19 December 2010

AGU 2010 – Day 2: Shoemaker Lecture and Icy Moons

Posted by Ryan Anderson

Whew! AGU is over, but fear not, my recap posts are just beginning! I was generally too busy and sleep deprived during the week to post any recaps but I kept taking lots of notes about the talks I saw, and now that I have some time to breathe I will be sharing the highlights with you over the next few days. Brace yourself, there’s a lot to cover!

I started off Day 2 with the Shoemaker Lecture, given by Carle Pieters. Pieters began with a corny poem about Gene Shoemaker, but thankfully it was actually somewhat interesting and it didn’t last long. He lecture itself was about why the Moon is an important place to study. Most of her talk was pretty familiar, especially to anyone with an astronomy background. She discussed the evolution of the solar system from a cloud of gas and dust to the planets we see today, and emphasized that the moon is the best place we can look to learn about the first billion years or so. She also described the moon as a place where you can do “pure” geology without pesky things like plate tectonics, water or an atmosphere messing things up. (Of course many geologists would argue that those things are what make geology interesting!)

The most important thing that I learned from Pieters’ lecture was that the famous South Pole-Aitken basin on the moon, one of the largest and oldest known impact basins in the solar system, is not at the south pole! It spans from the south pole to the crater called Aitken. Go figure.

The coolest thing in her talk was about the lunar swirls, which are patches of lunar terrain that have a swirly looking pattern of light and dark splotches. Pieters showed that these swirls are associated with magnetic fields, which I already knew, but also that they match up nicely with detections of water and OH absorption bands. This is cool because it strongly implies that the hydration seen on the moon is related to solar wind interactions with the surface: the magnetic fields deflect the solar wind, causing corresponding patterns in the hydration features.

Reiner Gamma, one of the most famous lunar "swirls". It turns out that there are strong magnetic fields and stronger-than-normal hydration features in the lunar swirls, suggesting that the patterns and the hydration are caused by the solar wind interacting with magnetic fields.

After the Shoemaker lecture, it was time to hear about icy moons, starting with my favorite: Titan. Bill McKinnon gave a talk about interpreting the Argon in Titan’s atmosphere, and suggested that the argon must be coming from the upper icy crust or an ocean under the ice, and that there are a variety of possible ways to get it out and into the atmosphere, including cryovolcanoes, and impacts.

Speaking of cryovolcanoes, Jeff Kargel described two main types of cryovolcanoes: the first is just the eruption of water or slush, while the second is hydrocarbon based, with the “lava” being something ranging from asphalt to candle wax? Water volcanoes have a couple of interesting problems, the first being that liquid water is more dense than solid water. That makes it relatively difficult to get water to want to erupt from under ice. There is also the problem that ice is pretty soft, so it can flow and convect heat to the surface without even melting, again making it difficult to get ice volcanoes. Mix a little ammonia into the water though, and it becomes easier to create and sustain a cryovolcano. Kargel also pointed out that hydrocarbon volcanoes aren’t as exotic as you might think: there are actually asphalt volcanoes on earth, and of course there are the La Brea tar pits.

Next up, Rany Kirk gave a talk about a putative new cryovolcano on Titan. He first gave some background: there have been a lot of things that were called volcanoes on Titan, but then topographic data revealed that they weren’t what they looked like in radar images. This newest candidate, called Sotra Facula, is actually a hill with a big hole in one side! He also pointed to a gap between two hills nearby and suggest that it might be a second volcano. Personally, I’m pretty skeptical, but that’s probably because I am spoiled by the fantastic data we have for Mars. I’m not used to working with grainy radar images and low-res topography, so I tend to take a lot of the geomorphology claims on Titan with a grain of salt. Kirk was very honest about this claim though, so I give him credit for that. He described the search for volcanoes on Titan as a sort of Rorschach test, where you see what you want to see. Take a look at the flyby of the “volcano” that Kirk was talking about. What do you think?

Ralph Lorenz gave the next talk on the organic cycle on Titan, but also pointed out that skepticism is warranted when doing geomorphology. Just because something looks like a familiar sort of feature doesn’t mean that it actually is. With that in mind, he went on to show images of the lakes on Titan and pointed out that they look a lot like lakes in karst terrain on Earth. Karst terrain, for those of you not in the know, is what happens when groundwater dissolves the (typically limestone) bedrock, forming lots of pits and lakes. But on Titan it would be weird for the liquid ethane in the lakes to dissolve the frozen H2O “bedrock”. Lorenz used this to suggest that maybe the crust of Titan is actually more organic-rich, possibly due to the “shock-synthesis” of hydrocarbons in the crust after a giant impact.

Next up, Alice LeGall talked about the expansive dune fields on Titan. Based on the radar reflection properties she found that their composition is probably hydrocarbons, and that the total sand volume is something like 50,000-500,000 cubic kilometers. Lorenz mentioned these hydrocarbon dunes also, saying that they represent something like 100-1000 times the organic content of all the coal on earth.

