EXO-LIGHTNING Part II: a planet and its radio signal

How big a lightning storm would we need to produce an observable signal on an exoplanet? What effect would such a storm have on the local atmosphere of the planet? These questions are explored in the paper by Gabriella Hodosán, Paul B. Rimmer and Christiane Helling for the mini-Neptune HAT-P-11b discovered by the Hungarian-made Automated Telescope Network (HATNet) in 2009.

 

Artist's impression of an exoplanet like HAT-P-11 (Credit: NASA/JPL-Caltech).

Artist’s impression of an exoplanet like HAT-P-11 (Credit: NASA/JPL-Caltech).

HAT-P-11b is a Neptune-like exoplanet, orbiting the host star at a distance of 20 times shorter than the Earth-Sun distance. It is about 5 times bigger in size than Earth and 26 times more massive. It was discovered by the transit technique. In 2009 a group of scientists (Lecavelier des Etangs et al. 2013) observed the planet at a radio frequency band centred at 150 MHz. The observation was timed so that the planet would be observed before, during and after its occultation. They found a radio signal of ~4 milliJansky (mJy), which vanished as the planet passed behind its host star. The French team suggested that this signal could come from the planet, as it disappeared when the planet was behind the star. In 2010 they had a re-observed the planet, but could not find the signal this time. If the radio signal in 2009 was real, than its absence in 2010 suggests that it was produced by a transient event.

A storm with millions of lightning flashes

In our work (Hodosán, Rimmer and Helling 2016) we assumed that the signal detected in 2009 was real and was coming from the planet, as was suggested by the original paper. The question that raised our curiosity was: Could such a radio signal be produced by lightning in the planet’s atmosphere, and if yes, how many lightning flashes would be needed for it? In order to answer this question, we assumed that the basic lightning physics is the same on Earth, on the Solar System planets and on exoplanets. We previously used this assumption in Bailey et al. 2014 (see the blog entry here). Based on these assumptions we calculated that ~4×106 lightning flashes over a km2 in an hour should have happened during the observations of HAT-P-11b, over its whole disk.

Artist's concept of lightning on Venus (Credit: ESA/image by Christophe Carreau)

Artist’s concept of lightning on Venus (Credit: ESA/image by Christophe Carreau)

Is this enormous number of lightning flashes unreasonable? Our group (e.g. Lee et al. 2015, Helling et al. 2016) and others (e.g. Heng & Showman 2015) have shown that exoplanets orbiting really close to their host star have very dynamic atmospheres, meaning that they change continuously, producing clouds of different sizes all over the planet’s surface (e.g. our posts here and here). HAT-P-11b, being so close to the star, is likely to have a hugely dynamic atmosphere, which could allow the formation of immense thunderclouds, similar to the big red spot on Jupiter, focusing the lightning activity to a certain regime of the planetary surface, such as the face of the planet, which was observed in 2009.

Screen Shot 2016-08-15 at 13.31.48

Flash densities and optical emission of lightning that would produced the observed radio signal (Hodosán et al. 2016)

Visible in the optical range?

Could we see such a huge storm with optical telescopes, in the visible range of the spectrum? Because a very large number of lightning flashes would be needed to produce the observed radio signal, they would produce a very high optical emission. We found that, depending on the energy emitted by the lightning flashes, the optical emission of this enormous thunderstorm would be comparable to that of the host star HAT-P-11, a bright K star, or would even outshine it. This is quite unlikely to happen, so we concluded that the radio signal was most probably not a result of lightning storms of HAT-P-11b.

Could we trace chemical signals of such a storm?

However, we were curious to find out how the local chemistry would change because of he large amount of lightning happening at a time. It is known from Earth that lightning has an effect on the chemistry of the atmosphere where it occurs. The process of discharging a lightning channel involves very high temperatures and the release of a large amount of energy. This results in chemical reactions that would not occur any other way. So-called „non-equilibrium” species are produced. These only stay in the atmosphere for a relatively short amount of time, then they turn into their equilibrium counterparts, unless the conditions became appropriate for their production. For example lightning on Earth produces NO2 in the atmosphere, which disappears after a while, unless more lightning occurs and produces more of this molecule.

