Petrol engine turns astro laboratory
(appeared in April 2018)

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“To see a World in a Grain of Sand...” was a flight of William Blake’s poetic fancy. Closer to the ground is the work of a group of scientists who leverage existing knowledge of the internal combustion engine, which powers motor cars, to make sense of the atmosphere of giant planets in close proximity of distant suns., says S.Ananthanarayanan.

O. Venot, E. Hébrard, M. Agúndez, M. Dobrijevic, F. Selsis, F. Hersant, N. Iro and R. Bounaceur, astronomers and applied combustion experts from the universities of Bordeaux and Lorraine in France, Keele University in UK, the CNRS lab in Floirac, France and the Observatory of Paris, describe in Astronomy and Astrophysics, the journal of the European Southern Observatory, the value of “importing well-tried methods from any other field whenever they exist,” to solve problems thrown up in research.

A large number of planets in solar systems that exist deep in space have been discovered in the last few decades. While the quest has been to find ‘earth-like’ planets, or planets with temperature that could support liquid water and hence life, many planets with different characteristics have been discovered too. One class of such planets is that of Hot Jupiters or planets the size of Jupiter, or over ten times the size of the earth and orbiting very close to the mother star. We have good methods to study the fiercely hot atmosphere of the sun and stars and we have some understanding of their interiors. But it is recently that we have started grappling with atmospheres that are not nearly as hot, in these planets that are nearly 50 times closer to their suns than the earth.

The temperature of the sun goes from some 5000°C at the surface to millions of degrees in the core. A feature of gases at such temperatures is that the atoms in molecules of the gases separate and lose their outer electrons, to behave as fast-moving, charged objects. Models of such objects in behaving as gases have been developed and features of such atmospheres, like the sun’s, where the outermost layer seems to be hotter than layers that lie below, have been explained. The atmosphere of Hot Jupiters, in contrast, are comparatively cooler, at 1000°C to 3000°C, and the behavior is not quite like a gas of charged particles. Nor do the usual laws of motion that work when applied to gases lead to satisfactory results. When pressures and speeds of movement get exceedingly high, as in Hot Jupiters, the approximations, like treating an atom as an object with zero dimensions, break down and chemical reactions that cannot take place in ordinary conditions become possible.

As Hot Jupiters are very close to their suns, which are at great distances from the earth, any reflected light from the planets is swamped by the glare of the sun. There are hence no methods of directly seeing what goes on in such atmospheres. These planets themselves have been detected only thanks to a wobble that such massive satellites impart to the mother star, or by the dip in the intensity of light from the star when the planet passes in front during its orbit. Any information about the atmosphere comes to us by analysing the starlight that passes through atmosphere of the planet when the planet goes past the star. Variations in the dim reflected light during phases of a large planet in orbit also indicate what gases the atmosphere contains.

The high pressures and temperature, the intense radiation from the mother star and the winds and currents, however, bring about rapid chemical changes in the composition of the atmosphere, the paper in the journal says. Understanding the course of changes calls for models that mimic the complex and rapid processes taking place. Creating such a model, or a sequence of causes and effects that could account for the information that is available, or could suggest features that should be looked for, would need two things, the paper says. One is a list of the different players and their interactions in the process. And the other is to have data of how the ensemble of reactants would behave in time. The available literature, however, is not good enough to plan, or experimental facilities sufficient to test possible models, the paper says.

Despite the lack of dedicated facilities to study conditions in Hot Jupiters, members of the team had noted that very similar conditions exist within the cylinders of motor car engines. The temperatures in internal combustion engines rise to some 2500°C and pressure rises to 1,500 psi, or over a hundred times the atmospheric pressure. In recent times, the drive to build more environment-friendly and less polluting petrol engines has led to wide research into the behavior of gases and the chemical reactions that are encouraged or inhibited at this range of temperatures and pressures.

As the fuels that burn in automobile engines contain hundreds of forms of hydrocarbon molecules the focus has been on finding models is that simulate the main combustion parameters , like when the fuel starts to burn, how the flame spreads, distribution of heat generation. These are the features that help design of engines or burners, to estimate the fuel consumption, and to visualize how some of the main pollutants (carbon monoxide, nitrogen oxides, unburned hydrocarbons and particulate matter) are formed. “Most of these models were developed for industrial applications and have been validated in a range of temperatures and pressures,” the paper says.

The paper notes that the fuels that burn in internal combustion engines consist of carbon, hydrogen, oxygen and nitrogen, which are also the main constituents of the molecules and molecular groups found in Hot Jupiter atmospheres. The existing models created for IC engines hence find ready extension to picture the dynamics of Hot Jupiter atmospheres.

Current observations of very hot planets are still tentative and can be interpreted in different ways. But with facilities that are under development, data is expected to become more elaborate and chemical modeling will become an important part of the analysis, the paper says. Even this bare access is now possible only with the very hot class of exoplanets. With better instruments, the atmosphere of even planets would come within reach. The nature of molecules present and the dynamics of the atmosphere would be different. Here, again, the validated models derived from automobile engine studies are the ones that would help model those distant atmospheres, the paper says.

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