6 March 2010
Thursday started off with a couple of talks about the possibility of oceans on Mars. The first one, given by Gaetano DiAchille looked at possible locations of deltas all over Mars to try to figure out the water level of a past ocean. Deltas form when a river hits a standing body of water and drops its sediment, so they are a reliable marker of the water level. DiAchille found that “open deltas” – that is, deltas that do not end in a closed basin like a crater, all appear at the same elevation. This might mean that they all fed into a large northern ocean.
In the second talk, Wei Luo described his work mapping where all of the valley networks on Mars are and found that the northern limit of the networks fits with elevations that had previously been considered as possible ocean shorelines. The valley networks also matched with locations that atmospheric models predict would get the most precipitation.
Neither of these studies is conclusive evidence for a northern ocean on Mars, but they are interesting and they suggest that the “ocean hypothesis” is becoming popular again after years of little interest.
Later that day I saw a talk by Nick Warner describing the possible thermokarst lakes that he discovered in Ares Vallis on Mars. I wrote an article on Universe Today about this discovery when it was first announced a couple months ago.
I ducked out of the Mars talks to go see a talk by my friend Debra Hurwitz about a lava channel in a crater in Elysium Planitia. The channel was formed when lava breached the rim of the crater, flowed down the inner wall and ponded in the bottom. She calculated that the lava probably flowed at about 17-35 meters per second and that 6,000 cubic meters per second flowed down the channel for about 15 days. She also found that the channel could have been eroded mechanically without the need for the lava to actually melt the underlying rock very much.
After that, I headed over to the Titan session to hear a talk by Ralph Lorenz about waves on Titan lakes. Most of what we know about the surface of Titan, including the presence of liquid hydrocarbon lakes, is based on radar images from Cassini that measure roughness. The lakes show up as perfectly smooth (and therefore dark) surfaces, which is weird because radar images of lakes on earth usually have slight roughness due to waves. On Titan the gravity is lower, so you would expect bigger waves. It’s possible the lack of waves is due to the viscosity of the lakes, which might be increased by bigger “tar-like” molecules dissolved in the thinner ethane and methane, but it might also be due to a lack of wind. The Cassini mission will be watching as the seasons at Titan change to see if the wind changes and kicks up any waves.
I did a lot of session hopping on Thursday! The next stop was the astrobiology session. Oleg Abramov presented some results of his investigation of what intense impacts might have done to early life on the earth or Mars. He found that even during the Late Heavy Bombardment, the crust is not sterilized by the impacts, and in fact it might be more habitable for early life because impacts deliver organic molecules and cause widespread hydrothermal activity!
The talks I was really interested in were two talks on the magnetite crystals discovered in the famous ALH84001 meteorite. I posted a while back about a new paper that claims these crystals are evidence of life on Mars, and these two talks were focused on the claim. The first talk, by Allan Treiman gave some good background on the debate over whether ALH84001 preserves evidence of life and then addressed some of the new claims about the magnetite crystals. He said that most of the attributes of biological magnetite crystals, such as their size, lack of flaws, and precise crystal structure were not observed in the ALH84001 crystals. The big question is why the crystals are so pure. Allan argued that you can get pure crystals just from the heating of iron carbonate, which is found in the meteorite.
The following talk was by Kathy Thomas-Kleptra, whose paper Treiman was responding to. She showed that Treiman had probably made an error in calculating the breakdown temperature for iron carbonate. She also pointed out that the crystals are found in carbonates without much iron and that there is no graphite observed, but it is also a byproduct of heating the carbonates.
I don’t know enough about petrology and geochemistry to know who is right here, and I was very disappointed that both Kathy and Allan used up all of their time talking, so there was no chance at all for questions! I wasn’t the only one. When the moderator said that there was not time for questions and that they had to get on with the next session, most of the room groaned and protested. But alas, the talks pressed onward.
I zipped back over to the Titan talks in time to catch the end of one pointing to features that they claimed were “deltas” in one of the lakes. I was very skeptical of this because the quality of the radar images is so low. What they avtuall observe is a dark branching channel that ends at a peninsula in one of the lakes. That’s not evidence for a delta in my book. This talk made me realize how spoiled I am with HiRISE, CTX, MOC and other high-resolution data on Mars!
Finally, I stopped by the asteroid session for two talks. The first was by Dan Scheeres and he talked about the role that tiny forces might play in holding asteroids together. He showed that Van Der Waals forces, normally ignored for all but the tiniest particles, actually might be important in holding particles together in asteroids. He made the analogy to powders like flour or cocoa powder on earth. These can clump together and when they are stressed the form fractures even though they are made of loos grains. The same thing might happen on a much bigger scale with the gravel and boulders in low-gravity asteroids!
The last talk I caught on Thursday was by my friend Seth Jacobson, who showed some simulations of asteroids that spin so fast they break apart. He showed that the ratio of sizes between the two bodies make a big difference in how the binary asteroid evolves. In some cases, the secondary asteroid even swings so close to the primary that it splats apart and forms a short-lived three-body system!