arXiv subjects

D. Grassi

Publications and source records attributed to D. Grassi.

5 recordsLinked to original sources

arXiv2025

Structure of Jupiter's High-Latitude Storms: Folded Filamentary Regions Revealed by Juno

Sprawling, turbulent cloud formations dominate the meteorology of Jupiter's mid-to-high latitudes, known as Folded Filamentary Regions (FFRs). A multi-wavelength characterisation by Juno reveals the spatial distribution, vertical structure, and energetics of the FFRs. The cloud tops display multiple lobes of stratiform aerosols, separated by darker, cloud-free lanes, and embedded with smaller eddies and high-altitude cumulus clouds. These cyclonic FFRs are microwave-bright in shallow-sounding wavelengths ($p<5$ bars) and microwave-dark in deep-sounding wavelengths ($p>10$ bars), with the transition potentially associated with the water condensation layer (6-7 bars). Associating microwave contrasts with temperature anomalies, this implies despinning of cyclonic eddies above/below their mid-planes. Despite deep roots (being detectable in wavelengths sounding $\sim100$ bars), they are ``pancake vortices'' with horizontal extents at least an order of magnitude larger than their depth. In the northern hemisphere, FFRs are most common in cyclonic belts poleward of $40^\circ$N (all latitudes are planetocentric), particularly a North Polar Filamentary Belt (NPFB) near $66-70^\circ$N that defines the transition from organised belts/zones to the chaotic polar domain. This distribution explains the high lightning rates from $45-80^\circ$N, peaking in a belt poleward of $52.3^\circ$N, which may trace the availability of water for moist convection. Many observed lightning flashes can be associated to specific FFRs containing bright storms, but some FFRs display no activity, suggesting quiescent periods during storm evolution. Analogies to Earth's oceanic eddies suggest that cyclones deform isentropic surfaces at their midplanes, raising deep water-rich layers upwards to promote moist convection, release latent heat, and inject clouds into the upper troposphere.

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arXiv2020

Jupiter's Equatorial Plumes and Hot Spots: Spectral Mapping from Gemini/TEXES and Juno/MWR

We present multi-wavelength measurements of the thermal, chemical, and cloud contrasts associated with the visibly dark formations (also known as 5-$\mu$m hot spots) and intervening bright plumes on the boundary between Jupiter's Equatorial Zone (EZ) and North Equatorial Belt (NEB). Observations made by the TEXES 5-20 $\mu$m spectrometer at the Gemini North Telescope in March 2017 reveal the upper-tropospheric properties of 12 hot spots, which are directly compared to measurements by Juno using the Microwave Radiometer (MWR), JIRAM at 5 $\mu$m, and JunoCam visible images. MWR and thermal-infrared spectroscopic results are consistent near 0.7 bar. Mid-infrared-derived aerosol opacity is consistent with that inferred from visible-albedo and 5-$\mu$m opacity maps. Aerosol contrasts, the defining characteristics of the cloudy plumes and aerosol-depleted hot spots, are not a good proxy for microwave brightness. The hot spots are neither uniformly warmer nor ammonia-depleted compared to their surroundings at $p<1$ bar. At 0.7 bar, the microwave brightness at the edges of hot spots is comparable to other features within the NEB. Conversely, hot spots are brighter at 1.5 bar, signifying either warm temperatures and/or depleted NH$_3$ at depth. Temperatures and ammonia are spatially variable within the hot spots, so the precise location of the observations matters to their interpretation. Reflective plumes sometimes have enhanced NH$_3$, cold temperatures, and elevated aerosol opacity, but each plume appears different. Neither plumes nor hot spots had microwave signatures in channels sensing $p>10$ bars, suggesting that the hot-spot/plume wave is a relatively shallow feature.

