Mars Global Surveyor
Mars Orbiter Camera

Over the past year and a half, the Mars Global Surveyor (MGS) and Mars Orbiter Camera (MOC) operations teams have been developing and testing a technique through which the MOC can acquire images that have a higher resolution than the camera was originally designed to achieve. The technique is tricky and the spacecraft does not always hit its target. However, when it does, the results can be spectacular. Two examples are shown here. The first (MOC2-862a), providing key evidence for the action of liquid water on Mars, is a view of megaripples formed in an ancient catastrophic flood in Athabasca Vallis. The second (MOC2-862b) shows the Mars Exploration Rover (MER-A), Spirit, and the tracks it made during the first 85 sols of work in Gusev Crater.

Under normal operating conditions, the highest resolution images the MOC narrow angle camera can obtain are about 1.4 to 1.5 meters per pixel (4.6-5.0 feet/pixel). An image of 1.4 m/pixel permits objects approximately 4 to 5 meters across (13-16 ft) to be clearly resolved. The new technique developed by the MOC and MGS operations teams, known as cPROTO for "compensated Pitch and Roll Targeted Observation," allows the camera to obtain images that have better than 1 meter per pixel resolution. Typically, the images will have about 1.5 meters per pixel resolution in the cross-track (east-west) direction, and about 50 centimeters (half a meter) per pixel in the downtrack (north-south) direction. These pictures also have an improved signal-to-noise ratio when compared to "normal" 1.5 m/pixel images, thus improving on the overall quality of a typical MOC full-resolution image. MOC cPROTO images allow objects of as small as 1.5 meters (5 feet) to be seen, including the Mars Exploration Rovers and the tracks they make on the surface.

The MOC narrow angle camera consists of a single line of 2048 detectors (which translates to 2048 pixels in a full-resolution, full-width image). The motion of the MGS spacecraft as it orbits Mars allows this single line of detectors to be swept over the planet's surface, building up an image one line at a time (i.e., much as a flatbed scanner attached to a computer builds up a picture). Because the MGS orbit is nearly circular, each of the 2048 pixels in a full resolution image correspond to a square ~1.5 by ~1.5 meters on a side, thus giving the typical "1.5 meters per pixel" or "1.5 m/pixel" resolution often cited for MOC's most detailed images.

During normal operations, the MGS spacecraft keeps one of its surfaces (the "nadir panel"), on which are mounted many of the instruments, always pointed towards the planet. To do so, the spacecraft must rotate (in this case about its Y-axis) exactly once per orbit. For cPROTOs, the spacecraft operations team changes this rotation rate by looking forward a bit and speeding up the spacecraft to stare at that location while MGS flies over it, before returning to the downward stare. In this way, the apparent forward speed of the spacecraft is reduced, allowing either a longer dwell time per MOC image line (which improves signal to noise and thus image quality), multiple-samples at a given dwell time (increasing the spatial sampling--the resolution--in the along-track direction), or both. Operationally, we command the spacecraft to dwell 6 times longer than normal over the area we're imaging, dividing this between sampling 3 times as the spacecraft covers a distance of 1.5 meters (downtrack) on the ground, and increasing the amount of time each sample represents by a factor of 2 (increasing the image quality by 40%). The result is a sharper image with ~50 cm/pixel downtrack and ~1.5 m/pixel crosstrack in which objects 2-4 times smaller than can be resolved in a typical 1.5 m/pixel MOC image are revealed.

