We analyze the onset and initial expansion of the 2018 Martian Global Dust Storm (GDS 2018) using ground‐based images in the visual range. This is the first case of a confirmed GDS initiating in the Northern Hemisphere. A dusty area extending about 1.4×105 km2 and centered at latitude +31.7°±1.8° and west longitude 18°±5°W in Acidalia Planitia was captured on 30 and 31 May 2018 (Ls = 184.9°). From 1 to 8 June, daily image series showed the storm expanding southward along the Acidalia corridor with velocities of 5 m/s and simultaneously progressing eastward and westward with horizontal velocities ranging from 5 to 40 m/s. By 8 June the dust reached latitude ‐55° and later penetrated in the South polar region, whereas in the North the dust progression stopped at latitude approximately +46°. We compare the onset and expansion stage of this GDS with the previous confirmed storms.

We study the 2018 Martian global dust storm (GDS 2018) over the Southern Polar Region using images obtained by the Visual Monitoring Camera (VMC) on board Mars Express (MEx) during June and July 2018. Dust penetrated into the polar cap region but never covered the cap completely, and its spatial distribution was nonhomogeneous and rapidly changing. However, we detected long but narrow aerosol curved arcs with a length of ~2,000–3,000 km traversing part of the cap and crossing the terminator into the nightside. Tracking discrete dust clouds allowed measurements of their motions that were toward the terminator with velocities up to 100 m/s. The images of the dust projected into the Martian limb show maximum altitudes of ~70 km but with large spatial and temporal variations. We discuss these results in the context of the predictions of a numerical model for dust storm scenario.

We report a previously unnoticed annually repeating phenomenon consisting of the daily formation of an extremely elongated cloud extending as far as 1,800 km westward from Arsia Mons. It takes place in the solar longitude (Ls) range of ∼220°-320°, around the Southern solstice. We study this Arsia Mons Elongated Cloud (AMEC) using images from different orbiters, including ESA Mars Express, NASA MAVEN, Viking 2, MRO, and ISRO Mars Orbiter Mission (MOM). We study the AMEC in detail in Martian year (MY) 34 in terms of local time and Ls and find that it exhibits a very rapid daily cycle: the cloud growth starts before sunrise on the western slope of the volcano, followed by a westward expansion that lasts 2.5 h with a velocity of around 170 m/s in the mesosphere (∼45 km over the areoid). The cloud formation then ceases, detaches from its formation point, and continues moving westward until it evaporates before the afternoon, when most sun-synchronous orbiters make observations. Moreover, we comparatively study observations from different years (i.e., MYs 29-34) in search of interannual variations and find that in MY33 the cloud exhibits lower activity, while in MY34 the beginning of its formation was delayed compared with other years, most likely due to the Global Dust Storm. This phenomenon takes place in a season known for the general lack of clouds on Mars. In this paper we focus on observations, and a theoretical interpretation will be the subject of a separate paper. HERNÁNDEZ-BERNAL ET AL.