In our study, we found that HCN would be the species to look for in the case of HAT-P-11b, if we want to observe lightning. Interestingly, we would not need the obtained number of lightning flashes to produce observable signatures, but orders of magnitude lower lightning activity would still be capable of creating observable quantities of HCN. These signatures would appear in the infrared band for 2-3 years after the occurrence of the storm, if lightning would have happened at a certain height of the atmosphere.

Lightning or aliens?

Since the press release (now showing previous, out-dated results) of the paper, several newsagents picked up the story. Some of them focused on our interpretation of an “unexplained radio signal” with a natural phenomenon (i.e. lightning) instead of aliens. This gave us the idea of comparing our study with previous ones looking for alien radio signals coming from space.

“Two possibilities exist: either we are alone in the Universe or we are not. Both are equally terrifying.” – Arthur C. Clarke

Allan Telescope Array (Credit: SETI Institute).

Allan Telescope Array (Credit: SETI Institute).

The question whether Earth is the only place in the Universe that hosts life on its surface has been around, unanswered, for thousands of years. We can go even further: are there other civilizations, intelligent races, out there? One of the first scientific papers on the question was written by G. Cocconi and P. Morrison, who wrote down the theory of ways of interstellar communication. Frank Drake started the so called “Project Ozma”, the first systematic search for alien signals with a radio telescope. His attempt was well designed, however, unsuccessful, such as all attempts have been so far. A year later, he organized a conference. While he was preparing for it, he came up with the idea of the famous Drake equation, which relates the number of intelligent civilization in our Galaxy to a series of factors. Since then several papers have been published about different theories of what the signatures could be of such intelligent civilizations, could we detect any of those signatures and why haven’t we detected any signs of them.

When looking for extraterrestrial intelligent life, we have to consider the signature most promising to look for. Radio waves propagate through space almost unimpeded and with the speed of light, making these wavelengths of the electromagnetic spectrum good candidates for ET signs. However, the electromagnetic spectrum contains a wide range of radio frequencies. Then where to look? The frequency band called “microwave window” is the favoured part of the spectrum, ranging from 1 GHz to 10 GHz. Physically this is the most quiet range of radio frequencies, meaning that this part is the less contaminated by natural radio emission. However, it is not free of it. But then how can we distinguish between a natural and non-natural radio emission? In theory, a radio signal produced by some kind of technology would be narrow-band. It would emit only in a very narrow range of frequencies. While natural phenomena would have broadband emission, emitting with a gradually increasing/decreasing power through out a wide range of frequencies.

Now, what about radio emission coming from lightning? Two main aspects of the technology-emitted radio signal and the lightning-caused radio signal are very different. Lightning emits in a broad spectral band, such as most of the natural phenomena we know of. The second main difference is the frequency at which it emits. While we expect other extraterrestrial civilizations to broadcast their signals at the GHz range, lightning emits mostly in the kHz up to a few hundred MHz frequency range. The radio signal from HAT-P-11b was observed in radio band centred at 150 MHz. This is much lower than what we would expect from an ET broadcast. However, this is the higher part of the radio band where lightning could still emit.

So in summary, the radio signal from HAT-P-11b is most probably not an alien signal, but either caused by a natural phenomenon, or simply instrumental noise. If we were the ones broadcasting radio signals towards other planets, we would not do it from a definitely uninhabitable planet, like the hot, Neptune-like HAT-P-11b. We would also not broadcast at frequencies, which would be difficult to observe due to natural noises, like at 150 MHz.

So the conclusion?

Credit: ESA

Credit: ESA

The conclusion is that there are various explanations, all from natural phenomena, which could cause a weak radio signal like the one observed by Lecavelier des Etangs et al. (2013) in 2009. However, most probably it is not lightning. We have to emphasize though, that lightning might be very different on an exoplanet from what we know in the Solar System. Exo-lightning may be much more energetic, producing more powerful signatures than has ever been observed in our Solar System. In which case, lower number of flashes, within the reasonable range, could produce observable signatures.

However, the importance of our study is that it showed that in the future, combined radio and infrared observations may lead to the first detection of lightning on an extrasolar planet, though this planet will most probably be closer to us than HAT-P-11b is. It also shows an original scenario for the explanation of radio emission observable on planets outside our Solar System.

If you have an opinion, please leave a comment below.

For more details check out the original paper on ADS:

 Hodosán, G.; Rimmer, P. B. and Helling, Ch, MNRAS, 461, 1222, 2016

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EXO-LIGHTNING Part I: what can we learn from the Solar System?

