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The Planetary Society Blog

By Emily Lakdawalla




Two Earth Years on Mars for Spirit, and a View of El Dorado

Jan. 4, 2006 | 18:50 PST | Jan. 5 02:50 UTC
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Although I feel like the first Mars year on Mars is a more significant cause for celebration, it is fun to mark Spirit's second Earth year on the surface. I heard several reports on National Public Radio about the milestone yesterday, and they reminded me of the drama in Building 264 at JPL when Spirit landed. It was an exhilarating experience -- seeing those pictures from Mars felt like I had been blind before, and now I wasn't -- at least in my mind's eye of Mars. Spirit was no longer a potential, it was in a place, a place that was unique among the other Mars landing sites, not because it was so different from Viking or Pathfinder but because I was there to watch it! Perhaps that's a self centered view, but I think it's an important truth about space exploration that people generally feel more emotionally connected to missions that they feel like they participated in.

Anyway, the New Year and the 2nd anniversary on Mars brought Spirit at last to a really interesting place, which the science team has taken to calling "El Dorado." El Dorado is glaringly visible on any orbital photo of the Spirit landing site. It is an incredibly dark-looking splotch on the side of the Columbia Hills. You can see it in this Mars Orbiter Camera view of the area that was captured on November 3:

MOC cPROTO image of Spirit high in the Columbia hills
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MOC cPROTO image of Spirit high in the Columbia hills
This detail from a MOC cPROTO view of Gusev crater was captured on November 2, 2005. Based on before-and-after comparison with earlier images, MOC scientists have identified a tiny dark spot to be Spirit, near the peak of Husband Hill. The dark splotch below Spirit's position is the dune field known as "El Dorado." Source Credit: NASA / JPL / Malin Space Science Systems
Once Spirit peaked Husband Hill, the rover drivers steered it toward this spot, and for a while it remained a black shadow on the flank of a distant hill. But as Spirit got closer it became clear that it was a strange rippled dune field. The people who enjoy watching Spirit's mission progress by downloading all of the raw images that are posted to the rover website tried to make panoramic views of the place, and this is what they came up with. Gorgeous -- but definitely very unreal looking.
El Dorado, Gusev Crater, Mars
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El Dorado, Gusev Crater, Mars
This panorama of El Dorado was produced by enthusiast Doug Ellison from the raw images available on JPL's Mars Exploration Rover website. Credit: NASA / JPL / Cornell / Doug Ellison
Finally, yesterday, the Cornell imaging team helped us all out by releasing the same panorama, only created from carefully color-calibrated data to show what the place would really look like if you were standing on Mars.
El Dorado, Gusev Crater, Mars
Click to enlarge >
El Dorado, Gusev Crater, Mars
This panorama from the Mars Exploration Rover Spirit was captured on sol 708 and 710 while Spirit was exploring a site named El Dorado. El Dorado is a field of dark, rippled sand that is visible from space as a dark spot on the side of the Columbia hills. You can download the full resolution view from the Planetary Photojournal (1.6 MB). Source Credit: NASA / JPL / Cornell
If you're used to looking at Spirit images, the El Dorado dunefield definitely has a different color to it -- it's a little darker and a little grayer than your typical Gusev dust-covered surface. But it's not as different as it looks in those raw images or from orbit. What gives?

This difference points out a pitfall in doing too much interpretation of the raw images that we get from the rovers and Cassini and other missions. They're pretty good for seeing morphology, that is, the shape of the terrain. El Dorado looks like a dunefield no matter what color it is. But they really are lousy for figuring out the true color of whatever it is the rovers are looking at. Most of the time, the raw images are "stretched," which is a way of saying that the brightest spots in the images have been automatically brightened to white, and the darkest spots in the images have been darkened to black, and everything else shifted in brightness to occupy the space in between. When you look at these pictures, you can accurately tell whether one spot is darker or lighter than another. But you can't tell how dark it really is. And when you combine these stretched images into color composites, you can get some really bizarre views. The only way of getting the right colors is to have access to "calibration" data, which the Cornell team has in the form of the original, unstretched images and a companion set of images of the Sundial, which has very well known color properties. The armchair image processors can only do so much with the stretched data.

One more curiosity for this discussion: when Spirit got to El Dorado it took out its Microscopic Imager and had a look, and found a soil unlike anything I've seen from Spirit before. This is a really cool spot, and I'm looking forward very much to hearing what the science team has to say about what it is, where the soil came from, and why it's all confined to one black shadow on the slope of the Columbia Hills.
El Dorado sand before compaction
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El Dorado sand before compaction
Credit: NASA / JPL / Cornell
El Dorado sand after compaction
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El Dorado sand after compaction
Spirit pressed down on the sand at El Dorado with its robotic arm and then used its Microscopic Imager to grab a picture. Unlike other soils on Mars, which tend to be made of stuff that breaks apart easily, the grains in this soil didn't break up. Credit: NASA / JPL / Cornell

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