Observation from Earth of an atypical cloud system in the upper Martian atmosphere^★

¹ Univ. Grenoble-Alpes, CNRS, IPAG, 38000 Grenoble, France
e-mail: jean.lilensten@univ-grenoble-alpes.fr
² Société de Planétologie des Pyrénées (S2P), 5 rue Gazan, 75014 Paris, France
³ Commission des Observations planétaires, Société Astronomique de France, 3 rue Beethoven, 75016 Paris, France
e-mail: chrispellier@sfr.fr
⁴ Université Paris-Saclay, LPS (UMR8502), 510 rue André Rivière, 91400 Orsay, France
e-mail: emmanuel.beaudoin@universite-paris-saclay.fr
⁵ Institut d’Astrophysique Spatiale, Université Paris-Saclay, Orsay, France
⁶ Royal Observatory of Belgium, Avenue Circulaire 3, 1180 Brussels, Belgium
⁷ Astronomical Society of Southern Africa, Centurion, South Africa
⁸ University of San Carlos - Physics Department, Nasipit, Talamban, Cebu City, Philippines
⁹ Hellenic Amateur Astronomy Association, Alopekis 42, 10676 Athens, Greece
¹⁰ Canopus Observatory, Ir-Rabat, Malta
¹¹ British Astronomical Association, Burlington House, Piccadilly, London W1J 0DU, UK
¹² Astronomical Society of Australia, PO Box 145 Rubyvale QLD 4702, Australia
¹³ Dept. Mathematics/Centre for mathematical Plasma Astrophysics Celestijnenlaan 200B, 3001 Leuven, Belgium
¹⁴ IMCCE, Observatoire de Paris, PSL Research University, CNRS UMR 8028, Sorbonne Université, 77 avenue Denfert-Rochereau, 75014 Paris, France
¹⁵ Institute of Physics, University of Maria Curie- Pl. M. Curie-Sklodowska 5, 20-031 Lublin, Poland

Received: 7 July 2021
Accepted: 14 February 2022

Abstract

Context. The atmosphere of Mars is characterised by a complex seasonal cycle of cloud formation related to the condensation of CO₂ and H₂O, and to the lifting of surface dust. Several decades of spacecraft observations have provided an impressive amount of data to constrain cloud properties. However, observations of a given cloud obtained from Mars orbit are typically limited in time sampling and spatial coverage. As a complement to this existing dataset, Earth-based telescopic observations have the potential to provide a global and dynamic view of some large-scale Mars clouds.

Aims. On 17 November 2020, Mars and Earth were close to opposition. We took advantage of this configuration to attempt observing large-scale high-altitude atmospheric phenomena from Earth with a high time sampling, over several hours.

Methods. Ten amateur astronomers were coordinated along with professional astronomers to observe Mars.

Results. We observed the occurrence of a large-scale high-altitude cloud system, extending over thousands of kilometres from the equator to 50°S. Over 3 h, it emerged from the night side at 92₋₁₆⁺³⁰ km and dissipated on the dayside. It occurred at a solar longitude of 316° (southern summer) concomitantly to a regional dust storm and west of the magnetic anomaly. Despite its high altitude, it was composed of relatively large particles (effective radius in the 1–2 µm range). While dust appears an unlikely candidate, possible composition by CO₂ or H₂O are both conceivable, although the whole properties of the cloud makes it atypical compared to previously reported clouds. We discuss the possible connections with the dust storm, along with the hypothetical role of nucleation from cosmic particle precipitation.

Conclusions. We continuously followed a high-altitude huge cloud system on Mars from Earth, emerging from the Martian night, from its appearance at the terminator until its complete dissipation. It is either a large-grained water ice cloud system or an extended mid-summer dawn CO₂ cloud system.