Sunrise in the north of Titan

The Cassini probe captured the reflection of the celestial body of the day in one of the many lakes in the northern hemisphere of Saturn’s biggest moon.

 

Titan’s crescent (in infrared light) taken by the Cassini probe. The bright spot at the top is caused by the reflection of the Sun in one of the lakes in Saturn’s moon’s northern hemisphere. Credit: NASA/JPL/Space Science Institute

Genuine gem of the solar system, this giant with its aesthetic rings takes a little more than 29 years to orbit the Sun. Saturn is at an angle in relation to its orbit plane. Just like the Earth, this planet therefore experiences a seasonal cycle where winter and summer periods alternatively follow one another in their northern and southern hemispheres.

A 15 year winter!
Orbiting in Saturn’s equatorial plane (and slightly angled in relation to it), Titan, the biggest of the giant’s moons, undergoes the same seasonal rhythm. As a result, its highest northern and southern latitudes are plunged into obscurity for several years as opposed to the few months of the terrestrial poles! With the Saturnine equinox which occurred in August 2009, the north of Titan is in the process of moving from winter to summer while the south is, logically, slowly succumbing to the rigours of winter and obscurity. A winter that lasts 15 years!
It was in July 2009, just before the equinox, that the Cassini probe captured the reflection of the Sun in the infrared range as it glinted off one of the lakes in Titan’s northern hemisphere where spring is coming (see the photograph at the beginning of this article). The image was published and explained yesterday by NASA’s Jet Propulsion Laboratory (JPL) and is to be presented today during a meeting of the American Geophysical Union in San Francisco.
The start of the summer period in the north of this moon is of great interest to astronomers since it is the first time that a probe has been in the area during the season “changeover”. In this distant world, summer marks the rainy season... But, do not go imagining showers of water: rain here is liquid methane (a hydrocarbon) at a temperature of -180°C; methane which is to be found in the impressive lakes, far more plentiful in the north than the south. Why such a difference? The reason would appear to be the eccentricity of Saturn’s orbit as it is slightly further away from the Sun during the “summers” in the north of Titan, thus lessening the phenomenon of evaporation during the rainy season when the lakes are filled, putting the south at a disadvantage.
But such orbit eccentricities vary over time and a few dozen thousands of years ago, the situation was probably the other way around with the south having a greater supply of lakes than the north! These climate changes caused by orbital parameter variations go by the name of Milankovitch cycles (Serbian astronomer and climatologist, 1879-1958). Applied to the Earth, these cycles could explain the alternation of long glacial and temperate climate periods.

Published on 18 December 2009