Abstracts
Exploring the atmosphere of Venus using a general circulation model
The satellite Venus Express (2006-2015), together with the Japanese spacecraft Akatsuki (still in orbit) and ground-based campaigns, are unveiling our neighbor planet. At the same time, those new measurements put in evidence the complexity and the high variability of the Venus atmosphere, opening new scientific questions (e.g. how does the interplay of planetary and small-scale waves control the circulation features? which processes occur in the region between the retrograde super-rotating zonal flow and the day-to-night circulation?is the polar vortex a permanent feature of the Venus atmosphere?). Sophisticated theoretical tools such as General Circulation Models (GCM) are essential to explore specific physical processes, to interpret the measurements and to help building a consistent picture of spatial and temporal evolution of Venus atmosphere.
In this talk, I will present some highlights from space observations of Venus atmosphere, and on-going work at the Instituto de Astrofísica e Ciência do Espaço (IA) in Lisbon, using a self-consistent Venus GCM. I will focus on the atmospheric layers above the clouds, a region difficult to sound and particularly challenging to be fully explained by current GCMs.
Comparative study of circulation regimes on planetary atmospheres
Understanding our Solar System Planetary Atmospheres is a significant step forward for paving the way for future studies of Extrasolar Planets' atmospheres. Notably, Venus and Mars are natural comparative laboratories to investigate diversity of circulation regimes of terrestrial planets' atmospheres. In this context, comparative studies are essentials to understand the evolution of climate on Earth, both in the past and in the future. Notably, Venus and Mars are natural comparative laboratories to investigate diversity of circulation regimes of terrestrial planets' atmosphere. Venus for example, is Earth’s closest sibling but it has ended up with a radically different climate. Venus atmospheric science is thus increasingly important in an era in which we are trying to understand the divergent evolutionary outcomes for terrestrial planets, whether we are considering the future of our Earth or the habitability in other planetary systems. Venus is a slowly rotating planet with a dense atmosphere. The mechanisms for the generation and maintenance of superrotation are still unclear and no model has been able to successfully reproduce its circulation in decades (Lebonnois 2013). A proper monitoring of Venus winds is crucial towards a full understanding of this phenomena. With this aim, we intend to conduct a synthesis effort that could provide important constraints on atmospheric models. In Venus’s mesosphere (65-85 km), visible observations of Doppler shifts in solar Fraunhofer lines, based on high-resolution spectra, have provided the only Doppler wind measurements near the cloud tops in recent years (Machado et al. 2014, 2017). We will present wind measurements based on VLT/UVES and CFHT/ESPaDOnS observations (around 70 km), wind measurements based on Akatsuki space probe data (and ESA's Venus Express archive data) with cloud tracking methods (from 48 km till 70 km), using an improved version of a cloud tracking tool based on phase-correlation between images. The objective of this work is to help constrain the planetary atmospheric characterization, and to take a step forward in the comparative studies of terrestrial planets.
Granular charging at a distance
Contact charging of granular materials can play a dominant role in effects as varied as lightning in sandstorms, the formation of extraterrestrial planetesimals, and segregation of pharmaceutical powders. Yet the physics underlying this charging is exceptionally poorly understood. For example we have no clear theory for how insulating grains recruit enough charge carriers to deposit charge but not enough to discharge. In this talk, we note that charging and discharging kinetics may be distinct, and from this observation we develop a mathematical model. The model surprisingly predicts that charging can decrease as contact frequency increases. We confirm this prediction experimentally in a vibrated bed and propose future steps.
Collisions of Dust, Sand, and Pebbles in Planet Formation
Planetesimals start growing from dust. After initial hit-and-stick, compact aggregates reach sand to pebble size. Further evolution might depend on the interaction of the aggregates colliding further. They might encounter a bouncing barrier and get stuck in size, might continue growing in the magnetic fields of the protoplanetary disk if ferromagnetic, might glue together if hot or might grow further as they charge in collisions. I will highlight some of our recent experiments on ground and under microgravity.
Physics of dunes on Earth and across the solar system
Sand dunes, which require a supply of granular particles at the surface and a boundary layer of sufficient efficacy to enable transport of these particles by fluid forces, have been detected in surprising locations of our solar system, including Mars and even Pluto. Extra-terrestrial dunes are important for the planetary science because their morphology, scale and dynamics provide proxies for local wind regimes, attributes of sediment, and climate changes. In this talk I will discuss the physics of sediment transport and dune formation on Earth and beyond. Our understanding of this physics has advanced much in the last years with the development of a numerical model for dunes, which combines a mathematical description of the average turbulent wind field over the topography with a continuum model for saltation transport. This dune model reproduces the shape of terrestrial dunes with quantitative agreement with measurements, and has proven to accurately describe the different morphologies of dunes on Mars as well. The model prediction that dunes can form and migrate on today’s Mars, where the atmospheric density is 2 orders of magnitude lower than the Earth’s, has been recently confirmed based on high resolution orbital data of Martian dune migration. Moreover, this year we have reported our discovery of dunes on Pluto, where the atmosphere is 100,000 times less dense than the Earth’s. While terrestrial dunes consist of quartz grains, Pluto dunes are made of methane ice. I will present insights we have gained about the formative wind regime and particle size of Pluto dunes from the numerical model.
