The 2018 Martian Global Dust Storm over the South Polar Region studied with MEx/VMC
J. Hernández-Bernal, A. Sánchez-Lavega, T. del Río-Gaztelurrutia, R. Hueso, A. Cardesín-Moinelo, E. Ravanis, A. de Burgos-Sierra, D. Titov, S. Wood
TThe 2018 Martian Global Dust Storm over the South Polar Region studied with MEx/VMC
Hernández-Bernal et al. 2019. Manuscript accepted for publication on Geophysical Research Letters This document is distributed under CC BY-SA 3.0 IGO license The 2018 Martian Global Dust Storm over the South Polar Region studied with MEx/VMC
J. Hernández-Bernal , A. Sánchez-Lavega , T. del Río-Gaztelurrutia , R. Hueso , A. Cardesín-Moinelo , E. Ravanis , A. de Burgos-Sierra , D. Titov , S. Wood Dpto. Física Aplicada I, EIB, Universidad País Vasco UPV/EHU, Bilbao, Spain Aula EspaZio Gela, Escuela de Ingeniería de Bilbao, Universidad del País Vasco UPV/EHU, Bilbao, Spain European Space Agency, ESAC, Madrid, Spain Instituto de Astrofísica e Ciências do Espaço, Obs. Astronomico de Lisboa, Portugal European Space Agency, ESTEC, Noordwijk, The Netherlands European Space Agency, ESOC, Darmstadt, Germany
Corresponding author: Jorge Hernández-Bernal ([email protected])
Abstract
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 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
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.
Plain Language Summary
Dust storms of different scales (local, regional…) are common on Mars. Some Martian years a regional storm activates secondary storms and dust encircles the planet, in a dust event usually called a Global Dust Storm. The last Global Dust Storm took place in 2018, and we are not currently able to predict when a new one will occur. Global Dust Storms affect the global dynamics of the Martian atmosphere, and the dynamics of the Polar Regions is a good proxy to the global situation. In this paper, we take advantage of the polar orbit of Mars Express to study the Southern Polar Region during 2018 Global Dust Storm using the Visual Monitoring Camera onboard the spacecraft. We show how the dust penetrated into the Polar Cap, the apparition of
Key Points: The 2018 Global Dust Storm propagated unevenly over the South Polar Region, not covering it fully, and forming elongated narrow dust arcs. Overall, dust moved towards the terminator, reaching velocities up to 100 ms-1 in the morning side. During June-July 2018, the top altitude of dust showed both spatial and temporal variability, ranging from 10 – 70 km. he 2018 Martian Global Dust Storm over the South Polar Region studied with MEx/VMC
Hernández-Bernal et al. 2019. Manuscript accepted for publication on Geophysical Research Letters This document is distributed under CC BY-SA 3.0 IGO license aerosol arcs curved around the pole, and the presence of winds blowing up to 100m/s, not following the usual patterns expected with no Global Dust Storm.
1. Introduction
Mars Global Dust Storms (GDS) are uncommon and nowadays unpredictable aperiodic events (Khare et al., 2017, Montabone and Forget, 2018, and references therein for recent reviews). On 30 May 2018 (Martian Year MY 34), a Dust Storm started in Acidalia Planitia, rapidly evolving to become a planet encircling dust storm (GDS 2018), as described from ground-based observations (Sánchez-Lavega et al., 2019) and from in situ measurements by Curiosity rover (MSL) (Guzewich et al., 2019). Ground-based images show that the storm reached the South Polar Cap edge at 60ºS on 6 June, and that by mid-June dust had penetrated the Polar Cap (Sánchez-Lavega et al. 2019). However, the nearly equatorial viewing geometry from ground-based telescopes prevented a detailed study of the dust propagation in the Southern Polar Region (SPR) and its interaction with the South Polar Cap. Mars Express (MEx) polar orbit allows a nearly nadir view of the poles, and images of both polar regions are regularly obtained using the Visual Monitoring Camera (VMC) (Ormston et al. 2011; Sánchez-Lavega et al., 2018a). Due to a technical pause in VMC operations, monitoring of the 2018 GDS started on 18 June, approximately 20 sols after its onset (Sánchez-Lavega et al., 2019). During the rest of June, and in July and August, VMC obtained a large set of images showing the SPR (Figure 1) at a typical resolution in the range of 9-13 km/pixel (details on the list of observations are given in the Supporting Information). The period under study spans from 18 June to 3 August 2018 (L s he 2018 Martian Global Dust Storm over the South Polar Region studied with MEx/VMC Hernández-Bernal et al. 2019. Manuscript accepted for publication on Geophysical Research Letters This document is distributed under CC BY-SA 3.0 IGO license
2. Dust Distribution in the Southern Polar Region (SPR)
In mid-June 2018, the edge of the Southern Polar Cap (the area of the South Polar Region covered by ice) was at latitude 60°S, in good agreement with previous measurements in this season (Ls he 2018 Martian Global Dust Storm over the South Polar Region studied with MEx/VMC Hernández-Bernal et al. 2019. Manuscript accepted for publication on Geophysical Research Letters This document is distributed under CC BY-SA 3.0 IGO license Figure 1.
