MESSENGER's Mercury Dual Imaging System (MDIS) is composed of two cameras, a wide-angle camera (WAC) and a narrow-angle camera (NAC). MDIS is scheduled to acquire more than 75,000 WAC and NAC images during the one-year orbital mission in support of MESSENGER's science goals.

A range of imaging campaigns achieves a balance between globally mapping the entire surface of Mercury and obtaining targeted higher-resolution images in support of specific science goals. Together, MDIS's imaging campaigns will provide a new view of Mercury and will address one of the mission's main science questions: What is the geologic history of Mercury?

Surface Morphology Base Map
During the first 176 days of the orbital mission, equal to one solar day on Mercury, MDIS will acquire images to produce a high-resolution base map for surface morphology (morphology is the term given to the shape and texture of the surface). This map will cover more than 90% of Mercury's surface at an average resolution of 250 m/pixel (0.16 miles/pixel or 820 feet/pixel) or better.

At this resolution, features about 1 km in horizontal scale are recognizable in the images. Images acquired for the surface morphology base map have off-vertical solar illumination and visible shadows so as to reveal clearly the topographic form of geologic features. Because of MESSENGER's highly elliptical orbit, the spacecraft passes close to the surface at high northern latitudes but is far above the southern hemisphere, so both the NAC and the WAC are being used to construct the global base map.

For the southern hemisphere, images are obtained with the NAC, which has a 1.5 degrees field of view and can acquire images at seven times greater resolution than the WAC. For the northern hemisphere, when the spacecraft is closer to and moving faster over the surface, the WAC is used, because its 10.5 degrees field of view enables good image coverage. Images from both the NAC and the WAC will be mosaicked together to produce the global map. Shown in Figure 1 is an example mosaic of four images acquired as part of the surface morphology campaign.

Color Base Map
In addition to the surface morphology base map, MDIS is currently acquiring a color base map during the mission's first 176 days. The color base map is composed of WAC images taken through eight different narrow-band color filters and will cover more than 90% of Mercury's surface at an average resolution of 1 km/pixel (0.6 miles/pixel) or better. In contrast to the imaging conditions best suited for seeing surface topography, the highest-quality color images of Mercury's surface are obtained when both the spacecraft and the Sun are overhead and shadows are limited.

The eight different color filters of the WAC that are used to create the color base map have central wavelengths of 430, 480, 560, 630, 750, 830, 900, and 1000 nm. The images acquired through these narrow-band filters are combined to create color images that accentuate color differences on Mercury's surface.

Stereo Base Map
After the surface morphology base map is acquired during the first Mercury solar day, a second, complementary near-global map, called the stereo base map, will be acquired during the second Mercury solar day of MESSENGER's one-year orbital mission. The stereo base map will be used in combination with the surface morphology base map to create high-resolution stereo views of Mercury's surface at an average resolution of 250 m/pixel (0.16 miles/pixel or 820 feet/pixel) or better.

As with the surface morphology base map, images are acquired under non-vertical solar illumination, so that shadows accentuate the topography of the surface. In addition, the stereo basemap images are acquired under viewing angles that differ from those for the morphology base map by about 20 degrees , allowing stereo information about the surface to be determined.

As the mission is currently in the first Mercury solar day, no images have yet been acquired in support of the stereo base map. However, different viewing conditions during MESSENGER's second and third Mercury flybys allowed stereo information to be obtained for a portion of Mercury's surface at an image resolution of 500 m/pixel.

South Polar Monitoring
In addition to the three global base maps, there is an MDIS imaging campaign to monitor the south polar region of Mercury. By imaging the south polar region once every four MESSENGER orbits (once every two Earth days) as illumination conditions change, features that were in shadow on earlier orbits can be discerned and any permanently shadowed areas can be identified over one Mercury solar day.

