The Scoop: No matter what planet you're on, lightning is about storms, and about ice and rain -- even if it's not necessarily water rain. I spoke with Timothy Dowling of the University of Louisville about some of the strange places lightning is made on other planets, and why lightning matters at all, in a planetary sense...
tedow: I think we are good to go.
larryohanlon: Well thanks again for your willingness to do this, and putting up with my chaotic morning. So I was referred to you because you study lightning on other planets. Is that right?
tedow: I'm a planetary scientist, and my specialty is planetary atmospheres. Lightning is a very specific subject. I develop an atmospheric model, the EPIC model, that simulates the gas giants (and the other planets). We added clouds to the model a couple of years ago (but not lightning!)
larryohanlon: But there is lightning on other planets, right?
tedow: Yes! Jupiter has amazing lightning. We can see it on the dark side. Both Voyager and Galileo saw large lightning flashes on Jupiter, and we've correlated it with convective storm activity in low-pressure regions.
larryohanlon: The dark side -- which we can only see with spacecraft, right, since we generally only see Jupiter's sunny side here in the inner solar system?
tedow: That's right. Even the Hubble Space Telescope only gets a sunny view of the gas giants. Spacecraft also can detect lightning indirectly, by the telltale signatures of radio and magnetic pulses it generates.
larryohanlon: Is it possible to compares Jupiter's lightning to Earth's in any way?
tedow: Yes, we have learned a great deal by intercomparing the lightning on Earth and Jupiter. Jupiter's water clouds are at a higher pressure, 5 bar, compared to 0.5-1 bar on Earth. And gravity is much stronger on Jupiter, which results in rain falling much faster. Add to this that the dry air on Jupiter is very light, just hydrogen and helium, whereas water is much heavier, and you have a recipe for very strong thunderstorms.
larryohanlon: So it's like the old fur rubbing a plastic comb thing we did in school to tear off electrons, only more so?
tedow: Exactly like that. I love rubbing balloons and sticking them on the wall. I think one of the most interesting aspects of all the atmospheres in the solar system is that the "ceiling", meaning the top of the troposphere and start of the stratosphere, is at 100 hPa (equals 100 mbar or 0.1 bar) on all the planets. This sets the top of all thunderstorms. On the other hand, the bottom depends on the temperature of the planet. Jupiter's thunderstorms are much taller than Earth's, because the water is seated where the air is 'room temperature' which is at 5 bar. The result is towering thunderstorms that are 3 times taller than the tallest cumulus towers on Earth.
larryohanlon: So Jupiter's storms are both super charged from those density and gravity differences, but scaled larger too. Wow, I'd think some meteorologists would be into studying this too. Has anything been seen like this on Saturn. I know I've heard it's harder to see what's happening in Saturn's atmosphere.
tedow: Saturn has giant lightning, too, but we haven't seen it directly yet. Being farther from the Sun and therefore colder, its water clouds are even deeper into the planet. But, we have seen the radio waves that are clearly from lightning storms. And, we see the convective clouds. There is a very active "storm alley" in Saturn's southern hemisphere at 35degS latitude.
larryohanlon: Cool! Would it pretty much work the same way on Saturn then? Same mechanism behind the lightning?
tedow: Yes. At least, that is what we surmise! On Earth, lightning requires both the liquid and solid (water and ice) phases, and we assume this holds for the planets as well. And, even though these planets have other types of clouds, namely ammonia and a mixture of ammonia and sulfur, those cloud types do not have nearly the same latent-heat kick that water has. Water is truly a special substance. That is why detecting lightning on the gas giants is so exciting, it implies plenty of water.
larryohanlon: That was my next question -- you beat me to it. Whether methane or another "rainable" compound could make lightning elsewhere.
tedow: In our present-day Solar System, Titan and Earth are the only wet "terrestrial planets", meaning planets where a person could stand (Jupiter and the other gas giants have no solid land!). And, Titan has monsoons and giant convective clouds. But, to my knowledge, the Cassini spacecraft, which has encountered Titan dozens of times now, has yet to detect any lightning signatures on Titan.
larryohanlon: Venus is too dry then. And I guess Mars. Weird that Earth has lightning in common with the gas giants.
