Driving in this stuff is an interesting challenge. We are literally in a maze. The easiest going is to stay within the troughs between the ripple crests, which run roughly north-south. So the best way to make southward progress is to follow a trough until it peters out, make a "lane change" to a nearby trough, and continue onward.

But then there's the issue of the "Erebus Highway". This is a stretch of light-toned terrain that we've seen from orbit, and that we're guessing may have some exposed bedrock in it that'll make for easier going. We also like the idea of getting to bedrock from a science perspective... it's been a long time since we've looked at any rock with Opportunity.

But there's a catch. The entrance to the Erebus Highway is not straight south of us, along the troughs... it's a bit off to the east, on a heading closer to 160 degrees or so. That means that in order to get to it, we'd have to head off cross-country, going up and over quite a few ripple crests. That's do-able, but it's also a lot slower than just bombing straight south down a nice trough.

So: If we do head for the highway, we make slow progress for awhile, but maybe we get to a better route to Erebus Crater. If we head down the troughs instead, they take us at a pretty decent speed directly to... Erebus Crater.

So it should work either way. We're planning the weekend drive as I type this, and after a bit of discussion we've decided to stay for now in this nice wide trough that we've found, and try to get as much southward distance within it as we can. Whether or not we're actually going to "hit the Highway" is an open question at this point, but Erebus isn't too far off either way you cut it.

And have you noticed how the terrain seems to be changing as we get south of Purgatory? Fewer tall dunes, more pebbles in the troughs, even what might be tiny outcrops of bedrock? I don't want to jump to conclusions, but the driving definitely looks a little nicer here than it did a hundred meters back.

Over at Gusev, Spirit has had a great couple of weeks. During our ascent of Husband Hill we stumbled fortuitously across a very cool outcrop of layered bedrock that we named Independence Rock. We've thoroughly worked it over with all of the arm instruments now, and it's very strange stuff... one of the oddest things that we've seen at Gusev. It's not like Peace and Alligator, and it's not like Methuselah either.

I'm not ready to go into much detail here about the chemistry and mineralogy yet, since we're still chugging through the data. But it's clearly highly altered, and it's got an unusually low iron content, which isn't something we've seen much of before.

The low iron is interesting and very cool, but it also slows us down a bit. Our Moessbauer spectrometer tells us what iron-bearing minerals are in a rock, and the more iron there is, the stronger the Moessbauer signal.

So when the iron content is low, like it is in Independence, the Moessbauer signal from the rock is weak. And when the signal is weak, we have to take a longer measurement to be able to get the information we want from the rock.

The other thing that's making Moessbauer measurements tough these days is that the radiation sources in the instruments are getting pretty weak. Each Moessbauer instrument has a tiny nugget of radioactive cobalt-57 in it that we use to irradiate the target with gamma rays. The half life of cobalt-57 is 271 days... in other words, every time 271 days goes by, the strength of the source is cut in half. Back when we thought the mission was going to be 90 days long, this wasn't a big deal.

But we have now been on Mars two entire cobalt-57 half lives, which means that our Moessbauer radiation sources are now only one-quarter the strength that they were when we landed. Combine that fact with the low iron content in Independence and we had to do a Moessbauer measurement on it that was four sols long.

We're done with Independence now, and we've resumed the climb. The ground is real solid here, and the climbing is good. I still don't know if we'll reach the summit or not, but the recent progress has been excellent. The coming week may reveal a lot.

Source: Steve Squyres Principal Investigator Mars Rovers

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