Mars Global Surveyor
Mars Orbiter Camera

During 2003 and 2004, the Mars Global Surveyor (MGS) Mars Orbiter Camera (MOC) operations team at Malin Space Science Systems (MSSS; San Diego, California) worked closely with the MGS operations teams at the Jet Propulsion Laboratory (JPL; Pasadena, California) and Lockheed Martin Astronautics (LMA; Denver, Colorado) to develop a new technique in which the MGS spacecraft does a maneuver that permits MOC to acquire images at a higher spatial resolution than normal. Usually, MOC images have a resolution of about 1.5 meters (5 ft) per pixel, and MOC can be commanded to acquire lower resolution (e.g., 3-12 m/pixel) data when desired. To obtain a higher resolution, the whole MGS spacecraft must be pitched at such a rate that MOC over-samples its view of the martian surface in the down-track direction. Called cPROTO, for Compensated Pitch and ROll Targeted Observation, this technique allows MOC to obtain images that have a resolution of about 50 centimeters (~1.6 ft) per pixel in the down-track direction, and 150 centimeters (~5 ft) per pixel in the cross-track dimension. Although it is usually quite challenging for MOC to hit its intended target using the cPROTO approach, some spectacular successes have been obtained, including images of the Mars Exploration Rovers Spirit and Opportunity, the Viking 2 lander, and various landforms such as flood ripples in Athabasca Vallis.

The picture shown here is a mosaic of two MOC cPROTO images acquired in January 2005 near 84.8°N, 356.4°W. The mosaic provides an incredibly detailed view of a scarp at the head of Chasma Boreale, a large trough cut by erosion into the martian north polar cap and the layered material beneath the ice cap. We consider this to be the best pair of cPROTO images MOC has yet acquired.

During each northern summer, there occurs a very narrow window in time of 2-3 months when conditions are ideal to image the north polar cap at high resolution. Throughout this ideal 2-3 month period, the atmosphere is generally clear over the cap, and the seasonal carbon dioxide frost from the previous winter and spring has sublimed away, permitting a good view of the surface geology. The two cPROTO images in this mosaic were acquired during this brief period during the most recent northern summer. Within a few weeks of when these images were acquired, dust storm activity picked up in the north polar region, making the atmosphere too dusty to obtain any more detailed views until late 2006.

Chasma Boreale is cut into the layered material that lies beneath the water ice of the north polar cap. For decades, these layered materials were assumed to consist of a mixture of ice and dust. MOC images obtained in 1999 and 2001 began to show that the layers also include some that are a source for windblown sand. The science objective for the two cPROTO images (MOC images S02-00486 and S02-01239) that are shown in this mosaic was to look for boulders in the debris shed from the steep slopes cut into the north polar layers by Chasma Boreale. Finding boulders would imply that the layers that are the most resistant to erosion in the polar region are as competent as solid rock, perhaps giving a new insight into the nature of the polar layered materials.

Examination of the high resolution cPROTO mosaic shows that there are indeed some large boulders that have eroded out of the layered materials and rolled down the slopes. It is possible, therefore, that the north polar layers are not simply a mixture of ice, dust, and sand. Some layers may actually be rock, cemented by minerals rather than by ice. Alternatively, if the materials are cemented by ice, then a future high resolution view (either another MOC cPROTO image or a picture from the Mars Reconnaissance Orbiter (MRO) high resolution camera, HiRISE) might show that the boulders have gotten smaller over time.

In addition to the observation of boulders eroding out of the polar layered materials, the cPROTO mosaic also helps confirm the view that dark sand is eroding out of the polar layered materials, and that there are three different groups of layers under the polar ice. The upper unit is light-toned, finely-layered, and more resistant to erosion (more competent, less easily destroyed by erosion) than the middle unit, which is rich in dark sand but also has several, individual, shelf-forming layers in it. Finally, below the dark, sandy layer is a third unit that is light-toned and has a different appearance relative to the other two units. Some of its layers have surfaces that have been broken by shallow fractures into polygonal and linear forms, also implying that they are hard, resistant rock.

Reference Material:

• The MGS MOC cPROTO technique was described in an extended abstract presented at the 2005 Lunar and Planetary Science Conference:
  Malin, M. C., and K. S. Edgett (2005), MGS MOC: First views of Mars at sub-meter resolution from orbit, Lunar and Planetary Science XXXVI, Abstract No. 1172, Lunar and Planetary Institute, Houston, Texas.
  
  
• The layers of the martian north polar region were discussed in a paper by K. S. Edgett and MOC colleagues in 2003:
  Edgett, K. S., R. M. E. Williams, M. C. Malin, B. A. Cantor, and P. C. Thomas (2003), Mars landscape evolution: Influence of stratigraphy on geomorphology in the north polar region, Geomorphology, 52(3-4), 289-297, 10.1016/S0169-555X(02)00262-3.