New Insights into the Atmospheric Chemistry of Venus from

Venus Express

Yuk L. Yung

Caltech

GISS Seminar, Mar 24 2012

Outline• Why Venus

• Venus Express

• Mesosphere

• Troposphere

• Unsolved Problems

Earth VenusSurface P, bar 1 90Surface T, °C +15 +460

Composition , %N2

O2

Atmospheric H2O Total H2O, cm

CO2

SO2

Clouds

0.780.21< 0.03~3×105

0.0003~10-9

H2O

0.035~ 00.00005~30.965~10-4

H2SO4 + ? (Sx, FeCl3…)

Different Twins

DeMore and Yung, Science, 1982

Upper Atmosphere

Outline• Why Venus

• Venus Express

• Mesosphere

• Troposphere

• Unsolved Problems

Titov et al., 2009

Venus Express Payloads

“Firsts” by Venus Express (1)• First global monitoring of the composition of the lower atmosphere in the near IR spectral windows

from orbit;– This has been done very well by VIRTIS. Abundance of CO, SO2, COS, H2O at ~35 km and H2O at the

surface at all latitudes. Indeed pioneering results. • First coherent study of the atmospheric temperature and dynamics at different levels of the atmosphere

up to the top of the cloud layer;– We have now a survey of temperature structures in the 40-120 km altitude range. From this the

thermal wind field in 50-80km range has been derived. This is complemented by direct wind tracking (clouds) at 70 km, ~60 km, and 50 km.

• First measurements of global surface temperature distribution from orbit;– VIRTIS has almost completely covered the Southern hemisphere. VMC is building surface maps from

~20 S to ~50 N.• First study of the middle and upper atmosphere dynamics from O2, O, and NO emissions;

– These emissions originating around the mesopause (~90-110 km) have been observed and mapped. The regions of maximum brightness of NO and O2 airglow are slightly displaced, leading to new insights to the dynamics in this region.

Svedhem

“Firsts” by Venus Express (2) • First measurements of the non-thermal atmospheric escape

– Great results from ASPERA: escape of O+, H+, and He++ ions is measured as well as spatial distribution of fluxes. The escape of O and H corresponds to water.

• First coherent observations of Venus in the spectral range from UV to thermal infrared;– Accomplished, but thermal range is limited to λ< 5 µm due to the non operational PFS.

• First application of the solar/stellar occultation technique at Venus;– Yes, the technique implemented by SOIR and SPICAV has proven to be extremely effective in sounding

the mesosphere (70-120 km).• Firsts use of 3D ion mass analyzer, high energy resolution electron spectrometer, and energetic neutral

atom imager;– Very good results on characterization of the plasma environment. Comparative studies with both

ASPERA-3 on MEX and ASPERA-4 on VEX.• First complete monitoring of the electromagnetic environment of the planet.

– MAG is providing excellent data on the structure and variability of the induced magnetosphere as well as on lightning. Particularly impressive as VEX has only one field instrument

Svedhem

Atmospheric composition results

SO2CO

H2SO4

H2O

H2O

SO2

SOIR/ SPICAVVIRTIS

VIRTIS

Outline• Why Venus

• Venus Express

• Mesosphere

• Troposphere

• Unsolved Problems

Belyaev et al., Icarus, 2011

SO & SO2

Why there is a problem

Yung and DeMore, Icarus, 1982 Mills, 1998

Volcanism?

Glaze et al., 2010, LPSCSmrekar et al., Science, 2010

Latitudinal Transport?

Bertaux et al., Nature, 2007Patzold et al., Science, 2007

Temperature Profiles

Global Circulation Regimes

Troposphere• Zonal superrotation (>100

m/s)• Poleward winds v ~ 10 m/s

Thermosphere• Zonal superrotation (~100

m/s) • Solar-antisolar circulation

(~200 m/s)

Titov et al., 2009

Aerosol Profiles

Wilquet et al., JGR, 2009

H2SO4 Photolysis?

