8/6/2019 Dissecting Saturn's Big Storm

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Jupiter'sGreatRedSpothasalwaysbeenthegranddaddyofstormsinoursolar

system.Butforthepastsixmonths,themoreexcitingactionforplanetaryobservershasbeenastormragingonSaturn.

BackonDecember5th,instrumentsaboardtheCassinispacecraftpickedupradio

andplasma-wavesignalsfromathunderstormbrewingintheplanet'snorthernhemisphere.Onlythreedayslater,amateurSadeghGhomizadehinIranspottedanewwhiteatmosphericdisturbanceontheplanet.Itrapidlygrewinaneast-west

directionfromitsvertexat41°northlatitude;withinamonthitwasspanning100°

oflongitude.

Sincethenamateurandprofessionalscopeshavebeentrackingtheevolutionofone

ofthelargeststormseverreportedonSaturn.Amateur-stackedvideoimagesand

high-resolutionpicturesfromCassini,takenasrecentlyasMay15th,showwhite

cloudymaterialwrappedaroundthenorthernhemisphereandspanning30&degof

latitude.

StormsofthismagnitudeseemtooccuronceeverySaturnianyear(29.5Earthyears).Duringtheseevents,gasesfromdeepinthetroposphereboilupintothe

lowerstratosphere,causinglargetemperaturechangesthatdisruptthesubtle

seasonalpatternsontheplanetandmodifythechemistryoftheupperatmosphere.

Suchspectacularsightshavebeenobservedonlyfiveprevioustimessince1876.Untilnowtheycouldbestudiedonlyinreflectedsunlight.Thistime,however,not

onlydowehaveaspacecraftonthescenebutastronomersonthegroundcanprobeintotheatmosphereatinfraredwavelengths.DatafromtheCompositeInfrared

Spectrometer(CIRS)onCassini,andthermalimagingfromtheVeryLargeTelescope

(VLT)atChile,havepresentedateamofscientistsledbyLeighFletcher(OxfordUniversity)anunprecedentedopportunitytoquantifythechangesinSaturn's

stormyzones.

TwospectroscopicreadingsbyCassini—thefirsttakenlastOctober22nd

(wellbeforethestormbegan)andthesecondonJanuary19th(45daysafterits

onset)-showa7Kto9Kdropintemperature.Thermalimagingshowsthatthe

cloudupheavalcausesadarkenedcoldspotoverthe3,000-mile-widevortexand

warmsareasaroundtheperiphery.Overthestorm'shead,theairmassappears

streakedwithwarmzones,dubbedstratosphericbeacons.

Ascloudscontinuedtospewupwardintothestratosphere,atmosphericlayerswerebeingdisplacedbyabout20plusorminus5kmovertheheartofthestorm,and10

plusorminus4kmatitsoutermargins.Thesedisplacementschurnedthegasesintheplanet'satmosphereviolently,causing,forinstance,adepletionofacetylene,a

tracerforstratosphericmotion,overtheheartofthestorm.Phosphineshoweda

suddenenhancementof20%inthestormyzones.Fletcher'steamconcludes,"Thethermalcontrastsresultingfromtheplanetary-scaledisturbancehavea

considerableaffectonatmosphericmotion."

8/6/2019 Dissecting Saturn's Big Storm

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Whileonlythebroadestpictureiswithinthereachofamateurtelescopes,it'sexcitingtopeekbeneaththeblanketofstormycloudsonSaturnbywayofimages

frommorepowerfulinstruments.Andtheshowwon'tendsoon-scientistsbelieve

thattheupliftvortexwilllingerforafewyearsandcontinuetopresentavisible

whitespotlongaftermostofthestormlosesitssteam.