Titan makes a fast getaway

ONE of the remarkable advances in astronomical research is the amazing precision with which some measurements can be made.

For example, astronomers can measure the relative speed at which a star is moving towards or away from us to a precision of about one metre per second, and determine the distance to the Moon to within about 2cm.

Now, astronomers have found that Saturn’s largest moon, Titan, is moving away from the famous ringed planet at a speed of about 11cm a year. It is about the rate at which human hair grows!

These measurements have been made using information returned by the Cassini spacecraft, which orbited Saturn for many years.

Its images showing the position of Titan in relation to background stars, and the gravitational effect of Titan on the spacecraft’s motion, gave researchers the data they needed.

However, the fact that astronomers have been able to make such a precise measurement is only part of the story. Earlier expectations were that the rate should be about 1mm per year − about a hundred times less.

It may seem surprising that the size of the orbit of a moon should be changing at all. However, we have known for some time that our own Moon was formed much closer to Earth than it is now.

Its recession from us is caused by tidal effects. The Moon is the main cause of the Earth’s tides, and this also imparts energy to the Moon, raising its orbit.

The rate of recession of our Moon is about 3.8cm per year, equal to about 3.8m each century. These accurate measurements of the distance to the Moon are made by using special laser reflectors left on the Moon’s surface by the astronauts of the Apollo 11, 14 and 15 missions.

One rather sad result of this is that in the distant future, there will be a time when there are no more total solar eclipses, because the Moon will be too far away from us to completely cover the Sun as seen from Earth − although that will not happen until about 600 million years from now.

Getting back to the Titan story, the general expectation that Titan’s outward migration would be only about 1mm each year was based on Saturn’s composition.

Saturn is a ‘gas giant’ planet, and overall it is even less dense than water.

Placed in a large enough sea, the entire planet Saturn would float! Saturn’s composition would normally cause Titan to move out at the expected minuscule rate, but there is another factor that comes into play. It is one that has been gaining increasing attention in relatively recent times, and it is called resonance locking.

Resonance is a fascinating and detailed subject on its own. Most people would have experienced resonance in some form, or made use of it. If you have ever pushed a child on a swing, you will know that if you apply each push at exactly the right moment, the amplitude of the swing will build up well.

The planet-moon resonance locking is far more complex, but the general idea is that the planet has internal oscillations, or vibrations, that become linked to the motion of the moon in question.

As part of investigating the background to this research, I was delighted to read a paper written four years ago on the topic of resonance locking.

Jim Fuller of CalTech in the US, one of the authors of the current paper, was lead author on a paper on the subject in 2016, foreshadowing that the outward movement of Titan would be found to be larger than the original expectations. It’s great science when a theory is tested by observations that support the idea!

Saturn can currently be seen in our Tasmanian night sky, to the east by mid-evening, below and to the right of the much brighter planet Jupiter.

Even a small telescope will show Titan, appearing as a starlike point of light near the planet.

Martin George is manager of the Launceston Planetarium – QVMAG