Photochemical Modelling of Exoplanetary Atmospheres

Atmosphere of the Earth
Biomarkers are compounds present in atmospheres which indicate the presence of life as we know it on Earth. They include molecules such as oxygen, ozone and nitrous oxide. Our work applies photochemical models to predict the concentrations of such molecules in Earthlike exoplanet environments. We try to understand how the biomarkers would change, e.g. for planets orbiting differing stars with different radiation output, or for planets with differing orbital properties. Our main goal is to develop a predictive tool providing biomarker profiles and theoretical spectra for Earthlike planets in our region of space which can be compared with observations from forthcoming missions.

The Habitable Zone (HZ) is the region around a star where life is favoured, usually defined as where liquid water can exist on a planet`s surface. In our solar system it is generally accepted that Venus lies just inward of the HZ whereas Mars lies just outside it. Clearly it is important to identify processes affecting HZ properties (location, thickness, lifetime, effect on biomarkers etc.). We are applying photochemical models with varying input parameters (e.g. stellar flux) to help address these issues.

We are developing models to deepen our understanding of the atmospheres of our sister planets, Venus and Mars. This work also sheds light on our understanding of the HZ since these planets are close to the inner and outer edges of this region. For example, we have suggested (Grenfell et al. 2010, PSS accepted) a mechanism to help explain the unusual stability of the Venus atmosphere, involving production of CO2 from CO occurring on the surface of the mineral hematite at high temperatures. This process is well-established in the chemical industry but we are the first to our knowledge to apply it to Venus conditions, where it is potentially important for the atmospheric budget. The mechanism may also play an important role for the emerging class of hot Superearth exoplanets.

A current focus is on the chemistry of the Early Earth. This is important because the atmospheres of Earthlike planets in our region in space may well resemble these conditions. Life began on the Earth more than two billion years ago. The atmosphere at the time was very different to today, containing very little oxygen and much more methane, hydrogen and carbon monoxide. An important question under discussion are possible mechanisms which shield the atmosphere from harmful UV radiation hence favour the development of life.

Another focus is the chemistry of planets orbiting M dwarf stars. These stars are important because they are very abundant in our region of space, where future observations will focus. They are cooler and smaller than our sun which means that any Earthlike planets orbiting them have to be close-in, about one fifth the Earth-Sun distance, if they are to support life. The close orbit leads to the planet becoming tidally-locked i.e. it rotates slowly and synchronously with the star. This means the planet has a constant-day face and a constant night-face. If it has an atmosphere then a circulation is formed which transports heat from day to night. The slow rotation also leads to a weakening in the planet's magnetosphere which means that bombardment from cosmic rays is much stronger than on the Earth. We are investigating these effects using a photochemical column model in collaboration with Dr James Kasting and Dr Antigona Segura. The cosmic rays have sufficient energy to break the strong nitrogen molecule which in the presence of some oxygen forms nitrogen monoxide. This compound can rapidly destroy ozone and also influences other biomarker molecules. Our work involves quantifying these effects.

We have recently initiated a modelling effort aimed at developing a photochemistry scheme for Venus. This work addresses questions relevant to the “Venus Express” mission which aims to observe key chemical species, e.g. for the sulphur and chlorine families, in the troposphere and stratosphere of Venus.

HGF report

The atmospheric modelling group contribute to the Helmholtz Alliance “Planetary Evolution and Life” which since its start in 2008, brings together scientists from many diverse fields e.g. planetary science, chemistry, biology, geology, impacts etc., all having the common goal of improving knowledge of habitable conditions in the universe and possible feedbacks with life. 

The atmosphere plays a major role both for the evolution of a terrestrial planet and its potential to sustain life on its surface. The atmosphere, in turn, is influenced by the biosphere, the planetary interior and surface, escape processes, and impacts from space, which makes it a complex, interconnected system. On Earth the atmosphere played a central role in the development and sustenance of life as we know it. Without the atmosphere, daytime temperatures would cause surface water to evaporate, whereas nighttime temperatures on Earth would plummet to far below freezing. Consider the (almost) atmosphere-free Moon, which lies at a similar distance from the Sun as the Earth, but whose surface temperature can vary from about minus 200°C (dark side) to above 100°C (lit side) (Cremers et al., 1971). The atmospheric greenhouse effect warms the Earth’s surface by about 30°C and transports heat from warm to cool regions, thus preventing potentially large temperature differences. However, the existence of an atmosphere supports and favors life in many other ways, too. For bodies without an atmosphere, or with a rather weak one at a pressure below 6.1mbar, the bulk liquid phase of water is not stable, and life as we know it is unimaginable without liquid water (e.g. Schulze-Makuch and Irwin, 2008). An oxygen-rich atmosphere is clearly necessary to maintain many of the complex lifeforms on the present Earth. In addition, our atmosphere plays a crucial role in protecting the surface from harmful radiation – in today’s atmosphere the ozone layer which peaks in the stratosphere at about 30km protects the surface from ultraviolet (UV) radiation which is harmful e.g. to DNA, whereas the early surface life may have been protected from UV by smog ozone or atmospheric haze (Grenfell et al., 2006; Wolf and Toon, 2010). Furthermore, the imprint of Earth's biosphere is clearly recognizable in today's atmospheric composition from its high content of photosynthetic oxygen (21%) and the presence of trace gases like nitrous oxide (laughing gas), produced by microorganisms.


