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Transcript
Isotopic Ratios In Titanʼs
Atmosphere: Clues and Challenges Titan Science Meeting#
St Jacut de-la-Mer, France, June 20th 2011#
Conor A. Nixon, #
Department of Astronomy#
University of Maryland College Park#
Overview •  This talk will focus on three main topics: 1.  Measurements of Titan isotopic ra8os: Cassini CIRS, Huygens GCMS and others. 2.  Comparison of Titan isotopic ra8os to other planets. 3.  Have the isotopic ra8os changed over 8me, and what were the star8ng values? •  Conclusions and areas of further work. Why Study Isotopic Ratios? •  Before discussing the measurements of isotopic ra8os, it is useful to remember why we are doing this, including implicit assump8ons. •  Two types: UNSTABLE (radiogenic) and STABLE. •  UNSTABLE ra8os (e.g. 40Ar from 40K tell us average degassing from interior, e. g. volcanism. •  STABLE ra8os may tell us about original composi8on, but are modified by addi8on and loss processes. May be complex to unravel. ʻStableʼ Isotopic Ratios Can Change! •  E. g. for the atmosphere, addi8on processes include: •  Degassing from interior •  Impacts •  While loss processes include: •  Impacts (again!) •  Jeans (and hydrodynamic) escape. •  Solar wind and other spuYering. •  Chemistry and sequestra8on into liquids/
solids on the surface/into interior. Part 1: Measurements Measuring Methane Isotopologues D/H and 12C/13C in methane •  CIRS has detected three minor isotopologues of methane( in addi8on to 12CH : 13CH , 12CH D and 4)
4
3
13CH D. 3
•  By measuring the abundances of all 3 minor species, we can make two es8mates of the two isotopic ra8os. •  Preliminary es8mates (subject to revision): 12CH
12C/13C 13CH
13CH
D/H 12CH
4
D/H 12CH 4
4
13CH
3D
3D
12C/13C 13CH 4
12CH
3D 86±7 (1.6 ± 0.3) x10-­‐4 (1.6 ± 0.4) 86±28 D
3
x10-­‐4 CIRS Isotopic Scorecard (as of June 2011) Detection of
13CH 12CH ?
2
2
~2σ significance
=> 12C/13C = 89±18 CIRS 12C/13C – hydrocarbon results Titan Isotopic Measurements D/Hx104 •  Voyager: 1.50 ± 1.00 •  ISO: 0.87 ± 0.26 •  Ground-­‐based: 0.78–1.25 •  Huygens-­‐GCMS: 2.30 ± 0.50 *
*in H2 not CH4 •  CIRS-­‐original: 1.17 ± 0.25 12C/13C 82.3 ± 1 77 ± 3 •  GCMS revised 1.35 ± 0.30 * •  CIRS revised 1.32 ± 0.13 •  EARTH (inorganic) 1.6 91.1 ± 1.4 (in progress) 89.4 Measurements - Time History Isotopic Ratios Summary •  D/H: values are largely compa8ble (or perhaps slightly lower by ~20%) than terrestrial. Emerging agreement between species (C2H2 and CH4) – difference from CH4 to H2 has vanished in GCMS reanalysis. • 
12C/13C: Measured in 7 or 8 gas species: largely compa8ble or perhaps slightly lower than Earth. • 
14N/15N: HCN value is substan8ally enriched vs terrestrial (x4.5) whereas N2 much less so (x1.5). • 
16O/18O: compa8ble with terrestrial given accuracy. Part 2: Solar System
Comparison Titan-Earth Isotope Comparison (II)
120%
Ratio (Titan/Earth)
100%
H2
80%
C2H2
CH4
C2H6
60%
HC3N
CO2
HCN
40%
N2
C4H2
20%
0%
D/H
12C/13C
14N/15N
Isotopic Species
16O/18O
•  Titan’s isotopic ratios vary in N, but much less in H and C.
•  All ratios except N near terrestrial – clue to volatile origin.
Titan & outer planets compared to Earth
Outer Solar System Isotopic Ratios
1.8
1.6
1.4
Ratio
1.2
Jupiter/Earth
Saturn/Earth
1.0
Titan/Earth
0.8
Uranus/Earth
Neptune/Earth
0.6
0.4
0.2
0.0
D/H
12C/13C
14N/15N
Isotopic Species
18O/16O
•  Titan’s ra8os closer to Earth ra8os than Saturn ra8os. •  Titan has best-­‐characterized ra8os in outer solar system. Terrestrial planets compared to Jupiter
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•  D/H much higher: evidence for ices, not gas? •  C and O ra8os do not vary greatly in solar system. •  15N enriched in terrestrial planets – evidence for escape? Part 3: Time Evolution of
Isotopic Ratios Interpretation – 14N/15N and 16O/18O 15N/14N
is higher in N2 than terrestrial by a factor ~1.5.
