The mineral clouds on the extrasolar giant gas planets HD 209458b and HD 189733b

The giant gas planets HD 189733b and HD 209458b are the two most studied extrasolar planets today. Both have been observed by several research groups with varies telescopes including the Hubble and the Spitzer Space Telescopes, and the super-high-precision HARPS spectrograph mounted on the 3.6 m telescope in La Silla in Chile. These extensive observational programs have reviled details about the atmospheres of these planets, like for example the presence of CO and CH4 in HD 189733b and CO and H2O in HD 209458b. Observations have further established that both gas giants form clouds inside their atmospheres (e.g. Sing et al. 2016). Are these clouds similar to clouds on Earth? What are they made of? Why does HD 209458b seem to have more water vapour than HD 189733b? How different are clouds between the two planets?

We use results from 3D radiative-hydrodynamics simulations of the atmospheres of HD 189733b and HD 209458b to answer the above questions and to derive cloud characteristics. We apply the same ideas about cloud formation as described in our Drift-Phoenix post. First, condensation seeds form with a certain efficiency. Once they are present, many solid materials (e.g. MgSiO3[s], Fe[s], SiO[s], TiO2[s], [s] meaning solid phase) can condense on these numerous but small surfaces. As these cloud particles grow, they fall into the atmosphere (gravitational settling). These raining cloud particles will encounter changing ambient conditions because the gas temperature and the gas pressure increase inwards the atmosphere. On their way, the cloud particles change in size but also in composition.

We now probe the atmospheric cloud formation in HD 189733b and HD 209458b by calculating the cloud structure for the vertical atmosphere at different longitudes and latitudes shown on Fig. 1.

Figure1. Points in the atmosphere where cloud formation was probed

Figure 1. Points in the atmosphere where cloud formation was probed

We find that both planets have the smallest cloud particles near the top of the cloud and the largest cloud particles at the bottom of the cloud, which is far inside the atmosphere beyond observable heights. This can be seen from the black solid line in all the panels in Fig 2.

Figure 2 demonstrates the vertical cloud structures for the daysides of the giant gas planets HD 189733b (top) and HD 209458b (bottom). We show how the material composition changes with height for different latitudes (Φ=270°, 315°, 0°, 45°, 90°) along the equator. The material composition is visualized by the lines of different colours, representing one material each: TiO2[s] – solid dark blue, Al2O3[s] – solid blue, CaTiO2[s] – solid purple, Fe2O3[s] – dashed light green, Fe[s] – dotted green, SiO[s] – dashed brown, SiO2[s] – solid brown, MgO[s] – dashed dirty orange, MgSiO3[s] – dashed orange, Mg2SiO4[s] – solid orange. The contribution of the different materials to the volume of the cloud particles, Vs/Vtot, is shown in percentage. For example, VMg2SiO4/Vtot=0.3 means 30% of the cloud particle is made of Mg2SiO4.

Figure 2. Dayside cloud particle material composition (colour coded, left axis) and mean grain sizes (black, right axis) for both exoplanets. For colour codes refer to the original paper at Helling et al. 2016, fig. 7 , or to the text above

Figure 2. Dayside cloud particle material composition (colour coded, left axis) and mean grain sizes (black, right axis) for both exoplanets. For colour codes refer to the original paper at Helling et al. 2016, fig. 7, or to the text above

To the left in Fig. 2, where the gas pressure is low, is the upper part of the atmosphere and the cloud. Here, the cloud particles are small (10-2 μm) and made of a rich mix of materials indicated by many coloured lines appearing in the plots of Fig 2. Letting your eyes wonder more to the right shows that most of the lines disappear, because these materials evaporate (like Fe2O3[s], MgO[s]). Most of the cloud particles are now made of MgSiO3[s], Mg2SiO4[s], SiO2[s] and a bit of Fe[s]. When moving further inwards the atmosphere where the temperature increases beyond thermal stability of the silicate materials, a larger fraction of cloud particles will be made of Fe[s].

Inspecting Fig 2 a bit closer by comparing the results for HD 189733b and HD 209458b shows that the cloud particles at the inner rim of HD 189733b are more Fe[s] rich than for HD 209458b. The cloud particles in the upper atmospheric regions appear rather similar in material composition: they are made of silicates and oxides with only very small contribution form iron.

A major result of our work is that all cloud properties are interlinked and that it is extremely difficult to guess correct combinations of cloud particle-sizes and their material composition that will occur at a certain place inside the atmosphere of an extrasolar planet.