Images of the SPR during GDS 2018. (a) Direct polar view: 18 June, 21.50 U.T and its polar projection (b). (b-f) Polar projections covering latitudes 50ºS-90ºS. (c) June 21, 12.40 U.T.; (d) 23 June, 11.50 U.T.; (e) 26 June, 18.25 U.T.; and (f) 11 July, 00.35 U.T. Orange and blue curves indicate respectively the morning and the evening terminator, and the grey line represents the subsolar meridian. In all projections, longitude 0ºE points upwards and east longitude increases clockwise. Hellas planitia, and Barnard and Secchi craters are identified by the letters H, B and S respectively. SP stands for South Pole. The blue arrows indicate morning hazes. See Supporting Information for further details on observations and image processing.
3. Arc-Shaped Aerosol Bands over the Southern Polar Region (SPR) seen in Twilight
The season under study follows the southern spring equinox (L s he 2018 Martian Global Dust Storm over the South Polar Region studied with MEx/VMC Hernández-Bernal et al. 2019. Manuscript accepted for publication on Geophysical Research Letters This document is distributed under CC BY-SA 3.0 IGO license the bands are seen in full length (Figure 2c-d). In the morning terminator, the bands are harder to distinguish since they mix with the morning hazes (Figure 2a-b). In Figure 2e we show the location of all measured bands against a map of the SPR. At the evening terminator, the bands are closer to the pole, at distances ranging between 300 and 1100 km from it. They extend across a typical length of 2000-3000 km, ending further from the pole, at about 500-1500 km in the morning terminator. The bands show irregular morphology with widths in the range he 2018 Martian Global Dust Storm over the South Polar Region studied with MEx/VMC Hernández-Bernal et al. 2019. Manuscript accepted for publication on Geophysical Research Letters This document is distributed under CC BY-SA 3.0 IGO license Figure 2.
Images and structure of the arc bands around the South Pole. (a), (c) Polar projected images of the southern polar region on 1 July, 17:30 U.T. and 22 July, 16:40 U.T. respectively. (b),(d) Schematic representations of previous images showing the night side (dark grey), the morning hazes (blue lilac), and the observed bands, with continuous orange lines indicating visible parts and dotted sections indicating the potential location in the night side, Numbers in orange indicate the estimated length of arcs. Red and blue lines indicate morning and evening terminators, and the brown line indicates the subsolar meridian. Notice the absence of morning hazes and the presence of a fully visible band on the 22 July. (e) Areographic distribution of measured bands over a grey topographic map made from MOLA data (Smith et al., 1999) (different colors represent different observations). (f) Graph showing the Latitude - Local Time distribution of all the observed bands. Different band colors indicate different periods starting on 18 June (blue), 1 July (green), and 18 July (red). Grey areas represent the night.
4. Tracking Motions over the Southern Polar Region (SPR)
Previous wind measurements of the SPR in a period (Ls = 337º-10º) with no dust storms were obtained by Wang & Ingersoll (2003) who analyzed Mars Obiter Camera images finding velocities from 10 to 20 ms -1 . Here we use VMC images to track motions of the dust during the 2018 dust storm (Figure 3). Previous wind measurements associated to cloud features using VMC images can be found in Sánchez-Lavega et al. (2018b). We have measured 10 pairs of VMC images taken between 1 July and 3 August separated by 20-40 minutes at he 2018 Martian Global Dust Storm over the South Polar Region studied with MEx/VMC Hernández-Bernal et al. 2019. Manuscript accepted for publication on Geophysical Research Letters This document is distributed under CC BY-SA 3.0 IGO license a spatial resolution of ~11km/px that allows us to retrieve velocities with an estimated error of 10 ms -1 . We use animations and blinking between the two images of a selected pair to identify moving features, and then point manually to their centers to track their motion. Sharpest aerosol features (suitable for cloud-tracking) are mostly found in regions at morning Local True Solar Time. We assume that features act as passive tracers, and that their motions mark the velocity of the underlying winds. Although other possibilities cannot be excluded, the short time interval in each pair makes advection the most likely cause of motion in features that do not change much in shape or area. The main trend visible in our measurements is that features move towards the terminator (Figure 3). The highest velocities occur at the edge of the polar cap, where they reach up to 110 ms -1 (Figure 3a and Supporting Information Fig. S5). At equal latitudes, velocities in the morning are higher than in the evening. Winds are slower over the polar cap, with typical velocities of 60 ms -1 in the morning (reaching 80-100 ms -1 only in exceptional cases), and still lower (~20-40 ms -1 ) around the pole and in the afternoon side. It is quite remarkable that the pattern of the wind field changes with date. Figure 3a, corresponding to 1 July, shows no hint of circulation around the pole, and the wind vectors are not oriented in the direction of the bands. On the contrary, on the 18 July (Figure 3b) wind vectors are suggestive of a circulation around the pole (longitudes he 2018 Martian Global Dust Storm over the South Polar Region studied with MEx/VMC Hernández-Bernal et al. 2019. Manuscript accepted for publication on Geophysical Research Letters This document is distributed under CC BY-SA 3.0 IGO license Figure 3 . Velocity vectors retrieved tracking discrete features in two different dates: (a) 1 July, 17:20 U.T. and (b) 18 July, 14:40 U.T. Morning terminator, evening terminator, and subsolar meridian are indicated in the same format as in Figure 1.
