Saturn’s rings in low-angled light

As a result of the Cassini probe observations, the rings of the second biggest planet in the solar system have once again surprised scientists.

Saturn, a day and a half after the exact moment of its equinox on 11 August 2009. The rings are therefore lit edge-on, in low-angled light. A photograph taken by the American-European Cassini. Credit: NASA/JPL/Space Science Institute

On 11 August 2009, Saturn was in equinox which means that the Sun was directly above its equator. Consequently, the gas giant’s rings were no longer illuminated from “above” or “below”, but head on. This is a golden opportunity for scientists as several weeks before and after this date, 11 August, the rings are lit by a low-angled light.

A huge surprise
Imagine a superb wooden table that, at first glance, appears to be perfectly flat. But, when equipped with a torch and using a low-angled light, you can see all the irregularities of the wood (lumps, hollows) highlighted by the shadowing effects.
This is exactly what is happening at the moment with Saturn’s rings and, moreover, planetologists have the Cassini (a programme associating NASA, the European Space Agency and the Italian Space Agency) probe on site.
Made up of blocks of rock and ice, the rings not only cover a large area (up to 140,000 km from the centre of the planet for the main ones - those visible through amateur telescopes), but are also very thin, ten or so meters thick. However, this thickness is greatly increased in places. In fact, thanks to the shadows which result from the Sun’s low-angled lighting, recent photographs from Cassini have, more than ever before, revealed the ruffles provoked by the small moons which move about within the rings. Thus, the 8 km diameter Daphnis creates waves extending 1.5 km with peaks going up to 4 km high due to the gravitational pull that its very presence causes (images below).

On the left, the Moon Daphnis (arrow), its shadow (vertical black line above) and the waves it provokes in the rings (wavy white line which projects a shadow). On the right, other much bigger undulations from Daphnis. Their shadow (arrow) indicates that the blocks which make up the rings form peaks here which are up to 4 km high above the plane of the rings. Credit: NASA/JPL/Space Science Institute

Even smaller moons, several hundreds of meters wide, incite the formation of “lumps” which are obviously not as high, but are, nevertheless, 200 m above the plane of the rings and 130 km in length.

This “lump”, caused by a small 400 m (estimation) diameter moon, is 130 km long and is more than 200 m above the plane of the rings. Credit: NASA/JPL/Space Science Institute

It is true that these two phenomena were already known, but the special lighting of the equinox has, this time, made it possible to accurately measure their size and the surprise was, therefore, the fact that they were much bigger than was ever suspected.

Published on 23 September