Is lightning a phenomenon only occurring on Earth? Or is it universal? How can the knowledge we learnt from Solar System lightning help with discovering lightning on exoplanets and to understand these very different worlds? The next two entries will be devoted to the work on extraterrestrial lightning carried out by Gabriella Hodosán, LEAP PhD student, under the supervision of LEAP PI Dr Christiane Helling, in collaboration with various LEAP group members. In Part I we apply results of lightning surveys of several Solar System planets, including Earth, to different groups of extrasolar planets. Part II will be about a specific planet, HAT-P-11b and the possibility of lightning detection in its atmosphere.

Lightning is one of the most spectacular phenomena on Earth. It has interested not just scientists but the general public for thousands of years. However, it is not a unique phenomenon to Earth. It has been observed before on several Solar System planets, such as Jupiter, Saturn, or Uranus and Neptune. Spacecraft like Cassini, Galileo, New Horizons or the Voyagers provided us with breath-taking images of the outer Solar System, including images and measurements of lightning occurring on the gas giant planets (Fig. 1).

Lightning on Jupier (right top and bottom) and Saturn (left bottom). (Credit: NASA/JPL-Caltech/SSI, NASA/Galileo)

Figure 1. Lightning on Jupiter (right top and bottom) and Saturn (left bottom). (Credit: NASA/JPL-Caltech/SSI, NASA/Galileo)

Since the late 90s, thousands of exoplanets have been discovered. These exoplanets show a large diversity (Fig. 2) in sizes, masses, even distances to the host star, much different to our Solar System planets: Jupiter-size planets orbiting other stars at the distance of Mercury; planetary systems with several planets inside the orbit of Mercury; terrestrial planets several times bigger than Earth, but still rocky and not made of gas. Could these planets host lightning in their atmospheres? Let’s look at Earth and Saturn. They have different composition, different sizes, masses, different atmospheres. And still, they both show lightning activity. So why couldn’t it occur on exoplanets?

Diversity of exoplanets and brown dwarfs on a mass-radius and distance-density plot (Hodosán et al. 2016).

Figure 2. Diversity of exoplanets and brown dwarfs on a mass-radius and distance-density plot. The lines on the top plot indicate the potential chemical composition of the bodies based on their obtained mass and radius  (Hodosán et al. 2016).

In our work, we were focusing on the statistical side of lightning occurrence on Solar System planets, then extrapolated to extrasolar objects. Lightning climatology explores the spacial and temporal distribution of lightning. It uses the quantity of flash rate [flashes/unit time, e.g. flashes/hour] or flash density [flashes/unit time/unit surface area, e.g. flashes/hour/km2] to quantify this distribution. It is a tool to estimate the lightning activity on the surface of an object. This is important in order to estimate the total energy released of lightning flashes, and to determine whether the signatures produced by lightning would be observable from Earth.

Earth

Lightning observing networks net the whole surface of the Earth and satellites continuously look for lightning flashes from near Earth orbits. Lightning monitoring is important because of the hazards (e.g. forest fires, large scale power outage, fatalities) it causes. Measurements from Earth provide the largest data sets we can work with. In our study we analyzed data from the Lightning Imaging Sensor (LIS)/Optical Transient Detector (OTD), which are optical instruments on board of satellites, and from two ground based radio networks, the Sferics Timing and Ranging Network (STARNET) and World Wide Lightning Location Network (WWLLN). Figure 3 shows an example of lightning climatology maps produced from LIS/OTD data. It shows an average of lightning occurrence over the period of 1995-2013. It shows clear trends of more lightning over continents than over oceans, and more lightning over lower latitude regions than higher latitude regions.

Figure 3. LIS/OTD lightning climatology map averaged from 1995-2013 (Hodosán et al. 2016).

Figure 3. LIS/OTD lightning climatology map averaged from 1995-2013 (Hodosán et al. 2016).

Lightning occurs not only in thunderclouds but in volcano plumes as well (e.g. see work by our group, and the eruption of Eyjafjallajökull). Based on literature research we collected flash densities from two volcano eruptions: Mt Redoubt in 2009 and Eyjafjallajökull in 2010. Some interesting fact: both eruptions show orders of magnitude larger flash densities than what thunderclouds produce. On average the LIS/OTD data showed a 2×10-4 flashes/km2/h flash density while Eyjafjallajökull showed 0.1 – 0.32 flashes/km2/h and Mt Redoubt produced up to 2000 flashes in an hour over a square kilometer.

