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The Planetary Society Blog

By Emily Lakdawalla

Recent gullies in Russell Crater, Mars, from MOC and HiRISE

Nov. 30, 2006 | 12:28 PST | 20:28 UTC

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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!

The first image I attacked was the one of recent gullies in Russell Crater. The image was both fascinating and familiar; MOC imaged this same location about a year ago. Here are the HiRISE (left) and MOC (right) views. The click-to-enlarge version of the HiRISE image gets you to 4 meters per pixel, an eighth the full resolution; the MOC image clicks to its full resolution of 2.96 meters per pixel.

Click to enlarge >

HiRISE image of recent gullies in Russell Crater, Mars
Credit: NASA / JPL / U. Arizona

Click to enlarge >

MOC image of recent gullies in Russell Crater, Mars
Credit: NASA / JPL / MSSS

The MOC image contains two different landforms that currently form on the surface of Mars. On the sloping face of a large dune within Russell Crater, in Mars' Noachis Terra in the far south, there are numerous, narrow, nearly straight gullies as well as tracks formed by dust devils. These types of gullies are typically only found in Noachis Terra, and almost exclusively form on south-facing slopes. The HiRISE team quotes the MOC team's hypothesis for the gullies: "CO2 (or maybe H2O) frost is deposited on the dunes in shadows or at night. Some frost may also be incorporated into the internal parts of the dunes due to natural avalanching. When the frost is eventually heated by sunlight, rapid sublimation triggers an avalanche of fluidized sand, forming a gully."

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.

Click to enlarge > Russell Crater gullies detail: gully heads (HiRISE)	Click to enlarge > Russell Crater gullies detail: gully heads (MOC)
Click to enlarge > Russell Crater gullies detail: gully toes (HiRISE)	Russell Crater gullies detail: gully toes (MOC)

There are several things that make these gullies really weird. They have a pretty constant width along their length; when tributaries join, the downstream gully is wider, which is what you'd expect. They are very, very straight, with very low sinuosity. They seem to have raised banks, or levees -- they aren't just incised into the slope. Whatever formed them pushed material up and to the sides. At their toes, they narrow and end very abruptly. I've heard of "theater-shaped" valley heads before -- you get those in glacial environments or where there's groundwater sapping -- but I don't believe I've ever heard of theater-shaped valley toes. Many of the valley toes have very short protrusions -- toes on the toes. Finally, it seems to me that since the gullies formed, their walls have been corrugated by winds into small duneforms.

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:

Click to enlarge >

Russell Crater gullies detail: dust devil tracks (HiRISE)

Click to enlarge >

Russell Crater gullies detail: dust devil tracks (MOC)

This made me scratch my head for quite a while. I thought that there was probably a clue in the different viewpoints of the two spacecraft. The shape of Mars Global Surveyor's orbit is such that it's approximately 2 pm local solar time on the ground where MOC takes its pictures. For Mars Reconnaissance Orbiter, it's later, approximately 3 pm. But I didn't think that that difference alone was enough to explain the difference between the two images. What's going on?

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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