InSAR Meteorology and the predictability of atmospheric storms
The quality of rain forecasts is limited by model shortcomings (in physics and numerics), but also by the uncertainty in the atmospheric initial state. A successful strategy to address that issue has been the development of powerful ensemble forecast techniques. Recent work, however, indicates that there is still some scope to improve forecasts by the clever use of new sources of data. The atmospheric noise in Synthetic Aperture Radar Interferograms (InSAR) can be used to produce very high resolution maps of integrated water vapor, and its assimilation by a numerical weather prediction model can improve the forecasts of water vapor and precipitation.
Simulations of soft matter: from sandstorms to granular electrostatics
The collective dynamics of soft matter has been the focus of intensive theoretical and experimental research of scientific and industrial interest. As opposed to experiments, direct computer simulations allow us to systematically modify at will the properties of the constituents and their interactions. This simultaneously offers theoreticians the possibility of a deep understanding of the mechanisms involved in the dynamics and to practitioners an exceptional tool for the rational development of practical applications. In this talk, we will discuss two examples to illustrate the possibilities and challenges of performing direct numerical simulations in soft matter.
We have all heard of sandstorms and their devastating effects. A question that has intrigued natural scientists for a long time is whether a sandstorm is stronger if the airborne sand grains do not collide with each other. Computer simulations offer the possibility of switching on/off the collisions between particles and of modifying the restitution coefficient to address this question. By doing so, we have found, surprisingly, that midair collisions do enhance the overall flux substantially. Following the individual trajectories of each grain, we explain this counterintuitive observation as a consequence of a soft bed of grains floating above the ground and reflects the highest-flying particles [1].
When rubbing two different insulating materials, they mutually charge in a process known as tribocharging. Several experimental and theoretical works provided a strong analytic foundation for charging mechanisms due to geometric or material asymmetries. But recent experimental results suggest that completely identical grains do also charge one another upon contact and the charge difference even increases with multiple contacts. In this case, the mechanism is not at all clear. What breaks the symmetry and sets the direction of charge transfer? How does charge transfer lead to the formation of a strong electromagnetic field in an agitated granular bed? We investigate these questions using particle-based simulations, mathematical modeling, and experiments. We simulated a discrete-element model including electrical multipoles and found that infinitesimally small initial charges can grow exponentially fast. We confirmed the exponential growth observed in experiments using vibrated grains under microgravity [2].
[1] M. V. Carneiro, N. A. M. Araújo, and H. J. Herrmann. Phys. Rev. Lett. 111, 058001 (2013).
[2] R. Yoshimatsu, N. A. M. Araújo, G. Wurm, H. J. Herrmann, and T. Shinbrot. Scientific Reports 7, 39996 (2017).
Charged grains: clusters and discharge
We investigate two scenarios involving charged grains. In the first, motivated by granular media charged by shaking, spherical particles constitute electric monopoles with random charges of both signs. Starting from a cloud, they settle under gravity on the bottom of a container. Switching off gravity, the stored elastic energy causes the sediment to disintegrate into clusters. We study properties, dynamics and statistics of the emerging clusters. The second scenario addresses the effect of a weak plasma on a single, positively charged grain of finite radius: The latter attracts negative charges from the plasma (while repelling the positive ones), causing a current which counteracts a continuous recharging of the grain. We study the properties of the steady state regarding the space charge and the currents.
Electrostatics in the growth of aggregates in protoplanetary disks
André Matias
Industrial as well as natural aggregation of fine particles is believed to be associated with electrostatics. Yet like charges repel, so it is unclear how similarly treated particles aggregate. To resolve this apparent contradiction, we analyze conditions necessary to hold aggregates together with electrostatic forces. We find that aggregates of particles charged with the same sign can be held together due to dielectric polarization, we evaluate the effect of aggregate size, and we briefly summarize consequences for practical aggregation.
Ground-based Doppler Velocimetry: wind measurements in Saturn’s atmosphere with UVES/VLT
Miguel Silva
I will present the latest Doppler wind velocity results of Saturn’s zonal flow at cloud level. The study of the planet’s global system of winds at the 0.7 bar region, promises to improve the characterization of the equatorial jet and the latitudinal variation of the zonal winds, as well the measurement (and monitorization) of its spatial and temporal variability, achieving a better understanding of the dynamics of Saturn’s zonal winds (which Sánchez-Lavega et al [2003] have found to have changed strongly in recent years). Finally, the complementarity with Cassini, has provided an independent set of observations to compare with and help validate the method.
