Australian Sky & Telescope - 04.2019

32 AUSTRALIAN SKY & TELESCOPE April 2019

thatthewatershootingoutofthatmoon’ssouthpolespreads

throughout the Saturn system, including to the top of Titan’s

atmosphere.Asthewatermoleculesdriftalong,solarphotons

break them up, producing ionised oxygen

atoms (O+). Calculations indicate that those

ionsaredepositedinthesameregionofthe

atmosphere where the heavy ions exist. That

meansthatoxygenmayalsobeparticipating

in the ionospheric chemistry, resulting in

theformationofveryheavyionsthatcontainCHON.

Indeed,laboratoryexperimentsthattrytosimulatethe

processesatworkintheupperatmosphereshow

thatsomeaminoacidsandnucleobasesmaybe

present. Therefore, there’s every indication that

complexmoleculesofprebioticinterestformin

Titan’s upper atmosphere.

TheincrediblyheavyionsthatCAPSdetectsare

attheverybeginningoftheirjourneyonTitan,nottheend.

Theywillstillundergoanumberofprocessesthatmayalter

theircompositionandphysicalpropertiesastheydescend

throughtheatmosphereandprecipitateontothesurface.

Mesosphere: Changing haze

Deeperdownintheatmosphere,Cassinirevealedother

secrets.Theregionbelowtheionosphere,themesosphere,

tendstobedifficulttomeasure:Theatmosphereisdense

enoughthataspacecraftcan’torbitthere—buttenuous

#7

Hydrogen

cyanide

enough that it’s difficult to study the region with common

techniques. Likewise, Earth’s mesosphere is at too high an

altitude to study with balloons and too low to study directly

with spacecraft. This region is therefore often called the

‘ignorosphere’.

Nevertheless, Cassini’s Ultraviolet Imaging Spectrograph

was particularly well suited to studying Titan’s mesosphere.

It finally provided us with our first detailed measurements of

this layer’s composition and density, furnishing important

inputs and tests for models of Titan’s atmosphere.

At the bottom of the mesosphere is something called the

detached haze layer, first discovered by the Voyager

1 spacecraft. When Cassini arrived in the Saturn

system, the detached haze layer was at a much

different altitude than where we’d left it during

Voyager’s 1979 flyby, which was very confusing to

Titan scientists. Luckily, the length of Cassini’s

mission allowed us to find out that the haze changes altitude

with season (a year in the Saturn system lasts nearly 30 Earth

years). When we returned to the same part of Titan’s year

that Voyager had explored, the layer returned to the same

location where it had been observed before.

We still don’t know exactly why the detached haze layer

changes altitude or very much about its composition, but we

do know that both chemistry and motion in the atmosphere,

as well as the feedback between them, play important roles.

For example, haze particles affect how energy moves through

The atmosphere is dense enough in the mesosphere that a spacecraft can’t

orbitthere—buttenuousenoughthatit’sdifficult to study with common

techniques. This region is therefore often called the ‘ignorosphere’.

POLAR BEAUTY MARK

Far right: This large vortex,

shown here in a natural-

colour composite, formed

at Titan’s south pole as the

hemisphere approached

winter. The cloud system

sticks above the surrounding

cloudtops. Near right: A

close-up reveals what might

be open-cell convection,

during which air sinks in the

centre of the cell and rises at

the edge, forming clouds at

cell edges.

NASA / JPL-CALTECH / SPACE SCIENCE INSTITUTE (2)

#8

Carbon

dioxide

ALIEN ‘EARTH’