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| Earth, Moon, and Jupiter, as Seen From Mars |
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NASA
May 22, 2003
What does Earth look like when viewed from Mars? At 13:00 GMT on
8 May 2003, the Mars Global Surveyor (MGS) Mars Orbiter Camera (MOC)
had an opportunity to find out. In addition, a fortuitous alignment of Earth
and Jupiter---the first planeary conjunction viewed from another
planet---permitted the MOC to acquire an image of both of these bodies and
their larger satellites. At the time, Mars and the orbiting camera were 139
million kilometers (86 million miles) from Earth and almost 1 billion kilometers
(nearly 600 million miles) from Jupiter. The orbit diagram, above, shows the
geometry at the time the images were obtained.
Because Jupiter is over 5 times farther from the Sun than Earth, two
different exposures were needed to image the two planets. Mosaiced
together, the images are shown above (top picture). The composite has
been highly contrast-enhanced and "colorized" to show both planets and
their satellites. The MGS MOC high resolution camera only takes grayscale
(black-and-white) images; the color was derived from Mariner 10 and
Cassini pictures of Earth/Moon and Jupiter, respectively, as described in
the note below.
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.
Jupiter/Galilean Satellites: When Galileo first turned his
telescope toward Jupiter four centuries ago, he saw that the giant planet
had four large satellites, or moons. These, the largest of dozens of moons
that orbit Jupiter, later became known as the Galilean satellites. The larger
two, Callisto and Ganymede, are roughly the size of the planet Mercury;
the smallest, Io and Europa, are approximately the size of Earth's Moon.
This MGS MOC image, obtained from Mars orbit on 8 May 2003, shows
Jupiter and three of the four Galilean satellites: Callisto, Ganymede,
and Europa. At the time, Io was behind Jupiter as seen from Mars, and
Jupiter's giant red spot had rotated out of view. This image has been
specially processed to show both Jupiter and its satellites, since Jupiter,
at an apparent magnitude of -1.8, was much brighter than the three satellites.
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, and a recent Cassini
image acquired during its Jupiter flyby was used to color the MOC Jupiter
picture. The procedure used was as follows: the Mariner 10 and Cassini
color images were converted from 24-bit color to 8-bit color using a JPEG
to GIF conversion program. These 8-bit color images were converted to
8-bit grayscale and an associated lookup table mapping each gray value
of that 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 their respective 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. Three separate color tables were used:
one each for the Earth, Moon and Jupiter. Jupiter's Galilean Satellites
were not colored.
To view the unprocessed Earth/Moon image, download the following to
your desktop and view with your favorite image viewing or processing tool:
R05-00763.gif.
To view the unprocessed Jupiter image, download the following to your
desktop and view with your favorite image viewing or processing tool:
R05-00762.gif.
All Images Credit: NASA/JPL/Malin Space Science Systems
Malin Space Science Systems and the California
Institute of Technology built the MOC using spare hardware from the
Mars Observer mission. MSSS operates the camera from its facilities in
San Diego, California. The Jet Propulsion Laboratory's Mars Surveyor
Operations Project operates the Mars Global Surveyor spacecraft with its
industrial partner, Lockheed Martin Astronautics, from facilities in Pasadena,
California and Denver, Colorado.
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