Coordinated Hubble Space Telescope and Venus Express Observations of Venus’ upper cloud deck

Abstract

International audienceHubble Space Telescope Imaging Spectrograph (HST/STIS) UV observations of Venus’ upper cloud tops were obtained between 20N and 40S latitude on December 28, 2010; January 22, 2011 and January 27, 2011 in coordination with the Venus Express (VEx) mission. The high spectral (0.27 nm) and spatial (40-60 km/pixel) resolution HST/STIS data provide the first direct and simultaneous record of the latitude and local time distribution of Venus’ 70-80 km SO and SO2 (SOX) gas density on Venus’ morning quadrant. These data were obtained simultaneously with a) VEx/SOIR occultation and/or ground-based James Clerk Maxwell Telescope sub-mm observations that record respectively, Venus’ near-terminator SO2 and dayside SOx vertical profiles between ∼75-100 km; and b) 0.36 μm VEx/VMC images of Venus’ cloud-tops. Updating the (Marcq, E., Belyaev, B., Montemssin, F., Fedoroav, A., Bertuax, J-L., Vandaele, A. C., Neefs, E. [2011], Icarus, 211: p. 58–69) radiative transfer model SO2 gas column densities of ∼ 2-10 μm-atm and ∼ 0.4-1.8 μm-atm are retrieved from the December 2010 and January 2011 HST observations, respectively on Venus’ dayside (i.e., at Solar Zenith Angles (SZA) < 60°); SO gas column densities of 0.1-0.11μm-atm, 0.03-0.31 μm-atm and 0.01-0.13 μm-atm are also retrieved from the respective December 28, 2010, January 22, 2011 and January 27, 2011 HST observations. A decline in the observed low-latitude 0.24 and 0.36 μm cloud top brightness also paralleled the declining SOx gas densities. On December 28, 2010 SO2 VMR values ∼ 280-290 ppb are retrieved between 74-81 km from the HST and SOIR data obtained near Venus’ morning terminator (at SZAs equal to 70° and 90°, respectively); these values are 10x higher than the HST-retrieved January 2011 near terminator values. Thus, the cloud top SO2 gas abundance declined at all local times between the three HST observing dates. On all dates the average dayside SO2/SO ratio inferred from HST between 70-80 km is higher than that inferred from the sub-mm the JCMT data above 84 km confirming that SOx photolysis is more efficient at higher altitudes. The direct correlation of the SOx gases provides the first clear evidence that SOx photolysis is not the only source for Venus’ 70-80 km sulfur reservoir. The cloud top SO2 gas density is dependent in part on the vertical transport of the gas from the lower atmosphere; and the 0.24 μm cloud top brightness levels are linked to the density of the sub-micron haze. Thus, the new results may also suggest a correlation between Venus’ cloud-top sub-micron haze density and the vertical transport rate. These new results must be considered in models designed to simulate and explore the relationship between Venus’ sulfur chemistry cycle, H2SO4 cloud formation rate and climate evolution. Additionally, we present the first photochemical model that uniquely tracks the transition of the SO2 atmosphere from steady to non-steady state with increasing SZA, as function of altitude within Venus’ mesosphere, showing the photochemical and dynam...

Composition and Chemistry of the Neutral Atmosphere of Venus

“…The HST/STIS instrument carried out high-resolution observations of both SO and SO 2 at cloud deck altitude during selected days in 2010 and 2011. The HST retrieved SO gas column densities (Jessup et al 2015) were positively correlated with both the temporal and latitudinal variability of SO 2 ; for example, the largest values of both species were observed at the terminator. This implies that the SO x balance in the atmosphere is not purely photochemically driven, but that an additional source for SO 2 is available in the 74-81 km region; otherwise the two species would exhibit anti-correlated densities.…”

“…This analysis provides SO 2 number density and volume mixing ratio vertical profiles in the 65 to 100 km region at the Venus terminator, covering all latitudes, in the period May 2006 to February 2013. Figure 5 summarises the different measurements of SO 2 performed above the clouds using different techniques, from ground-based to space-borne observations, including detection using the STIS (Space Telescope Imaging Spectrograph) UV spectrometer on the HST (Hubble Space Telescope) (Jessup et al 2015).…”

“…negligible abundance below a cut-off altitude, Observations and model profiles for SO 2 . Green crosses are SO 2 observations by SOIR, blue asterisks are solar occultation observations by SPICAV-UV, red diamonds are HST observations (Jessup et al 2015), and magenta vertical dotted lines are JCMT observations above 84 km with an indicative upper limit below 84 km. Typical uncertainties on the observations are indicated by the half-error bars at the top.…”

Long-term Variations of Venus’s 365 nm Albedo Observed by Venus Express, Akatsuki, MESSENGER, and the Hubble Space Telescope

“…An error of 5-10% is estimated at 340-390 nm from star observations during MESSENGER's cruise phase (Holsclaw et al 2010). Other near-UV Venus spectra were taken with the Space Telescope Imaging Spectrograph (STIS) of Hubble Space Telescope with the G430L grating mode (290-570 nm, spectral resolution 0.54 nm) on 2011 January 2 (Jessup et al 2015). 5% error is estimated for STIS measurements from based on regular standard star observations (Jessup et al in press).…”

Identification of OSSO as a near‐UV absorber in the Venusian atmosphere

“…Ground‐based studies of Venus have also taken place [ Barker , ; Sandor et al , ; Travis , , and references therein]. Many features of the Venusian atmosphere have been studied, such as the atmospheric distribution of SO and SO 2 and the interannual variation in their concentration, the sulfuric acid cycle, and carbonyl sulfide (OCS) chemistry [ Esposito et al , ; Seiff et al , ; Belyaev et al , ; Sandor et al , ; Esposito , ; Esposito et al , ; Marcq et al , ; Encrenaz et al , ; Marcq et al , ; Encrenaz et al , ; Jessup et al , ; Zhang et al , ; Palmer and Williams , ; Yung et al , ; Krasnopolsky , ; Arney et al , ; Marcq et al , , , ]. Extensive modeling of the atmospheric chemistry on Venus has also been done [ Krasnopolsky , ; Zhang et al , ].…”