June 2, 2005

Spirit, The Problem Child

In their explorations of Mars, both the Spirit and Opportunity rovers found evidence that liquid water was once on the planet's surface. Joy Crisp, project scientist for NASA's Mars Exploration Rovers, discussed the rovers' long journey and their surprising discoveries at a public lecture on May 19, 2005. In this contribution, Crisp explores why the Spirit rover might be called a problem child.

This edited transcript of the lecture is Part 2 of a 4-part series.

Astrobiology Magazine -- "Spirit is the problem child, so to speak. It took us a lot longer to reach our science objectives with Spirit, but it's really coming into its own right now.

In the first three months, all that Spirit encountered was volcanic basalt lava. Here we were, trying to find lakebed sediments or river sediments, some clues to past liquid water. We found evidence for little trickles of water, clues that tiny amounts of water had gotten into these volcanic rocks, but we weren't finding any evidence of a past environment that would have been favorable for life. At this point at Bonneville Crater, we saw that the rover was still in good health, so we made the bold attempt to try to drive to the Columbia Hills.

We weren't sure we would make it. We decided to drive as fast as we could, and five months after landing, on martian day 156, we made it to the base of the Hills. We immediately found a very different kind of rock that had signs of past liquid water.

There's a diversity of rocks in the Hills. They range from grey to orange, which is telling you how oxidized or altered they are, how rusty they are. You see some rocks that look massive and smooth, and some that look corroded. This might be due to variations in water alteration.

The "smoking gun" evidence for liquid water was found in the rock Clovis. This rock was soft compared to the basalt lava rocks out on the plains. If you've ever been to Hawaii, the basalt lava there is very hard. It has been difficult to grind into this sort of rock on Mars.

The results from the Mössbauer spectrometer told us that the mineral goethite was abundant in this rock. Goethite's formula is FeO(OH) -- it contains OH, which only forms in the presence of liquid water. That water may have been groundwater alteration"¦it may not have been flowing on the surface. We aren't sure.

We have argued about all the rocks in the Hills, as to whether they are sandstones laid down by liquid water, or volcanic eruption ash tuffs or airfalls, or whether they were formed by impacts.

We found pyroxene, olivine, and magnetite, which are volcanic minerals, but we also found hematite, goethite, and nanophase oxides, so it's like a volcanic rock that has been altered, or volcanic rock that maybe got transported by water, or impacted.

There is some layering in these rocks in the Hills. We've been trying to test the hypothesis as to whether these layers were put down by water, one layer at a time, or if they represent volcanic eruption blasts, one blast at a time, or one impact event at a time. We haven't been able to figure that out yet.

One of the unusual things that we've found in some of the rocks in the Hills is that, in the infrared, we get a signature that is similar to fresh volcanic glass. This has been puzzling to us because we're finding that in conjunction with this is highly altered, oxidized minerals like the goethite and the hematite. The chemistry is showing high bromine sometimes and high chlorine and high sulfur, which aren't in normal volcanic amounts.

So how could we have fresh volcanic glass in the rock? One possibility is that instead of fresh volcanic glass, maybe there is impact glass in here - minerals that were impacted and shocked and converted into a glassy phase. Or, maybe they're so altered that there's some kind of cruddy, amorphous phase that was formed when the water affected these rocks - we don't know yet.

I hope you're getting a sense of how puzzling these rocks have been. If we're going to get an answer, we'll probably get it from the textures of the rocks. As to whether they are sandstones, sedimentary, laid down by water, whether they're volcanic airfall deposits that came out of an eruption column and landed where they are, or whether they formed by impact blast, when an incoming asteroid hit Mars and broke things up. Or it could be a combination of some of these processes. That could be why we're so stumped. It's hard to sort things out when there was more than one process going on.

But we're very happy because we are seeing signs of past liquid water. Even if this was groundwater and the rocks were just soaked by it, that does represent an environment where life could have possibly existed.

You can walk for very long distances on the Earth and keep encountering the same kind of rock. When you send a rover to Mars, you don't know if you're going to have the same thing happen. So a great thing about Spirit is, having finally made it to the Hills after that long drive, we are now encountering, over very short distances, striking changes in the chemistry and the minerals that we're finding in the rocks.

There are also some signs in the soil of the action of liquid water. One place where the rover wheel had disturbed the soil was very white. We started taking measurements, and there was a striking amount of salt. I'm not referring to table salt - NaCl - but to sulfate salt. This was iron sulfate salt, and it was the first time we've seen iron correlating with sulfur.

This is a hydrated iron sulfate, so this was our second "smoking gun" evidence indicating past liquid water. The soil is about 50 percent salt, so water must have been percolating in this soil, soaking it, and these salts precipitated out of it. Or that happened somewhere else and they were transported here and formed the soil.

When Spirit climbed to Larry's Lookout, and looked back at where we had come, we saw layered rocks that were connected with one another. We saw that these layers were dipping at about a 20 degree angle, at about the same dip as the slope of the hill we had come up. These exposed layers, to a geologist, are very exciting, because you can follow the layers at great length to see if they change, and you can tell what's younger and what's older.

We're about halfway up to the top of the Hill, but we found such good stuff right where we are that we're spending some time here, looking at these rocks before we continue our way up. We're exploring, so if we see something that looks good we might head off into a different direction. We may change our minds as we go."

Part 1: Mars: Windows on the World


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