Thursday 3 March 2011

Ciel & Espace

A reporter for French popular astronomy magazine "Ciel & Espace" recently contacted me about the relationships between Jupiter and the exploding numbers of exoplanets being discovered.  I thought I’d reproduce my answers here, to add some technical perspective to some of the recent news items about Jupiter!

Has our knowledge of Jupiter’s atmosphere improved recently ? Or is it because it is easily accessed, and under constant scrutiny of astronomers, including amateurs ?

The weather layer of Jupiter's atmosphere (specifically the upper troposphere above the top-most cloud decks) is the only atmospheric region easily accessible from Earth.  Amateur observers can track the evolution of dynamic phenomena (impact scars, disappearing and reviving belts, storms and plumes) using a backyard telescope, and professional observers can use reflected sunlight in the near-infrared and thermal emission in the mid-infrared to diagnose the temperature, composition and cloud structure.  But in all cases, we struggle to get any deeper than the upper troposphere, and must infer the details of 'deep' processes from their visible manifestations in the cloud tops.  The rise of the internet has allowed easy sharing of digital images by amateur observers, so that the professional community has easy access to a gigantic volume of data to track the dynamics of Jupiter's atmosphere.  Their hard work and dedication gives us a near complete temporal coverage of the evolving atmospheres of Jupiter and Saturn.

What makes some features (vortices) particularly long-lasting (eg the Great Red Spot), vs others (small white ovals...) ?

Once a storm like the GRS is established, it continues to feed off injections of energy and momentum from absorption of smaller vortices and eddies that it encounters as it drifts in longitude.  The size and momentum of an established storm, and the absence of viscous frictional forces which would dissipate their energy, allows them to be long lived.  Small white ovals have a good chance of encountering other ovals of similar magnitude - when they interact, they can either merge (as with the formation of Oval BA) or sometime shear themselves apart.  A continuing mystery about the spots is why some are red and some are white, and my favourite hypothesis is that the larger storms also penetrate deep below the clouds, with access to some chemical that reddens when exposed to UV irradiation.  But that's only a theory, and we've never been able to identify the chromophore responsible for the red colours.

The SEB recently "reappeared" after disappearing for a while. I read that this has happened many times before. Is it known what drives such cyclical phenomena ?

The fade of the SEB was apparently caused by the formation of an upper tropospheric 'haze' or 'coating' of the existing red particles, possibly due to a fresh supply of ammonia vapour which condensed to form a new, whiter haze layer.  When a powerful convective plume punched through the white cloud on November 9th, it triggered a chain of upwelling and subsiding motion.  The sinking air is warmed, causing sublimation of the 'whiter' ices and revealing the original darker colours of the belt.  The fade may have been caused when the complex atmospheric flows surrounding the GRS were altered somehow (and we don't know why).  The usual turbulence seen northwest of the GRS vanished in 2009, so the atmosphere had to transfer energy in some other way, possibly by large scale upwelling that caused the 'clouding over' or fade.  The revival signals a return to the usual turbulent activity within the SEB.  

The SEB life cycle is reasonably predictable, but we don't know when it will start.  It's almost like trying to predict earthquakes.  Until we can peer beneath the clouds to see the deep workings of the troposphere surrounding the GRS (either with observations on new missions, or with sophisticated models), we may not know why a fade and revival cycle is triggered.

How important is the contribution or amateur astronomers ? I'm thinking of reporting possible impacts, missing belts... Are there other ways ?

The most important contribution is their continuing, reliable, consistent coverage of atmospheric dynamics.  Time on giant observatories is costly, and space missions are short lived and often starved for time (think of the competition between the different Cassini instruments at Saturn).  Amateur observers provide a day to day record of the atmosphere that we simply couldn't get elsewhere.  In the coming years, with more sophisticated webcams, digital cameras and image processing, amateurs will provide crucial supporting observations to missions in flight.  And if they happen to spot an impact or two while they're at it, so much the better!

It seems to be quite common now for researchers who study giant planets - such as you - to be interested in exoplanets as well. Do you have some examples of how the knowledge of Jupiter's atmosphere (composition, dynamics...) contributed to the study of exo-Jupiters' atmospheres ?

As we began to study planets in other solar systems, we used our own collection of giant planets as the template, or archetype, for what a gas giant should look like.  With no way to directly visualise these exoplanets as anything more than specks of light, it's logical to think of them as alien versions of our own planetary system.  Models of their chemistry, dynamics, and the way that light interacts with their atmospheres are all based on those used in solar system studies.  However, we've had a to adapt them to the extreme conditions of exoplanets (high stellar irradiance, hot temperatures, etc.).  It's hard to imagine what would happen if we took our four gas giants and moved them in closer than Mercury, but it's a sure bet that they'd look very different!  If we can understand the huge diversity between Jupiter, Saturn, Uranus and Neptune, then we might have some chance at predicting what a planet will look like (circulation, composition, etc.) under the extreme conditions of exoplanets.

On the other hand, did some discoveries about exoplanets lead to rethink models of Jupiter ?

Certainly!  When the first exoplanets were discovered with orbital periods on the order of days, no one believed them!  Our understanding of planet formation couldn't fully cope with these new discoveries, because they weren't designed to.  The idea that planets migrated after their formation is now commonplace, given that so many giant planets are found close to their parent stars.  If the explosion of exoplanet systems has taught us anything, its that the range of system configurations is enormous.  The ultimate model for the origins of our solar system (and hence the bulk compositions of the giant planets) must be consistent with the possibilities raised by exoplanetary systems.