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The Planetary Society BlogBy Emily Lakdawalla
Jan. 9, 2009 | 16:14 PST | Jan. 10 00:14 UTC One last plea for donationsI haven't posted about my fund drive in a bit because I figured people would be preoccupied with December holidays. Donations have matched the $600 challenge issued by the unmannedspaceflight.com moderators, for which I thank you all very much. In total, we've made a few thousand bucks, which will help support the work that I and the other web staff do to keep the information flowing on this website, but of course it's a long way from paying for our costs; we still rely on the constant generosity of our loyal members, whose annual dues and occasional donations keep the Society going. And it is primarily dues and donations that keep us going -- no government funds, and not much in the way of grant funding here; it's individuals joining together in a tens-of-thousands-strong association of space enthusiasts who drive the Society forward, and who are our reason for existence. We here in Pasadena are here to serve as a voice for all our members. So I'm going to make one last plea for you to donate in whatever amount you think my efforts are worth, whatever you can spare, and I'll wrap up the fund drive next week. Thank you, again, to everyone who's donated already, and thank you also for your patience in putting up with my begging! Jan. 9, 2009 | 16:02 PST | Jan. 10 00:02 UTC The wind blows rocks on Mars?There's a press release that's making the rounds today that neatly explains the regular spacing of rocks on the plains on Mars. Everywhere we've landed spacecraft on Mars, they arrive on relatively flat surfaces that are covered with rocks. This in itself is odd. On Earth, you mostly see rocks where there are steep slopes. Where things are flat, you mostly don't see rocks (with the notable exception of places where there have been glaciers). It has to do with how much energy it takes to transport rocks. When you have steep slopes, gravity does most of the work in transporting rocks around; flowing water helps things along. Once the slope levels out, though, gravity doesn't work to move rocks anymore, so it's rare to see large rocks in a flat area. On Mars there are rocks everywhere. The difference is that Mars' landscape is shaped in large part by impact processes. Far-away impacts can toss rocks for miles, and they fall where they land. So it's not particularly surprising that you see rocks everywhere, even in flat places on Mars. What is a bit surprising is their even spacing. Here's an example of a rock-strewn landscape selected more or less at random from the early part of Spirit's mission, when it was dashing across the flat plains to the east of its landing site toward the Columbia Hills.
Apparently it's been suggested that wind actually pushed the rocks around, though I find that notion just a little hard to believe. Remember the air is a hundred times thinner on Mars than it is on Earth. Anyway, we don't need that explanation, because Jon Pelletier and coworkers have suggested a more elegant explanation. It does involve the wind, but the wind doesn't move the rocks, at least not directly. What the wind does do is lift sand; sand particles jump (or "saltate") along the ground, knocking into each other and launching more sand particles. When the wind runs into a rock, it loops and whirls, scouring the area right in front of the rock. Over time, it digs a pit in front of the rock. At the same time, the sand that was scoured from in front of the rock gets deposited in the wind shadow behind the rock. Do this for long enough, dig a steep-sided enough pit, and one random day the rock will tip forward, rolling in the upwind direction, into the pit. Rinse and repeat, and you get rocks trooping across the landscape over time. (The press release didn't give a time scale for this process.) That explains how rocks can move, but how does it explain an even spacing? Well, according to the release, when you have a cluster of rocks, "those in the front of the group shield those in the middle or on the edges from the wind, Pelletier said. Because the middle and outer rocks are not directly hit by the wind, the wind creates pits to the sides of those rocks. Therefore, they roll to the side, not directly into the wind, and the cluster begins to spread out." Neat. It can't be a fast process. But just think: it's still windy on Mars today, so it's probably a process that is going on today. And think about how much windy plains there is covering Mars, and how many millions (billions? more?) small rocks there are on those plains. Let's say that for any given rock, there's only a one-in-a-billion chance that it will tip over in any given second. If there's a billion rocks on Martian plains, that'd be one rock tipping over every second, all over Mars. Mars may look dead, but it's in constant motion! Jan. 8, 2009 | 21:48 PST | Jan. 9 05:48 UTC The Santorini panoramaA tip of the hat to Ryan over at Martian Chronicles for posting this lovely version of the Santorini panorama, which Opportunity captured just before Mars dipped too close to the Sun in late November of last year. If you click to enlarge you'll get a nice view of some large sand ripples that have wandered over the odd, flat bedrock of Meridiani Planum. Actually what struck me about that rock in this image was that in many places, that bedrock isn't perfectly flat; slabs of rock are tipped up here and there, poking above the otherwise flat surface. I wonder why that happens?
Anyway, if you're wondering how a Mars rover can have a hangover, here's what they mean. Opportunity went in to solar conjunction with a lengthy list of things to do, mostly weather measurements and long-duration observations of the composition of a cobble called Santorini with the spectrometers on the end of its robotic arm. This was after it had acquired the images for the panorama above, but before it had a chance to send all that data back to Earth. During the few weeks of conjunction, there was no way to send any more science data back to Earth. When Opportunity had its first communications pass with Odyssey after conjunction was over, it revealed that it had 6,448 different data products in its onboard memory, a worrisome number. It's not that there were too many bits of data, filling up storage -- Opportunity wasn't near that limit. The problem is that having too many different files can trigger a pretty horrible fault on the rovers, the same fault that crippled Spirit for a couple of weeks beginning on sol 18 and which Mark Adler has written about here in the past. The update said that the potential for the sol 18 fault to happen "becomes a concern when the number of on-board data products is greater than 6,000." 6,448 is way over that limit. Eek. Opportunity had to be nursed carefully through the subsequent couple of days in order to return that valuable conjunction data to Earth, verify its receipt, then delete onboard data products, all without accumulating many more onboard products. It's fine now though. Jim was a little sheepish about causing Opportunity that post-conjunction hangover, but I will say he has a lovely panorama to show for it! Jan. 8, 2009 | 19:01 PST | Jan. 9 03:01 UTC The Antarctic Deep Field Is Not for the ImpatientI've posted another week plus worth of updates on the 2008-2009 Antarctic Search for Meteorites expedition, in which the bloggers seem to be growing increasingly punchy as they spend more time in tents far from civilization without such amenities as showers or warmth or dark nights in which to sleep. They seem to be keeping their spirits up though, and when the weather gods are kind to them, they have productive meteorite-gathering days. I am also really enjoying their device of ending each day's post with a summary of what Shackleton and his poor team were suffering through on the same date, one century ago. The latest updates are posted on two pages here: The Antarctic Deep Field Is Not for the Impatient 237 Meteorites Await the Long Drive to Houston
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