[酷图] 卡西尼号最新图片(2004年04月08日)

Merging Saturnian Storms

Only a month and a half into its long approach to Saturn, the Cassini spacecra
ft captured two storms, each a swirling mass of clouds and gas, in the act of
merging. With diameters close to 1000 kilometers (621 miles), both storms, whi
ch appear as spots in the southern hemisphere, were seen moving westward, rela
tive to the rotation of Saturn's interior, for about a month before they merge
d on Mar. 19-20, 2004.

Merging is one of the distinct features of storms in the giant planet atmosphe
res. On Earth, storms last for a week or so and usually fade away when they en
ter 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 planet
s end their lives by merging. How they form is still uncertain.

The series of eight images shown here was taken between Feb. 22 and Mar. 22, 2
004; the image scale ranges from 381 kilometers (237 miles) to 300 kilometers
(186 miles) per pixel. All images have been processed to enhance visibility. T
he top four frames, spanning 26 days, are portions of narrow angle camera imag
es that were taken through a filter accepting light in the near-IR region of t
he spectrum centered at 619 nanometers, and show two spots approaching each ot
her. 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 wes
tward relative to the flow to the south. Consequently, the northern storm move
s westward at a slightly greater rate than the southern one: 11 vs. 6 meters p
er second (25 and 13 miles per hour), respectively. The storms drift with thes
e currents and engage in a counterclockwise dance before merging with each oth
er.

The bottom four frames are from images taken on Mar. 19, 20, 21, and 22, respe
ctively, in a region of the spectrum visible to the human eye and illustrate t
he storms' evolution. Just after the merger, on Mar. 20, the new feature is el
ongated in the north-south direction, with bright clouds on either end. Two da
ys later on Mar. 22, 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 a
ltitude is uncertain.

The new storm is a few tenths of a degree farther south than either of its pro
genitors. There, its westward velocity is weaker and it is almost stationary r
elative to the planet's rotation. Although these particular storms move slowly
westward, storms at Saturn's equator move eastward at speeds up to 450 meters
per second (1000 mph), which is ~10 times the speed of the Earth's jet stream
s and ~ three times greater than the equatorial winds on Jupiter. Saturn is th
e windiest planet in the solar system, which is another mystery of the ringed
giant.

The Cassini-Huygens mission is a cooperative project of NASA, the European Spa
ce Agency and the Italian Space Agency. The Jet Propulsion Laboratory, a divis
ion of the California Institute of Technology in Pasadena, manages the Cassini
-Huygens mission for NASA's Office of Space Science, Washington, D.C. The imag
ing team is based at the Space Science Institute, Boulder, Colorado.

For more information about the Cassini-Huygens mission, visit http://saturn.jp
l.nasa.gov and the Cassini imaging team home page, http://ciclops.org.

Credit: NASA/JPL/Space Science Institute
Released: April 8, 2004 (PIA 05386)