16 October 2008

DPS 2008 Day Five: Icy and Not-so-Icy Moons

Posted by Ryan Anderson

Today was the final day of the DPS meeting here at Cornell, but the sessions were still very interesting. They served to remind me just how little we know about the outer solar system. Also, remember you can go and watch all the sessions yourself! I believe the plan is to transfer all the videos to a more permanent location soon, so I will keep you posted.

The first few talks I saw today were about Jupiter’s large Galilean satellites. One talk confronted the mystery of Ganymede’s magnetic field, but found that it is still pretty confusing. It has been known for a long time that there is not enough heat for thermal convection (i.e. hot stuff floats, cool stuff sinks) to be the power source for the magnetic field, so they considered some other possibilities such as convection due to changes in composition or some past tidale event injecting a bunch of new heat into the moon but neither scenario really worked.

A second talk looked at “pit craters” on Mars and Ganymede. Pit craters are craters with a pit at the center, either in the floor or in the central peak. They are only found on icy or potentially icy bodies but nobody is sure exactly how they form. One theory is that the pits form when the central peak collapses because the icy ground in which the crater forms is too weak. But then you have to ask why all craters on icy satellites aren’t pit craters and why some craters have a pit but no peak. Another possibility that has been suggested for Mars is that melted rock interacting with subsurface ice could form collapse pits, but that wouldn’t explain why there are pits on icy moons. A third possibility is that the craters are puncturing through into some subsurface liquid layer, but the craters are not localized it is hard to imagine a global layer. The fourth possibility, and the one that the speaker though was most likely, is that vaporization of ice during the crater’s formation causes the pit to form. But what are the details of that process, and why aren’t all craters in the icy solar system pit craters? So far, nobody knows.

Ok, so Ganymede is icy and is pretty confusing, but it’s got nothing on Io when it comes to awesomeness. I saw a very interesting talk about ground-based observations of Io attmpting to measure the activity of one of the larger volcanoes, Loki. They used infrared telescopes and observed Io when it was in Jupiter’s shadow so that all the signal was from the warmpth of the lava. By very carefully studying the way that Io’s brighness changed over a period of months, they were able to estimate how much energy was being released by Loki, and also suggested that the variations in brightness are consistent with a lava lake that occasionally overturns, exposing fresh, hot lava to space! Awesome!

Io’s volcanoes spew particles into space, which get caught in Jupiter’s magnetic field and do all sorts of fun things. There is even a tube of plasma that connects Io to Jupiter’s north and south poles. With the right telescope, you can see where the plasma hits Jupiter’s atmosphere because it causes a bright spot in Jupiter’s aurora that follows Io around as it orbits. Nick Schneider presented new evidence that there is a new plume of plasma coming from Io and pointed directly at Jupiter. The thing is, nobody knows why there’s this extra plume! This means that scientists have been missing some key physics in how Io interacts with Jupiter’s magnetic field!

Moving out to Saturn’s icy moons, there were several interesting talks about Iapetus, Saturn’s thoroughly bizarre moon. It has a ring of mountains around its equator as high as Mt. Everest. Half of it is as black as coal, the other half is bright white like snow. It orbits more than three and a half million kilometers from Saturn in a tilted orbit, and yet it is tidally locked so that one side always faces Saturn. It’s probably my favorite atmosphereless object in the solar system other than Enceladus because it is just so strange.

One of the talks today may have solved one of these problems! They suggested a two-step process to explain the strange black and white appearance of the moon. First, dust from the numerous retrograde-orbiting small moons collects on the leading hemisphere of Iapetus, tinting it slightly reddish. Once that hemisphere became dark, it tended to heat up more in sunlight, which would drive off ice and leave behind the dust. This would lead to a runaway process as the dirty spots darken and therefore heat up more, drive off more ice and get even darker. It’s a pretty interesting theory! Still, I wonder why other Saturnian moons aren’t affected to the same extent.

A second talk about Iapetus presented evidence from the magnetometer on Cassini that Iapetus is deflecting the magnetic field of the solar wind! They showed that the sort of deflection observed is not what you would expect from an inert ball of ice and rock. It also doesn’t match what you would expect for a moon that is outgassing into space. The only way to get the observed deflection would be if Iapetus had a magnetic field, or if it had a layer of conducting fluid inside! Neither one of these seems very likely, so this is a real mystery! Yet another reason that Iapetus is fascinating.