  Enrichment by photolysis and escape over 4.6 Gyr is a
likely explanation (e.g. Lunine et al. 1999).
  Implies atmospheric loss of factor 2-10 (Niemann ‘05)
The 15N/14N is significantly less enriched in Titan’s N2,
compared to HCN.
  Liang et al. (2007) have shown that differences in the
photolysis of 15N14N compared to 14N14N can drive large
fractionation effects in daughter species.
  However the effect appears to over-enrich 15N unless
an additional process in invoked (e.g. impact
dissociation.)
18O:
preliminary, but seems to agree with other planets.
Interpretation – D/H Titan’s D/H in methane is ~7-8x protosolar (like Earth).
Several possible explanations:
 Primitive enrichment: the building blocks of
Titan were substantially enriched versus giant
planets, due to fractionation of D/H in ices
(enriched) versus H2 (e.g. Mousis et al. 2002).
 Enrichment by evolution: preferential escape,
sputtering or chemical depletion of 1H in methane
(many papers: Pinto et al. 1986, Lunine et al.
1999, Cordier et al. 2008, Mandt et al. 2010).
 … or some combination of both!
ess
the
700 km that to form ethane from the recombination of methyl
radicals, they have to be transported down to below 600 km
(Figure 1), outside of the region where these processes are
effective.
Chemical effects on CH4 Isotopes S1)
Figure from Wilson and Atreya, 2009. R6)
ion
H 3)
et
”H
ofugh
hey
ane
and
1. Red solid line is the chemical destruction rate profile for
• Figure
Chemical destruc8on of CH4 peaks rate
at profile
~200 km, by the CH , the orange solid line is the methane destruction
due
to photolysis,
and + theCdashed
line
isHthe+ production
rate profile
reac8on: C
H
H 
C
C
H
(
cataly8c, recycles C2H). 4 2
3 2 2
for C H .
•  Can be mass-­‐selecHve (‘kine8c isotope effect’). 4
2
6
Time Evolution of D/H •  Recent work by Cordier et al. (2008) and Mousis
(2009) have shown that KIE, escape,
serpentization and other D/H fractionation effects
for methane are insufficient to reach 7-8x
protosolar in 4.6 Gyr.
⇒  Methane must have been enriched to begin
with.
⇒  We do not know the initial D/H, so we can pick
some enrichment model and work backwards to
estimate it.
⇒  So everything is rosy now? Not so fast…
What about
12C/13C? •  From our attempts to enrich D/H, we have
convinced ourselves that processes exist which
select between methane isotopes.
⇒  Therefore same principal (different strength)
applies to carbon isotope ratio.
⇒  E.g. KIE (13CH4) ~1.01, vs. KIE (13CH3D) ~1.16.
⇒  12C/13C evolves over time, because carbon
from methane irreversibly converted to higher
hydrocarbons, and lost.
⇒  But 12C/13C is same as every other body?
Time evolution of
12C/13C? •  Brings us to work of Mandt (2010) – who showed
that two solutions are possible:
1.  Methane has just been released, hence exhibits ‘normal’ 12C/
13C.
⇒  Tight constraint on time since release, e.g. if KIE ~1%,
not very many ‘lifetimes’ allowed before noticeably
different from 89.
2.  Methane constantly replenished: we can show that carbon
ratio stabilzes at (89/f) – where f (enrichment per lifetime) is
not much above unity.
•  May pose a problem for a single release episode
model if not very recent (unless efficient recycling?)
Noble Gas Abundances
(Huygens GCMS, Niemann et al. 2010) • 
36Ar: unexpectedly low abundance of (2.1±0.8)×10-­‐7; must be priordial so some amount of N2 also trapped? • 
38Ar/36Ar ~ 0.2: very approximate. • 
40Ar: abundance of (3.39±0.12)×10-­‐5 indicates significant outgassing has occurred, but below 0.05% level that would correspond to terrestrial equivalent. •  Ne: 20Ne not detected due to much higher 40Ar++; 22Ne detected at (2.8±2.1)×10-­‐7; requires 20-­‐25K trapping. •  Kr and Xe: non-­‐detec8ons may not be significant as abundances expected to be below instrument sensi8vity (upper limits are 1×10-­‐8). Conclusions and Further Work Conclusions:
–  Titan made from different building blocks than Saturn.
–  Light element ratios near-terrestrial, except N.
–  Evidence for both outgassing and cold accretion from
the noble gas ratios?
Further work:
–  Continue to refine measurements with CIRS and other
instruments: more accurate the better.
–  Detailed isotope-photochemical models must be used
that fully account for C2H2, and ion chemistry.
–  Also want to know isotopes in bulk ices: especially
H2O.
–  Another measurement of noble gases required.