Mercury
AmongMESSENGER’sbiggestsurpriseswasthe
discoveryofwatericeonMercury.Thisseemsodd,
giventheextremelyhighsurfacetemperaturesin
directsunlight.Buttherearespotsinsidesomeof
thecratersneartheplanet’spolesthatneverseethe
Sunatall,andconsequentlythetemperaturethere
goestotheoppositeextreme– closertominus 200
degreesCelsius(minus 328 degreesFahrenheit).
Therehadbeenhintsoficeinsidethese
permanentlyshadowedcratersaslongagoasthe
1990s,whenradarmeasurementsindicatedthe
presenceofhighlyref lectivematerial.Theissue
wasn’tclinched,however,untilMESSENGER
detectedhydrogenatomsinthesamelocations.
“Watericeistheonlycandidatewe’vegotthatfits
allthoseobservations,”asprincipalinvestigator
SeanSolomonexplained.It’snotjusta few
tracesofice,either– accordingtoSolomonit’s
“enoughicetoencaseWashingtonDCina
frozenblocktwo-and-a-halfmilesdeep”.
Anotherunexpectedfindingwasa
brand-newtypeofsurfacefeaturethat
isn’tseenanywhereelseintheSolarSystemexceptMercury.Called‘hollows’,these
areshallowdepressionsfoundinsidemanyofthe
planet’scraters.They’rebelievedtohaveformed
whenvolatilecomponentsinthesurfacematerial
evaporated,causingtheremainingmaterial
tocollapse.“Thehollowsareoneofthemost
viscerallyinterestingdiscoveriesfromthemission,”
accordingtoanotherMESSENGERscientist,Steve
Hauck.“Theywerecompletelyunexpected– a new
landform,andonethatappearstoformbylossof
rocktospace.”
Theideathatvolatilematerialsboiledoff
Mercuryinthedistantpastseemsreasonable
enough,givenitshotlocationclosetotheSun.
Whatwouldbemoresurprisingwouldbetofind
volatilematerialsstillontheplanet’ssurface
today– andyetthat’sexactlywhatMESSENGER
didfind.It’sa subjectBrianCoxtalkedaboutin
hisTVseriesThePlanetslastyear.“Thediscovery
ofrelativelylargeconcentrationsofelementslike
sulphurandpotassiumonMercury’ssurfacewasa
hugesurprise,”hesaid.“SoMercuryisanenigma,
anddiscoveriesliketheseforceustocompletely“Mostofwhatweconsideredtobegospel
aboutMercury turnedout tobea little different
thanwethought”WilliamMcClintock
Oncourse
forMercury
Thetwospacecraft
makinguptheexciting
BepiColombomission,
fromtheEuropeanand
Japanesespaceagencies,
werelaunchedinOctober
2018 andshouldgointo
orbitaroundMercury
inlate2025.Rethinking
Mercury’sorigin
Itusedtobethought
thatMercurysuffered
a hugeimpactwhich
strippedoffitsouter
layers,butthat’slesslikely
nowweknowthereare
volatilesonthesurface.© Adrian Mann© NASA