From Carbon Planets to the Lakes of Titan
FAJARDO, Puerto Rico—Part of the fun of coming to astronomy conferences is seeing the nonplussed reaction of local people in restaurants and bars--it's not often you get a crowd of people sweeping through your town to talk about Titanian lakes, lunar soil, and the prospects for life beyond the solar system. People get over it soon, though, and start peppering you with questions they've clearly been storing up for a while. That's already been the pattern at this year's Division for Planetary Sciences meeting, being held this week in Fajardo, Puerto Rico, hosted by Arecibo Observatory. Here's a flavor of some of the topics that have been discussed already.
On Monday, Jade Bond of the University of Arizona was among those trying to figure out what Earthlike planets elsewhere in the galaxy might be like. No one has yet seen such a world; the closest they've come so far is a body about five times the mass of Earth orbiting inhospitably close to its star. So researchers have to make guesses informed on theory and on trends observed for larger planets. One pitfall is that the theory of planet formation commonly focuses on dynamics-- how planetary building blocks stick together to form progressively larger objects--to the neglect of chemistry: what bodies actually consist of. Bond and her colleagues have been trying to tie the two together. They find that although our Earth is silicon-rich, other Earths may instead be carbon-rich, like giant versions of carbon-rich asteroids and comets. Rather than silicate rocks, they'd have carbonaceous ones. In fact, Bond suggests that such Earths may be the majority: most stars with planets contain proportionately more carbon than our sun does. Yet scientists have barely begun to think about how their geology would differ. "No one has looked at it," Bond says.
Later in the afternoon, David Choi of the University of Arizona talked about winds on the planet Jupiter. The jovian atmosphere has some of the most visually spectacular examples of turbulence in the solar system -- looking like cream swirling in a coffee cup, except that the cup is larger than the entire Earth. Turbulence occurs on all scales, from tiny little eddies to the planet's notorious storms and spots. Choi and his graduate advisor Adam Showman analyzed images of the giant planet taken by the Cassini space probe during a flyby in 2000. They traced cloud features to estimate windspeeds and thus the kinetic energy of the atmosphere. Then they calculated the amount of kinetic energy represented by turbulent features of various sizes and discovered that energy is being injected at a characteristic size of about 3,000 kilometers. This energy propagates down to smaller eddies and up to larger storms. Presumably the energy originates in the interior of the planet and wells up to the top of the atmosphere.
On Tuesday morning, Clark Chapman of the Southwest Research Institute updated the assembled multitudes about the MESSENGER space probe, which made its latest flyby of the planet Mercury last week and filled in much of the remaining terra incognita. On its first flyby, MESSENGER had surprised scientists by seeing a fairly recent impact basin just over 200 kilometers across, named Raditladi. Hopes it might be a fluke have now evaporated with the discovery of yet another such basin, as yet unnamed. "Recent" in this context means within the past billion years. Chapman says these giant craters are surprising in two ways. First, impacts of the requisite size (involving a projectile about 10 kilometers in diameter) should be very rare, so its looks weirdly like a case of lightning striking twice. Even weirder, the basins appear to have been modified by volcanic activity, even though Mercury should have died geologically billions of years ago. When MESSENGER finally enters orbit around Mercury in March 2011, it might find more than even its most enthusiastic proponent dreamt of.
Finally, much of Tuesday afternoon was turned over to the wonders of Saturn's moon Titan, a land of lakes, rivers, and rain, where hydrocarbons such as methane and ethane play the role that water does on Earth. The Cassini probe has been studying Titan for five years now, long enough that it has re-imaged many areas several times to look for any changes. In the lake district near the south pole, Alex Hayes of Caltech, Jonathan Lunine of the University of Arizona, and their colleagues discovered that one lake has receded and two others have dried up altogether. It's summer now in Titan's southern hemisphere, and every year about a meter's worth of liquid evaporates off the lakes.
One interesting thing about lakes on Titan is that the lake district around the north pole has a lot more lakes than its southern counterpart. This isn't just because the north is in the throes of winter, Oded Aharonson and Sonja Graves of Caltech argued yesterday. Because Saturn's orbital is elliptical -- its distance from the sun varies by about one astronomical unit over the course of its 30-year orbit--the summer shorter is hotter than the northern one, which favors lake formation in the north. But over tens of thousands of years, the ellipse itself shifts around, so the northern summer becomes the more intense one. Then the north lake district should recede and the south one should grow. Thus the lake asymmetry appears to be a direct product of long climate cycles--meta-seasons--on Titan.
Much to talk about with my Puerto Rican friends over dinner and breakfast!