Spirit Sol 169

Driving Spirit is about to get harder. Up until now, we've been spoiled: we've been driving a vehicle that does exactly what you tell it, in the exact kind of terrain it was designed for. That's about to change. Not only is the right front wheel in danger of seizing up, which will make Spirit much less maneuverable and much less predictable, we're also about to start driving uphill, which means we'll be slipping more. No more 100m drives for us: we'll be inching our way up along "fire roads," shallow-sloped, relatively rocky paths that generally take the long way uphill. (Thanks to slip, more direct but steeper routes would actually require us to spin our wheels more times to get to the same location.) Increasing the difficulty, we'll also be trying to find a route that keeps the solar panels aimed toward the sun.

Fortunately, the Opportunity team's experience in Eagle and Endurance craters gives us some useful data for driving on slopes. Because of their situation, Randy Lindemann has already done a whole bunch of slip testing, producing graphs that show how the rover slips on various slopes, from zero up to 20 degrees. His results match up well against Opportunity's actual performance on Mars, making them the baseline for our expectations. So we Spirit rover drivers (sans John) sit in a meeting, talking over the results with Randy, trying to figure out how we're going to drive this rover under the new conditions.

I've decided to look at this as a challenge -- it's like we have a new vehicle and have to start all over learning about it. Pity it's not as good as the old one, but what are you gonna do?

Or they could take my suggestion, which I make half-jokingly: why not swap the Spirit and Opportunity rover-driving teams? There's a move afoot to pull Opportunity back out of its crater, which means they'd go back to driving on flat terrain. Why not let us deal with the flat-terrain driving we're used to, and let them deal with slipping on slopes, as they're used to?

I don't think it'll happen, though.

As it happens, Randy is also doing some slip testing this afternoon in the Mars Yard. I still have about an hour before I'm on shift, so I go check it out. They've set up a ten-foot-tall ramp, held up by a forklift and supports, which they can (laboriously) adjust to various angles. On this ramp they've placed a model of the rocky lip Opportunity is perched in front of, and they're testing whether the rover can drive over the lip and get back out. I have to leave before I see whether they succeeded (although the first test looks promising). But I do get to see a NOVA crew hanging around -- apparently they're doing a follow-up show on us. I'm looking forward to seeing it. It's fascinating to see events in person and then see how they're portrayed on TV; it sort of calibrates you. You also get to see how people you know react to the cameras -- who shies away, and who seeks them out.

When I get back to the SMSA, Jeng tells me the drive executed successfully. PotOfGold is RATtable at last!

But before we start drilling holes in it, it's time for a science lecture. Happily, the science team has agreed to resume these. The first one -- today's -- is given by Squyres, who uses the time to bring everyone up to speed on what's happened since the lectures were suspended (around the end of the nominal mission). He does it off the cuff ("No slides, no PowerPoint, no headsets, all analog"), and of course does a terrific job.[1]

He starts by reviewing the history of our explorations in Gusev -- Spirit's stuff. The whole point of landing at Gusev was to try to find sediments laid down by water, which (we think) flowed in through what is now the Ma'adim Valley. So we went hunting for sedimentary rocks, but found only basalt. All the rocks were identical -- interesting rocks, but not sedimentary ones.

"Mars," he says, "had faked us out. The sediments were there, but they were buried by lava." So we went to Bonneville, which should have punched a nice hole through the basaltic stuff into the underlying sedimentary rock, but all we saw when we got there was more basalt. Such a disappointment!

The only remaining prospect for anything fundamentally different was the Columbia Hills, 2.5km away. This was much farther than the rovers were supposed to be able to travel, but what else could we do? So we laid out a plan to get there by sol 160, and made great time.

There was one surprise along the way. We stopped long enough to dig a couple of trenches, in order to ground-truth the results from Odyssey's gamma-ray experiment (which Steve is also a member of). The gamma-ray experiment sees 20-30cm below the ground, and the trenches see maybe 10cm down. As it turns out, this was just the right depth to see something that can't be seen from space: highly elevated levels of sulfur and correlated elevated levels of magnesium, peaking about 5cm down. The reason they care: the only plausible explanation for this combination is a reaction between sulfuric acid and the basalt. The likely model is that Mars's groundwater was dilute sulfuric acid, which reacted with the basalt rocks in Gusev's floor over a long period. This isn't proof of large amounts of standing water -- it might have just been just a kind of watery sulfuric acid fog, reacting with the basalts over three billion years.

