Australian Sky & Telescope - April 2016__

http://www.skyandtelescope.com.au 19

GET TO

KNOW

JUPITER

For more

informa-

tion about

the study

of Jupiter’s

cloud features

and the Great

Red Spot in

particular,

see The Giant

Planet Jupiter

(Cambridge

University

Press, 1995),

authored by

John Rogers

of the British

Astronomical

Association.

First, the storm has gotten smaller not just in its

longitudinal width but also in its north-south extent.

Inaddition,itoccasionallymovesslightlynorthward

or southward,affecting which wind jet it deflects

more. HST’s2014 measurements show the spot

locatednearitsnormalcentrallatitude,butbothits

north and south edges contracted centreward as the

entire system shrank.

The westward jet still experiences significant

deflection around the GRS’s northern edge, though

perhapsslightlylessthanpreviously.However,the

eastwardjetisnowbarelydeflectedsouthwardbythe

GRS.Thismeansthatanyentrainededdiesinthis

regionarenotinteractingcloselyenoughtobecaught

inthespot’sflow.Wenowstronglysuspectthatthese

jetinteractionsareaffectingnotonlythecolourbut

alsopossiblythesizeoftheGreatRedSpot.Intime,

ourcomputermodellingoftheGRS’sinteractionwith

the nearby wind jets should help us understand the

energy exchange between them.

The future

Sowhatdoesthefutureholdforthemostfamous

stormintheSolarSystem?That’sdifficulttopredict,

as sudden outbreaks could always alter the energy

balance and wind flow near and within the GRS and

changeourviewcompletely.Atmosphericspecialists

describe the shape of Jupiter’s anticyclonic storms

(high-pressure systems like the GRS) by the ratio of

theirlengthtowidth,andit’snotunusualforthiskey

aspect ratio to decrease in such storms, particularly

right after they form.

Forexample,thewhiteovalsjustsouthoftheGRS

firstappearedinthelate1930snotasdiscretespotsbut

rather as high-altitude clouds and haze that suddenly

ALTERED BY

UPHEAVAL

Compare how Jupiter

looked in 2010 (left)

— with its dark

South Equatorial Belt

‘missing’ and its GRS

intensely coloured

— with the view two

years later, after the

high-altitude clouds

obscuring the SEB had

dissipated and the GRS

had become paler.

inundated their latitude band, probably related to

a large convective event. The brightened zone then

coalesced into three distinct, oblong white storms,

which observers designated BC, DE and FA. By the

Voyager flybys in 1979, each had an aspect ratio of

about 2. They continued to shrink until merging into

a single oval in 2000, with the new, combined oval BA

having an aspect ratio of 1.25. In 2005, it turned red —

earning the moniker ‘Red Spot Jr.’ — and by 2015 its

aspect ratio was 1.3. Meanwhile, the GRS’s aspect ratio

has changed from about 3 during the Voyager flybys in

1979 to the 1.4 it has now.

Computer models suggest that Jupiter’s anticyclonic

vortices should exhibit preferred aspect ratios, but

those depend somewhat on interactions with adjacent

wind jets, especially for a large storm that can deflect

the jets. So the GRS might eventually reach a shape

that is ‘just right,’ allowing it to stabilise.

Studies of its internal dynamics will help us to

model its fate, so we continue to make yearly Hubble

observations. And amateurs’ observations are critical

for monitoring the GRS. Their regular reports help

us to fill in the gaps between studies with HST and

professional telescopes. They can also alert us to new

happenings that require rapid follow-up.

All things considered, we expect that the Great Red

Spot will continue to shrink for some time to come.

We don’t expect it to disappear completely — but for

now what happens in the years ahead is anyone’s

guess. ✦

Amy Simon, an atmospheric dynamicist who works at

NASA’s Goddard Space Flight Center, leads the observing

team for HST’s Outer Planet Atmospheres Legacy

monitoring program.

DAMIAN PEACH

September 2010 February 2012

NTrZ NTB

NEB

EZ

SEB

STrZ

STB

GRS

“Red Spot Jr.”