ODAA3

The cloud discontinuity of Venus is a planetary-scale phenomenon known to be recurrent since, at least, the 1980s. It was initially identified in images from JAXA’s orbiter Akatsuki.  This disruption is associated to dramatic changes in the clouds’ opacity and distribution of aerosols and is interpreted as a new type of Kelvin wave. The phenomenon may constitute a critical piece for our understanding of the thermal balance and atmospheric circulation of Venus. The  reappearance on the dayside middle clouds  four years after its last detection with Akatsuki/IR1 is reported in this work. We characterize its main properties using exclusively near-infrared images from amateur observations for the first time. The discontinuity exhibited tempοrаl variations in its zonal speed, orientation, length, and its effect over the clouds’ albedo during the 2019/2020 eastern elongation in agreement with previous rеρorts. Moreover, amateur observations are compared with simultaneous observations by Akatsuki UVI and LIR confirming that the discontinuity is not visible on the upper clouds’ albedo or thermal emission. While its zonal speeds are faster than the background winds at the middle clouds, and slower than winds at the clouds’ top, it is evidencing that this Kelvin wave might be transporting momentum up to upper clouds.

How to cite: Kardasis, E., Peralta, J., Maravelias, G., Imai, M., Wesley, A., Olivetti, T., Naryzhniy, Y., Morrone, L., Gallardo, A., Calapai, G., Camarena, J., Casquinha, P., Kananovich, D., MacNeill, N., Viladrich, C., and Takoudi, A.: Results from the professional-amateur collaboration to investigate the Cloud Discontinuity phenomenon in Venus’ atmosphere, Europlanet Science Congress 2022, Granada, Spain, 18–23 Sep 2022, EPSC2022-208, https://doi.org/10.5194/epsc2022-208, 2022.

The early 2020 eastern (evening) elongation of Venus was a particularly favourable one for northern hemisphere observers, and the early 2022 western (morning) elongation was favourable for southern observers. A number of observers in both hemispheres achieved IR imaging at around 1000nm of the night-time surface of Venus when in crescent phase. This presentation reviews the equipment and methods used by successful imagers of the night side, including the sky criteria necessary (altitude, solar altitude and phase), and the results obtained.

IR narrowband images taken around 1000nm show relief on the night-time surface of the planet due to differential cooling; the mountain-tops cool faster than the valleys and so emit less IR radiation. In several bands this radiation is not fully absorbed by the atmosphere and can be detected with small earth-based telescopes (at least 200mm aperture) so long as the sky is dark enough, light scatter from the illuminated planet in telescope and camera is minimised, and there is sufficient blocking of shorter wavelengths. This has been demonstrated since 2009. The signal detected by this method is not 100% correlated with surface relief, but seems to be modulated by some atmospheric effects also.

There is a key observational trade-off of sky darkness against altitude of the planet. Acceptable results are not obtained until the signal from the surface is at least 50% above the noise due to the sky background, but below 10º altitude the noise is increased to unacceptable levels by atmospheric absorption.

The design of the sensor is found to be critical. Most sensors marketed to amateurs have too much internal scatter at these wavelengths to be usable at such low s/n ratios. However, good results have been obtained with a number of commercial cameras.

 The relationships between Venus’s rotation period and Earth and Venus’ orbital periods conspire to mean that essentially only one range of longitudes is imageable at all eastern elongations, and another at all western elongations. One reason for doing this work is the possibility that Venus could still have active vulcanism. If it does, and if it were on a large enough scale, this technique could potentially reveal. it. Observations in the period in question, however, did not generate any evidence of this being the case.

How to cite: Arditti, D., Lewis, M., Miles, P., and Wesley, A.: Amateur observations of the surface of Venus in 2020-22, Europlanet Science Congress 2022, Granada, Spain, 18–23 Sep 2022, EPSC2022-1268, https://doi.org/10.5194/epsc2022-1268, 2022.

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. 17^th, 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.