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.
Key Points
May‐June 2018 ground‐based images show the onset and early evolution of a Martian Global Dust Storm (GDS)
The outbreak took place at location (North hemisphere) and time (solar longitude 184.9°) unusual for most GDSs
The expansion involved horizontal velocities in all directions in the range 5‐40 m/s
We investigate the long‐term motion of Saturn's north pole hexagon and the structure of its associated eastward jet, using Cassini imaging science system and ground‐based images from 2008 to 2014. We ...show that both are persistent features that have survived the long polar night, the jet profile remaining essentially unchanged. During those years, the hexagon vertices showed a steady rotation period of 10 h 39 min 23.01 ± 0.01 s. The analysis of Voyager 1 and 2 (1980–1981) and Hubble Space Telescope and ground‐based (1990–1991) images shows a period shorter by 3.5 s due to the presence at the time of a large anticyclone. We interpret the hexagon as a manifestation of a vertically trapped Rossby wave on the polar jet and, because of their survival and unchanged properties under the strong seasonal variations in insolation, we propose that both hexagon and jet are deep‐rooted atmospheric features that could reveal the true rotation of the planet Saturn.
Key Points
Hexagon wave steady motion
Jet stream unchanged to seasonal changes
Saturn's rotation
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.
Plain Language Summary
Dust storms of different scales (local, regional, etc.) 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 aerosol arcs curved around the pole, and the presence of winds blowing up to 100 m/s, not following the usual patterns expected with no global dust storm.
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 toward the terminator, reaching velocities up to 100 m/s in the morningside
During June–July 2018, the top altitude of dust showed both spatial and temporal variability, ranging from 10–70 km
In June 2015, Cassini high-resolution images of Saturn's limb southwards of the planet's hexagonal wave revealed a system of at least six stacked haze layers above the upper cloud deck. Here, we ...characterize those haze layers and discuss their nature. Vertical thickness of layers ranged from 7 to 18 km, and they extended in altitude ∼130 km, from pressure level 0.5 bar to 0.01 bar. Above them, a thin but extended aerosol layer reached altitude ∼340 km (0.4 mbar). Radiative transfer modeling of spectral reflectivity shows that haze properties are consistent with particles of diameter 0.07-1.4 μm and number density 100-500 cm
. The nature of the hazes is compatible with their formation by condensation of hydrocarbon ices, including acetylene and benzene at higher altitudes. Their vertical distribution could be due to upward propagating gravity waves generated by dynamical forcing by the hexagon and its associated eastward jet.
Convective storms occur regularly in Saturn's atmosphere. Huge storms known as Great White Spots, which are ten times larger than the regular storms, are rarer and occur about once per Saturnian year ...(29.5 Earth years). Current models propose that the outbreak of a Great White Spot is due to moist convection induced by water. However, the generation of the global disturbance and its effect on Saturn's permanent winds have hitherto been unconstrained by data, because there was insufficient spatial resolution and temporal sampling to infer the dynamics of Saturn's weather layer (the layer in the troposphere where the cloud forms). Theoretically, it has been suggested that this phenomenon is seasonally controlled. Here we report observations of a storm at northern latitudes in the peak of a weak westward jet during the beginning of northern springtime, in accord with the seasonal cycle but earlier than expected. The storm head moved faster than the jet, was active during the two-month observation period, and triggered a planetary-scale disturbance that circled Saturn but did not significantly alter the ambient zonal winds. Numerical simulations of the phenomenon show that, as on Jupiter, Saturn's winds extend without decay deep down into the weather layer, at least to the water-cloud base at pressures of 10-12 bar, which is much deeper than solar radiation penetrates.
In March 2020 a convective storm erupted at planetographic latitude 76°N in the southern flank of Saturn’s long-lived hexagonal wave. The storm reached a zonal size of 4,500 km and developed a tail ...extending zonally 33,000 km. Two new short-lived storms erupted in May in the hexagon edge. These storms formed after the convective storms that took place in 2018 in nearby latitudes. There were no noticeable changes in the zonal profile of Saturn's polar winds in 2018-2020. Measurements of the longitude position of the vertices of the hexagon throughout this period yield a value for its period of rotation equal to that of System III of radio-rotation measured at the time of Voyagers. We report changes in the hexagon clouds related to the activity of the storms. Our study reinforces the idea that Saturn’s hexagon is a well rooted structure with a possible direct relationship with the bulk rotation of the planet.
This review describes the dynamic phenomena in the atmosphere of Mars that are visible in images taken in the visual range through cloud formation and dust lifting. We describe the properties of ...atmospheric features traced by aerosols covering a large range of spatial and temporal scales, including dynamical interpretations and modelling when available. We present the areographic distribution and the daily and seasonal cycles of those atmospheric phenomena. We rely primarily on images taken by cameras on Mars Express.
We present the first systematic study of clouds observed during twilight on Mars. We analyze images obtained by the Visual Monitoring Camera on Mars Express between 2007 and 2020. Using an automated ...retrieval algorithm, we found 407 cases of clouds observed at twilight, in which the geometry of the observations allows to derive the minimum altitude, revealing that many of these clouds are in the mesosphere (above 40 km and up to 90 km). The majority of these mesospheric clouds were detected in mid‐latitudes at local autumn and winter, a new trend only hinted at by previous studies. In particular, we find a massive concentration of clouds in the southern mid‐latitudes between Terra Cimmeria and Aonia, a region where high altitude events have been previously observed. We propose that there is an unknown mechanism in these regions that enhances the probability to host high altitude clouds around the southern winter solstice.