Venus, Jupiter and Saturn

Is there lightning on Venus? The long debated question has not yet been fully answered, however, more and more clue indicate the existence of such phenomenon on Earth’s sister planet. If lightning does exist on Venus, it is probably not very energetic and it appears deep within the atmosphere, since no optical observation of it has been made to date. Radio data from the Venus Express mission, however, shows a possible ~10-11 flashes/km2/hour flash density.

Figure 4. Lightning distribution on Jupiter. Triangles: Galileo data from 1997. Circles: New Horizons data from 2007.

Figure 4. Lightning distribution on Jupiter. Triangles: Galileo data from 1997. Circles: New Horizons data from 2007.

Jupiter and Saturn are more interesting, since, apart from Earth, these are the only planets in the Solar System where lightning has been observed directly. We used published data from the Galileo (1997, Jupiter), New Horizons (2007, Jupiter) and Cassini (2009, 2011, Saturn, e.g.) spacecraft. Figure 4 shows and example lightning distribution map for Jupiter. It shows and increased lightning activity around the +- 50-degree latitude regions, most probably due to the increased affect of internal heating on convection and cloud formation. Until 2009, only radio signals of lightning were observed on Saturn. The so-called SEDs (Saturnian Electrostatic Discharges) are short and strong radio bursts from lightning, detected by e.g. the Voyagers and Cassini spacecraft. The two giant gas planet, on average, shows a flash density of 10-6 – 10-7 flashes/km2/hour.

Exoplanets

Now the most exciting part: How does this all relate to exoplanetary research? The idea behind our paper is to use the statistics from Solar System planets to estimate a possible lightning occurrence on exoplanets with similar environments to those described in the previous sections. E.g. let’s take possible ocean planet: Kepler-62f. Then apply the flash density derived for Earth for above oceans: ~5 x 10-5/km2/hour. Combining this, we estimate the flash density on Kepler-62f, throughout its whole surface, to be the same: ~5 x 10-5/km2/hour. Another example is to apply the flash densities in order to estimate the total lightning occurrence during the transit of a planet. 55 Cancri e is a close-in rocky planet, most probably cover by lava. How much lightning could occur during its transit on the projected disk of the planet, if we assume continuous flashing from volcano activity? Depending on which eruptions we consider as the template, 55 Cnc e could produce as many as 108-1012 flashes on its whole disk during its 1.5-hour transit. This enormous amount of lightning would produce radio emission that might be observable during the transit.

In Part II we introduce our original idea of estimating whether a weak and tentative radio signal observed on the mini-Neptune, HAT-P-11b, could have been caused by lightning or not.

If you have an opinion, please leave a comment below.

For more details check out the original paper on ADS:

 Hodosán, G.; Helling, Ch; Asensio-Torres, R.; Vorgul, I.; Rimmer, P. B., MNRAS, 461, 3927, 2016

The LEAP Group can be found here:

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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, eprint arXiv:1603.04022

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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, eprint arXiv:1601.04594

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The Chemistry of Lightning and Life

What chemical effects does lightning have on other planets? How does life affect the atmospheres of other worlds? The ground-breaking experiments of Miller and Urey showed that in some cases, lightning can produce the chemicals necessary for the origin of life. Did lighting succeed in starting off the chemistry that resulted in life on the Early Earth? Does it do so for other planets? These are the questions that a paper by Paul Rimmer & Christiane Helling (2015) wrestle with.

Chemistry leaves a very distinctive footprint on exoplanet atmospheres, thanks to the quantum mechanical precision of spectroscopy. Generally, the spectra of exoplanets can be determined in two ways. The most straightforward way to determine the spectrum of an exoplanet is to look directly at the object, and to observe its emitted light. This can be done only for exoplanets that are bright and far away from their host star, so that the starlight can be blocked out, leaving the planet’s light for astronomers to study. The light from the exoplanet is emitted at characteristic wavelengths, mostly in the infrared. The other way to obtain a spectrum is to look at a transiting planet, a planet that, from our perspective, passes in front of its host star, blocking out the star’s light. At some wavelengths, more light is blocked and the planet looks larger. At other wavelengths, less light is blocked and the planet looks smaller. These wavelengths are determined to exceptional precision by the quantum mechanical properties of the chemical species within the exoplanet’s atmosphere. Physics influences the chemistry, and changing the chemistry changes the light that the exoplanet emits and absorbs. Chemistry gives us a window into the lives of these alien worlds.

