Metal Detectors and Alien Oceans

I remember watching footage from the Voyager space probes when I was a child. These were two spacecraft sent to study the outer solar system. They sent back incredible images of Jupiter, Saturn, and Neptune.  

When the Voyager missions were planned, the interest was in the planets of the outer solar system, but when the images came back, the moons caught people’s attention. These turned out to be a fantastic array of hugely different worlds, giving scientists many years' worth of data to study. 

The images of the moon's surfaces gave clues about their structure and what they could be made from. Astronomers have spent hundreds of years studying our moon (above), which is covered in mountains, craters and flat plains called 'mare', which show that it has had a violent past. Some of the moons of the outer solar system are like our moon, see Mimas below, but others caused a great deal of excitement.

Io (below), for example, had a multicoloured surface of reds and oranges. Scattered across the surface were dark, circular structures. Craters perhaps? No, what they were was much more exciting. Voyager flew past Io taking photos, and one of these caught a scientist's eye. In the picture, there was a strange bulge behind Io. None of the known moons was in the right place for this to be one of those. Was it a new moon? No, when she looked closer, she concluded that it was the plume of a volcanic eruption, the first time a volcano had been seen anywhere other than earth. Not only was Io not just a lump of rock, but it was active and changing. 

Titan has a dense, opaque atmosphere, hydrocarbon lakes and methane rain; it is the only other place in the solar system with liquid rain.

Europa (below) and Enceladus were icy, with tantalising signs that something was moving this ice around. What were these hints? Well, the solar system can be a very violent place. It was far more damaging in the past compared to today. Our moon shows evidence for this, with the enormous number of craters covering its surface.

Some of the moons are strangely smooth. Have they somehow missed any collisions? The explanation is that the moons have liquid oceans underneath a thick crust of ice. The ocean moves around, cracking the ice, allowing the liquid to flow towards the surface and then freeze. This resurfacing covers the evidence of collisions but, more excitingly, provides proof of oceans somewhere other than Earth.

The puzzle was, why wasn't the water on these small objects frozen solid? How were some of this water hot enough to be liquid so far from the sun? 

A surprising discovery had been made on earth a couple of years before the Voyager missions. A submersible had gone down to the deepest part of the ocean and discovered a rocky ridge with volcanic activity. This ridge was along the boundary between two tectonic plates that make up the Earth's crust. The heat from the mantle heats up the water above the fault line, forcing it out of vents. The surprise was that a whole life ecosystem was around these vents, which were incredibly hot, under pressure and spewing toxic chemicals. That life could survive in these extreme conditions, in absolute darkness, was astonishing. It made scientists rethink where to look for life in the solar system. 

Could heat from the inside of the moons be keeping the oceans liquid? But where was this heat coming from? The Earth's mantle is hot and liquid because radioactive elements in the core let out heat as they decay. Much larger than these moons, Mars has cooled to the point where it is now solid. Earth is still liquid inside and volcanically active because it is more extensive and cools more slowly than Mars. Why would these tiny moons hold on to their heat when Mars hadn't? The answer is tides. 

Tides on earth are caused by gravity. As the moon orbits, the earth's gravity attracts the nearby ocean, causing a bulge. This is high tide on this part of the earth. You may not realise that the earth's crust is also pulled towards the moon, lifting it by about 30cm daily. Earth is also pulling on the moon, deforming it too.

Saturn and Jupiter are much larger than the earth, so they have much higher gravity. As their moons orbit, they get stretched and squashed by tidal forces, which warm them up a little. This heat could be keeping the oceans liquid under a thick layer of ice (and causing the volcanos on Io). 

Scientists like to have evidence; the more, the better. Liquid oceans were one explanation for what they saw on the surface of these moons, but could they back it up? Could they find out what they were made from even? How can you determine what oceans are made from under thick layers of ice millions of miles away? It's not like you can just pop down there with a bucket. 

The answer is magnetism. Yes, magnetism! They detected the oceans in the same way that metal detectors find coins underground.

The metal detector uses some science you will learn in school. Electricity and magnetism are linked; you often have the other when you have one. This is what happens-

• the coil of the detector is an electromagnet, which produces a moving magnetic field
• moving magnetic fields produce (induce) a voltage in the conductor, which produces currents. These currents swirl like water going around a rock in a metallic object. Currents like this are called eddy currents.
• Electric currents produce magnetic fields. The metal detector has a sound then it picks this up.

The scientists used this metal detector science to determine if the moons had oceans. Like Earth, the outer planets have magnetic fields on a much larger scale. As the moons orbit, they move through their planet's magnetic field. This would induce a voltage in an ocean (salty water is a conductor), and the voltage produces eddy currents in the surface of the oceans, which induce a magnetic field.

The scientists collected information about the magnetic fields of the planets and moons, predicting what effects they would see if eddy currents were also producing a magnetic field. This is called modelling. They then looked to see if the magnetometer data from Voyager matched their predictions. It did. There are salty oceans beneath the ice on Europa, Enceladus, and Calisto, among others.

We are now so sure that there are salty oceans on some of the moons in the solar system that a mission is being planned to study one of them. The plan is to land instruments on the surface of Europa and drill through the icy crust. They then hope to be able to send a submarine down to study the ocean. What will we find, I wonder? Perhaps there will be strange ecosystems around hydrothermal vents? 

To discover more about electromagnetic induction, click here.