Atmospheric Chemistry of Titan Options

[titanicrivers]

Thanks for the discussion and the link Mike. One point that troubles a bit regarding Titan atmospheric simulation experiments is the lack of evidence so far for lightning on Titan. Is it safe to conclude that the sun's effect on the upper atmosphere of Titan is ionizing enough that lightning is not required to produce active species that can form into organic compounds. Do the plasma discharges and rf zaps of these experiments simulate the sun's effects or is there a presumption of lightning going on in Titan's troposphere that Cassini has just not detected as yet. (Hmm ... one wonders with the T72 storm whether the RPWS instrument lightning sensors were positioned to pick up a discharge?)

[Juramike]

Most of the formation chemistry seems to be happening at very high altitudes, likely above any cloud-based lightning. (sprites?)

I think the laboratory experiments are trying to get molecular nitrogen to ionize, so either Extreme UltraViolet radiation (got beam source?) or plasma discharge are the best ways to do it. My simplistic mind views a plasma discharge experiment as a scaled up version of a mass spectrometer antechamber.

I'd suppose the major ionizing sources in Titan's upper atmosphere would be sunlight (very shortwave radiation). Not sure of the role that cosmic rays, energized particles (solar and trapped in Saturn orbit) or other electronic discharges would play.

[Juramike]

LPSC 2011 abstract describes continuing efforts to identify heavier molecules from the Huygens GCMS using the flight spare at GSFC.

Trainer et al. LPSC 2011, Abstract 1399. "Laboratory Simulations of the Titan Surface to Elucidate the Huygens Probe GCMS Observations."

Why is this cool? Identifying heavier ions, and their relative amounts, can help constrain the atmospheric models and give a clue what's really down there on the surface.

A recent paper many of the same authors (Niemann et al. JGR (2010) E12006), showed that C2H6, C2H2, C2N2, and CO2 were detected at the surface after Huygens landed and volatilized materials in the muds. (Benzene (C6H6) was detected but couldn't be ruled out as atmospheric origin, maybe with more simulation analysis it can be confirmed?)

But even with mole fractions of these four components, if you normalize this and the atmospheric models to ethane, it can be seen that the Krasnopolsky model fits best for C2H2 and C2N2 (consistently 3x too low). The Cordier/Lavvas and Wilson and Atreya model have the predicted flux rate of C2N2 too low. See chart below:


(also attached as a spreadsheet):

Attached File(s)

 Huygens_GCMS_data_vs_lit_models.xls ( 22K ) Number of downloads: 232

 

[Juramike]

Detection of high altitude cirrus clouds in Titan's atmosphere: http://www.nasa.gov/mission_pages/cassini/...tan-clouds.html

"One of those is cyanoacetylene, a member of the nitrile family. The compound's distinctive signature made it the first to be picked up in the northern ice clouds by Voyager 1 and by Anderson and Samuelson"

[centsworth_II]

This gives hope of much greater understanding of Titan's atmosphere in the years to come.

NASA’s “COSmIC” Simulator Helps Fingerprint Unknown Matter in Space
Located at NASA’s Ames Research Center, Moffett Field, Calif., this specialized facility, called the Cosmic Simulation Chamber (COSmIC)...

The chamber is the heart of the system. It recreates the extreme conditions that reign in space...

...Ames scientists delivered their first major milestone by coupling COSmIC with a cavity ringdown spectrometer, an extremely sensitive device that can detect the spectral fingerprint of matter at the molecular level.

Now, another major milestone has been achieved by coupling COSmIC with a time-of-flight mass spectrometer, an ultra-sensitive device that detects the mass of matter at the molecular level....

To understand Cassini’s data, scientists need this very powerful, very sensitive new tool. They will begin their analysis by forming molecules and species in the lab, measuring them in situ (inside their environment without disturbing them), and then trying to match their identity to Titan’s unknown aerosol molecules.

“Titan’s upper atmosphere data shows a rich spectrum. We will recreate those data in the lab and compare them to Cassini’s data. If they fit, great. If not, we will try something else. We will know when we are coming close to understanding them. We now have the right tool to do this,” said Salama.

[ngunn]

Interesting link posted on the Cassini Huygens Yahoo group:
http://dx.doi.org/10.1038/ngeo1147

[Juramike]

The Cool Way to PAH's in Titan's Upper Atmosphere

A recent article provides a new mechanism to get to fused ring systems at Titan-like temperatures and low pressures in the upper atmosphere. It is called EAM, or ethynyl addition mechanism. (The previously described HACA mechanism was adapted from high-temperature combustion studies).



The sequence starts with styrene, which can be formed by phenyl radical reacting with ethylene. This is known at high temperatures, but may or may not be valid at Titan upper atmosphere conditions. There may be an ion neutral chemistry route to styrene as well, but a a quick glance at the previous Titan atmospheric models, I didn't see styrene specifically mentioned.

Ethynyl radical (generated from EUV photodissociation of acetylene) attacks styrene at the ortho position to the vinyl substituent. The double bond opens, then the radical closes back in to kick out hydrogen radical, and you get vinylacetylenebenzene. This is almost barrierless and is pure downhill, so it can happen at very low temperatures.