Why would HD 209458b show more water absorption than HD 189733b? Hence, why does HD 209458b seem to have more water vapour than HD 189733b according to observations? Water is the most abundant absorbing species in the gas phase and maybe one would not expect any differences between two relatively similar planets like HD 189733b and HD 209458b. However, our research shows that a considerably larger portion of the atmosphere of HD 209458b is affected by the cloud than for HD 189733b. The clouds in HD 209458b reach into regions of lower atmospheric pressure, hence lower gas densities, compared to HD 189733b. It should therefore be more difficult to observe water on HD 209458b than on HD 189733b. Therefore, the more a cloud layer extends into the low-density upper part of the atmosphere, the shallower the gas-absorption features will be in the observed spectrum (Fig. 3).

Figure 3. More water molecules accumulate above clouds at higher atmospheric pressure. Clouds on HD 209458b form lower in the atmosphere (right), therefore have more water molecules above them resulting in a deeper water absorption feature in the spectrum, than on HD 189733b (left)

Figure 3. More water molecules accumulate above clouds at higher atmospheric pressure. Clouds on HD 209458b form lower in the atmosphere (right), therefore have more water molecules above them resulting in a deeper water absorption feature in the spectrum, than on HD 189733b (left)

 

For more details check out the original paper on ADS:

 Ch. Helling, G. Lee, I. Dobbs-Dixon, et al. 2016, MNRAS, 460, 855

The LEAP Group can be found here:

http://leap2010.wp.st-andrews.ac.uk/

And finally, don’t forget to like us on Facebook:

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Interdisciplinary thinking: Atmospheric electrification in the Solar System and beyond

According to Wikipedia, interdisciplinarity involves the contribution of two or more academic disciplines to allow progress through recognition of different ways of thinking. Driven by curiosity, a group of researchers from the disciplines of plasma physics, meteorology, volcanology and astrophysics (observations and modelling of brown dwarfs, exoplanets, protoplanetary disks) met in the Scottish Highlands in Pitlochry in 2014 to discuss their research on ‘Electrification in dusty atmospheres inside and outside the solar system’. This workshop was the inspiration for a review articles ‘Atmospheric electrification in dusty, reactive gases in the solar system and beyond’ accepted for publication in ‘Surveys of Geophysics’, which aims to stimulate a closer interaction between the communities involved. A short summery of aim and content is given here.

The last few decades have taken us from a Universe with only a single planetary system known, to one with thousands, and maybe millions, of such systems. We are now entering the time when we explore theories and results derived for the Solar System and for Earth in application to unknown worlds. As such, it is more and more important for the different science communities, in this case earth sciences and astronomy and astrophysics, to share the knowledge they have gathered, in order to combine their approaches to explore new worlds.

“Planets are the coldest and smallest objects in the universe known to possess a cloud-forming and potential life protecting atmosphere”. In Figure 1 we see Jupiter in the astrophysical context. It is compared to the coolest stellar objects, M-dwarfs and brown dwarfs, while these are compared to the Sun representing a regular star. Brown dwarfs bridge the stellar and the planetary regime as their atmospheres can be as cold as those of planets but they form like stars. The Sun, including its corona, the hot plasma surrounding it, is well studied by satellites like SOHO and HINODE. However, such high-resolution monitoring is not yet possible for Solar System planets, moons, comets and for extrasolar objects. In case we want to learn about their cold cloud-forming atmospheres, which may host electrical phenomena, we need to combine experimental work on Earth, Earth observations, modelling and comparative studies for the Solar System and extrasolar objects.

Figure1. M-dwarfs, brown dwarfs and giant gas planets in comparison. Teide 1 is an example for a late M-dwarf, GD 165B for a cloud-forming brown dwarf of spectral type L, Gliese 229B is a cooler cloud-forming brown dwarf of spectral class T, and Jupiter is the example for a giant gas plane.

Figure 1. M-dwarfs, brown dwarfs and giant gas planets in comparison. Teide 1 is an example for a late M-dwarf, GD 165B for a cloud-forming brown dwarf of spectral type L, Gliese 229B is a cooler cloud-forming brown dwarf of spectral class T, and Jupiter is the example for a giant gas plane (Helling et al. 2016).

Plasma and discharge experiments are essential in providing a controlled environment in contrast to observation of atmospheric phenomena. An atmospheric environment that is only partially ionized may show plasma character on only local scales compared to the global scale of a comet, moon, planet, brown dwarf or protoplanetary disk. Volcanic eruptions on Earth have been shown to produce significant electrostatic charging and subsequent lightning. It is also possible that similar charging mechanisms occur on Jupiter’s moon Io, for example. Understanding dust-charging processes is important for space exploration because the local ionization changes on the surface of a moon or an asteroid as a result of the variability of the solar wind hitting these objects. When a spacecraft, like the Rosetta lander Philae, lands on the surface of such objects, it creates a very similar effect. The ionization of the local environment influences the spacecraft’s operation on the object and the landing itself.

