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MESSENGER's Earth flyby on Aug. 2, 2005, not only adjusted the
spacecraft's path to Mercury - the gravity assist maneuver allowed the
spacecraft team to test several MESSENGER science instruments by observing
its home planet.
The Mercury Dual Imaging System's wide-angle camera passed with flying
colors—several of them, actually—snapping a number of images
across its full multispectral capability. The camera is designed to
characterize minerals that may have formed in Mercury's crust. Telescope
measurements from Earth suggest that Mercury's surface resembles the
highlands on our moon: abundant feldspar (anorthite) with limited amounts
of iron-rich minerals such as pyroxene and olivine. The MESSENGER team
carefully picked 11 filters across visible and near-infrared wavelengths
(400 to 1,100 nanometers) known to indicate these and other common
silicate minerals.
This side-by-side look at Earth illustrates the value of
multiple-wavelength imaging. The three-band composite at left is made from
filters with peak sensitivities near 480 nm, 560 nm and 630 nm. These
filters help distinguish materials with distinct visible color differences
(ilmenite, volcanic glasses) but are also very close to the sensitivity of
the human eye. (Natural color is somewhat subjective, so this combination
of bands is "approximate" natural color.) The spacecraft was 63,950 miles
(102,918 kilometers) above Earth when the images were taken.
The camera's computer can mix red, green, and blue (RGB) light in various
proportions to create a full spectrum of colors. Infrared images are
visualized by substituting one of the RGB components. On the right, the
red component is the 750 nm band and green and blue are formed from the
630 nm and the 560 nm bands. Despite the substitution of only one band,
the results are dramatically different. Continental areas are mostly red
due to the high reflectance of vegetation in the near-infrared. Short
wavelength light (blue) is easily scattered in the Earth's atmosphere,
producing our blue skies, but also obscuring the surface from MESSENGER's
viewpoint. Infrared light is not easily scattered, so images of the Earth
remain sharp. The red coloring in the center of the image is a reflection
of the Brazilian rain forests and other vegetation in South America.
Once in orbit around Mercury, MESSENGER will map the entire surface in all
eleven wavelengths—and at resolutions ten times better than seen in
this view of Earth.
These images are from MESSENGER, a NASA Discovery mission to conduct the
first orbital study of the innermost planet, Mercury. For information
regarding the use of images, see the MESSENGER image use policy.