Titan: a new astrobiological vision from theCassini-Huygens data

François Raulin

Titan

Temperature : Surface : ~ - 180 °C Tropopause : ~ - 160 °C

Stratosphere : ~ - 105 °C

Atmosphere : gas + aerosols Surface Pressure : 1.5 bar!! Nitrogen N2 98 % Methane CH4 2 %

Distance to the sun : ~10 a. u. Surface radius: 2575 km

Astrobiology: Life in the universe Extraterrestrial life Origin of life on Earth Extraterrestrial organic chemistry

Titan : prime target for Astrobiology => 3 main aspects

1) Extraterrestrial organic chemistry

Active through methane chemistry, in gas + aerosol phase

_________________________________________________________________

Stratosphere Mixing Ratio Production inCompounds (E=Equ.; N=North Pole ) Simulation

Experiments _________________________________________________________________Main constituentsNitrogen N2 0.98 – 0.95 Methane CH4 0.02 - 0.05Hydrogen H2 0.0006 - 0.0014

HydrocarbonsEthane C2H6 1.3 x 10-5E Maj. Acetylene C2H2 2.2 x 10-6E Maj. Propane C3H8 7.0 x 10-7E ++Ethylene C2H4 9.0 x 10-8E ++Propyne C3H4 1.7 x 10-8N +Diacetylene C4H2 2.2 x 10-8N +Benzene C6H6 few 10-9 +

N-Organics Hydrogen cyanide HCN 6.0 x 10-7N Maj.Cyanoacetylene HC3N 7.0 x 10-8N ++Cyanogen C2N2 4.5 x 10-9N +Acetonitrile CH3CN few 10-9 ++Dicyanoacetylene C4N2 Solid Phase N +

O-Compounds Carbon monoxide CO 2.0 x 10-5 Carbon dioxide CO2 1.4 x 10-8E Water H2O few 10-9

low pressure (~ 1 mbar)

low temperature (~ 200K)

Titan’s atmospheric conditions

The experimental setup for global simulation at LISA

Laboratory analogues of Titan’s aerosols

• Generic name (introduced by Carl Sagan in the late 70ties)

SOLID PHASE : tholins

Molecular composition : still very badly known

• HCN –C2H2 –HC3N oligomers or co-oligomers

• Macromolecule of largely irregular structure

In the aerosols ?

no direct clue (are “tholins” real laboratory analogues of Titan’s haze particules)?

Still many questions on the complexity of this chemistry

In the troposphere ?

any lightning inducing chemical transformations

On the surface ?

episods of liquid water or water/ammonia mixtures ?

In the subsurface ? in a water-ammonia ocean ?

1) Extraterrestrial organic chemistry

In spite of strong differences : • atmospheric composition (CH4 instead of CO2)• temperature ranges => no permanent bodies of liquid water on the

surface

2) Origins of Life on Earth

Many analogies :• Vertical atmospheric structure• N2 rich atmosphere• CH4 cycle looking similar to the water cycle on Earth• Presence of organics playing a key role in terrestrial

prebiotic chemistry (HCN, HC3N, …. and their oligomers)

crucial to look at processes of prebiotic interest in the whole complexity of a real planetary environment

laboratory experiments and theoretical modelling need the constraints of ground truth (prebiotic Earth not available now)

• Life = subtile and complex cooking implying 2 main ingredients : liquid water + organic matter

3) Search for Extraterrestrial Life

• Titan’s interior models

* liquid water (presence of a subsurface H2O-NH3 ocean)

* organics may have been present (chondritic material)

=> life may have originated in this ocean

Temperature

pH

Pressure

Salinity

-20ºC +121ºC

0 ~12

? 5,5 M (NaCl)

? ~100 MPa

~1 GPa ??

On Earth, life can develop in extreme conditions * high pressure* high salinity & pH* low temperatures

=> life may still be present in this hypothetical Titan subsurface water ocean

=> Possible biological origin of CH4 cannot be excluded

July 1rst 2004 : Cassini Saturn Orbit Insertion

Cassini (http://www.jpl.nasa.gov/cassini)– Exploration of Saturn and its environnement during 4 years (2004-

2008)– 12 experiments onboard including

Imaging Science Subsystem (ISS)Filters between 0,2 and 1,1 m

Composite Infrared Spectrometer (CIRS)

= 7,2 m-1 mm (10-1400 cm-1)Spectral resolution 8 times better

than Voyager/IRIS

Visible and Infrared Mapping Spectrometer (VIMS)