In situ measurements of the chemically active Earth-atmosphere offer insight in charge and discharge processes, their local properties, and their global changes. These measurements in the natural atmospheric environment lead to an understanding of the role of electrons, ions and dust involved in the ionization of the atmosphere. Such observations allow an understanding of atmospheric processes on Earth that can only be gained for Solar System and extrasolar bodies from intensive modelling efforts in combination with observations and experiments.

Ionization processes also have implications for industry. One example of plasma technology development is included in our review to demonstrate the impact of the theme of this paper beyond academic research. The paper gives an overview of electrification processes inside and outside the Solar System. It moves from small-scale to large-scale charge processes in different types of environments, such as the terrestrial atmosphere, the Moon and asteroids, and also extrasolar planetary and brown dwarf atmospheres and protoplanetary disks.

Interdisciplinary thinking: Meteorological balloon experiment launch (Credit: Giles Harrison); laboratory volcanic lightning experiment (Cimarelli et al. 2014); temperature variations between the day and night side of the exoplanet HD 189733b (Credit: Graham Lee)

Interdisciplinary thinking:
Meteorological balloon experiment launch (Credit: Giles Harrison); laboratory volcanic lightning experiment (Cimarelli et al. 2014); temperature variations between the day and night side of the exoplanet HD 189733b (Credit: Graham Lee)

The paper first sets the stage for the interdisciplinary exchange: it introduces the fundamental physics of charging processes, defines general terms, and shows the field of experimental dust-charging works to the reader. The next chapter explains the electrification and discharging of planetary atmospheres. Explains the role of the Wilson Global Circuit (continuous movement of electric current between the ionosphere and the surface of a planetary object), the production of thundercloud lightning and its subsequent phenomena, the transient luminous events and how the electrification of volcano plumes lead to volcanic lightning. We get an insight on the chemical changes in Solar System planetary atmospheres caused by lightning discharges. Those who are interested in the Moon or asteroids in the Solar System can learn about charging processes in the environments on these objects from the next big section. The paper finishes with the very new topic of charging in extrasolar environments, such as exoplanetary and brown dwarf atmospheres and protoplanetary disks. Each of these topics could be the core of individual blog entries. This blog can, therefore, only provide a very minimalistic introduction to the whole paper with which we hope to inspire further interdisciplinary communications.

This paper was born as collaboration between scientists from various fields of earth sciences and astrophysics. It intends to show the importance of such multi-disciplinary works. To help the readers of different background, it includes a glossary at the end.

 

For more details check out the original paper on ADS:

 Ch. Helling, R. G. Harrison, F. Honary, D. A. Diver, K. Aplin, I. Dobbs-Dixon, U. Ebert, S. Inutsuka, F. J. Gordillo-Vazquez, S. Littlefair, 2016, Surveys in Geophysics, 37, 705

The LEAP Group can be found here:

http://leap2010.wp.st-andrews.ac.uk/

And finally, don’t forget to like us on Facebook:

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Electrification in dusty atmospheres inside and outside the solar system, Pitlochry, 8-11 Sep. 2014

The workshop ‘Electrification of dusty atmospheres inside and outside the solar system’ hosted by the LEAP Group took place in Pitlochry, Scotland. The cross-disciplinary nature of the workshop attracted scientists from fields of plasma physics, volcanology, meteorology, and astrophysics from all over the world.

The meeting started with a welcome barbeque on Sunday evening: people were talking in small groups, catching up with old friends and meeting new colleagues.

The welcome barbeque in the garden of the hotel

_DSC1519 The welcome barbeque in the garden of the hotel (Credit: Rubén Asensio Torres)

On Monday morning Christiane Helling summarized the scientific idea that lead to the organization of this workshop: she talked about the benefits of the meeting for both astrophysicists and scientists from other fields. She also introduced a new proceeding idea, which is planned to be published in Surveys of Geophysics.

The first talk was given by Alan Phelps who discussed laboratory studies of crystalline-like ordered structure in dense dusty plasmas, with the potential to investigate similar behaviour in substellar atmospheres. In this context, the exciting possibility exists of identifying a unique observable signature associated with plasma crystals that could be used to diagnose the charged environment.

The difficulties of the inter-disciplinary nature of the workshop appeared right after the first talk when it turned out that the definition of ‘dust’ is not the same in every field. However, after discussing the issue, the speakers and participants quickly got used to the fact that most of the people are from a different field than they are and explained their fields in a way, which was understandable for everyone.

Keri Nicoll and Corrado Cimarelli gave exciting talks on volcanic lightning. Nicoll gave an overview on the different charging mechanisms in volcanic plumes and reported that broad particle size distributions of volcanic ash clouds are more susceptible to triboelectric charging, which give an analogy to substellar clouds with atmospheric regions with the appropriate particle size distribution. Cimarelli described a laboratory experiment where they reproduced volcanic lightning strikes, and explained how the particle size and distribution affects the charge separations on plumes.

