Australian Sky & Telescope - May 2018

http://www.skyandtelescope.com.au 59

NASA / AMY SIMON (6)

gotten redder and darker over the

past decade. (Historically, the spot’s

colour seems to intensify when its

westward drift accelerates.) The cause

of the darkening isn’t clear, but Simon

suspects it has to do with conservation

of angular momentum. As the spot

shrinks, its internal circulation should

speed up — but it hasn’t. Instead, the

swirling gas might be forced to go to

higher altitudes (and there’s evidence

this is occurring), where lower pressures

and temperatures might be driving

‘colourful’ chemical reactions.

Theactualchemicalcompound

responsible for the spot’s ruddy hue

is still unknown. Some candidates,

likeredphosphorousderivedfrom

phosphine (PH 3 ), aren’t wholly

consistentwiththechemicalmakeup

oftheJovianatmosphere.In2016

Robert Carlson (Jet Propulsion

Laboratory) and others suggested that

thecolourresultsfromthereactionof

ammonia (NH 3 )andacetylene(C 2 H 2 ).

Recently a team led by Lawrence

Sromovsky (University of Wisconsin

at Madison) argued that this same

compound could explain the hues of

many other Jovian cloud features.

Timing Is everything

Chemistryaside,theRedSpot’snow-

deepercolourmakesthisagreattimeto

trytospotit.Butdon’tjustdashoutside

withyourscope—alittlepreparationis

called for.

Mostcritically,you’llneedtoknow

whentheGRSisfrontandcentredon

Jupiter’s rapidly spinning globe. Look at

the list of times on page 57 to find when

the spot crosses the planet’s central

meridian (the north-south line through

its poles). Don’t forget to convert those

values from Universal Time to your

local time zone. Plan to look within an

hour of a listed time — otherwise the

spot will be close to the planet’s limb

and difficult to identify.

Ideally, chose a meridian-crossing

time that occurs when Jupiter is high

in your sky. You’ll always see less detail

when the planet is close to the horizon,

both because of atmospheric turbulence

alongyourlineofsightandalsodueto

thethickair’sprism-likedispersionof

colour in Jupiter’s disk.

Next,usetheRedSpot’scolourto

youradvantage.Ablueorgreenfilter

will make it stand out more readily

from the surrounding clouds.

Finally, make sure your telescope

is big enough. AS&T’s Sean Wa lker

suggests that you’ll need at least

100mmofaperturetospotthe

GRSreliably.DavidArditti(British

Astronomical Association) adds, “For

moreexperiencedobserversIhavelittle

doubtthata75-mmrefractorwouldbe

enough,undertypicalconditions,with

amagnificationof80×.”

Tony Flanders notes that the task

will be much easier if the atmosphere is

stable.“Ifindthevariationinvisibility

fromnighttonightduetoseeing

swampsthevariationsdependingon

the GRS’s appearance,” he cautions.

“Onahalf-decentnight,itshouldbe

prominentthroughagood100-mm

refractoror15-cmreflector.”

In fact, if you have two scopes,

try setting them up side by side at

comparable magnifications to see

which one most easily coaxes the

GRSintoview.Andifthespotitself

isn’tapparent,lookforthe‘RedSpot

Hollow,’abrightindentationthatthe

spot makes in the broad, dark South

Equatorial Belt to its immediate north.

SNatural-colour Hubble Space Telescope images show how the Great Red Spot’s hue has darkened and deepened since 2014. The spot creates

pronounced turbulence in the westward low of the South Equatorial Belt (lower right in each image).

SThese violet-iltered Hubble images

accentuate the dramatic change in the

GRS’s size and interior structure over a

23-year period.

July 1994

July 2017

April 2014

Red

Spot

Hollow

January 2015 February 2016 April 2017

“Use the Red Spot’s colour to your advantage.

A blue or green filter will make it stand out.”