= 0,35-5 m

Ultraviolet Imaging Spectrograph (UVIS)

= 56-190 nm

Radio Science Subsystem (RSS) Radar

To

• Huygens (http://sci.esa.int/huygens)– Descent probe in Titan’s atmosphere (January 14, 2005)– Measurements in the atmosphere (0-150 km) and on the

surface during ~ 2:20 and ~1:10 – 6 experiments onboard :

Atmospheric Structure Instrument (HASI) Pressure, temperature, lightning

Gas Chromatograph & Mass Spectrometer (GCMS) Gas composition, analysis of the pyrolyzed

aerosols

Aerosol Collector & Pyrolyzer (ACP) collect and pyrolysis of the aerosols

Descent Imager/Spectral Radiometer (DISR) Cameras, visible and near IR spectrometers

(0,35 - 1,70 m) Energy flux, CH4 concentration profil,

physical properties of clouds and aerosols, physical data of the surface

Surface Science Package (SSP) physical state and composition of the

surface

Doppler Wind Experiment (DWE)

First Huygens data

=> no evidence for macroscopic life on Titan’s surface

=> Except one data the origin of which is far from certain

12C/13C on Earth:

* Reference standard for carbon-13 : Belemnite of the Pee Dee Formation => 12C/13C = 88.97

What about a microscopic life and the possible biological origin of CH4 ?

* Biological processes produce 12C enrichment : Biotic carbon from biosynthesis => 12C/13C ~ 91 – 94

In Titan low atmosphere: * GCMS data => 12C/13C = 82

* Preliminary conclusion: non biological origin for CH4

HASI • Vertical T,P profiles with high precision• Surface T = 93.65±0.25 K P = 1467±15 hPa • Surface permittivity• No lightning detected: no important organic syntheses processes in the troposphere (but one location only …)

Extraterrestrial Organic Chemistry

SSP Surface state : crust above a softer material (“crème brûlée” or pebble)

DISR• Atmospheric composition : CH4 from ~2 to ~ 5%• Surface composition : Water ice + ?? no fit with (pure) tholins….

Altitude Profiles (using HASI P,T ) & Surface Measurements=> Methane mixing ratio = 2% up to ~5% => 14N/15N ~ 183=> 12C/13C isotope ratio ~82.3=> 40Ar mixing ratio ~4x10-5 ( 40K : atmosph. origin )

=> Trace species on surface

• Low atmosphere poor in gas phase organics (except methane) at noticeable concentration

• Surface much richer

• Methane increases to saturation in the low troposphere

• A wet (methane) surface

GCMS

Thus preliminary retrieving of ACP data : supports the tholins hypothesis.

More work to be done (error bars + low temps.)

H,C,N Tholins

Condensates including

HCN & NH3

ACPlow atmosphere aerosols include an organic refractory part + condensed volatiles, including ammonia and HCN

ACP & GC-MS teams:

Guy Israel, Hasso Niemann, Tobias Owen,

LISA-GPCOS Team:

Patrice Coll, Mai-Julie Nguyen & Eric Hébrard

Yves Bénilan, David Coscia, Marie-Claire Gazeau, Claire Romanzin, Robert Sternberg

ENSC Paris:

Sylvie Derenne

Tholins from last experiments at LISA (Coll, Nguyen & al.)

CH4 => Tholins : from + 42 °/oo to -36/-57 °/oo

= ~ - 80 / -100 °/oo

* centesimal analysis

C/N = 1.7 - 2.0 & C/H = 0.7

* isotopic analysis

CH4 : 12C/13C = 85.5

Tholins : 12C/13C = 92.3 to 94.3

Titan (ACP) : Gas / aerosols enrichment in 12C

ACPsampling

Alti

tud

e (

km)* T

ime

(min)

0

50

100

150 0

20

40

80

120

Aerosolsdistribution

Photochemicalaerosols

Clouds

Filterposition

Out

In

InOut

Ovenheating

250°C

250°C

600°C

600°C

Pumping

On

Off

OffOn

4.2 mbar *

202.8 mbar *

364.7 mbar *

497.8 mbar *

Pressure*

*Data provided by HASI experiment

To GCMS

ACP

ACP sample #2 (temp.: 600 C)analyzed by GCMS

Counts/sec

m/zCredits: ESA/CNRS/GSFC

SSP : ACC-E Penetration Force

The impact speed for both of these drops was ~ 3.7 ms-1.

Impact Simulations

A crust or pebble impact on the surface = most likely scenarios so far …