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On November 17, 2020, a 3,000 km long cloud was observed on the surface of Mars following a joint effort by a group of amateur and professional astronomers. Never before has a planet been so closely targeted by space missions, with 7 robots currently active in orbit and on the surface, but paradoxically almost none of them offers a global view of the planet (with the exception of the Emirates Mars Mission). So it was from Earth that this discovery was made (Lilensten et al., 2021). The Red Planet is not continuously monitored by professional ground-based astronomers. In fact, at present, only amateur astronomers carry out this work around periods of opposition. They contribute to several participatory science databases, such as Association of Lunar and Planetary Observers in Japan (ALPO, http://www.alpo-j.sakura.ne.jp/Contents/homeE.htm) or Planetary Virtual Observatory and Laboratory (PVOL, http://pvol2.ehu.eus/pvol2/). Some of these enthusiastic persons have joined the monitoring program. In 2018, they set up a network of 10 observers spread across all continents to be able to follow the Red Planet at all times.On November 17, 2020, 2 of the 10 observers were able to photograph Mars: Frenchmen Christophe Pellier and Emmanuel Beaudoin. Their data, acquired under excellent conditions, made it possible to follow a huge cloud structure located at the terminator for 3 hours in a row through different filters. The images obtained show the formation emerging from the night, clearly separated from the terminator. Its evolution was followed until the sun rose over the terrain beneath the cloud. The cloud dissipates shortly afterwards. What was observed here is atypical in two respects: not only is the cloud complex gigantic in relation to the planet, it is also located at an altitude of 92 km, at the gateway to the interplanetary void. This altitude is comparable to that of noctilucent clouds regularly observed on Earth at high latitudes around the summer solstices. The altitude was assessed by 3 independent methods, notably by Marc Delcroix, head of the Socit Astronomique de Frane (SAF, https://saf-astronomie.fr/) planetary observations commission.Detailed analysis of the photometric data showed that the cloud scatters light at all visible wavelengths, with a maximum in the red. This suggests that the light is scattered by dust, water ice or CO2 ice particles. The dust hypothesis has been ruled out as incompatible with observations, but water and CO2 are good candidates. Water ice clouds, for example, have already been observed at this altitude and in this season. However, water is rare on Mars, and spectroscopic detections to date show that water ice crystals are usually very small at this altitude, with a typical size of around 0.1 to 0.5 m. However, the photometric data obtained on November 17, 2020 suggest that they are rather larger particles of 1 to 2 m, which would make this cloud atypical, beyond its unusual dimensions. CO2, on the other hand, is abundant, and is the main component of the Martian atmosphere. Previously observed CO2 clouds at these altitudes may be composed of ice crystals of this size. However, the observation was obtained outside the typical season for CO2 clouds, and these are usually a few hundred km or even 1000 km across, but never 3000 km, which would also make this cloud an atypical event.At the same time, on November 17, 2020, a dust storm was developing on Mars, questioning the possibility that this activity could be contributing to a rise in atmospheric layers, favoring the formation of cloud structures atypical in size. We also propose that cosmic rays play a role in the nucleation of ice crystals at this altitude, especially as the structures observed are on the edge of a magnetic zone. The initial aim of the program was however fairly different: it was to detect polar aurorae on Mars, following the prediction made in Lilensten et al., 2015. This hypothesis was seriously considered to explain the observation of November 17, 2020, but then dismissed because the structures observed cast a shadow on the ground. Moreover, the phenomenon tends to occur at the edge of areas where Mars' magnetic field is most disturbed, and solar activity was low that day. On the other hand, a review of previous amateur observations published in 2015 by Sanchez et al. shows that the 2012 observations they report take place above this magnetic zone and coincide with coronal mass ejection episodes. We are therefore potentially dealing with two different phenomena.Both phenomenon high altitude clouds and aurorae are observable from the Earth while the planets are in opposition. This occurs about every 2 years. We were lucky in November 2020 but the weather conditions were unfavorable in 2022. We are now preparing for the next opposition (January 16, 2025). The best periods for these observations are around November 15-20, 2024, around Christmas, and the first ten days of March 2025. In this lecture, we will therefore give a call for participation to this program over the 5 Earth continents, so that whatever the local time and the local weather, observers can help either to better characterize these clouds and/or to make the first observation of the Mars visible aurorae.
Lilensten et al. (Wed,) studied this question.