Identifying areas of permanent shadow are of interest to understand the unusual materials at Mercury's poles and whether these highly radar-reflective materials consist of water ice. During MESSENGER's one-year mission, the WAC is used to monitor the polar region south of 70 degrees S at 1.5 km/pixel for the first Mercury solar day. On the second Mercury solar day, the NAC will be used for imaging the polar region south of 85 degrees S at 300 m/pixel.

Limb Imaging
Once per week, MDIS captures images of Mercury's limb (the edge of the sunlit planet with space), with an emphasis on imaging the southern hemisphere limb. The spacecraft was high above Mercury's south polar region when capturing this image. However, even when the spacecraft is at its highest altitude above Mercury, a single WAC image cannot capture the entire limb of Mercury.

Consequently, two images are taken and mosaicked together to image Mercury's entire limb. These limb images will provide information about Mercury's shape and will complement measurements of topography made by the Mercury Laser Altimeter (MLA) of Mercury's northern hemisphere.

Targeted Observation
MDIS also acquires targeted images of small areas on Mercury's surface at resolutions much higher than those of the morphology, stereo, or color base maps. It is not possible to cover all of Mercury's surface at such high resolutions during MESSENGER's one-year primary mission, but several areas of high scientific interest are generally imaged in this mode each week.

Additionally, as new features of particular science interest are imaged from orbit, targets are added to a database list and will be imaged if possible at higher resolution by MDIS, or with multiple instruments, the next time that area of Mercury is in view from the spacecraft.

Calibrations
In addition to the science imaging campaigns described above, MDIS also acquires a few images each week in support of calibration of the instrument. Some of these images are of star fields, to verify the pointing of the camera and to assess any changes in instrument characteristics during the mission. These star images resemble those acquired in support of the vulcanoid search effort, carried out during MESSENGER's journey prior to orbit insertion.

Other calibration images include repeated imaging of the same portion of Mercury's surface under different lighting and viewing geometries. These image sets provide information about the photometric corrections that must be applied to compare images acquired under a range of lighting and viewing conditions.

related report
100 Orbits and Counting
MESSENGER has begun its 100th orbit around Mercury. Since its insertion into orbit about the innermost planet on March 17, the spacecraft has executed nearly 2 million commands.

The data gathered so far include more than 70 million magnetic field measurements, 300,000 visible and infrared spectra of the surface, 16,000 images, and 12,000 X-ray and 9,000 gamma-ray spectra probing the elemental composition of Mercury's uppermost crust.

"As the primary orbital phase of the MESSENGER mission unfolds, we are building up the first comprehensive view of the innermost planet," states MESSENGER Principal Investigator Sean Solomon, of the Carnegie Institution of Washington.

"The surface is unraveling before our eyes in great detail, and the planet's topography and gravity and magnetic fields are being steadily filled in. As the Sun becomes increasingly active, Mercury's extraordinarily dynamic exosphere and magnetosphere continue to display novel phenomena."

MESSENGER continues its science-mapping phase in orbit around Mercury. All spacecraft systems remain safe and healthy, and all science instruments are on and continue to collect data according to the baseline observation plan.

"Over the next several weeks, MESSENGER's subsystems and instruments will experience their hottest temperatures yet as the spacecraft crosses between the planet's surface and our Sun at high noon close to the planet, preceded by hour-long eclipses near local midnight with only the spacecraft battery to keep the spacecraft alive in the dark of Mercury's night," notes MESSENGER Project Scientist Ralph McNutt.

"All of this was planned in great detail more than seven years ago, as was the orbit insertion burn that went so flawlessly," he adds. "Theory is one thing and reality another, and the sense of thrill leading to 'firsts' is always followed by a sense of relief, especially in the challenging environment of interplanetary space, far from home."

With less than one-sixth of its primary orbital mission completed, MESSENGER is already rewriting our books on what is known (and unknown) regarding the innermost planet, McNutt says. "By exploring our near - and far - neighbors in our solar system, we touch new knowledge, new understanding, and new wonderment about not only our own origins and place but of the other worlds circling the stars we see in our night sky."

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