1. Titan’s High Altitude South Polar (HASP) Cloud at 200 cm-1
85°N (T4) – northern winter
75°S (T112) – late southern fall
ISS/VIMS polar cloud
HASP Cloud
CIRS Far-IR LIMB Scan
Add 300 km level – point it out (label VIMS/ISS Polar Cloud). The ISS cloud is not the same as the HASP cloud. The temperatures and relative abundance of the vapors are different at 300 and 200 km. So the ISS cloud will finally disappear as the temperature warms up. The ISS cloud itself does not get transported down on an elevator.
Carrie M Anderson
NASA Goddard Space Flight Center

2. HASP
cloud
This is mostly the Haystack but the aerosol is also contributing.

3. Saturation Vapor Pressures
Condensation first occurs where the saturation vapor pressure curves intersect the temperature curve.
The HASP cloud is observed at the altitudes where C6H6, HCN, and HC3N will condense.
As a result of subsidence, condensation will occur over an extended altitude range.
Co-condensation (or co-deposition) occurs as a result of simultaneous condensation over the shared altitude range.
Co-condensation results when the gases condense simultaneously over this shared altitude range

4. SPECtroscopy of Titan-Related ice AnaLogs (SPECTRAL) chamber
My lab at NASA GSFC
SPECtroscopy of Titan-Related ice AnaLogs (SPECTRAL) chamber

5. HCN and HC3N
Ice Analogs

6. HC3N and C6H6
Ice Analogs

7. HCN and C6H6
Ice Analogs
The mixing ratios of the ices shed some light of the order of condensation. Over the shared altitude range, there is tremendous variation in the abundance of the various gases – as a result of the gases condensing out. The vapor abundance is rapidly reduced as subsidence carries the gas downward and condense it out – so you loose the vapor in a short amount of time.
He condensation/evaporation rates vary at each level as a function of mixing ratios. So the different mixtures at the different altitudes get averaged out and we see the best match of a 4:1 ice mixing ratio.
As you are condensing as you descend, the reltive abudnances of the 2 co-condesned ices are different as a function of altitude. So these relative abundances chnge as the ice particle descends – and they may change quite a lot. This change is dependnet on the rate of subllimaton and condensation. So we essentially get an average spectral shape for the co-condensing ice spectral feature. In reality, the mixing ratio of the condensed ices at each level will be much more complex.

8. Far-IR Corroboration of HASP Cloud Chemical Composition
Titan far IR winter polar latitude limb integration spectra
at 225 km in 2005 and 2015
HAYSTACK
Excess signatures near 220 cm-1 are due to the Haystack
The Haystack spectrally pollutes the HASP cloud
This is a ramification of the large CIRS far IR FOV
The two clouds are actually separated in altitude by ~100 km

9. Mid-IR Corroboration of HASP Cloud Chemical Composition

10. Take Away Message:
The benzene (C6H6)—hydrogen cyanide (HCN) HASP cloud is a brand new, never before seen, type of noxious hybrid ice cloud observed in Titan’s early southern winter polar stratosphere.
The HASP cloud is chemically different from that of the stratospheric ice clouds observed ten years ago at northern winter polar latitudes – these clouds consisted of co-condensed hydrogen cyanide (HCN) and cyanoacetylene (HC3N).
Titan’s meridional circulation reversal is the main cause for these observed seasonal variations – this drives different temperatures and relative abundances of the organic gases.
HCN adsorption is a way to deplete the HCN gas abundance at warmer temperatures above the condensation level for pure HCN
Need to better constrain the co-condensed ice binding energies, the co-condensed sticking coefficients, co-condensed ice structure, etc.