[The authors calculated the collision rate at a simulated Titan upper atmosphere conditions, and found that the radiacal lifetime is shorter than any expected collision rates. It was estimated at 70 ms between molecular collisions at 90 K and 1E-6 mbar. So there is little chance ('bout 10%) another molecule can bump into the vinylacetylenebenzene and help remove all that excess energy. No relaxation, so the compound kicks out hydrogen radical. For those lucky molecules that do hit something, the benzene radical could become the final product of reactions and can then go on to do other things with other molecules...]

Another attack of acetylene radical, this time at the beta carbon (interior) of the alkyne substituent, generates a new radical that undergoes cyclization to form a bicyclic napthalene, but with a bonus alkyne substituent hanging off the end. It turns out this sequence is pretty much barrierless and downhill as well.

Why doesn't it form napthalene in the first sequence? It turns out that there is a pretty big activation energy barrier. So while that end product would be thermodynamically more downhill, the transition state energy mountain you need to go over is too big. So it shunts to the vinylacetylene benzene.



Here is a free link to the full paper. It is a pretty hardcore computational mechanism paper, but it does a great job of showing the snapshot-by-snapshot molecular movements and gyrations, including calculated energetics of the EAM process and alternatives that don't happen.
http://www.chem.hawaii.edu/Bil301/Kaiser%20Paper/p244.pdf

[scalbers]

I saw a talk given at the AGU conference last week mentioning that far infra-red spectral observations suggest that the atmospheric hazes probably have different composition than the traditional Titan tholins. More lab work will be helpful in comparing various compositions to the observations. This is related to post #79 I believe. Here is the abstract...

ABSTRACT FINAL ID: P33F-0
TITLE: Spectral and vertical distribution properties of Titan’s particulates from thermal-IR CIRS data: Physical Implications
SESSION TYPE: Oral
SESSION TITLE: P33F. Titan: An Earth-Like World II
AUTHORS (FIRST NAME, LAST NAME): Carrie M Anderson1, Robert Samuelson2, 1, Sandrine Vinatier3
INSTITUTIONS (ALL): 1. Solar System Exploration Divis, NASA---GSFC, Greenbelt, MD, United States.
2. Astronomy, University of Maryland, College Park, MD, United States.
3. LESIA , Observatoire de Paris-Meudon, Meudon , France.
Title of Team:
ABSTRACT BODY: Analyses of far-IR spectra between 20 and 560 cm-1 (500 and 18 μm) recorded by the Cassini Composite Infrared Spectrometer (CIRS) yield the spectral dependence and the vertical distribution of Titan’s photochemical aerosol and stratospheric ice clouds. Below the stratopause (~300 km) the aerosol appears to be incompletely mixed for the following reasons: 1) the altitude dependence of the aerosol mass absorption coefficient is larger at higher altitudes than at lower altitudes, 2) the aerosol scale height varies with altitude, which implies some kind of layering effect, and 3) the aerosol abundance varies with latitude.

The spectral shape of the aerosol opacity appears to be independent in altitude and latitude below the stratopause, even though inhomogeneities in the abundance appear to be prevalent throughout this altitude region. This implies that aerosol chemistry is restricted to altitude regions above the stratopause, where pressures are less than ~0.1 mbar. The aerosol exhibits an extremely broad emission feature with a spectral peak at 140 cm-1 (71 μm), which is not evident in laboratory simulated Titan aerosols (tholin) that are created at pressures greater than 0.1 mbar.

A strong broad emission feature centered roughly around 160 cm-1 corresponds very closely to those found in nitrile ice spectra. This feature is pervasive throughout the region from high northern to high southern latitudes. The inference of nitrile ices is consistent with the highly restricted altitude ranges over which these features are observed, and appear to be dominated by HCN and HC3N. At low and moderate latitudes these clouds are observed to be located between 60 and 100 km, whereas at high northern latitudes during northern winter these clouds are observed at altitudes between 150 and 165 km. The ubiquitous nature of these nitrile ice clouds is inconsistent with a simple meridional circulation concept, suggesting that the true dynamical situation is more complex.



KEYWORDS: [6281] PLANETARY SCIENCES: SOLAR SYSTEM OBJECTS / Titan, [5405] PLANETARY SCIENCES: SOLID SURFACE PLANETS / Atmospheres, [5422] PLANETARY SCIENCES: SOLID SURFACE PLANETS / Ices.
(No Image Selected)
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SPONSOR NAME: Carrie Anderson

Additional Details
Previously Presented Material: Most of the material was published in Icarus 212, 762-778 in 2011.

Contact Details
CONTACT (NAME ONLY): Carrie Anderson
CONTACT (E-MAIL ONLY): carrie.m.anderson@nasa.gov

[Paolo]

not sure this is the best topic to post it to, this interesting paper was published on today's Nature:
Polar methane accumulation and rainstorms on Titan from simulations of the methane cycle