Oh weather; a joy, a pain, the making of a beautiful day or a miserable evening. Our planetary environment is constantly shuffling through a deck of thermodynamic cards and local conditions reflect a small part of the resultant lofting, pouring, steaming, streaming and meandering of atmospheric contents. All planets with atmospheres walk this same basic walk. However, the most spectacular places for weather phenomena are the giant worlds. Of these the hot Jupiter exoplanets hold the current record for wild and crazy weather action, from extremes of heat and cold that take you from frigid to iron-melting across nightside to dayside and possibly supersonic airflow pumped by the input of stellar thermal energy to these tightly orbiting worlds.
Although they inhabit a much less intensely irradiated part of the solar system, our own giant planets are no slouches in the weather department. Jupiter, spinning roughly every 10 hours, has a multitude of latitudinal jet streams - about a dozen. It also hosts an old friend, the Great Red Spot. Although somewhat diminished over the past 10 years it's still a huge tower of cool cloud that could hold 2-3 Earths within its swirls. For at least 300 years it has sat glaring at the universe from the southern Jovian hemisphere. Neptune, and even bland Uranus, harbor dark vortices and light clouds of what may be methane condensates. Saturn is also a hive of atmospheric activity. In fact one of the most startling episodes of Saturnian weather was recently reported from Cassini observations.
Last December was springtime on Saturn. In the midst of its otherwise boring northern hemisphere a white spot began to erupt like an unfortunate tumescent pimple. Growing to span the diameter of the Earth this storm system left an extraordinary trail in the atmosphere, eventually nearly wrapping around the planet a couple of months later. Observations of this incredible activity were recently reported in Nature by Sanchez-Lavega et al. and Fischer et al. along with a great review by Peter Read. Over the past 130 years five comparable events have been recorded on Saturn, though of course this is the first with a flagship planetary mission conveniently in orbit to record the activity. This Great White Spot event seems to consist of a nest of massive thunderstorms, complete with upwelling atmospheric flow and an astonishing frequency of lightning - as many as ten huge electrical discharges per second across thousands of kilometers and with intensities 10,000 times anything seen on Earth.
Just like terrestrial thunderstorms the Saturnian equivalent involves massive updrafts, probably driven by moist (water) laden atmosphere. Unlike on Earth these up-wellings seem to be rising as much as 250 kilometers above the deeper atmosphere. The white color that we see? Fresh particles of ammonia ice, squirted up from the warmer environs below and brightly reflecting sunlight as they waft on the plumes. Exactly what triggered this event is yet to be understood, but it's a clear reminder that during our paltry 400 years of telescopic astronomy we've likely seen only a fraction of what our neighboring planets have to offer in terms of dynamic change.
Far, far further afield the detection of weather patterns like these on exoplanets is in its nascent stages. Rudimentary observations of the effective temperature of gas giant atmospheres can be made by exploiting the on again/off again nature of planetary transit and secondary eclipse (the latter when the star obscures a suitably aligned planet). In a few cases the data has enough fidelity to enable estimation of the texture of atmospheric temperature distribution, for example the measurement of a 'hot spot' on HD 189733 b and changing patterns on HD 80606 b. At an even more fledgling stage is the use of the atmospheric transmission spectrum of a transiting planet. If that makes you glaze over, let me explain. All planets, rocky or gaseous, are effectively 100% opaque out to a certain radius. For an Earth-type world that opacity is simply due to the solid rock of the Earth and its oceanic layer. But above that surface our thin shell of atmosphere is transparent at many wavelengths of light. Similarly, a gas-giant will be opaque up to the thinner outer atmosphere, sometimes up until the gas becomes just tenuous enough to allow some photons through or until we get above any cloud layers - condensed material like water vapor or other exotica like titanium oxide that can block light.
When a planet transits - passing between us and its parent star by chance geometric alignment - any outer transparent atmosphere filters the starlight passing through it. By carefully extracting the spectrum of this light during the planetary transit we can sniff for the signs of atoms and molecules in the atmosphere - the fingerprints of composition. In this way we've spotted molecules like methane and water in the upper zones of several giant exoplanets. But what happens when one of these planets gets extra cloudy, perhaps with a storm system like that on Saturn, lofting great light-blocking droplets or crystals high into the atmosphere? First the overall size of the planet, the net amount of light it blocks, will appear to increase across many wavelengths of light. But we can also use the wavelength-specific blocking of light in an otherwise transparent atmosphere to extract more details. Together with theoretical models of planetary atmospheres we can begin to ask questions about the altitude of these clouds and their general distribution.
This kind of study is very much in its infancy, it takes a lot of very high quality data and great care. Nonetheless the possibilities are intriguing. Planets on short period orbits could be monitored during every transit, and the shifting, changing weather patterns would begin to reveal themselves - both through the varying atmospheric transparency and through spatially resolved maps of temperature. The same basic tools can apply to gas giants and eventually terrestrial-sized worlds. Observing changes on exoplanets will be like diagnosing how an engine works - next to impossible if you have only one snapshot, a lot more feasible if you can watch it running.
All of which brings me to the guilty pleasure of outrageous speculation. Over the years there have been a wide variety of ideas about how cosmic civilizations (should such things exist) might choose to either announce their presence or attempt real communication across the void. A few discussions have considered the role of "signposts", including the construction of a distinct geometrical structure in space - a vast sunshield that would appear as a peculiar non-natural transit signature for anyone monitoring stars in the galaxy. It's a nifty idea, but it's a bit of a one-shot deal, "ooh, aliens, so now what?". So here's a new twist. Let's suppose we wanted to exploit the same basic idea (transits of our Sun as seen by someone else) but also wanted to send a real message. Why not use our planet itself? Without getting into pesky details of practicality it seems that it would be possible to encode a message within our own planetary atmosphere that exploits the same kind of opacity blocking signatures that cloud condensates produce in exoplanetary atmospheres. Of course, by extension one can speculate that some bunch of ingenious aliens might do the same thing and that we could search for this.
How would it work? Picture an old-fashioned music box. These often use a metal cylinder with pins sticking out. As the cylinder rotates past a set of musical keys, or tuning forks, the pins pluck out the notes of a ditty. The beauty of this is that it doesn't really matter where the stop or start is, the cylinder just keeps spinning and the notes keep on playing.
Now go to a planetary scale. Perhaps artificially engineered weather systems (admittedly on a huge scale) could loft opaque condensates high up at a series of latitudes and longitudes like the pins in the music box cylinder. The natural planetary spin carries these notes into and out of sight - perhaps only one note seen during a single transit by some distant observer, but sequenced to be clearly artificial, clearly structured and non-random. Every new transit bringing a new signature - but ultimately repeated, a big "We are here, how's your weather?" Or maybe instead of clouds, giant arrays of atmospheric blimps, doubling up as carriers of solar panels and so necessarily spread across longitude. Terrible eyesores but what a grand enterprise, messaging the cosmos.
Obviously this is a wee bit fantastical*, but it does raise an important question. As we get better and better at extracting data from exoplanets, particularly those more similar to the Earth, should we at least be on the lookout for the deliberate or inadvertent signs of technological inhabitants? It is not inconceivable that curious patterns and repeated structures might represent more than just another stormy season, they could be smoke signals across the interstellar valley.
(* you may or may not consider this an understatement)