Icy plumes on moons far away

To celebrate my first week at work- a post on Enceladus, my favourite icy moon! My colleagues will know this, because I have spoken of little else this week, apologies...

Enceladus is one of dozens of moons orbiting Saturn, the famous ringed gas giant planet in our solar system. It has an icy crust, and is thought to host a subsurface ocean beneath, possibly with hydrothermal vents on its seafloor at the rock to liquid boundary. Cracks and holes on the icy surface allow the subsurface ocean to escape as plumes of icy droplets shooting out like geysers. I think the reason I'm so fond of this moon is because it is so simple to visualise what is going on and the science behind it, which can be understood in simple terms just by looking at an image of it. In fact, here is one of my favourites below. But how did the cracks get there in the first place?

Plumes of icy droplets escaping the ice crust of Enceladus. Credit: NASA/JPL/Space Science Institute

Everybody knows planets are spherical, but this is not a given for moons. An object's gravity increases with its mass, and an icy moon less than ~400km in diameter won't have enough mass to generate a sufficiently strong gravitational force to overcome the body's elastic resistance and pull itself into a sphere, hence it will be slightly deformed. Rocky moons have a stronger resistive force, so their minimum diameter to be massive enough to be spherical is slightly bigger at ~600km. Tidal forces are a consequence of the inverse square nature of gravity- which just means that gravity gets weaker the further away from an object you go. Specifically, gravity gets weaker with distance squared, so going 3x further away makes your gravity 9x weaker, for example. We see the consequences of tidal forces on Earth, where our Moon exerts a small but notable gravitational force on the Earth, which is stronger on the nearside (since it is closer to the Moon) than the far side. This causes small bulges in our oceans, which can be seen in the tides of the sea! An exaggerated sketch of this phenomenon can be seen below.

Tidal forces on planet Earth caused by the pull of the Moon, as seen in ocean tides.

Similarly, planets can exert tidal forces on moons, particularly if the planets are massive. This effect is particularly pronounced for elliptical orbits. The orbiting moon's tidal bulge will always be directed towards the planet, but the moon spins as it orbits around the planet (shown below). Hence, the tidal bulge moves around the moon's surface, distorting the surface and occasionally causing cracks to form over time. Tidal distortion causes internal friction, causing the interior of the moon to become warm, which is called tidal heating. Tidal heating is also responsible for the molten layers beneath the rocky or icy crusts of large moons, causing active ice volcanoes (cryovolcanoes) as seen on the surface of Enceladus. If the moon’s internal elastic forces to resist deformation are not strong enough, tidal forces will dominate and break up the moon.

Tidal forces exerted by a massive planet on an orbiting moon. The strength of the deformation is weakest at the greatest distance between the bodies in the orbit, as the planet’s gravity weakens with the separation squared. As the moon spins, the deformation moves around the surface, potentially causing cracks, as well as providing tidal heating, a potential energy source for life other than starlight.

Tidal heating is one possible mechanism for generating the heat that is thought to be needed for life, which is why even though there is no sunlight reaching the subsurface ocean of Enceladus due to the crust, it may be possible for life to occur there anyway, as the stretching and compressing of the crust and core could provide an alternative source of heat and energy. Furthermore, hydrothermal vents (discussed in earlier posts here and here) act as underwater volcanoes, potentially feeding microbes with rich nutrients from which to metabolise as well as providing a toasty environment. Such microbes, for example methanogens, exist at the bottom of the sea of Earth. Perhaps they may even exist on Enceladus too, an ocean buzzing with microbes hidden beneath an icy surface.

While the ocean is too far beneath the crust to be accessed directly, we can infer its chemical composition and other properties by flying a probe through one of the plumes, where dredged up ocean droplets spew out from cracks in the crust's surface. This has already been done by the Cassini-Huygens mission (2004-2017), however it could be very interesting to send a more specialised instrument to make further observations of this moon's ocean, which some astronomers argue is the most promising body in the Solar System to find other life. Other space missions (JUICE and Clipper) have in the last two years been sent to probe Jupiter's icy moon Europa, which is another promising candidate, and the telescopes should arrive there within the next few years, and among other things, they will be looking for potential habitable environments and tracers of life on icy moons. Given Earth is currently the only place we know of to contain living organisms, this would be quite the discovery.