LPSC, Day 1: Phoenix, morning session
Posted by Emily Lakdawalla
23-03-2009 16:13 CDT
So the Lunar and Planetary Science Conference (LPSC) started this morning in Houston, and I am seriously bummed about the fact that I can't be there this year. I'm hopeful that I'll receive a couple of messages from friends attending the conference describing interesting talks, posters, or hallway conversations, but it won't substitute for actually attending.
One small consolation is that the "abstracts" written for LPSC are nothing at all like the single-paragraph notes that announce talks at most conferences; they're almost like miniature scientific papers, two full pages of small type, often illustrated (though it should be noted that unlike scientific papers, LPSC abstracts are NOT peer-reviewed). So if I can't be there, at least I can read through the abstract volume and find out about what the presenters were planning on presenting at the conference a couple of months ago.
Today, the schedule includes a full day of oral sessions on Phoenix, morning sessions on the Moon, comet Wild 2, and astrobiology, and afternoon sessions on planetary formation and Venus' atmosphere. If I were attending, I'd probably have focused on the Phoenix sessions, so here's what I got out of reading the abstracts. Links mostly go to PDF-formatted abstracts, which may be a few hundred kb to a few Mb in size.
In "In Situ Analysis of Ice Table Depth Variability under a Rock at the Phoenix Landing Site, Mars," Hanna Sizemore and coauthors report on the results of one little experiment performed with the robotic arm: what did they learn when they looked under Headless? Headless was one of the rocks within reach of the arm. On sol 117, they used the arm to scoot Headless from its former position into the Neverland trench. On Earth you might move a rock to see if any bugs were underneath it, but on Mars they were looking for something different: how did the area underneath a rock look different from dirt-covered areas at the landing site, and, specifically, what did the subsurface ice table do under that rock? Sizemore reports that underneath Headless, the ice table dipped. This dip tells them two important things:
First, diffusion is likely the primary mechanism mobilizing water in the shallow regolith in the current epoch. Locally shallow ice beneath the rock might indicate recent water transport via thin films or liquid water, but is not observed. Second, the apparent equilibrium between ground ice and the atmosphere is a long-term phenomenon (>103 yr). A recent shift in climate conditions would likely cause the magnitude of the ice table depression associated with the rock to deviate from theoretical predictions. Again, this is not observed. Taken together, the range of ice table depths measured at the Phoenix landing site and the detailed behavior of the ice table near Headless argue strongly for a prevailing equilibrium between ground ice and atmospheric water, and indicate that physical models of ground ice employed for the past four decades are largely appropriate."It's always good to find out that models in wide use are borne out by empirical studies. I'll note that the following abstract on polygon scales by Mike Mellon and coauthors also supports the idea that the depth to the ice table is controlled by diffusion and not by motion of occasionally formed liquid water. And the next abstract, on soil spectral properties, by Diana Blaney and coworkers, found that there was no evidence for layering within the soil, in particular that there was no evidence for any kind of salt-rich layer, which strongly "argues against models involving perchlorate-rich brines mobilizing liquids to form these [ice] deposits." Next up was an abstract by Walter Goetz and a long list of coauthors including the rest of the Danish team who have provided magnets to practically every landed Mars mission. They looked at Phoenix optical microscope images of soil samples and suggest that a couple of the grain types have strong similarities to materials observed by the rovers. They observe three main types of particles: brown silt-sized particles, black silt-sized particles, and very fine-grained orange particles. The orange stuff forms the bulk of the Phoenix samples, and its color is in remarkable agreement with the spectral observations made by the orbiting CRISM instrument of the whole region near Phoenix' landing site. They say that the black particles are very similar to ones observed to cling to Opportunity's Capture magnet (one of the large magnets located at the base of the rover's mast), and possibly to the dark sands of the El Dorado dune field studied by Spirit. The brown stuff appears glassy. Glass happens when you melt rock and cool it quickly; Goetz and coauthors say that this stuff may be unique to the Phoenix landing site, and that it could either be impact glass from the nearby Heimdall crater or volcanic glasses from one of the big volcanoes nearby, such as Alba Patera. At that point I probably would have taken a break and returned in time to see the presentation by Christina von Holstein-Rathlou and coauthors on what the motions of that little telltale on the tip of the meteorological mast tell us about winds at the Phoenix landing site. Winds at night were fairly calm and easterly at around 4 meters per second, but during the day they were gusty, anywhere from 1 to 12 meters per second. During the day the winds completed a full circle -- they started out northeasterly in the morning, and shifted over to the east, then south, then west, then north throughout the daytime before returning to their easterly direction overnight. Finally, this comment was curious: "Among the interesting features at the landing site is the total lack of any aeolian features such as dunes as has been observed at all other landing sites on Mars. [Optical Microscope] images show that the building material to make dunes exists at the landing site, and Telltale data show that conditions for particle movement are (barely) met. It is suggested that the surface is active on a timescale faster than the time needed to build up aeolian features e.g. around rocks."
As that rounds out the morning Phoenix sessions, I think I'll wrap up the post there!
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