Today they turned on the scientific fire hose at the Division of Planetary Sciences / European Planetary Science Congress meeting happening here in Nantes, France. My brain already feels full and I still have four more days! As I write this there is a poster session going on, but I had to take a break and try to reflect a bit on the presentations I've seen today. I have no hope of summarizing everything now, but will try to hit a few high points.
In the morning I mostly attended Enceladus talks. One interesting presentation by Frank Postberg and followed up on by Schmidt showed Cassini Cosmic Dust Analyzer (CDA) results from a very close pass by Enceladus. Enceladus makes the E ring out of particles formed from water vapor condensating into crystals. But deep in the plume there are much, much larger grains that contain lots of salt. What that means is that most of the mass coming out of Enceladus' plumes is these salty grains. So much salt can't come from vapor condensation, so that pretty much requires there to be a briny liquid that's the source of those particles.
Ralf Jaumann showed some beautiful high-resolution VIMS data on Enceladus that showed that large particle size in Enceladus's icy surface is highly correlated with regions that have had tectonic deformation.
Paul Schenk had a lovely presentation on Enceladan snow, and showed that in places it may be up to 100 meters thick; in other places, very little has fallen.
In the afternoon I went to the Vesta session, where the Dawn team presented their preliminary results. For the most part, these were REALLY preliminary results. Vesta presents a lot of puzzles so the development of science is not going to be quick.
Chris Russell said that talks at this meeting only cover results through the Survey Orbit period; at AGU they will present results from High Altitude Mapping Orbit, and at LPSC and EGU they will present results from Low Altitude Mapping Orbit.
Russell also reported on the results of their search for moons. Dawn found none, which pretty much eliminates the possibility of Vesta having moons larger than 10 meters in diameter. They did observe asteroids moving in the background, so that proves that they could detect small objects in orbit at Vesta, they just didn't.
He also reported some physical parameters derived from Dawn data. Here they are:
- GM: 17.2884 ± 0.0005
- Mass 2.591 x 10^20 kg
- Volume 7.532 ± 0.073 cubic km (with the caveat that they haven't seen the extreme north pole, so they had to assume a shape to the north pole to calculate that)—this volume is equivalent to a sphere with diameter 524 km
- Average density 3440 kg/cubic m
- J2 [this is a descriptor of the oblateness (or equatorial fat-ness) of a body as it affects its gravitational field]: 0.03178
- Meteoritic data suggest a core density of 7400 kg/cubic m, crust and mantle 3170 kg/cubic m. Assuming these densities for Vesta's core and mantle, you calculate a core volume of 6.28 million cubic km, equivalent to a sphere with diameter 214 km.
Everybody who presented spectral data appeared to be overwhelmed by just how colorful Vesta is. Usually you have to work extremely hard to tease any color variation out of terrestrial planetary images -- everything is gray gray gray because of space weathering. But Vesta has blazing color variations, which is very exciting. Still, it's too early to get very specific compositional information out of those color variations.
One conclusion: overall, Dawn's spectral data say that Vesta's surface is more like howardite meteorites than it is like the other two types of meteorite known from Vesta, diogenites and eucrites.
Another conclusion: there is a huge difference in color between the south polar terrain and the equatorial terrain.
Vesta appears to be unusually rough (as rough as Phoebe). It has low thermal inertia, meaning its surface behaves more like rock and less like dust in response to heating.
One of the three science instruments, GRaND, has only recently started even detecting Vesta -- it will really only get good data once Dawn is in Low Altitude Mapping Orbit.
Preliminary results of crater counting indicate a wide range of surface ages and a complex geologic history. For instance, the south polar impact basin is about a billion years younger than the surface of the more heavily cratered northern hemisphere, but there have been major landslides since then that have altered the appearance of the south polar terrain.
That's all I have time to write for now -- stay tuned for more, and of course follow me on Twitter for live updates from Nantes!