Merging Saturnian Storms
Only a month and a half into its long approach to Saturn, the Cassini spacecraft captured two storms, each a swirling mass of clouds and gas, in the act of merging. With diameters close to 1000 kilometres, both storms, which appear as spots in the southern hemisphere, were seen moving westward, relative to the rotation of Saturn's interior, for about a month before they merged on 19-20 March 2004.
Merging is one of the distinct features of storms in the giant planet atmospheres. On Earth, storms last for a week or so and usually fade away when they enter the mature phase and can no longer extract energy from their surroundings. On Saturn and the other giant planets, storms last for months, years, or even centuries, and instead of simply fading away, many storms on the giant planets end their lives by merging. How they form is still uncertain.
The series of eight images shown here was taken between 22 February and 22 March 2004; the image scale ranges from 381 kilometres to 300 kilometres per pixel. All images have been processed to enhance visibility. The top four frames, spanning 26 days, are portions of narrow angle camera images that were taken through a filter accepting light in the near-IR region of the spectrum centered at 619 nanometres, and show two spots approaching each other. Both storms are within half a degree of 36 degrees south latitude and sit in an anti-cyclonic shear zone, which means that the flow to the north is westward relative to the flow to the south. Consequently, the northern storm moves westward at a slightly greater rate than the southern one: 11 vs. 6 metres per second, respectively. The storms drift with these currents and engage in a counterclockwise dance before merging with each other.
The bottom four frames are from images taken on 19, 20, 21, and 22 March, respectively, in a region of the spectrum visible to the human eye and illustrate the storms' evolution. Just after the merger, on 20 March, the new feature is elongated in the north-south direction, with bright clouds on either end. Two days later on 22 March, it has settled into a more circular shape and the bright clouds have spread around the circumference to form a halo. Whether the bright clouds are particles of a different composition or particles at a different altitude is uncertain.
The new storm is a few tenths of a degree farther south than either of its progenitors. There, its westward velocity is weaker and it is almost stationary relative to the planet's rotation. Although these particular storms move slowly westward, storms at Saturn's equator move eastward at speeds up to 450 metres per second, which is ~10 times the speed of the Earth's jet streams and ~ three times greater than the equatorial winds on Jupiter. Saturn is the windiest planet in the solar system, which is another mystery of the ringed giant.