Opportunity’s Escape from Dune

Empty nest view back to landing petal from the mobile Opportunity rover, which has ventured to the crater’s rim
Credit: NASA/JPL

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.

This edited transcript of the lecture is Part 3 of a 4-part series.

(Part 1 * 2 * 3 * 4 )

“Opportunity rover landed in Eagle crater, and we immediately found an exposed rock outcrop. We spent a lot of time working up and down this outcrop, trying to read the record in the rocks. It was very unusual looking rock — finely layered, with these slightly bluish gray spheres the size of peppercorns that we nicknamed blueberries. They were all over this soil, and when we finally got a close-up look, we saw the blueberries were also embedded in the rock.

Our chemical analyzer showed high amounts of sulfur, chlorine, and bromine. Also, the sulfur correlated strongly with magnesium, and somewhat with calcium. The clues told us that this rock was very rich in sulfate salt.

The Mössbauer spectrometer found its “smoking gun” evidence for liquid water right away. The first clear clue of liquid water found by Opportunity was the mineral jarosite. This mineral, packed with OH, can only form in the presence of liquid water. It’s basically a hydrated sulfate.

The other thing about jarosite that’s very special is that it only forms in acidic water. So, it was telling us something about the conditions of the water when the water was there.

Click image to enlarge overhead view of Endurance Crater Credit: Mars MGS/NASA/JPL

We studied the rocks in that outcrop very carefully, took lots of microscopic images, put them together in mosaics, and then we traced the faint lineations, the curved lines that we could see in this rock. The sedimentologists on the team compared these lineations against what we see on the Earth, in rocks that are deposited by wind, water, and volcanic blasts. It turned out that liquid water had to have made the shapes and geometries of these curving lines. So that tells us that these rocks were deposited in flowing liquid water on the surface of Mars.

After the rocks were deposited, they had additional water soaking. It wasn’t enough that they formed in flowing liquid water, these rocks continued to be soaked in groundwater, and formed these little blueberries in the rocks that are rich in hematite. These concretions form when you soak rocks that are porous, and the iron precipitates out and forms these hard spheres.

There are also holes which were probably made by minerals easily dissolved in water which first precipitated from water and then later may have also been removed by water. So, there were very good signs that this rock was soaked in water for quite a bit of time. Just so you know I’m not making this up, these are rocks in Utah that are similar, layered sandstone with iron-rich concretions that formed by the same kind of mechanism.

A feature called “Burns Cliff”, part of the rocky outcrop in Endurance Crater.
Credit: NASA/JPL

After we finished with Eagle crater, we popped our heads out and said, where next? Looking at the orbital images, we decided we wanted Opportunity to go to Endurance crater, a big, 150 yard diameter crater, one and a half times a football field length.

In the orbital images, Endurance crater has a white rim. Our geologic hypothesis was that we might find the same kind of rocks that we saw in Eagle crater in the rim. We might also find older rocks as we looked down into that crater.

We took measurements of the rock at the top of the rim with our robotic arm, and sure enough, it was the same kind of rock we saw in Eagle crater: finely layered, with jarosite, that mineral that can only form in water, high sulfur, high chlorine, high bromine. They are sedimentary rocks deposited in flowing liquid water, evaporite-containing sandstones.

We went down into the crater, and it was a fairly steep slope: from 18 to 30 degrees. So it wasn’t easy, and the hardest part was making sure we could get back out. There was much more of an exposed rock record here than in Eagle crater. We did a very careful job of checking out the layers as we went down and ground 11 holes along the way, and took close-up microscopic images of them. We did see differences in texture, although essentially the rock all the way down was the same kind of rock. It had the same kinds of minerals, with subtle differences in texture, and some not-so-subtle changes in chemistry.

Rover computer rendering on the edge of a depression, much like Opportunity’s perch on the edge of Endurance Crater.
Credit: Maas/NASA/JPL

Wind deposited a lot of the rocks in Endurance crater. The good news is, they’re full of evaporite minerals, minerals that precipitated out of liquid water somewhere else, in water that was very salty, forming rock very rich in sulfate salts and jarosite. So there is a story here of a lot of liquid water on Mars, in an area the size of Oklahoma. That’s a good thing if you’re looking for environments that could have been favorable for life.

Now, both here and at Gusev crater for Spirit, there are some obstacles to being a really favorable place for life, if life ever got started here. And one of these is water chemistry. Both here and at Gusev, the signatures that we see are that the waters were acidic. Now, on the Earth, we do have microorganisms that have adapted to those kinds of environments and can thrive. So that doesn’t rule it out, it just makes it a bit harder for life to exist, and it may be a difficult environment to start life.

The other thing is that both places are salty, with these sulfate salts. And that can be difficult also for life, but not impossible. We have very many examples on the Earth of microorganisms that can thrive in salty waters.

There is also an issue with the persistence of water at Meridiani, where we see sand dune and sand sheet deposits. When those were being deposited, were times when water wasn’t around. So the water may not have hung around continuously, and that also might have made it difficult for life.

Mars Clouds
Seasonality over Martian sols

After Opportunity visited Eagle crater, we headed south. The orbital view of this terrain shows some ripples, and as we headed south the ripples get bigger and bigger. Finally we encountered an extremely large dune 30 centimeters tall that happened to be right in a place where the terrain stepped up about 25 centimeters. We had been happily commanding the rover to drive a certain distance, and the wheels plowed into this deep dune, and the rover thought everything was fine. “Hey, my wheels are turning, I’m moving, I’m making progress.” Then we got the pictures back, and said, “Oh my goodness, we’re not where we thought we were. And look at the wheels, they’re almost completely buried.”

We didn’t want to do something that would get us into worse shape, so we had to engage in a crash test program in the testbed with our Earth rover, and try to simulate the conditions with the kind of soil that we were seeing, with the soil caking up on the wheels and making the treads disappear.

We’ve done about five movements on Mars, and have gone about five inches. So we are not stuck, we just have to get out slowly so we don’t get into more trouble. We’re making very good progress even though it’s just an inch at a time. I’m sure we’ll get out of it. When we do, we want to try to head south, going from Endurance crater down to the much bigger Victoria crater.

The terrain heading down toward Victoria crater looks brighter in the orbital images, and it looks somewhat rougher. It is possible that this brighter color is bigger dunes that have fine dust on them that give them a brighter color, and we’ve started encountering that. If it becomes too difficult to drive through we might choose another path to take us to Victoria crater. We don’t know how long it will take us to get there, so stay tuned.”

Related Web Pages

JPL Rovers
Spirit’s Sol images and slideshow
Opportunity image gallery and slideshow
Mars Berries Once Rich in Iron-Water
NASA’s RATs Go Roving on Mars

Water Signs
Microscopic Imager
Gusev Crater
Pancam– Surveying the Martian Scene
Mössbauer spectrometer
Alpha Proton X-ray Spectrometer