Cassini minded the gap
IT would be great to travel back in time more than three centuries to meet the great astronomer Giovanni Cassini, also known as JeanDominique Cassini.
I’d love to tell him that the spacecraft named after him orbited Saturn for more than a decade.
And that even after its demise – it was deliberately crashed into Saturn in September – astronomers still have a great deal of analysis to perform on its observations, with some surprising results near the end of the mission.
Cassini, the astronomer, was a keen observer of Saturn. In 1675, he was the first to notice that there was a gap in what was at first thought to be a single ring.
This is the celebrated Cassini Division in the rings, which appears to be a range of orbital distances that separates two rings but which we now know is not devoid of material.
It is now becoming quite clear that the observations that the spacecraft made during the final few months of its mission are reaping amazing scientific rewards, especially the information that it obtained during its daring dashes between the planet and its famous ring system.
When we observe Saturn, even a small telescope will show a clear separation between the planet and its ring system. Cassini flew through this gap this year, and made measurement that showed this region is not empty.
This information was gathered using a device called a mass spectrometer. Such instruments are a very important part of scientific analysis, as they can be used to determine the types of atoms and molecules present in a sample.
Cassini’s mass spectrometer, a 9kg device mounted on the spacecraft, had also been used to make measurements of the upper atmosphere of Titan, the largest of Saturn’s moons.
Cassini’s recent measurements as it flew between the planet and its rings have scientists puzzled. Although we can’t be sure exactly what substances were detected, one of them has a mass of 16 atomic mass units and is likely to be methane, which is composed of one carbon atom plus four hydrogen atoms. This certainly seems to fit: a methane molecule has 16 atomic mass units. This is less massive than water, which is made up of two atoms of hydrogen and one of oxygen (with the famous formula H2O), for a total mass of 18 units.
But why should methane be present there? We certainly did not expect methane to be escaping from Saturn itself, nor did we expect it to be coming from the rings, which are made mostly of water ice.
In addition, we did expect some water to be present in this gap, because relatively simply physics dictates that material should be falling in toward the planet from the innermost part of the rings. However, there was much less water than expected in the samples.
Another mystery has been the detection of a substance having 28 mass units, which is rather likely to be carbon monoxide.
In all, this set of observations will need to be considered more deeply in context of what we know, or thought we knew, about Saturn.
Saturn’s beautiful rings have been admired for centuries, but they have also posed a puzzle. How did they form and how old are they?
We still don’t know the answer for sure but it is hoped that Cassini’s observations, when analysed further, may help us understand the history of this fascinating system.
Measuring the mass of the rings has been important in achieving this aim, and Cassini’s information now seems to suggest that they may be less massive than previously thought. We shall have to wait and see what information further analysis produces.
However, there is no doubt that the rings are made up of a huge number of individual pieces, mostly made up of large and small pieces of water ice. In addition, there are very many gaps in the system where there is much less material. Indeed, the rings look rather like the surface of a vinyl LP record, although with far less regular grooves.
Next week: more about Saturn and its famous rings.