Sulfur Chemistry above 80 km

Or…

Zhang et al., Icarus, 2011

H2SO4 Case

H2SO4 V.S. Sx

• A sulfur source is required to explain the SO2 inversion layer above 80 km.

• The evaporation of the aerosols composed of sulfuric acid or polysulfur above 90 km could provide the sulfur source.

• Measurements of SO3 and SO (a1∆→X3∑) emission at 1.7 μm may be the key to distinguish between the two models.

Summary

Conclusions (1)

• Recent observations of enhanced amounts of SO2 at 100 km by Venus Express suggest a hitherto unknown source of gaseous sulfur species in the upper atmosphere of Venus. Highly variable correlated with temperature.

• The photolysis of H2SO4 vapor derived from evaporation of H2SO4 aerosols provides a source of SO3, which upon photolysis yields SO2.

• The predicted concentrations of SO and SO3 could be detected by future measurements.

Conclusions (2)

• More experimental work is needed to investigate the molecular dynamics of the photolysis of H2SO4 and its hydrates, as well as the saturation vapor pressure of H2SO4 in the low temperature range (150-300 K).

• A more detailed microphysical aerosol coupled photochemical model is needed.

• The proposed mechanism may play an important role in the recycling of H2SO4 in the terrestrial stratosphere, where the Junge layer (composed of H2SO4 aerosols) is a regulator of climate and the abundance of O3.

Outline• Why Venus

• Venus Express

• Mesosphere

• Troposphere

• Unsolved Problems

Atmospheric composition results

SO2CO

H2SO4

H2O

H2O

SO2

SOIR/ SPICAVVIRTIS

VIRTIS

Novel Chemistry

• SO3 + CO → CO2 + SO2

• SO3 + OCS → CO2 + (SO)2

• (SO)2 + OCS → CO + S2 + SO2

• CO + (1/n)Sn → OCS

• OCS + S → CO + S2

• Krasnopolsky, Pollack, Fegley, Yung

Polysulfur Chemistry

S2 OCS

S3 S4S8

hv

S

S

hv

hv

hv

hv

CO

S4

S2

S

S3

S4

hv S

Carlson, R. W. Venus' Ultraviolet Absorber and Sulfuric Acid Droplets.International Venus Conference, Aussois, France, 44 (2010).

Hadley Circulation

• Venus = 2000×109 kg/s ≈ 10×Earth

• Earth = 180×109 kg/s

OCSOCS

OCS

Conclusions

Novel Chemistry of OCS and CO via polysulfur photochemistry for converting CO to OCS

Integrated destruction rate of OCS is 23,000 Tg-S/yr [Earth Pinatubo = 10 Tg-S/yr]

Flux ~ 1012 cm-2s-1

Comparable production and flux for CO

Outline• Why Venus

• Venus Express

• Mesosphere

• Troposphere

• Unsolved Problems

Atmospheric composition results

SO2CO

H2SO4

H2O

H2O

SO2

SOIR/ SPICAVVIRTIS

VIRTIS

Conclusion: Julie Moses is the greatest science fiction writer since Paul Asimov!

Liang and Yung (2009)

Liang and Yung (2009)

Long term evolution of SO2

1970 1975 1980 1985 1990 1995 2000 2005

101

102

103

SO2 vs. Time

Anderson et al.Owen&Sagan

Balloon

Venera 15

HSTRocket

IUE

PV

SOIR (68 km)SOIR (70 km)

SPICAV UV(at 100 km)

SO

2 a

bu

nd

ance

at

leve

l 40

mb

ar (

~69

km

), p

pb

upper limit

YEAR

SPICAV UV NADIR(~70 km)

51

Li, …Yung 2009Time 15 June 2009

52

• Co-authors: Liang, M. C., Mills, F. P., Belyaev, D. A., Arthur Zhang

• Marcq, E., Parkinson, C., Bougher, S., Brecht, A., Ingersoll, A., Yang, D., Zeng, R., Gerstell, M., Line, M.

• NASA Grant • Venus Express Project

Acknowledgement

Thanks