Australian Sky & Telescope - 04.2019

28 AUSTRALIAN SKY & TELESCOPE April 2019

theorganicchemistryoccurringinTitan’satmospherewas

complex, we knew we needed to study Titan’s atmosphere

furthertoinvestigatethesequestions.

ToexplorewhattheCassiniorbiterandHuygensprobehave

taughtusaboutchemistryonTitan,let’stakeatripintothe

atmosphere,startingatthetopandworkingourwaydown.

Ionosphere: Surprise super-molecules

The Ion and Neutral Mass Spectrometer (INMS)

wastheprimaryinstrumentdesignedtomeasure

thecompositionofTitan’sionosphereasCassini

repeatedlyflewthroughitduringthespacecraft’s

mission. INMS measured the composition of

theatmospherebymeasuringthemassesofthe

moleculesitingested.Theteamhaddesignedthe

instrumenttocoveramassrangeof1to99atomic

mass units, which roughly covers atomic hydrogen (1) to

moleculeswithapproximatelysixtoseven‘heavyatoms’such

ascarbon,nitrogenoroxygen.Thismassrangeeasilyincluded

theheaviestmoleculeknowntoexistinTitan’satmosphere

TTITAN’S SURFACEThese infrared images combine 13 years of Cassini’s visible and near-infrared imagery into smooth mosaics of the surface

thathidesbeneathTitan’sthickatmosphere.Reddishbrownmarksthemoon’sequatorialdunefields,whilethebluishandpurplishregionsmight

beenrichedinwaterice.Thecentrallongitudesofeachvieware(fromlefttoright)approximately30°,60°,120°,210°,300°and330°west.

We need to know what kinds of

molecules are made only by life so that

we can use them to detect life both in

the Solar System and elsewhere.

Whydowecarehowcomplextheatmosphericchemistry

is?Oneofthebigquestionsweareinterestedinansweringis

howfarorganicchemistrycanproceedintheabsenceoflife.

Weknowfromanalysingmeteoritesthatcompoundssuchas

amino acids (the building blocks of proteins) can be produced

byabioticprocesses.WithTitan,wecan

begintounderstandhowfarabioticorganic

chemistrycanproceedinanatmosphere.

Thisisimportantfortworeasons.First,

we need to know what kinds of molecules are

madeonlyby life so that we can use them to

detect life both within the Solar System and elsewhere in

theuniverse.ManyofthemoleculesusedbylifeonEarth,

such as amino acids and nucleobases (the building blocks of

DNAandRNA),arenotuniquetobioticprocesses:They’re

alsoproducedwithoutlife’sinvolvementandarefoundin

meteorites and comets. But their relative amounts

intheseenvironmentsaredifferentthanwhat

wefindwhenthey’recreatedbylifeonEarth.So

we need to study abiotically produced organic

molecules in order to be able to tell the difference

between abiotic sources and biotic ones when

lookingatotherplanets.

Second,wewanttounderstandtherolethat

planetary atmospheres play in the origin and evolution of

life.Onepotentialroleistoprovidematerial(suchasorganic

molecules) to help life get started and/or to evolve. Since we

realised,evenbeforethearrivalofCassini-Huygens,that

#2

Carbon

monoxide

NASA / JPL-CALTECH / ASI / USGS; S. M. HÖRST /

JOURNAL OF GEOPHYSICAL RESEARCH: PLANETS

2017

#3

Ethane

ALIEN ‘EARTH’