The Mercury Dual Imaging System (MDIS) consists of a pair of color cameras—a narrow angle imager and a wide angle imager—which will take pictures of Mercury in visible and near-infrared light. Many images will be captured during the flybys, and more detailed pictures will be taken during the time in orbit (one Earth year). In total, the cameras will image the entire planet twice to create global topographic maps of Mercury.
First, light must enter MDIS through a small (12 cm x 12 cm) window, where filters allow only the visible and near-infrared wavelengths of light to pass. From there the light can enter either camera, although only one may operate at a time due to temperature constraints.
The wide angle camera takes pictures with a larger field of view, but with less detail. This camera employs 12 different filters to better define the light that enters it over a range of different wavelengths. Different wavelengths of light can provide information about the composition of a feature, as well as its thermal properties.
For example, the images of the Old Faithful Geyser at Yellowstone National Park, at right, were taken using a visible light filter (top) and an infrared light filter (bottom). The infrared image reveals that the water erupting from the geyser is hot (red and white areas), while the surrounding ground is cooler (bluer areas).
Additionally, light in different wavelengths can be combined to produce color pictures. Your television uses red, green, and blue filters in concert to create the image you see on the screen.
Images obtained through different filters in the MDIS wide angle camera will provide information about the composition of the rocks on the surface of Mercury, allowing us to understand more about the geological processes which have taken place there. Learn more about filters and color here!
The narrow angle camera has a smaller field of view, and therefore can take more detailed pictures of landforms and surface features. This imager has only one visible light filter.
Both cameras store images using technology similar to fax machines, scanners, and digital still or video cameras. Images are recorded using charge-coupled devices (CCDs), which consist of many, many (e.g., up to 16 million!) small pixels or light-sensitive squares. Light strikes these tiny pixels, causing an electric charge to build up that is proportional to the intensity of the light. Data from all of the pixels is stored in this digital form.
The pictures above are both digital images of Earth’s Moon. The picture on the left has only about 140 pixels per row and column (a total of 140x140 = 19600 pixels), whereas the picture on the right has almost 2,000 pixels per row or column (4 million pixels). It’s easy to see how increasing the number of pixels increases the amount of detail in the image.
Both of the MDIS cameras have 1024 pixels in each column and 1020 in each row, for a total of over a million pixels per image. Learn more about CCDs here!
Presently, only 45% of Mercury has been photographed via spacecraft; Mariner 10 orbited Mercury in 1974, capturing black and white photographs of the planet. We have yet to see an entire hemisphere of Mercury! The MESSENGER MDIS will greatly expand our imaging coverage and therefore our knowledge of Mercury’s surface topography and surface composition. The entire planet will be imaged twice, at two different angles.
Photographs will be in color and monochrome, as well as in visible light and near infrared light. Most of the globe will be photographed from two different viewing angles, allowing stereo maps to be created (in the same way that each of your eyes sees a slightly different view, and your brain combines these into a 3-dimensional view). By combining these stereo maps with data acquired by the Mercury Laser Altimeter, a global topographic map will be constructed.
Several images were combined to create these views of Mars from the Mars Exploration Rover, Spirit. The image on the left is a monochrome (black and white), whereas the image on the right is a stereo image. If viewed through 3-D glasses, the stereo image will appear in 3-dimensions!
Instruments similar to the MDIS—digital cameras—are used here on Earth to capture photographs of friends, family, landscapes, and adventures. Sophisticated versions of our digital cameras, such as the MDIS, are used on virtually every NASA mission.
The image on the left is a mosaic of 18 pictures taken by the Mariner 10 spacecraft in 1974. The image on the right is a 27-picture mosaic taken in 2000 from the Cassini spacecraft, and is “the most detailed color portrait of Jupiter ever produced”. Certainly MESSENGER will include in its list of accomplishments the most detailed portrait of Mercury ever produced!
In order to create an accurate topographic map of the planet, MDIS images will be used with Mercury Laser Altimeter (MLA) data. In addition, MDIS spectral data will complement that collected by the Mercury Atmospheric and Surface Composition Spectrometer (MASCS), the X-Ray Spectrometer (XRS), and the Gamma Ray and Neutron Spectrometer (GRNS) to reveal surface composition characteristics.