Isotopic Ratios In Titanʼs Atmosphere: Clues and Challenges

Transcription

1 Isotopic Ratios In Titanʼs Atmosphere: Clues and Challenges Titan Science Meeting# St Jacut de-la-mer, France, June 20 th 2011# Conor A. Nixon, # Department of Astronomy# University of Maryland College Park#

2 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.

3 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. 40 Ar from 40 K 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.

4 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.

5 Part 1: Measurements

6 Measuring Methane Isotopologues

7 D/H and 12 C/ 13 C in methane CIRS has detected three minor isotopologues of methane( in addi8on to 12 CH 4) : 13 CH 4, 12 CH 3 D and 13 CH 3 D. By measuring the abundances of all 3 minor species, we can make two es8mates of the two isotopic ra8os. Preliminary es8mates (subject to revision): 12 C/ 13 C 12 CH 4 13 CH 4 12 CH 3 D D/H 13 CH 3 D D/H 12 C/ 13 C 13 CH 4 12 CH 3 D 12 CH 4 86±7 (1.6 ± 0.3) x CH 3 D (1.6 ± 0.4) x ±28

8 CIRS Isotopic Scorecard (as of June 2011)

9 Detection of 13 CH 2 12 CH 2? ~2σ significance => 12 C/ 13 C = 89±18

10 CIRS 12 C/ 13 C hydrocarbon results

11 Titan Isotopic Measurements Voyager: ISO: D/Hx ± ± C/ 13 C Ground- based: Huygens- GCMS: 2.30 ± 0.50 * 82.3 ± 1 *in H 2 not CH 4 CIRS- original: 1.17 ± ± 3 GCMS revised 1.35 ± 0.30 * 91.1 ± 1.4 CIRS revised 1.32 ± 0.13 (in progress) EARTH (inorganic)

12 Measurements - Time History

13 Isotopic Ratios Summary D/H: values are largely compa8ble (or perhaps slightly lower by ~20%) than terrestrial. Emerging agreement between species (C 2 H 2 and CH 4 ) difference from CH 4 to H 2 has vanished in GCMS reanalysis. 12 C/ 13 C: Measured in 7 or 8 gas species: largely compa8ble or perhaps slightly lower than Earth. 14 N/ 15 N: HCN value is substan8ally enriched vs terrestrial (x4.5) whereas N 2 much less so (x1.5). 16 O/ 18 O: compa8ble with terrestrial given accuracy.

14 Part 2: Solar System Comparison

15 Titan-Earth Isotope Comparison (II) 120% 100% Ratio (Titan/Earth) 80% 60% 40% 20% H2 C2H2 CH4 C2H6 HC3N CO2 HCN N2 C4H2 0% D/H 12C/13C 14N/15N 16O/18O Isotopic Species Titan s isotopic ratios vary in N, but much less in H and C. All ratios except N near terrestrial clue to volatile origin.

16 Titan & outer planets compared to Earth Outer Solar System Isotopic Ratios Ratio Jupiter/Earth Saturn/Earth Titan/Earth Uranus/Earth Neptune/Earth 0.0 D/H 12C/13C 14N/15N 18O/16O Isotopic Species Titan s ra8os closer to Earth ra8os than Saturn ra8os. Titan has best- characterized ra8os in outer solar system.

17 Terrestrial planets compared to Jupiter #!!!"!$!"#$#%&'()*+#"(,-.&'/01-$(21."3"(43%&$1.(#.(5#6*.( #!!"!$ #!"!$ 12345$ 6789:$ ;785$ <=973$ #"!$ %&'$ #()&#*)$ #+,&#-,$ #./&#0/$!"#$ D/H much higher: evidence for ices, not gas? C and O ra8os do not vary greatly in solar system. 15 N enriched in terrestrial planets evidence for escape?

18 Part 3: Time Evolution of Isotopic Ratios

19 Interpretation 14 N/ 15 N and 16 O/ 18 O 15 N/ 14 N is higher in N 2 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 15 N/ 14 N is significantly less enriched in Titan s N 2, compared to HCN. Liang et al. (2007) have shown that differences in the photolysis of 15 N 14 N compared to 14 N 14 N can drive large fractionation effects in daughter species. However the effect appears to over-enrich 15 N unless an additional process in invoked (e.g. impact dissociation.) 18 O: preliminary, but seems to agree with other planets.

20 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 H 2 (e.g. Mousis et al. 2002). Enrichment by evolution: preferential escape, sputtering or chemical depletion of 1 H 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!

21 ss he 1) 6) 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 CH 4 Isotopes Figure from Wilson and Atreya, on 3) et H f- gh ey ne nd Figure 1. Red solid line is the chemical destruction rate profile for Chemical CH 4 destruc8on of CH,theorangesolidlineisthemethanedestructionrateprofile 4 peaks at ~200 km, by the due reac8on: to photolysis, CHand the dashed line is the production rate profile for C 2 H C 2 H CH 3 + C 2 H 2 (cataly8c, recycles C 2 H). Can be mass- selechve ( kine8c isotope effect ).

22 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

23 What about 12 C/ 13 C? 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 ( 13 CH 4 ) ~1.01, vs. KIE ( 13 CH 3 D) ~ C/ 13 C evolves over time, because carbon from methane irreversibly converted to higher hydrocarbons, and lost. But 12 C/ 13 C is same as every other body?

24 Time evolution of 12 C/ 13 C? Brings us to work of Mandt (2010) who showed that two solutions are possible: 1. Methane has just been released, hence exhibits normal 12 C/ 13 C. Tight constraint on time since release, e.g. if KIE ~1%, not very many lifetimes allowed before noticeably different from 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?)

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27 Noble Gas Abundances (Huygens GCMS, Niemann et al. 2010) 36 Ar: unexpectedly low abundance of (2.1±0.8) 10-7 ; must be priordial so some amount of N 2 also trapped? 38 Ar/ 36 Ar ~ 0.2: very approximate. 40 Ar: 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: 20 Ne not detected due to much higher 40 Ar ++ ; 22 Ne 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 ).

28 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 C 2 H 2, and ion chemistry. Also want to know isotopes in bulk ices: especially H 2 O. Another measurement of noble gases required.