No abstract

<p><strong>1. Introduction</strong></p><p>The European Space Agency (ESA) mission ‘Mars Express’ (MEX) launched in 2003 equipped with seven instruments. The Visual Monitoring Camera (VMC) on board MEX was designed to monitor the release of the Beagle 2 lander, but was switched back on again in 2007. In the following years, in addition to helping engage the general public with the MEX mission [1] VMC images were used for atmospheric studies [2,3] and subsequently the camera was ‘upgraded’ to a scientific instrument in 2016. Hence, the mission ‘gained’ a scientific instrument in the form of the VMC. The scientific success [4] of this small camera is a part of the larger success story of Europe’s first Mars mission, serving as an example of how planetary missions can exceed and build upon their original expectations. This work details the journey of VMC from an engineering to a scientific instrument, including how VMC is operated, how the data is calibrated, and examples of the scientific work that has been undertaken with VMC data, images of which are exemplified in Figure 1.</p><p><img src="https://contentmanager.copernicus.org/fileStorageProxy.php?f=gepj.09f38c8413fe52188892951/sdaolpUECMynit/0202CSPE&app=m&a=0&c=be7c52c6916c63e67ea6d89a3e01936a&ct=x&pn=gepj.elif" alt=""></p><p><strong>2. Instrument Operations</strong></p><p>The VMC is a 640x480 pixel camera with a large field of view (FOV) of ~40 x 31°. The wide FOV allows the camera to capture both the entire disk of Mars within the image and to perform observations over a wide portion of the limb. When taken in combination with the elliptical orbit of MEX, this enables observations at different local times and distances. VMC has a different data protocol and is offset from other instruments by 19°, and for these reasons cannot observe at the same time as other instruments on MEX. Since 2018, planning for the VMC has been integrated with planning for the other payload instruments, which takes place at the European Space Astronomy Centre (ESAC). This integration has increased both the quantity and the types of observations performed by VMC (Figures 2 and 3).</p><p><img src="https://contentmanager.copernicus.org/fileStorageProxy.php?f=gepj.664a359413fe51288892951/sdaolpUECMynit/0202CSPE&app=m&a=0&c=cdd07e37c78f7d340d3bbe0a9a96ee0f&ct=x&pn=gepj.elif" alt=""></p><p><img src="https://contentmanager.copernicus.org/fileStorageProxy.php?f=gepj.cf829f9413fe51388892951/sdaolpUECMynit/0202CSPE&app=m&a=0&c=065e933e2ecfeee5895c54a72354302d&ct=x&pn=gepj.elif" alt=""></p><p><strong>3. Data Calibration</strong></p><p>The VMC team has performed in situ calibration for VMC since no on-ground calibration exists for the instrument (discussed in [5]). Observations of dark sky were taken to create a master dark-current file for dark-current correction. Dark-corrected images of flat portions of Mars taken at pericentre that were well and uniformly illuminated, as free as possible from large structures and as flat as possible were used to create a file for flat-field correction. The boresight offset of VMC has also been calculated by comparing the location of stars in VMC images with the stars’ known positions given by the SPICE geometry information system.</p><p><strong>4. Data Processing and Archiving</strong></p><p>Since [5], the VMC pipeline has been updated in collaboration with the science team at UPV-EHU Bilbao. VMC data are dark-corrected, flat-fielded, and are now provided in raw, FITS and PNG formats. The VMC pipeline runs at ESAC and is utilised by the VMC science team, and the current dataset from 2007 to the present totals ~50,000 images distributed across ~3000 observations. VMC data for scientific usage have been prepared for ingestion into the Planetary Science Archives (PSA) over the summer of 2020. This will be the first science data release from the instrument, thereby augmenting the already extensive wealth of data obtained from Mars Express over the last 17 years. Data from the VMC instrument continue to be available for outreach purposes through Twitter and Flickr (@esamarswebcam, Flickr: https://www.flickr.com/photos/esa_marswebcam/).</p><p><strong>5. Scientific Success</strong></p><p>The regional and global scale atmospheric dynamics of Mars are fast-paced and so a high temporal resolution of observations at various local times is required to help us understand and constrain how such dynamics develop. As previously mentioned, the wide FOV of VMC coupled with the highly elliptical orbit of MEX allows VMC to take observations at diverse local times and therefore to capture these large-scale atmospheric phenomena (Figure 4). VMC images are taken approximately every ~48 seconds depending on exposure time, and so the science team has been able to stack images from the same observation and also produce mosaics and videos showing the movements of aerosols. VMC data has been used for the analysis of the Arsia Mons cloud [6]; a recurrent double cyclone in the north polar region [7]; the 2018 global dust storm [8] and local dust storms in 2019 [9]; and ‘twilight clouds’ in the Martian night [10].</p><p><img src="https://contentmanager.copernicus.org/fileStorageProxy.php?f=gepj.80b53aa413fe52488892951/sdaolpUECMynit/0202CSPE&app=m&a=0&c=e3cd785599ffe99c03074a74bc71d1fc&ct=x&pn=gepj.elif" alt=""></p><p><strong>References:</strong></p><p>[1] Ormston, T., et al. (2011). An ordinary camera in an extraordinary location: Outreach with the Mars Webcam. Acta Astronautica, 69, 703-713.</p><p>[2] Sánchez-Lavega, A., et al. (2016). Limb clouds and dust on Mars from VMC-Mars Express images. DPS 48,  16-21 October 2016, 409-01.</p><p>[3] Sánchez-Lavega, A., et al. (2018). Limb clouds and dust on Mars from images obtained by the Visual Monitoring Camera (VMC) onboard Mars Express. Icarus 299: 194-205.</p><p>[4] Cardesín-Moinelo, A., et al. (2017). A “NEW” SCIENTIFIC CAMERA AROUND MARS, GETTING SCIENCE WITH VISUAL MONITORING CAMERA ONBOARD MARS EXPRESS. Sixth International Workshop on the Mars Atmosphere: Modelling and Observations, 17-20th January 2017, Granada, Spain.</p><p>[5] Ravanis, E.M., et al. (2019). Mars Express Visual Monitoring Camera: New Operations and Data Processing for more Science. EPSC2019, 15-20 September, Geneva, Switzerland.</p><p>[6] Hernández-Bernal et al. (2020) An Extremely Elongated Cloud over Arsia Mons Volcano on Mars: Life Cycle. Submitted to Journal of Geophysical Research.</p><p>[7] Sánchez-Lavega, A., et al. (2018) A seasonally recurrent annular cyclone in Mars northern latitudes and observations of a companion vortex. Journal of Geophysical Research: Planets 123.11: 3020-3034</p><p>[8] Hernández‐Bernal, J., et al. (2019). The 2018 Martian Global Dust Storm over the South Polar Region studied with MEx/VMC. Geophysical Research Letters 46.17-18: 10330-10337.</p><p>[9] Sánchez-Lavega, A., et al. (2020). Patterns in textured dust storms in Mars North Pole. EPSC2020, 21 September – 9<sup>th</sup> October 2020, Virtual.</p><p>[10] Hernández-Bernal, J. et al. (2020). A long term study of twilight clouds on Mars based on Mars Express VMC images. EPSC2020, 21 September – 9<sup>th</sup> October 2020, Virtual.</p>