Australian Sky & Telescope - 04.2019

34 AUSTRALIAN SKY & TELESCOPE April 2019

XATMOSPHERE IN PROFILEShown is a representative

temperature profile for Titan’s atmosphere (black line), along

with some of the major chemical processes. Shorter (more

energetic) ultraviolet wavelengths reach shallower depths in

the atmosphere. Relativistic particles called galactic cosmic

raysmightreachallthewaydownintothetroposphere.

the atmosphere. Energy’s movement in turn

affectsatmosphericdynamics,whichcanthen

affect where the haze particles are located. So

by studying the haze, we’ll have a better idea of

what’s going on energy-wise in the atmosphere.

Stratosphere and troposphere:

Abundant methane

Lower down, in Titan’s stratosphere,

additional chemistry is driven by sunlight’s

less-energetic ultraviolet photons, which

break up some of the larger molecules

originally formed higher in the atmosphere

that have sunk to this level. Those molecular fragments

then react with the widespread CH 4 ,destroyingmoreof

it. The major net chemical reaction in Titan’s atmosphere

istheconversionofCH 4 into H 2 , which escapes to space

because of Titan’s low gravity, and C 2 H 6 (ethane), which

condenses into droplets that fall onto the surface, where the

liquid stays because it cannot evaporate in Titan’s surface

conditions.

This is a crucial reaction sequence: It means

that CH 4 is irreversibly destroyed in Titan’s

atmosphere. Computer models indicate that

all the existing methane in Titan’s atmosphere

should be used up in 10 to 100 million years.

Scientists generally assume this means that CH 4

is somehow resupplied, perhaps from outgassing

of methane trapped in cage-like surface

compounds called clathrates, and not that it is merely what

isleftofamuchlargeroriginalabundanceofmethaneinthe

atmospherethatisintheactofdisappearing.

The less-energetic photons may also drive photochemistry

within ices that have condensed out of the atmosphere. One

ofthemoststrikingexamplesofcondensationinTitan’s

atmosphere occurred as the south pole moved into winter:

A polar vortex formed and a giant ice cloud, later identified

asHCNice,appeared.Thisisoneofmanyexamplesofthe

seasonal changes that were observed by Cassini, a benefit of

having such a long-lived mission, which provided data for

almosthalfofaSaturnyear.

As we descend into the troposphere, more condensation

canoccur.HereCH 4 cloudsandstormsappearseasonallyand

change latitude as they chase the sunlight that drives their

formation. Titan’s dense atmosphere and low gravity result

in raindrops that fall slowly and grow to larger sizes than on

Earth. Rain appears to be relatively infrequent on Titan — but

Hydrocarbon

condensation Troposphere

Stratosphere

Mesosphere

Temperature (K)

60 80 100 Lakes 140 160 180 200

Main haze

Detached haze

Post-equinox

Pre-equinox

Galactic

cosmic rays

Sunlight Electrons Ions

Escape

H 2 CH 4 (?)

Hydrocarbon

reactions

CH 4 broken down

Heavy ions

HCN C 2 H 2

Thermosphere

Ionosphere

N 2 and CH 4

ionized

CH 4 clouds

200

0

400

600

800

1,000

1,200

1,400

Altitude (kilometers)

Extreme ultraviolet

Far ultraviolet

Lyman –

_

UV

whenitdoesrain,itpours,resultinginflashfloodsthatcarve

streams and rivers into the organic-coated, water-ice bedrock.

The Huygens probe descended through all these regions

of the atmosphere, finally landing near the equator in a

relativelydrywash(eg.astreambedthatonlysometimes

fills with liquid). As the Huygens probe sat on the surface, it

measured methane evaporating from beneath it due to the

heat of the probe. This indicates that liquid methane (and

likely other liquids as well) had ponded just

belowthesurface,perhapsthememoryofthe

last rainstorm.

All of the liquid and solid organic compounds

produced in the atmosphere eventually end up

onthesurface,wheretheyareeitherfurther

modified by chemical processes or instead

modify the surface themselves, or both. In

thisway,Titan’ssurfaceandatmosphereare

uniquely connected. To fully understand what’s going on in

the atmosphere — and what it could tell us about both our

planetandothersbeyondtheSolarSystem—we’llneedmore

information about Titan’s surface, especially the composition.

Several teams are working on

mission concepts for returning us to

Titan to explore these questions.

In the meantime, scientists will

digdeeperintothewealthofdata

returned by Cassini-Huygens, point

powerful telescopes at Titan to search for new molecules,

andrunlaboratoryexperimentstobetterunderstandhow

organic materials behave at cryogenic temperatures. With

hard work we will crack the enigma of Titan.

„When not dreaming of parasailing through Titan’s

atmosphere, planetary scientistSARAH HÖRST(Johns Hopkins)

investigates organic chemistry throughout the SolarSystem.

#9

Methylacetylene

#10

Diacetylene

GREGG DINDERMAN /

S&T

, SOURCE: S. M. HÖRST /

JOURNAL OF GEOPHYSICAL RESEARCH: PLANETS

2017

ALIEN ‘EARTH’