DPS 2012, Monday: Icy moons and a four-star exoplanet
Posted by Emily Lakdawalla
15-10-2012 23:31 CDT
Where do I begin? I've only been at the Division of Planetary Sciences (DPS) meeting for one day, and already I feel like my brain is full. As it's nearly 9 pm, and I'm only just sitting down to write, I'm afraid I'm not going to be able to offer any deep reflections on the talks or posters I saw today. I think that the best I can manage is some highlight notes.
Today (Monday, October 15) featured concurrent sessions on exoplanets, asteroids, and icy outer solar system bodies. I knew that asteroid astronomer Andy Rivkin would be Tweeting the asteroid sessions, and that planetary polymath Sarah Horst would be in the exoplanet sessions, so I relied upon their coverage and sat myself in the icy satellite sessions.
David Paige talked about some modeling work on Ganymede, where he took Ganymede's topography and considered what happens when it gets heated. Even though icy bodies are (by definition) cold, solar heating can make liberate ice molecules and they float around a bit and then return to the surface. Since the poles are colder than the equator, these kinds of thermal processes usually wind up concentrating and trapping mobile icy molecules at the poles. Paige showed that the annual average temperature -- which would be the temperature that prevails at a few meters' depth below the surface -- is so cold that not only would the poles be trapping mercury, water, ethanol, methanol, HCN, and ammonia, they could even trap xenon.
Kevin Hand reported on Keck II infrared observations of Europa to map the distribution of hydrogen peroxide, H2O2 on the surface. I was not sure why this was important until I listened to the next talk (more on that in a moment). Galileo performed only one observation of H2O2 abundance. Although Keck's spatial resolution is lower, the spectral resolution is far higher than Galileo, and by making four observations of Europa at different positions in its orbit they were able to determine peroxide abundances ranging from 0.018 to 0.142 percent. He said that the peroxide abundance correlates very well with the fraction of water ice that makes up the surface in the same location.
One of my favorite talks of the morning was given by Reggie Hudson, about experimental models of chemical reactions in Europa's ice shell. He talked about previous work, beginning with a mixture of water and sulfur dioxide ices in a vacuum and heating it up, showing that the process would produce bisulfite ion (HSO3-). He called this an "easy result" and wondered why nobody had done the work before and mentioned in the same breath that performing this experiment destroys your experimental apparatus. (The water, he said, is "sticky" and coats the innards of the apparatus, making it difficult to achieve a good vacuum; the sulfur dioxide or bisulfite corrodes the vacuum pump's parts.) This got a laugh from the audience.
Hudson built on this previous work by taking bisulfite ion-bearing water ice, mixed also with hydrogen peroxide, and heating that. He produced sulfate ion, which is interesting from an astrobiological perspective because we know that some microbes on Earth make a good living consuming sulfate ions. This is a purely thermal process, with no ultraviolet irradiation required, which means it could happen throughout the thickness of Europa's ice shell, not just in the top couple of millimeters, where ultraviolet radiolysis occurs.
Another talk I enjoyed was Frank Postberg's. He talked about an improved dust detector (compared to Cassini CDA) being developed for the European Space Agency's JUICE mission. He pointed out that icy moons are surrounded in clouds of dust because of impacts knocking dust off of their surfaces. He demonstrated that a really good dust detector could actually make compositional maps of Ganymede's surface with 10-kilometer resolution. These wouldn't be compositional maps like you get by studying spectra; these compositional maps would be based on mass spectrometry of actual particles lifted off of the surface, essentially a global surface sampling mission. Wow.
Tilmann Denk showed that some puzzling aspects of the lightcurve of an irregular outer Saturnian moon, Ymir, can be explained by Ymir being triangle-shaped. The lightcurve has three peaks and three valleys with two magnitudes of variation between peak and valley. A triangle shape with three sides of length 25, 24, and 20 kilometers fits the observations well.
Dan Tamayo presented the first optical detection of the Phoebe ring of Saturn, an amazingly difficult thing to do. He acknowledged the challenges of this observation, but gave a pretty convincing argument that their tricky data reduction techniques resulted in a detection of the ring. Future work will include probing the radial structure of the ring, seeking to validate the hypothesis that the ring ends at Iapetus (because Iapetus sweeps up the infalling Phoebe material, and that's what darkens Iapetus' leading hemisphere.
Julie Castillo-Rogez showed that the Uranian and Saturnian ring and satellite systems have some striking similarities. Uranus has a blue ring analagous to Saturn's E ring, which is of course supplied by Enceladus' geysers. Uranus' ring is associated positionally with the satellite Mab, which is way too tiny to have any hope of Enceladus-like activity. She suggested that there has been migration of the moons and/or ring material and that Ariel could originally have been the source and may relatively recently have been geologically active.
Two really cool talks gave completely independent verification of extremely localized heat sources within Enceladus' tiger stripes, specifically Baghdad Sulcus. John Spencer described an amazing observation in which they employed Cassini's CIRS instrument as an interferometer, achieving resolutions down to something like 40 meters per pixel (although this is dependent on modeling the locations of Enceladan plumes, and the solutions are not unique). So you can't believe the precise locations of plumes picked out by the CIRS data, but what you can say is that the plume sources appear to be extremely narrow, narrower than that 40-meter pixel width, and they fall right in the bottom of the Baghdad sulcus.
Jay Gougen reported on a similar effort with VIMS, performing extremely high resolution sampling of the surface during a very close flyby. They found exactly one pixel that contained infrared thermal emission as they crossed Baghdad sulcus. He made the argument that VIMS was detecting thermal emission at temperatures of near 200 Kelvin, which is really very hot for something at that distance from the Sun.
Mark Perry shared numerous Ion and Neutral Mass Spectrometer (INMS) measurements taken during close flybys near and through Enceladan plumes, and showed that if there is any time variation of plume output, it is very weak and difficult to detect.
As I was not in the exoplanets session I missed Meg Schwamb talking about her exciting work where the Planet Hunters citizen science project is reporting its first exoplanet discovery, an announcement made extra cool by the fact that it is the first known planet in a four-star system. The planet orbits an eclipsing binary. The two stars in that binary pair are only about 0.1 or 0.2 AU apart, and the planet orbits them at 0.6 AU. While following up this discovery with Keck II radial velocity observations, Meg found that there was another star orbiting this pair at a great distance, 1000 AU. Upon closer investigation, this distant star actually turned out to be a binary, with the two components separated by 40 or 50 AU. What would this all look like if you could stand on the planet? The distant binary would provide about as much illumination as a quarter-phase moon does on Earth, but they would be point sources, at least visually. With binoculars, you could resolve them as disks.
And that is absolutely all I can write tonight. I must head to bed, and get ready for another day of science. Tomorrow it's Titan, Dawn, and Mars for me.
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