5. Altitude Reached by Dust During GDS 2018
In addition to apocenter observations of the SPR, some VMC images were programmed to observe the limb of the planet from a closer distance. The orbit of MEx during this period allowed limb images of equatorial and northern latitudes with resolutions of
10 to 70 km) depending on the date and location, reflecting the complex dynamics involved in the GDS 2018. he 2018 Martian Global Dust Storm over the South Polar Region studied with MEx/VMC
Hernández-Bernal et al. 2019. Manuscript accepted for publication on Geophysical Research Letters This document is distributed under CC BY-SA 3.0 IGO license Figure 4 . Upper two panels: Dust projected at limb in two different dates corresponding to a planet wide expansion of the GDS 2018 (Sánchez-Lavega et al., 2019). Lower panel: Location of the limb observations over a map of Mars. Top altitudes are indicated using a color code.
6. Comparison with Model Predictions
The Mars Climate Database (Forget et al., 1999; Millour et al., 2015) provides easy access to a statistical summary of results of the Global Climate Model of Mars developed by the Laboratoire de Météorologie Dynamique (LMD) under different dust and solar activity scenarios. In order to find out how the GDS affected the dynamics of the SPR, we have compared our measurements with he 2018 Martian Global Dust Storm over the South Polar Region studied with MEx/VMC
Hernández-Bernal et al. 2019. Manuscript accepted for publication on Geophysical Research Letters This document is distributed under CC BY-SA 3.0 IGO license the predictions from the model. Since the GDS developed under solar minimum, we considered two solar minimum scenarios: climatology (low dust) and dust storm. The dust storm scenario is not expected to describe the situation with accuracy, but it might be a first approach to the actual dynamics of the atmosphere during the dust storm. For the orbital Solar Longitude and time of the day in Mars corresponding to the observation on 1 July at 17:20 U.T (Figure 3a) the MCD winds are shown in Figure 5. Since our cloud-tracking measurements lack altitude information, we present the velocity field from the model in the altitude range of 0-50 km, which is consistent with the altitude measurements in the previous section. Above 10km, the MCD predicts winds moving towards the terminator, mostly in the evening side; winds in the morning side are slower. These winds seem to result from a displacement of a predicted polar vortex in the direction of the morning side, relative to the terminator. In fact, a similar shift of the north polar vortex was observed in a reanalysis of a regional dust storm in MY 26, Ls~320º (Mitchell et al., 2015). The prediction of motions toward the terminator above 10 km is globally consistent with our measurements shown in Figure 3a, although our measured values do not agree with the predicted velocities, as MCD predicts faster winds in the evening side than in the morning side at any altitude over 10km, while we observe faster winds in the morning side. he 2018 Martian Global Dust Storm over the South Polar Region studied with MEx/VMC Hernández-Bernal et al. 2019. Manuscript accepted for publication on Geophysical Research Letters This document is distributed under CC BY-SA 3.0 IGO license Figure 5.
Wind field retrieved from MCD for dust storm solar minimum scenario, showing winds over the South Pole on 1 July at 17:10 (same time as for Figure 3a) at different altitudes. Equivalent figure for Figure 3b is available in Supporting Information. A white line in the maps shows the terminator. The darker area corresponds to the night side and the lighter are to day side. he 2018 Martian Global Dust Storm over the South Polar Region studied with MEx/VMC
Hernández-Bernal et al. 2019. Manuscript accepted for publication on Geophysical Research Letters This document is distributed under CC BY-SA 3.0 IGO license
7. Conclusions
The GDS 2018 penetrated the southern polar cap region, as already reported by Sánchez-Lavega et al. (2019), but dust coverage was not complete or homogeneous and we do not observe the south polar cap fully covered by dust in any image. In the north, the GDS expansion apparently stopped at he 2018 Martian Global Dust Storm over the South Polar Region studied with MEx/VMC Hernández-Bernal et al. 2019. Manuscript accepted for publication on Geophysical Research Letters This document is distributed under CC BY-SA 3.0 IGO license Acknowledgments, Samples, and Data
This work has been supported by the Spanish project AYA2015-65041-P (MINECO/FEDER, UE) and Grupos Gobierno Vasco IT-1366-19. JHB was supported by ESA Contract No. 4000118461/16/ES/JD, Scientific Support for Mars Express Visual Monitoring Camera. We acknowledge support from the Faculty of the European Space Astronomy Centre (ESAC) VMC images are available at https://archives.esac.esa.int/psa/
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