Equinox on Saturn

About every 15 Earth years, Saturn’s experiences an equinox, much like Earth’s equinox. On Earth it occurs twice in Earth’s orbit—two times every Earth year—when the Sun lies in the Earth’s equatorial plane. As a result, the duration of day and night are approximately equal, which is also the origin of the term equinox.

Saturn also experiences this twice in its orbit, but its orbit lasts about 30 Earth years, meaning that the equinox takes place every 15 Earth years. Plus, this event is even more special on Saturn. Saturn has a very famous and widely recognized set of rings. These rings lie in Saturn’s equatorial plane, and therefore, when Saturn undergoes an equinox, the Sun also lies in the plane of the rings. Since the rings are very thin (at least compared to the size of Saturn), during the equinox the shadow from the rings disappears. The rings themselves also are barely visible because only the edges catch the light of the Sun.

The Cassini Spacecraft was present to capture photos from the most recent equinox last August. The below photograph was just recently released by the Jet Propulsion Laboratory, captured on July 18, 2009, just a few weeks before the equinox. The barely visible rings cast a thin shadow on the body of Saturn, creating a stunning view. In fact, the rings captured in the original images from Cassini were so dim that it was necessary to brighten them by a factor of 9.5 relative to the planet in order to produce the result (I’d be very interested to see the picture where the rings are not brightened).

The picture below is from just after equinox, when the shadow from the rings on Saturn is barely visible. Again, the brightness of the rings relative to the planet had to be increased in order to generate the image.

This equinox was also important in allowing Cassini to gather important data about the structure of the rings. When the rings are at a large angle to the Sun, small bumps and features are very hard to detect. However, when the Sun’s rays are parallel to the plain of the rings, the bumps and features create large shadows which are much easier to detect. And the results from this work were surprising. Some areas of Saturn’s rings ripple up and down, forming vertical formations about 800 km high. This phenomenon has not been explained yet. Meanwhile in some other areas, the particles in the ring are affected by the gravity of moons of Saturn, towering above the plain as high as 4 km. So while the rings may appear simple and peaceful in the images above, they have some very complex and strange features.