If studying the Earth's atmosphere and all the processes which control it sounds very complicated, imagine doing this for all the planets of the Solar system. Then imagine that, using mathematical formulae, you could build climate models which reproduce the different atmospheric phenomena that occur, for example on Venus or on Titan (the largest moon of Saturn). All this is possible, and these models can even be used to improve our understanding of the Earth's climate and even, in the last analysis, to find exoplanets with the characteristics necessary to support life. This is the research field of Sebastien Lebonnois, a researcher at the Laboratory of Dynamical Meteorology in the National Centre for Scientific Research (CNRS), in France, who is an expert in the atmospheres of Venus and Titan. He is the first lecturer at the XXVIII Canary Island Winter School of Astrophysics, organized by the Instituto de Astrofísica de Canarias, (IAC), which this year is dealing with the exploration of the Solar System.

By Elena Mora (IAC)

“Studying other atmospheres help us verify that the basic laws we use to understand the climate of the Earth are valid when extrapolated to different situations.”

“We consider Venus as a sister planet to the Earth. Almost the same size and similar distance to the Sun. However, the atmosphere is so different.”

“Though extremely cold, the surface of Titan exhibits a cycle of methane that mimics the hydrological cycle on Earth, with rain, rivers, seas and thunderstorms.”

Question: Are there many differences between planetary atmospheres in the Solar System? Why?

Answer: Every atmosphere is unique, but there are also common characteristics. From the primordial material originally present in the solar nebula, each atmosphere evolved along its own way, depending on the size of the planet, its position in the Solar System, and hazards, such as the event that created our Moon. Though their composition, thermal and dynamical structures, are different, the physical processes controlling their behavior are the same.

Q: What can we learn about the study of planetary atmospheres?

A: The goal is to understand the processes that explain their properties. Some are very basic, and it is quite easy to get the first approximation right. But when we want to master the atmosphere we live in, and decipher its future, the devil is in the details.

Studying other atmospheres help us verify that the basic laws we use to understand the climate of the Earth are valid when extrapolated to different situations. We can learn a lot about our own environment through observation and modeling of this panel of different atmospheres.

With additional confidence in our tools, we can also learn more about the history of these atmospheres, and the conditions that lead to the appearance of life on Earth. We can search for planets elsewhere, around other stars, and explore whether they could also present favorable conditions for life.

Q: Global climate models are able to simulate, based on universal physical equations, real climate models for any terrestrial planet. Specifically, what are these models?

A: These models are built from bricks representing mathematical formulations of laws of physics applicable to these systems. The core solves the equations of fluid dynamics, in the spherical geometry characteristic of planetary atmospheres. Then the main driver is the radiative transfer, i.e. how the solar radiation coming from the Sun, and the infrared radiation emitted by the atmospheric system, are absorbed and emitted by the atmosphere, and how they drive the circulation.

Additional bricks include how the surface layers will interact with the atmosphere, processes that may happen at small scale, such as turbulence or convection, photochemical equations that will tell you how the composition of the atmosphere evolves, laws of microphysics to model the build-up and evolution of clouds, and how they will interact with the atmosphere.

All these laws couple the variables that are characteristics of the atmosphere. It is necessary to define a grid on which these coupled laws may be mathematically solved, in order to follow the evolution of the atmospheric variables with time.

Q: What scientific research can be done with these models? Is it one of your objectives to study exoplanets climate to find life on them?

A: When we observe the planetary atmospheres, we can get access to some characteristics, such as the distributions of temperature, winds, clouds, etc over time. The climate models help to reproduce these observations, then interpret them: what physical mechanism can explain the vertical profile of temperature, or the variability of the winds? Many processes are coupled in the atmospheric system, so the models help to decipher these couplings, and explain why, for example, the atmospheric circulation in Titan's stratosphere is so peculiar, or why we observe past glaciers on the slopes of Martian volcanoes.

These climate models have been adapted now to all the planetary atmospheres of the Solar System, from the giant planets to the tenuous atmosphere of Pluto. Based on the success of these models for the atmospheres we can study close to our home, we can extrapolate to the potential atmospheres detected around exoplanets. Finding places suitable for life is of course an objective, a first step towards detecting possible life elsewhere in the neighborhood.

Q: Your work focuses on the study of the atmospheres of Venus and Titan. What was the decisive factor for awaken interest in these bodies in the Solar System?

A: Venus has always triggered a lot of interest. It is a very bright object in our evening or morning sky, observing it with a telescope shows that it has phases, like our Moon, and variable apparent diameter. We have also seen it pass before our Sun (the famous transits), which lead to first proof of the presence of an atmosphere, in 1761. With additional observations, we now consider Venus as a sister planet to the Earth. Almost the same size, similar distance to the Sun... But the atmosphere is so different! Understanding the reason why Venus evolved so differently from Earth is a fundamental question that drives continuous interest.

Titan's atmosphere was mainly investigated after the Voyager missions, in the 1980's. But it is the many discoveries of the Cassini-Huygens mission that really triggered interest: though extremely cold, the surface of Titan exhibits a cycle of methane that mimics the hydrological cycle on Earth, with rain, rivers, seas, thunderstorms. This is fascinating!

Q: It is known that the planet Venus rotates slowly on itself –one day on this planet is 243 Earth days- but its atmosphere rotates 60 times faster. Is there a physical model able to reproduce the super rotation of Venus’ atmosphere that helps to understand these dynamics? Why?

A: This has been a very difficult challenge for a long time, and a severe test to climate models. It seems that the processes that can explain this phenomenon are quite subtle to model, and small uncertainties or approximations prevented the development of this superrotation in many models. However, with the progress of numerical simulations, the current models of Venus (and also Titan, which does exhibit similar superrotation) can simulate the extremely high winds present in the atmospheric circulation. It seems now validated that the superrotation results from a quite sensitive balance between the transport of angular momentum by the mean meridional circulation (driven by the solar insolation) and the one done by the waves that arise from instabilities in the circulation, and from thermal tides.

Q: Titan is the largest natural satellite of Saturn and, in addition, the only one with atmosphere. Is it this atmosphere similar to the Earth? Why scientists decided to classify as “seas” some parts of its surface?

A: The atmosphere of Titan is composed mainly of nitrogen, like the atmosphere of Earth. The surface pressure is almost identical to Earth's. However, the surface temperature is very cold, due to the far distance of Saturn to the Sun. On Titan, water ice is the surface rocks. And it does not have any oxygen.

The second constituent is methane, and in the conditions at the surface of Titan, it can be liquid. Therefore, it can rain, form liquid seas in the Polar Regions (where precipitations are favored by the atmospheric circulation), evaporate, condense as clouds in the troposphere...

It has a cycle that is very similar to the cycle of water on Earth. These large bodies of liquid methane that were observed, especially in the Northern polar region of Titan are indeed very similar to our water seas (much more in fact that the so called “seas” on the Moon, that are absolutely not liquid-covered areas !). Titan is the only place in the Solar System other than Earth to possess such seas.

Some similarities are possible between the atmospheric composition of Titan's current atmosphere, and the early atmosphere of Earth. However, the temperature conditions have always been extremely different.