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

The LEAP Group can be found here:

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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)

Lightning on exoplanets and brown dwarfs: How extended and energetic could these events be?

Lightning events are very spectacular phenomena on Earth. They are one of those beautiful plays of nature, which interest both scientists and non-scientists equally. But can lightning occur on objects outside our Solar System? The St Andrews student Rachel Bailey studies the scales that large-scale gas discharges can develop, what atmospheric volume might be affected, and what amount of energy may be deposited into the atmospheres of brown dwarfs and planets under the supervision of Dr Christiane Helling. This study was performed in collaboration with Gabriella Hodosán, Camile Bilger and Craig Stark.

Lightning strike above the Arabian Peninsula captured from aboard the ISS. (Credit: NASA)

Fig. 1. Lightning strike above the Arabian Peninsula captured from aboard the ISS. (Credit: NASA)

Atmospheric electrical discharges (like lightning) are observed not only on Earth. Lightning on Jupiter was observed both in optical and radio wavelengths of the electromagnetic spectrum. In the late ‘70s early ‘80s the Voyager 1 spacecraft recorded impulsive events in the radio band which were called SEDs or Saturnian Electrostatic Discharges. These events were identified as lightning discharges, although the optical confirmation did not come until 2009. Electromagnetic signatures associated with lightning activity were also detected on Uranus and Neptune by Voyager 2.

Lightning activity on Saturn captured by NASA’s Cassini Spacecraft. For the animated version check out Space.com  (Credit: NASA/ JPL-Caltech/ SSI/ University of Iowa)

Fig. 2. Lightning activity on Saturn captured by NASA’s Cassini Spacecraft. For the animated version check out Space.com (Credit: NASA/ JPL-Caltech/ SSI/ University of Iowa)

Not all processes involved in lightning are known in detail. The simplest idea is the following: first, a strong electric field needs to be present for long enough. This electric field, or potential difference, builds up by various ionisation processes. Processes like particle encounters with cosmic rays would make the electrons break away from their ‘parent’ atom or molecule, which leads to the formation of negative and positive ions and the atmospheric gas becomes conductive. Second, a large-scale separation of these charges over large enough distances is needed. One process causing large-scale charge separation is gravitational settling (also known as‘rain’). If the distance between the charged cloud layers is large enough, the electric field can grow so strong that it exceeds a threshold, which results in the acceleration of electrons to very high energies. These energetic electrons will ionise their surroundings by freeing more electrons resulting in an avalanche of high-energy electrons and, as a final step, a lightning discharge. This process is called runaway breakdown.

Charge separation and different types of lightning discharges (inter- and intra-cloud, and cloud-to-ground discharges) (thunder.msfc.nasa.gov/primer)

Fig. 3. Charge separation and different types of lightning discharges (inter- and intra-cloud, and cloud-to-ground discharges) (thunder.msfc.nasa.gov/primer)

Other interesting phenomena related to lightning activity are Sprites. Sprites appear above thunderclouds as extended red discharges, right after a lightning strike. These luminous events on Earth can be observed from space.

It is very likely that lightning also occurs outside the Solar System. Both exoplanets and brown dwarfs host clouds, which are made of minerals or gemstones. Why is it important to know how extended and energetic lightning events are on extrasolar bodies of interest? First, lightning affects the local chemistry of the atmosphere creating molecules that otherwise would not appear, such as prebiotic molecules responsible for the origin of life on the young Earth. On the other hand it is of interest to know whether the discharge energy is large enough to be detectable from Earth.

Atmospheres of cool objects (brown dwarfs and gas giant planets with global temperatures between 500 and 2700 K) are cold and dense enough for mineral clouds to condense. In this paper we adopt the idea that a large-scale discharge is initiated by an electron avalanche that develops into a streamer (electrically conducting channel). We apply our discharge-propagation model to one-dimensional Drift-Phoenix atmosphere models, which provide information about the local gas temperature, pressure and chemical composition.

Streamer properties in laboratory experiments as used in this paper: the segment length, L, is the length of a single segment of the streamer. The minimum diameter, dmin, is the minimal segment diameter as streamer can reach. The energy per length is the amount of dissipated energy per length of single segment. (Briels, T. M. P. et al. 2008, JPhD, 41, 234008; ©IOP Publishing. Reproduced with permission. All rights reserved.)

Fig. 4. Streamer properties in laboratory experiments as used in this paper: the segment length, L, is the length of a single segment of the streamer. The minimum diameter, d_min, is the minimal segment diameter as streamer can reach. The energy per length is the amount of dissipated energy per length of single segment. (Briels, T. M. P. et al. 2008, JPhD, 41, 234008; ©IOP Publishing. Reproduced with permission. All rights reserved.)