Finally, Alex Hayes gave a information-dense talk about his work on the Titan lakes, describing how he has used the attentuation of radar signals in the shallow parts of Ontario Lacus to infer some of the properties of the lakes, confirming their mostly-ethane composition. He also looked at how the lake levels are changing over time and found that although the southern lakes are losing about one meter per year, the northern lakes don’t seem to be changing much, and predicted an increase in precipitation in the north soon. (That’s right, we’re forecasting the weather on one of the moons of Saturn. We live in the future!)

After all that fun with Titan, it was time for the second most famous Saturnian moon to steal the show. Francis Nimmo gave the first talk about Enceladus, setting the stage for a bunch of interesting talks to follow. He reminded us that there are at least 7 Gigawatts of power pouring out of the south pole of Enceladus, but that tidal heating can only produce about 1.1 GW if Enceladus is in a steady state. So that means it isn’t! In other words, either the heat is produced only episodically, or it is produced all the time but only release occasionally. He also pointed out that since the heat production and dissipation in a moon can influence the moon’s orbit (a fact that still blows my mind whenever I think about it), it may be possible to carefully study Enceladus’ orbit and learn more about its heat production.

This false-color view of Enceladus shows the parallel tiger stripes at the south pole, which are linked to the geysers of water vapor that make Enceladus so interesting.

Another talk about Enceladus by Gabriel Tobie looked at the stability of liquid water beneath the moon’s icy shell. A moon with liquid inside is much better at dissipating tidal energy, so the fact that Enceladus is putting out so much energy argues in favor of some liquid beneath the surface. Tobie found that if the sub-ice ocean on Enceladus were global, most of the heat would be dissipated at the equator, but if the water was just a regional sea at the south pole, it could produce heating similar to what is observed.

Hunter Waite, who was my research adviser in undergrad and is in charge of the Ion Neutral mass Spectrometer (INMS) on Cassini gave an interesting talk about INMS observations from multiple flybys of Enceladus. Apparently, in higher-velocity flybys the instrument detected more carbon. Waite suggested that this might be caused by really large organic molecules – so large that INMS couldn’t normally detect them – breaking up into pieces when they hit the detector at really high speeds. So far there aren’t that many flybys of the plumes but future measurements will help test the hypothesis.

Amanda Hendrix was up next with a talk about occultations of the Enceladus plumes. Essentially she used the UV spectrometer on Cassini to watch as the plume passed in front of three stars and the sun. She was looking for the nitrogen that had been detected by INMS, but didn’t see any, implying that nitrogen isn’t the propellant for the plumes. She was also able to tell that the gas in the jets was going at Mach 5-8 (~2 kilometers per second!) which was much higher than previous estimates and is consistent with models of the gets based on boiling from a liquid reservoir of water.

Cassini ISS view of the Enceladus plumes.

Carolyn Porco also gave a talk about the Enceladus jets, accompanied by the ever-spectacular images from the Cassini Imaging Science Subsystem (ISS). New images have allowed her to identify the sources of the jets, and to identify thirty new ones! All of the jets seem to originat from the huge cracks in the south pole known as “Tiger Stripes”. Porco showed that the average particle velocity in the plumes was 20 meters per second, and that 96% of the particles in the plume do not escape Enceladus’ gravity. Finally, she showed that the estimated particle to vapor ration in the plumes is large (0.45), which is difficult to do with sublimation, but easy to achieve with a liquid water source for the plumes.

But wait! There’s more! There are more icy moons than just Titan and Enceladus, and there were several talks about the others. Hauke Hussmann gave an overview of the diversity of oceans in our solar system, starting with the possible soruces of heat to keep them liquid. These include radioactive decay, primordial heat of formation, heat released as the body differentiates and heavy material sinks to the core, and of course, tidal heating. He also gave an overview of the possible ways to detect an ocean on an icy moon. An induced magnetic field, tides, decoupling of the outer shell of a moon or planet, and vapor plumes all can hint at an internal ocean. A lot of people have heard of the possible oceans on Europa, but fewer realize the possibility of oceans on Ganymede, Titan, Enceladus, Triton, and even out on the Kuiper belt objects like Pluto!

Jason Goodman gave a talk about the vertical structure of icy oceans, emphasizing that temperature and salt content can both change the density of ocean water. On an icy ocean, any freeze or thaw of the ice layer is going to have an effect on the salinity of the water in the upper layers of the ocean, which could cause instabilities.

A view of the "chaos" terrain on Europa, as seen by the Galileo orbiter.

Focusing on Europa, Britney Schmidt gave a talk about the interesting “chaos” regions. These regions are strange: they are made of what looks like big blocks of ice that have been rotated and then frozen in place again, surrounded by finer-grained ice. This isn’t too strange, but oddly enough the chaos regions tend to be topographic highs. Schmidt suggested a model where the ice crust breaks but the water underneath acts like a hydraulic fluid, with pressure sending it up into the fractured area, causing it to rise and then re-freeze.

Well, there you have it. That’s about what Tuesday was like if you were like me and camped out in the planetary science room for most of the day. Next up is Wednesday, which won’t be nearly so long since I spent a lot of the time presenting my own poster.