tedow: Ah, but it doesn't have to be water. Venus does have lightning! This was suspected but not verified for decades. Only with the currently active Venus Express Mission by the European Space Agency has a clear lightning signature finally been detected on Venus. The magnetic detector on Venus Express has now seen 100 Hz pulses that last a fraction of a second. The best estimate now is that cloud-to-cloud lightning is occurring on Venus a good 25% of the time. What we really need, though, is a photograph!
larryohanlon: Whoa! What causes it on Venus? I' guessing it'd have to be speculation at this point.
tedow: Yes, it isn't clear what the lightning mechanism is on Venus, but it is very intriguing! Venus has an amazingly deep atmosphere, with a bottom pressure of 90 bar and a surface temperature hotter than a pizza oven. And, it has very strong wind shears that seem to change with time. The tannish-yellow color of the clouds on Venus are from sulfuric acid, so there is plenty of liquid drops of an exotic nature.
larryohanlon: So water is not likely the culprit in Venus' case, right? Way more exotic case?
tedow: Water vapor is present in the Venus atmosphere, but it is at an extremely low humidity. Probably we are taking about a slightly different mechanism than on Earth/Jupiter.
larryohanlon: In any case, lightning sounds like a window into a planet's atmosphere, even when all you get is a radio signal of it. Are you able to get data from Jupiter from radio telescopes? I mean, you aren't limited to what spacecraft tell you, are you?
tedow: Yes, Jupiter is a strong emitter at essentially every wavelength useful to astronomers, including being one of the strongest radio sources in the sky. If you could see Jupiter's magnetic field, it would be the size of the full moon in the sky---the biggest object in the solar system (Saturn actually passed through Jupiter's magnetotail during the Voyager encounter!).
tedow: But, when it comes to lightning, by far the best data come from close encounters with spacecraft.
larryohanlon: And I haven't heard of any new spacecraft plans for Jupiter/Saturn. Are there any?
tedow: We have one major spacecraft to Jupiter nearly ready to launch, called the Juno mission. This mission will fly very close to Jupiter in order to detect and map the details of the gravity and magnetic field. This will allow us to reconstruct in some detail the internal structure of Jupiter, including telling us how deep its famous belts and zones (jet streams) go into the planet.
larryohanlon: That sounds major. Jupiter has always been the easiest planet to admire with my telescope, but the most mysterious because it seems we only know it in a skin-deep kind of way. So I'm guessing you and your colleagues have a million questions that Juno could help answer.
tedow: An alien astronomer studying our Solar System would quickly realize that Jupiter contains more planetary mass than all the other planets combined. Still, he/she/it would probably be very intrigued by that one pale-blue dot between Venus and Mars.
2:07 PM Yes, Juno will be a major advance in gas-giant science. Fortunately, we will also get something similar from Saturn, because the "extended extended" mission of Cassini, in orbit around Saturn, has "Juno" type observations.
larryohanlon: Wow. Didn't know that about Cassini. I heard that New Horizon's made a pass on it's way to Pluto.
tedow: Yes, we get a free Jupiter encounter with each and every outer-planet mission, because it is always used to gain a large gravity assist. Each spacecraft has a camera on it that is obsolete! Meaning, it is designed and space-rated years before it is flown. Voyager's cameras were not nearly as good as the cameras on most people's cell phones. But, the view was better!
larryohanlon: New Horizon's will be downright ancient by the time it reaches Pluto then!
tedow: Ah, but the view will be magnificent. Location, location, location!
larryohanlon: I''m sure looking forward to it! Hey I know we're coming on an hour, so should wrap up. Any last thoughts about extraterrestrial lightning that you'd like to leave readers with?
tedow: We've discovered several hundred planets outside our Solar System, with more being discovered every few months. And, we have every reason to expect to find planets with plenty of water on them. One of the critical aspects of lightning is its effect on chemistry. How important is it to the formation of life for there to be lightning on a planet? We might someday actually be able to answer that question.
larryohanlon: Thanks for raising the chemistry matter. Lightning is still considered part of the formula for life, last I heard.
tedow: Gotta get that zap from somewhere! Thanks for the chat, it was great. -Tim
larryohanlon: thank you!