Ground-based mapping of SO2 and HDO on Venus in the thermal infrared

HAL Id: hal-02323313

https://hal.archives-ouvertes.fr/hal-02323313

Submitted on 21 Oct 2019

HAL is a multi-disciplinary open access

archive for the deposit and dissemination of sci-entific research documents, whether they are pub-lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.

L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Ground-based mapping of SO2 and HDO on Venus in

the thermal infrared

Thérèse Encrenaz, T. Greathouse, Emmanuel Marcq, Hideo Sagawa, Thomas

Widemann, Bruno Bezard, Thierry Fouchet, Franck Lefèvre, Sébastien

Lebonnois, Sushil Atreya, et al.

To cite this version:

Thérèse Encrenaz, T. Greathouse, Emmanuel Marcq, Hideo Sagawa, Thomas Widemann, et al.. Ground-based mapping of SO2 and HDO on Venus in the thermal infrared. EPSC-DPS joint meeting, Sep 2019, Genève, Switzerland. pp.EPSC-DPS2019-83. �hal-02323313�

Ground-based mapping of SO

2

and HDO on Venus in the

thermal infrared

T. Encrenaz (1), T. Greathouse (2), E. Marcq (3), H. Sagawa (4), T. Widemann (1), B. Bézard (1), T. Fouchet (1), F. Lefèvre

(3), S. Lebonnois (5), S. Atreya (6), Y. J. Lee (7), R. Giles (2), S. Watanabe (8)

(1) LESIA, Paris Observatory, PSL, France (therese.encrenaz@obspm.fr), (2) SwRI, San Antonio, TX, USA, (3) LATMOS, (4) Kyoto Sangyo University, Kyoto, Japan, (5) LMD, IPSL, Paris, France, (6) University of Michigan, Ann Arbor, MI, USA, (7) University of Tojyo, Kashiba, Japan, (8) Hokkaido Information University, Hokkaido, Japan

Abstract

Since January 2012 we have been monitoring the behavior of sulfur dioxide and water on Venus, using the Texas Echelon Cross-Echelle Spectrograph (TEXES) imaging spectrometer at the NASA InfraRed Telescope Facility (IRTF, Mauna Kea Observatory). Data were recorded around 1345 cm-1

(7.4 µm) and 530 cm-1 _{(19 µm). The molecules SO} 2,

CO2, and HDO (used as a proxy for H2O) were

observed at the cloudtop of Venus at 7.4 µm, and a few km below at 19 µm. The volume mixing ratio of SO2

was estimated using the SO2/CO2 line depth ratios of

weak transitions; the H2O volume mixing ratio was

derived from the HDO/CO2 line depth ratio, assuming

a D/H ratio of 200 times the terrestrial value (VSMOW). The SO2 mixing ratio shows strong

variations with time and over the disk, showing evidence for the formation of SO2 plumes with a

lifetime of a few hours; in contrast, the H2O abundance

is remarkably uniform over the disk and shows moderate variations as a function of time. We performed a statistical analysis of the behavior of the SO2 plumes, using all TEXES data at 7.4 µm between

2012 and 2018. The plumes appear mostly located around the equator. Their distribution as a function of local time seems to show a depletion around noon, which remains to be confirmed. There is a good agreement between the TEXES results and those obtained in the UV range (SPICAV/Venus Express and UVI/Akatsuki) at a slightly higher altitude. A comparison of TEXES data at 7.4 and 19 µm can be used to retrieve information about the vertical distribution of SO2, which shows a depletion above the

cloudtop [1-4].

1. Short-term variations of SO

Figure 1 shows examples of SO2 and HDO maps

recorded with TEXES. The two pairs of SO2 maps,

separated by 2 hours, were taken on two consecutive days. It can be seen that the SO2 distribution is very

patchy; the SO2 plumes sometimes follow the 4-day

rotation of Venus at the cloudtop over a timescale of 2 hours, but disappear within 24 hours. In contrast, the HDO distribution is very uniform over the disk.

Figure 1: Examples of TEXES maps of SO2 (left and

middle) and HDO (right)

2. Long-term variations of SO

and

H

O

Figure 2 shows the variations of the disk-integrated mixing ratios of H2O and SO2 between January 2012

and September 2018. While the H2O abundance shows

a slow decrease by a factor of about 2 (from about 1 ppmv to 0.5 ppmv) between 2016 and 2018, the SO2

abundance exhibits changes by as much as a factor 20, with a minimum of 30 ppbv in February 2014 and a maximum of 600 ppbv in July 2018.

July 7, 2014 July 8, 2014 January 21, 2017 17:00 UT 17:00 UT

July 7, 2014 July 8:2014 July 12, 2017 19:00 UT 19:00 UT

Figure 2: Long-term variations of the disk-integrated mixing ratios of H2O (top) and SO2 (bottom)

measured by TEXES between 2012 and 2018.

