Venus’s ultraviolet absorber:
Cyclo-octal (S8) and polymeric sulfur (Sx)
and their latitudinal behavior.
R. W. Carlson1, G. Piccioni2, G. Filacchione2 , R. Mehlman3,
M. S. Anderson1, Y. Yung4, and P. Gao4
1Jet
Propulsion Laboratory, California Institute of Technology, Pasadena, California USA
([email protected]),
2Istituto
di Astrofisica e Planetologia Spaziali, Roma, Italy
3Institute
of Geophysics and Planetary Physics, Univ. California, Los Angeles, USA (retired)
4California
Institute of Technology, Pasadena, California USA
Some suggested Venus UV absorbers:
Formaldehyde CH2O
Carbon suboxide C3O2
Nitrosylsulfuric acid NOHSO4
Nitrogen dioxide NO2 and nitrogen tetroxide N2O4
Ammonium nitrite NH3NO2
Ammonium sulfate (NH4)2SO4
Ammonium pyrosulfate (NH4)S2O5
Ammonium chloride NH4Cl
Amides
Chlorine Cl2
Sulfur dichloride, SCl2
Iron chloride FeCl2•2H2O, FeCl3
Perchloric acid HClO4
Disulfur monoxide S2O
Sulfur
Ferric chloride at
Venus?
Forms sulfuric acid cloud droplets with
FeCl3 in solution
FeCl3
The (uncommon) mineral
Molysite
Ferric chloride, a salt, is deliquescent and FeCl3
aerosols can act as a condensation nuclei
Associated with terrestrial
fumaroles
Dimer condenses at 315 K (54 km)
Considered by Kuiper (1969)
Resurrected by Zasova,
Krasnopolsky, & Moroz (1981)
Computed profile matches
Mode-1 particles in lower and
middle clouds (Krasnopolsky,
2006)
FeCl3 in plume
Molysite (FeCl3)
deposits
(FeCl3)2 gas formed at 400 K (42 km)
FeCl3 gas from plume
and sublimation
Spectroscopy of FeCl3 solutions
Obtain absorption coefficient and imaginary
index
Transmission relative to empty cell
Transmission spectra by Mark Anderson
1.2
In sulfuric acid solution,
1.0
1% FeCl3 in
80% sulfuric acid
0.8
56 µm
500 µm
0.6
0.4
0.2
0.0
200
300
400
500
600
700
800
Wavelength, nm
get mainly Fe3+, FeSO4+ , FeHSO4 2+
106
105
FeSO4+ and FeHSO4 2+ bands at about
290 and 360 nm.
Extends into visible region
Hosokawa et al., 200 C
104
Absorption coefficient, cm-1
Fe3+ absorption bands at 194 nm and `
245 nm (Brown & Kester, 1980)
103
S8 α-sulfur
Fuller et al.
1% FeCl3 in 80% sulfuric acid
Hapke & Nelson
102
101
S8 fit
100
10-1
10-2
200
300
400
500
600
Wavelength, nm
700
800
Spectroscopy of FeCl3 solutions
Obtain absorption coefficient and imaginary
index
Transmission relative to empty cell
Transmission spectra by Mark Anderson
1.2
In sulfuric acid solution,
1.0
1% FeCl3 in
80% sulfuric acid
0.8
56 µm
500 µm
0.6
0.4
Very little absorption > 500 nm
0.2
0.0
200
300
400
500
600
700
800
Wavelength, nm
get mainly Fe3+, FeSO4+ , FeHSO4 2+
106
105
FeSO4+ and FeHSO4 2+ bands at about
290 and 360 nm.
Extends into visible region
Hosokawa et al., 200 C
104
Absorption coefficient, cm-1
Fe3+ absorption bands at 194 nm and `
245 nm (Brown & Kester, 1980)
103
S8 α-sulfur
Fuller et al.
1% FeCl3 in 80% sulfuric acid
Hapke & Nelson
102
101
S8 fit
100
10-1
10-2
200
300
400
500
600
Wavelength, nm
700
800
FeCl3 at Venus? Normalized geometric
albedo spectra for various concentrations.
