Mercury is one of the most mysterious planets within our solar system because it is so close to the Sun. The Sun creates a strong glare that makes it very difficult for astronomers here on Earth to take clear photos of the small planet. Only one spacecraft has taken a close-up look at Mercury so far. NASA's Mariner 10 performed three fly-bys of the planet during 1974 and 1975 on a carefully planned trajectory that allowed the probe to capture images of Mercury's lighted hemisphere. Unfortunately, Mariner 10 was only able to photograph about 45% of the planet's surface, so over half of the planet has never been seen by human eyes to date.
The Mariner 10 photos show that Mercury looks much like our own Moon. The surface is predominantly gray and heavily cratered, attesting to the large number of impacts the world suffered early in its history. Some of these craters are surrounded by lighter-colored regions created by the material thrown outward by the impacts that created them. Similar features can also be seen on the Moon. Unlike the Moon, however, Mercury does not appear to have darker areas, or "seas," that were created on the Moon by lava flows.
The second planet, Venus, is also shrouded in mystery thanks to the planet's thick clouds that block its surface from view. Over two dozen spacecraft have visited Venus so far, including landers that have set down on the planet's extremely hot surface. Most probes to visit Venus have seen little more than a hazy, pale yellow ball. Lighter white-colored clouds create some variation in the planet's appearance, but its atmosphere is generally featureless in visible light. NASA's previously mentioned Mariner 10 took the following photo of Venus in 1974.
The only way to get a good view of the planet's surface from space is by radar. NASA launched the Magellan probe to Venus in 1989 to make a detailed survey of the planet using a mapping radar. The data from these surveys has been used to create computer-generated world maps of the planet's topography. These maps usually portray the surface of Venus in a reddish brown color. This is not the true color of the surface but was chosen to highlight features in the planet's geology. Actual photos of the surface were collected by several Soviet craft that visited Venus, such as the following image taken by the Venera 13 lander in 1982.
These true-color pictures give the planet's surface more of a reddish yellow color. However, analysis shows that the atmosphere causes the yellowish hue by filtering out other colors. If the atmosphere could be removed, the Venutian surface would actually be a dull gray color.
The next planet from the Sun is our own home Earth. From space, Earth is a predominantly blue world due to the atmosphere as well as the large bodies of water that cover most of its surface. White is also a common color created by clouds of water ice in the atmosphere as well as large ice packs near the polar regions. Depending on cloud coverage, regions of brown terrain and green plant life may also be visible to an observer from space.
Most of these colors can be seen in the above photo of Earth taken by astronauts aboard Apollo 17 in December 1972. This photo was taken from over the southern hemisphere and gives a clear view of Africa, the Arabian peninsula, and Antrarctica. The motions of clouds, flow of water, changes in temperature with seasons, and effects of life on the planet's surface give Earth the most dynamic and changing appearance of any world in the solar system.
As discussed in a previous article about Martian climate, Mars may have once looked much like Earth with large regions of water on its surface. Today, however, Mars is a cold and arid wasteland. The planet is still a popular destination for exploration though and has been visited by 40 or so spacecraft so far. These probes have shown that Mars is overwhelmingly reddish brown with some darker patches in volcanic regions.
The red color is caused primarily by the presence of iron in surface rocks. The dry planet is also regularly subjected to strong winds that stir up dust from the surface to give the atmosphere a reddish hue. Mars too has polar caps similar to Earth's except those on Mars include frozen carbon dioxide, or dry ice, in addition to water ice. These ice caps plus wispy water ice clouds that occassionally form in the atmosphere add some white color to the planet.
After Earth, probably the second most colorful and dynamic planet in the solar system is our largest neighbor Jupiter. Jupiter is one of the gas giants, which have no solid surface, so the colors we see are created by gases and clouds in the upper layers of the atmosphere. The primary colors in Jupiter's atmosphere are orange and white. The white bands are created by clouds of ammonia while the orange regions are ammonium hydrosulfide clouds.
Many photos of Jupiter show more vibrant coloring with deeper reds and yellows, but these were created using filters to reveal more detail in the cloud structure. Though not quite as vibrant in true color, Jupiter's atmosphere is still a dynamic sight filled with many storms and other regions of turbulent mixing. The most amazing of these storms is the Great Red Spot that has been raging across Jupiter's southern hemisphere for at least 300 years. Only a half dozen spacecraft have visited Jupiter so far, including the highly successful Pioneer and Voyager probes. Other spacecraft have also made flybys of Jupiter to take advantage of gravity-assist maneuvers on their way to other destinations in the solar system. NASA's Cassini took the above image of Jupiter during its 2000 flyby as the craft journeyed onward to Saturn.