 Our analysis shows that the electrical breakdown can occur inside the cloud layer (lightning) and/or above the cloud layer (sprite). From these locations the discharge propagates through the atmosphere while subsequent branches appear until it reaches a minimum diameter and the discharge terminates.

Our results show that a lightning strike reaches longer distances in a brown dwarf than in an exoplanet, which means it affects larger atmospheric volumes in the former than in the latter. The total energy that dissipates from one such event is less then 1012 J. (For comparison, on Jupiter and Saturn this value is around 1012-1013 J while on Earth it is ~108-109 J.) This energy causes an increase in the local gas temperature, which results in changes in the local chemistry as well. In the paper we showed the increase of small carbohydrate molecules such as CH and CH2.

Total lengths, L_discharge, that a large-scale discharge can reach in different atmospheres (left), and total dissipated energy for different model atmospheres (right) (top panels – giant gas planet, bottom panels – brown dwarf). Solid lines indicate solar metallicity, dashed lines show sub-solar metallicity. Left: results for two different value of a constant number of charges (Q). (Bailey et al. 2014, Fig. 9, Fig 11.)

Fig. 5. Total lengths, L_discharge, that a large-scale discharge can reach in different atmospheres (left), and total dissipated energy for different model atmospheres (right) (top panels – giant gas planet, bottom panels – brown dwarf). Solid lines indicate solar metallicity, dashed lines show sub-solar metallicity. Left: results for two different value of a constant number of charges (Q). (Bailey et al. 2014, Fig. 9, Fig 11.)

Considering mineral clouds, the closest alternatives on Earth we can investigate are volcano plumes, which are composed of small silicate ash particles. After explosive eruptions, volcano plumes host lightning activity that is orders of magnitudes larger than in a common thundercloud on Earth. Taking these arguments into account it is suggested that we provided a lower limit of the dissipation energy and that, in reality lightning can be stronger and more frequent on fast rotating extrasolar objects.

Volcanic lightning captured over the Puyehue-Cordon Caulle volcanic chain in southern Chile on 4 June, 2011. (Credit: Francisco Negroni/Agenciauno /EPA)

Fig. 6. Volcano lightning captured over the Puyehue-Cordon Caulle volcanic chain in southern Chile on 4 June, 2011. (Credit: Francisco Negroni/Agenciauno /EPA)

For more details check out the original paper on ADS:

R. L. Bailey, Ch. Helling, G. Hodosán, C. Bilger & C. R. Stark 2014, ApJ, 784, 43.

Also look at the University press release of the paper:

http://www.st-andrews.ac.uk/news/archive/2014/title,242028,en.php

Christiane Helling gives a press conferenc on 30 April, 2014 (9 am):

 http://client.cntv.at/EGU2014/?play=31

The LEAP Group can be found here:

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

leap-2010.eu

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Electrostatic activation of prebiotic chemistry in substellar atmospheres

How do prebiotic molecules, necessary to the origin of life, form? What energies are needed for the formation of e.g. formaldehyde, ammonia or glycine? Do dust grains of exoplanetary atmospheres have a key role in these processes? These and similar questions were investigated by Dr. Craig Stark and his collaborators.

Dust is present all over the Universe, growing in a variety of diverse environments, for example in the atmospheres of gas giant exoplanets, where mineral dust clouds form, as earlier works of our group have demonstrated. In this atmospheric plasma the dust grains become charged and attract positive ions accelerated from the plasma. “The energy of the ions upon reaching the grain surface may be sufficient to overcome the activation energy of particular chemical reactions that would be unattainable via ion and neutral bombardment from classical, thermal excitation. As a result, prebiotic molecules or their precursors could be synthesized on the surface of dust grains that form clouds in exoplanetary atmospheres.”

Miller-Urey_experiment-en.svg

Figure 1. Miller-Urey experiment

The famous experiment of Stanley Miller and Harold Urey was one of the first to show how important electricity is in the synthesis of prebiotic molecules. (Fig. 1.) They showed that in a planetary atmosphere composed of H2, CH4, NH3 and H2O it is possible to form prebiotic amino acids and other biological molecules important for life if electrical discharge is present. However nowadays scientists believe that the atmospheric composition used in the above mentioned experiment does not correspond to the one existed on the young Earth, although a recent study showed that during volcano outbursts, where reduced gases and lightning processes are also present, it is possible to produce prebiotic molecules. As our paper says: “For atmospheres more representative of primitive Earth, no significant organic molecules are produced using electrical (sparking) discharges. However, the presence of hydroxy-acids in the famous Murchison meteorite indicate that the so-called Strecker amino-acid synthesizing mechanism (triggered by a Miller-Urey-type electrical discharge) may be responsible for the extraterrestrial synthesis of amino acids.”