1.2
Expected FeCl3 signature at Venus
Cloud droplets of FeCl3 in sulfuric
acid
Mie scattering for single scattering
albedo, asymmetry factor
Radiative transfer a la Hapke
Normalized geomtric albedo
1.0
0.8
0.01
0.02
0.05
0.6
FeCl3 in 80% H2SO4 at various
percentage concentrations
0.10
0.20
Spherical grains, 1-µm radius,
log-normal distribution
0.4
Mie scattering calculations using
experimental indices of refraction
0.2
Geometric albedo

0.0
200
300
400
500
Wavelength, nm
600
700
800
Elemental sulfur
Elemental sulfur is one component
of Venus’s sulfur cycle
Elemental Sulfur in Venus's Lower Atmosphere
60
H2SO4, SO2, SO, OCS, SO3, Sx
Condensation lines (Lyon, 2008)
50
SO + hν → S + O
OCS + hν → CO + S
(SO)2 + OCS → CO + SO2 + S2
Sublimation from surface
Mills, Esposito, & Yung 2007
S8
Condensed
S7
40
ALTITUDE, km
SO2 + hν → SO + O,
S6
Gaseous
S8 Yung et al., 2009
30
20
S8, Bertaux et al., 1986
(but see Bertaux et al.,
1996 & Yung et al., 2009)
ΣS, Krasnopolsky, 2007
10
S2, Fegley et al., 1997
0
10-7
10-6
10-5
10-4
SULFUR VOLUME MIXING RATIO
10-3
Spectral properties of S8 and Sx
1.0
The stable form of sulfur is cyclo-octal S8
0.8
Reflectance
which has absorption bands at ~ 220, 260, and
280 nm,
0.6
100-nm coating of S8 on
0.4
10-µm SiO2 spheres
and a gaussian tail that extends to ~450 nm.
0.2
Similar to FeCl3, little absorption > 500 nm
106
0.0
300
400
500
BUT
Absorption in these bands breaks the S8 ring.
…absorption extends to longer wavelengths
Proposed for Venus by Hapke & Nelson (1975)
Spectral shape can evolve with UV exposure
104
Absorption coefficient, cm-1
New bands at longer wavelengths, so…
700
105
Forms amorphous sulfur a-S
Similar structure to polymeric sulfur Sx produced
in liquid sulfur (long chains, large rings)
600
Wavelength, nm
Hosokawa et al., 200 C
Meyer
et al.
103
102
101
S8 α-sulfur
Fuller et al.
Hapke & Nelson
(polymeric portion)
S8 fit
100
10-1
10-2
200
300
400
500
600
Wavelength, nm
700
800
1.2
FeCl3
Polymeric sulfur Sx and FeCl3 compared
Normalized geomtric albedo
1.0
0.01
0.02
0.8
0.05
FeCl3 in 80% H2SO4 at various
percentage concentrations
0.10
0.6
0.20
Spherical grains, 1-µm radius,
log-normal distribution
0.4
Mie scattering calculations using
experimental indices of refraction
0.2
Computed to match observed UV
absorber band depth
0.0
200
300
400
500
600
700
800
Wavelength, nm
S 8 & Sx
1.2
Geometric albedo, normalized
Can discriminate using the long-wave
region > 500 nm
S8
1.0
S8 + Sx
0.8
Sx Hosokawa et al.
Hapke & Nelson
0.6
Two-component cloud
sulfuric acid aerosols +
sulfur grains (2% by number)
Both 1-µm radius, log-normal dist.
Mie scattering
0.4
0.2
0.0
200
300
400
500
Wavelength, nm
600
700
800
Venus Express VIRTIS hyperspectral
image cube
Entrance slit
256 samples
Vega calibration
position (Sample 126)
VV0459_03, MTP016
-45°
Image uses 380 to 400-nm UV bands and
is illumination-corrected and stretched
Black denotes UV absorption
Scanning mirror moves slit in ~ N - S scan
240
lines
Use Vega calibrated sample (126)
60 spectra, each for 1 sample × 4 lines
Spectra normalized to “white” polar cloud
~ -80°
256 samples
300
Absorption extending
to 640 nm and well
beyond!
400
500
600
700
-45
1
So not predominately
FeCl3 nor pure S8.
Polymeric sulfur
implicated
(but may be
consistent with
Krasnopolsky’s lower
and middle cloud
FeCl3/H2SO4
aerosols)
As south latitude
increases, slope
increases
0
-75
300
400
500
Wavelength, nm
600
700
300
Two examples
VV0436_03:
Barker’s ground-based spectra
400
500
600
700
400
500
600
700
1
1.0
ALBEDO
25 July 1974
Average
0.5
Venus albedo
Barker et al., J. Atmos. Sci. 32, 1205 (1975)
0.0
0.3
0.4
0.5
0.6
WAVELENGTH, µm
To investigate further:
Take ratio of 500 – 700 nm slope
to band depth at 350 nm
0
300
Wavelength, nm
S8
Polymeric sulfur
0
-45
Somewhat polymeric at 45°
-50
Polymeric content
increases with south
latitude…
-55
…and time in Hadley cell
Spectrum number
10
20
-60 Latitude
30
-65
0.5
1.0
Slope / Band depth
1.5
2.0
…cloud top descends in
cold collar…
-70
…and overlying hazes
provide complications.
-75
Bright white polar hazes
here
40
50
0.0
Decrease in amount of
sulfur and relative slope
starting at ~ -65° where….
Summary
Provides good evidence for polymeric sulfur as Venus’s UV absorber
Supports Hapke & Nelson’s 1975 suggestion
Consistent with the UV absorber brought up from depth in low latitudes as
suggested by Titov et al. 2008.
Polymerization appears to proceed as sulfur aerosols move poleward
Curious and unexplained behavior at high latitudes
(annealing, mixing effect, temperature effect, stratification?)
