and comes flying towards the Earth, people will ask ‘why didn’t we know it was going to do that?’ Nothing so apocalyptic is going to happen in our lifetimes, or probably in the lifetime of the human race, but the orbits and rotations of moons do change, and it is important to keep tabs on that. For instance, the most recent cracks in the surface of the ice of Europa are exactly what we would expect from the tidal stresses between that moon and Jupiter. With increasing age, however, the cracks gradually change in their orientation, indicating that either the moons has changed in its orbit and rotation, or something else is going on. Mathematical models show that something else is indeed going on, and in fact the surface of Europa is rotating slightly faster than its core, something that could only happen if they were separated. What separates them? Water, liquid water. An ocean of liquid water exists beneath the icy crust of Europa, and that is one of the most exciting discoveries in our solar system in the last 10 years.
Moons are life givers
What I personally find most exciting about our constant lunar companions, is that their vast diversity in composition, location and characteristics provides the best opportunity we have of finding extraterrestrial life in our solar system.
It is generally accepted that life as we know it needs a liquid in order to get started and keep going. On Earth, that liquid is water, and liquid water is a precious commodity in the solar system. It can only exist at the surface of a planet within narrow zone at a specific distance away from the sun – the habitable zone. Too close, and the water boils away, too far and it is frozen solid into ice. Outside this habitable zone, finding life as we know it has been considered extremely unlikely owing to the lack of water. But all is not lost. The habitable zone is not the last call for our terrestrial life. There are two more ways that we can make liquid for our spa-loving beasties, and moons have it all.
Firstly, while liquid water may not exist on the surfaces of bodies in the solar system, it may be maintained beneath the surface. The heat generated by the tidal friction between moons and their giant host planets like Jupiter and Saturn is enough to melt lower layers of ice into sub-surface ocean, as in Europa and Enceladus (see above) and possibly Ganymede. Contact between these extensive oceans and a warm rocky core would also provide minerals and nutrients needed for the maintenance of life, and occasional cracking of the overlying ice will provide access to rare gaseous elements and oxidisers in the thin atmospheres. Europa is usually considered the main target for our next step in the search for extraterrestrial life, and amino acids, the building blocks of life, have been detected in the icy plumes of Saturn’s moon Enceladus.
But liquid water is not the only way. Titan, the largest moon of Saturn, has a strange and intricate landscape of ice that has been shaped by weather. Weather that is entirely methane and ethane, rather than water. Methane clouds, methane rain, methane rivers and methane lakes. Because of its lower freezing point, methane can exist as a liquid on the surface of Titan, and this liquid may be all that an extraterrestrial life form needs. The metabolism of such a life form would have to be radically different to the metabolisms of Earth-based life – having to make its food from methane and nitrogen, instead of water and carbon dioxide, but scientists are still optimistic. Titan is up there on the list for ET-hunters.
Moons are awesome, I hope this little essay has helped convince. They are the hottest topic in solar system research, and in astrobiology at the moment, and I hope the love and passion and attention that they receive in the future will continue to contribute to the escalating bank of knowledge and marvelous discoveries. At the very least, there will be another lunar eclipse in a couple of years. Maybe the weather will be good this time.