Characterising Atmospheric Gravity Waves on the lower and upper cloud bank using Venus Express VMC and VIRTIS images
José Silva
An atmospheric gravity wave is an oscillatory disturbance on an atmospheric layer in which buoyancy acts as the restoring force. It can only exist in a stably stratified atmosphere, that is, a fluid in which density varies mostly vertically (Holton, 2004).
Gravity waves manifest themselves as regular cloud structures or quasi-periodic disturbances on atmospheric temperature profiles (Piccialli et al., 2014). Though their origin is not clear, possible theories include Kelvin Helmholtz instability, surface topography and convective instability below the upper cloud (Peralta et al., 2008; Piccialli et al.,2014).
Reports of observations of features interpreted as gravity waves are frequent on Earth's atmosphere (Sanchez-Lavega, 2011), on the atmosphere of Mars (Maattanen et al.,2010; McConnochie et al. 2010), on Jupiter's temperature profile (Young et al., 2005) and at cloud level (Arregi et al. 2009).
On Venus' atmosphere, gravity waves have been detected both on temperature profiles acquired by the Pioneer Venus Probes (Seiff et al., 1980; Counselman et al., 1980) and visually on the base (44-48 km altitude) and upper (62-70 km) cloud deck with ultraviolet, visible and infrared observations with VIRTIS (Peralta et al., 2008) and VMC (Markiewicz et al., 2007; Piccialli et al. 2014), both onboard Venus Express.
Atmospheric gravity waves are very important since they can transport energy and momentum by propagating both vertically and horizontally within the atmosphere (Holton, 2004) and could play a key role in the maintenance of the atmospheric circulation on Venus.
This study aims to continue the systematic search and analysis of gravity waves on Venus performed previously by Piccialli et al.2014 and Peralta et al., 2008 using archived data from the Venus Express instruments VMC and VIRTIS. We visually inspect each image in search of wave patterns on selected layers of cloud (at different wavelength ranges) and further characterise its observable features such as geographical position, wavelength, wave packet length and width, orientation and where possible, phase speed.
Venus’ cloud top wind measurements with TNG/HARPSN (Doppler velocimetry) and coordinated Akatsuki observations
Ruben Gonçalves
We present wind velocity results based in the measurements of the horizontal wind field at the cloud top level of the atmosphere of Venus, near 70 km altitude, in the visible range on the dayside. At this altitude the wind circulation is dominated by the retrograde zonal superrotation. The ground observations were carried out, on the 28 and 29 of January 2017, at the 3.58-meter “Telescopio Nazionale Galileo” (TNG) using the “High Accuracy Radial velocity Planet Searcher” spectrograph (HARPS-N) in the visible range (0.38-6.9 µm). It was the first use of this high-resolution (R≈115000) spectrograph to study the dynamics of a solar system atmosphere. The sequential technique of visible Doppler velocimetry is based on solar light scattered by cloud top particles in motion. This technique was developed over the last decade (Widemann et al. 2008, Machado et al. 2012, 2014) and has proven to be a reference in the retrieval of instantaneous zonal and meridional winds - it has already been successfully used using two high resolution spectrographs: (1) slit spectrograph UVES at VLT (Machado et a.l 2012) and (2) fiber-fed spectrograph ESPaDOnS at CFHT (Machado et al. 2014, 2017). In this work we successfully adapt this technique to the HARPS-N fiber-fed spectrograph with consistent results. The cloud-tracking space observations were carried out, between 26-31 January 2017, by the “Ultra Violet Imager” (UVI) onboard Akatsuki’s Venus Climate Orbiter (VCO), using the 365 nm filter. The cloud-tracking technique we used was evolved from a phase correlation method between images developed by Peralta et al. 2007. The HARPS-N ground observations focused on the meridional wind field between 60 S and 55 N latitude and zonal wind field near equator (latitudes between 10 S and 10 N). HARPS-N results present an unprecedented high-precision meridional wind latitudinal profile, which is essential for the understanding of the superrotation atmosphere mechanisms. The Akatsuki/UVI observations provided 3 high-quality images per observation day, separated by ~2h interval. Due to its low inclination orbit (<10º), Akatsuki’s images offer a great range in Venus’ dayside, allowing us to track cloud features from 60º N to 70º S latitude and from 7:30 to 17:00 local time. This has enable a study of spatial and time variability of both zonal and meridional wind. This work intends to contribute to the characterization of Venus’ cloud top zonal and meridional wind by studying latitudinal behavior on hour and day timescales as well as wind temporal and spatial variability. Similar studies have proven the relevance of both space-based cloud tracking observations (Sánchez-Lavega et al. 2008, Hueso et al. 2012, Hourinouchi et al. 2018) and ground-based doppler velocimetry (Machado et al. 2014, 2107), as well as the usefulness of coordinated observations in the cross validation of both technique results.