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arXiv2020

Global maps of Venus nightside mean infrared thermal emissions obtained by VIRTIS on Venus Express

One of the striking features about Venus atmosphere is its temporal variability and dynamics, with a chaotic polar vortex, large-scale atmospheric waves, sheared features, and variable winds that depend on local time and possibly orographic features. The aim of this research is to combine data accumulated over several years and obtain a global mean state of the atmosphere focusing in the global structure of the clouds using the cloud opacity and upper cloud temperatures. We have first produced global maps using the integrated radiance through the infrared atmospheric windows centred around 1.74{\mu}m and 2.25{\mu}m, that show the spatial variations of the cloud opacity in the lower clouds around 44-48 km altitude and also provide an indirect estimation of the possible particle size. We have also produced similar global maps using the brightness temperatures seen in the thermal region at 3.8{\mu}m and 5.0{\mu}m, which provide direct indication of the temperatures at the top of the clouds around 60-70 km altitude. These maps have been generated using the complete dataset of the Visible and InfraRed Thermal Imaging Spectrometer mapping channel (VIRTIS-M) on board Venus Express, with a wide spatial and long temporal coverage in the period from May 2006 until October 2008. Our results provide a global view of the cloud opacity, particle size and upper cloud temperatures at both hemispheres, showing the main different dynamical regions of the planet. The profiles obtained also provide the detailed dependencies with latitude, local time and longitude, diagnostic of the global circulation flow and dynamics at various altitude layers, from about 44 up to 70 km over the surface.

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arXiv2017

Properties of a Martian local dust storm in Atlantis Chaos from OMEGA/MEX data

In this study we present the analysis of the dust properties of a local storm imaged in the Atlantis Chaos region on Mars by the OMEGA imaging spectrometer on March 2nd 2005. We use the radiative transfer model MITRA to study the dust properties at solar wavelengths between 0.5 {\mu}m and 2.5 {\mu}m and infer the connection between the local storm dynamics and the topography. We retrieve maps of effective grain radius (reff), optical depth at 9.3 {\mu}m ({\tau}9.3) and top altitude (ta) of the dust layer. Our results show that large particles (reff = 1.6 {\mu}m) are gathered in the centre of the storm (lat=33.5{\deg} S; lon=183.5 W{\deg}), where the optical depth is maximum ({\tau}9.3 > 7.0) and the top altitude exceeds 18 km. Outside the storm, we obtain {\tau}9.3 < 0.2, in agreement with the estimates derived from global climate models (GCM). We speculate that a low thermal inertia region at the western border of Atlantis Chaos is a possible source of the dust storm. Moreover, we find evidence that topography plays a role in confining the local storm in Atlantis Chaos. The vertical wind component from the GCM does not provide any hint for the triggering of dust lifting. On the other hand, the combination of the horizontal and vertical wind profiles suggests that the dust, once lifted, is pushed eastward and then downward and gets confined within the north-east ridge of Atlantis Chaos. (continues in article)

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arXiv2011

EChO - Exoplanet Characterisation Observatory

A dedicated mission to investigate exoplanetary atmospheres represents a major milestone in our quest to understand our place in the universe by placing our Solar System in context and by addressing the suitability of planets for the presence of life. EChO -the Exoplanet Characterisation Observatory- is a mission concept specifically geared for this purpose. EChO will provide simultaneous, multi-wavelength spectroscopic observations on a stable platform that will allow very long exposures. EChO will build on observations by Hubble, Spitzer and groundbased telescopes, which discovered the first molecules and atoms in exoplanetary atmospheres. EChO will simultaneously observe a broad enough spectral region -from the visible to the mid-IR- to constrain from one single spectrum the temperature structure of the atmosphere and the abundances of the major molecular species. The spectral range and resolution are tailored to separate bands belonging to up to 30 molecules to retrieve the composition and temperature structure of planetary atmospheres. The target list for EChO includes planets ranging from Jupiter-sized with equilibrium temperatures Teq up to 2000 K, to those of a few Earth masses, with Teq ~300 K. We have baselined a dispersive spectrograph design covering continuously the 0.4-16 micron spectral range in 6 channels (1 in the VIS, 5 in the IR), which allows the spectral resolution to be adapted from several tens to several hundreds, depending on the target brightness. The instrument will be mounted behind a 1.5 m class telescope, passively cooled to 50 K, with the instrument structure and optics passively cooled to ~45 K. EChO will be placed in a grand halo orbit around L2. We have also undertaken a first-order cost and development plan analysis and find that EChO is easily compatible with the ESA M-class mission framework.

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