Why may dust grains have a significant role in the occurrence of particular chemical reactions? Because in a plasma containing dust particles the absorption of different kind of species is electrostatically driven, which means that energies can easily exceed the activation energy required for the formation of prebiotic molecules.

“Consider a dusty plasma in the atmosphere of a substellar object such as a giant gas exoplanet. The dust particles will be negatively charged and as a result a plasma sheath (an electron depleted region) forms around the particle. As a consequence, the ionic flux at the grain surface increases as the plasma ions are attracted to and are accelerated towards the grain surface. Upon reaching the surface the ions have fallen through an electrostatic potential and have been energized. In comparison to the neutral case, the ionic flux is enhanced and the ionic energy amplified, increasing the probability that chemical reactions will occur and that reactions with higher activation energies are accessible. In this way, charged particle surfaces help catalyze chemical reactions otherwise inaccessible at such low-temperatures present in planetary atmospheres.”

In this paper we investigated the energization of ions as they are accelerated to the surface of a charged dust grain. We were mainly interested in the electrostatic activation of particular chemical reactions in the atmospheres of exoplanets.

Simulations were made using an example substellar atmosphere, which was created by the Drif-Phoenix atmosphere and cloud formation code. The atmosphere was defined by the following parameters: Surface gravity (log(g)) = 3.0; effective temperature (Teff) = 1500 K and solar metalicity ([M/H]=0.0).

Figure 2. was taken from the paper. As it says, the figure “shows the mean grain size  as a function of atmospheric pressure pgas. In the nucleation-dominated upper atmosphere (pgas ≈ 10-11 bar) seed particles form with a mean grain size <a> ≈ 10-7  cm. The dust particles gravitationally settle and grow as they fall, increasing in size. In the lower atmosphere (pgas ≈ 1 bar) the mean particle size is <a> ≈ 10-5 cm.”

Figure 2.  Mean grain particle size  as a function of gas pressure.

Figure 2. Mean grain particle size as a function of gas pressure (Stark et al. 2013)

As the electron temperature increases the local electrostatic field will rise as well which will result in that the ions accelerated by this field gain more energy. The more energy the ions gain the more likely to form molecules with higher activation energies.

We used the formation of glycine (NH2CH2COOH) to give an example “of the electrostatic activation of prebiotic chemistry on the surface of a charged dust grain.” The following chemical sequence shows the individual steps, which lead to the formation of glycine, with the formation energies (an indicator of the activation energy of the chemical reaction)

Screen Shot 2014-01-28 at 14.24.12

Figure 3. Synthesis of glycine

Figure 3. Synthesis of glycine

As our results show, “high in the atmosphere where pgas ≈ 10-15 bar, the ion temperature is ≈ 600 K and the resulting thermal energy of an ion is ≈ 0.08 eV, which is lower than the energies required to form the reaction products above. However when the electron temperature Te = 1 eV the ions are accelerated to Etot ≈ 7.8 eV which surpasses the required formation energies, increasing the likelihood that these reactions will occur. At lower atmospheric pressures where it is hotter, the thermal energy can reach ≈ 0.45 eV (≈ 5200 K) and there will exist a population of higher energy ions that, once neutralized on the surface, may be energetic enough to activate the required chemical reactions to form glycine.”

We showed in our paper that in atmospheric plasma where dust grains become negatively charged, ions can be accelerated to energies high enough to produce chemical reactions which would be inaccessible via classical thermal ion and neutral fluxes. As a consequence the creation of prebiotic molecules on grain surfaces may increase significantly.

The importance of this paper is that it “establishes the feasibility of the electrostatic activation of prebiotic chemistry. This idea can be developed to explicitly model the surface chemical kinetics, describing the incoming accelerated ions interacting with the grain surface. In this way, the effect of the plasma ionic species, the composition of the grain surface and the effect of the grain charge on the resulting surface chemical reactions can be quantified.”

For more details check out the original paper on arXiv:

C. R. Stark, Ch. Helling, D. A. Diver, and P. B. Rimmer 2013, IntJAstrobio 13, 165

Also look at the press release of the paper:

http://www.st-andrews.ac.uk/news/archive/2014/title,233471,en.php