3. Statistical analysis of the SO

plumes: comparison with space data

We have analyzed the behavior of the SO2 plumes as

a function of latitude, longitude and local time. This study has shown that they are mostly located around the equator, with a depletion around noon and two possible maxima around the terminators. More data will be needed to confirm the existence of a semi-diurnal wave. The depletion around noon is also observed in the SO2 distribution retrieved in the UV

by SPICAV/VEx [5] (Fig. 3); a very good agreement is also observed between the SO2 measurements

recorded by TEXES and the ones obtained in the UV by UVI/Akatsuki (Fig. 4). This comparison shows that the SO2 measurements obtained in the thermal infrared

can be used to complement, in the night side, the space data recorded in the UV on the dayside.

Figure 3: The SO2 abundance recorded by SPICAV

(top) and the probability of the SO2 plume occurrence

as measured by TEXES (bottom) as a function of local time.

Figure 4: The SO2 abundance measured in the UV by

UVI/Akatsuki (left) and in the IR by TEXES (right)

4. Vertical distribution of SO

Simultaneous observations of TEXES at 7.4 µm and 19 µm have allowed us to probe SO2 at two different

levels, at the cloudtop and a few kilometers below within the clouds. In addition, the widths of the SO2

lines, broader than the CO2 lines, have allowed us to

constrain the SO2 vertical profile which shows a clear

depletion above the cloudtop [2]. By combining the use of weak and strong CO2 lines, we have shown that,

in the polar collars, the morning terminator is colder than the evening one, showing evidence for a cold diurnal longitudinal wave [2]. In a forthcoming study, we are going to analyze the whole TEXES dataset at 7.4 and 19 µm to refine the vertical distribution of SO2

as a function of latitude and local time.

Acknowledgements

T.E. and T.K.G. were visiting astronomers at the NASA Infrared Telescope Facility, which is operated by the University of Hawaii under Cooperative Agreement no. NNX-08AE38A with the National Aeronautics and Space Administration, Science Mission Directorate, Planetary Astronomy Program. We wish to thank the IRTF staff for the support of the TEXES observations.

References

[1] Encrenaz, T., Greathouse, T. K., Roe, H. et al. 2012,

A&A, 543, A153. [2] Encrenaz, T., Greathouse, T. K., Richter, M. J. et al. 2013, A&A, 559, A65. [3]Encrenaz, Greathouse, T. K., Richter, M. J. et al. 2016, A&A, 595, A74.

[4] Encrenaz, T, Greathouse, T., Marcq, E. et al. 2019, A&A, 623, A70. [5] E. Marcq, K. Jessup, L. Baggio et al., submitted to Icarus (2019).

Encrenaz, T. et al.: HDO and SO2mapping on Venus 21

21 January 2017 13 July 2017

Fig. 14. Maps of the HDO/CO2line depth ratio, using the HDO transition at 1344.90 cm1to

the CO2transition at 1345.22 cm1_{, on January 21, 2017 and July 12, 2017. The subsolar point}

is shown as a white dot.

0 0.5 1 1.5 2 2.5 3 -1 0 1 2 3 4 5 6 7 8 0 100 200 300 400 500 600 700 -1 0 1 2 3 4 5 6 7 8 Jan. Oct. Feb. Jul. Mar. Jan. Dec. Jan. Jul. Jul. Sep. 2012 2014 2015 2016 2017 2018 SO2 vm r( ppb v) H2 O vm r( ppm v) 3 2 1 0 600 400 200 0

Fig. 15. Long-term variations of the H2O volume mixing ratio (top), inferred from the HDO measurements, and the SO2volume mixing ratio (bottom), measured at the cloud top using the

TEXES data at 7.4 µm.

UV ( = 283 nm) albedo, as measured by the UV imager (UVI) onboard Akatsuki on January 21, 2017. The observation time was 03:43 - 04:18 UT and 01:46 UT for TEXES and Akatsuki, respectively. The UV radiances measured by Akatuki UVI were corrected for the incident and emission angle dependences of reflection assuming the Lambert

24 Encrenaz, T. et al.: HDO and SO2mapping on Venus

Fig. 18. The SO2volume mixing ratio at an altitude of 70 km, inferred from the SPICAV data aboard Venus Express. Superimposed: TEXES measurements of SO2rescaled for an altitude of 70 km.

6. Conclusions

In this paper, we have presented the data of our SO2and HDO monitoring at the

cloud top of Venus using the TEXES instrument at 7.4 µm between January 2016 and

Local Time (hour) -0.2 0 0.2 0.4 0.6 0.8 0 5 10 15 20

Probability of SO2 plume appearance

Local time (hours)

0 5 10 15 20

Probability of SO2plume occurrence

0.8 0.6 0.4 0.2 0.0 -0.2 SPICAV TEXES

Fig. 19. Top: The SO2volume mixing ratio, as observed by SPICAV aboard Venus Express at an altitude of 70 km as a function of local time, including all data between 2006 and 2015. Bottom: Probability of occurrence of an SO2plume as a function of local time, including TEXES

data between 2012 and 2018 (see Fig. 11).

UV albedo SO2/CO2map -7.4 µm

UVI/Akatsuki TEXES