Australian Sky & Telescope - April 2016__

16 AUSTRALIAN SKY & TELESCOPE APRIL 2016

G

enerations of amateur and professional

astronomers have viewed and studied Jupiter’s

Great Red Spot (GRS), one of the giant planet’s

most easily recognised features. It joins Saturn’s rings

andtheMartianpolarcapsas‘mustsee’featuresfor

SolarSystementhusiasts.ButtheGRSisaninconstant

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.

not-so-great

Jupiter’s

Red Spot

After shrinking in size for decades, is Jupiter’s iconic Great Red Spot facing an identity crisis?

The past

Historical records reveal that observers regularly

measured the GRS from the1870s onward. It became

a popular target for study starting about 1878, in part

becauseitwasextremelyredandprominentatthetime.

The earliest accurate size measurements employed

transit timings of its red edges, often with finely scored

micrometers in the eyepiece. These consistently showed

thattheGRShadaneast-westlengthofabout34° in

longitude. On Earth, this longitudinal extent would be

almost the size of Australia. But on far larger Jupiter,

this expanse corresponds to about 40,000 km, three

times Earth’s diameter.

GIGANTIC STORMArguably no planetary feature in

the Solar System is better known than Jupiter’s Great Red

Spot,orGRS,seenhereinacontrast-enhancedcomposite

of Voyager images from 1 979. Note the vast regions of

disturbed cloud flow to its north and south, along with the

Earth-size white oval (designated BC) gliding nearby.

NASA / JPL / BJÖRN JÓNSSON

AMY A. SIMON

Shrinking Storm