Saturn’s hexagon

A night-time view of Saturn's north pole showing the strange hexagon-shaped structure in its uppermost clouds. The image was taken by the visual and infrared mapping spectrometer on board NASA's Cassini orbiter.

IN 1980 and 1981, NASA’s Voyager 1 and 2 space probes passed for the first time over the planet Saturn, which is located 1,500 million kilometres from the sun. The probes observed a strange, hexagon-shaped structure in the planet’s uppermost clouds surrounding its north pole. Nothing similar with such a regular geometry had ever been seen on any planet in the solar system. The hexagon remained virtually static, without moving relative to the planet’s overall rotation. Saturn is the only planet in the solar system whose rotation time remains unknown. Interestingly, the images captured by the probes showed that the clouds were moving rapidly inside the hexagon in an enclosed jet stream and were being dragged by winds travelling at over 400 km/h.

Thirty years later, the equivalent of one Saturn year, a research team of the Planetary Sciences Group of the University of the Basque Country, Leioa, Spain, has been able to study and measure the phenomenon and also establish its rotation period. The researchers determined that its movement remains extremely stable and that the jet stream inside it remains unchanged. For this study, the researchers used images taken from the earth between 2008 and 2014 and the very high resolution images obtained by NASA’s Cassini spacecraft, which has been orbiting Saturn since 2004. Owing to Saturn’s tilt of approximately 27°, its polar atmosphere undergoes intense seasonable variations, with polar nights lasting over seven years followed by 23 years of variable illumination. However, the seasonal variations do not affect the hexagon and its jet stream at all. So, both are part of an extensive wave, deeply rooted in Saturn’s atmosphere. The researchers suggest that the hexagon and its stream are the manifestation of a “Rossby wave” similar to those in the jet stream in the mid-latitudes of the earth, which meanders from west to east and is associated with areas of low pressure and anticyclones.

On Saturn, a hydrogen gas planet 10 times the size of the earth, cold in its upper clouds, without a solid surface, and with an atmosphere as deep as that of an ocean, “the hexagonal wavy motion of the jet stream is expected to be propagated vertically and reveal to us aspects of the planet’s hidden atmosphere”, said Agustín Sánchez-Lavega, the research group’s head. “The movement of the hexagon could therefore be linked to the depths of Saturn, and the rotation period of this structure, which, as we have been able to ascertain, is 10 hours, 39 minutes and 23 seconds, could be that of the planet itself.” The work was published in a recent issue of the journal Geophysical Research Letters.