New HGF projects

As part of our ongoing involvement in the Helmholtz Alliance "Planetary Evolution and Life" we are collaborating with scientific expertise over a wide range of subject material, related to the challenging theme of understanding planetary habitability over a wide range of planetary conditions. These include the following cross-topic collaborations:

Early Mars - Geological Features

We are collaborating with geologists to learn more about atmospheric conditions on Early Mars, particularly with regard  to investigating how geological features observed today on Mars' surface could have formed during possible Early Mars habitable scenarios.

Early Earth - Biomass Emissions

We are working together with biologists in order to constrain biogenic surface emissions on the Early Earth hence better constrain our understanding of the development of atmospheric climate and composition for our home planet.

Early Earth and Early Mars - Outgassing Rates

We are collaborating with planetary interior physicists who are developing modelling tools to estimate outgassing rates of e.g. radiatively-active atmospheric species. Our goal here is to include such outgassing rates as a lower boundary condition in our atmospheric coupled climate-chemistry column model for a range of planetary  scenarios e.g. for the Early Earth and Early Mars, in order to better understand how such processes affect atmospheric climate and composition.


Ongoing work and recent results

Stefanie Gebauer is developing a new coupled climate-chemical scheme with variable O2-N2-CO2 surface fluxes to be applied to the Early Earth for three periods, namely before, during and after the Great Oxidation Event.

Mareike Godolt is adapting the ECHAM5 GCM to study the global atmospheric dynamics of earthlike exoplanets orbiting different star classes.

John Lee Grenfell has modelled the atmospheric chemistry of the Early Earth Proterozoic period (Grenfell et al., 2011) and has shown that the biomarker nitrous oxide (N2O) can have a relatively strong spectral signal is such atmospheres due to possible high N2O surface emissions in such atmospheres. Also, this work has shown that there may exist chemical feedbacks between N2O and the biomarker Ozone (O3), which could lead to a minimum level of O3 in such atmospheres, which shields the surface and favours the development of life.

Alexander Hölscher is updating the photochemical comet model with chemical kinetic and photolytic-cross sections with the goal of investigating e.g. production mechanisms for organic species containing 2 and 3 carbon atoms (C2 and C3 respectively) in cometary comae.

Daniel Kitzmann has studied climatic effects of multi-layered clouds for earthlike planets orbiting different stellar spectral classes (Kitzmann et al., 2010). Their paper showed, for example that the greenhouse effect of the high level clouds responded sensitively to the stellar spectrum whereas for the low level cloud the effect was weaker. Clouds have a potentially significant impact on habitability and also on calculated theoretical spectra in such scenarios.

Philip von Paris has performed model studies of the potential habitability of the planet Gliese 581d (von Paris et al. 2010) with an improved coupled radiative-climate column model which can calculate climate conditions over a wide range of atmospheric CO2 abundances and pressure. Their results suggested that assuming e.g. CO2 rich (95% CO2) atmospheres with 5,10 and 20 bar surface pressures would produce a sufficient greenhouse warming to make the planet habitable. This planet is therefore the first candidate for habitability assuming atmospheric compositions and masses which are reasonable compared with terrestrial planets in the Solar System.

Joachim Stock is investigating photochemical cycles on Mars with the Pathway Analysis Program (PAP, Lehmann 2004) (Stock et al. 2011). The PAP tool automatically identified and quantified such cycles based on output from a photochemical model. First results with this unique tool have shown that hydrogen oxide (HOx) cycles are potentially important for stabilising Mars' CO2-rich atmosphere against photolytic destruction by regenerating CO2 via catalytic cycles.  Their paper also identified a new cycle which not only generated CO2 but also produced the atmospheric biomarker O3. A collaboration which builds on this work is underway with Dr Chris Boxe and Prof Yuk Yung (Caltech/JPL).

Heike Rauer has investigated the spectral detectability of (Super)earthlike planets orbiting in the HZ of a range of M-dwarf star spectral classes (Rauer et al. 2011 accepted) for future satellite missions such as the JWST. Their paper calculated theoretical emission and transmission spectra for such planets assuming an earth biomass with 1bar surface pressure and varying gravity (1g and 3g cases).  For the M0-M3 (1g) cases, they found, for example, that the emission spectra showed enhanced absorption for e.g. CH4 due to enhanced CH4 abundances. For the M5-M7 (1g) cases, results suggested that increased stratospheric temperatures led to a weakening in the emission spectral signal.

Barbara Stracke is updating model parameterisations of convection and radiative transfer for hot wet conditions near the inner boundary of the habitable zone.

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