Amateur observation of an atypical martian atmospheric feature: when serendipity leads to identify an atypical cloud system
- 1French Astronomical Society (SAF), Planetary observations commission, Tournefeuille, France (delcroix.marc@free.fr)
- 2Univ. Grenoble-Alpes, CNRS, IPAG, 38000 Grenoble, France
- 3Société de Planétologie des Pyrénées (S2P) 5 rue Gazan, 75014 Paris, France
- 4Université Paris-Saclay, LPS (UMR8502), 510 Rue André Rivière, 91400 Orsay, France
- 5Institut d’Astrophysique Spatiale, Université Paris Saclay, Orsay, France
Introduction
Amateur observations of atmospheric features on the limb or night side of Mars proved their interest ([1], [2]). This led JL, specialist in aurorae, to collaborate with JLD, advanced amateur astronomer, to coordinate ten amateurs for attempting the first observation from Earth of aurora above the limb or on the night side of Mars.
Observations
On Nov. 17th, 2020 (316° solar longitude), one of those amateurs, CP observed a suspect phenomenon over the night side of Mars. We identified an exceptional quality simultaneous observation by EB, over a three-hour timespan (fig. 1). Observation of the data set shows a 3000 km (from equator to South) detached layer on the night side, which seems to rotate with the planet on the day side, casting shadows before disappearing.
Fig. 1. Detached layer from 20H25 to 21H26 UT through red (R), green (G), and blue (B) filters. The disk is overexposed to better show the phenomenon.
Analysis
This feature could be an aurora, or a cloud system made from dust, H2O or CO2. With MV, specialist in Mars clouds, the collaborative team worked to characterize its altitude, its photometric properties, and its possible composition to determine its type.
a. Altitude
It was determined through several methods, using measures of the apparent position of the features on the images. Assuming the detached layer is seen at the time when the cloud emerges from night side, we used both a simple 2D method (fig. 2) and the 3D equations of [1] on the emergence images’ measures. Another method used the measure of the length of the shadow casted by the features. A last method measured the clouds fronts’ position following the features when it rotates on day side.
Amateurs MD, JLD and EB worked out those different methods which led overall to an altitude of 92 (+30/-16) km.
Fig. 2: Detached layer altitude determination with 2D geometric method.
b. Colour profile and albedo
Amateur CP performed UBVRI photometry ([3]) of the planet and the layer. Reference stars observed at the same airmass as the observation were used. Different features were measured (bright and dark terrains, polar cap) as well as the overall globe, and one part of the detached layer. Fig. 3 shows the respective albedos measured, showing how the different zones measured reflects sunlight. The detached layer reflectance is twice brighter in red than in blue (while bright reddish terrain like Amazonis is five times).
Fig. 3. Albedo of different Martian structures compared to those of the full globe and of the observed detached layer.
c. Size and optical depth of particles constituting the layer
Colour profile shows that the layer scatters light over the whole spectrum, inconsistent with Rayleigh scattering or single wavelength emission, suggesting a layer consisted of dust aerosols or ice crystals.
Professional MV used [4] to model ice scattering reflectance of CO2 and H2O, resulting in fig. 4, showing that the reflectance profile of the layer is compatible with either 1-2µm CO2 or 2(+/-1) µm H2O particle sizes.