JUPITER鈥橲 Great Red Spot resembles a lopsided wedding cake, say astronomers
who have worked out the most detailed models so far of the planet鈥檚
atmosphere.
Infrared spectra from the Galileo spacecraft allowed Kevin Baines, a
planetary scientist at NASA鈥檚 Jet Propulsion Laboratory in California, and his
colleagues to peer into the giant planet鈥檚 complex atmospheric stew. They found
that the Great Red Spot, a hurricane-like storm system twice the diameter of
Earth and at least 300 years old, is capped by a tilted tower of ammonia clouds
about 20 kilometres tall.
As he will tell a meeting of the American Astronomical Society in Padua,
Italy, in October, these clouds form a raised oval about two-thirds the diameter
of the base. 鈥淚t looks like a wedding cake,鈥� Baines says. Spirals of clouds
extend out across the top of the upper tier, and deeper layers can be glimpsed
through clearings.
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Fred Taylor, a physicist at Oxford University, thinks the spiral helps
explain how the Red Spot works. 鈥淚t would have a relatively small region below
it of rapidly rising gas rich in water, ammonia and other quite exotic things,鈥�
he says. 鈥淎t a certain height, they spread out, like the cap of a terrestrial
丑耻谤谤颈肠补苍别.鈥�
Baines thinks the upwelling acts like a rock in the middle of a stream. 鈥淵ou
get a bow wave where the east side gets lifted up,鈥� he says. 鈥淭hen the clouds
descend as the core rotates.鈥�
Taylor, Baines and others have pieced together tentative models of the upper
layers of Jupiter鈥檚 atmosphere鈥攆rom a high, thin hydrocarbon haze, through
layers of ammonia and hydrogen sulphide clouds, down to clouds of water vapour
at six times the pressure of Earth鈥檚 atmosphere.
The Cassini probe is expected to transmit even sharper images when it passes
Jupiter next year. These may help answer questions such as what makes the spot
red, and why it has lasted so long. 鈥淭heir work is one piece of the puzzle,鈥�
says Andrew Ingersoll, a planetary scientist at the California Institute of
Technology, 鈥渂ut on Jupiter there are many pieces.鈥