Mineral clouds and the conditions for lightning in atmospheres of extra-solar, planetary objects

Seminar Date: 
02 May 2013 - 14:00 to 15:00
Christiane Helling
University of St Andrews, Scotland

The pioneering paper by Jones & Tsuji (1997) on 'Spectral Evidence for
Dust in Late-Type M Dwarfs' started a new research area of cloud
formation in substellar objects. Very low mass objects (M-dwarfs,
Brown Dwarfs, planets) are cool enough that mineral dust, and subsequently
clouds, can form already inside the atmosphere. These clouds form from
a chemically rich ambient gas made of molecules like CO, H2O but also
TiO2, MgO, SiO2, and iron oxides. These cloud particles, which are
made of minerals rather than of water, have a strong impact on the
atmosphere's structure due to the particle's large absorption cross
section, the depletion of the gas phase due to condensation, and
gravitational settling.

This talk will present results from a phase-non-equilibrium model
which allows us to model the formation of heterogeneous dust grains
and the formation of clouds by seed formation, growth/evaporation,
gravitational settling, element depletion and convective mixing. Our
model predicts, for example, grain size distributions and material
composition of the cloud particles. The coupling of our cloud
formation model (DRIFT) with a radiative transfer code (PHOENIX)
resulted in a grid of DRIFT-PHOENIX model atmospheres. The talk will
discuss a comparison of model atmospheres with different approached to
cloud, i.e. dust, modelling.

Clouds on Earth are easily associated with lightning. However, clouds
in substellar objects are not made of water and it is therefore
necessary to investigate if charge and discharge processes similar to
Earth can be expected.  The talk will finish with presenting first
ideas on ionisation processes for ultra-cool atmospheres that have
chemical compositions different to what we know from Earth.