Some of the most interesting things in the universe are far outside equilibrium. Equilibrium is a way of describing a system by certain bulk quantities, such as temperature, pressure, volume, and available elements. If you know these properties, you have learnt a lot about the system. You know its chemical composition and its colour. But most of the interesting places in the universe exist outside equilibrium. Take Earth’s atmosphere, for example. It’s comprised of 78% molecular nitrogen, 21% molecular oxygen, and then small amounts of carbon dioxide, methane, hydrogen and water. If the carbon held within the biomass were present in the atmosphere, then our atmosphere would be made up of almost entirely nitrogen and carbon dioxide, with virtually no oxygen or methane. Even if the biomass were not liberated into the atmosphere, all the methane would be destroyed. Methane shouldn’t exist in our atmosphere at all at equilibrium. Life keeps the methane locked out of equilibrium. Additionally, there would be no ozone layer at all. Ozone is produced when molecular oxygen reacts with ultraviolet light from the sun.

Life and light are both written into our planet’s chemistry. And not just our planet’s. Directly imaged planets have chemical features that cannot be explained without invoking strong winds carrying equilibrium mixtures of molecules into places where they are out of equilibrium, and where they are replenished at a rate much faster than they are destroyed. Some hot Jupiters indicate abundances of acetylene and ammonia that are far from equilibrium. The search for life on other worlds has just started, and is focused entirely on the detection of just the kind of non-equilibrium chemistry that indicates the presence of life. There have been a variety of chemical models used to explain this non-equilibrium chemistry, either for Earth and Earth-like planets, or for hot Jupiters or directly imaged exoplanets. The models that work for one object generally cannot be applied to an object with a completely different composition or temperature, and none of these models considers the detailed chemistry of lightning.

Fig. 1. Artistic representation of the types of exoplanets discovered (Credit: Harvard-Smithsonian CfA)

Fig. 1. Artistic representation of the types of exoplanets discovered (Credit: Harvard-Smithsonian CfA)

An object like Jupiter is dominated by hydrogen, with trace amounts of carbon, oxygen and nitrogen, among other elements. Earth, on the other hand, is dominated by nitrogen and oxygen, with trace amounts of carbon in the atmosphere. The exceedingly thin atmosphere of Mars is primarily composed of carbon dioxide and nitrogen. Jupiter, Earth and Mars all would have formed from the same disk of gas and dust, with the gas mostly made of hydrogen. Why do they look so different? Why does Jupiter have so much hydrogen and the Earth and Mars don’t? The most important reason for this is that Mars and Earth are too light to retain hydrogen. They probably did have a hydrogen atmosphere very early in their lifetimes, but lost it long ago. Super-Earths, rocky exoplanets that are more massive than the Earth, have been discovered in abundance, and would have sufficient mass to retain much more hydrogen.

Paul Rimmer and Christiane Helling have developed a comprehensive chemical network called STAND, that incorporates hydrogen, carbon, nitrogen and oxygen chemistry, complete up to glycine, as well as simple chemistries for a variety of other species, including titanium, silicon, sodium, calcium, and potassium. It is robust for a wide range of temperatures and compositions, so that it can be applied to both hot Jupiter planets and directly imaged planets far from their host star, to rocky planets like Mars, Earth, or even to moons like Titan. Additionally, it includes UV and X-ray photochemistry, and chemistry induced by cosmic ray ionization or by interaction with other charged particles. The combination of these tools makes it so STAND can model in detail the chemistry of lightning, to determine both what observational effect it might have on exoplanet atmospheres and to determine whether lighting on different worlds might effectively open up the pathways to prebiotic chemistry.

