Post on 17-Mar-2021
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 …