Saturn is another extraordinary sight on our tour of the planets. Like Jupiter, Saturn's atmosphere is predominantly hydrogen and helium gas. However, it is small amounts of other substances that account for the color in Saturn's atmosphere. Sulfur, in particular, gives the planet an overall yellowish hue. The atmosphere also contains small amounts of nitrogen and oxygen that combine with hydrogen to create a smoggy haze that obscures much of the lower atmosphere from view.
Similar to Jupiter, Saturn also features storm systems and alternating bands of color, though they are much less distinct on Saturn than Jupiter. Ammonia clouds high in the planetís atmosphere are responsible for the whiter bands. The extent of storms and bands in Saturn's is difficult to appreciate in true color but becomes far more obvious in other wavelengths. Many of the photos from the Voyager probes were taken in ultraviolet light to show attributes of the clouds, often resulting in rich blue, red, and violet colors. Cassini has also observed many storms in Saturn's atmosphere using an infrared camera.
Though its atmosphere may be somewhat bland in appearance, what makes Saturn a far more remarkable sight is the complex set of rings surrounding the planet. Composed primarily of water ice and dust particles, the rings consist of several different bands that reflect light differently. These differences give each of the major rings a different brightness and shade of color. Though the rings are gray overall, the shade can vary from very dark at the innermost rings to a very light, almost white color towards the middle. The rings have been one of the primary topics of study for the four spacecraft that have visited Saturn to date. In addition to Pioneer 11 and the two Voyagers that made brief flybys of the planet, NASA's Cassini is currently orbiting Saturn to make a detailed survey of the planet and its many moons.
The seventh stop along our tour of the solar system is distant Uranus. Uranus is difficult to see from Earth because of its great distance, and NASA's Voyager 2 is the only probe that has visited the world so far. Unfortunately, the images returned by Voyager reveal little detail in the planet's fuzzy atmosphere. The atmosphere, like those of the other gas giants, is predominantly composed of hydrogen and helium gases. It is small amounts of methane gas in atmospheric clouds that give the world the light blue color shown in the following Voyager 2 photo.
Wispy white clouds can also be seen in some of Voyager's photos as well as in more recent views of the planet collected by observatories on Earth. Voyager 2 visited Uranus in 1986 and no further missions to the large planet are currently planned.
Even more distant from Uranus is another blue-tinted gas giant, the planet Neptune. Voyager 2 is also the only spacecraft to visit distant Neptune thus far having completed its flyby mission in 1989. In so doing, Voyager found a world with a thick atmosphere of hydrogen, helium, methane, and ammonia gases plus water ice. Methane clouds give Neptune a bluish color similar to that of Uranus, but Neptune appears somewhat darker because it receives less light from the Sun.
Voyager 2 also viewed a much more dynamic atmosphere during its visit to Neptune than the craft had seen at Uranus. Neptune's atmosphere is marked by cirrus clouds and large storms, most notably the Great Dark Spot. This darker bluish-gray region circling the planet's southern hemisphere is surrounded by white clouds of ammonia and is similar to storms on Jupiter. More recent images of Neptune taken by the Hubble Space Telescope, however, indicate that the massive storm has disappeared.
All of the planets we have discussed so far have been visited by spacecraft that have sent back close-up photos telling us much about the world's appearance. That is not the case with our final port of call, the tiny and distant planet of Pluto. The ability of telescopes here on Earth to see Pluto continues to improve, but we will not be able to get a clear picture of what this world looks like until the New Horizons probe reaches Pluto in 2015.
Shown above is the best photo astronomers have yet been able to take of Pluto. The imagery indicates that Pluto is predominantly a tan or light brown color. The planet is probably composed mostly of ice and rock, and the brownish hue is likely due to methane frozen on its surface. Similar in structure to a comet, Pluto is often described as a "dirty snowball" with a composition that could change considerably depending on the relative quantities of ice and rock that make up different regions on its surface. The darker area near the equator in the above photo, for example, could indicate a greater concentration of darker rock along that portion of the planet. Since Pluto appears to be much brighter than its large moon Charon, scientists are eagerly awaiting the arrival of New Horizons to learn more about the structure and geology of these unique worlds.
In closing, it should be noted that most astronomers do not refer to the colors of the planets using common names
like blue and brown as we have done in this discussion. Names like these are very imprecise since what is orange
to one person might be red to another. Astronomers will typically refer to the specific spectra emitted by a
planet to remove any ambiguity in describing its color.
- answer by Justine Whitman
- answer by Molly Swanson, 5 March 2006
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