The "PROTO" part of "cPROTO" refers to the movements the MGS spacecraft must make to acquire the image-- it must be 'pitched' in the downtrack direction to obtain the 50 cm/pixel view, and rolled to allow the spacecraft to point at the specific target of interest. The "c" in "cPROTO" stands for planetary motion compensation. While MGS is pitching, rolling, and moving along its orbit, Mars is rotating underneath it (just as Earth is rotating right now, as you read this, such that the Sun, Moon, and stars appear to move in the sky). The pitch and roll of MGS are timed in such a way as to account for the rotation of Mars, as well as the desired image resolution and target location. The animation in Figure MOC2-862c, above, provides a visual representation of the movements that MGS must undergo in order to acquire a cPROTO image. Opportunities to do this are limited by spacecraft communication schedules, because the spacecraft cannot communicate with Earth during a cPROTO maneuver, and by spacecraft solar power, because the solar panels cannot point at the Sun when a cPROTO is being executed. The size of a cPROTO image is limited by how much data the MOC can collect and place in its internal buffer---typical cPROTO images are about 3 km wide by 3 to 4 km high (about 1.9-2.5 miles); and the location of a cPROTO image is limited by atmospheric clarity, solar illumination of the surface, and protection of MOC's optical system from direct sunlight. Because of the complexity of movements involved, it often takes 2-3 attempts before a cPROTO image hits is intended target. For example, the image shown here of Spirit's rover tracks was attempted twice in March 2004, but only hit the target area once (the picture shown here); it was tried again in June 2004, but that image also missed the target.

While acquisition of cPROTO images remains challenging, the results are worthwhile. The goal of the first cPROTO image shown here, MOC2-862a, was to look for boulders in the flood-deposited megaripples in Athabasca Vallis. These ripples were recognized in earlier, lower-resolution MOC narrow angle images, and they are the only really good example of ripples formed in a giant catastrophic flood anywhere on Mars. Their presence indicates that large amounts of water poured very rapidly through the area. Strange, somewhat circular features on top of some of the ripples and the adjacent plains are the products of erosion and removal of an overlying layer--that is, the ripples in Athabasca Vallis were buried for some period of time and later exhumed.

Finding boulders in the ripples would help constrain the power of the floods. The cPROTO image, however, did not show boulders in the ripples, implying either that the rocks that make up these features are smaller than about 1 to 2 meters in diameter, or that the ripple sediments have not been completely exhumed. Similar megaripples are known from catastrophic flood sites on Earth, including the Channeled Scabland in Washington State and a variety of flood features in Iceland. Figure MOC2-862d shows an example of megaripples formed by a flood in Iceland; Figure MOC2-862e shows what these same ripples look like from the ground (with a person for scale). In the foreground, in front of the person, a plethora of cobbles can be seen. A covering of grasses was stripped away to reveal these cobbles, which are the sediments that make up the megaripples. In a normal river or stream, ripples would be made of sand---in a catastrophic flood, they can be made of cobbles or even boulders. An additional MOC view of flood landforms in Athabasca Vallis can be seen in MOC's 12 December 2002 release, "Athabasca Vallis Streamlined 'Islands'". The cPROTO image of Athabasca Vallis megaripples covers an area 3 km wide, is illuminated by sunlight from the lower left, and is located near 9.5°N, 203.7°W.

Approximately 1,700 km (~1,060 mi) southwest of the Athabasca Valles megaripples lies the Gusev Crater landing site of the Mars Exploration Rover (MER-A), Spirit. The rover spent most of its 90-sol Primary Mission operating in the terrain located between its lander and the southwest rim of nearby Bonneville Crater. An overview of the MER-A site, acquired before the rover had moved away from the lander, was obtained in an earlier, January 2004, MOC cPROTO image, "MGS MOC Image of Mars Exploration Rover, Spirit, on Mars". The new image shown here, obtained on Spirit's 85th sol (30 March 2004) of surface operations, shows the same features as the January cPROTO image (the lander, parachute, backshell, and heat shield impact site). It also shows the tracks made by Spirit, and it shows the little rover itself. Figure MOC2-862f is a view of Spirit's tracks made by the rover's navigation cameras on Sol 62. The tracks are darker than the surrounding surface, and this allows the track to show up nicely in the MOC image obtained from orbit. Figure MOC2-862g provides a look at the full-sized cPROTO image in which the rover site is found. The rover cPROTO image is illuminated by sunlight from the upper left; the full image covers an area 3 km wide and is located near 14.8°S, 184.6°W.

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