Plain Language Summary
During twilight, when the sun is below the horizon, its light can still reach clouds or mountains high above the surface, making them bright features on the dark background. This effect is sometimes seen on noctilucent clouds on Earth, and also in the mountains on the Moon. On Mars, it was first observed by ground based observers in the 1890s, and occasional observations have been later reported. We present here the first systematic study of such clouds on Mars as observed from space by the Visual Monitoring Camera (also known as the Mars webcam) onboard Mars Express. The study of clouds at twilight reveals information about their altitude, and the state of the atmosphere at this moment of the daily cycle. We analyze the occurrence of these clouds and find some new trends that previous observations had only hinted at.
Key Points
We present a new methodology to detect clouds at twilight and measure their altitude. We find 407 cases, some at altitudes over 80 km
High altitude clouds appear most often in mid‐latitudes during the local winter. This is a new trend when compared to previous studies
High altitude clouds concentrate aerographically in a southern belt that includes Terra Cimmeria, and in clusters on northern planitias
In a previous work (Hernández‐Bernal et al., 2021, https://doi.org/10.1029/2020je006517) we performed an observational analysis of the Arsia Mons Elongated Cloud (AMEC), which stands out due to its ...impressive size and shape, quick dynamics, and the fact that it happens during the Martian dusty season. Observations show that its morphology can be split in a head, on the western slope of the volcano of around 120 km in diameter; and a tail, that expands to the west reaching more than 1,000 km in length, making the AMEC the longest orographic cloud observed so far in the solar system. In this work we run the Laboratoire de Météorologie Dynamique Mesoscale Model to gain insight into the physics of the AMEC. We note that it is coincident in terms of local time and seasonality with the fastest winds on the summit of Arsia Mons. A downslope windstorm on the western slope is followed by a hydraulic‐like jump triggering a strong vertical updraft that propagates upwards in the atmosphere, causing a drop in temperatures of down to 30 K at 40–50 km in altitude, spatially and temporarily coincident with the observed head of the AMEC. However the model does not reproduce the microphysics of this cloud: the optical depth is too low and the expansion of the tail does not happen in the model. The observed diurnal cycle is correctly captured by the model for the head of the cloud. This work raises new questions that will guide future observations of the AMEC.
Plain Language Summary
This is the second paper of our research on the Arsia Mons Elongated Cloud (AMEC), which is a visually impressive cloud on Mars. It appears on the western flank of the Arsia Mons volcano during a specific season right at sunrise. For 3 hr it grows, developing a thin elongated tail that has been observed to be as long as 1,800 km. In our previous work we described available observations. In this work we run a high resolution atmospheric model that captures the effect of the Arsia Mons volcano on the atmosphere. This model shows that due to the presence of the volcano and its effect on the wind, air is forced upwards next to the volcano, leading to a drop in temperatures of 30°C, which causes the formation of the cloud under extreme conditions of humidity. This is a success of the model that provides a new understanding of this outstanding cloud, however, the accurate physics behind the extreme expansion of the AMEC are not fully understood yet. This work solves some questions and raises many new ones, which will be an aid in the planning of new observations.
Key Points
We performed mesoscale model dynamic simulations of the Arsia Mons Elongated Cloud observed in the Martian southern solstice
Topography‐induced circulation causes temperatures to drop by about 30 K at the observed origin location and local time of the cloud
The cloud tail is much more elongated in the observations than in the model, which challenges our understanding of winds and microphysics
Dynamics of Saturn's polar regions Antuñano, A.; del Río-Gaztelurrutia, T.; Sánchez-Lavega, A. ...
Journal of geophysical research. Planets,
February 2015, Letnik:
120, Številka:
2
Journal Article
Recenzirano
Odprti dostop
We analyze data retrieved by the imaging science system onboard the Cassini spacecraft to study the horizontal velocity and vorticity fields of Saturn's polar regions (latitudes 60–90°N in ...June–December 2013 and 60–90°S in October 2006 and July–December 2008), including the northern region where the hexagonal wave is prominent. With the aid of an automated two‐dimensional correlation algorithm we determine two‐dimensional maps of zonal and meridional winds and deduce vorticity maps. We extract zonal averages of zonal winds, providing wind profiles that reach latitudes as high as 89.5° in the south and 89.9° in the north. Wind measurements cover the intense polar cyclonic vortices that reach similar peak velocities of 150 m s−1 at ±88.5°. The hexagonal wave lies in the core of an intense eastward jet at planetocentric latitude 75.8°N with motions that become nonzonal at the hexagonal feature. In the south hemisphere the peak of the eastward jet is located at planetocentric latitude 70.4°S. A large anticyclone (the south polar spot, SPS), similar to the north polar spot (NPS) observed at the Voyager times (1980–1981), has been observed in images from April 2008 to January 2009 in the south polar region at latitude −66.1° close to the eastward jet. The SPS does not apparently excite a wave on the jet. We analyze the stability of the zonal jets, finding potential instabilities at the flanks of the eastward jets around 70°, and we measure the eddy wind components, suggesting momentum transfer from eddy motion to the westward jets closer to the poles.
Key Points
Velocity and vorticity fields of Saturn's polar regions are presented
Cloud morphology at north and south poles is studied and compared
Zonal wind profiles for the polar vortices are retrieved and analyzed
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