We describe a huge planetary‐scale disturbance in the highest‐speed Jovian jet at latitude 23.5°N that was first observed in October 2016 during the Juno perijove‐2 approach. An extraordinary ...outburst of four plumes was involved in the disturbance development. They were located in the range of planetographic latitudes from 22.2° to 23.0°N and moved faster than the jet peak with eastward velocities in the range 155 to 175 m s−1. In the wake of the plumes, a turbulent pattern of bright and dark spots (wave number 20–25) formed and progressed during October and November on both sides of the jet, moving with speeds in the range 100–125 m s−1 and leading to a new reddish and homogeneous belt when activity ceased in late November. Nonlinear numerical models reproduce the disturbance cloud patterns as a result of the interaction between local sources (the plumes) and the zonal eastward jet.
Key Points
A planetary‐scale disturbance developed in the highest‐speed Jupiter jet at 23.5°N latitude during October and November 2016
Four “plumes” were involved in the outbreak moving with speeds between 155 and 175 m s−1, the fastest features at cloud level
Nonlinear numerical models reproduce the disturbance from the interaction between local sources (the plumes) and the zonal eastward jet
Martian planet-encircling dust storms or global dust storms (GDS), resulting from the combined influence of local and regional storms, are uncommon aperiodic phenomena: with an average frequency of ...approximately one every 3–4 MY, they produce a substantial rise in the atmospheric dust loading that lasts from weeks to months and have a significant impact on the atmospheric properties, energy budget, and global circulation. During the 2018/MY34 global dust storm, initiated at LS = 185° (30–31 May 2018), an intensive atmospheric science campaign was carried out by the Mars Science Laboratory (MSL) rover to monitor the environmental parameters at Gale Crater. We contribute to previous studies with independent retrievals to constrain the dust opacity and characterise the aerosol particle properties, including: size, shape and single scattering phase function. An iterative radiative transfer retrieval procedure was implemented to determine the aerosol parameters that best fit the angular distribution of sky radiance at forward and backward scattering regions observed by MSL Navigation Cameras (Navcams) during the 2018/MY34 GDS. The MOPSMAP aerosol database and Double Henyey-Greenstein (DHG) analytical single scattering phase functions were used to model the Martian dust aerosol. Outcomes of this study show a steep rise in dust opacity from pre-storm levels of 1.2 up to τ > 9, correlated to particle size variations from 1 to 4 μm. DHG phase functions are characterised with an average asymmetry parameter of g = 0.60 ± 0.11 during the storm, diverging from values of around 0.71 ± 0.06 for the same period in previous MY. Best fitting simulations to backscatter observations for high-opacity periods were generated by a mixture of spheroids following a log-normal distribution of aspect ratios centred on 2.8 ± 0.9, in contrast to values of 1.8 in post-storm sols, thus pointing to more irregular particle shapes at the peak of the dust storm.
We present a reanalysis (using the Minnaert limb‐darkening approximation) of visible/near‐infrared (0.3–2.5 μm) observations of Uranus and Neptune made by several instruments. We find a common model ...of the vertical aerosol distribution i.e., consistent with the observed reflectivity spectra of both planets, consisting of: (a) a deep aerosol layer with a base pressure >5–7 bar, assumed to be composed of a mixture of H2S ice and photochemical haze; (b) a layer of photochemical haze/ice, coincident with a layer of high static stability at the methane condensation level at 1–2 bar; and (c) an extended layer of photochemical haze, likely mostly of the same composition as the 1–2‐bar layer, extending from this level up through to the stratosphere, where the photochemical haze particles are thought to be produced. For Neptune, we find that we also need to add a thin layer of micron‐sized methane ice particles at ∼0.2 bar to explain the enhanced reflection at longer methane‐absorbing wavelengths. We suggest that methane condensing onto the haze particles at the base of the 1–2‐bar aerosol layer forms ice/haze particles that grow very quickly to large size and immediately “snow out” (as predicted by Carlson et al. (1988), https://doi.org/10.1175/1520-0469(1988)045<2066:CMOTGP>2.0.CO;2), re‐evaporating at deeper levels to release their core haze particles to act as condensation nuclei for H2S ice formation. In addition, we find that the spectral characteristics of “dark spots”, such as the Voyager‐2/ISS Great Dark Spot and the HST/WFC3 NDS‐2018, are well modelled by a darkening or possibly clearing of the deep aerosol layer only.
Plain Language Summary
Previous studies of the reflectance spectra of Uranus and Neptune have concentrated on individual, narrow wavelength regions and the conclusions have been difficult to compare with each other. Here, we analyse a combined set of observations from three different instruments covering the wavelength range 0.3–2.5 μm to arrive at a single aerosol model that matches the observations at all wavelengths simultaneously for both planets. We conclude that photochemical haze produced in the upper atmospheres of both planets is steadily mixed down to lower layers, where it forms part of a vertically thin layer in a statically stable region above the methane condensation level at 1–2 bar. We suggest that methane condenses so rapidly upon these haze particles that it efficiently “snows” out at the base of this layer, falling to lower, warmer levels, where the methane evaporates, releasing the core haze particles to “seed” H2S condensation. For Neptune we need to add an additional layer of moderately large methane ice particles at ∼0.2 bar. Intriguingly, we find that a darkening (or perhaps clearing) of the lowest H2S/haze layer matches very well the observed properties of the dark spots seen occasionally in Neptune's atmosphere and very occasionally in Uranus's atmosphere.
