Séminaire IPAG

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Observation of a cloud system on Mars

jeudi 15 septembre 2022 - 11h00

Jean Lilensten - IPAG
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The Mars ionosphere and thermosphere are submitted to the solar activity, which results in the existence of local aurorae, specifically around the magnetic anomaly in the southern hemisphere. These aurorae have been discovered in the UV range in 2005, occurring typically between 100 and 200 km height. However, we have shown in 2015 both experimentally and theoretically that a series of visible auroral emissions can occur with sufficiently high intensities to be observed by the naked eyes.  This has been the starting point for a series of observations from the ground of the Earth, conducted by a group of 10 amateur astronomers, scattered over the planet. 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. We got a positive observation which was first though to be the aurorae that we were looking for. However, a detailed analysis of these observations showed that they are actually a large-scale high-altitude cloud system, extending over thousands of kilometers from the equator to 50°. Over 3 hours, it emerged from the night side at about 92 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 micrometers 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. Its location, close to the magnetic anomaly, suggest a link with charged particles, and we therefore suggest the hypothetical role of nucleation from cosmic particle precipitation. Such a cloud system cannot be observed by the existing space probes; only a large scale observation from a large distance permitted its discovery. The next opposition will occur in December 2022. A new set of experiments is foreseen with a larger group of observers, so that we may reproduce this cloud observation and better characterize them, but also with the expectation that we can see the aurorae, accounting for the increase in the solar activity.

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Hôtes : Julien Milli

Salle Manuel Forestini, 414 rue de la piscine, 38400 Saint Martin d'Hères