Euan Bennet’s talk on isolating different sized bacteria using electrostatic disruption of water droplets was an interesting part of the conference. It showed some of the unexpected applications that can arise from the study of aerosol electrification.

During the afternoon session Ute Ebert introduced us into the mechanism of lightning development and gave an overview of streamer propagation. The following talks were about Transient Luminous Events (TLEs) such as sprite modelling and the possibility of TLE initiation on gas giant planets like Jupiter.

The afternoon ended with the poster pop-up, where each poster presenter was given one minute to advertise his or her work, which was followed by the poster session itself. Delicious pretzels and Guinness accompanied the session.

_DSC1609

_DSC1623 Poster session (Credit: Rubén Asensio Torres)

On Tuesday we started with a very interesting talk by Farideh Honary on Lunar dust charging and how this can affect future (and past) landing missions. Karen Aplin introduced us a similar approach but with asteroids. She raised the question of what would happen if a, possibly, oppositely charged landing spacecraft (negatively charged) and the surface of an asteroid (positively charged) interact with each other and showed a model of how the electrostatic effects can be best measured in situ.

The afternoon session started with Ian Dobbs-Dixon’s presentation on dynamical modelling of the atmospheres of tidally locked hot Jupiters. Michael Rycroft introduced the audience to the conditions a planet would need in order to host a global electric circuit.

In the evening we had the workshop dinner in the hotel. In a short dinner speech, Christiane Helling also thanked all the participants for their exciting contributions to the workshop. Towards the end of the dinner Craig Stark announced the winners of the poster contest, Graham Lee and Karen Aplin. Congratulations!

Wednesday was the day of brown dwarfs (BDs) and ionization processes. Sara Caswell talked about two White Dwarf–Brown Dwarf systems and showed how different the spectra of the day and night side of an irradiated BD can be. Irena Vorgul gave a talk on how flash ionization processes (such as lightning) could be detected through cyclotron maser emission going through the affected atmospheric volume. Craig Stark summarized the concept of the LEAP Project, then talked about the basics of Alfvén ionization, a process where a low density magnetized plasma is hit by a high speed flow of neutral gas. He then talked about the possibility of creating prebiotic molecules (like glycen) on the surface of dust particles in plasmas. An impressive talk was given by Takayuki Muranushi, how proposed to use ion lines width for detection lightning occurring within protoplanetary disks.

On the last day of the workshop we learnt a lot about cosmic ray (CR) air showers and their ionizing effects. However, due to a change in the schedule, the first talk was about multi-wavelength observations of BDs given by Stuart Littlefair. He showed that consistent cyclotron emission detection shows very good correlation with optical observations, suggesting an aurora-like mechanism for the radio emission. There is though some variation in radiated power for different periods of rotation, which might also be attributed to undergoing transient processes in the atmosphere (like lightning).

Alan Watson talked about the work at the Pierre Auger Observatory, an ultra-high-energy CR detector in Argentina. He showed us an unusual phenomenon observed by multiple detectors and asked the opinion of the audience on the topic. Large variety of ideas came including possible lightning events, and military missile activity as well. Although the question has not been answered unequivocally, the response from the audience showed how beneficial such a multi-disciplinary meeting can be for the different scientific fields. Paul Rimmer went into the details of CR ionization in BD atmospheres and proposed the possibility of using Jupiter as a giant gamma-ray detector through the extensive CR air showers occurring in its atmosphere.

The last talk of the day and the workshop was given by Scott Gregory who showed us how stellar magnetic fields can affect the habitability of a planet orbiting that star. He also pointed out that the magnetic field structures differ for different stars.

The afternoon was rounded off with a whiskey tour and tasting in the Blair Atholl Destillery where we learnt a lot on how whiskey is made, what are the main ingredients, how is the alcohol content regulated and how much time the infusion spends in the barrels.

A few of the participants had the opportunity to tour the Blair Castle and its extensive grounds on Friday. The fresh apples and pears from the trees in the Hercules garden were especially enjoyable.

On the whole the workshop was a great experience for all of us, the talks were very diverse still related to our work in the LEAP Group. All speakers made great efforts to allow the audience to appreciate their contribution to the workshop’s theme. We had a great opportunity to meet scientists from other fields and discuss our projects, concerns, works with them.

We would like to thank all of the participants for their contribution to the success of the workshop. The high quality of the talks and posters gave an insight for the audience into the different disciplines.

 

Participants of the workshop (Credit: Rubén Asensio Torres)

Participants of the workshop (Credit: Rubén Asensio Torres)