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Planetary News: Cassini-Huygens (2008)When Titan's Winds Blow, Mountains Move:The Moon's Entire Crust May Slide Over Subsurface OceanBy Emily LakdawallaMarch 20, 2008
In a paper to be published in tomorrow's issue of the journal Science, members of the Cassini RADAR team announce evidence that Saturn's moon Titan hides an ocean beneath its surface. The presence of a global ocean means that the entire crust of Titan is "decoupled" from the moon's interior. Outside forces acting on Titan's crust, most importantly the force of the global circulation of the atmosphere, can apparently slide the entire crust around on Titan's ocean, so that the crust does not rotate at exactly the same rate that the moon orbits Saturn. These conclusions are based upon the RADAR instrument's high-resolution Synthetic Aperture Radar (SAR) images of Titan's surface. Since the acquisition of the first SAR swath across Titan in October, 2005, there have been 19 regions on Titan that have been imaged more than once by the RADAR instrument. When the RADAR team assumed that Titan's rotation was synchronous -- that is, that it rotates precisely once with each orbit around Saturn -- features seen during one flyby were observably offset when imaged during another flyby, by as much as 30 kilometers (19 miles). "We had an erroneous spin model," explains Bryan Stiles, one of the coauthors on the Science paper. "We had some assumptions built in: that the spin rate was exactly at the synchronous rate, and that the pole of Titan was perpendicular to the orbit of Titan. When you use those assumptions, locations of features appear to move. What that tells us is that Titan was spinning differently than what we had initially guessed. It doesn't always spin at exactly the synchronous rate, and the pole is not perpendicular, but is off by 0.3 degrees." The measured offset of the surface features, relative to the prediction for synchronous rotation, means that, over the time period measured in the Cassini data (October 2005 to May 2007), Titan's surface was shifting by 0.36 degrees per year. For there to be this rapid of a shift in the position of Titan's surface requires the surface to be able to move freely about the rest of the moon, sliding around atop a liquid interior ocean. The presence of a liquid ocean within Titan would be no surprise to Titan scientists; geophysical models for the interior of Titan (and other large icy bodies in the solar system) suggest it's highly likely that many of these worlds contain liquid water layers. Magnetometer measurements by Galileo strongly suggest that Jupiter's moons Europa, Ganymede, and possibly Callisto have interior oceans. And the presence of abundant nitrogen in Titan's atmosphere suggests that it may have ammonia (which contains nitrogen atoms) in its interior, which would lower the melting temperature of water to make an ocean even more likely there. However, this RADAR data represents the first empirical evidence for the existence of that ocean.
In fact, the observation of Titan's asynchronous rotation is the first of its kind. Similar observations were attempted for Europa by members of the Galileo science team, including Greg Hoppa and coworkers, who compared Galileo images of features along Europa's terminator (night-dark boundary) in Voyager images to later ones with nearly the same geometry from Galileo. They were unable to report any observable shift in surface features, which implies that Europa's surface had not shifted by more than a few tenths of a degree in the 17 years that separated the two observations. The measurement was limited by the relatively low resolution of the Voyager images; higher-resolution data from a future mission could allow Europa's nonsychronous rotation to be measured. What is making Titan's surface move? It's the wind, says Ralph Lorenz, first author on the Science paper. Lorenz explained that a pair of atmospheric scientists, Tetsuya Tokano and Fritz Neubauer, had predicted at an American Geophysical Union meeting in 2005 that if Titan had an ocean, the atmosphere could be capable of shoving surface landmarks around by distances of up to 100 kilometers compared with positions predicted from the assumption of a constant rotation rate. "I thought, 'Hundreds of kilometers? You've got to be kidding,'" Lorenz remembers. Since each individual RADAR image of Titan is a long "noodle" that is only approximately 200 kilometers wide at its center, employing the wrong assumptions about Titan's rotation would mean that features on Titan would be very noticeably offset in RADAR images of the same locations taken on different dates, which is exactly what the RADAR team found. How could the atmosphere push around the entire surface of a moon? It's not unheard of; in fact, "The same thing occurs on Earth," Lorenz points out. "The Earth length-of-day changes by about one millisecond over the course of a year because of winds speeding up and slowing down." That's a tiny amount, but Earth is much more rigid than Titan (it does have an internal molten layer, the outer core, but this is at great depth, so the winds on Earth have to push around all of Earth's crust and all of its mantle, a comparatively huge mass), and Earth's atmosphere is far less dense than Titan's. Titan's relatively near-surface ocean means that a comparatively small fraction of its mass is acted upon by the winds. On Titan, the wind blows in predominantly one direction at any given time, always generally east-west, though whether the wind is easterly or westerly at the surface depends upon the season. The RADAR team's observations sampled less than two Earth years, or only about five percent of a Saturn year, so the winds were probably blowing constantly in one direction throughout the period of study. Lorenz explained that the wind acts upon the surface in two ways. One is simply drag, or friction, between the moving air and the surface. The other is that there may be so-called "mountain torques." "If you have a high-pressure system on one side of a mountain range and a low-pressure system on the other, then there's a net push on the mountain range, even if there's no flow of the atmosphere over the mountains. That certainly is important for Earth, particularly with north-south trending ranges like the Urals and the Andes. But we don't know enough about Titan's topography to know whether those are significant." If it's true that Titan's winds shift direction seasonally, then the RADAR team should see the observed asynchronous rotation reverse with time -- if, that is, Cassini observes the Saturn system for long enough. Its prime mission ends in June of this year, and an extension will take its operations through Titan's (and Saturn's) equinox in August 2009. It's possible that may be long enough to detect another change in rotation rate relative to the synchronous assumption, but of course the longer Cassini spends observing Titan, the more likely the team will be able to pick up such long-term changes. Lorenz also stated that the best way to measure the asynchronous rotation of the surface would be with a future mission employing a long-lived lander, whose radio beacon would provide very exact measurements of the day-to-day drift of Titan's crust.
The effects of the atmosphere on the surface are not the whole story, Lorenz says. "There are probably several things going on; this is likely the first of many papers about Titan's rotation." And he also points out that, even if Titan scientists can chalk up the apparent asynchronous rotation of Titan's surface to the atmosphere, they can't currently explain why Titan's atmosphere blows west-to-east near the surface instead of the other direction. "There's a fundamental difficulty with Titan global circulation models right now -- all of them -- which is that they predict that the predominant winds at low latitudes near the surface would be easterly, from east to west. Yet all the sand dunes point in exactly the opposite direction. There's something we do not understand about Titan's circulation." It's a difficult problem, but one that Lorenz has found "fun," he says, especially because of the interdisciplinary nature of the present study. "Measuring the rotation rate plays into geophysics, but only with meteorology to push things around. People who study global circulation models are not generally into studying the deep interiors." On Titan, it seems, you have to consider the entire world, from the top of its atmosphere through its hydrocarbon-rich surface to an interior ocean, just to explain how long its day is. It's a world as complex and dynamic as Earth, and likely will continue to surprise Cassini's science team for as long as the mission lasts. |
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