DPS 2008 Day One: Mars, Exoplanets, Defining Planets and Enceladus
Today was the first day of the Division for Planetary Sciences conference here at Cornell. All the talks are being live web-streamed, but since most of you probably don’t have time to sit and watch esoteric scientific talks online all weekend, here are the highlights from the sessions I saw today.
In the morning, the first session that I went to was Mars Surface and Surface/Atmosphere Interactions. DPS is an astronomy conference, so many of the more geology-oriented people I’m used to seeing at conferences weren’t there and the Mars session seemed oddly empty. The talks were interesting though!
Eldar Noe Dobrea gave a presentation about the mineralogy of Arabia Terra, which is a huge region on Mars southeast of the Mawrth Vallis potential MSL landing site. I had heard a lot of what he said at the recent MSL landing site workshop, but he had a new result showing that the observed spectrum in many places is fit by the minaral chlorite which is a low-temperature metamorphic mineral. The scenario that he favored to form what may be a sheet of chlorite spanning many thousands of km is the deposition of some sort of volcanic material (ash would work well) and then burial, alteration to chlorite, and then leaching away some of the minerals in the top layer by water to form more aluminum-rich clays. This leaching could be caused by rainfall on the surface…
The next session was about extrasolar planets, which are always interesting. Jason Wright gave a talk discussing the fact that multiple planet systems seem pretty common: ~27% of the known extrasolar planet systems have more than one planet! He also pointed out that hot jupiters were more common in single planet systems and that systems with low-mass planets also tended to have more circular orbits. This is sort of coutnerintuitive because normally we think of planets forming in circular orbits and then getting bumped into more elliptical orbits. You would think that it would be easier to perturb a small planet, but the observations seem to disagree.
Another talk in the exoplanets session discussed “tidovolcanoplanetology” (and acknowledged that this term is ridiculous). Basically they looked at tidal heating of terrestrial planets and what implications it has for habitability. The bottom line seemes to be that it depended heavily on the mass of the star and the distance from the star where the “habitable zone” occurs.
After lunch there was an interesting “special session” about the dynamical classification of planetary bodies (which is code for “why isn’t pluto a planet?”). The first talk basically just reviewed what it means for an object to dominate its orbit, and emphasized that in the IAU definition it says that planets must “clear”their orbit, but it really should say that a planet “dominates” its orbit. For example, there are asteroids called Trojans that are trapped in the lagrange points of Jupiter, so Jupiter hasn’t technically cleared its orbit, but it clearly dominates it.
A plot of the number of objects considered to be “planets” over time.
A second talk brought up the interesting point that a good criterion for being a planet is that the object should not fit into a continuous size distribution. In other words, it should stand out in size from other objects around it. This way, all you need to do to identify a planet is look for other things of the same size in similar orbits. No need to estimate masses or roundness or anything else.
The third talk in this session dove home the point that any classification is going to have some arbitrariness and we should really just get used to it. As an example, it pointed out that there are populations of asteroids that are classified by arbitrary orbital parameters, like how close they get to the sun, and nobody gets upset when one of them gets reclassified when its orbit changes. He pointed out that far in the future, mercury’s orbit may destabilize and it could move around the solar system. Does it then stop being a planet? What if it messes up other planet orbits? Do they become non-planets? During the formation of the solar system, there were planet sized objects flying all over the place: which where planets and which weren’t?
My take on all of this is that it’s fun to talk about but is ultimately irrelevant. People constantly try to attach neat labels to things, and not just in science. And then the real world goes and comes up with things that don’t fit the neat categories. Fundamentally we have to just get over the fact that the world comes in all sorts of shades of grey and that our categories are going to be somewhat arbitrary. We can work hard to make them as non-arbitrary as possible, but it’s probably not best to dwell too long on classification. I’d rather do some real science than sit around worry what to call the thing I’m studying.
After the special session, there was a series of talks about Enceladus. This entry is getting pretty long so I’ll be brief. One talk took a look at the possible models of geysers and showed that it is very likely that there is ice melting in geyser formation on Enceladus. Another talk reviewd the awesome high-resolution images from the flyby in August, and another showed that the highest surface temperature measured during that flyby was about 160 K.