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Earth/Moon: This is the first image of Earth ever taken from another planet that actually shows our home as a planetary disk. Because Earth and the Moon are closer to the Sun than Mars, they exhibit phases, just as the Moon, Venus, and Mercury do when viewed from Earth. As seen from Mars by MGS on 8 May 2003 at 13:00 GMT (6:00 AM PDT), Earth and the Moon appeared in the evening sky. The MOC Earth/Moon image has been specially processed to allow both Earth (with an apparent magnitude of -2.5) and the much darker Moon (with an apparent magnitude of +0.9) to be visible together. The bright area at the top of the image of Earth is cloud cover over central and eastern North America. Below that, a darker area includes Central America and the Gulf of Mexico. The bright feature near the center-right of the crescent Earth consists of clouds over northern South America. The image also shows the Earth-facing hemisphere of the Moon, since the Moon was on the far side of Earth as viewed from Mars. The slightly lighter tone of the lower portion of the image of the Moon results from the large and conspicuous ray system associated with the crater Tycho.
A note about the coloring process: The MGS MOC high resolution camera only takes grayscale (black-and-white) images. To "colorize" the image, a Mariner 10 Earth/Moon image taken in 1973 was used to color the MOC Earth and Moon picture. The procedure used was as follows: the Mariner 10 image was converted from 24-bit color to 8-bit color using a JPEG to GIF conversion program. The 8-bit color image was converted to 8-bit grayscale and an associated lookup table mapping each gray value of the image to a red-green-blue color triplet (RGB). Each color triplet was root-sum-squared (RSS), and sorted in increasing RSS value. These sorted lists were brightness-to-color maps for the images. Each brightness-to-color map was then used to convert the 8-bit grayscale MOC image to an 8-bit color image. This 8-bit color image was then converted to a 24-bit color image. The color image was edited to return the background to black.
The High Resolution Imaging Science Experiment (HiRISE) camera would make a great backyard telescope for viewing Mars, and we can also use it at Mars to view other planets. This is an image of Earth and the moon, acquired on October 3, 2007, by the HiRISE camera on NASA's Mars Reconnaissance Orbiter.
At the time the image was taken, Earth was 142 million kilometers (88 million miles) from Mars, giving the HiRISE image a scale of 142 kilometers (88 miles) per pixel, an Earth diameter of about 90 pixels and a moon diameter of 24 pixels. The phase angle is 98 degrees, which means that less than half of the disk of the Earth and the disk of the moon have direct illumination. We could image Earth and moon at full disk illumination only when they are on the opposite side of the sun from Mars, but then the range would be much greater and the image would show less detail.
On the day this image was taken, the Japanese Kayuga (Selene) spacecraft was en route from the Earth to the moon, and has since returned spectacular images and movies (see www.jaxa.jp...).
On the Earth image we can make out the west coast outline of South America at lower right, although the clouds are the dominant features. These clouds are so bright, compared with the moon, that they are saturated in the HiRISE images. In fact the red-filter image was almost completely saturated, the Blue-Green image had significant saturation, and the brightest clouds were saturated in the infrared image. This color image required a fair amount of processing to make a nice-looking release. The moon image is unsaturated but brightened relative to Earth for this composite. The lunar images are useful for calibration of the camera.