In combination with ARGO, a photochemistry and diffusion model, STAND has been tested against various planets and exoplanets to find out whether it achieves results consistent with observation and with other models. STAND itself was also tested against laboratory tests similar to those of the Miller experiment, and found the same general trend concerning the hydrogen bonds of carbon and nitrogen, measured in the paper by a quantity called Rr. When they form mostly hydrogen bonds (Rr is small), they produce significant concentrations of glycine; and when carbon and nitrogen are bonded to oxygen or to each other instead of hydrogen (Rr is big), they produce small concentrations of glycine. This is true up until a certain point, when virtually all the nitrogen and carbon share hydrogen bonds, and none are bound to oxygen (when oxygen is bound either to itself as molecular oxygen or is in the form of water). In this case, interestingly, virtually no glycine is formed at all. This result from the paper is shown here in colour (Fig. 2)

Fig. 2. Mixing ratio of glycine as a function of time, for five lab simulations (Rimmer & Helling 2015)

Fig. 2. Mixing ratio of glycine as a function of time, for five lab simulations (Rimmer & Helling 2015)

The STAND network is now being compared to work from leading origin of life research groups, and is also being applied to explain the observed chemical impact of aurorae on brown dwarfs. Rimmer & Helling’s 2015 paper show how important it is to understand the connection between physics and chemistry in exoplanet atmospheres, because often chemistry is the only thing that can be determined very precisely or in much detail. They also provide the community with a new tool to model this chemistry at its most interesting, when it’s far from equilibrium.

For more details check out the original paper on ADS:

 Paul Rimmer & Christiane Helling 2015, eprint: arXiv:1510.07052

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Why exoplanets should have ionospheres and brown dwarfs chromospheres

Do exoplanets have an ionosphere? What does a brown dwarf need to form aurorae even without a companion? Isabel Rodriguez-Barrera and colleagues, including Christiane Helling and former member Craig Stark from the LEAP group, investigated whether it is possible to create a magnetized plasma, a medium composed of positive and negative charges but with an overall neutral electric property. The production of magnetized plasma would allow the creation of ionospheres and electromagnetic phenomena such as aurorae.

The ionosphere is the upper part of a planetary atmosphere created by the ionizing effects of stellar UV and X-ray radiation. Its importance inheres in atmospheric electricity and radio wave propagation, but also the shielding of the inner atmosphere from stellar UV radiation. The following study showed what conditions a planetary atmosphere (that is an atmosphere from a planet or a planet like object like a brown dwarf) should fulfil to produce an ionosphere or, in case of brown dwarfs, a chromosphere, such as the spectacular example of the solar chromosphere.

Radio, X-ray and Hα emission from brown dwarfs have been observed in the recent years (e.g. 2MASS J10475385+2124234 by Route & Wolszczan 2012; 2MASS J13153094-2649513AB by Burgasser et al. 2013). Similar observations are not yet available for extrasolar planets. In case of the Sun, observations of these emissions (radio, X-ray and Hα) are tracers of the solar chromosphere. These observations suggest that brown dwarfs contain ionized gas and host very strong magnetic fields, which are both needed to explain, for example, the radio emission. The aim of our study is to identify ultra-cool objects (with effective temperatures less than ~3000 K; Fig. 1.) that are most susceptible to processes leading to instabilities that trigger the emergence of strong plasma, a neutral state of matter composed of equal number of positive and negative ions.

Our theoretical work proposes a method of analysing the ionization and magnetic coupling state of objects with ultra-cool atmospheres. Our particular interest focuses on late M-dwarfs, brown dwarfs and giant gas planets.

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.

To determine the fraction of atmosphere that can be ionized, first Rodriguez-Barrera et al. consider thermal ionization only. Thermal ionization results from collisions between the gas particles according to the local gas temperature, therefore here we do not consider external ionizing affects from companions. Sources of possible irradiation are the host star in the case of planets, and a white dwarf in the case of a white dwarf – brown dwarf binary (for example WD0137-349B, Casewell et al. 2015). Such external affects can be later compared to the results of our reference study. We use the Drift-Phoenix model atmosphere grid where the local atmospheric structure is determined by the following global parameters: Teff (effective temperature), log(g) (surface gravity) and [M/H] (metallicity).

Rodriguez-Barrera et al. show that ultra-cool atmospheres with high Teff , or with high metallicity and low log(g) have large fraction of atmospheric volume where plasma processes occur, and so they are the best candidates for radio, X-ray and Hα emissions, observed from various objects as was mentioned above.

