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The Planetary Society BlogBy Emily LakdawallaRecent gullies in Russell Crater, Mars, from MOC and HiRISENov. 30, 2006 | 12:28 PST | 20:28 UTC
So I spent most of yesterday playing around with the newly released Mars Reconnaissance Orbiter images. This work was made much easier thanks to some suggestions I received in an email from Trent Hare, who is the master of planetary Geographic Information Systems (GIS) at the United States Geological Survey. Trent said that they, too, had had issues trying to use Photoshop to open the images. Instead, he recommended a freeware viewer called OpenEV. OpenEV is included in a freeware package called FWtools, which you can download here. I've tried out FWtools and it can open even a 1-Gigabyte JPEG2000 image incredibly quickly. You can use scroll bars or arrow keys to scroll around an image, and there's a quick and easy scale feature to zoom in and zoom out. Most importantly for me, there is an "Export" dialog in the File menu that allows you to export a selected part of the image, at full resolution or downsampled, which is what I needed Photoshop for. Thanks so much to Trent for the suggestions!
On the MOC image you can see the gullies as dark streaks on the dune slope, but their morphology is mysterious. On the HiRISE images you can see the morphology much more clearly -- but I don't think it's any less mysterious! Here are a two segments of the images showing the heads and toes of these gullies, comparing the HiRISE (left) and MOC (right) views. Clicking to enlarge delivers you the full HiRISE resolution of 50 centimeters per pixel and images that are several hundred kb in size.
Looking at the gully heads, these things also start pretty abruptly. Upslope from the heads there's a region of dune slope that is very finely dissected into little channels, down to the resolution of the camera -- and remember the image has a resolution of 50 centimeters per pixel! We're looking at little erosional features that are narrow enough to walk across without lengthening your stride. One other thing confused me a lot as I was comparing the HiRISE and MOC images. The gullies are visible to both cameras. But the MOC image contains hundreds of squirrely, dark dust devil tracks that are completely absent in the HiRISE image. Take a look:
It finally occurred to me to ask where on Mars this place was. It turns out it's at 55 degrees south latitude. And that finally led me to the answer: the two images were taken at a different season. At such a southern location, the difference in solar illumination with season can result in extremely different views. The HiRISE image was taken during southern winter, a solar longitude of 136.3 degrees, while the MOC image was taken in the spring, at a solar longitude of 266.7 degrees. (Read about solar longitude and Mars' seasons here.) In the winter HiRISE image, the Sun sits at only 5 degrees above the horizon, while the spring MOC image is illuminated with the Sun 51 degrees above the horizon. The glancing light in the HiRISE image emphasizes topography but hides subtle details of color. The overhead light in the MOC image shows us the color differences that make the dust devil tracks visible, at the expense of topographic detail. This is why it's useful to view the same location at different times of the year -- and why the loss of Mars Global Surveyor is a blow, even though we have great new spacecraft like Mars Reconnaissance Orbiter there.
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