Mapping the thermal structure and minor species of Venus mesosphere with ALMA submillimeter observations^⋆

LESIA, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Universités, UPMC Univ. Paris 06, Univ. Paris Diderot, Sorbonne Paris Cité, 5 place Jules Janssen, 92195 Meudon, France
e-mail: arianna.piccialli@aeronomie.be

Received: 3 April 2017
Accepted: 27 June 2017

Abstract

Context. Water vapor and sulfur compounds are key species in the photochemistry of Venus mesosphere. These species, together with mesospheric temperatures, exhibit drastic temporal variations, both on short timescales (diurnal and day-to-day) as well on long timescales, far from being understood.

Aims. We targeted CO, SO, HDO and SO₂ transitions in the submillimeter range using the Atacama Large Millimeter Array (ALMA) to study their spatial and temporal variations.

Methods. Four sets of observations were acquired on different dates in November 2011 during the first ALMA Early Science observation Cycle 0. Venus angular diameter was about 11′′ with an illumination factor of 92%, so that mostly the day side of the planet was mapped. Assuming a nominal CO abundance profile, we retrieved vertical temperature profiles over the entire disk as a function of latitude and local time. Temperature profiles were later used to retrieve SO, SO₂, and H₂O. We used HDO as a tracer for water assuming a D/H enrichment of 200 times the terrestrial value.

Results. We derived 3D maps of mesospheric temperatures in the altitude range 70−105 km. SO, SO₂, and H₂O are characterized by a negligible abundance below ~ 85 km followed by an increase with altitude in the upper mesosphere. Disk-averaged SO abundances present a maximum mixing ratio of 15.0 ± 3.1 ppb on November 26 followed the next day by a minimum value of 9.9 ± 1.2 ppb. Due to a very low S/N, SO₂ could only be derived from the disk-averaged spectrum on the first day of observation revealing an abundance of 16.5 ± 4.6 ppb. We found a SO₂/SO ratio of 1.5 ± 0.4. Global maps of SO reveal strong variations both with latitude and local time and from day to day with abundance ranging from < 1 to 15 ppb. H₂O disk-averages retrievals reveal a steady decrease from November 14 to 27, with the abundance varying from 3.6 ± 0.6 ppm on the first day to 2.9 ± 0.7 ppm on the last day. H₂O maps reveal a slightly higher abundance on the evening side compared to the morning side and a strong depletion between the first and the second day of observation.