John Spencer is a staff scientist at Southwest Research Institute's Department of Space Studies in Boulder, Colorado and is a member of the New Horizons and Cassini science teams. His research interests include the moons of the outer planets, particularly the Galilean satellites of Jupiter and the icy moons of Saturn. His favorite moons are the active ones, Io and Enceladus. When he's not staring at a computer screen, he enjoys Colorado with his wife Jane and their dog Maggie, and loves tooling around Boulder on his bicycle.
Ah, Hawaii! The palm trees, the gentle breezes, the hula dancing, the spam musubi, the 1 mm precipitable H2O... The Aloha State means different things to different people, and to me, having lived there for four memorable years as a postdoc 20 years ago, it means all of the above. What's relevant here, though, is the last item. I'm nearing the end of an observing run at the NASA Infrared Telescope Facility at Mauna Kea Observatory, at 13,500 foot elevation on the Big Island, where the breezes are rarely gentle but the transparency of the atmosphere to infrared light is spectacular, due to the thin, cold, air which holds very little moisture. We need that transparency for our current project, because we're looking at the atmosphere of Jupiter's volcanic moon Io, which we can only observe at infrared wavelengths that are largely blocked by our own atmosphere from most places on this planet.
Io's atmosphere is one of the most peculiar in the solar system. By some standards it's barely an atmosphere at all- just one-billionth of Earth's atmospheric pressure-but for those of us who love it, it's still a real atmosphere. The wind blows, frost evaporates and condenses, volcanos belch their sulfurous exhalations into it. And it's way more of a proper atmosphere than those of the other Galilean satellites, or our own Moon, where a few molecules bounce lazily across the surface and rarely encounter each other during their flight. Io's atmospheric composition is unusual (largely sulfur dioxide, SO2), but what's most peculiar about it is that it's the only atmosphere in the solar system (so far as we know) that is patchy. Where there's a source of gas, a patch of sun-warmed SO2 frost or a volcano, there's an atmosphere, but in places that are too cold and too far from volcanic plumes, like the poles much of the time, there's barely an atmosphere at all. You would expect winds to blow towards these holes in the atmosphere to fill them up (in seventeenth-century parlance, nature abhors a vacuum), and indeed the winds do blow, at supersonic speeds. But the atmosphere attempting to fill the holes gets frozen in its tracks, condensing onto the surface before it can equalize the pressure.
One of the things we've been puzzling over for decades is, what holds the atmosphere up? Are we looking at gas directly blown out of the volcanos, or does the volcanic gas first condense onto the surface and then evaporate to supply the atmosphere as the frost is warmed by the sun? It's an important question. For instance, if the volcanos support the atmosphere, the atmosphere will be much the same day or night, while a frost-supported atmosphere is likely to freeze out onto the surface at night as the frost cools. So far, we've amassed a fair amount of evidence in favor of both options, so we're still confused.
Unfortunately, we don't yet have a way to directly observe the atmosphere at night. We tried to measure the nighttime atmosphere with the New Horizons spacecraft in 2007, but were foiled when Jupiter's radiation temporarily shut down the ultraviolet instrument that we were planning to use. So we need to use indirect techniques, and that's what we're doing on Mauna Kea at the moment. We are looking at the absorption of the infrared (heat) radiation emitted by Io and its volcanoes by SO2 molecules in the overlying atmosphere- the amount of the absorption gives us a measure of the density of the atmosphere. We need a specialized instrument for this job, and our weapon of choice is TEXES, the Texas Echelon Cross Echelle Spectrometer, built and operated by John Lacy of the University of Texas and his team.
We've been looking at Io with TEXES for eight years now. We consistently see a lot more SO2 gas on the anti-Jupiter side (the side of Io that always faces away from Jupiter) than on the Jupiter-facing side, which is another confirmation that the atmosphere is indeed patchy. Most of the long-lived volcanic plumes are on the anti-Jupiter side, but so is most of the SO2 frost, so this fact alone doesn't tell us what holds up the atmosphere. Instead, what gives us a handle on the problem is that Io's distance from the sun has been changing significantly during those eight years, due to Jupiter's eccentric orbit around the sun. In 2005 Io was at its maximum distance of 5.45 astronomical units (AU) from the sun (an AU is the average Earth-Sun distance), but now the distance is down to 5.06 AU. Any frost that was sun-warmed to 118 Kelvin in 2005, for instance, should now be heated to a toasty 122.4 Kelvin by the closer sun. Not a big temperature change, but that slightly warmer frost will evaporate nearly four times faster than it did during the cold days of 2005. If the atmosphere was entirely supported by frost, and life was simple, we should be seeing four times as much gas as we did back in 2005.
We're not seeing such a big change. In fact, for most of the past eight years the atmospheric density at any particular place on Io seems to have held pretty steady, and has ignored the changing distance from the sun. But this time we're looking at Io when it's closer to the sun than at any time since we started the program, so I've been eagerly anticipating our observations. And in fact we seem to be seeing a change, finally! The absorption bands are stronger than we've ever seen before, not by a factor of four, but by up to 50%. Maybe the atmosphere was supported by the volcanos during the cold season far from the sun, but now the warming frost is starting to contribute. We'll know even more next year, if we are granted telescope time, because Io will then be a mere 4.97 AU from the sun, and any frost-warming effect will be even more obvious.
You are maybe imagining me huddled in a parka on the summit of Mauna Kea, or perhaps at the end of the observing run, sitting by a Hawaiian beach with a Mai Tai in hand. But neither is the case- I'm home in Boulder, and have been participating in the observing run remotely while the TEXES team (John Lacy, Tommy Greathouse, Gina Buffalo, and Matt Richter) endure the thin air, cold, and long nights at the summit. Sometimes from home, sometimes from my office, I make suggestions by phone and periodically pull the latest spectra over the Internet to see how we're doing. Due to the time difference, I don't even have to get up too early- my shift starts at 2am Hawaii time, which is 6am here, so it's already light when I first check in.
Remote observing like this is becoming more and more common, as computers become more sophisticated and the Internet gets faster. Often the remote participation is much more extensive, allowing us to directly operate the telescope and its instruments from thousands of miles away. This is good for our research budgets, good for our carbon footprints, and good for our family lives. But I do miss the romance of the long drive to the desolate mountain top, the Southern Cross hanging low over Mauna Loa, the orange glow of the Kilauea volcano lavas reminding me of the connections between Io and our own volcanic world, the whir and rumble of the great telescope swinging into position. And yes, I miss the beaches, palm trees, and hula dances too. Not the spam musubi, though. Plug of the Day
If you're interested in the history of planetary exploration, space historian David Portree's blog Robot Explorers makes for fascinating reading. He writes about all the planetary missions that never made if off the drawing board, or at least never made it off planet Earth. It's a reminder that this exploration business is a difficult one, and the path to the planets is littered with false starts and dreams deferred. The many successes are all the more remarkable when seen in the context of the missions that didn't make it.