This finding helps ties the Gusev and Meridiani sites together. Small amounts of water on this side of the planet, large amounts on the other side, producing different results in the two locations. (At Meridiani, the evaporating water left behind large amounts of many different kinds of salts.)

So now we're at the hills. "It would have really sucked to get all this way, only to find the same old stuff," he says. Happily, that's not what happened. Upon our arrival at the base of the hills, we immediately found something morphologically different -- PotOfGold (née EndOfTheRainbow), the rock that's been such a pain to investigate. As Steve describes it, PotOfGold looks like a potato with toothpicks stuck in it, and jellybeans stuck on the ends of the toothpicks.

This shows differential erosion, a rock made of different materials of varying hardness being weathered over time. But what it's made of, we're not sure -- except that we're sure it includes hematite, revealed by the MB. That's our first sign of hematite -- more proof of water -- at Gusev. Oddly, the MTES doesn't show hematite in this rock, but there might be an explanation for that. Hematite comes in different forms, and some small-grained forms of hematite can be visible to the MB but not the MTES. So maybe that's what we're dealing with.

The rock will be a challenge to RAT, but the results will be worth it. ("It's so far outside the design specs of the RAT, it's not funny," Steve says. "But what the hell, it's sol 168, so let's go for it.") For one thing, they'll tell us whether it's rich in hematite all the way down, or only on the surface. The rock also shows signs of elements commonly deposited by water (though not olivine). The conclusion, Steve says, is that the rock started as basalt and has seen water somewhere along the way.

After RATting this rock, we'll find out what the hills are made of. It'll be the first Martian mountaineering expedition.

Then Steve starts relating the progress on Opportunity. At Eagle Crater, Opportunity was looking only at the very top layer of the geological record of Mars -- "like a book with all but the last chapter torn out."

They suspect there's a layer of soft, loose basaltic sandstone below the top layer. There are lots of impact craters, but no ejecta to speak of. But there is a ton of basaltic sand. That suggests that the asteroid impacts simply pulverize soft rock. (This is in contrast to Gusev, where the asteroid impacts throw out big chunky rocks like Humphrey.)

But they don't see this layer at Eagle Crater, because it's not deep enough. So they went to the much larger and deeper Endurance Crater in search of the basaltic layer. In contrast to Bonneville, they found what they were looking for. They wanted to jump right in and start measuring, but first we had to set up an extensive test program to evaluate the safety of entering the crater. So while that was going on, they had Opportunity start running a lap around the crater, taking pictures as it went. Once they worked out how to do it, they dipped their toes into the crater, using the RAT, MB, and APXS to produce a stratigraphic survey (the first such survey of another planet).

So far, the survey is showing lots of sulfates, not much basalt. They've also demonstrated that the evaporites go down for meters, which implies that there was lots of water here.

At this point I'm interrupted to help out with a minor problem they're having on Opportunity. It turns out to be a non-issue, and by the time I get back, the lecture is pretty much over. Ah, well. It's time for work anyway.

Thisol's emphasis is on the IDD, specifically the RAT. We're going to RAT PotOfGold or know the reason why. PotOfGold is a small rock in an awkward spot, maybe just barely large enough to fit the RAT on. But we're going for it; we're at least trying everything we can do. Besides -- as I point out at the CAM -- with all the trouble we've gone through to get to this stupid rock, seeing that RAT hole tomorrow is going to be so satisfying.


[1] I recently took a memorable trip to the Scott Joplin House in St. Louis -- his former residence, now a museum to the famous ragtime composer. It struck me that there were people alive at the time who could sit in a room with Joplin and listen to the master play music for fun. Listening to Squyres give an impromptu science lecture is exactly like that, just exactly like that, and don't think for an instant that I don't know it.

No comments: