Sunday, May 4, 2014

Lightning in Exotic Superheated Atmospheres

In the Solar System, lightning activity is present on all planets that have clouds in their atmospheres. With the sheer number and diversity of exoplanets discovered in recent years, it is worth considering if lightning activity is also present on these worlds. A study by Christiane Helling et al. (2012) examined the possibility of lightning being present in the exotic superheated atmospheres of hot-Jupiters and brown dwarfs. Hot-Jupiters are giant gaseous planets that orbit very close to their host stars, while brown dwarfs are objects that bridge the gap between the most massive planets and the least massive stars.

Figure 1: Artist’s impression of a brown dwarf.

Both hot-Jupiters and brown dwarfs (provided the brown dwarf has not cooled sufficiently) have atmospheric temperatures that are much higher than on Earth or on Jupiter. Temperatures in these atmospheres can reach thousands of degrees Kelvin. Such temperatures are too hot for water to exist outside the vapour phase, thereby ruling out the moist thundercloud charging process that is responsible for generating lightning here on Earth, and on the gas giants Jupiter and Saturn. Nevertheless, the superheated atmospheres of hot-Jupiters and brown dwarfs are dominated by mineral clouds, consisting of a number of mineral species in the form of liquid mineral droplets and/or mineral dust particles. For example, at sufficiently high temperatures, droplets of liquid iron (Fe) or droplets of liquid titanium oxide (TiO2) can fall as rain.

Figure 2: Dust cloud material composition in volume fractions in the atmosphere of a hot-Jupiter. The compositional changes with atmospheric height are indicated by the local temperature. Source: Christiane Helling et al. (2012).

In order for lightning to be present, the mineral clouds need to undergo some form of ionisation. A number of mechanisms can lead to sufficient ionisation of the mineral clouds. These mechanisms include energetic radiation from the host star, triboelectric charging and cosmic rays. Of particular interest is triboelectric charging, involving dust-dust collisions. Although dust-dust collisions from gravitational settling (i.e. mineral dust particles raining out) are not energetic enough to induce triboelectric charging, atmospheric turbulence can enhance the dust-dust collisions such that triboelectric charging becomes possible.

Nevertheless, gravitational setting enables large-scale charge separation in the mineral clouds and establishes the condition necessary for lightning to occur. The large-scale charge separation develops as particles of different sizes fall at different rates. Smaller, less negatively charged particles fall slower and remain longer in the upper cloud layers. Conditions for lightning to occur are created when large-scale charge separation eventually causes the electric potential between two regions of the mineral cloud to exceed the breakdown field.

The study by Christiane Helling et al. (2012) concludes that mineral clouds in the atmospheres of hot-Jupiters and brown dwarfs should generate lightning since the breakdown fields are lower than on Earth or on Jupiter. Hot-Jupiters and brown dwarfs do not have solid surfaces like on terrestrial planets such as the Earth. As a result, lightning activity occurs in the form of intra-cloud discharges.

- Christiane Helling et al. (2012), “Dust cloud lightning in extraterrestrial atmospheres”, arXiv:1207.1907 [astro-ph.EP]
- Christiane Helling et al. (2008), “A comparison of chemistry and dust cloud formation in ultracool dwarf model atmospheres”, arXiv:0809.3657 [astro-ph]