Lots more data available for further analysis
Also need photolysis measurements of micron-size S8 grains!
Rate of polymerization as function of excitation wavelength
Absorption spectra changes with temperature
Annealing rate at various temperatures
Backup Material
(Previous work and notes)
I/F ratio
1.0
Virtis VV0459_03
0.9
FeCl3 (0.25%) in sulfuric acid
0.8
S8 + Sµ (0.4:0.6) ) grains +
sulfuric acid droplets
0.7
300
400
500
Wavelength, nm
600
700
0.2
0.3
0.4
0.5
0.6
0.7
REFLECTANCE
1.0
α-S
UV
RED
0.5
295 K
BROWN
106
-1
ORANGE
77 K
0.0
ABSORPTIVITY, cm
X-RAY
105
104
103
102
101
100
10-1
10-2
Venera 9
In-situ & orbital measurements
inferring sulfur
S elemental
X-ray fluorescence, (Andreichikov et al. 1987; see
Krasnopolsky 1989 for review of Vega cloud results)
S elemental
Gas chromatography of cloud refractory material (Porshnev et al.
1987; Surkov et al. 1987)
S2, S3
Descent spectra, S2 ~ 2 ´10−8, S3 ~ 3 ´10−11 to 10 ´10−11
(Moroz, 1979; Sanko 1980; Moshkin 1983; Krasnopolsky 1987,
Malorov et al. 2004)
S2-8, S8
Descent active absorption spectroscopy,
25 ppm at 45 km, 5 ppm at 25 km (Bertaux et al. 1986) But see Bertaux
et al., 1996.
S12
Venera 15 infrared spectroscopy (Spankuch & Schuster 1990)
Sulfur Models and Thoughts
S8 + Sµ
Hapke & Nelson 1974, 1975
S2n
Prinn 1975
S8
Young 1977
One 10-µm Sx grain per 670 H2SO4 droplets.
(sulfur mass > H2SO4 mass!)
Simultaneously suggested sulfur as the absorber
from photochemical S2
Can be S8 if concentration increases with depth
Not S8
Pollack et al., 1979;
Tomasko et al.1979
S8 band edge too sharp, and shifts on cooling.
(but UV irradiation shifts in opposite direction)
S8+S3+S4
Toon et al. 1982
Sulfur grains act as condensation nuclii, forming
cores with H2SO4 mantles. But S3 and S4 are unstable.
Sulfur doesn’t “wet” sulfuric acid, so can’t form cores.
suggests “Gumdrop” Model
H2SO4 cloud model seems to require soluble
condensation nucleii
Young 1983
James et al. 1997
Hapke & Nelson
Young
Two-component cloud
Venus 2-component cloud
Sulfuric acid & sulfur
S8:Sµ ~ 1:1
S grain sizes ~ ½ µm
Relative number ~ 3%
Elemental S/H2SO4 ~ 1%
NORMALIZED GEOMETRIC ALBEDO
1.1
1.0
S8
0.9
S8:Sµ = 0.4:0.6, r = 0.6 µm, f = 0.032
0.8
0.7
0.6
0.5
Barker et al. 1975
0.4
0.3
0.30
0.35
0.40
0.45
0.50
WAVELENGTH, µm
0.55
0.60
Composite particle
If one assumes that all cloud particles are
nucleated with monodisperse soluble sulfur
grains (e. g. thionates), then
The models don’t match observations!
Inclusion of smaller sulfur grains may help.
Probably would require a bi-modal
distribution.
Or allow the presence of non-nucleated
droplets.
NORMALIZED GEOMETRIC ALBEDO
Composite Mode 1 Particles
1.0
0.8
0.1 µm
0.6
0.2 µm
S8:Sµ = 0.4:0.6 (solid lines)
S8:Sµ = 0.6:0.4 (dashed)
0.4
rcore = 0.3 µm
0.2
0.0
0.30
0.35
0.40
0.45
0.50
WAVELENGTH, µm
0.55
0.60
"Gumdrop" model for sulfur-sulfuric acid aerosols
Gumdrop Model
1.2
ALBEDO
1.0
Droplets “decorated” with many
small sulfur grains does not work
(too much absorption).
0.8
f = 0.1
Sulfur radius = 0.3 µm
Sulfuric acid radius = 1 µm
0.6
f = 0.3
0.4
0.2
0.30
0.35
0.40
0.45
0.50
0.55
0.60
1.2
OK for low grain/droplet ratio, with
some fraction “decorated” with one
S grain, most not.
ALBEDO
Elemental S/H2SO4 ~ 1%
1.0
0.8
f = 0.1
0.6
Sulfur radius = 0.6 µm
Sulfuric acid radius = 1 µm
0.4
Results preliminary; more work
needed for very small S grains.
0.2
0.30
f = 0.3
0.35
0.40
0.45
0.50
WAVELENGTH, µm
0.55
0.60