Blue Dot, Red Planet
The first image of Earth ever taken from another planet that actually shows our home as a planetary disk was captured by the Mars Orbital Camera on May 8th.
|What does Earth look like when viewed from Mars? At right shows the map outline of North and South America, compared to the reflectance from the May 8 photo from MOC|
Credit: NASA/JPL/Malin Space Science Systems
The evocative phrase describing the Earth as a ‘pale blue dot’ was coined by Carl Sagan after seeing our planet as a single pixel. The view was taken from the departing Voyager spacecraft. The entire earth could be encompassed as a flicker of light.
As Anne Druyan , Sagan’s widow, later described this perspective image to Astrobiology Magazine, that Voyager shot made "us look at this tiny planet, at the pale blue dot, and to see it in its real context, in its actual circumstances, in its true tininess. I don’t know anyone who’s able to really see that one-pixel Earth and not feel like they want to protect the Earth; that we have much more in common with each other than we’re likely to have with anyone anywhere else."
The Mars Orbital Camera has revealed more than a single pixel. 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 the Mars Global Surveyor (MGS) the Earth was imaged on May 8th, 2003 at 13:00 GMT (6:00 AM PDT), as both the Earth and the Moon appeared in the evening sky.
|Comparison of Mars, Venus and Earth in water bands, showing the clear presence of water on Earth uniquely |
Credit: NASA Workshop, Pale Blue Dot
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.
The Earth, as seen from a distant vantage point, has long captivated the imagination of planet finders. And in 1993, a team of researchers inspired by Carl Sagan, used an Earth fly-by of the Galileo spacecraft on its way to Jupiter to catch a glimpse of how the Earth might appear from afar. For astrobiologists, Sagan’s results were surprising.
Rather than seeing the Earth as an obvious candidate for life, the Galileo pictures gave surprisingly few clues of the biological potential of our own planet.
From afar, how Galileo missed the obvious signs of terrestrial life as we would have expected to see them, was at first disconcerting to the scientific community, because future missions aim to observe more distant extrasolar planets and detect what would be visible in the spectra–the ‘pale blue dot’ scenario.
|Image of the Earth and Moon taken by Galileo. |
One answer may lie in the fact that the spacecraft made its observations while still quite close to the Earth.
|Earth as seen by the departing Voyager spacecraft: a tiny, pale blue dot.|
"The spectrograph was designed to look at small areas of Jupiter, so the field of view of the spectrograph was quite small," said Nick Woolf of Arizona, in earlier discussions with the Astrobiology Magazine.
"Also, since the surface brightness of Jupiter [the Gaileo's intended visual target] is far less than the Earth, the spectrograph detectors saturated except when the spectrograph was pointed at the darkest area of Earth – a cloud-free section of sea," Woolf noted. The cloud-free sea is considered very dark relative to the dominance of bright clouds in a global picture of Earth. Thus it should come as no surprise that Galileo was successful in only imaging a relatively dark and lifeless planet, mainly because its design was not intended to look at Earth, but to probe Jupiter instead.
For most planet finders, the real challenge is to identify faint planets in the glare of their much brighter parent stars. To overcome the distortion of how our own atmosphere may further obscure this detection, both large land-based telescopes and space missions will likely combine in the future to complete the picture.
Both NASA and the European Space Agency (ESA) propose space missions to look for Earth-like planets in the infrared. NASA is developing the Terrestrial Planet Finder (TPF) project, part of the Jet Propulsion Laboratory Navigator Program, and ESA is developing its DARWIN project. The European Southern Observatory is exploring the possibility of ground-based searches using the future Overwhelmingly Large Telescope (OWL) project.
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.
Related Web Pages
The Earth as Seen by Galileo
Crescent Moon and EarthShine–courtesy Chris Cook, astrophotography
Mars Exploration Rovers
Jet Propulsion Laboratory
Mars Exploration Program