MARCI Bands Composite
The Mars Color Imager (MARCI) camera on NASA's Mars Reconnaissance Orbiter
acquired a seven-band color, wide-angle view of Mars on March 24, 2006, as
part of a checkout of the orbiter's payload. This image shows a color
composite made from the MARCI red, green, and blue bands. The view looks
northward and includes the large Argyre Basin in Mars' southern hemisphere.
One use of the test imaging is an opportunity to fine-tune calibrations
used for processing the separate bands into "true" color -- as it would
appear to a human eye looking down from orbit. Further calibration will
be needed. Regular use of MARCI and the other science instruments on Mars
Reconnaissance Orbiter will begin in autumn 2006, after the spacecraft's
orbit has been reshaped to a nearly circular, low-altitude path.
The March 24 test produced images from each color band. Illustrated here
are some of these test images. In figure 1, three views acquired by
MARCI are compared to a color composite of two views acquired about four
hours later by the wide-angle imager of Mars Orbiter Camera (MOC) on
NASA's Mars Global Surveyor spacecraft. The MARCI imaging occurred during
the morning on Mars, while the MOC observations were made at about 2 p.m.
local solar time. The region of Mars imaged by MARCI was south of the
Valles Marineris. It includes the Argyre Basin's interior plains, Argyre
Planitia, and mountains forming the basin rim, Nereidum Montes to the
northwest (middle of images) and Charitum Montes to the southeast (bottom
of images).
The color composite from MARCI differs from the MOC wide-angle color
composite because, to create a color image with MOC data, camera-team
members synthesize (fake) a green channel by adding the red and blue
channels together and dividing by two. The slightly greenish tint of the
MARCI image shows that the approximation used for MOC images
underestimates the amount of green.
The test image labeled 260 nm shows how the planet appears at an
ultraviolet (UV) waveband of 260 nanometers, where ozone absorbs the UV
light. Relatively darker areas in this band normally will indicate the
presence of ozone, and relatively lighter areas will indicate the absence
of ozone. Water vapor in Mars' atmosphere is in an inverse relationship
with ozone; where there is more of one, there is less of the other. So,
lighter areas in images can be used to track water vapor. The term
"relatively" is used here because Mars itself is very dark in the UV
owing to absorption of UV light by iron-bearing minerals, and sunlight is
deficient in UV relative to visible light, so in general Mars will always
look dark in the UV. A second UV band on MARCI (not shown in the figure
above) at a longer wavelength allows these differences to be quantified.
The MOC wide-angle image shows wispy, light water-ice clouds to the
northwest of Argyre in the afternoon, but researchers cannot yet correlate
these clouds with the UV information from MARCI, especially because the
times of day are different. When in its final mapping orbit, Mars
Reconnaissance Orbiter will view the same area as Mars Global Surveyor
separated by only one hour, and such correlations will be much more
direct.
For more details of how MARCI images are acquired and processed, see the
companion release, MARCI2-3, and be certain to examine the 15.6 Mbyte animated Gif movie.
The pictures shown here are the first views of Mars acquired by the Mars
Color Imager on Mars Reconnaissance Orbiter. This is a re-flight of a
similar instrument that was aboard the Mars Climate Orbiter, which was
lost in September 1999 during its orbit insertion activity. In the primary
science phase of the Mars Reconnaissance Orbiter, MARCI will routinely
acquire daily global maps of the planet. These data will be used to help
track storms, monitor clouds and water vapor, and track seasonal changes
in surface albedo (bright and dark) patterns and the polar caps.