Sky and Telescope - March 2016 - 18

Jupiter's Shrinking Storm

The Not-So-Great

Red Spot

After shrinking in size for decades, is Jupiter's iconic Great Red Spot facing an identity crisis?
NASA / JPL / BJÖRN JÓNSSON

Amy A. Simon
Generations of amateur and professional
astronomers have viewed and studied Jupiter's Great Red Spot (GRS), one of the giant
planet's most easily recognized features. It joins Saturn's
rings and the Martian polar caps as "must-see" features
for solar system enthusiasts. But the GRS is an inconstant icon. In fact, in recent years observers have become
increasingly alarmed about how small (relatively speaking) the spot has become. Visually it's far less impressive
than it was two or three decades ago.
So planetary scientists have begun to ask whether
this huge cloud system is really shrinking and, if so,
might it someday disappear completely? The answer to
the first question is unequivocally "yes," but the second
one remains a mystery. Given that Jupiter has other
small, enduring, red-hued ovals, perhaps the real question should be whether the GRS no longer qualifies as a
"great" red spot.
Let's examine the history of this iconic storm and
what we do - and don't - know about its evolution.
18

March 2016 sky & telescope

GIGANTIC STORM Arguably no planetary feature in the
solar system is better known than Jupiter's Great Red Spot,
or GRS, seen here in a contrast-enhanced composite of
Voyager images from 1979. Note the vast regions of disturbed
cloud flow to its north and south, along with the Earth-size
white oval (designated BC) gliding nearby.

The Past
Historical records reveal that observers regularly
measured the GRS from the 1870s onward. It became
a popular target for study starting about 1878, in part
because it was extremely red and prominent at the time.
The earliest accurate size measurements employed
transit timings of its red edges, often with finely scored
micrometers in the eyepiece. These consistently showed
that the GRS had an east-west length of about 34° in
longitude. On Earth, this longitudinal extent would be
something not quite the size of Australia. But on far
larger Jupiter, this expanse corresponds to about 40,000
km (25,000 miles), three times Earth's diameter.



Sky and Telescope - March 2016

Table of Contents for the Digital Edition of Sky and Telescope - March 2016

Contents
Sky and Telescope - March 2016 - Cover1
Sky and Telescope - March 2016 - Cover2
Sky and Telescope - March 2016 - 1
Sky and Telescope - March 2016 - Contents
Sky and Telescope - March 2016 - 3
Sky and Telescope - March 2016 - A
Sky and Telescope - March 2016 - B
Sky and Telescope - March 2016 - 4
Sky and Telescope - March 2016 - 5
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Sky and Telescope - March 2016 - Cover3
Sky and Telescope - March 2016 - Cover4
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