LPSC, Day 1: Spirit and Phoenix
Posted by Emily Lakdawalla
02-03-2010 9:35 CST
Where to begin with the Lunar and Planetary Science Conference (LPSC)? I sat in sessions all day yesterday (Monday), leaving only briefly to enjoy hallway conversations with former grad school buddies, people I've recently interviewed, mission project scientists and principal investigators, NASA headquarters staffers, and on and on. I am beginning this post (but likely will not finish it) in a few moments stolen before I'm going to attend the third (I think) annual "Women in Science" breakfast, an event funded by NASA headquarters.
I wish I could be posting a little bit more from within the sessions, but that's made difficult by the lack of Wi-Fi in the session halls. I got an interesting note from the conference organizers yesterday, telling me that the lack of Wi-Fi is not their choice, nor is it the hotel's. "For the record," they said, "it is the conference co-chairs who have deliberately made the decision not to allow Wi-Fi in the oral session rooms." Grrr. I can still Tweet from my phone, which I did regularly. Do go check out my Twitter feed for those notes, because all of them will not end up on the blog, though of course the things that I do cover in the blog will be reported in more detail here than they ever could be via the 140-character-long updates on Twitter. I am afraid that time constraints will prevent me from doing as much as I usually do to try to explain the more technical bits of this work -- I hope that some of you can follow what I'm talking about!
Looking over my notes from yesterday, the easiest thing to contain into a brief post will be an update on the rovers and on Phoenix. I attended Ray Arvidson's talk on recent results from Spirit's observations of sulfate sands on the side of Scamander Crater, Columbia Hills, Mars. Probably the most notable thing from his talk was the fact that Spirit moved 39 centimeters since being declared a lander; he thinks (as does pretty much every rover team member I've talked with) that a couple of weeks of work in the spring should win Spirit free of the sand trap.
Being stuck has allowed Spirit to make a much more detailed analysis of the soils than she's managed at other locations. Ray said that the analysis suggests occasional tiny amounts of wetting of the soils at Gusev. Frost and the occasional thin film of water remove soluble materials from the surface of the soil, so soluble compounds aren't visible on undisturbed soil. Wherever Spirit has disturbed the soil, millimeters to centimeters beneath the surface, you start seeing the soluble species like hydrated sulfates. Yet these materials, which are produced by altering basaltic minerals in the presence of water, sit immediately next to other unaltered basaltic minerals like olivine, which is really unstable in the presence of water. That suggests that we're also looking at mechanical mixtures -- sands stirred by other geological processes.
The other rover talk I saw was given by Albert Yen. He used the simple device of comparing the proportion of iron to manganese in Mars Exploration Rover APXS elemental abundance measurements to get at the weathering history of a rock. The story is this: iron and manganese ions behave almost identically in melted rock. They go into the same minerals, substituting freely for each other in crystal structures. For basaltic rocks from all over Mars (basalt is an igneous rock formed from a melt), the ratio of iron to manganese is pretty constant, about 50. This is true both for Martian meteorites and for the basalt rocks observed by the rovers.
But weathering changes that proportion. Manganese and iron behave differently in solution depending on the pH, the temperature, the redox potential, and many other factors. In general, manganese seems to be more soluble under Martian weathering conditions than iron is. So when these rocks get into contact with water, the manganese goes into solution more readily than iron. In some places, like in the Clovis class rocks from West Spur, or in the sulfate sands near home plate, the rocks have been weathered, the manganese has gone into solution, and been carried away, leaving the rocks relatively enriched in iron. In other places, like Wishstone in the Columbia Hills, the rocks were isochemically weathered; the minerals were changed, but the proportion of iron to magnanese was unaffected.
After the two rover talks I tuned in to Sam Kounaves' talk on some Wet Chemistry Laboratory results from Phoenix. You may recall the Phoenix polar lander had a four-cell wet chemistry lab into which they dropped samples, mixed with water, then titrated with various chemicals to determine the presence and abundance of several soluble compounds in the Martian soil. One of the four cells was never filled, because the sticky soil failed to fall through the hopper into the cell. So, Kounaves, said, this cell wound up being a "blank."
This was fortuitous, because some things happened in that blank cell that should not have. In particular, the empty cell, cell 3, recorded increasing levels of barium chloride with time -- something that was also seen in all three of the filled cells. Barium chloride is one of the chemicals they took along for titration. Kounaves said they don't understand why or how yet, but it seemed that barium chloride was slowly leaking from within the WCL instrument into the wet cells. Obviously this affects their chemical analyses in the other cells. Kounaves put a positive spin on this, saying that it actually led to their being able to constrain which sulfate compound was present in the soil to being magnesium sulfate.
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