Figure 2. The volume fraction of the atmosphere that is thermally ionized, V^th_gas/V_atm, for f_e>10-7 and for M-dwarf, brown dwarf and gas giant planet atmospheres. f_e measures the extent to which a gas is ionized. (Rodriguez-Barrera et al. 2015)

Figure 2. The volume fraction of the atmosphere that is thermally ionized, V^th_gas/V_atm, for f_e>10-7 and for M-dwarf, brown dwarf and gas giant planet atmospheres. f_e measures the extent to which a gas is ionized. (Rodriguez-Barrera et al. 2015)

M-dwarfs have a considerable degree of ionization throughout the whole atmosphere, the degree of thermal ionization for a L-dwarf is low but high enough to seed other local ionization processes like Alfven ionization (see Stark et al. 2013) or electrostatic discharges, such as lightning, as seen on Fig. 2.

We show that the first criterion to form chromospheres, ionospheres or an aurora on an extrasolar planets or brown dwarf, namely a small but sufficient degree of ionization, can be fulfilled by thermal ionization alone without the need for additional processes. But is it possible to magnetise this ionized plasma? The second part of our study says yes! The results also give an idea of how well the different types of atmospheres can be magnetized (Fig. 3): The minimum threshold for the magnetic flux density required for electrons and ions to be magnetised is smaller than typical values of the global magnetic field strengths of a brown dwarf and a giant gas planet. This means the ionized plasma inside the atmosphere can be magnetised quite easily. A considerably lower magnetic flux density is required for magnetic coupling of the atmosphere in the rarefied upper atmosphere than in the dense inner atmosphere, meaning magnetising the plasma in the upper atmosphere is easier than in the inner parts of the atmosphere. The magnetic coupling works equally for electrons and atomic ions like Mg+ and Fe+ (Fig. 3).

Figure 3. The magnetic flux density required for electrons, Be (lower set of lines), and ions, Bi (upper set of lines), to be magnetically coupled to a background magnetic field in the object (B=10 G - giant gas planets (GP), B=103 G M-dwarfs (MD), brown dwarfs (BD); black horizontal/vertical lines). If B>Bi (or B>Be) the gas is magnetized by the external magnetic field B. (Rodriguez-Barrera et al. 2015).

Figure 3. The magnetic flux density required for electrons, Be (lower set of lines), and ions, Bi (upper set of lines), to be magnetically coupled to a background magnetic field in the object (B=10 G – giant gas planets (GP), B=103 G M-dwarfs (MD), brown dwarfs (BD); black horizontal/vertical lines). If B>Bi (or B>Be) the gas is magnetized by the external magnetic field B. (Rodriguez-Barrera et al. 2015).

To sum it up, our results suggest that it is not unreasonable to expect ultra-cool atmospheres (M-dwarfs and L & T brown dwarfs) to emit Hα or even in radio wavelength. We showed that, in particular, the rarefied upper parts of the atmospheres fulfil quite easily the plasma criteria despite having low degrees of ionization also in the case of giant gas planets. Therefore the results suggest that an ionosphere may emerge also in brown dwarf and giant gas planet atmospheres, and that the built-up of a chromosphere on brown dwarfs is likely too. Both effects will contribute to atmospheric weather features and to space weather occurrence in extrasolar, planet-like objects. An interesting result is that ultra-cool atmospheres could also drive auroral emission without the need for a companion’s wind (e.g. aurora on Earth triggered by solar wind) or an outgassing moon (e.g. aurora on Jupiter is triggered by its outgassing moon Io).

fig4

Figure 4. The dominating thermal electron donors for a subset of effective temperatures for log(g)=3,0 and solar element abundances, against the local gas pressure (Rodriguez-Barrera et al. 2015).

We further studied which of the gas species might be the best electron donors. Na+, K+ and Ca+ are the dominating electron donors in low-density atmospheres (low log(g), solar metallicity) independent of Teff. Mg+ and Fe+ dominate the thermal ionization in the inner parts of M-dwarf atmospheres. Molecules remain unimportant for thermal ionization. Chemical processes (e.g. cloud formation, cosmic ray ionization) that affect the abundances of Na, K, Mg, Ca and Fe will have a direct impact on the state of ionization in ultra-cool atmospheres.

For more details check out the original paper on ADS:

 Rodríguez-Barrera, I.; Helling, Ch; Stark, C. R. and Rice, A. M. 2015, MNRAS 454, 3977

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Mapping the sparkling clouds of the extrasolar planet HD 189733b

How do clouds look like on alien worlds? Graham Lee and Christiane Helling in collaboration with Ian Dobbs-Dixon (New York University Abu Dhabi) and Diana Juncher (University Copenhagen) took the first steps in modelling the formation of clouds for the giant gas planet HD 189733b, a member of a class of exoplanets commonly called ‘hot Jupiters’.

