Mars: Windows on the World
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.
Astrobiology Magazine — 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 1 of a 4-part series.
“In January 2004, Spirit and Opportunity landed on Mars. In May of 2004, four months into the expected three month mission, Nagin Cox gave a talk in this lecture series on the status of the rover mission.
And now, we are a year beyond that. I’m thrilled that I can come here and tell you that these rovers are still working on Mars, and that we’ve had a whole year’s more worth of exciting discoveries.
I’m also amazed to see how many people are in the audience here tonight, and not at the new Star Wars movie.
Mars today is a harsh place for life. It’s cold, there’s very little atmosphere, and what atmosphere is there is mostly carbon dioxide (CO2). Liquid water is not stable on the surface today. The water that’s there is either in the form of ice or vapor.
Life as we know it requires liquid water. As you’ve probably heard, we are interested in whether life ever got started on Mars.
We had some evidence from orbit that there was liquid water on the planet in the past. We’ve seen valley networks that were either carved by rivers similar to what we have on the Earth, or by near-surface groundwater sapping. We’ve also seen very intricate, fan-shaped delta deposits on Mars that are very similar to Earth’s delta deposits.
So we sent the two rovers to two different places on Mars, on opposite sides of the planet. We picked two landing sites that had good evidence, from orbit, that there should have once been liquid water there on the surface. To be sure, we needed to go to the surface to see if there had been liquid water in the past, and to see if we could find clues in the rocks that would tell us whether it would have been a favorable environment for life.
We sent Spirit rover to Gusev Crater, where the evidence for past liquid water is in the landforms. From orbit, we saw this long sinuous dry river bed leading up to a crater. We thought water must have flowed through this valley and ponded in that crater, leaving lakebed sediments in that crater and possibly also river sediments. We wanted to send a rover there because the floor in the crater was nice and safe for landing, and we thought we might have a chance of finding water lain sediments.
We sent the other rover, Opportunity, to the plains of Meridiani, which is a nice smooth region, also very safe for landings. We had spotted the mineral hematite from orbit, using the Mars Global Surveyor’s Thermal Emission Spectrometer. Hematite is iron oxide, Fe2O3. On Earth, hematite often forms in the presence of liquid water – not always, but often.
The mapped region on Mars showing hematite was a very large region, almost the size of Oklahoma. In high resolution images, along the edges of the flat smooth area, we saw layers in the rocks. We wanted to check to see if the hematite had been deposited by liquid water as a stack of rock layers. We have similar analogues to this sort of thing on the Earth.
Doing field geology is a lot like doing detective work. We are hunting for clues, trying to put the story together to build a case. In this example, we are trying to determine how these rocks formed. Was liquid water around when they formed, or later on when they sat around near the surface of Mars?
There’s a TV detective show called “Monk,” and the boss in the show once said, “How does he do it? I have two eyes, I see everything he sees. But I don’t see everything he sees.” The same kind of thing happens when a scientist looks at a rock. Scientists can look at a rock and say, “Wow! Isn’t that amazing?” Then you look at it and say, “Well, there’s some scratch marks and bumps on it. I don’t know what that means.” You don’t see the same thing as a trained geologist, who has studied geologic processes and learned to read the clues.
And so what I’m going to do in this talk is give you some feel for the kind of clues that we look for, and then what we have found with the Spirit and Opportunity rovers.
The main objective of our mission was try to find evidence of past liquid water and try to figure out what was it like when the water was around. One kind of clue to past liquid water is textures in rocks. We can look for that with camera images taken with the rover.
We knew before we got there that we wanted to look for things like ripple marks, rounded grains that looked like they were cemented together, crossbeds, and rock layers that were at angles to each other. We see those kinds of things on the Earth when rocks are laid down by water.
The other kind of clues that we wanted to look for are specific types of minerals that can only form in the presence of liquid water. Either we know they have precipitated out of liquid water, or they’re hydrated and actually contain bound water (OH or H2O).
A few examples from the Earth include calcite, dolomite, clays, salts, iron oxides that are hydrated, and sulfates that are hydrated. So we outfitted our rover to try to look for these things.
Another clue is to measure the abundance of the different elements. So we wanted to keep an eye out for differences in chemistry that we might expect if water was around, to look for higher abundances of elements that are easily dissolved in water and can then be re-precipitated in rocks.
The first thing that a field geologist does when they go out in the field is survey the scene, looking around at the landforms and the shapes of the rocks. The rovers can also survey the scene, because cameras are at the top of a mast that is about five feet off the ground. The two science cameras have the clarity of 20/20 vision. So the pictures we get back are very much like what it would be like if you were there, standing on Mars.
The mast can rotate, so we can get a complete 360 degree panorama, we can point it upwards and downwards, command it to look where we want it to look, and then take pictures from blue to green to red to the slightly near infrared – we can choose which filters to put in front of the eyes– and with red, green, and blue we can put them together and form the color images that we put out on the web.
Also at the top of the mast we have a thermal infrared emission spectrometer. You can think of infrared spectra as little squiggle fingerprints that help us figure out what minerals are present. We compare the infrared spectra from Mars with lots minerals we have measured on the Earth. So we can look at the rocks and soil around us, just as a field geologist would look around and say, “I think I see different kinds of rocks.” We did that kind of survey with the rovers, to see where the different types of materials were, and to see what looked like the best places to go for clues to past liquid water.
A field geologist would walk over to something of interest. Because the rovers have wheels, we can send them to specific places, where they can investigate by using instruments on the end of their robotic arm.
If you know any field geologists, they always carry a rock hammer out into the field. The reason you want to break open rocks is so that you can see the textures inside. The outside of rocks, especially on Mars, can be dust covered, and they also can have alteration coatings on the outside as they interact with the atmosphere and soil.
So we put a grinding tool on the end of the robotic arm that clears away a spot, grinds into the rock, and gives us a window into the rock – just like a rock hammer would do.
The other thing a geologist always carries with them is a hand lens to look at shapes of grains, to help them identify what kind of minerals are there and to look at the texture of the rocks. So another thing that we put on that robotic arm is a microscopic imager, which gives us a close-up view of the rocks.
We have two other instruments on the rovers that are extras that geologists don’t normally carry into the field — we usually have to come back to a laboratory with rock samples to take these kinds of measurements. We put on a chemical analyzer that gets us the abundances of the different elements in the periodic table. We also put on a Mössbauer spectrometer that tells us which iron-bearing minerals are in the rocks and soils, and how oxidized, or how rusty the rocks are. Mars is rich in iron, so it’s a useful instrument to have.
With that assortment of instruments, including hazard cameras and navigation cameras for driving, we are equipped to go out and do some really exciting science.”
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