28 March 2009
LPSC 2009: Day 2
Posted by Ryan Anderson
Day 2 was all about ice in the mars sessions: the morning focused on the polar caps and the afternoon focused on the subsurface. I also managed to catch a few non-mars talks.
One of the first talks I saw was by Ken Tanaka, famed for his geologic maps of Mars. He showed the results of his studies of the north polar cap, and identified at least two major hiatuses. The official geologic definition of hiatus is: “A cessation in deposition of sediments during which no strata form or an erosional surface forms on the underlying strata; a gap in the rock record.” Tanaka showed examples of locations that demonstrate the different ages of layers, but the main take-home message of his talk was that most of the time, things are not being deposited on the polar cap.
Ken Herkenhoff gave an interesting presentation about HiRISE observations of active processes at the poles. He emphasized that there are many processes that they will understand better after another year of repeated observations, but still had some interesting results. He talked about active streaks in gullies and the strange fans and spots that form on defrosting dunes. Interestingly, these spots tend not to form on the base suface beneath the dines, implying that the material underneath the ice has to be mobile enough to blow around once the ice is partially removed.
HiRISE also observed avalanches in action on the north polar cap! As ice begins to thaw in the spring, debris from the ice cap can come loose and cause an avalanche. These are apparently pretty common, because multiple avalanches were actually caught in action in the same image!
Finally, Herkenhoff showed a picture that apparently has everyone baffled. There are lots of streaks seen on the polar cap, but some like the ones in the following image don’t make much sense. It looks like most of the streaks in the image are going down-slope, so you might think they were formed by small avalanches of dust or something. But why do they have such sharp edges? And, more importantly, why are some of them diagonal compared to the rest?!
I darted out of the Mars talks just in time to catch an interesting Enceladus presentation by Sue Kieffer. She took a look at the thermodynamics of warm ice in a vaccum and believes that the ratio of ice to water vapor in Enceladus’ famous plumes is half of what was originally reported. Kieffer claimed that the original calculation made a faulty asumption about the range of particle sizes in the plume, which led to a very different estimate. Why is this a big deal? Because it turns out that Kieffer’s calculations fit much better to sublimation than boiling liquid water! Enceladus might not have liquid water at its pole, it might just have warm ice! I’m sure the icy moons community will be looking into this some more and trying to figure out which calculation is correct as soon as possible. It would be really cool if we could prove that there’s liquid water on Enceladus, but the universe doesn’t care what we think is cool, so maybe the little moon is just a warm ice-ball.
Finally, I saw Steve Wood give a really interesting talk in the afternoon about atmospheric collapse on Mars and its effects. Mars’ tilt changes over millions of years, and occasionally it decreases to the point that the global temperatures drop, and CO2 from the atmosphere is dumped on the surface in a thick layer. This talk considered what that blanket of CO2 would do the the martian subsurface and concluded that it would indeed act as a blanket. CO2 ice has a lower thermal conductivity than rock, so the ice and icy soil would act to trap the geothermal heat of the planet, and might cause a subsurface warming of 20 degrees: enough to melt ground ice and drive off the water from some hydrated minerals. This is a really interesting effect and I had never heard of it before!
I agree that Steve Wood’s talk was very interesting. I am hoping that he turns this early work into a paper as soon as possible.
About the figure showing ice distribution over a range of obliquities: first, the figure is showing water ice deposition, not CO2 ice; and second, during high obliquity, the equatorial ice would be a ring that covered the entire equator, not a patch like you get at the poles (and as erroneously shown in the figure). I wonder who made this figure?
Thanks for pointing out the error in the figure. I’ve deleted it to avoid confusing other readers.
I know I have seen a better figure. If I find it, I will post a link here in the comments.