Observations (e.g. Pont et al. 2013; Sing et al. 2015, ) suggest that many hot Jupiters contain a large dust cloud component in their atmosphere because they obscure the absorption signatures of the atmospheric gas underneath the cloud layers. These clouds are made of mineral compounds such as TiO2[s], MgSiO3[s], SiO[s], Al2O3[s], Fe[s] (‘s’ meaning solid particles) (see previous blog post), and not of water like on Earth.

Inspired by previous research which proved clouds exist in brown dwarf atmosphere (see our post on DRIFT-PHOENIX Atmosphere Models) we set out to investigate if the same family of clouds could reside in hot Jupiter atmospheres.

We applied our cloud formation model to a 3D radiative-hydrodynamic simulation (RHD) of HD 189733b (Dobbs-Dixon & Agol 2013), to prove that the temperature and pressure conditions on these planetary atmospheres are suitable for cloud formation. We took temperature, density and pressure data in 1D “slices” of the 3D simulation as input for our cloud formation model (Figure 1). This is like using an atmospheric probe to sample the local conditions of the atmosphere during descent. The combination of a sophisticated 3D RHD atmospheric model and our 1D cloud formation model allowed us to create cloud “maps” of the HD 189733b atmosphere.

Left: Illustration of the sample trajectories (black points) taken from the 3D radiative-hydrodynamic (RHD) model atmosphere of HD 189733b (Dobbs-Dixon & Agol 2013). Right: Input temperature and pressure profile for the cloud formation model at the equator of the 3D RHD model.

Left: Illustration of the sample trajectories (black points) taken from the 3D radiative-hydrodynamic (RHD) model atmosphere of HD 189733b (Dobbs-Dixon & Agol 2013).
Right: Input temperature and pressure profile for the cloud formation model at the equator of the 3D RHD model. The dayside profiles are φ= 0°, 45°, 315°, nightside profiles φ = 135°, 180°, 225° and day-night terminator regions φ = 90°, 270°.

Our results show how cloud properties change between different regions of the planet. First we noticed that the size of cloud particles changes with the location on the globe. Grains found on the dayside generally grow faster and larger than those on the nightside. However, because of the lower temperatures on the nightside, more grains form on the nightside. This leads to an cloud structure where numerous small grains reside on the nightside while larger (but less abundant) grains reside on the dayside face of the planet.

We converted the cloud properties across the globe into a map of global cloud properties: Figure 2 depicts the mean particle size at an atmospheric pressure of 10-2 bar across the globe of HD 189733b, where the difference between nightside and dayside is most apparent.

With our simulations, we show that the maximum reflectivity of mineral clouds correspond to the 8 micron Spitzer global flux maximum observed by Knutson et al. (2007). Our results therefore suggest that clouds can significantly contribute to the infrared flux from these planets by scattering photons back into space.

We further found that the clouds on the hot Jupiter HD 189733b reflected more efficiently in blue than red spectral range. This suggests that the clouds on this planet will appear midnight blue in colour if viewed with human eyes. Figure 3 shows an RGB colour estimation for HD 189733b clouds  by interpolating the light scattering result. Evans et al. (2013) presented observations with the Hubble Space Telescope suggesting a bluish appearance of HD 189733b which our work now supports on the basis of detailed cloud formation modelling.

RGB scale and colour estimate of the cloud particles on the dayside face of the planet. Hubble Space Telescope observations found the planet to be a deep blue colour.

RGB scale and colour estimate of the cloud particles on the dayside face of the planet. Hubble Space Telescope observations found the planet to be a deep blue colour.

Could these clouds sparkle? Mineral cloud particles are thought to form crystalline structures as they travel through the atmosphere (Helling & Ritmeijer 2009). This means that the mineral particles that form the clouds on HD 189733b are likely to “sparkle” similar to gemstones on Earth, such as sapphire.

 

 

 

 

For more details check out the original paper on ADS:

Lee, G., Helling, Ch., Dobbs-Dixon, I., Juncher, D. 2015, arXiv:1505.06576

The LEAP Group can be found here:

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