SETI bioastro: Fw: Jupiter's Shrinking Red Spot

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Date: Fri Dec 28 2001 - 20:52:54 PST

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Subject: Jupiter's Shrinking Red Spot

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Telescopic observers from the 19th century may not have had the
technological wizardry available to modern-day skywatchers -- but they
apparently had an easier time spotting Jupiter's signature feature,
its Great Red Spot. According to Amy Simon-Miller (NASA/Goddard Space
Fight Center), today this giant cyclonic storm is only about half as
big as it was in the 1880s. Simon-Miller and three colleagues
confirmed the shrinkage during a careful comparison of historical
records and contemporary images from the Voyager, Galileo, and Cassini
spacecraft. She presented their results last month at a meeting of
planetary scientists.

Astronomers have known since the early 1900s that the Great Red Spot's
longitudinal extent has been decreasing. Late in the 19th century the
spot was nearly 35 degrees wide, which corresponds to about 40,000
kilometers, or more than three times Earth's diameter.

By 1979, when Voyagers 1 and 2 swept past, it had shrunk to 21 degrees
(about 25,000 km), yet its latitudinal "height" remained essentially
unchanged, about 12,000 km from top to bottom.

Simon-Miller has discovered that the contraction seems to have picked
up steam since the Voyager visits: at its present rate of shrinkage
(0.19 degree in longitude per year), the spot will become the "Great
Red Circle" by the year 2040. However a perfectly round shape is
unlikely, she explains, because the strong, opposing jet streams that
confine the spot's northern and southern boundaries will always
distort it into an oval.

No one knows why the not-so-Great Red Spot has shrunk -- or, for that
matter, why its color intensifies and fades over time. One clue is
that the winds around its circumference are whirling 70 percent faster
now (about 700 km per hour) than they were in the Voyager era. Some
historical observations suggest that the Red Spot grows and shrinks in
a decades-long sequence. "I'm not sure the behavior is really
cyclical," Simon-Miller comments, "but I certainly would not be
surprised in the least if this shrinking trend slowed or reversed."

One possible explanation is that deep-seated bursts of
thunderstorm-like convection periodically energize the overlying cloud
layers, causing the spot to bloat in size, then gradually contract as
the turbulence subsides. "All of the weather on Jupiter seems to have
sporadic increases in activity," she notes, "so whatever feeds the
Great Red Spot likely will too."


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