Key Points
Ice Giant reflectivity spectra from 0.3 to 2.5 μm well approximated by a single aerosol model comprised of three to four distinct layers
Static stability region at 1–2 bar, caused by methane condensation, seems to lead to build‐up of haze and seeds CH4 snow at its base
Darkening of deepest H2S/haze layer, based at p > 5–7 bar, found to account well for spectral properties of dark spots
In this paper we show that Sun-viewing images obtained by the Mars Science Laboratory (MSL) Navigation Cameras (Navcam) can be used for retrieving the dust optical depth and constrain the aerosol ...physical properties at Gale Crater by evaluating the sky brightness as a function of the scattering angle. We have used 65 Sun-pointing images covering a period of almost three Martian years, from MSL mission sol 21 to sol 1646 (MY 31 to 33). Radiometric calibration and geometric reduction were performed on MSL Navcam raw image data records to provide the observed sky radiance as a function of the scattering angle for the near-Sun region (scattering angle from 4° to 30°). These curves were fitted with a multiple scattering radiative transfer model for a plane-parallel Martian atmosphere model using the discrete ordinates method. Modelled sky brightness curves were generated as a function of two parameters: the aerosol particle size distribution effective radius and the dust column optical depth at the surface. A retrieval scheme was implemented for deriving the parameters that generated the best fitting curve under a least-square error criterion. The obtained results present a good agreement with previous work, showing the seasonal dependence of both dust column optical depth and the effective particle radius.
We present zonal and meridional wind measurements at three altitude levels within the cloud layers of Venus from cloud tracking using images taken with the VIRTIS instrument on board Venus Express. ...At low latitudes, zonal winds in the Southern hemisphere are nearly constant with latitude with westward velocities of 105 ms−1 at cloud‐tops (altitude ∼ 66 km) and 60–70 ms−1 at the cloud‐base (altitude ∼ 47 km). At high latitudes, zonal wind speeds decrease linearly with latitude with no detectable vertical wind shear (values lower than 15 ms−1), indicating the possibility of a vertically coherent vortex structure. Meridional winds at the cloud‐tops are poleward with peak speed of 10 ms−1 at 55° S but below the cloud tops and averaged over the South hemisphere are found to be smaller than 5 ms−1. We also report the detection at subpolar latitudes of wind variability due to the solar tide.
We analyze Jupiter observations between December 2015 and August 2016 in the 0.38–1.7 μm wavelength range from the PlanetCam instrument at the 2.2 m telescope at Calar Alto Observatory and in the ...optical range by amateur observers contributing to the Planetary Virtual Observatory Laboratory. Over this time Jupiter was in a quiescent state without notable disturbances. Analysis of ground‐based images and Hubble Space Telescope observations in February 2016 allowed the retrieval of mean zonal winds from −74.5° to +73.2°. These winds did not change over 2016 or when compared with winds from previous years with the sole exception of intense zonal winds at the North Temperate Belt. We also present results concerning the major wave systems in the North Equatorial Belt and in the upper polar hazes visible in methane absorption bands, a description of the planet's overall cloud morphology and observations of Jupiter hours before Juno's orbit insertion.
Plain Language Summary
We present a characterization of Jupiter clouds and their dynamics prior to Juno's arrival. We present results based on observations with our own high‐resolution instrument PlanetCam, Hubble Space Telescope images acquired in February 2016, and analysis of high‐quality images provided by amateur astronomers using small telescopes. Before Juno's arrival to Jupiter its atmosphere behaved in a normal way without major convective outbreaks or changes in its usual belts and bands. Juno's first perijove characterized Jupiter at a time of usual behavior of the planet. Zonal winds were similar to previous years except for high values of the wind velocity at the most intense jet stream in the planet in the North Temperate latitudes which reached values of 157 m/s. We also study two systems of planetary‐scale waves in the planet: The first system is north to the equator in the North Equatorial Belt and is characterized by a regular pattern of large dark features in the planet. These are regions of reduced cloud content formed by an atmospheric wave. The second system is an undulatory pattern in the polar hazes that is best observed in the southern hemisphere. Both are characterized for later comparisons with results from the Juno mission.
Key Points
We present the overall cloud morphology of Jupiter over 2016 from images in the spectral range 0.38–1.7 micrometers
We characterize the equatorial hot spots and present snapshots of the polar regions providing context for observations from Juno
We measured zonal winds from December 2015 to June 2016 spanning the latitude range 74.5°S–73.2°N without variability over this period
The ubiquitous dust in the Martian environment plays a key role in its weather and climate: it must be taken into account in the interpretation of remote sensing data and observations, and could pose ...a potential risk to surface equipment and operations. In this study, we use observations retrieved by the Instrument Context Camera (ICC) onboard the InSight lander to evaluate the accumulation of dust on the camera lens and estimate the size of the deposited dust particles. Dust contamination is revealed as mottled pattern image artefacts on ICC observations. These were detected using a template matching blob detection algorithm and modelled with a first-order optical model to simulate their size and optical density as a function of the particle diameter. The results show a deep decay in the first 70 sols (LS = 295–337°, MY34) during which dust particles deposited at landing were mostly removed. The subsequent gradual decrease and stable behaviour in the number of detected particles is only interrupted by accumulation and removal periods around sols 160 (LS ∼ 23°, MY35) and 800–1100 (LS = 9–150°, MY36). The estimated particle sizes follow a similar trend, with deposited particles due to wind-driven forces (average diameter < 50 μm) being smaller than the ones deposited by other forces during landing, with particles of up to 220 μm of diameter. The results of this study provide an additional source of information for evaluating aeolian dust processes in Mars, with quantitative results on dust accumulation and removal activity, and may contribute to a better determination of dust entrainment threshold models by constraining susceptible dust particle sizes.
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